We traveled to Nevada to visit my dad, Devin Thomas, and make some Damascus with Pop’s ProCut. Along the way we saw some sites around my hometown and my wife tries to prove that her town is better.

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Video

Here is the video version of the following information:

What is Tamahagane?

Tamahagane is the name used in Japan for steel made with their centuries-old process. There are several videos available online to see the process such as this one. It starts with “iron sand” which is a naturally occurring form of iron mostly composed of magnetite, Fe₃O₄, though can also contain some hematite (Fe₂O₃) and other constituents like quartz and titanium dioxide [1]. One nice thing about magnetite (as the name suggests) is that it is magnetic, so it can be relatively easily separated from other minerals with magnets. The iron sand is then “smelted,” which is the process of reducing the oxide ore to the base metal, in this case, iron. The smelting is performed in a traditional furnace called a tatara. A clay tub is used which is layered with charcoal and the iron sand [2]. The charcoal acts as a reducing agent to turn the magnetite into iron, and also is a carbon source to turn the iron into steel (steel is iron plus carbon). The charcoal reduces the steel because carbon monoxide and carbon dioxide forms when it burns in air. The carbon monoxide (CO) reduces the magnetite to iron [3]. At the end the clay vessel is broken and the tamahagane is removed, which is in the form of rough and porous sponge-like pieces that also contains slag (large undesirable inclusions). The swordsmith separates pieces by “grade” which corresponds to the approximate content [4]. The grade is determined by color and also the fracture appearance [4]. The swordsmith then forges the pieces gently, and then folds and forge-welds them multiple times to eliminate porosity, remove some slag, even out the carbon content, and break up inclusions and impurities.

The “sponge-like” pieces of tamahagane after smelting. Picture from Wikipedia.

Soo all of that describes the manufacturing process. Ultimately tamahagane is a simple carbon steel. It is even simpler than a typical carbon steel like 1095 or White #1, because it is lacking intentional additions to those steels of manganese and silicon. So the steel has carbon of some content (described next) and then only trace amounts of other elements.

While there are a few individual swordsmiths that make their own tamahagane, most of the tamahagane produced in Japan comes from Nittoho tatara in Shiname Prefecture, the last of its kind. All tamahagane production had ceased by World War II. But this facility was restarted in 1977 and continues to produce the traditional steel for Japanese swordsmiths.

Published Tests of Tamahagane

Carbon Content and Composition

The cutting edge of Japanese swords has typically been found to be around 0.5-0.8% carbon [2, 5-9]. However, there are different types of construction used in Japanese swords, such as the “kobuse” method that uses a low carbon core and a higher carbon sides and cutting edge. Thus, the low-carbon pieces of tamahagane can be used in these low-carbon portions, giving the blades greater ductility and reducing the chance of catastrophic breakage. Measurements of the core carbon content have returned values between 0.01% [7-8] and 0.2% [2]. Sometimes steel with a carbon content in between these two extremes is used on the sides [2,7-8].

Schematic of “kobuse” construction from [3]

However, the carbon content is not always perfectly distributed throughout. Analysis of a modern kogatana knife (utility knife) produced by Sadanao Mikami had a range of carbon from 0.6% on one end to 2.0% on the other [5-6]. The uneven carbon content can come from distributions within the original pieces, mixing of low carbon and high carbon pieces (often done intentionally), and performing an insufficient number of forge-folding operations to even out the carbon. They reported that the average carbon content was 1.03% for the entire knife [5]. There were also small amounts of other elements:

Composition of modern kogatana by Sadanao Mikami [5]

Microstructure and Hardness

Japanese swords are produced to have a “hamon,” a visible line that distinguishes between a soft spine and the hard cutting edge. Thick clay is applied to the blades at the spine to slow the quench rate so that it does not harden. The hard phase at the edge is martensite, and the soft phase is pearlite. At the hamon transition, there is a combination of martensite and pearlite. The low carbon core contains large amounts of soft ferrite. The pearlite content of the core is determined by the carbon content, more carbon means more pearlite.

Images of microstructure from a “modern” sword (1945) and an “old” sword (~600 years old) at different locations [9]

Micrographs of a ~600 year old sword [7]

Studies on the hardness of Japanese swords correspond to these differences in microstructure and carbon content. Typically, the hardness at the edge is between 600-900 HV [2,3, 5,7, 9] and the spine is between 100-300 HV [2, 3, 7, 9]. This corresponds to about 55-67 Rc at the edge and 0-30 Rc at the spine (Note: Rockwell C hardness only reliably goes down to 20).

Microhardness values of ~600 year old sword [7]

Microhardness of a “Koto” style Japanese blade (before 1596 AD) [3]

Microhardness from the 1945 and 600 year old swords [9].

Impurities, Inclusions, Forge Welding, and Folding

A study was performed along with swordsmith Sadanao Mikami on the effect of forging and folding on impurities and inclusions in tamahagane [10]. The process is shown in the figure below:

The forge welding and folding process used in Ref. [10]

The swordsmith started by hammering chunks of tamahagane into sheets, then cutting those sheets into chips. The chips are sorted by carbon content by hitting them with a hammer. High carbon chips break and low carbon chips don’t. The low carbon chips were stacked together and forged together into a bar, then cut and restacked six times (A1-A6); this was repeated with the high carbon chips (B1-B6). Then the low and high carbon bars were stacked together and forge welded together, followed by cutting, restacking, and forge welding six more times (C1-C6). The microstructure became more evenly distributed through the forge welding process as the carbon content became more equalized, as shown in the following images:

Microstructure evolution of tamahagane after forge-folding operations [10]

One common claim of folding and forge welding is the elimination of inclusions, oxides and slag that are present in the original tamahagane pieces. The researchers found that while bands of FeO were present in A1 and B1 from oxides present in the original pieces, they were not found in A6 or B6. In other words, the FeO bands were eliminated by further forging and forge welding. They were originally present in bands from forge welding flat pieces together. They also found silica (SiO₂) and Fayalite (Fe₂SiO₄) inclusions in the steel. The silica particles were less than 1 micron and the Fayalite inclusions were 16-20 microns in length in the A1 and B1 samples. The thickness of the Fayalite bands was significantly smaller than the FeO, about two microns.

Iron oxide bands forge welded into tamahagane during initial forge welding

The Fayalite was reduced from 16-20 microns long in the A1/B1 specimens to 14-15 microns in the A6/B6 specimens. These were further reduced to 13.1 microns in the C1 specimens and down to only 8.2 microns in the C6 specimens. So while forge welding and folding did not significantly reduce the volume fraction of Fayalite inclusions, they were reduced in size, more isotropic (round), and more evenly distributed after that process.

Inclusions in tamahagane steel in the C1 condition (one forge weld of low and high carbon steel), left, and after six folding and forge welding operations, right [10]

In a study on a kogatana knife produced by Sadanao Mikami had relatively homogenous oxide inclusions in the size range of 5-10 microns [5]. They reported a volume of 4-7% inclusions.

Oxide inclusions in modern tamahagane knife [5]

Analysis of a ~600 year old sword [7] found an inclusion content of 0.8-1.9%, which they found to be much higher than the content in a modern production steel (labeled “ordinary steel”).

Volume percent of inclusions in ancient sword vs modern steel [7]

Micrographs of inclusions in ancient sword [7]

Analysis of another ancient sword found an inclusion volume of 1.1% [3]. The authors claimed that this inclusion content is low enough to avoid a “significant reduction in its toughness.”

Inclusions in pre-1596 AD sword (cross-hair scratch added intentionally) [3]

Strength and Toughness

Tensile tests of a sword by swordsmith T. Takaichi were performed and compared with a conventional carbon steel with similar carbon content of 0.55% C [2]. They found that in testing of the steel before heat treating that the conventional steel (S55C) had greater utility than the tamahagane sword steel (S1). When observing the fracture surfaces, they found oxides in the tamahagane and they speculated that it was these oxides that led to the lower ductility.

Stress-strain curves and fracture appearance of tamahagane sword steel (S1) and conventional steel (S55C) [2]

In tests of a ~600 year old sword and a 1945 sword, they measured the strength and ductility of the swords with a 4-point bending test [8-9].

4-point bend test of tamahagane swords [9]

They found the ancient sword to form a crack at around 0.88 ton-force, but the blade did not fully break and continued bending until final fracture. The 1945 sword broke at 2.62 ton-force and did not have the load drop from cracking that the ancient sword had. They calculated the bending strength of the swords as 2552 MPa for the ancient sword and 4645 MPa for the 1945 sword. They claimed that these results are “in a range of performance tool steel” [8]. The hardness of the cutting edges were similar in the two swords. Instead, they said the difference in measured strength was because the 1945 sword had a 5 mm hardened area while the ancient sword had only 1 mm of hardened area (see earlier microhardness charts).

I didn’t find much other toughness testing of tamahagane so comparisons between this traditional steel and modern carbon steels are in short supply. I found one study of miniature charpy specimens taken from a Japanese sword [11], but they did not compare with similar tool steels. Furthermore, the values at the edge of the knife where we would probably be more interested were essentially zero:

Image taken from [11]

Effect of Carbon Content on Final Properties

While there seems to be a consensus that the cutting edges of tamahagane Japanese swords are in the range of 0.5-0.8% carbon or so, I am not sure if the carbon content was and is that consistent. There was the one study on the tamahagane knife where they found carbon values as high as 2% within the same knife that read 0.6% in other locations [6]. I also found a study on different iron sand use at the Nittoho tatara (the last major facility remaining), and found that they had moved to a different source of sand that resulted in higher carbon and lower impurities [4]. So I think there is reason to believe that carbon content of tamahagane has gone up in recent decades.

The carbon content is very important to steel properties. One of the best known is the effect of carbon on the resulting as-quenched hardness. The higher the carbon content, the higher the hardness, though it levels out around 0.6% carbon:

Image from [12]

Carbon has an equally strong effect on impact toughness, even when tested at the same hardness for different low alloy tool steels:

Higher carbon also gives steel more iron carbide (cementite) which gives it greater wear resistance. This can give knives greater edge retention because edge wear is slowed. However, for a sword this wear resistance is not really beneficial, and toughness is a much more important property. So the 0.5-0.7% range of carbon would be about optimal for a sword, given the high combination of hardness/strength and toughness that you get from this microstructure. But for smaller knives, kitchen knives, etc made with tamahagane higher carbon could be beneficial for wear resistance.

1.25%C steel with significant carbide content (white particles)

0.6%C steel with almost no visible carbide

My Tamahagane for Testing

I recently went to Japan and took a class from Yuya Nakanishi, also called Masahiro. You can watch a video of our trip to Japan and making the “kogatana” (a utility knife). We took home our two knives that we made with him. However, I also wanted to test the steel for toughness which would be difficult with just the knife, as the standard toughness test uses rectangles. So I asked him if I could have a piece for toughness testing and he let me have one. This piece had already been through the forge welding and folding process. I also asked him what temperatures to use for heat treating and he told me 780°C for austenitizing and 180°C for tempering. I normalized and annealed the piece first, and followed his recommendations by austenitizing for 10 minutes, quenching in water, and tempering twice for one hour at each time.

Image from https://morinokyoto.jp/masahiro_en/

Carbon Content

I measured the broken toughness specimen with LECO combustion for carbon and sulfur content and got 1.06% carbon and 0.001% sulfur. This carbon content puts it into a range similar to White #2 (1.05-1.15%C). This is a good range for a utility knife, though as explained above would be on the high side for a sword. I am not sure if this is because Masahiro intentionally used higher carbon tamahagane for these smaller knives, or if it is due to carbon fluctuations, or higher carbon tamahagane being made at the Nittoho tatara. If I hear back from Masahiro I will update the article. This is very close to the carbon content in the kogatana by Mikami of 1.03% [5], which perhaps provides evidence that the carbon content of tamahagane has increased from older blades. I wasn’t able to measure other elements because of the size of the steel that I had.

Micrographs

The carbide structure looks consistent with the measured carbon content of 1.05%. It has a significant amount of small and evenly distributed carbides, but a smaller volume than the higher carbon 26C3. It looks closer to O1 and 52100, though the carbides are even finer in the tamahagane.

Micrograph of the heat treated tamahagane

O1 steel

52100 steel

Looking at a lower magnification micrograph, the carbide structure appears relatively consistent throughout. There are no visible layers from the forge-folding process. There is a “bright spot” in the upper left portion of the image. Higher magnification reveals that this is not carbide clusters but either an etching artifact or retained austenite.

Lower magnification of tamahagane

Impurities and Inclusions

The inclusion content of the tamahagane provided to me is very low, lower than reported inclusion contents in the journal articles that I cited. Using threshold measurements I estimate the inclusion content to be around 0.1%. I am not sure why this steel is so much “cleaner” than those analyzed in the other studies.

Polished (unetched) surface of tamahagane. Black particles are inclusions.

There is a small amount of orange, square/rectangular particles in the steel. These are likely titanium nitride (TiN) as these particles are typically yellow/orange and angular [13]. Iron sand typically has TiO₂ in it [1], and the Mikami blade had 0.03% Ti in it [5], so the presence of TiN is not entirely unexpected. Titanium and nitrogen have a strong affinity for each other, and these particles are often found in steels with small titanium additions.

Titanium nitride particles (TiN)

Hardness

The hardness of my toughness coupon was 64.6 Rc. This is relatively high in hardness, maybe even a bit high for a utility knife. However, it is within a reasonable range, and would provide very good cutting and edge retention. This lines up closely with tempering of other simple high carbon steels, such as 1.25% carbon steel 26C3, shown below. The 26C3 is somewhat lower in hardness, though this steel was tempered in another oven. My furnace is very good at maintaining a consistent temperature during tempering, so “overshooting” is less common.

I am not able to measure the hardness of the edge of the knife that I made at Masahiro Tantojo, as it is only hardened at the edge. It would require cutting it apart and using microhardness measurements to see. Since the knife was tempered by eye by the swordsmith it could be significantly different in hardness. The tempering was performed for a very short period, until it reached a temperature where water would sizzle when he touched the blade. My guess is that it is higher in hardness, somewhere between 65-67 Rc. However, if the carbon content is significantly lower in the blade it is possible that it would also be lower in hardness.

Toughness

Given the relatively high hardness of 64.6 Rc, the toughness was not chart topping. I measured 5.8 ft-lbs. However, this is a relatively high value for that level of hardness, as you can see when compared against other low alloy steels here:

As you can see the toughness falls right on the same line as 26C3 (green line in the upper image). So the tamahagane appears to be behaving similarly to conventionally produced low alloy high carbon steels. Overall I think this is a good result considering that some studies I cited before showed a potential reduction in toughness from oxides.

Edge Retention

Comparing the edge retention is a bit tricky since the kogatana design is different than the standard test knife that I use for CATRA testing. This knife is thinner behind the edge, about 0.010″ (0.25 mm) after sharpening, while our test knives are around 0.015″. This gives the thinner knife an advantage for a given edge angle, as shown in this earlier study on 154CM steel:

There is also the complication that I do not know the hardness of the kogatana knife, my guess is it is in the range of 65-67 Rc. When I tested it with CATRA I got 370 mm, which would also lend credence to the hypothesis that it is on the upper end of hardness.

There aren’t many low-alloy steels I have tested that would be in that hardness range. The light grey dotted lines show the approximate relationship between hardness and edge retention. So you will notice that the tamahagane is on the same line as most of the other low alloy steels, especially after accounting for the knife being thinner behind the edge. This is not too surprising given that most of the low alloy and carbon steels have tested in a similar range after accounting for hardness.

Tim Zowada Information

Tim is a bladesmith who makes his own bloomery steel using iron sand from Lake Superior. He was an author on one of the cited papers above along with Dr. John Verhoeven. He had the following comments after reading my report:

Here are a couple things that might be helpful. In general:

1. The final carbon content is determined by the average initial carbon content and the number of folds. More folds decrease the carbon content. The outside of the billet will decarburize during forging. That decarb layer is folded in with each weld, decreasing the average carbon content. Plus, the oxidation of the billet (scale) will decrease the overall size of the billet during all the folding and welding. For high quality steel, the typical number of of folds will be between 10 and 15. It ends up being a balancing act based on experience. Yoshindo Yoshihara prefers about 0.7% carbon for his swords. His brother prefers 0.6%. Maybe I have that backwards. The Art of the Japanese Sword (Yoshihara) is an excellent book that you should own. I’m not sure why the carbon content varied so much in your blade. My best guess is a low number of folds. A quick nital or ferric chloride etch would tell. You would be able to see lighter and darker bands of varying carbon content. Typically, the carbon content is pretty much homogeneous through the bar by about eight folds.

2. High hardness and low abrasion resistance is an advantage for my straight razor work. At 65 HRC, the blades hold up very well against whiskers. But, with the low abrasion resistance, a clean leather strop will remove enough metal to keep them sharp. They don’t need to go to the stones very often.

3. Like many things, the final carbon content and quality of the steel will vary from smith to smith, and batch to batch. I think what you got was a good representation of high carbon (1.0%) tamahagane.

Summary and Conclusions

Reviewing the prior studies on tamahagane, researchers of this material have been pretty realistic about its properties. At its best, tamahagane performs like other simple carbon steels. The potential for higher oxide inclusion content potentially limits its toughness, though in my own experiments it was roughly in line with the toughness you would expect for a steel made with modern methods. It is very interesting to learn about traditional methods for making steel and how they compare to the more advanced methods that we have developed since.

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[1] Tanii, Hiroshi, Tadahiro Inazumi, and Keiichi Terashima. “Mineralogical study of iron sand with different metallurgical characteristic to smelting with use of Japanese classic iron-making furnace “Tatara”.” ISIJ international 54, no. 5 (2014): 1044-1050.

[2] Okayasu, Mitsuhiro, H. Sakai, and T. Tanaka. “Mechanical Properties of Samurai Swords (Carbon Steel) Made Using a Traditional Steelmaking Technology (tatara).” Journal of Material Sciences & Engineering 4, no. 2 (2015): 1-6.

[3] Verhoeven, J. D., and Tim Zowada. “Comparison of Two Swords of Antiquity: The Japanese Sword and the Muslim Crucible Damascus Sword.” Metallography, Microstructure, and Analysis 12, no. 6 (2023): 934-943.

[4] SUZUKI, Takuo, and Kazuhiro NAGATA. “Effect of the charge of” Komori” iron sand on the properties of” Tamahagane” steel produced by” Tatara” operation.” Tetsu-to-hagané 85, no. 12 (1999): 911-916.

[5] Das, Ananda Kumar, Takuya Ohba, Shigekazu Morito, and Muneo Yaso. “Evidence of Lath Martensite in High-C Japanese Sword Produced from Tamahagane Steel by Tatara Process.” In Materials Science Forum, vol. 654, pp. 138-141. Trans Tech Publications Ltd, 2010.

[6] Matsumoto, Chihiro, Ananda Kumar Das, Takuya Ohba, Shigekazu Morito, Taisuke Hayashi, and Go Takami. “Characteristics of Japanese sword produced from tatara steel.” Journal of Alloys and Compounds 577 (2013): S673-S677.

[7] Yaso, Muneo, Toshifumi Takaiwa, Yoshihiro Minagi, Kunichika Kubota, Shigekazu Morito, Takuya Ohba, and A. K. Das. “Study of microstructures on cross section of JAPANESE SWORD.” In European Symposium on Martensitic Transformations, p. 07018. EDP Sciences, 2009.

[8] Yaso, Muneo, Yoshihiro Minagi, Toshifumi Takaiwa, Kunichika Kubota, Tsuyoshi Kanaizumi, Takuya Ohba, Shigekazu Morito, and Taisuke Hayashi. “Study of Metallurgy and Mechanical Property on Japanese Sword.” In Materials Science Forum, vol. 738, pp. 222-227. Trans Tech Publications Ltd, 2013.

[9] Yaso, Muneo, Toshifumi Takaiwa, Yoshihiro Minagi, Tsuyoshi Kanaizumi, Kunichika Kubota, Taisuke Hayashi, Shigekazu Morito, and Takuya Ohba. “Study of Japanese sword from a viewpoint of steel strength.” Journal of Alloys and Compounds 577 (2013): S690-S694.

[10] Takami, Go, Takuya Ohba, Shigekazu Morito, and Ananda Kumar Das. “Microstructural observation on materials of the japanese sword under fold-forging process.” In Materials Science Forum, vol. 654, pp. 134-137. Trans Tech Publications Ltd, 2010.

[11] Misawa, Toshihei, and Shin-Ichi Komazaki. “Ductile-brittel transition evaluation of Japanese sword and weld metals using miniaturized impact specimens.” In European Structural Integrity Society, vol. 30, pp. 119-125. Elsevier, 2002.

[12] Krauss, George. “Martensitic transformation, structure and properties in hardenable steels.” Metallurgical Society AIME,(1978): 229-248.

[13] Descotes, Vincent, Thibault Quatravaux, Jean-Pierre Bellot, Sylvain Witzke, and Alain Jardy. “Titanium nitride (TiN) germination and growth during vacuum arc remelting of a maraging steel.” Metals 10, no. 4 (2020): 541.

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Video

Here is the video version of the following information:

Misunderstandings about Hardness and Corrosion Resistance of MagnaCut

I have been seeing a disturbing number of comments around the internet saying something like the following: “Larrin says that 60 Rc MagnaCut has better corrosion resistance than 64 Rc MagnaCut, so there are different hardness levels for different applications.” I have never said such a thing. It is true that there are different levels of hardness for various applications, but hardness does not dictate corrosion resistance; therefore, it is not one of the parameters by which they would make this decision. I have a video where I have talked about the pros and cons of higher and lower hardness. How you heat treat a given steel can affect its corrosion resistance but a whole range of hardness values can be achieved with both good or bad heat treatments. I will give a variety of examples for this. A separate but related topic is about how hard different steels can be heat treated to when related to their corrosion resistance. For example, the very corrosion resistant Vanax and LC200N top out around 59-61 Rc. I will explain why they don’t get any harder and why this is partially due to those steels having very high corrosion resistance.

Chromium and Corrosion Resistance

When iron is in a corrosive environment it forms rust – an iron oxide. If you add chromium to iron, the corrosion resistance is increased the more chromium you add. The chromium forms with oxygen to make a passive film at the surface which prevents rust from forming:

The following chart shows an old classic study [1] of corrosion rate of steel in high humidity environments. You will see that the corrosion rate went down with increasing chromium until about 12% Cr. Sometimes the cutoff for a steel being “stainless” is given as 10.5, 11, or 12% chromium. There isn’t any real agreement as far as I can tell. But corrosion resistance will increase with even greater amounts of chromium; it isn’t an on/off after some arbitrary cutoff.

Data adapted from [1]

Heat Treating and Corrosion Resistance

In the annealed (soft) state stainless steel is not yet stainless. When a knifemaker or manufacturer receives steel it is in this annealed condition so it is easy to drill, grind, machine, etc. Most of the chromium is in the form of carbides in the steel. When the chromium is tied up with carbon as a carbide it cannot form the chromium oxide layer at the surface.

There are three major steps to heat treating: austenitizing, quenching, and tempering.

Austenitizing

During austenitizing the steel is heated up to a high temperature and then held, allowing the chromium carbides to be dissolved (or partially dissolved) to put more chromium in solution so that the chromium oxide passive layer can be formed. Below shows the increase in chromium vs austenitizing temperature for Elmax and Vanax:

So one of the key variables for corrosion resistance is the austenitizing temperature because heating it hotter means more chromium in solution. However, the amount of chromium in solution is also controlled in part by the composition. The chromium in solution for Elmax at a very high temperature of 2200°F (the datasheet recommends no higher than 2010°F) is still below Vanax when it is austenitized at 1800°F (well below what the datasheet recommends). Austenitizing at a higher temperature also leads to increased hardness, so in that way higher hardness can sometimes mean improved corrosion resistance.

Quenching

During quenching the steel is rapidly cooled to form the hard phase of steel, martensite. Martensite formation is controlled by temperature rather than time. There is a “martensite start” temperature and a “martensite finish” temperature. In some cases martensite finish can be below room temperature and in that case the steel does not fully transform, meaning some austenite remains in the steel. This is called “retained” austenite. If there is too much retained austenite the hardness is reduced in the steel, and also the edge performance is bad and the knife would be very difficult to sharpen. One of the major factors for martensite start (Ms) and martensite finish (Mf) temperatures is the carbon content, as shown in the chart below.

With higher austenitizing temperatures and more carbide being dissolved, this does not only put more chromium in solution but also more carbon, as seen in this chart of Vanax and Elmax. Or in the case of Vanax, I included the carbon and nitrogen, since nitrogen also contributes to hardness in a similar fashion to carbon.

All of the above happens as long as the quench is sufficiently fast to avoid other competing transformations that will occur with slow cooling. Carbides can precipitate during slow cooling which would also reduce corrosion resistance. This can happen with the relatively common gas quenches that are used in large vacuum furnaces by many knife manufacturers. I covered this in a previous article/video about custom vs production heat treating.

Cryo and Cold Processing

You will notice that the Vanax hardness dropped at 2025°F while the Elmax hardness increased all the way up to 2150°F. These heat treatments included a cryo step in liquid nitrogen after the quench. If cryo had not been used the Vanax would have seen a drop below 2025°F and the maximum hardness would not have been as high. The Elmax likely also would have seen a hardness drop in this tested range. As an example, the following chart shows AEB-L steel when quenched to room temperature vs placed in a freezer or in liquid nitrogen:

AEB-L steel vs austenitizing temperature

The AEB-L achieved its peak hardness around 1975-2000°F just like the Vanax and then dropped in hardness with further increases in austenitizing temperature. It reached a max hardness of around 64 Rc, though when no cold treatment was used it maxed out around 62 Rc. The reason for the drop in hardness above a certain temperature is because of retained austenite. In other words, the martensite finish temperature was below room temperature. Using liquid nitrogen after the quench means cooling the steel to a lower temperature getting closer to martensite finish. However, some retained austenite will stabilize and not be transformed even with the very low temperatures of liquid nitrogen, which is why the hardness still drops with an austenitizing temperature that is too high.

So the use of cryo can allow both higher hardness and corrosion resistance if the austenitizing temperature is increased. For a fixed austenitizing temperature, typically there is a 1-2 Rc increase but the corrosion resistance would not be affected.

Tempering

After quenching, steel is tempered to improve its toughness. The tradeoff is that hardness is also reduced through tempering. Below shows a tempering chart for MagnaCut with different austenitizing temperatures.

Different austenitizing-tempering combinations can be used to achieve the same level of hardness. For example, to achieve ~60 Rc, you could use 1950°F and 300°F, 2000°F and 400°F, or 2100°F and 500°F. Because the higher austenitizing temperature potentially means more chromium in solution we would expect the 2100°F and 500°F combination to achieve the best corrosion resistance for that hardness. A unique aspect of MagnaCut is that all of its chromium carbide is dissolved around 2050°F, as shown in the chart below. So if instead we were heat treating to ~63 Rc, 2050°F and 300°F, 2150°F and 400°F, and 2200°F and 450°F would all have approximately equal corrosion resistance. So austenitizing above 2050°F still leads to an increase in hardness for a fixed tempering temperature but the corrosion resistance would be unaffected.

There is an important aspect to tempering and corrosion when it comes to a different regime of tempering, however. Stainless steels and other high alloy steels see a bump in hardness by tempering above about 750°F, as shown in the following chart for Elmax:

You can see that the hardness sees a peak with tempering around 500°C (930°F). This increase in hardness comes from precipitation of small chromium, molybdenum, tungsten, and vanadium carbides. Because chromium is coming out of solution as a chromium carbide this “secondary hardening” also leads to a decrease in corrosion resistance. Below shows Vanax steel that I tested with a 1% saltwater spray test for 24 hours where one was tempered at 400°F and the other was tempered at 1000°F. The 400°F temper Vanax showed no corrosion while the 1000°F temper led to significant rusting.

Vanax tempered at 400°F (left) or 1000°F (right) before a 1% saltwater spray test

It is relatively common for knifemakers and knife manufacturers to temper at 950-1000°F. They do this for several reasons, though the main one is that the higher tempering temperature means the steel is less sensitive to overheating. This allows them to apply certain coatings to steel (that require higher temperatures to apply than a typical “low” temper of 400°F). It also means they don’t have to be as careful with grinding because the steel can be heated up much higher before approaching the tempering temperature (where the steel would soften).

The high tempering temperature can be used with many stainless steels to achieve similar levels of hardness as the low tempering temperature. In other words, you can have two knives at 60 Rc, 62 Rc, etc. with very different corrosion resistance depending on whether they tempered in the low range or the high range.

Hardness vs Corrosion Resistance for Different Steels

So after all of that we can return to our discussion of what makes one steel more corrosion resistant than another. Below shows Vanax and Elmax after a 1% saltwater spray test for 72 hours:

The Elmax rusted while the Vanax did not. Both were austenitized at 1975°F which is the recommended temperature for both steels according to their datasheets, and both were given a temper in the low range (400°F). As shown in the earlier chart, the difference in corrosion behavior is primarily due to higher chromium in solution for Vanax, which would roughly be 11.5% for Elmax and 14.5% for Vanax. As seen in the hardness chart, Vanax would actually be a bit harder than Elmax for this identical heat treatment. However, Elmax can be austenitized at higher temperature and achieve significantly higher hardness than Vanax which reaches its maximum around 61 Rc. The reason is because chromium also reduces martensite start and finish temperatures. Vanax has more chromium in solution for a given carbon/nitrogen in solution:

This means that for a given austenitizing temperature, Ms and Mf are lower in Vanax than in Elmax. So as seen below with an austenitizing temperature of 2025°F, the Ms was about 150°C. The Mf is sufficiently low that even with liquid nitrogen not all of the austenite is transforming to martensite, and thus hardness was lower with 2025 than 2000°F. Elmax, however, still has a predicted Ms above 150°C all the way to 2200°F, which is why its hardness increased all the way to 2150°F (the highest temperature I tried).

Thus an approximate relationship is created when comparing different steels to each other for their maximum hardness vs their corrosion resistance:

This trendline is usually correlated with chromium in solution. For example, Vanax and LC200N have >14% chromium in solution, which gives them a very high corrosion rating but also limits their potential hardness. On the other end of the spectrum is ZDP-189, which achieves very high hardness and is advertised as a stainless steel, but I discovered a few years ago that it is not, in fact, stainless. I measured only 8.6% chromium in solution with ZDP-189, which is quite low. D2, for example, I measured at 7.7% with a relatively conventional heat treatment, which is famously called a “semi-stainless” steel.

There are other complicating parts to this trendline, however. It does not always perfectly correlate with chromium in solution. Molybdenum, for example, is an element known to improve pitting resistance and to strengthen the chromium oxide passive film. In tests I did with 1% saltwater there was an improvement in corrosion resistance for a given chromium in solution if the Mo was increased:

CPM-154 with its high molybdenum content (4%) had acceptable corrosion resistance even with only 9.5% chromium in solution. (Note: the “corrosion ratings” in this chart are a rating of how much the samples rusted and not the same as the “corrosion rating” in the hardness trend plot which are from my knife steel ratings).

You may have also noticed that S90V, S125V, M390, and S110V are all above the hardness-corrosion resistance trendline. In other words, they have unexpectedly high maximum hardness for their corrosion rating. These steels all have a very high volume of carbide (>20%). Carbides are very hard particles and when the volume of them is reaching these high levels it can affect the bulk hardness that is measured. You can see that for the following two charts comparing non-stainless tool steels with different amounts of carbide (Vanadis 4 Extra, 10V, and 15V):

So while M390 and 14C28N both have similar Cr in solution (depending on austenitizing temperature), the M390 reaches higher hardness because it has so much more carbide:

M390 has a lot of carbide

14C28N not much carbide

The other steel above the trendline for hardness-corrosion resistance is MagnaCut. It does not have particularly high carbide volume, about the same as Vanadis 4 Extra:

MagnaCut also does not have particularly high chromium in solution, around 11%. Instead, MagnaCut has superior corrosion resistance to those other steels because all of its chromium carbides are dissolved in heat treatment. With typical stainless steels they have many chromium carbides, and the chromium in solution is lower around each carbide:

Thus the chromium carbides act as corrosion initiation sites. This is somewhat similar to the phenomena called “sensitization” in low carbon stainless steels that are cooled too slowly.

Image from [2]

Instead, all of the carbides in MagnaCut are vanadium or niobium carbides (after heat treating). Therefore those corrosion initiation sites are missing and so it has superior corrosion resistance for a given level of chromium in solution.

Comparing Steels vs Comparing Heat Treatments

Perhaps it is the above discussion that confuses some people, and they mix the low potential hardness of Vanax and LC200N with heat treating of other steels and assume that lower hardness means better corrosion resistance. However, as I have discussed, hardness and corrosion resistance don’t really correlate for an individual steel. If anything, higher hardness sometimes correlates with higher corrosion resistance. If you use higher austenitizing temperatures in combination with a fast quench and cryo, and temper at a low temperature, you get maximum corrosion resistance. That is also a recipe for high hardness (as long as the austenitizing temperature isn’t too high). However, tempering can be increased without detriment to corrosion resistance to heat treat to lower hardness levels (as long as the >750°F range is avoided).

Summary and Conclusions

Austenitizing temperature, quench rate, and tempering can all affect corrosion resistance. Austenitizing higher leads to both higher hardness and better corrosion resistance. Beyond a certain temperature cryo is necessary to get higher hardness, and even then there is a temperature beyond which hardness decreases. Quenching too slowly, as happens often in industry, can lead to a reduction in corrosion resistance. Tempering in the high temperature range (>750°F), also relatively common, significantly reduces corrosion resistance. Combinations of heat treating variables can be used to achieve a range of hardness values while maintaining high corrosion resistance. In other words, hardness does not correlate with corrosion resistance for a given steel. However, when comparing different steels to each other, there is a trend where more corrosion resistant steels typically have a lower maximum hardness. Molybdenum alloying, avoiding chromium carbides (such as in MagnaCut), and having a high volume of carbide all lead to higher potential hardness for a given level of corrosion resistance when comparing different steels to each other.

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[1] Binder, W. O., and C. M. Brown. “Atmospheric Corrosion Tests on High-Chromium Steels.” In Proceedings of ASTM, vol. 46, pp. 593-606. 1946.

[2] https://newzelindustries.com/what-is-sensitization-in-austenitic-stainless-steel/

The post Corrosion Resistance vs Hardness in Knife Steels appeared first on Knife Steel Nerds.

My steel research is supported through Patreon. Patreon supporters saw the news on Pop’s ProCut before anyone else. There is currently an article about the upcoming MagnaMax only available on Patreon.

Video

Here is a video version of the following information:

Development

In March 2024 Joey Berry of Pop’s Knife Supply called me and said he wanted to develop a new steel. He said that their most popular steel was 80CrV2 and so he wanted to make “80CrV3.” “You mean 80CrV2 but with a little more vanadium?” He said no but some kind of “sequel” to 80CrV2 that would be more exciting. I told him that doesn’t give me much to go off of but I would think about whether I had any good ideas along those lines. I thought about gaps in the market in the area of low alloy knife steels (“Carbon steel”) that would also be usable by the knifemaker that is buying 80CrV2. It occurred to me that our selection of high nickel steels is very limited; 15N20, L6, and 8670 are pretty much it. These steels are high in toughness and offer good hardness to go with it, but have no real wear resistance to speak of. 80CrV2 is in a similar position just without the nickel. I thought if we added some tungsten and vanadium to a high nickel steel we could make the steel more balanced; give it some wear resistance along with the high toughness. Those carbide pinning elements would mean that the steels are more beginner friendly for forge heat treating. 15N20, for example, already sees grain growth around 1500°F (815°C) and so its toughness drops very rapidly even when only slightly overheated. If the tungsten and vanadium were kept in check the forgeability, grindability, and polishability would remain high. This would also offer an alternative to other tungsten/vanadium steels like Blue #1, V-Toku2, Wolfram Special, 1.2519, and others. Those steels don’t have much toughness to speak of, so we could combine the best of the nickel steels with the best of the tungsten/vanadium steels. Another exciting element with the high nickel is the possibility to use the steel in pattern-welded Damascus as a “bright” layer. This gives an option for a higher wear resistance steel with better edge retention for that component of the Damascus. I told Joey about my idea and we decided to move forward with it.

We took the composition I came up with to a steel mill in Europe and they agreed to make it. I spent a bunch of time working on optimal annealing for the somewhat similar 15N20 based on the limitations of their production annealing process. Based on that we generated an annealing procedure to make the steel respond well to forge heat treating.

Composition and Tungsten/Vanadium Carbides

Here is the composition of Pop’s ProCut compared with other grades in its category. The far right column says “MC (%)” which is a calculation of how much total vanadium and tungsten carbide each steel contains after heat treating. One thing you will notice is that since tungsten is a heavy element it does not contribute as much to the MC as you might expect. For example, CruForgeV with 0.75% vanadium has more MC than Wolfram Special which has ~2.25% tungsten. ProCut uses a combination of vanadium and tungsten in a similar fashion to O7 and 1.2519 steels. So it has similar MC to steels like Blue #1, 1.2519/O7, and Wolfram Special, and more MC than Blue #2 and O1. The other nickel nickel steels (15N20, L6, and 8670) of course do not have any MC. These very hard carbides give the steel wear resistance.

Austenitizing, Tempering, and Hardness

A major goal for this grade was to be “easy” to heat treat with a forge. This requires a wide range of austenitizing temperature where full hardness is achieved while avoiding a toughness drop with higher temperatures. 80CrV2 steel varies a lot with starting microstructure, and two of the mills that make the steel lead to very different heat treating response:

So we wanted to avoid this issue that 80CrV2 sees. We did this by keeping the chromium low and also working with the manufacturer to dial in the annealing procedure so that carbides aren’t too coarse (making them difficult to dissolve). I also compared the austenitizing response with my recommended anneal (described later):

You can see that the hardness reaches its maximum around 1475°F and then does not change above that temperature. If using the anneal I recommend the microstructure is a bit finer and so this can be dropped even further to 1375°F. Thus for heat treating in a forge you can heat the steel to “a shade brighter” after reaching nonmagnetic and the steel will fully harden. If you forge and follow the recommending normalizing and annealing procedure you can even quench from nonmagnetic if you wish.

If the steel is austenitized in that range (1475°F or higher) the tempering is roughly the same regardless of the austenitizing temperature. Here are datapoints for both 1550°F and 1625°F:

You can see that the hardness is relatively high, still 64 Rc after tempering at 300°F, which is generally as low as I recommend for tempering most any steel. With a 450°F temper the steel is still above 60 Rc.

Toughness

Austenitizing

In our heat treating and toughness experiments we found three different regions of toughness behavior:

Below 1500°F, the steel shows an increase in toughness with increasing temperature. From 1500-1575°F the toughness is roughly flat. 1625°F and above shows a big jump in toughness where it is flat again up to at least 1675°F. The reason for this behavior we figured out by looking at the microstructure. These images were taken by knifemaker Shawn Houston of Triple B Knives:

1425°F, 400°F temper

1475°F, 400°F temper

1600°F, 400°F temper

You can see that the carbide content is significantly reduced by austenitizing at 1475°F as opposed to 1425°F. This significant change in cementite (iron carbide) raised toughness. Then when the temperature is further increased to 1600°F and above, virtually all of the cementite is gone and all that remains is the small volume of hard vanadium and tungsten carbides. The presence of these small carbides is what prevents grain growth even at high temperatures. The carbides “pin” the grain boundaries.

15N20 and L6, in contrast, show a drop in toughness at much lower temperatures because they do not have those carbides to pin grains. The steel 1.2519 does have the W/V carbides but it sees a drop in toughness above 1500°F because it gets excess carbon in solution, leading to plate martensite. The carbon content in ProCut is controlled so that the matrix carbon does not reach excessive levels even at high temperature. This gives ProCut a very wide austenitizing range.

Tempering and Toughness

Like many other knife steels, ProCut sees a peak in toughness with a tempering temperature of around 450°F (230°C). Above that the toughness drops due to a phenomena called “tempered martensite embrittlement” which happens in all steels. There is lower toughness with tempering at lower temperatures as well, of course, corresponding with the higher hardness. The behavior of toughness with tempering temperature is roughly similar whether using the higher or lower austenitizing temperature range.

Hardness vs Toughness

This creates two different heat treating ranges, where the steel can be austenitized high (1625-1675°F) to max out toughness with some cost to wear resistance and edge retention, or austenitizing lower (1475-1575°F) to retain more carbide for wear resistance. The steel still maintains a toughness advantage vs previous tungsten/vanadium steels due to the nickel addition and controlled carbon content with the lower austenitizing range. However, with the high austenitizing range it achieves levels of toughness similar to 8670, 15N20, and L6 but with enhanced wear resistance due to the small tungsten and vanadium carbides.

To compare with other steels you can look at the following chart:

Grinding and Polishing

I don’t have a quantitative test for grindability. Reports from knifemakers so far say that grinding and polishing is very easy with the steel. Some low alloy steels developed for higher wear resistance like CruForgeV are more difficult to polish due to relatively large vanadium carbides that are found relatively frequently. This steel we controlled the W/V content to try to avoid large carbides. Because of the limitations of standard steelmaking (as opposed to powder metallurgy) there are still very occasional larger carbides but they are much rarer than a steel like CruForgeV. So far those few carbides don’t appear to be affecting polishing and finishing.

A lower magnification image of ProCut showing that large carbides aren’t observed most of the time

One of the rare large carbides in ProCut

A micrograph of CruForgeV showing large carbides. Notice there is a magnfication difference vs the ProCut images.

Quenching, Oil Selection, and Hamon

The “hardenability” of ProCut is relatively high, which I also found in testing of 15N20. The high nickel content gives it this hardenability. Air cooling from 1600°F with 1/8″ steel resulted in 60.7 Rc, though from lower temperatures like 1350°F the hardenability is lower (I measured 35.3 Rc). This means that ProCut can be quenched in virtually any quenching oil, even canola, and with relatively thick cross-sections.

This high hardenability means that ProCut is not well suited for developing a hamon. The best choices for a hamon are low hardenability steels like 1095, W2, and 26C3. I won’t say it is impossible to develop a hamon but there are better choices. It is likely still fine for other differential methods like an edge quench.

Forging, Thermal Cycling

Maximum forging temperatures are most greatly affected by the carbon content. High carbon steels are easier to overheat, leading to crumbling while forging. The carbon content of ProCut is limited to ~0.87% so this is not a huge factor with ProCut. For safety I put in the datasheet to limit forging to 2200°F (1200°C). Some knifemakers, especially Damascus makers, push the boundaries of forging temperatures and this can be dangerous. Like with other steels, if you continue to forge when the steel gets too cold this is also a danger in terms of cracking. I haven’t heard a lot of feedback in this area so let me know how the steel behaves for you. The high hardenability means that the steel can sometimes harden in air while cooling from the forging temperature. This can lead to cracking if there are stress risers in the blade.

The “thermal cycling” procedure for ProCut is relatively simple. Like with other steels I prefer to do a single normalize and anneal, so only two steps. I have written about this procedure in past articles. The normalizing step is for dissolving any undesirable carbides and other structures. For this steel it would be in the range of 1600-1650°F where the cementite is dissolved. With a furnace you can hold at this temperature for 10-15 minutes before air cooling. With a forge by eye you just need to heat somewhere into that range or a bit higher.

The annealing procedure is then done by heating to nonmagnetic and slow cooling. I recommend faster cooling rates than is typical for datasheets and this type of anneal is called a “Fast DET” anneal. For simple heat treatments you can slow cool either in a furnace or in an insulating media like vermiculite (available in the garden section of home improvement stores). I tried a range of cooling rates in my furnace. I held the steel for 30 minutes at 1350°F and slow cooled at different rates to measure the annealed hardness. At 50°F/hr cooling the resulting hardness was 23.9 Rc. After 500°F/hr the hardness was 24.5 Rc. I also tried setting the furnace to 1000°F/hr but it only maintained that cooling rate until about 1250°F and slowed from there, the average rate of cooling was about 680°F/hr. That resulted in 26.2 Rc, which is plenty soft enough. Shawn Houston did an anneal with 250°F/hr and he measured 21.2 Rc. Here is the microstructure after normalizing and annealing at 1350°F for 30 minutes, and cooling at 250°F/hr:

Normalized and “Fast DET” annealed microstructure of ProCut (21.2 Rc)

“As-received” annealed microstructure of ProCut (12 Rc)

You can see that the faster annealing procedure results in a finer microstructure which is why the austenitizing response was different (see the prior as-quenched hardness chart). The toughness was still relatively similar despite the very different starting microstructure (see the toughness vs austenitizing temperature chart). Those datapoints were generated using a 100°F/hr anneal which is what I recommended in the datasheet. Somewhat faster and slower cooling rates would have a similar result.

Use in Damascus

The high nickel content means that ProCut is a good replacement for 15N20 or L6 as a bright layer in pattern-welded Damascus. This gives an option for providing higher edge retention whereas 15N20 and L6 have almost no wear resistance apart from their hardness. An initial forging experiment with 1084 and ProCut resulted in excellent contrast:

Edge Retention

Pop’s ProCut did surprisingly well in the CATRA test. The heat treatments performed were 1650°F with a 450°F temper (61.2 Rc), and 1500°F with a 300°F temper (64 Rc). As I have written about before, CATRA is not the best for low alloy steels because the sand particles in the test media are harder than cementite (iron carbides). Even with different media they wouldn’t be at the top of the chart but they would be a bit better. It could be that the relatively low amount of cementite in ProCut helped for the CATRA test. But even if that were the case if we compare with another steel with low cementite content like 8670 or 1095, there was a significant boost to edge retention through the tungsten and vanadium additions. Perhaps with the lower carbon compared with other W/V steels meant that there was less of tungsten and vanadium found in the cementite, allowing them to form more of the hard WC and VC carbides.

Toughness-Edge Retention Balance

With the high toughness plus the unexpectedly good CATRA numbers the ProCut looks very good compared with other low alloy steel options:

Corrosion Resistance

This is a non-stainless steel and should not be expected to be stainless. People say that the 2% nickel in 15N20 gives it somewhat better corrosion resistance than carbon steels but I have not developed a test for comparing corrosion resistance of low alloy steels.

Cryo

There is a certain lore out there with knifemakers claiming that some steels “need” cryo and other steels “don’t benefit” from cryo. Part of this lore is that low alloy and carbon steels are in the “don’t benefit” category. Simple carbon steels and low alloy steels still see an increase in hardness with cryo, typically 0.5-2 Rc depending on the steel and the heat treatment. There is a small cost to toughness because of the increase in hardness. ProCut is the same. As an example of another low alloy steel we experimented with see 52100. I did experiments from 1500°F and 1650°F and found, as expected, that hardness increased. The higher temperature led to a bigger bump from cyro which is also typical. None of the edge retention or toughness tests I showed in the article so far used cryo in any of the heat treatments. But if you want even higher hardness for edge retention and strength you can add a cryo step after quenching.

Comparisons to Other Steels

1084 and 15N20

These steels are relatively “easy” to heat treat in that you can heat them up and quench them and get full hardness. However, they are very easy to overheat leading to grain growth and a drop in toughness. They also have almost no carbide thus having very little wear resistance.

80CrV2

This steel is also lacking in wear resistance though overheating is not much of an issue because of the vanadium addition. It also varies a lot between manufacturers and needs more temperature prior to quenching making it more difficult to perform a forge heat treatment.

52100

In the lower austenitizing range (for higher wear resistance), ProCut compares favorably with 52100 in terms of properties. 52100 has a very good combination of toughness and wear resistance. ProCut is easier to heat treat for an amateur knifemaker and also has the option of the high toughness heat treatment.

O1 and 1095

These are old standard steels. O1 has the benefit of being “oil hardening” so it is easier to quench. I have found both to be very sensitive to overheating because of excess carbon in solution. See my article on O1.

5160, 8670, and L6

These steels have significant chromium additions so they are more difficult to heat treat in a forge. They also don’t have any carbide left over after heat treating and thus their wear resistance is very low.

Blue #2, Wolfram Special, 1.2519

These steels have similar wear resistance for edge retention but significantly lower toughness and do not have the benefits of being beginner friendly.

ApexUltra

This is another low alloy steel I helped develop. It has significantly higher wear resistance and attainable hardness than ProCut, which also means lower toughness. I would recommend ApexUltra for those looking for maximum performance in the “carbon steel” category. Though I wouldn’t call it a difficult steel, it is best for makers that have a bit of experience first.

Heat Treatment Recommendations

In a Forge

Heat to “one shade brighter” after reaching nonmagnetic, quench in oil (most types are fine), and temper twice for one hour each time at 300-450°F (150-230°C) to desired hardness.

In a Furnace

Maximum toughness: 1650°F (900°C) for 10-15 minutes.

Higher edge retention: 1500°F (815°C) for 10-15 minutes

Quench in oil (most types are fine), and temper twice for one hour each time at 300-450°F (150-230°C) to desired hardness.

Thermal Cycling after Forging

1625-1675°F for 10-15 minutes, air cool. Without a controlled furnace, heat into that rough range and establish an even temperature distribution prior to cooling.

1350°F for 30 minutes, cool at 100°F/hr to 1100°F. After that it can be cooled more rapidly. Without a controlled furnace, heat to nonmagnetic and slow cool such as in vermiculite.

Summary and Conclusions

I am happy with how Pop’s ProCut turned out. It offers good balanced performance when compared with other low alloy non-stainless steels. Better edge retention than 1095, O1, 80CrV2, L6, 15N20, etc. And depending on the heat treatment its toughness approaches 8670, 15N20, and 5160. And on top of this it is beginner friendly, being easy to heat treat, grind, and finish.

The post Pop’s ProCut – A New Carbon Steel for Knives appeared first on Knife Steel Nerds.

Thanks to my Patreon supporters for supporting knife steel research. Patreon supporters saw this interview a couple of days ago. You can see the latest results of my testing early if you join at Patreon.com/KnifeSteelNerds

Video Interview

Transcript

I made some tiny edits for clarity and removed a lot of “um”s and “you know”s and repeated words but otherwise I left the interview alone.

Larrin Thomas:

I’m Larrin. This is Knife Steel Nerds. I have with me Bob Shabala of Niagara Specialty Metals. And we have a lot to talk about in the wake of the Crucible bankruptcy, and a lot of things that are happening right now. So, Bob, could you start out by telling us your position at Niagara and some of the history of Niagara Specialty Metals?

Bob Shabala:

Sure. Yeah, thanks for having me on. I’m the president and CEO of Niagara Specialty Metals. I’ve been here for 31 years. Niagara Specialty Metals started in 1982 by two gentlemen, Barry Hemphill and Lou Valery, primarily as a mill to service the tool steel industry. They bought slabs of steel and hot rolled it into sheet or plate. Quickly, it grew into more of a service company where people would send them material to roll and ship back to them. Then over time, we expanded capabilities to be able to process stainless steels, aerospace alloys, titanium, maraging, and the cutlery grades that are of interest here today.

In 2009, Crucible was our biggest customer. They went bankrupt the first time, and we came to an agreement where they would supply us with all the CPM alloys, and we could market them and sell them to whoever we want.

So from 2009 until present day, they were our sole supplier of PM alloys. They were great to work with, they were very responsive. But we could see the writing on the wall starting last year that things weren’t going well and that they were in trouble. So that’s when we started reaching out to other suppliers to, uh, to get us the steel we need for, for this market.

In addition to that, we have 40 employees. In 2004, Barry Hemphill was the remaining sole owner of the company. He essentially gave it to the employees. He turned it into a 100% ESOP (Employee Stock Ownership Plan). So everybody working here is an owner of the company. So when they retire, we buy the shares back from them, and it’s worked out very well for all the employees that are here. So when it comes time to fill positions, it’s not easy, but it’s easier than most places. And typically, once we get somebody in here working and they understand the value of the ESOP and working for a small employee-owned company, they tend to stay for a long time, which is great. We have very minimal turnover, so we can take our time training somebody because it takes three to five years to train somebody on all the jobs we have here. So the model has worked out very well for us.

Larrin:

So you mentioned PM alloys. That refers to powder metallurgy alloys, or specifically (in this case) steels for the knife industry. And Crucible invented that technology back in the late 60s, early 70s. And so they had a great legacy of making those steels. You know you had other suppliers for regular conventional steels, which are ingot cast, right? And so, powder metallurgy, a lot of the best knife steels are made with powder metallurgy. So anything with a CPM in front of it was powder metallurgy. So many of the best steels like S30V, S35VN, MagnaCut, you know, those are all powder metallurgy. So, Crucible going away is kind of a blow.

Another thing I want to hear about a little bit is the unique capabilities of Niagara. There’s really not any competitors in the USA for what you’re doing, right? What makes it so challenging to hot roll steel for knives?

Bob:

So for our initial rolling operation, we would just call it the plate mill. It’s in a fairly small area compared to most big mills. So we have the unique ability to reheat. So, take a grade like Rex 121 or Rex 76, we might have to reheat those plates every two or three passes. So we’re able to do that, whereas some mills, once you take it out of the furnace, they can only roll it so far and then that’s it. It’s also a hand mill, so we can take anything from 100 pounds up to 3,000 pounds and roll that into a different dimension. When we buy slabs, they start out anywhere from three to six inches thick. So if we’re rolling down to anything less than a half inch, what we do is roll it to an intermediate size. Then we anneal it to soften it back up. We’ll de-scale it to clean the surface, send it down to our sheet mill where we then reheat it again, finish roll it to size, anneal it, flatten, and ship.

I guess people don’t do what we do because the quantities are relatively small. We take on anything from one piece to a truckload, and our ability to deliver on time and relatively quickly is hard to beat. So that’s why we have this small segment of the market kind of, not to ourselves, but, why we’re doing well with this kind of product.

Larrin:

So I know for the steel that I developed, MagnaCut, a major reason why I was able to make it happen is because I got your buy-in early on. Crucible was a bit skeptical of trying a grade that I designed since I had not designed any tool steels or stainless steels before. So I know you guys have had a big commitment to the knife industry. Getting steels from other companies is often challenging because a lot of times, they’re sitting around in several-inch-thick pieces. So I think that’s awesome that Niagara has been so good. And the reason why we have a lot of these awesome grades is because Niagara is able to roll them and deliver them, at a good pace, like you said.

So Crucible, you hear, they’re maybe not doing well. So, what was your approach next? What did you start doing to make sure that we weren’t losing out on everything?

Bob:

Sure. Yeah. So last January, January 24, we started increasing our inventory with Crucible. So we started ordering more than we needed to, to kind of boost the inventory. And then, last fall, just in discussions with Crucible, we knew they were for sale. They had announced that. And I asked the president of the company in my position, should I start looking for other places to buy steel? And he said, yes. So I give him a lot of credit that he didn’t try to hide the fact that they were in a desperate time.

So we already had relationships with Carpenter Technology. We’ve processed millions of pounds for Carpenter and Erasteel for other industrial customers. So we, we reached out to both mills, explained our situation, and they were happy to help. So they both right away started looking into the ability to make MagnaCut. Because that’s our most popular grade right now. Erasteel already makes a version of CPM 154. So we ordered test heat of those grades from both mills. And they’ve been fantastic. You know they basically have delivered when they said they were going to deliver. We know the quality is good. It’s just a matter of ensuring that chemistries from different mills, you know, certs are, they don’t read right or they don’t read exactly the same. There might be some offset between mills. So, before we start releasing anything to the public, we want to make sure that it performs like it should. And we really should have everything ready to go here in the next next quarter or so.

Larrin:

So what you’re talking about is that different mills have different equipment for measuring the composition of the heats. And those pieces of equipment can be calibrated somewhat differently. And so one mill might measure 14% chromium and another might read 14.5% chromium for the same thing. And so if you’re then trying to transition grades over between companies, it might behave slightly differently if they have the same composition target. So we’re trying to work on any offset that might be necessary. Not that there would be a problem with a grade with a slightly different composition, but if all of your customers are used to heat treating at specific temperatures, and then it starts heat treating as if you’re 25 or 50 degrees hotter, you know that could be an issue for them if all the steel’s coming out different.

But the quality is high from Erasteel and Carpenter. They have a good reputation for making powder metallurgy steel.

Bob:

Yeah. Yeah. For years, Carpenter would send us their cutlery alloys in addition to aerospace and tool steel. So I think all of the XHP that’s ever been processed and sold to the market has been rolled here in Niagara Specialty, and we’re now stocking it. They felt that they would have better success or move more XHP if they would just sell us the steel and let us sell it to whoever we want. So that’s starting to take off. We also have their version of S90V, 10V, M4. So, yeah, it rolls really nice. There’s really no issues with it at all.

Larrin:

Well, I know XHP, which is a powder metallurgy stainless steel used in knives, that it has had a reputation for being difficult to obtain or having inconsistent availability. And I do think that’s one thing that Niagara has maybe been the best at is just having steel in the sizes that you want available to buy. When you call up these steel companies and they say, yeah, we can get you XHP, just you need to order many tons of it, and then you need to wait a year, it’s not very enticing. And if you’re a small custom knife maker, then obviously it’s just a total no-go. It’s impossible.

So what about our ability to make new alloys? We’re trying to make a new steel with the same concept of MagnaCut, but with improved wear resistance called MagnaMax. Are we going to be able to make that happen now that Crucible is gone?

Bob:

Yeah. So that’s a grade that, thanks to you, we had a prototype heat or an initial two heats made at Crucible, and the feedback’s been great. So, we patented it with you. We also trademarked it. So that gives us the ability to buy from both mills. And they both are getting ready to make it. So by, I think, the fourth quarter this year, we should have heats from both mills ready to go. As far as future alloys go, it’s something we’ll have to discuss with them. You know, our primary concern right now is getting the grades that we’ve always sold and making sure they test out and we get them locked in. And then we can have discussions with them about making new grades as you develop them.

Larrin:

So I guess we haven’t said explicitly that Erasteel bought the IP of Crucible CPM alloys, so all their powder metallurgy alloys. So that includes trademarks and also patents. Though there’s not very many patents remaining because all of the patents date back to before Crucible’s first bankruptcy in 2009. Is there any chance that Crucible will come back, or are they just Erasteel now?

Bob:

I think they’re just Erasteel. I don’t know all of the particulars of what’s going on inside the Crucible facility, but I just don’t see that getting started back up. In fact, I don’t see any domestic companies getting ready to atomize iron-based alloys that would be used in cutlery. There’s atomizing for nickel alloys and maybe titanium, but, um, iron base, I think, is just such a small portion of the overall steel industry. Nobody’s going to put the kind of money into it that they would be required to, and to bring that back on shore.

Larrin:

Well, that would be very disappointing. Nickel and titanium alloys, they have higher profit margins, which might explain why they prefer to make those, even if they have atomization equipment.

So, are we going to continue to make all these grades? I mean, Crucible had… a ton of grades, S30V, S35VN, S45VN, S90V, (etc.) there are so many. Are we going to be able to keep all of those, or are we going to need to simplify the portfolio?

Bob:

We could, in theory, buy them all. Except for 15V, I think that the atomizing temperature on that is higher than what Carpenter or Erasteel can do in their atomizing furnace. They both have said that they can’t make 15V. So until we come up with an alternative there, I think that grade’s going away. But yeah, everything else we could buy. I mean, I think that once MagnaMax comes out, we will take a look at the existing grades in our portfolio and decide if some should be obsoleted. You know, we tried to bring S6 V back a few years ago. We bought a small heat and half of that heat is still here. That didn’t work out. Sales of 20CV have slowed down. So that might be a potential candidate to not buy anymore. But we still sell a lot of CPM-154, S30V, S35VN, S45VN. So I don’t see any of the major grades going anywhere anytime soon. And they’re all on order. We have all those grades on order with Erasteel at this point.

Larrin:

So, how is branding going to work going forward? Are you going to use the same names, or are some grades going to have different names?

Bob:

Yeah, so anything we buy exclusively from Erasteel will retain the CPM grade name. We’re not going to enforce (the prefix) CPM. So if people want to buy S30V, I think we’ll just be calling it S30V. And that satisfies everybody. CPM CruWear, because of an existing agreement they have with another company, we’re going to have to rebrand that. You know, that’s still a big moving grade for us. They currently make that same composition. They have been making it for years. So we’re going to buy that, and it would just be rebranded NSMWear, unless somebody can give me a better name. S90V is not a real big grade for us. And we have a lot of that in stock from Carpenter that we bought during COVID as a way to let Crucible focus on cutlery grades. We had placed orders for industrial grades with Carpenter. So their name for that is CTS 420CW MOD.

Larrin:

And that is not a good name.

Bob:

And I don’t see anybody putting that on a knife blade. So I think we’re going to rebrand that as NSM90 and just try to make sure everybody understands it’s the exact same composition, or virtually the same exact composition, as CPM S90V. So those two grades we’re going to rebrand and see how it goes. And that gives us the ability then to buy from both mills and be single source like we were with Crucible.

Larrin:

So we’re going to have continued availability. Niagara, they’re a great American company specialized in rolling these difficult grades to thin sizes that we need for knives. Everything is in motion to continue. Is there a chance that there is a gap in availability, or is it going to be a smooth transition?

Bob:

Yeah, there’s going to be some disruptions, almost certainly. I think once people found out that Crucible is going out of business, there’s some panic buying. So hopefully everybody has what they need for the next five or six months, but everything except for a CPM-154, MagnaCut, we still have plenty of stock to get us through for the next several months. It’s just a matter of getting in these initial heats, getting them approved. And once that happens, it’s full speed ahead. So I think that the next quarter or so, there are going to be disruptions almost certainly. But once approved, I think that Erasteel and Carpenter both are really looking forward to participating in this in a meaningful way. And they’re here to support us and the whole knife community.

Larrin:

Well, that’s great. I am biased, but I think Niagara is an awesome company and a major reason why we have so many knife steels available in a good range of sizes.

Bob:

Thank you.

Larrin:

So I’m happy things are continuing. I’ve gotten messages from a bunch of knife companies that are freaking out, worried that stuff isn’t going to be available anymore. I keep telling them that Niagara is going to keep stuff going and not to worry.

So hopefully we can quell some of those fears. Supply is going to continue. I think people didn’t always realize that, yes, like Crucible was making the powder and sending slabs to Niagara, but Niagara is the company processing all this steel and selling it all. So hopefully we can get the word out that Niagara is a great company that is delivering all these steels, and Niagara is still here and Niagara is doing well and continuing to make these steels.

Bob:

Yeah, we’re financially very sound. You know, the people we have working here, they’re awesome. I mean, they really make it happen. You know when we have something important, like a new heat of MagnaCut coming in from Erasteel and we need to get it approved, they’re right on it. Literally unloading it from the truck and putting it on the saw and cutting it into blocks to get it rolled the next day. So they really are invested in (the) success of the company and the industry as a whole. So yeah, we’re here for the long haul.

Larrin:

And Niagara’s made some upgrades in recent years, right?

Bob:

Yeah. We did our biggest project ever last year. It’s still in process, but for us it was big. I mean, it is about a $7 million investment. So we added about 15,000 square feet to our sheet mill building. And then we had six small electric box furnaces that they did the job for several years, but the heat in them could be uneven. The elements were delicate and expensive to replace. And it was a bottleneck. The guys could work faster than the furnaces. So lots of times they’re just waiting for the steel to heat up. So once we got the building built, we put in a continuous gas furnace. So now the sheets are in the furnace for 25 to 35 minutes instead of three hours. And the temperature on the blanks is really uniform from front to back and side to side. So the gauge coming off our mill is much more even and easier to control. The surface is a lot cleaner. There’s no scale because it’s not in the furnace as long.

And then the last step of the process is going to be a new housing for the rolling mill itself. So the housing is essentially the structure that holds the rolls and the rolls are, rotating and feeding; that’s where the steel goes through to get reduced. The new housings are complete, but we’re not ready to put them in here yet. We need another month to get all the wiring in place and control panels. So around Labor Day, we’re going to install that. So there we’re going to increase our separating force. There’s going to be some machine learning on it so we can put in recipes. Right now, we just turn the dial and get it to where we think it needs to be, and then we have to check the gauge. So in the end, once that comes in, with the much stronger housing, it should be higher throughput, even more uniform gauge from piece to piece.

So that was a real big investment for us. In addition, we’re getting ready to put on another, I think about 12,000 square foot building for our grinding operation. We have a Blanchard grinder for grinding our sheets to size that’s starting to get a bit of a bottleneck. So we already bought another Blanchard grinder, 84 inch diameter that’s in storage until the building gets built.

We have a fiber optic laser that we can cut up to about 3/8 of an inch and we use high-pressure nitrogen in that. So we cut a lot of knife blanks for the industry now. And that’s starting to get busier. So we cut for small shops that give us a DXF file, we can cut their blanks for them. And (that) saves them shipping whole sheets to a laser shop and then taking the blanks to their place. So kind of a one-stop shop here. So with that filling up, we want to get another laser to put in the new area. And then outside of the cutlery industry, we do a lot of bandsaw cutting. So we roll a lot of big heavy plate, or people send us their plates, and we saw cut it into either small rectangular shapes or strips or full-size plate. So over the past couple years, we’ve spent two to three million dollars in in bandsaws just upgrading our cutting capabilities.

Larrin Thomas

Wow, on bandsaws.

Bob

Yeah. There we have two of them now, they’ll cut in two directions. So all of our bandsaws to date either just cut along the length of a plate or across the end. These two axis ones, we can cut the ends first, then the head goes back, and then it’ll just strip it out. So just put the plate on there one time and you’re done, essentially let the machine do the work. So it just makes it easier to get accurate cuts. The guys like it cause it’s safer, a lot less handling moving plates around. So, yeah, that’s been a big part of our business.

Larrin

Well, thanks, Bob. Hopefully, we answered people’s questions and didn’t just add to the confusion. So we’ll have to bring you back on if we get a lot more questions based on the interview. But thanks for joining me and talking about Niagara and the whole plan for keeping knife steel going.

Bob

All right, anytime. Thanks, Larrin. I appreciate it.

The post The Future of Knife Steel with Bob Shabala of Niagara Specialty Metals appeared first on Knife Steel Nerds.

Still working on getting an update to everyone about the status of knife steel after Crucible’s bankruptcy and the sale of many of the assets to Erasteel. Still more i’s to dot and t’s to cross.

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History of CPM-3V

The thing most don’t know about CPM-3V is that it is a powder metallurgy version of an older conventional steel called Vasco Die. Vasco Die was patented by Harry Johnstin of VASCO in 1964 [1]. This was the first popular “8% Cr die steel” which created a new category. The most popular cold work die steels were (and are) A2 and D2. D2 is used for higher wear resistance and A2 is used for higher toughness. The 8% Cr is right in the middle between D2 (12%) and A2 (5%). And with the 2.5% vanadium for wear resistance and the relatively low carbon of 0.8% it has an interesting combination of properties, better overall than either A2 or D2. The steel was advertised as twice as tough as D2 while have ten times the wear resistance of A2. This created a new category of cold work die steel which is still seen today in steels like DC53 and Sleipner. You can read more about the history of Vasco Die and all of the developments that led up to it in my book The Story of Knife Steel. Vasco Die was never really used in knives, though the higher carbon version, Vasco Wear, did get used by some knifemakers, and by Gerber in some production knives. And of course CPM CruWear, the PM version of Vasco Wear, has also developed some popularity in recent years.

In the mid-1990s, Kenneth Pinnow and William Stasko of Crucible were looking to make a high toughness powder metallurgy tool steel. Up until that point, powder metallurgy steels had focused on high hardness high speed steels and creating new steels with higher wear resistance than was possible with conventional steelmaking like CPM-10V. Crucible’s highest toughness PM steel was CPM-M4 and that was basically an accident. Uddeholm had released Vanadis 4 in the late 1980’s to be a more balanced steel and its toughness didn’t even match CPM-M4. Uddeholm would later redesign this steel and call it Vanadis 4 Extra. Pinnow and Stasko realized they needed a steel with lower carbide volume. Powder metallurgy leads to higher toughness because it keeps the carbide size small, but to achieve the highest levels of toughness they needed to have less overall carbide. They experimented with a few compositions and found that a PM version of Vasco Die had excellent toughness and wear resistance.

It is somewhat surprising to me that they managed to patent this steel in 1997 [2] since Vasco Die had already previously been patented (though the patent had expired). It seems their claim was that the powder metallurgy process significantly changed the steel, and in more ways than just making the carbides smaller. With conventional casting of steel there is significant segregation of elements, and this led to relatively large chromium carbides which couldn’t be dissolved in Vasco Die. With powder metallurgy, the heat treated microstructure is free from chromium carbide and has only vanadium carbide instead.

Images from 3V patent showing CPM-3V (Fig. 1) and Vasco Die (Fig. 2). The larger carbides labeled “C” are chromium carbides. 3V is only small vanadium carbides.

CPM-3V became Crucible’s go-to steel for applications requiring high toughness; they released it around 1999. Surprisingly this steel didn’t really take off in knives. And I would argue it never really has. Jerry Hossom was the first custom knifemaker I noticed using it regularly. In more recent years perhaps the best known user of the steel is Nathan Carothers with his “Delta” heat treatment. I don’t mean to suggest no one is using 3V, I see currently available knives from a range of manufacturers including Bark River, ESEE, Cold Steel, and others, but it never seems like there are as many 3V knives as there could be. I think it being non-stainless has held it back as knife manufacturers tend to stick with stainless steels.

Heat Treating and Hardness

I did a new set of heat treating coupons for CPM-3V as I wanted to go a bit higher on austenitizing temperature than coupons I had done for the book Knife Engineering. The hold time at 1850°F was 45 minutes, 1950°F was 30 minutes, 2050°F was 20 minutes, and 2150°F was 10 minutes. While CPM-3V is typically heat treated in the 58-62 Rc range because of its excellent toughness, it can be heat treated to higher hardness if desired. This is significantly higher than the 61 Rc the datasheet shows the steel maxing out at. This is because I used the low temper range (300-500°F), rather than the datasheet recommended high temper range (1000-1050°F).

Microstructure

CPM-3V has about 5% vanadium carbide, and is free of chromium carbide despite the relatively high chromium content. This is in contrast to Z-Tuff which has about 3% chromium carbide and only 1% vanadium carbide. CPM-1V also has about 1% vanadium carbide but about 2% M6C (molybdenum/tungsten carbides found in high speed steels). So 3V has only a bit higher carbide volume than those two steels but it is all the hard vanadium carbide type which should mean more balanced properties.

3V (1925°F) – 5% carbide volume

Z-Tuff (1925°F) = 4% carbide volume

CPM-1V (2050°F) – 3% carbide volume

Toughness

One mystery about CPM-3V toughness comes from a patented Crucible steel that was never released commercially, a modified version of 3V. The steel was patented in 2006 by Alojz Kajinin and Andrzej Wojciesczynski. I actually wrote about this modified steel in my very first article on Knife Steel Nerds in February 2018. I didn’t even advertise the article because it was a test before I wrote a series on austenitizing that I did promote with the announcement of the website. The modified 3V was designed by partially replacing vanadium with niobium, which made the carbides smaller. They showed experiments in the patent that the longitudinal toughness of 3V was much higher than the transverse toughness. With transverse toughness the crack grows along the rolling direction, and carbides and impurities are aligned along the rolling direction giving easier paths for crack growth. However, with the modified 3V they found the transverse toughness to be roughly equal to the longitudinal toughness.

So for a new experiment on CPM-3V I decided to test both the longitudinal and transverse toughness and see if the transverse toughness was really as bad as it was shown in the 3V mod patent. I used an austenitizing temperature of 2000°F, plate quench, cryo, and a temper at 400°F. This resulted in about 61.5 Rc, which is significantly higher in hardness than our original 3V toughness tests. Those coupons were heat treated by Warren Krywko who heat treated a whole bunch of toughness coupons that I tested early on. I think maybe the cooling rate in quenching he was performing was somewhat slower than what I did, maybe because he was doing many specimens at the same time in the plate quench. That is significant because a slower cooling rate leads to both lower hardness and lower toughness, as I wrote about in last month’s article. All of the coupons we heat treated were tempered at 400°F rather than the 1000°F temper the datasheet recommends, because of our earlier findings with CPM-CruWear/Z-Wear.

Comparing the hardness-toughness to the previous CPM-3V tests I found significantly better properties. The transverse toughness I measured was only ~7% worse than the longitudinal, which is quite impressive. The transverse toughness reported in the 3V mod patent looks like an anomaly, I have never seen such a large difference between longitudinal and transverse toughness. Though I have to say, the very small drop between longitudinal and transverse toughness I measured here is quite impressive.

With the improved toughness measurements 3V nearly rivals the results of Z-Tuff and CPM-1V which both have lower carbon and vanadium. This actually does make some sense, as the carbide volume is similar between the three steels. To me the composition designs of Z-Tuff and CPM-1V are not totally optimal because they have a significant amount of non-vanadium carbides. The best toughness-wear resistance balance is generally achieved by having all vanadium carbides. Perhaps in the future we could explore higher hardness heat treatments like in the 62-64 Rc range.

Wear Resistance and Edge Retention

I tested the CATRA edge retention of 3V way back in my giant CATRA study. It did exceptionally well for its level of toughness at 463 mm cardstock cut, significantly higher than CPM-1V and CD1/Z-Tuff. Looking back I do think the heat treatment used was cheating a bit, as the tempering temperature was only 300°F. If the tempering temperature was a more appropriate 400°F, the austenitizing temperature for the same hardness would be higher and thus there would be a bit less carbide in the steel. However, this would only marginally affect the final result.

Edge Retention-Toughness Balance

The combination of wear resistance and toughness of 3V is quite impressive. I added a new datapoint for 3V with its newly higher toughness test. I had to adjust up the CATRA edge retention based on the predicted change with hardness for the chart since the CATRA knife was 1.4 Rc lower than the toughness coupon. CPM-3V is really in a class of its own for the level of toughness it has for the level of edge retention. Of course it doesn’t do anything too crazy when it comes to edge retention, but it is sufficient for many knives and has excellent toughness to go with it.

Corrosion Resistance

I have never tested the corrosion resistance of CPM-3V. With its 7.5% Cr and no chromium carbide its corrosion resistance can be quite good for a non-stainless. Some people talk about “semi-stainless” steels and if any steel were to fit in this category it would be 3V. It actually has more chromium in solution than D2, which is perhaps the best known “semi-stainless” steel. Z-Tuff and CD#1 also have a similar chromium content but because they have some chromium carbide in the heat treated condition we would expect 3V to have somewhat better corrosion resistance. In a corrosion test performed by Scott Larimore and Kelly W., CPM-3V was found to survive “in the elements” without rust. However, this was only the case with the low temper condition (<750°F) but not the high temper condition (>750°F). This is expected, though the stark difference never fails to surprise me. The low temper condition is labeled “3VL” and the high temper condition is labeled “3VH.”

Corrosion test by Scott Larimore and Kelly W.

Summary and Conclusions

I think CPM-3V has an excellent set of properties with very high toughness and decent wear resistance and edge retention for that level of toughness. With the 7.5% Cr and lack of chromium carbides its level of corrosion resistance is also quite good for a non-stainless steel. I don’t see anyone out there criticizing the steel, but somehow it seems to never quite get the recognition and popularity it probably deserves.

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[1] Johnstin, Harry G. “Alloy steels and articles thereof.” U.S. Patent 3,219,442, issued November 23, 1965.

[2] Pinnow, Kenneth E., and William Stasko. “Wear resistant, powder metallurgy cold work tool steel articles having high impact toughness and a method for producing the same.” U.S. Patent 5,830,287, issued November 3, 1998.

The post CPM-3V – Still the Best High Toughness Steel appeared first on Knife Steel Nerds.

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Heat Treating Steps and Equipment Types

There are three major steps of heat treating performed by a custom knifemaker or a factory, and an optional fourth:

1. Austenitizing – heating the steel up hot and soaking
2. Quenching – Rapidly cooling the steel
3. Tempering – reheating the steel to a lower temperature such as 400°F (200°C) for 1-2 hours. This is typically performed 2-3 times (cooled to room temperature in between).
4. Cryogenic or cold processing is an optional step that can be performed after the quench or in between tempering steps. It involves cooling the steel to as low a temperature as possible.

Fundamentally these steps can be performed with a wide range of equipment, and that equipment can have an effect on the results. Below I have written about some of the basic equipment setups.

Austenitizing

The austenitizing step is performed somewhere between 1450 and 2250°F (785-1230°C), so this is not happening in a kitchen oven. The biggest difference between small batch heat treating done by an individual knifemaker and a factory is the size of the furnace. Knifemakers will use a benchtop unit such as those made by EvenHeat or Paragon. Therefore the number of knives that are in the furnace at once is much greater in a factory furnace. There are pros and cons to both situations apart from just the number of knives that are heat treated at once. A furnace is not perfectly evenly heated, it is going to be slightly colder at the door, for example, and there will be some thermal “gradient” and relatively hot and cold spots. Typically there is a “control” thermocouple that measures the temperature and the furnace is constantly adjusting to hit the target temperature. If you are heat treating one knife at a time that is directly next to the thermocouple you can be sure the temperature the knife sees is very close to what the thermocouple is measuring. With a loaded up furnace there will be some variation in temperature. That could be only a few degrees or in extreme situations there could be a 20-30°F+ difference between blades in the center of a furnace vs the “corners.”

Custom knifemakers do not always use furnaces. It is relatively common for knifemakers to use a forge which is relatively uncontrolled and the goal is to pull the blade after it is heated into the right range. Some knifemakers will try to hold a specific temperature within the forge or improve the uniformity of the forge by using a “baffle,” also called a “muffle.” This can be as simple as a large piece of pipe. And less common, though still done by some, is to use a torch to heat the edge or the entire blade. Again, this is relatively uncontrolled compared with a furnace.

Quenching

Quenching also varies quite a bit. Steels are rated by “hardenability” in terms of how fast they need to be quenched. The general categories are “air hardening,” “oil hardening,” and “water hardening.” An air hardening steel can be quenched with pressurized gas or even just in still air. Oil hardening knives are quenched in oil, and water hardening steels are quenched in water. Factory knives tend to use air hardening steels, in part because they are better suited to large batch heat treating, and in part because all stainless steels are air hardening. With custom knives, “plate quenching” has become popular in the past couple decades. Plate quenching involves placing the steel between two plates (usually aluminum) and the plates draw heat out of the blade. This is faster than sitting in air and also helps maintain flatness during quenching. The most common quenching method for factories is a pressurized nitrogen gas quench. The pressure of the gas is typically rated in “bars” where a “1 bar” quench would be atmospheric pressure, and 2 bars would be double atmospheric pressure. The higher the pressure, the faster the quench. However, this is not the only factor that matters. How “loaded up” the furnace is will also dictate the quench rate. If many blades are stacked with each other, that increases the volume of each and thus it takes longer to cool it down. You can easily imagine how a 4 inch thick block of steel takes longer to cool than a 1/8″ thick blade. And if a relatively small number of blades are in the furnace, and they are well separated, that will mean a faster quench rate even with the same pressure quench. The heat treaters usually want to load up the furnace as much as possible for efficiency. Here is a random YouTube video that shows vacuum furnaces:

Slower quenching means that hardness and toughness is reduced. Below shows a published study [1] on M2 high speed steel tested with 2 bar, 6 bar, and 9 bar quenching in a vacuum furnace. They tested the hardness and also toughness with a bend fracture test. With higher quench pressure, both properties were improved. With slow cooling, carbides are precipitated at the grain boundaries. This depletes carbon from the matrix, reducing hardness. And those precipitated carbides are brittle and reduce toughness, especially since they are present on grain boundaries.

Data adapted from [1]

In a study on Uddeholm Dievar tool steel [2] they compared toughness between air cooling and oil quenching. They found the same 48 Rc hardness with both conditions but the oil quenched steel had 25% better toughness. They took micrographs of both conditions, and you can see the grain boundaries in the air cooled steel. The dark grain boundaries are visible because of the carbides formed along those boundaries.

Image from [2]

Vacuum furnaces are not the only method used by production knife facilities but are by far the most common. One counterexample is Paul Bos heat treating, which is located at Buck Knives and also performs all of the heat treating for Buck. See the video below. They use a conveyer belt furnace and the steel comes out the other side and cools in air. This type of cooling is generally faster than a pressure quench in a vacuum furnace because of how much time it takes to cool down a fully loaded vacuum furnace.

Tempering

Tempering has similarities to austenitizing in that we are heating to some temperature and holding there. The main difference is that tempering is from a much lower temperature, typically in the range of 300-1050°F (150-565°C). This means that it can be performed in less specialized equipment. Many custom knifemakers will use a kitchen oven or toaster oven.

Cold or Cryogenic Processing

Cryo processing generally involves the use of liquid nitrogen. This can be simple, like dipping a knife into a liquid nitrogen Dewar, to fancier setups that spray liquid nitrogen, or cool down a refrigeration unit with liquid nitrogen. There are also refrigeration units that can get very cold without liquid nitrogen, though are generally at a somewhat higher temperature. Liquid nitrogen is about -320°F (-196°C). Dry ice is also relatively common for subzero temperatures, it is about -109°F (-78°C). I have also tested the use of household freezers, which are in the range of 10°F to -15°F (-12 to -25°C). This is not as effective since it isn’t as cold, though it can still effect the transformation of steel.

Measured Properties Comparing Commercial and Custom Heat Treating

Below I have collected several examples of measured properties between commercial heat treating and small shop heat treatments. This type of data is not widely available so hopefully it is enlightening.

Hardness

The final hardness of the steel comes from several factors. Austenitizing from a hotter temperature, or tempering from a lower temperature, usually means higher hardness. Cold or cryo processing can increase the hardness another 0.5-3 Rc depending on different factors. However, even with the same austenitizing, tempering, and cryo setup, the quench rate will affect the final hardness. This is one of the biggest differences between typical factory and typical custom heat treating.

For the MagnaCut datasheet I measured hardness for a whole range of austenitizing and tempering combinations with “plate quenching” with no cryo, a household freezer, and liquid nitrogen. We also sent steel to Peters Heat Treating, which has typical factory vacuum furnaces that use a pressurized gas quench. They tested with a “2 bar” nitrogen gas quench. You can see that the hardness after the pressurized gas quench by Peters (bottom) is typically 0.5-1 Rc lower than when I plate quenched individual pieces.

Different steels can be more or less sensitive to these differences. MagnaCut has 2% Mo in it which increases its hardenability (thus it is less sensitive to quench rate). However, AEB-L has no Mo and thus it can benefit more from a fast quench. Of course an oil hardening steel would be very soft even with a high pressure quench in a vacuum furnace. For air hardening steels I have heard reports of up to 2 Rc difference between a plate quench and a 2-bar gas quench in a vacuum furnace.

Of course with oil hardening steels they require an oil quench. This is more difficult to do in large batches than air hardening steels. However, factories that are setup for oil quenching will have a relatively similar quench rate to a custom knifemaker. Both are quenching in oil so there isn’t much difference.

Toughness 

I have toughness data for vacuum furnace heat treating of MagnaCut from three different heat treaters that use vacuum furnaces, which I have compared against my own experiments with plate quenching.

The “2-bar” and “Unknown pressure” heat treaters were targeting the same hardness with the same temperatures. I am not sure the reason for the discrepancy in hardness. For toughness, both are a similar offset below the “Plate Quench” numbers because of the slower quench speed. Presumably they had a similar quench rate. The slower quench rate led to a reduction in toughness for a given hardness. The faster 10-bar quench also used a higher austenitizing temperature and a higher target hardness, so the faster quench is not the only reason for higher hardness. However, for a given hardness this heat treatment was closer in toughness to the “custom” heat treatment using a relatively rapid plate quench.

Carbon and Low Alloy Steels

Below are results from heat treating of 52100 with a heat treatment performed by knifemaker Warren Krywko and also two heat treatment facilities that can do oil quenched blades:

The austenitizing temperatures and quench rate were similar in the case of 52100 since all were quenching in oil. Therefore the hardness and toughness was similar whether it was done in a small shop or by a commercial heat treatment company.

Forging and Thermal Cycling by Custom Knifemakers

One way that custom knifemakers will try to make their heat treatments better than factory knives is in forging and thermal cycling of their blades prior to the steps we outlined above (austenitizing, quenching, and tempering). While knifemakers typically will claim that forging, and especially thermal cycling, is for the purpose of refining the grain size, instead the biggest differences are from affecting the carbide structure prior to austenitizing. The steel manufacturers typically want the steel to be as soft as possible for easy machining and grinding by the end customer. The very soft steel is achieved by having a relatively coarse annealed microstructure. If instead the carbides are smaller and the steel is a bit harder in the annealed condition, then the hardness-toughness balance can be improved somewhat. By modifying the normalizing and annealing process (called “thermal cycling” by knifemakers) you can achieve these results. Click here for an article about normalizing and annealing. This thermal cycling could likely be replicated to some extent even in a factory setting, but typically the time, logistics, and cost make this less feasible when hundreds or thousands of blades are being produced. Below I have the same 52100 chart as before but this time I added in results from 52100 that was forged and thermal cycled by my father Devin Thomas. To be clear, it was the modification to the carbide structure through a different normalizing and annealing process that led to the change in properties, not the forging. You can read about the process that was followed in this article on 52100.

Some steel manufacturers will have a coarser starting structure than others, leading to differences in how the steel responds to heat treatment. In my study on 80CrV2, I found that one steel manufacturer’s steel needed significantly higher temperatures than the others to achieve the same hardness. Notice how the “Buderus” material is softer than the steel labeled “Jantz” or “AKS” which were also heat treated as-received from the manufacturer. The “KSN cycling” used my own recommended normalizing and annealing. The cycling procedure I performed did not start with forged steel, showing that the improved microstructure can be achieved without forging.

We found that this result lined up with the starting microstructure:

Buderus-annealed 80CrV2

Jantz-Supplied 80CrV2

80CrV2 with “KSN cycling”

The smaller carbide size also led to improved toughness for a given hardness with 80CrV2. Notice that toughness was similar for each condition but at higher hardness when the starting carbide size was smaller:

Forging and Annealing Stainless Steels

While “thermal cycling” is much, much more common with simple carbon and low alloy steels, there are improvements to be had even with stainless steels by refining the carbide structure. Below are results for AEB-L and MagnaCut:

You can see that in both cases we had relatively small improvements to the hardness-toughness balance by improving the annealing process that was used for those two stainless steels. You can read about how those were optimized in this article on forging and annealing stainless steels.

When Custom Heat Treatments are Worse

In many cases the difference is pretty small between a “basic” heat treatment and one with more involved thermal cycling. Usually with more complex and “dialed-in” heat treatments the potential improvements are small, on the order of 10-20%. When I refer to a “dialed-in” heat treatment I am also referring to other aspects like optimizing austenitizing and tempering temperatures. I have many articles on my website for various steels, such as when I found that CPM-CruWear has better toughness with a low temperature temper of 400°F/200°C despite the fact that the datasheet recommends the high temperature range (~1000°F/540°C) instead. However, these types of temperature optimizations can be (and sometimes are) done by either factories or by individual custom knifemakers.

When using complex or “fancy” heat treatments, the chance of making the steel worse is usually greater than the chances of making it better. I think every buyer should be skeptical of knifemakers that are claiming their heat treatments are significantly better; without quantitative, controlled testing methods the knifemaker is unlikely to know whether an improvement has been achieved. I have heard many knifemakers claim improvements based on changes they made that basically have no chance of improving anything. And some of those modifications are actually making it worse. For Knife Steel Nerds I have tested heat treatment changes I was sure would improve the steel, but instead found worse properties. For example, I did a rapid triple quench heat treatment of 1084 using salt pots. The hardness ended up the same, but I was shocked to see that the toughness had been significantly reduced, especially since the grain size was small in all three tested conditions. The normal furnace heat treatment is labeled “Furnace in air,” the short salt pot heat treatment is labeled “Salt pot, 3 mins,” and the triple quench is the same label with “(x3)” added. In this case the best result was with the basic and boring heat treatment.

But the above examples I have given on improved (or worsened) custom heat treatments all assume furnace heat treating with well controlled temperatures. There is a much more common and pernicious form of custom heat treating which is largely accepted in custom knifemaker circles: forge heat treating. The biggest problem with forge heat treating is that the temperatures are not well controlled. Typically the knifemaker does not even know for sure what the temperature is. There are some methods, like using a magnet, that are used to try to be more consistent, but none are as good as using a furnace with well controlled temperatures. Some steels are more sensitive than others, and some methods are better at being consistent, but none are perfect. Knifemaker/metallurgist Juha Pertulla did an experiment with forge heat treating of 1075 steel where he found that holding the steel for only one second beyond reaching nonmagnetic had already led to increased grain size and reduced toughness. Using 80CrV2 with its vanadium addition made the steel less sensitive to overheating. In my own experiments with forge heat treating I also found 1084 steel (similar to 1075) to be sensitive to overheating:

I have had multiple knifemakers argue with me that I am wrong about the unreliability of forge heat treating. Three of them have sent me steel they heat treated to show me how their forge heat treatments are consistently high in performance. Every case I have tested so far has resulted in poor toughness from overheating. Here is one example:

When Custom and Factory Heat Treating is the Same

Many custom knifemakers do not perform their own heat treating and instead send to commercial heat treaters. This can be for a variety of reasons, but three common scenarios are: 1) the knifemaker doesn’t have the cash for investing in heat treating equipment, 2) the knifemaker doesn’t trust himself to learn how to properly heat treat and/or trusts the commercial heat treater more, or 3) the knifemaker works at a high enough volume that a commercial heat treater makes more sense. In these cases there is unlikely to be much difference between the heat treatment performed on the knives for an individual knifemaker and those performed on “factory” knives. Of course, the lines are sometimes blurred between “custom” and “factory” knives where knifemakers may outsource many steps beyond heat treating. There isn’t anything inherently wrong with this it just means we can’t always neatly categorize things.

Edge Retention

Many years ago I analyzed a set of CATRA edge retention data from a major knife manufacturer and wrote two articles about it: Part 1 and Part 2. Later I obtained my own CATRA tester and did a large set of tests on a wide range of steels. I have had some knife enthusiasts argue that because I was performing my own custom heat treatments that my tests should not be accepted out of hand. Usually this is because they think a steel over- or under-performed relative to where they had already decided this steel should be on the chart. To study this I compared a range of steels that I had tested to the knife manufacturer’s dataset:

The relative position of each steel was basically the same whether it was my tests with “custom” heat treatments or the knife manufacturer with “factory” heat treatments. However, you will notice that my results were consistently somewhat higher than the knife manufacturer, averaging about 17% better. This is not because my heat treating was superior, but rather due to differences in the design of the blades, the edge geometry, and how the blades were sharpened. The “behind the edge” thickness was greater with the production-made test knives. Also, the knives that I tested were sharpened with an Edge Pro which keeps the edges very “triangular” without rounding. The factory sharpened knife edges instead have a more convex shape, which makes them behave more like an edge sharpened to a more obtuse angle. The factory knives were given a polishing step at the end of sharpening. My knives were sharpened to 400 grit, relatively coarse, which gives the steels enhanced slicing edge retention. I have tested the effect of edge finish on CATRA edge retention in the past:

Another clue is seen when we plot the percent difference between my “custom” knives and their factory knives vs the total CATRA edge retention:

If we ignore Maxamet, the percent difference is greater with lower edge retention steels. I promise I do not have secret heat treatments that can make 55 Rc 420 steel cut 50% longer. Instead it was because of the differences in sharpening and edge geometry. With the low wear resistance of 420, how much cardstock it cut was largely controlled by its cutting ability (sharpness and edge geometry) rather than its wear resistance.

To illustrate just how important the edge geometry is, here is a chart showing the effect of total edge angle vs CATRA edge retention:

You can see that AUS-6 with a 27 degree edge (13.5 degrees per side) cut about 400 mm of cardstock. This matched the performance of the significantly more wear resistant CPM-154 with a 34 degree edge, which is only 3.5 degrees per side greater. And that same 400 mm is measured with the very wear resistant S110V with an edge angle of about 41 degrees. If CPM-154 is given a 27 degree edge instead, it matches Maxamet with a 41 degree edge! So small differences in sharpening can make a big difference in the result of an edge retention test.

Custom knives are often given thinner edges than factory-made knives. In general, when superior edge retention performance is measured with custom knives, it is due to better edge geometry rather than any kind of super, unmatchable heat treatment. When a custom heat treatment does perform better with identical edge geometry and steel it can usually be attributed to higher hardness. In my own CATRA experiments, the effect of hardness is greater than other changes to the heat treatment. This is in contrast with toughness measurements where I often find significant differences when changing heat treatment variables.

Seattle Ultrasonics Kitchen Knife CATRA Study

For one more dataset that shows the importance of edge geometry I can show the “Quantified Knife Project” of Seattle Ultrasonics. I performed the CATRA tests for this project. We measured the cutting performance of a range of low- to high-end factory knives and one custom made knife. For the initial cutting ability (how much cardstock cut in the first back and forth strokes), the only difference in performance was edge geometry and sharpening. In this case the correlation with edge angle is very strong as we would expect:

However, even when we plot edge angle vs CATRA edge retention, we find overall the trend with edge angle is still very strong:

For the majority of the knives, the strong effect of edge geometry completely washes out any potential analysis for the effect of steel type and heat treatment. This is despite the fact that there is a wide range of steel from 56 Rc low wear resistance stainless steels (1.4116) in many of the European knives to ~60 Rc VG10 in many of the Japanese knives. The main exception to the trend are those values at ~820 and ~1050 mm. Those are for the steel advertised as “FC61” steel which is a proprietary name. I have seen speculation that the FC61 is similar to AEB-L or 13C26 (relatively low wear resistance), but this very high performance would seem to point towards it being something more wear resistant. However, not many of the knives were below 20 degrees so there aren’t many other comparisons.

Another specific knife I want to point out is the custom Moritaka knife, the only custom knife that we tested. It was one of the best performers at ~650 mm. This was in Blue Super steel which in my testing with a 30 degree angle was only at 338 mm. So the superior results for this custom knife were not from a super heat treatment or magical forging but instead from a smaller edge angle. Of course it should be noted that the very low edge angle also makes this edge more delicate; it is more likely to chip in use. Everything is a tradeoff.

Sharpenability, Edge Stability, and “Mushy Edges”

In my prior CATRA studies which have included over 50 steels, I have only had significant issues with sharpening a small handful of them. This is in part due to the use of CBN abrasives which makes sharpening even high vanadium steels very easy. Instead, the difficult to sharpen knives were from heat treating, not steel. Those difficult-to-sharpen knives had high retained austenite, which meant that deburring would leave behind a ragged edge which wasn’t very sharp. High retained austenite comes from austenitizing at too high of a temperature, especially in combination with slow quench speeds and when cryo isn’t performed. You can learn more about retained austenite in this article on cryogenic processing.

Both small batch and commercial heat treaters can have issues with retained austenite. But there are a couple areas where commercial heat treaters are at a disadvantage. One is the slower cooling rate we have been discussing throughout this article. Slower cooling stabilizes the retained austenite which makes less of it transform during cold and cryo treatments. The other is that commercial heat treaters often like to perform cryogenic steps in between the two tempering cycles, where cryo is less effective. The first tempering step also helps stabilize austenite just like slow cooling does. They prefer to do it between tempering steps because the cryo processing is less likely to lead to distortion, warping, or cracking. But it is less likely to do so in part because the cryo processing isn’t doing as much. To be clear, some knifemakers doing their own heat treating will also do cryo in between tempering, and some commercial heat treaters will perform cryo directly after the quench when it is more effective.

High retained austenite also makes steel behave as if it is softer than it is. Two knives with the same hardness will behave differently if one has significantly more retained austenite than the other. One area is in sharpening as already mentioned, but the high RA knife will have edges that deform more easily.

With proper selection of austenitizing temperatures and cryo processing, retained austenite can be limited to levels where it doesn’t affect the performance of the knife. Some steels have more issues than others. For example, I found that the datasheet-recommended austenitizing temperature for CPM-S110V was quite high. Steels like Vanax and LC200N are difficult to heat treat beyond 58-60 Rc so the heat treaters are forced to austenitize at the very peak of the curve where it is easy to “overshoot.” For example, here is a comparison between Vanax and Elmax steel with a range of austenitizing temperatures, using cryo and a 300°F (150°C) temper:

You can see that while you can use a pretty wide range of austenitizing temperatures with Elmax and achieve 61+ Rc (and then temper down to target hardness), the range for Vanax is much narrower. The peak hardness is around 1975-2000°F but already at 2025°F the hardness has dropped. When there is a hardness drop from austenitizing too high this is the point where there is too much retained austenite, and in some cases the very peak can also be questionable. So Vanax is difficult to heat treat for 60+ Rc while also avoiding excess retained austenite. When the composition of the steel is slightly different from batch to batch, the austenitizing temperature where peak hardness is reached will vary slightly. This in combination with variation in temperature within a furnace, and between furnace batches, which means that it is usually better to be a bit conservative with the temperature and go a bit lower than the absolute peak.

Corrosion Resistance

The carbides precipitated during a slow quench are also detrimental to corrosion resistance. Of course, other heat treating variables also matter. A higher austenitizing temperature means more chromium carbide is dissolved, putting more chromium in solution for better corrosion resistance. And it is relatively common in industry to use the high tempering range of 950-1050°F (510-565°C) which reduces corrosion resistance. This can make the very high corrosion resistance Vanax behave more like a “normal” stainless steel. I have an article on my corrosion resistance testing here. This high temperature tempering range can be used by both custom knifemakers and commercial heat treaters but I see it somewhat more often with commercial heat treating.

Vanax tempered at 400°F on left and 1000°F on right. Sprayed with 1% saltwater for 24 hours.

Who Knows More About Heat Treating?

In many cases the knowledge of the person performing the heat treatment is more important than the equipment being used. A heat treatment without any issues gives you 90-95% of the potential performance. It is when there is some problem with the heat treatment where the performance is noticeable to the end user. I have written about this in an old article called “What a Good Heat Treatment Can and Cannot Do.”

When a knifemaker or knife company outsources their heat treatment to a commercial heat treater, they typically ask for a target hardness and leave it up to the heat treater. The commercial heat treater presumably has significant experience with heat treating, especially with obtaining consistent results, avoiding catastrophic issues, and with troubleshooting common problems. However, many heat treatment facilities lack metallurgists, and when it comes to measuring properties, the hardness test is by far the most common. As I have shown multiple times in this article, hitting the target hardness is only one tiny piece of the puzzle. Some heat treatment facilities are better than others.

When it comes to custom knifemakers doing their own heat treating, the level of knowledge varies widely. Of course it goes without saying that the average knifemaker is not a metallurgist. In general, I would say that the average knifemaker has knowledge of the basic steps of heat treating, but their knowledge of the mechanisms behind each step is relatively poor. I reacted to knifemaker explanations of heat treatment in this video. If the knifemaker knows how to heat treat correctly and how to follow a datasheet they are usually fine. It is when they are trying to diagnose issues or to modify the heat treatment that they get in trouble without knowing how things “work” in the steel. I have an article and video about the basic steps of heat treating and how to follow a datasheet. For those that want to learn more about what is happening in the steel and how to modify heat treatments for different purposes I have many articles on my website as well as my book Knife Engineering. There are a few knifemakers that have put in the time required to understand heat treating and gained hands-on experience to become well-informed, excellent heat treaters.

Differential Hardening and Hamon

This article is already too long but I did want to mention specialized heat treatment techniques that are almost exclusively performed by custom knifemakers. One is developing a hamon, which is often for artistic purposes rather than strictly performance. Various types of other “differential hardening” techniques are used for specific performance goals like edge quenching or tempering back the spine with a torch. These are famously done to pass the 90 degree bend test required for Journeyman or Master Smith performance testing.

Summary and Conclusions

We went over a lot in this article so I will try to sum up. One of the biggest differences between commercial and custom heat treating is the quench rate. The slower quench rate in many vacuum furnaces used by knife production facilities and large custom knife makers can often lead to a reduction in hardness and toughness. A faster quench that can be accomplished with other equipment gives better properties. The degree to which this matters can be debated, but there is a potential for small batch heat treating to be superior. The potential for improving edge retention with heat treatment is somewhat overhyped. Hardness is the main area where edge retention can be improved, and this can generally be achieved with commercial heat treating. The biggest differences between “good” and “bad” heat treatments come when there is a problem in the heat treatment. It is the heat treaters who can avoid these problems (and diagnose them when they come up) that perform the best heat treatments.

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[1] Gonçalves, Cristiane, André Slaviero, Rafael Mesquita, André Tschiptschin, and Paulo Haddad. “Effect of cooling rate during quenching on the toughness of high speed steels.” Journal of ASTM International 8, no. 4 (2011): JAI103483.

[2] Taljat, B., J. Tušek, D. Klobcar, P. Boscarol, and Giorgio Scavino. “Heat and surface treatment of hot-work tool steel for optimum in-service performance.” In The Use of Tool Steels: Experience and Research: Proceedings of the 6th International Tooling Conference, vol. 1, pp. 67-80. 2002.

The post Factory vs Custom Heat Treating of Knives appeared first on Knife Steel Nerds.

Atlantic 33 Steel

There are a few places to purchase this Atlantic 33 “non-tempering” steel so I have been getting questions about it over the past couple years. The information is pretty scant and the descriptions are mysterious and confusing to anyone who knows steel [1]. In the advertising it says things like: “Drawing of temper to suit different requirements is unnecessary and completely eliminated.” Also there is no composition listed and the heat treating instructions are very barebones. “Suitable hardness is obtained by merely heating the tool and quenching it in water.” And, “May be heated to almost a melting heat without danger of distortion.” It basically just says to heat treat it however you feel like and skip the tempering. If it’s true that it doesn’t matter how you heat treat that is great, but otherwise it would be nice to know how to best heat treat the steel.

What is a Non-Tempering Steel?

While the advertising online calls Atlantic 33 “a new departure alloy steel” it is in fact not new, it has been around since the mid 1930s [2]. Here is an ad for the steel from 1937:

1937 ad for Atlantic 33

However, “non-tempering” steels actually go back even further than Atlantic 33. The earliest I found was the steel “ABC” made by Darwin and Milner starting in 1909 [3][4]. This steel company was run by Paul Kuehnrich who developed several tool steels including D3 and D5 (later slightly modified to be the common D2 in 1928). He was an early pioneer in tool steel who I wrote about in my book The Story of Knife Steel.

Another early non-tempering steel I found was called Simplex which in 1915 had some advertising which uses a lot of language similar to what we still see for Atlantic 33:

Composition  of Atlantic 33 and Other Non-Tempering Steels

There were several other non-tempering steels that would be released over the following years though they never gained much prominence. Here are several that were available in 1954 [5]:

You can see that all are medium carbon, roughly 0.3-0.4%. Each has a chromium addition and either W or Mo (including the original ABC). There is also a Cu addition in some of them. The reason for the copper addition is somewhat of a mystery. Copper additions are not unheard of but are relatively uncommon in low alloy steels, especially back in the 1930s. Copper can help with grain size control from copper precipitates but I have no insight into the 1930s thought process by Atlantic Steel. In any case there were several non-tempering steels without any copper so it does not seem to have been necessary.

What Makes Them “Non Tempering”?

Even in the 1915 ad for Simplex they do not claim that nothing happens during tempering of these steels. It says that “No tempering is necessary except in the case of very small tools, such as punches, when it may be drawn to a straw color with advantage.” In other words, the steels benefit from tempering just like any other steel. However, they selected the carbon content to be in the range of 0.3-0.4% so that they would obtain sufficient hardness for the intended tools while not being brittle. Lower carbon steel is tougher and more ductile than high carbon steel, so you can achieve similar toughness by tempering less with a lower carbon steel. It is kind of an odd idea to recommend skipping tempering altogether. Heating to 300-500°F degrees doesn’t seem all that onerous even in these supposed simple shops they are talking about. So it seems to be a case of marketing more than anything else. Other 0.3-0.4% carbon steels could be used in a similar way. The Ryerson Non Tempering steel could even fit within the 4135 steel specification though the chromium content is borderline for 4135 (0.8-1.1% Cr).

What Happened to Non Tempering Steels?

So Non-Tempering steels saw some use in the industry, though were never particularly popular. Seemingly they were the most useful in shops that have very simple (in other words, bad) heat treating facilities. Those shops that wanted to make quick blacksmith tools, punches, shear blades, and other tools that are subjected to impact at room temperature. The steels fell out of favor by the 1960s and were largely replaced by silicon (S2, S4, S5, S6) or tungsten shock-resisting steels (S1) [6].

What is a Flutagon?

Initially the Atlantic 33 steel was advertised on its own. However, now you often see the steel called “Flutagon Atlantic 33” or “Atlantic 33 Flutagon” and even some people calling it only “Flutagon.” Flutagon was a trademark term by Atlantic steel in 1940 [7] for a new shape of the steel bars they were using that they claimed was more convenient, etc. Because the new shape was advertised for Atlantic 33 the two names became somewhat conflated. However, I think it is better to call the steel “Atlantic 33” and use Flutagon to refer to the shape of the bars instead. After all, the steel is also available in hex bars. Here is a 1941 ad for the new Flutagon shape for Atlantic 33:

1941 ad for flutagon shape

Datasheet for Similar Steel

I did not find a datasheet for Atlantic 33 but I did find heat treating information for Bethlehem’s similar “Non-Tempering” steel from 1958 [8].

Forging and Annealing

The forging temperature range recommendation is pretty standard, 2100-2150°F (1150-1175°C) and finish at 1700°F (925°C).

The datasheet says annealing is recommended if machining will be performed after forging. It says to start at 1400°F (760°C) and cool at a very slow rate of 20°F/hr (11°C/hr) to 900°F (480°C). It is pretty standard for datasheets to recommend such low cooling rates.

It is somewhat strange they recommend such a specific procedure for annealing given the intention of the steel is simple heat treating setups. The steel could be annealed by heating to nonmagnetic and putting in slow cool media like vermiculite.

Normalizing?

There is no mention of normalizing in the Bethlehem datasheet. Perhaps with the low carbon content they were less concerned about it. If you do normalize, the similar 4135 steel has a pretty typical 1600°F (870°C) normalizing temperature.

Austenitizing Temperature

One of the selling points for the steel being viable for simple heat treating setups is that it can be quenched from a range of austenitizing temperatures and still have decent properties. The extra alloy added to the steel helps it resist grain growth, thus having decent toughness even with relatively high austenitizing temperatures. However, perhaps we shouldn’t over-exaggerate this point too much. There is a small drop in toughness and ductility, and a small increase in grain size by austenitizing hotter than the optimal range, which appears to be about 1500-1600°F (815-870°C), see page 170 above. 4135 is typically austenitized at 1600°F (870°C) which is consistent with those results. The steel is relatively insensitive to overheating but if using a furnace I would stick with that range.

Quenching

This is typically an oil hardening steel. Tool Steels by Roberts [6] recommends using water if the cross-section is larger than 2-1/2 inches in diameter.

Tempering Temperature

While the steel is advertised as “non-tempering,” you can see on the page 170 chart the effects of tempering. Comparing “As-quenched” (not tempered) to 300°F (150°C) you can see that the low temper leads to the same hardness (555 Brinell or about 54 Rc), but an increase in yield strength, as well as elongation and reduction in area (measures of ductility). Increasing the tempering temperature to 400°F provides a further increase in yield strength, elongation, and reduction in area at the cost of a little hardness (~1 Rc). So unless I was in a big hurry I would probably still temper these “non-tempering” steels in the range of 300-400°F.

Summary

“Non-Tempering” steels have been around since at least 1909, while Atlantic 33 came out in the 1930s. The steel became connected to their trademark “Flutagon” shape in 1940. The main advantages of non-tempering steels are simple heat treatment where a wide range of austenitizing temperature is acceptable for good properties. While they are advertised as not requiring tempering, the properties actually are improved by tempering. However, the relatively low carbon content is what gives the steels decent properties even when skipping tempering. Other steels with 0.3-0.4% carbon would also behave similarly if tempering was skipped and there isn’t anything particularly special about non-tempering steels apart from the marketing. Austenitizing is best in the 1500-1600°F (815-870°C) range and I would temper Atlantic 33 at least at 300°F/150°C for the small improvement in properties.

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[1] https://www.canonballforge.com/about-a33

[2] https://archive.org/details/sim_industry-week_1936-04-06_98_14/page/66/mode/2up?q=%22non+tempering%22+

[3] https://books.google.com/books?id=WH8zAAAAIAAJ&pg=PR57&dq=%22non+tempering+steel%22

[4] https://babel.hathitrust.org/cgi/pt?id=wu.89074782335&seq=282&q1=3332

[5] https://archive.org/details/sim_american-machinist_mid-november-1954_98_24/page/149/mode/1up?q=%22atlantic+33%22

[6] https://babel.hathitrust.org/cgi/pt?id=uc1.b4418480&seq=426

[7] https://books.google.com/books?id=logbAQAAMAAJ&pg=PA823&dq=%22flutagon%22

[8] https://babel.hathitrust.org/cgi/pt?id=uiug.30112008811041&seq=172

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Stainless and San-Mai

This article got too long so I will be discussing stainless steels, high alloy steels, and San-Mai laminates in a future article.

Historical Steel Combinations

I have a book on the history of steel in knives in “modern” times called The Story of Knife Steel: Innovators Behind Modern Damascus and Super Steels. I have a very short summary below of some of the content in that book but if you want to learn more about steel combinations and patterning in Damascus, you should read the book.

Early Bill Moran pattern-welded Damascus used O1 and mild steel. Moran and other 1970s and 1980s-era Damascus smiths believed that a large carbon difference would lead to the difference in etching behavior to provide a contrast. Dr. John Verhoeven and bladesmith Howard Clark in 1998 did a study on pattern-welded Damascus [1] where they found that carbon would diffuse evenly between two steels and thus have the same carbon content throughout. Instead they found that other alloying elements led to contrast after etching. Carbon is a very small element which is “interstitial,” meaning it sits between iron atoms. Larger elements like manganese, chromium, and nickel are “substitutional” atoms that replace iron atoms and thus diffusion is much slower.

Before the study by Verhoeven and Clark, Damascus smiths had already moved on to other combinations. Instead of mild steel a high nickel, low carbon steel A203E became popular for the “bright” layer in Damascus. This recommendation was made in 1977 [2] by the “Damascus Steel Research Team” of Daryl Meier, Jim Wallace, and Robert Griffith. They also recommended pure nickel metal as a bright layer. Anciently, nickel steel was used in some blades from metallic meteorites, which typically have 5-10% nickel. Early “dark” layer choices in the 1970s and 1980s were typically simple high carbon steels such as W1, W2, and 1095. Along with low alloy high carbon tool steels like O1 and O2.

A203E has only 0.1% carbon and there was an increasing desire by bladesmiths to use two high carbon steels for better hardness and wear resistance. Tim Zowada began using O2 and L6 high carbon steels from Carpenter around 1990-1991. It became increasingly common to use bandsaw blades which were typically made with high nickel steels such as 15N20. However, it was more common at that time to call this material “L6,” despite the fact that L6 has never been used in bandsaw blades as far as I can find. L6 is somewhat similar because it has 1.5% Ni but it is different than bandsaw blade steels.

Popularization of 1084 and 15N20

The steel combination of 1084 and 15N20 has become ubiquitous, though it did not become popular until the mid-to-late 1990s. For example, the Jim Hrisoulas book The Pattern Welded Blade from 1994 does not even mention 15N20, and he mostly wrote in that book about combining high carbon and low carbon steels. The 1084/15N20 combination was popularized in part by the “Montana Mafia,” made up of Damascus smiths such as Rick Dunkerley and Shane Taylor. Dunkerley reported that he began using this mix in 1995 from a suggestion from Devin Thomas [3]. I asked my father Devin why he suggested these steels and he said that a lot of bladesmiths would ask him for help with forge welding and etching issues but a lot of people were using difficult steel combinations. He felt that 1084 and 15N20 were easy to forge weld and would give a good contrast.

15N20 has 0.75% carbon along with 2% nickel, so like other nickel alloyed steels it can act as the “bright” layer in Damascus. 1084 has only slightly higher carbon at 0.84% and has elevated manganese when compared with 1095 (0.75 vs 0.4% Mn). 1075 has the same 0.75% carbon of 15N20 so I also asked my father why it was 1084 and not 1075. He said that 1075 was typically available as wide sheet which would require shearing down to narrow pieces for the billet. 1084 was available as narrower “bar” which could be bandsaw cut for the stack with 15N20. For some knifemakers it was appealing that 1084 had more carbon than 1075. Regardless, 1075, 1080, and 1084 are roughly equivalent.

Rick Dunkerley folder in 1084 and 15N20

Etching Response

Notes on Comparisons

Comparing etching response of different combinations can be somewhat tricky because of a few factors:

1. Different etchants may lead to different levels of contrast
2. Etching and polishing technique affect contrast
3. Photography has a big effect on contrast. You may have experienced times when you saw a Damascus knife in an image with very clear contrast but once in person you see it is much more subtle.

So despite those difficulties I am going to make some generalizations about level of contrast with different combinations. If you disagree with my assessment feel free to say so in the comments.

Nickel

It has long been understood that the nickel content in steels like 15N20 allows them to resist etching and thus provide the “bright” layer in Damascus. However, when you etch 15N20 on its own it etches dark just fine so I think it may be some kind of anode/cathode effect at work where the 1084 is etched preferentially rather than the 15N20 resisting etching on its own. Higher nickel seems to lead to greater contrast than lower nickel, so 15N20 with its 2% nickel does pretty well. There are some rarer steels with 4% nickel, such as 4800 steel powder or Bohler K600 (1.2767). L6 has somewhat lower nickel at 1.5% and 8670 even lower with ~0.9%. Unfortunately there aren’t any low alloy nickel steels with significant vanadium or tungsten additions so wear resistance typically needs to come from the other steel used in the combination.

Nickel knife steels. Note: I found conflicting information on 4600 and 4800 compositions so I left the Mn blank. Some list there being 0.5% Mo. The most common versions have little or no carbon, and the “KC” versions have carbon added to them, usually graphite. I have also heard that some distributors take 1080 powder and add nickel powder.

Manganese (and Carbon)

Higher manganese leads to darker etching. For example, in the Verhoeven/Clark study they found that with 1018 (0.75% Mn) and 1086 steel (0.4% Mn) the 1018 was the “dark” layer despite its lower carbon content. This demonstrates that the old view that carbon differences led to etching contrast was incorrect.

Image from [1]. Notice the higher carbon steel is the “light” layer.

The higher Mn in 1084 relative to 1095 and 1075 leads to it being somewhat darker when in combination with 15N20. Some steels with even higher Mn etch even darker such as O1 (1.2%) and O2 (1.6%). The European version of O2 called 1.2842 has 2% Mn and etches darker than perhaps any other common steel.

Chromium

Chromium is a bit more mysterious as to its effects. In the Verhoeven/Clark study they found that 52100/L6 had poor contrast. 52100 has high chromium (1.5%) but also low manganese (0.35%). I have seen knives here or there with 52100 and 15N20 and the contrast can be “ok” in some of them, such as this feather pattern bowie by Aaron Wilburn:

Aaron Wilburn feather Damascus in 52100 and 15N20

1095 steel has similar carbon content to 52100 and slightly more Mn (0.45%) but without chromium. It typically etches darker than 52100 when in combination with 15N20, as seen in this knife:

Blake Nichols knife with Greg Shahan Damascus in 1095 and 15N20

O1 still etches quite dark despite having 0.5% Cr and so does 1.2842 despite having 0.35% Cr. 80CrV2 is also generally considered sufficiently dark in combination with 15N20 even though it has 0.5% Cr and only 0.4% Mn. 5160 has relatively high Mn (~0.85%) and Cr (0.8%) and it can also etch relatively dark despite the Cr addition.

So I think overall it appears that low amounts of Cr do not have a large effect on etching response, but higher amounts like the 1.5% in 52100 may affect how dark steel etches.

Vanadium, Tungsten, Molybdenum

As far as I can tell these elements do not effect etching. However, high vanadium and/or tungsten additions are good for higher wear resistance and edge retention in Damascus so I have included the composition of several of them below:

“3-color Damascus”

Sometimes steel combinations can be used that give multiple shades of grey/white. A common example would be a dark etching steel in combination with 15N20 and pure nickel. While 15N20 does resist etching it doesn’t do so nearly to the extent of pure nickel so three different shades are achieved.

Sometimes an intermediate shade steel can be found between 15N20 and a dark etching steel. One example I found was 1.2842/80CrV2/15N20 Damascus [4]. There can also be some contrast between 2% Ni 4600 and 4% Ni 4800 as seen comparing the eyes (4800) and beak (4600) in the roadrunner by Cliff Parker below:

Cliff Parker Damascus roadrunner. Bright lines are nickel sheet, eyes are 4800, beak is 4600, and the rest is 1084/1018 mixed.

Forge Welding Temperatures – Crumbling, Melting

It is quite common for Damascus smiths to use very high forge welding temperatures such as 2300, 2350°F (1260-1290°C) or sometimes even hotter. This typically works for steels like 1084 and 15N20. However, with higher carbon the melting temperature of the steel is reduced. The grain boundaries melt first. After the grain boundaries melt the steel will crumble during forging. This is especially important to keep in mind when steels have higher than 1.2% carbon like 26C3, ApexUltra, 1.2562, Blue Super, White #1, etc. However, even 1% carbon steels like W1 and 52100 can cause problems for Damascus smiths that are used to pushing the temperature as much as possible with lower carbon mixes like 1084 and 15N20. With these very high carbon steels, a reduced temperature is likely necessary such as 2150-2200°F (1175-1205°C). A longer soak time can make up for reduced forge welding temperatures.

Ease in Forging

I have a separate article on difficulty in forging of different steels. I have a loose ranking of some typical steels in terms of difficulty in forging in that article. Typically, higher carbon and higher alloy steels are more difficult to forge. One reason is because those steels are more likely to have undissolved carbides at the forging temperature. Those carbides don’t deform as well as steel which increases the difficulty.

Steels below the eutectoid (~0.77% carbon) are very easy to dissolve the carbides. The carbides would likely be dissolved well below the forge welding temperature. Steels with very high carbon like 26C3 (1.25% C) may still have some carbide near the forging temperature. 52100, even though it only has 1% carbon, also has 1.5% chromium which raises the temperature at which carbides dissolve. However, even 26C3 and 52100 will have very little carbide when compared to something like D2, and the remaining carbides would be much smaller. Some low alloy steels have vanadium and tungsten additions for greater wear resistance like CruForgeV and ApexUltra. Those vanadium and tungsten carbides are still present at forging temperatures and can affect forgeability.

How Well Do They “Stick”?

When forge welding in an oxygen atmosphere, oxides form on the surface which prevents diffusion bonding. Flux dissolves the oxides, allowing the two pieces of steel to bond. Some alloying elements will change the type of oxide that will form. The most common example is chromium in stainless steels, which famously cannot be forge welded with the conventional flux method. The chromium oxides are not dissolved by flux and therefore forge welding is not typically possible. Manganese and nickel do not seem to have a large effect which is why 1084 and 15N20 are easily forge welded using flux. The higher the chromium content, the more difficult the steel is to be forge welded. However, if the steel is in an oxygen-free environment then difficult steels can still be forge welded.

Cracking After Forging

High hardenability steels can partially harden after forging and this can lead to cracking, sometimes several hours later. Oil hardening steels like O1, O2, and L6 are more sensitive to this behavior. It helps to slow cool from below a certain temperature (like 1400°F/760°C or so). Slow cooling from very high temperatures can lead to carbides forming on the grain boundaries that are difficult to eliminate. Low alloy steels can be normalized to dissolve the carbides though air hardening steels cannot be normalized. After cooling to room temperature (whether slowly on purpose or unintentionally quickly), the steel can by cycled or annealed to remove the stresses in the material so that cracking doesn’t happen.

Austenitizing

Hardness

Semi-frequently I get questions from knife enthusiasts (and even some Damascus smiths) about combining non-typical steels together. Sometimes bizarre combinations like MagnaCut and 15N20. Often times the reason these steels are not compatible is because of the austenitizing (hardening) temperature range. 15N20 is optimally austenitized from about 1400-1475°F (760-800°C), while MagnaCut is austenitized from 1950-2200°F (1065-1205°C). As you can see, these two ranges do not overlap. If the MagnaCut was austenitized from 1475°F it would be very soft (and corrosion resistance would be lower than 1950+ as well). If the 15N20 was austenitized at 1950°F or higher it would have very poor toughness from grain growth. Some steels can be pushed a bit higher or lower than typically recommended in a datasheet but this is obviously not an example of where that can happen.

MagnaCut and 15N20 is of course an extreme example but austenitizing ranges can also be relevant even with low alloy non-stainless steels. For example, Mn and Ni push down the temperature at which steel transforms to austenite, and Cr increases the temperature where steel transforms to austenite. You can see this in the chart below where I austenitized steel in 25°F increments and quenched in oil (from a normalized microstructure).

You can see that 15N20 with the 2% Ni addition is fully austenitized (from a normalized, pearlite microstructure) from 1325°F (720°C) while 1084 was still dead soft after quenching from that temperature. The 1084 instead showed full hardness at 1350°F (732°C), only 25°F hotter. 80CrV2 has 0.5% Cr, however, which increases the temperature at which carbide dissolves, and full as-quenched hardness was not achieved until about 1400°F (760°C). This is from a normalized microstructure, not an annealed microstructure. The annealing procedure can have a large effect on heat treating behavior. Below is a chart of 80CrV2 after austenitizing from different temperatures:

As you can see, there is a significant difference between a pearlitic starting microstructure (normalized) and annealed. And in this case even a sizeable difference in the annealed microstructure of the two different suppliers. The Cr addition makes the starting microstructure have a particularly large effect on the heat treating response. This is why Cr alloyed steels often have a somewhat higher recommended austenitizing temperature than Cr-free steels such as 1500-1550°F (815-845°C).

15N20, only alloyed with Mn and Ni, shows a much smaller effect of starting microstructure on heat treating response:

The “Fast DET” anneal was my own annealing procedure with a faster cooling rate. The “Steel company anneal” is the steel as it was received from the manufacturer. You can see that with the steel as-received the peak hardness is not achieved until about 1400°F which is ~75°F higher than a normalized microstructure. But 1400°F is a much lower temperature than was necessary for factory annealed 80CrV2. This helps explain why Cr-free steels are usually austenitized from a somewhat lower temperature such as 1475°F (800°C).

Toughness – Grain Growth

However, there is also an upper end of acceptable temperature, which is where grain growth occurs. For chromium-alloyed steel, toughness typically peaks around 1500-1550°F (815-845°C) and drops from there. This was found with 5160, 52100, and 8670, for example. Carbides can “pin” grain boundaries and prevent grain growth. Because chromium stabilizes the carbides to higher temperatures, grain growth does not occur until somewhat higher temperatures than steels alloyed only with Mn and Ni. Vanadium can extend this temperature range further, as the vanadium carbides do not dissolve until higher temperatures. This is why small vanadium additions (~0.2%) are made to certain steels like W2 and 80CrV2. In my tests of 80CrV2, toughness had not dropped even with an austenitizing temperature of 1580°F (860°C), though I did not try any higher. But 15N20 does not have any chromium or vanadium so we did a new series of tests with 15N20. We also added two other steels, L6, a 1.5% Ni steel with a chromium addition; and 1.2519, a steel with chromium, tungsten, and vanadium (all three of those elements can help prevent grain growth). Each of them were heat treated from the annealed condition as-received from the steel manufacturer.

In terms of hardness, the steels behave in ways I have been describing. The L6 and 1.2519 increased in hardness with higher austenitizing temperature. This is because the steels were alloyed with chromium, delaying the dissolution of carbides. 15N20, lacking chromium, was flat in hardness between 1400 and 1600°F.

The L6 peaked in toughness at 1500°F which is similar to other chromium-alloyed steels. The toughness actually increased between 1400 (760°C) and 1500°F (815°C), which is because we are dissolving some carbide (carbides are brittle),  but not getting too much carbon in solution or grain growth. So the optimal austenitizing temperature for L6 would be around 1500°F, as that gives us a good combination of hardness and toughness. 1.2519 had flat toughness between 1400 and 1500°F though hardness also increased significantly over that range. Therefore, the hardness-toughness balance peaked at 1500°F even though the toughness did not change between 1400 and 1500°F. Toughness dropped slightly at 1550-1600°F (845-870°C), which is either due to grain growth or perhaps too much carbon in solution (I will explain that next).

With 15N20, the toughness dropped significantly even with 1500°F. In an earlier test with 15N20 I also found good toughness from 1475°F (800°C). So grain growth and a drop in toughness was found even with 1500°F, the same temperature that gave the peak properties for L6! I was surprised that toughness dropped off so rapidly for 15N20 at 1500°F, which I did not think sufficiently high to cause significant problems, even in a steel with little carbide like 15N20. This may have affected the toughness results of two 15N20 combinations I tested in my prior study, one with 1.2419 and the other with CruForgeV. 1084 is also very sensitive to overheating, grain growth, and a reduction in toughness. So despite how easy it is for 1084 and 15N20 to be hardened, I recommend using a furnace to avoid grain growth and a toughness drop.

80CrV2 and 15N20

So 15N20 needs to be austenitized at 1475°F (800°C) or below to avoid poor toughness, but 80CrV2 is more optimal from 1500-1525°F (815-830°C). Perhaps L6 is the more optimal steel to combine with 80CrV2, and 15N20 is more optimal to be used with 1084. However, the 80CrV2 can be processed to be more compatible with the 15N20. If starting from a normalized microstructure, the steel can be austenitized from a lower temperature to match the 15N20. Normalized steel is somewhat harder, and therefore more difficult to drill and bandsaw cut. With normalized 80CrV2 I found it to be about 25 Rc. This value can vary some based on the cooling rate (greatly affected by the size of the piece). If another grain refining step was added by heating to 1400°F (760°C) and air cooling, the hardness is dropped to about 19 Rc without significantly affecting the hardening response. Generally I prefer to use a “Fast DET” anneal before austenitizing instead, but this is one of the exceptions to that general recommendation.

Toughness – Carbon in Solution and Plate Martensite

Grain growth is not the only source of reduced toughness with higher austenitizing temperatures. The higher you austenitize, the more carbide is dissolved, putting more carbon “in solution,” which is why hardness is increased with temperature. However, as carbon in solution increases past 0.6% you get more “plate martensite” which is brittle and reduces toughness. You can read more about plate martensite in this article. With a steel like 15N20, the carbon content is only 0.75%, so this is not as much of an issue. However, when steels have over 0.85% carbon, they are more in danger of low toughness from plate martensite. Below is the example of O1 steel, austenitized from 1425 (775°C), 1475 (800°C), and 1550°F (845°C). Despite 1475°F not being hot enough for grain growth in O1, there was a significant reduction in toughness by austenitizing at 1475 instead of 1425°F. This was further reduced by austenitizing at 1550°F. So this is yet another reason to pay attention to optimal austenitizing ranges for different steels before combining them in pattern-welded Damascus.

Normalizing and Annealing

I know it seems slightly out of order to talk normalizing and annealing after austenitizing (to quench), but we are typically austenitizing the steel to perform these two steps as well. Compensating for normalizing is not too difficult with various Damascus combinations. It is generally best to take the higher required normalizing temperature of the two. For example, if you had combined 26C3, which needs something like 1700°F (925°C) to normalize, and combined it with 15N20 which only needs 1550°F (845°C) or so, you would normalize at 1700°F. The temperature needs to be sufficiently high to dissolve all the carbides, and the 1550°F for 15N20 would not be sufficiently high for the 26C3.

For annealing, similar rules apply. You generally want to use the higher required annealing temperature to ensure that both are annealed. If too much carbide is dissolved in one of the steels, that can mean you get pearlite at the end instead of spheroidized carbide and thus a little higher annealed hardness. But that is usually an acceptable tradeoff. As I mentioned in the 80CrV2/15N20 section, for some combinations a pearlitic, normalized structure may make the steels more compatible in heat treatment. In this case you would normalize like normal at the required temperature for 80CrV2 (1550-1650°F). Then heat to the annealing temperature for 80CrV2 (1400°F), but instead of slow cooling do air cooling. This is a grain refinement cycle instead of annealing. Normally grain refinement cycles aren’t necessary when annealing because the low temperature anneal also refines the grain. Generally I prefer annealed steel outside of specific cases such as mixing chromium and non-chromium steels. Or steel that will be heat treated in an uncontrolled forge.

I have a separate article on normalizing and annealing with recommended procedures and temperatures.

Quenching and Hardenability

Steels also have different levels of “hardenability,” or how fast they need to be quenched to be fully hardened. For example, there are “water hardening,” “oil hardening,” and “air hardening” steels. Even within low alloy non-stainless steels there can be significant differences. For example, L6 is a high hardenability oil hardening steel, while W2 is a very low hardenability water hardening steel. The two can be used together, but there is a significant gap between the two steels for hardenability. This means that to fully harden the W2/L6 Damascus you need to quench very rapidly for the W2 to ensure it fully hardens. If a slow oil was used you would likely end up with hardened L6 but soft W2. Even 1084 and 15N20 have a difference in hardenability because the 2% Ni addition in the 15N20 means its hardenability is higher despite the lower Mn. I have a loose ranking of hardenability of different steels in this article.

Tempering

Tempering leads to darker etching than untempered steel [1]. Some people using Damascus for decorative purposes (thus not needing high hardness) will skip the heat treatment but this leads to poorer contrast. Martensite etches better than ferrite, and tempered martensite etches even better.

In terms of compatibility, tempering is usually not a big deal. Most steels can be tempered at ~400°F and achieve good properties. Some steels have a slightly different range than others. 15N20 and 8670 needed to be tempered at least at 350°F for achieving high toughness and 5160 needed 375°F. The steels were not necessarily brittle below those temperatures, but did not have the high toughness the steels are known for.

You can also see that the toughness of 15N20 dropped when tempering at 450°F, which is called “Tempered Martensite Embrittlement.” The temperature at which TME is first seen changes some by steel, and some do not see the phenomena until 500°F or even higher. However, 400°F is safe for any steel I have so far tested.

Toughness and Edge Retention

I already have a whole article on the testing of toughness and edge retention of pattern-welded Damascus so I won’t rehash all of that here. But there are a few key points to mention here when it comes to compatibility:

1. Combining a low toughness and high toughness steel generally seems to lead to toughness similar to the low toughness steel. It fractures at the “weakest link” and there isn’t much benefit to adding a high toughness steel in terms of overall toughness.
2. High wear resistance steels with high slicing edge retention generally still cut very well even when combined with a steel with lower wear resistance. So combining CruForgeV and 15N20 or ApexUltra and L6 led to edge retention similar to CruForgeV or ApexUltra. However, this was due in part to the improvement in edge retention seen in ladder patterning.
3. While ladder patterning benefited edge retention it also slightly reduced toughness.
4. A “Damascus Cutting Effect” was seen in a 1095/Nickel Damascus. Most steel combinations do not have hard and soft layers but using nickel does provide this potential benefit.

When not attempting to have the “Damascus Cutting Effect” it can be beneficial to have two steels with good wear resistance. For low alloy non-stainless steels it makes sense to look at steels with significant vanadium and/or tungsten additions like ApexUltra, CruForgeV, 1.2519, 1.2419, 1.2562, Blue Super, V-Toku1, etc. However, for low alloy steels the main choices for a “bright” layer are 15N20 and L6 and neither has much wear resistance.

If making a Damascus combination for high toughness there must be two high toughness steels. 1084 and 15N20 are a decent combination for this. 1084 is somewhat lower in toughness but the two together still tested at ~34 ft-lbs in a random pattern (as opposed to 45-50 ft-lbs for 15N20).

Summary and Conclusions

It is best to choose steels that have similar temperatures for heat treating. High manganese is best for a dark layer after etching while Nickel is best for a bright layer. Other alloying elements can make forge welding difficult when using conventional flux methods, especially high levels of chromium. It is important to adjust heat treatment based on the steels that are combined together. For example, 15N20 sees a large toughness drop when austenitized at 1500°F. So when 1084 and 15N20 are used together it is best to heat treat with a furnace so that the steel isn’t overheated. The optimal temperature range for austenitizing 80CrV2 is too high for 15N20. If 80CrV2 and 15N20 are used together it is better to use a normalized starting structure so that a lower austenitizing temperature of 1475°F (800°C) can be used with the 80CrV2 and still achieve high hardness. L6 may be more optimal than 15N20 to use along with other chromium-alloyed steels though it has lower nickel than 15N20 (thus contrast is not as good), and is also more difficult to work with because of its alloy additions. High carbon steels are more sensitive to overheating because the grain boundaries melt, leading to crumbling during forging. High vanadium and tungsten steels are best for high wear resistance combinations, though they typically need to be combined with 15N20 or L6 which unfortunately do not have much wear resistance.

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[1] Verhoeven, John D., and Howard F. Clark. “Carbon diffusion between the layers in modern pattern-welded Damascus blades.” Materials characterization 41, no. 5 (1998): 183-191.

[2] Meilach, Dona Z., George Martin, E. A. Chase, and Theodore Davidson. “Decorative and sculptural ironwork: tools, techniques, inspiration.” 1977.

[3] Dunkerley, Rick. “His Forge Burns Hot for Mosaic Damascus.” Kertzman, Joe, ed. Blade’s Guide to Making Knives. Krause Publications, 2005.

[4] https://www.kovares.com/product-page/165mm-damascus-petty

 

The post Is 1084-15N20 the Best Pattern Welded Damascus? appeared first on Knife Steel Nerds.

Patreon

Several new experiments were done to support this article. I am able to do these experiments thanks to the contributions of Patreon supporters. If you would like to support knife steel research visit Patreon.com/KnifeSteelNerds

YouTube Video

Here is the video version of the following information:

Stainless and San-Mai

This article got too long so I will be discussing stainless steels, high alloy steels, and San-Mai laminates in a future article.

Historical Steel Combinations

I have a book on the history of steel in knives in “modern” times called The Story of Knife Steel: Innovators Behind Modern Damascus and Super Steels. I have a very short summary below of some of the content in that book but if you want to learn more about steel combinations and patterning in Damascus, you should read the book.

Early Bill Moran pattern-welded Damascus used O1 and mild steel. Moran and other 1970s and 1980s-era Damascus smiths believed that a large carbon difference would lead to the difference in etching behavior to provide a contrast. Dr. John Verhoeven and bladesmith Howard Clark in 1998 did a study on pattern-welded Damascus [1] where they found that carbon would diffuse evenly between two steels and thus have the same carbon content throughout. Instead they found that other alloying elements led to contrast after etching. Carbon is a very small element which is “interstitial,” meaning it sits between iron atoms. Larger elements like manganese, chromium, and nickel are “substitutional” atoms that replace iron atoms and thus diffusion is much slower.

Before the study by Verhoeven and Clark, Damascus smiths had already moved on to other combinations. Instead of mild steel a high nickel, low carbon steel A203E became popular for the “bright” layer in Damascus. This recommendation was made in 1977 [2] by the “Damascus Steel Research Team” of Daryl Meier, Jim Wallace, and Robert Griffith. They also recommended pure nickel metal as a bright layer. Anciently, nickel steel was used in some blades from metallic meteorites, which typically have 5-10% nickel. Early “dark” layer choices in the 1970s and 1980s were typically simple high carbon steels such as W1, W2, and 1095. Along with low alloy high carbon tool steels like O1 and O2.

A203E has only 0.1% carbon and there was an increasing desire by bladesmiths to use two high carbon steels for better hardness and wear resistance. Tim Zowada began using O2 and L6 high carbon steels from Carpenter around 1990-1991. It became increasingly common to use bandsaw blades which were typically made with high nickel steels such as 15N20. However, it was more common at that time to call this material “L6,” despite the fact that L6 has never been used in bandsaw blades as far as I can find. L6 is somewhat similar because it has 1.5% Ni but it is different than bandsaw blade steels.

Popularization of 1084 and 15N20

The steel combination of 1084 and 15N20 has become ubiquitous, though it did not become popular until the mid-to-late 1990s. For example, the Jim Hrisoulas book The Pattern Welded Blade from 1994 does not even mention 15N20, and he mostly wrote in that book about combining high carbon and low carbon steels. The 1084/15N20 combination was popularized in part by the “Montana Mafia,” made up of Damascus smiths such as Rick Dunkerley and Shane Taylor. Dunkerley reported that he began using this mix in 1995 from a suggestion from Devin Thomas [3]. I asked my father Devin why he suggested these steels and he said that a lot of bladesmiths would ask him for help with forge welding and etching issues but a lot of people were using difficult steel combinations. He felt that 1084 and 15N20 were easy to forge weld and would give a good contrast.

15N20 has 0.75% carbon along with 2% nickel, so like other nickel alloyed steels it can act as the “bright” layer in Damascus. 1084 has only slightly higher carbon at 0.84% and has elevated manganese when compared with 1095 (0.75 vs 0.4% Mn). 1075 has the same 0.75% carbon of 15N20 so I also asked my father why it was 1084 and not 1075. He said that 1075 was typically available as wide sheet which would require shearing down to narrow pieces for the billet. 1084 was available as narrower “bar” which could be bandsaw cut for the stack with 15N20. For some knifemakers it was appealing that 1084 had more carbon than 1075. Regardless, 1075, 1080, and 1084 are roughly equivalent.

Rick Dunkerley folder in 1084 and 15N20

Etching Response

Notes on Comparisons

Comparing etching response of different combinations can be somewhat tricky because of a few factors:

1. Different etchants may lead to different levels of contrast
2. Etching and polishing technique affect contrast
3. Photography has a big effect on contrast. You may have experienced times when you saw a Damascus knife in an image with very clear contrast but once in person you see it is much more subtle.

So despite those difficulties I am going to make some generalizations about level of contrast with different combinations. If you disagree with my assessment feel free to say so in the comments.

Nickel

It has long been understood that the nickel content in steels like 15N20 allows them to resist etching and thus provide the “bright” layer in Damascus. However, when you etch 15N20 on its own it etches dark just fine so I think it may be some kind of anode/cathode effect at work where the 1084 is etched preferentially rather than the 15N20 resisting etching on its own. Higher nickel seems to lead to greater contrast than lower nickel, so 15N20 with its 2% nickel does pretty well. There are some rarer steels with 4% nickel, such as 4800 steel powder or Bohler K600 (1.2767). L6 has somewhat lower nickel at 1.5% and 8670 even lower with ~0.9%. Unfortunately there aren’t any low alloy nickel steels with significant vanadium or tungsten additions so wear resistance typically needs to come from the other steel used in the combination.

Nickel knife steels. Note: I found conflicting information on 4600 and 4800 compositions so I left the Mn blank. Some list there being 0.5% Mo. The most common versions have little or no carbon, and the “KC” versions have carbon added to them, usually graphite. I have also heard that some distributors take 1080 powder and add nickel powder.

Manganese (and Carbon)

Higher manganese leads to darker etching. For example, in the Verhoeven/Clark study they found that with 1018 (0.75% Mn) and 1086 steel (0.4% Mn) the 1018 was the “dark” layer despite its lower carbon content. This demonstrates that the old view that carbon differences led to etching contrast was incorrect.

Image from [1]. Notice the higher carbon steel is the “light” layer.

The higher Mn in 1084 relative to 1095 and 1075 leads to it being somewhat darker when in combination with 15N20. Some steels with even higher Mn etch even darker such as O1 (1.2%) and O2 (1.6%). The European version of O2 called 1.2842 has 2% Mn and etches darker than perhaps any other common steel.

Chromium

Chromium is a bit more mysterious as to its effects. In the Verhoeven/Clark study they found that 52100/L6 had poor contrast. 52100 has high chromium (1.5%) but also low manganese (0.35%). I have seen knives here or there with 52100 and 15N20 and the contrast can be “ok” in some of them, such as this feather pattern bowie by Aaron Wilburn:

Aaron Wilburn feather Damascus in 52100 and 15N20

1095 steel has similar carbon content to 52100 and slightly more Mn (0.45%) but without chromium. It typically etches darker than 52100 when in combination with 15N20, as seen in this knife:

Blake Nichols knife with Greg Shahan Damascus in 1095 and 15N20

O1 still etches quite dark despite having 0.5% Cr and so does 1.2842 despite having 0.35% Cr. 80CrV2 is also generally considered sufficiently dark in combination with 15N20 even though it has 0.5% Cr and only 0.4% Mn. 5160 has relatively high Mn (~0.85%) and Cr (0.8%) and it can also etch relatively dark despite the Cr addition.

So I think overall it appears that low amounts of Cr do not have a large effect on etching response, but higher amounts like the 1.5% in 52100 may affect how dark steel etches.

Vanadium, Tungsten, Molybdenum

As far as I can tell these elements do not effect etching. However, high vanadium and/or tungsten additions are good for higher wear resistance and edge retention in Damascus so I have included the composition of several of them below:

“3-color Damascus”

Sometimes steel combinations can be used that give multiple shades of grey/white. A common example would be a dark etching steel in combination with 15N20 and pure nickel. While 15N20 does resist etching it doesn’t do so nearly to the extent of pure nickel so three different shades are achieved.

Sometimes an intermediate shade steel can be found between 15N20 and a dark etching steel. One example I found was 1.2842/80CrV2/15N20 Damascus [4]. There can also be some contrast between 2% Ni 4600 and 4% Ni 4800 as seen comparing the eyes (4800) and beak (4600) in the roadrunner by Cliff Parker below:

Cliff Parker Damascus roadrunner. Bright lines are nickel sheet, eyes are 4800, beak is 4600, and the rest is 1084/1018 mixed.

Forge Welding Temperatures – Crumbling, Melting

It is quite common for Damascus smiths to use very high forge welding temperatures such as 2300, 2350°F (1260-1290°C) or sometimes even hotter. This typically works for steels like 1084 and 15N20. However, with higher carbon the melting temperature of the steel is reduced. The grain boundaries melt first. After the grain boundaries melt the steel will crumble during forging. This is especially important to keep in mind when steels have higher than 1.2% carbon like 26C3, ApexUltra, 1.2562, Blue Super, White #1, etc. However, even 1% carbon steels like W1 and 52100 can cause problems for Damascus smiths that are used to pushing the temperature as much as possible with lower carbon mixes like 1084 and 15N20. With these very high carbon steels, a reduced temperature is likely necessary such as 2150-2200°F (1175-1205°C). A longer soak time can make up for reduced forge welding temperatures.

Ease in Forging

I have a separate article on difficulty in forging of different steels. I have a loose ranking of some typical steels in terms of difficulty in forging in that article. Typically, higher carbon and higher alloy steels are more difficult to forge. One reason is because those steels are more likely to have undissolved carbides at the forging temperature. Those carbides don’t deform as well as steel which increases the difficulty.

Steels below the eutectoid (~0.77% carbon) are very easy to dissolve the carbides. The carbides would likely be dissolved well below the forge welding temperature. Steels with very high carbon like 26C3 (1.25% C) may still have some carbide near the forging temperature. 52100, even though it only has 1% carbon, also has 1.5% chromium which raises the temperature at which carbides dissolve. However, even 26C3 and 52100 will have very little carbide when compared to something like D2, and the remaining carbides would be much smaller. Some low alloy steels have vanadium and tungsten additions for greater wear resistance like CruForgeV and ApexUltra. Those vanadium and tungsten carbides are still present at forging temperatures and can affect forgeability.

How Well Do They “Stick”?

When forge welding in an oxygen atmosphere, oxides form on the surface which prevents diffusion bonding. Flux dissolves the oxides, allowing the two pieces of steel to bond. Some alloying elements will change the type of oxide that will form. The most common example is chromium in stainless steels, which famously cannot be forge welded with the conventional flux method. The chromium oxides are not dissolved by flux and therefore forge welding is not typically possible. Manganese and nickel do not seem to have a large effect which is why 1084 and 15N20 are easily forge welded using flux. The higher the chromium content, the more difficult the steel is to be forge welded. However, if the steel is in an oxygen-free environment then difficult steels can still be forge welded.

Cracking After Forging

High hardenability steels can partially harden after forging and this can lead to cracking, sometimes several hours later. Oil hardening steels like O1, O2, and L6 are more sensitive to this behavior. It helps to slow cool from below a certain temperature (like 1400°F/760°C or so). Slow cooling from very high temperatures can lead to carbides forming on the grain boundaries that are difficult to eliminate. Low alloy steels can be normalized to dissolve the carbides though air hardening steels cannot be normalized. After cooling to room temperature (whether slowly on purpose or unintentionally quickly), the steel can by cycled or annealed to remove the stresses in the material so that cracking doesn’t happen.

Austenitizing

Hardness

Semi-frequently I get questions from knife enthusiasts (and even some Damascus smiths) about combining non-typical steels together. Sometimes bizarre combinations like MagnaCut and 15N20. Often times the reason these steels are not compatible is because of the austenitizing (hardening) temperature range. 15N20 is optimally austenitized from about 1400-1475°F (760-800°C), while MagnaCut is austenitized from 1950-2200°F (1065-1205°C). As you can see, these two ranges do not overlap. If the MagnaCut was austenitized from 1475°F it would be very soft (and corrosion resistance would be lower than 1950+ as well). If the 15N20 was austenitized at 1950°F or higher it would have very poor toughness from grain growth. Some steels can be pushed a bit higher or lower than typically recommended in a datasheet but this is obviously not an example of where that can happen.

MagnaCut and 15N20 is of course an extreme example but austenitizing ranges can also be relevant even with low alloy non-stainless steels. For example, Mn and Ni push down the temperature at which steel transforms to austenite, and Cr increases the temperature where steel transforms to austenite. You can see this in the chart below where I austenitized steel in 25°F increments and quenched in oil (from a normalized microstructure).

You can see that 15N20 with the 2% Ni addition is fully austenitized (from a normalized, pearlite microstructure) from 1325°F (720°C) while 1084 was still dead soft after quenching from that temperature. The 1084 instead showed full hardness at 1350°F (732°C), only 25°F hotter. 80CrV2 has 0.5% Cr, however, which increases the temperature at which carbide dissolves, and full as-quenched hardness was not achieved until about 1400°F (760°C). This is from a normalized microstructure, not an annealed microstructure. The annealing procedure can have a large effect on heat treating behavior. Below is a chart of 80CrV2 after austenitizing from different temperatures:

As you can see, there is a significant difference between a pearlitic starting microstructure (normalized) and annealed. And in this case even a sizeable difference in the annealed microstructure of the two different suppliers. The Cr addition makes the starting microstructure have a particularly large effect on the heat treating response. This is why Cr alloyed steels often have a somewhat higher recommended austenitizing temperature than Cr-free steels such as 1500-1550°F (815-845°C).

15N20, only alloyed with Mn and Ni, shows a much smaller effect of starting microstructure on heat treating response:

The “Fast DET” anneal was my own annealing procedure with a faster cooling rate. The “Steel company anneal” is the steel as it was received from the manufacturer. You can see that with the steel as-received the peak hardness is not achieved until about 1400°F which is ~75°F higher than a normalized microstructure. But 1400°F is a much lower temperature than was necessary for factory annealed 80CrV2. This helps explain why Cr-free steels are usually austenitized from a somewhat lower temperature such as 1475°F (800°C).

Toughness – Grain Growth

However, there is also an upper end of acceptable temperature, which is where grain growth occurs. For chromium-alloyed steel, toughness typically peaks around 1500-1550°F (815-845°C) and drops from there. This was found with 5160, 52100, and 8670, for example. Carbides can “pin” grain boundaries and prevent grain growth. Because chromium stabilizes the carbides to higher temperatures, grain growth does not occur until somewhat higher temperatures than steels alloyed only with Mn and Ni. Vanadium can extend this temperature range further, as the vanadium carbides do not dissolve until higher temperatures. This is why small vanadium additions (~0.2%) are made to certain steels like W2 and 80CrV2. In my tests of 80CrV2, toughness had not dropped even with an austenitizing temperature of 1580°F (860°C), though I did not try any higher. But 15N20 does not have any chromium or vanadium so we did a new series of tests with 15N20. We also added two other steels, L6, a 1.5% Ni steel with a chromium addition; and 1.2519, a steel with chromium, tungsten, and vanadium (all three of those elements can help prevent grain growth). Each of them were heat treated from the annealed condition as-received from the steel manufacturer.

In terms of hardness, the steels behave in ways I have been describing. The L6 and 1.2519 increased in hardness with higher austenitizing temperature. This is because the steels were alloyed with chromium, delaying the dissolution of carbides. 15N20, lacking chromium, was flat in hardness between 1400 and 1600°F.

The L6 peaked in toughness at 1500°F which is similar to other chromium-alloyed steels. The toughness actually increased between 1400 (760°C) and 1500°F (815°C), which is because we are dissolving some carbide (carbides are brittle),  but not getting too much carbon in solution or grain growth. So the optimal austenitizing temperature for L6 would be around 1500°F, as that gives us a good combination of hardness and toughness. 1.2519 had flat toughness between 1400 and 1500°F though hardness also increased significantly over that range. Therefore, the hardness-toughness balance peaked at 1500°F even though the toughness did not change between 1400 and 1500°F. Toughness dropped slightly at 1550-1600°F (845-870°C), which is either due to grain growth or perhaps too much carbon in solution (I will explain that next).

With 15N20, the toughness dropped significantly even with 1500°F. In an earlier test with 15N20 I also found good toughness from 1475°F (800°C). So grain growth and a drop in toughness was found even with 1500°F, the same temperature that gave the peak properties for L6! I was surprised that toughness dropped off so rapidly for 15N20 at 1500°F, which I did not think sufficiently high to cause significant problems, even in a steel with little carbide like 15N20. This may have affected the toughness results of two 15N20 combinations I tested in my prior study, one with 1.2419 and the other with CruForgeV. 1084 is also very sensitive to overheating, grain growth, and a reduction in toughness. So despite how easy it is for 1084 and 15N20 to be hardened, I recommend using a furnace to avoid grain growth and a toughness drop.

80CrV2 and 15N20

So 15N20 needs to be austenitized at 1475°F (800°C) or below to avoid poor toughness, but 80CrV2 is more optimal from 1500-1525°F (815-830°C). Perhaps L6 is the more optimal steel to combine with 80CrV2, and 15N20 is more optimal to be used with 1084. However, the 80CrV2 can be processed to be more compatible with the 15N20. If starting from a normalized microstructure, the steel can be austenitized from a lower temperature to match the 15N20. Normalized steel is somewhat harder, and therefore more difficult to drill and bandsaw cut. With normalized 80CrV2 I found it to be about 25 Rc. This value can vary some based on the cooling rate (greatly affected by the size of the piece). If another grain refining step was added by heating to 1400°F (760°C) and air cooling, the hardness is dropped to about 19 Rc without significantly affecting the hardening response. Generally I prefer to use a “Fast DET” anneal before austenitizing instead, but this is one of the exceptions to that general recommendation.

Toughness – Carbon in Solution and Plate Martensite

Grain growth is not the only source of reduced toughness with higher austenitizing temperatures. The higher you austenitize, the more carbide is dissolved, putting more carbon “in solution,” which is why hardness is increased with temperature. However, as carbon in solution increases past 0.6% you get more “plate martensite” which is brittle and reduces toughness. You can read more about plate martensite in this article. With a steel like 15N20, the carbon content is only 0.75%, so this is not as much of an issue. However, when steels have over 0.85% carbon, they are more in danger of low toughness from plate martensite. Below is the example of O1 steel, austenitized from 1425 (775°C), 1475 (800°C), and 1550°F (845°C). Despite 1475°F not being hot enough for grain growth in O1, there was a significant reduction in toughness by austenitizing at 1475 instead of 1425°F. This was further reduced by austenitizing at 1550°F. So this is yet another reason to pay attention to optimal austenitizing ranges for different steels before combining them in pattern-welded Damascus.

Normalizing and Annealing

I know it seems slightly out of order to talk normalizing and annealing after austenitizing (to quench), but we are typically austenitizing the steel to perform these two steps as well. Compensating for normalizing is not too difficult with various Damascus combinations. It is generally best to take the higher required normalizing temperature of the two. For example, if you had combined 26C3, which needs something like 1700°F (925°C) to normalize, and combined it with 15N20 which only needs 1550°F (845°C) or so, you would normalize at 1700°F. The temperature needs to be sufficiently high to dissolve all the carbides, and the 1550°F for 15N20 would not be sufficiently high for the 26C3.

For annealing, similar rules apply. You generally want to use the higher required annealing temperature to ensure that both are annealed. If too much carbide is dissolved in one of the steels, that can mean you get pearlite at the end instead of spheroidized carbide and thus a little higher annealed hardness. But that is usually an acceptable tradeoff. As I mentioned in the 80CrV2/15N20 section, for some combinations a pearlitic, normalized structure may make the steels more compatible in heat treatment. In this case you would normalize like normal at the required temperature for 80CrV2 (1550-1650°F). Then heat to the annealing temperature for 80CrV2 (1400°F), but instead of slow cooling do air cooling. This is a grain refinement cycle instead of annealing. Normally grain refinement cycles aren’t necessary when annealing because the low temperature anneal also refines the grain. Generally I prefer annealed steel outside of specific cases such as mixing chromium and non-chromium steels. Or steel that will be heat treated in an uncontrolled forge.

I have a separate article on normalizing and annealing with recommended procedures and temperatures.

Quenching and Hardenability

Steels also have different levels of “hardenability,” or how fast they need to be quenched to be fully hardened. For example, there are “water hardening,” “oil hardening,” and “air hardening” steels. Even within low alloy non-stainless steels there can be significant differences. For example, L6 is a high hardenability oil hardening steel, while W2 is a very low hardenability water hardening steel. The two can be used together, but there is a significant gap between the two steels for hardenability. This means that to fully harden the W2/L6 Damascus you need to quench very rapidly for the W2 to ensure it fully hardens. If a slow oil was used you would likely end up with hardened L6 but soft W2. Even 1084 and 15N20 have a difference in hardenability because the 2% Ni addition in the 15N20 means its hardenability is higher despite the lower Mn. I have a loose ranking of hardenability of different steels in this article.

Tempering

Tempering leads to darker etching than untempered steel [1]. Some people using Damascus for decorative purposes (thus not needing high hardness) will skip the heat treatment but this leads to poorer contrast. Martensite etches better than ferrite, and tempered martensite etches even better.

In terms of compatibility, tempering is usually not a big deal. Most steels can be tempered at ~400°F and achieve good properties. Some steels have a slightly different range than others. 15N20 and 8670 needed to be tempered at least at 350°F for achieving high toughness and 5160 needed 375°F. The steels were not necessarily brittle below those temperatures, but did not have the high toughness the steels are known for.

You can also see that the toughness of 15N20 dropped when tempering at 450°F, which is called “Tempered Martensite Embrittlement.” The temperature at which TME is first seen changes some by steel, and some do not see the phenomena until 500°F or even higher. However, 400°F is safe for any steel I have so far tested.

Toughness and Edge Retention

I already have a whole article on the testing of toughness and edge retention of pattern-welded Damascus so I won’t rehash all of that here. But there are a few key points to mention here when it comes to compatibility:

1. Combining a low toughness and high toughness steel generally seems to lead to toughness similar to the low toughness steel. It fractures at the “weakest link” and there isn’t much benefit to adding a high toughness steel in terms of overall toughness.
2. High wear resistance steels with high slicing edge retention generally still cut very well even when combined with a steel with lower wear resistance. So combining CruForgeV and 15N20 or ApexUltra and L6 led to edge retention similar to CruForgeV or ApexUltra. However, this was due in part to the improvement in edge retention seen in ladder patterning.
3. While ladder patterning benefited edge retention it also slightly reduced toughness.
4. A “Damascus Cutting Effect” was seen in a 1095/Nickel Damascus. Most steel combinations do not have hard and soft layers but using nickel does provide this potential benefit.

When not attempting to have the “Damascus Cutting Effect” it can be beneficial to have two steels with good wear resistance. For low alloy non-stainless steels it makes sense to look at steels with significant vanadium and/or tungsten additions like ApexUltra, CruForgeV, 1.2519, 1.2419, 1.2562, Blue Super, V-Toku1, etc. However, for low alloy steels the main choices for a “bright” layer are 15N20 and L6 and neither has much wear resistance.

If making a Damascus combination for high toughness there must be two high toughness steels. 1084 and 15N20 are a decent combination for this. 1084 is somewhat lower in toughness but the two together still tested at ~34 ft-lbs in a random pattern (as opposed to 45-50 ft-lbs for 15N20).

Summary and Conclusions

It is best to choose steels that have similar temperatures for heat treating. High manganese is best for a dark layer after etching while Nickel is best for a bright layer. Other alloying elements can make forge welding difficult when using conventional flux methods, especially high levels of chromium. It is important to adjust heat treatment based on the steels that are combined together. For example, 15N20 sees a large toughness drop when austenitized at 1500°F. So when 1084 and 15N20 are used together it is best to heat treat with a furnace so that the steel isn’t overheated. The optimal temperature range for austenitizing 80CrV2 is too high for 15N20. If 80CrV2 and 15N20 are used together it is better to use a normalized starting structure so that a lower austenitizing temperature of 1475°F (800°C) can be used with the 80CrV2 and still achieve high hardness. L6 may be more optimal than 15N20 to use along with other chromium-alloyed steels though it has lower nickel than 15N20 (thus contrast is not as good), and is also more difficult to work with because of its alloy additions. High carbon steels are more sensitive to overheating because the grain boundaries melt, leading to crumbling during forging. High vanadium and tungsten steels are best for high wear resistance combinations, though they typically need to be combined with 15N20 or L6 which unfortunately do not have much wear resistance.

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[1] Verhoeven, John D., and Howard F. Clark. “Carbon diffusion between the layers in modern pattern-welded Damascus blades.” Materials characterization 41, no. 5 (1998): 183-191.

[2] Meilach, Dona Z., George Martin, E. A. Chase, and Theodore Davidson. “Decorative and sculptural ironwork: tools, techniques, inspiration.” 1977.

[3] Dunkerley, Rick. “His Forge Burns Hot for Mosaic Damascus.” Kertzman, Joe, ed. Blade’s Guide to Making Knives. Krause Publications, 2005.

[4] https://www.kovares.com/product-page/165mm-damascus-petty

The post Is 1084-15N20 The Best? Pattern-Welded Damascus appeared first on Knife Steel Nerds.

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Video

YouTube video for the following information:

The Current Production Path for MagnaCut and CPM Steels

Crucible Industries makes steel (duh), specializing in powder metallurgy tool steels. They are located in Solvay, NY, near Syracuse. Any steel with a “CPM” in front of it comes from Crucible. This includes steels like (CPM) S30V, 3V, 4V, 10V, 15V, S35VN, S45VN, M4, CruWear, Rex 45, Rex 121, and many others. However, they also make “conventional” tool steels and stainless steels, notably 154CM, 440C, A2, D2, and others. They are one of the few companies that makes powder metallurgy steel, and in fact, they were the company that developed the process back in the late 1960s. Read this article to learn more about powder metallurgy.

However, Crucible is not the only company responsible for this wide selection of steels available to the knife industry. Niagara Specialty Metals hot rolls, anneals, etc. Crucible steel to the thin sizes necessary for knife steels. All of the Crucible steel made into knives also goes through Niagara Specialty Metals. Niagara then distributes the steel directly to knife companies, larger knifemakers, and suppliers that deliver smaller orders. A big reason for the wide availability of knife steels in many varieties at the sizes that knifemakers want is because of Niagara Specialty Metals.

The Current State of Crucible

Crucible has been having a difficult time monetarily due to a slowdown in steel orders [1]. I don’t know all of the reasons for this decline in sales, so I won’t speculate too much. The company has several million dollars in debt which it is unable to pay. Sometime in 2024, they began trying to sell the company to remain operational. Four companies were interested, but none would commit to buying without a bankruptcy sale process. Crucible issued a “WARN” notice to the state of New York on December 5th, stating they would close the plant and lay off their 158 employees by March 2025. They filed for Chapter 11 bankruptcy protection shortly after.

There will be an auction on February 4th for Crucible’s assets [1]. Erasteel has made a bid for $17.3 million for the plant and all intellectual property. Erasteel is currently headquartered in France. However, historically a big chunk of the company is Swedish and they still have many of their plants in Sweden including their powder metallurgy facilities. Erasteel primarily makes high speed steel, both conventional and powder metallurgy grades. They are a well-respected steel company.

Past Bankruptcy

Crucible has actually gone through bankruptcy before in 2009, which they said was due to a reduction in sales to the automotive industry. At that time they had 675 employees. They were purchased by J.P. Industres, Inc. for $8 million, and ultimately, Crucible was able to survive. However, even that bankruptcy was not without pain. Crucible got rid of its research and development center in Pittsburgh [2]. They also sold off their service centers to SB Specialty Metals [3]. Service centers warehouse steel, handle sales calls, and perform value-added steps like cutting and surface grinding. This left Crucible as the steel plant only.

Patents and Trademarks of Crucible

When Crucible had R&D, it was very active in patenting new steel compositions, but it hasn’t patented anything since the bankruptcy. So, almost all of its patents are expired. One notable exception is CPM S110V, which won’t expire until 2028 [4]. Patents protect technology like steel compositions and processes, so I don’t think potential buyers would see the purchase of Crucible as useful from a patent standpoint.

Trademarks, however, can be extended indefinitely. Crucible owns trademarks for many of its grades, such as 10V, S30V, and S35VN. It also owns the trademark for “CPM,” which it puts at the front of its powder metallurgy grades. Potential buyers could see these trademarks as valuable.

What Niagara Plans to Do

Niagara is sort of stuck in the middle on this since Crucible has been a major steel supplier of theirs to be able to hot roll and deliver steel to knifemakers. On December 10th, Bob Shabala of Niagara Specialty Metals posted to BladeForums that they have a plan in place no matter what happens to Crucible [5]:

We’ve taken proactive steps to maintain a strong supply chain and ensure uninterrupted service for the cutlery industry.

We have already placed orders with Erasteel, Carpenter Technologies, Ellwood Quality Steels and Universal Stainless who will provide the same high-quality products and the same exact chemistries. We have been processing their cutlery and/or aerospace alloys for several years and their quality has always been outstanding. Although there may be a brief adjustment period during this transition, our team is committed to minimizing any disruptions and maintaining the service and product quality you have come to expect from us.

It is also important to note that Crucible Industries is making efforts to remain in business during this process. We have steel on order with them and they are continuing to process everything they can.

Bob later added a couple of other important points:

Crucible has the trademarks for CPM, MagnaCut and most of the other cutlery grades we buy from them. We can’t use those names without the express written consent of Crucible.

The grades are not patented so we can get the same composition from other mills as needed. New names would have to be used unless Crucible granted us permission. (CPM S90V is the same at Carpenter’s CTS 420CW Mod)

Crucible has filed for Chapter 11 bankruptcy protection and would like to sell to someone who will keep the operation going. We don’t know when that will happen but we are in constant contact with Crucible and looking forward to working with the new ownership.

What About MagnaCut?

Crucible owns the trademark to MagnaCut but Niagara is able to order the steel from Erasteel and Carpenter instead. Niagara will try to either purchase the trademark for MagnaCut or to get permission from whoever the new owner is to use the trademark. If that fails we would have to use a new name. That would be a real bummer since MagnaCut has significant name recognition but we will do what we have to do.

Common Questions

If Crucible owns trademarks on these grades how will Niagara be able to order them somewhere else?

Trademarks are names and symbols not compositions. Without separate protections on compositions (patents) there is nothing stopping another steel company from making them.

So many knives use Crucible steels, how can they be bankrupt?

From what I understand knife steel is only a relatively small percentage of Crucible’s steel production. Even if it were a sizable percentage that wouldn’t necessarily guarantee the company is making money.

Will knife steel supply be disrupted?

I think Niagara has a good plan in place for keeping the knife steel coming even if Crucible stopped making it so I think any disruptions will be minimal. Hopefully Crucible continues to operate and we won’t need to worry at all.

Would the same steels made by a different company perform the same?

Erasteel and Carpenter are known for high quality powder metallurgy steels. They already make several steels of identical composition to those that Crucible makes, and they are known in the market for being equivalent. I have previously compared powder metallurgy steels of identical composition and found no differences in properties. One previous example was tests on Z-Wear made by Crucible and a European steel company. Another study was on stainless powder metallurgy steels made by Bohler, Uddeholm, Crucible, and Carpenter.

Would we be fine without Crucible and Niagara?

I have seen a few people say not to worry because we can just buy knife steel from competitors like Bohler, Uddeholm, and Carpenter. I am not quite so optimistic on that front.

Carpenter has historically had somewhat variable interest in making knife steels. For example, Cold Steel switched away from CTS-XHP to CPM S35VN in 2018 because of availability issues [6]. I am optimistic that with Niagara being the one ordering (relatively large) quantities from Carpenter, and also being the company that does the final hot rolling (to have the range of sizes necessary), that this inconsistent supply issue with Carpenter will be solved.

In terms of Bohler and Uddeholm (same parent company but they operate relatively independently) their steels are also not always available in the sizes we want for knives. We want very thin steel for knives and those are often viewed by these companies as being “special runs” just for knives. The tool and die industry, plastics processing industry, etc. often use blocks that are several inches in thickness. Several new or rare Bohler and Uddeholm steels I have tested in the past were only obtained because knifemakers were able to pay the company to take slices off thick blocks of the steel. These are very expensive and not really viable for any significant amounts.

Overall, I would say that without Crucible and Niagara we would still find steels to use. But we would see a large reduction in varieties of steels available, and getting a wide ranges of sizes would also be much more difficult. And getting steel when you want it (ie “delivery”) would also be much worse. The future is much better for knife steel with Niagara in it. Niagara is a major driving force behind the wide availability of exotic knife steels.

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[1] https://www.syracuse.com/business/2024/12/crucible-industries-steel-plant-to-be-sold-early-next-year.html

[2] https://www.aist.org/allegheny-technologies-completes-acquisition-of-crucible-powder-metals

[3] https://sbisteel.com/wp-content/uploads/2012/04/12-18-2009-Crucible-Service-Centers-sold-to-SBI.pdf

[4] https://patents.google.com/patent/US7615123B2/en

[5] https://www.bladeforums.com/threads/crucible-industries-update.2017101/

[6] https://knifenews.com/cold-steel-to-transition-from-cts-xhp-to-s35vn-blade-steel/

The post Crucible Steel is Bankrupt! What is Next for MagnaCut? appeared first on Knife Steel Nerds.

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Video

There is a video version of the following information:

Why Analyze AI Capabilities?

I’ve had multiple people over the past couple years send me steel compositions that ChatGPT came up with. People want to know what I think, and if AI is going to be taking over everything now, including the design of new steels. I took a few of the steel compositions people sent me so I can analyze them to see if they would work. Can ChatGPT come up with new and fresh ideas to revolutionize knife steel? Let’s look at them and see:

Infinisteel

ChatGPT has some grandiose claims for this first steel:

Introducing the revolutionary super steel, InfiniSteel! This cutting-edge material boasts unparalleled toughness, edge retention, and corrosion resistance, surpassing any other steel on the market. By exploiting a unique interplay between various alloying elements, InfiniSteel exhibits remarkable properties that set it apart from conventional steel grades.

The InfiniSteel composition consists of:

Iron (Fe): 75%
Carbon (C): 1.2%
Chromium (Cr): 20%
Vanadium (V): 2.5%
Molybdenum (Mo): 0.8%
Tungsten (W): 0.5%

The biggest issue with this steel is that it won’t harden. When heat treating knife steel the main step is “austenitizing,” the high temperature step before the steel is quenched (rapidly cooled). You can read more about austenitizing in this article. It is called the austenitizing temperature because the steel transforms from the low temperature ferrite phase to the high temperature austenite phase. Ferrite is the normal “phase” of iron at room temperature which has a certain set of properties, such as being magnetic and soft. When you heat up iron to high temperature the atoms rearrange themselves into a different structure called austenite, which has different properties, one of them being that it is not magnetic. When you quench austenite it transforms to the hard steel phase martensite. If you don’t transform to austenite at your heat treatment temperature you will still have soft ferrite after heat treating and thus it will not make a good knife edge.

Some elements are “ferrite stabilizers” and some are “austenite stabilizers.” This steel has a relatively high content of ferrite stabilizers including 20% Cr, 2.5% V, 0.8% Mo, and 0.5% W. This would be ok if there were enough austenite stabilizers, but the only austenite stabilizer in the steel is carbon, and 1.2% is not enough for all of those ferrite stabilizers. ChatGPT is just adding elements in because when it reads random articles online that say that each element does some beneficial thing. It doesn’t know anything about balancing different elements together and it has no idea that it needs to have enough carbon or other austenite stabilizers to go with all of those ferrite stabilizers.

Here’s a detailed explanation of the role each component plays in the alloy and how they contribute to the exceptional performance of InfiniSteel:

Iron (Fe): As the primary element in steel, iron provides the backbone of the material’s structure. It imparts strength and toughness to the alloy, ensuring InfiniSteel can withstand substantial loads and resist deformation under various conditions.

Carbon (C): The addition of 1.2% carbon significantly improves the strength and hardness of the alloy. This results in superior edge retention, allowing InfiniSteel to maintain a sharp edge even after extensive use. Additionally, the high carbon content enables the formation of carbides, which further enhance wear resistance.

Chromium (Cr): Chromium contributes to InfiniSteel’s exceptional corrosion resistance. At 20%, it forms a stable, adherent, and self-healing passive film on the steel’s surface, effectively protecting it from oxidizing agents. This film also enhances the steel’s resistance to various corrosive environments, such as those with high humidity or in contact with aggressive chemicals.

Vanadium (V): Vanadium’s primary function is to refine the steel’s grain structure, leading to enhanced toughness and resistance to impact. At 2.5%, it forms hard vanadium carbides that strengthen the material and improve its edge retention properties. Additionally, vanadium aids in stabilizing the passive film, further enhancing the steel’s corrosion resistance.

Molybdenum (Mo): Molybdenum strengthens the steel matrix and improves its resistance to high-temperature creep and stress rupture. At 0.8%, it also contributes to the formation of a more stable passive film, increasing the corrosion resistance of InfiniSteel.

Tungsten (W): The addition of 0.5% tungsten enhances the high-temperature performance and hardness of InfiniSteel. It forms carbides that provide excellent wear resistance, contributing to the alloy’s outstanding edge retention properties.

Through this unique interplay of alloying elements, InfiniSteel achieves an unprecedented level of performance in terms of toughness, edge retention, and corrosion resistance. The combination of carefully selected constituents enables the alloy to outshine any other steel on the market, making it the ideal choice for a wide range of demanding applications, such as aerospace, automotive, marine, and cutting tools industries.

The explanations of what the different elements are doing are also very dumb and obviously based on random articles from non-experts online. For example, under iron it says that the iron “imparts strength and toughness to the alloy.” This is a very odd thing to say about the steel since steel is made up of iron. Iron is pretty soft before we add other elements to it, especially carbon. So why would you describe iron as imparting “strength”? Sounds like something a dumb AI would write.

It also says that “Vanadium’s primary function is to refine the steel’s grain structure, leading to enhanced toughness and resistance to impact.” Small amounts of vanadium, like 0.1-0.2%, are added to prevent grain growth. In knife steels when you have 0.3% or more added, that is for wear resistance. In that case it is adding wear resistance and reducing toughness. So the claim by the AI that the vanadium is added for enhanced toughness is nonsense.

The AI also says that molybdenum was added for “its resistance to high-temperature creep and stress rupture.” High temperature creep is when a material is held under stress at relatively high temperature for long periods of time. The material slowly deforms, which is called creep. Resisting creep is totally unnecessary in a knife, and the AI is just pulling things from various places to support its choices in a seemingly random way.

No manganese or silicon was added to the steel. These are not always shown on datasheets but are present in all tool steels that I am aware of. I would say it just didn’t mention them, but it also gave the percentage of iron (75%) which leaves no more room for Mn and Si.

User
Please provide a detailed analysis of Infinisteel vs Magnacut?

ChatGPT
InfiniSteel and MagnaCut are both high-performance steels designed for demanding applications. Here’s a detailed analysis comparing their properties and performance:

Composition:

MagnaCut:
Carbon (C): 1.15%
Chromium (Cr): 17.0%
Manganese (Mn): 0.6%
Nickel (Ni): 0.3%
Nitrogen (N): 0.18%
Vanadium (V): 4.0%
Molybdenum (Mo): 2.0%

Comparison:

Toughness: Both InfiniSteel and MagnaCut boast high levels of toughness due to their vanadium content, which refines the grain structure of the steel. InfiniSteel contains 2.5% vanadium, while MagnaCut has a higher vanadium content at 4.0%. The higher vanadium content in MagnaCut might result in slightly better toughness compared to InfiniSteel.

Corrosion resistance: InfiniSteel has a higher chromium content (20%) than MagnaCut (17%). This results in better passive film formation and, consequently, superior corrosion resistance. Additionally, MagnaCut’s nitrogen content (0.18%) further enhances its corrosion resistance. However, InfiniSteel is likely to have the edge in this category due to its higher chromium content.

…

The person who sent me this also asked ChatGPT how the steel would compare to MagnaCut. Somehow ChatGPT managed to get the composition of MagnaCut wrong. MagnaCut has 10.7% chromium but ChatGPT says it has 17%. It also has 0.2% nitrogen as a target, so the 0.18% nitrogen that ChatGPT reports could technically be a nitrogen content that would be seen in an individual bar of steel, but that is not the target composition. And it also reports that MagnaCut has 0.3% nickel, even though nickel is not added to the steel and is not reported in the datasheet.

ChatGPT also repeats its mistake from earlier, saying that vanadium in higher amounts leads to more toughness. So it says that MagnaCut would have higher toughness because it has 4% vanadium instead of the 2.5% in “Infinisteel.” That is not how it works.

My Grade: D-

It is difficult to give the steel high marks when you can’t harden it.

Ultravium Steel

— create a unique composition of elements for a great blade steel that does not exist yet…
ChatGPT:
Creating a unique composition for a great blade steel requires a deep understanding of metallurgy and the desired properties in the resulting steel. While this composition doesn’t exist yet, we can propose an innovative blend of elements to produce an exceptional blade steel:

Name: Ultravium Steel

Composition:

Carbon (C): 2.5%
A higher carbon content contributes to increased hardness, making the blade exceptionally sharp and able to hold an edge for extended periods.
Vanadium (V): 7%
Vanadium adds wear resistance, enhancing the blade’s durability and edge retention. It also aids in fine-grain structure formation, crucial for sharpness.
Molybdenum (Mo): 3%
Molybdenum boosts the steel’s strength, toughness, and resistance to corrosion. It also helps maintain blade sharpness under high-stress conditions.
Tungsten (W): 5%
Tungsten further improves hardness, wear resistance, and overall blade performance. It enhances the steel’s ability to withstand extreme temperatures.
Niobium (Nb): 1.5%
Niobium promotes grain refinement, which contributes to sharpness and overall blade strength. It also adds stability to the steel’s microstructure.
Cobalt (Co): 2%
Cobalt increases the steel’s strength and high-temperature performance, making it ideal for blades subjected to extreme conditions.
Copper (Cu): 0.5%
Copper adds a touch of corrosion resistance to the steel while maintaining its strength and hardness.
Silicon (Si): 1%
Silicon enhances the steel’s toughness and resistance to wear, improving its overall durability.

ChatGPT starts out by saying that a unique composition for knife steel “requires a deep understanding of metallurgy,” which I agree with. However, this composition shows that ChatGPT also does not have this required deep understanding. The approach appears to have been adding large amounts of most of the major elements. This steel is a good example of how ChatGPT “reads” things online that say all the positive things that different elements do. When a layman reads about different elements this is also typically what they see. It says what an element does to improve properties but it doesn’t usually explain the tradeoffs or how elements are balanced or how they work together. It says that molybdenum “helps maintain blade sharpness under high-stress conditions.” I don’t know what that means or how molybdenum is helping. Is a “high stress” condition one where an individual is under attack? Or is “high stress” referring to “engineering stress” where a load is being applied to a knife? It says that tungsten is added “to withstand extreme temperatures.” What kind of temperatures? Is this a knife for use in volcanoes? It says that niobium is for grain refinement, but 1.5% is way more than necessary for grain refinement. That amount is for wear resistance and ChatGPT didn’t even mention wear resistance. Cobalt was added again for “high-temperature performance,” “for blades subjected to extreme conditions.” And again I say that if the temperature is too high for the knife it is way, way too hot for humans. Copper is added for “a touch of corrosion resistance” for some reason.

There is a big omission with this steel, however, which is chromium. ChatGPT didn’t add any of it. This means the steel does not have any corrosion resistance to speak of (that 0.5% copper isn’t doing much). But chromium is also added to steels for other reasons. It is present in nearly all high alloy tool steels, in part so that it has high “hardenability” so that it can be quenched more slowly and still fully harden. Not adding it is very strange for this steel.

Heat Treatment:

Ultravium Steel would undergo a precise heat treatment process, including quenching and tempering, to maximize its hardness, toughness, and fine-grain structure. The ideal hardness level would be around HRC 65-66.
Properties:

Ultravium Steel would be renowned for its exceptional hardness, wear resistance, and edge retention. It would maintain sharpness even under heavy use and extreme conditions.
The steel’s fine-grain structure would contribute to its ability to take and hold a razor-sharp edge.
Superior strength and toughness would make it suitable for a wide range of cutting applications, from fine slicing to heavy-duty chopping.
Ultravium Steel would exhibit excellent corrosion resistance, reducing the need for extensive maintenance and ensuring longevity.
It would be an ideal choice for high-performance knives, cutting tools, and blades used in demanding environments.
Applications:

Chef’s knives: Ultravium Steel would excel in the kitchen, making it a favorite among professional chefs and culinary enthusiasts.
Survival and outdoor knives: Blades made from this steel would prove highly reliable in rugged outdoor conditions.
Industrial cutting tools: Ultravium Steel would be sought after for its ability to withstand heavy use in manufacturing and construction.
Military and tactical knives: Its strength and durability would make it suitable for combat and tactical applications.
Artistic blades: Bladesmiths and artisans would appreciate its ability to hold intricate patterns and designs in damascus or layered steel.
While this hypothetical Ultravium Steel may not exist yet, advancements in metallurgy continue to push the boundaries of what’s possible. Such an innovative blend of elements could revolutionize the world of blade steel, offering exceptional performance and versatility to knife makers and users alike.

ChatGPT then claims that Ultravium would have “excellent corrosion resistance” which it certainly would not have without any chromium. It also says that bladesmiths would like it for “its ability to hold intricate patterns and designed in damascus or layered steel.” I don’t know what it means to “hold” an intricate pattern. But bladesmiths would likely not prefer this steel for Damascus because the high alloy content would make it more difficult to forge weld.

My Grade: C-

Since this steel hardens I will give it a C-. Can’t give higher than that since it skipped chromium while claiming the steel would have high corrosion resistance.

MagnaCut+

— ChatGPT
Hypothetical Composition for “MagnaCut Plus”
• • •
Element Proposed Content

Carbon (C) 1.05%
Chromium (Cr) 11.5%
Vanadium (V) 3.0%
Molybdenum (Mo) 2.0%
Tungsten (W) 0.5%
Nitrogen (N) 0.3%
Niobium (Nb) 0.5%
Cobalt (Co) 1.5%

Expected Improvements
• Wear Resistance: The increase in vanadium and tungsten would lead to harder vanadium and tungsten carbides, resulting in better edge retention.
• Toughness: The slight decrease in carbon and the introduction of cobalt would help maintain or even improve toughness.
• Corrosion Resistance: The increase in chromium and nitrogen should enhance resistance to rust and corrosion.
• Grain Refinement: More niobium would aid in creating a finer grain structure, resulting in better overall balance.
Potential Drawbacks
• Higher Cost: Adding tungsten and cobalt increases the cost of production.
• More Challenging Heat Treatment: The changes may require more precise heat treatment to unlock the full potential of the alloy.
• Diminished Ease of Sharpening: Higher vanadium content can make sharpening slightly more difficult.
This proposed “MagnaCut Plus” would aim to push the limits of what MagnaCut offers without compromising the balanced properties that make it so desirable. It would likely excel in applications requiring even greater edge retention and toughness, such as high-performance cutting tools and hard-use knives.

This steel was requested by the user to be an improvement on MagnaCut. However, ChatGPT offers from very strange reasoning:

“The increase in vanadium and tungsten would lead to harder vanadium and tungsten carbides, resulting in better edge retention.”

The vanadium content was reduced from MagnaCut, down to 3% from 4%. So I don’t know why ChatGPT thinks it has increased vanadium. It also claims that it added tungsten to form “tungsten carbides.” In these types of high alloy steels, adding a tiny amount of tungsten does not lead to the formation of any carbides that are tungsten-rich. The tungsten incorporates into other carbide types such as vanadium-rich or chromium-rich carbides. And those carbides would not be any harder. A small addition of tungsten seems to be a theme for ChatGPT and I don’t really see the point. To be fair, there are several stainless steels from Takefu where they added small amounts of tungsten and claimed similar reasoning. But I have also been on record saying I think that the tungsten is unnecessary in these steels.

“More niobium would aid in creating a finer grain structure.” The niobium content is less than MagnaCut, 0.5% vs 2%. so it does not have “more niobium.”

“The changes may require more precise heat treatment to unlock the full potential of the alloy.” Why? Based on what?

Other criticisms of the design require more in-depth analysis with a thermodynamics software that predicts carbide types and alloy in solution. ChatGPT claims an improvement in corrosion resistance because of the higher chromium, but in fact the chromium in solution is reduced in the “MagnaCut+” alloy because of the other elements. There is also  3-5% chromium carbide in the heat treated structure which reduces corrosion resistance. The improvement in corrosion resistance with MagnaCut came from reducing chromium carbide content to near-zero and this steel dropped that innovation for no reason because ChatGPT doesn’t know what it is doing.

The carbide content of MagnaCut+ and MagnaCut would be similar but MagnaCut+ would have half of its carbide be the larger chromium-rich type, reducing toughness. It also claims that cobalt improves toughness, which it doesn’t.

My Grade: C

The steel can be hardened and would be nearly stainless. But calling it MagnaCut+ while reducing corrosion resistance, wear resistance, and toughness from MagnaCut is bad so it gets a C.

Some General Thoughts

In none of the proposals that ChatGPT gave did it ever say that it would make these steels by powder metallurgy. The very high alloy content of these steels means they would not be very good if made with conventional steelmaking. Or at the very least these would not be an improvement on existing steels if they were not made with powder metallurgy. So to not state explicitly that the steel should be made with powder metallurgy is a big omission.

ChatGPT provided no heat treating information. In one case ChatGPT claimed its new alloy would require “more precise heat treatment” but it did not provide any information on temperature ranges the steel might heat treat from. I very much doubt it could give accurate ranges even if asked. Of course in the one case the steel wouldn’t be able to be hardened at all so it wouldn’t be able to give a temperature that would work.

As I have stated several times, the approach ChatGPT takes is to add a lot of every element. It seems to think that elements only improve steel, so add some of all of them and you have a new super steel. It has no real concept of balancing different elements together or what the drawbacks are for adding different elements. It is basically what happens when “internet experts” try to design their own steel. So maybe this is a win for AI because it is now as smart as “internet experts”?

Could a Different AI Design Knife Steel?

ChatGPT is a “large language model,” meaning it was trained on huge amounts of written text from humans. It uses the statistical information about how humans put words together to generate coherent text. However, the model is not good at collecting data about different things, analyzing and comparing that data to find unique insights and solutions, and then to turn that into information. The other problem is that information about how to design tool steels is not really available on the internet, especially not in a form that ChatGPT could use. To really develop a good model we would need a database of information specific to knife steel design. So not a “general AI” but a “narrow AI” for our specific task. This would still be challenging, because the exact data we want does not really exist. The more data we can feed into an AI model the better the results can be. This is the classic example of “garbage in, garbage out.” A lot of information on tool steels and stainless steels are locked within specific steel companies, or even inside the heads of different metallurgists. The scientific literature, including publicly available journal articles and reported experiments, are not usually in a format that would easily be usable in a large database for an AI. Each study is done with different conditions: they hot rolled the steel with a different amount of reduction, they tested different sized coupons, they reported microstructure values but not mechanical properties, they tested Izod toughness instead of charpy toughness, etc.

For my own development of steel I use several different things, including:

1. Journal articles and books which collect different studies to find trends with different variables. Changes to composition, heat treatment, etc.
2. My own data which I have collected the last several years on Knife Steel Nerds – toughness, corrosion resistance, hardness, edge retention, etc.
3. Thermodynamics software – this software does not use machine learning or AI but uses databases to come up with expected microstructure at different temperatures. When it works perfectly you can know what carbides you have and which elements are “in solution” and how much.
4. Simple models for different properties – I have created simple equations which predict properties and there are some that are reported in journal articles and books. Some that I have created have been reported on this website such as this study on corrosion resistance or this study on edge wear.

With those pieces of information I can come up with approximate property targets and balance elements together. A similar process could be used for a “narrow AI.” Existing thermodynamics software could also be plugged into this AI, which is already happening. I think to do much better than humans would likely require a bigger database of experimental data than we currently have. For example, small tweaks could be made to existing steels such as the content of Mo, Si, Mn, N, etc. Currently, without experimentation by making these variations, we don’t know which changes could potentially lead to improvements. If our database was big enough, perhaps an AI could make predictions about which changes to try and thus reduce the number of experiments. But we are not there yet.

The post Can Artificial Intelligence Design a New Knife Steel? appeared first on Knife Steel Nerds.

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Video

I also have a video covering this same material:

Cold Forging and Rolling

I already have a separate article about the mechanisms behind cold forging and rolling, so I will not recap much of that here: Cold Forging of Steel.

While it is called “cold” forging it doesn’t usually refer to freezing or subzero temperatures. Most commonly, cold forging refers to room temperature forging. There are several major differences with high temperature forging; here are a few important ones:

1. The steel requires higher forces to forge.
2. The steel increases in hardness through forging and eventually will fracture from too much deformation.
3. The surface of the steel remains untarnished – no scale or decarburization.

Cold forging is not particularly rare or exotic. In fact some knife steels are available in the cold rolled condition. However, some knifemakers will cold forge their blades, at least partially. This is a somewhat less common practice. Cold forging works the same way in practice as cold rolling though of course cold rolling leads to a more even distribution of the cold reduction to the steel.

Some knifemakers have claimed performance benefits from the cold forging. In the previous article I wrote, I cited previous studies performed with high carbon steels showing some small potential benefits to heat treating steel from a cold forged/rolled condition. These improvements are generally explained by a reduction in grain size by performing the cold reduction before the quench and temper procedure. However, I had not done any experiments to see the improvements compared with other processing changes we have tried. Now we have done a study to see it for ourselves.

The New 52100 Study

We used Uddeholm 52100, close to the mid-point composition. There were two conditions tested:

1. As-received – The steel as it arrives from Uddeholm, annealed and ready to use.
2. Normalized at 1700°F (925°C) for 20 minutes and air cooled. Grain refinement treatment from 1460°F (795°C) for 30 minutes, air cool. Annealed at 1460°F for 30 minutes, cooled at 650°F/hr (350°C/hr). We call this anneal a “Fast DET” anneal where DET stands for “Divorced Eutectoid Transformation.” It is “fast” because the cooling rate is much faster than generally recommended in datasheets. You can read more about normalizing and annealing in this article.

Each of those two conditions were then cold rolled either 20% or 40% and compared to zero cold reduction. Cold rolling is usually best performed from an annealed condition so that it is as soft as possible to avoid fracturing during cold reduction. Perhaps you can get away with a normalized condition depending on the steel and how much cold reduction will be performed. Steels that have large carbides and relatively low toughness in the annealed condition are not good choices for cold forging.

We then performed a final quench and temper heat treatment with 1500°F (815°C) for 15 minutes, quenched in Parks 50 oil, and tempered twice at 400°F (200°C) for one hour each time.

Hardness After Cold Rolling

The annealed hardness is somewhat higher with the fast anneal. However the finer microstructure from this anneal leads to a better hardness-toughness balance after the final heat treatment as you will see. However, somewhat surprisingly the hardness after cold reduction was very close for the two different prior conditions. The increase in hardness was roughly similar to a study I cited in the cold forging article with A8 Mod tool steel [1]:

Data adapted from [1]

Hardness After Final Heat Treatment

After the quench and temper, you can see that the cold reduction led to higher hardness. The difference was relatively small, however, even with 40% cold reduction, 0.7 Rc for the as-received and 0.3 Rc for the normalized and annealed. Cold reduction leads to faster dissolution of carbides putting more carbon in solution and therefore higher hardness after heat treating. In this case the finer microstructure achieved through our modified annealing procedure led to a bigger difference in hardness than cold rolling.

Toughness

The cold reduction led to lower impact toughness for either the as-received or thermal cycled conditions. This is perhaps expected based on the increase in hardness though we had hoped that there would be a reduction in grain size to help improve toughness despite the small increase in hardness. The two different conditions, as-received or thermal cycled, had similar toughness despite the thermal cycled condition leading to higher heat treated hardness.

Hardness-Toughness Balance

The thermal cycled condition (Normalized, Fast DET anneal) had ~1 Rc higher hardness but similar toughness to the as-received condition. Thus the hardness-toughness balance was significantly better in the thermal cycled condition. However, the cold reduction did not improve hardness-toughness to the same extent as an improved initial carbide structure did.

The effects of cold reduction look more similar to a typical reduction in toughness (with higher hardness) from a lower tempering temperature or adding a cryo step as shown below. You can see where the following numbers came from in the earlier article about heat treating 52100.

Why No Improvement?

Because there are a few former studies that showed a small improvement in toughness from cold reduction before heat treating I wouldn’t totally discount it. However, positive results are more likely to be published and I did not find an improvement in this testing. Looking at those studies the measured improvements were quite small and perhaps were just within the scatter of the test.

Maybe in this case the increase in carbon in solution was detrimental enough to overwhelm any other improvement we might have obtained. In the previous Cold Forging article I wrote there was a case with D2 where they found a reduction in hardness from cold reduction because extra carbon in solution meant more retained austenite. So as with any change to processing there can sometimes be unintended consequences. Very rarely are there changes that only improve things without any potential detrimental effects. Perhaps if I slightly reduced the austenitizing temperature of the cold rolled material that would lead to similar hardness and thus either similar toughness or slightly improved toughness.

Summary and Conclusions

Cold reduction led to an increase in hardness of the annealed 52100 as well as the quench and tempered 52100. However, the increase in hardness led to a small reduction in toughness. There was no clear improvement in the hardness-toughness balance. The increase in hardness comes from higher carbon in solution, so perhaps a small reduction in the austenitizing temperature would help to compensate. The thermal cycled 52100, with or without cold reduction, had superior hardness-toughness balanced to the as-received steel. The fast annealing procedure utilized in its processing led to a finer carbide structure which improved its properties.

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[1] Ghasemi-Nanesa, Hadi, Mohammad Jahazi, Majid Heidari, and Tom Levasseur. “The influence of deformation-induced microvoids on mechanical failure of AISI A8-Mod martensitic tool steel.” In AIP Conference Proceedings, vol. 1896, no. 1, p. 020021. AIP Publishing, 2017.

The post Testing Cold Forged 52100 Knife Steel appeared first on Knife Steel Nerds.

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YouTube Video

I have a video version of the following information:

Why Protect the Surface?

There are two things we are trying to prevent by protecting the surface during heat treating: scale and decarburization. I frequently see knifemakers confusing these two things. Scale is not decarburization, and removing scale does not mean that the layer of decarburization is removed. Scale is the black iron oxide that forms on steel at high temperature in the presence of oxygen. During forging of steel it is common to see this scale come off in flakes.

A bar of 4340 steel with black scale on it

However, the oxygen doesn’t only interact with the iron, but also carbon. The carbon leaves the steel at the surface after forming carbon dioxide or carbon monoxide. This leads to a layer of steel with very low carbon, called a “decarburization layer.”

Problems from Decarburization

This decarb layer doesn’t harden because of the lack of carbon. However, unlike scale, a decarburization layer is still steel so it can be difficult to see without etching in acid first. So sometimes knifemakers are measuring low hardness or easily scratching with a file because they are unaware of this decarburization layer. That soft layer will also obviously affect performance, if an edge is sharpened within the “decarb” layer it will be soft and the knife won’t hold an edge. It can also affect aesthetics because the soft steel will polish differently. Another common issue can be warping and distortion from the decarb layer. This layer being free from carbon means that it will transform to other phases at a different temperature than the rest of the steel. The uneven transformation leads to different size changes within the steel leading to warping.

Protective Atmosphere and Salt Pots

If no oxygen is present, scale will not form and decarburization will not occur. So if the steel is heat treated in a vacuum furnace or within a protective atmosphere like argon then scale and decarb are not issues. With salt pots the steel is within molten salt without an atmosphere, so again there is no scale or decarb. However, these solutions are not always practical so other ways of protecting steel are often used by individual knifemakers with simple heat treating equipment.

Foil

A common method for preventing scale and decarb is to use a heat treating foil. The steel is wrapped in the foil and then each exposed side is folded over at least twice to prevent air from entering the packet. The foil is thin and on the outside so it heats up first and oxidizes which further eliminates oxygen left within the packet.

There are two major types of heat treating foil: 321 and 309 stainless steels. The 321 foil is rated up to 2000°F and the 309 is rated up to 2240°F. The 309 is more expensive than 321. The 321 stainless also has a small titanium addition which is also claimed to help with reacting with oxygen inside the packet [1]. Foil is very sharp so gloves should be worn when handling it. Some people recommend adding paper or oil to burn up inside the packet but this is not necessary. Some people recommend talcum powder to prevent sticking though I haven’t tried it. Reusing foil is not recommended because the foil has already oxidized and also becomes brittle.

The main limitation of foil is for oil or water hardening steels because removing the foil prior to quenching is very difficult within a reasonable time. With air hardening steels the foil works well with a “plate quench” because when the plates are held tight the heat will steel conduct through the foil.

Coatings

Various coatings are also available that are designed to protect steel from scale and decarb during heat treating. These are especially useful for oil and water hardening steels since no foil needs to be removed. I used several different coatings for my recent experiments:

ATP-641 [2] – a water based ceramic coating that is rated up to 2300°F. It can be sprayed, dipped, or brushed.

NoScale2000 [3] – another water based coating that is rated up to 2000°F. The coating is made by Daniel O’Connor for knives. It can be sprayed or brushed. O’Connor also makes a version made for developing a hamon called Hamon1800.

Turco Pretreat [4] – This product is also named Bonderite L-FM Pretreat Aero. It is a paint, so the texture is quite different than the two above. The primary recommendation is to spray this coating though I used it by dipping. The datasheet does not list a maximum temperature.

Condursal Z1100 [5] – Another paint coating like Turco. Rated up to 1100°C (2012°F). Can by applied by brushing, dipping, or spraying.

Experiments

I did three experiments with the different coatings. Each used a 1/8 x 3/4 x 1-1/2 inch (19 x 38 mm) coupon. They were finished to 120 grit and cleaned with soap and water prior to dipping in one of the coatings, followed by letting them dry for 24 hours. The 1095 coupons were only finished to 60 grit and the Condursal Z1100 coupons were only finished to 60 grit. I also tried brushing the NoScale2000 which I will explain in that section. A “bare” sample was also tested with each without a coating.

1. 1095 steel heated to 1475°F for 10 minutes, quenched in Parks 50 oil. This is to test the effect of each coating on the quench since 1095 is a water hardening steel and needs a fast quench.

2. 52100 steel heated to 1550°F for thirty minutes, quenched in Parks 50 oil. This is a relatively standard datasheet heat treatment for 52100. It is on the high end for temperature and time for oil/water hardening steels so it serves as a good test of how much decarb happens for these types of steels.

3. AEB-L steel heated to 1975°F for 30 minutes, then quenched in Parks 50, then placed in liquid nitrogen for an hour. For the AEB-L I also added a sample that was  wrapped in foil prior to a plate quench and then liquid nitrogen. This was a test to see how the coatings did near their temperature limit. Also to see how much decarb happens at this relatively high temperature.

For each coupon I took pictures after cleaning them to see the amount of scale from each. I did microscopy of the cross-section for ATP-641, NoScale2000, and Turco but I didn’t get the Condursal until later so no microscopy for that coating. I also measured hardness for all of them.

Uncoated Coupons

The 1095 coupon formed only a small amount of scale because it was heated to a relatively low temperature of 1475°F (800°C) for a short time of 10 minutes.

1095 coupon heat treated without a coating

The 52100 coupon heat treated at 1550°F for 30 minutes had significantly more scale and it was very dark in color. Some came off when cleaning the oil off the coupon.

52100 coupon heat treated without a coating

The AEB-L coupon heat treated at 1975°F for 30 minutes was not as dark as the 52100 coupon and the scale was a bit less even.

AEB-L coupon heat treated without a coating

Looking at the cross section, the 52100 has a thin decarb layer and thin layer of scale. The AEB-L, however, had a very thick decarb layer measuring over 0.2 mm (~0.01 inches).

52100 uncoated coupon cross section

AEB-L uncoated coupon cross section

1095 Quenching Experiment

I decided to do the quenching experiment based on an earlier experience. A knifemaker contacted me about difficulty with heat treating 80CrV2; it wasn’t hardening. We talked through the process and he told me he was coating each blade with ATP-641. I recommended he try it without the coating and he didn’t have any problems after that.

All of the 1095 specimens fully hardened to 66-67 Rc and this was true at the surface as well as after grinding 1 mm into the specimen. The one exception was the NoScale2000, which was about 36 Rc. I repeated the experiment to make sure it wasn’t a fluke and got 33 Rc. In both cases the coating did not come off during quenching and I had to clean it off to be able to test hardness. I saw that O’Connell recommended brushing on the NoScale2000 rather than dipping so I tried it again by brushing it on. In this case the steel fully hardened.

1095 NoScale2000 coupon after quenching in Parks 50 oil. The coating remained on the steel.

I did not find an issue with hardening the 1095 with ATP-641 despite the prior issue the knifemaker experienced with 80CrV2. However, I do think that there is significant “danger” to quenching slowly with coatings applied. Ensuring the coating is not too thick helps with quenching as expected.

NoScale2000

Another puzzling thing was that the NoScale2000 appeared to lead to even more scale than when heat treating it without any coating. The repeat experiment of 1095 also led to a lot of scale. When I brushed on the NoScale2000 there was still scale that formed it was just more “spotty.”

1095 “bare” coupon (heat treated without a coating)

1095 NoScale2000 dipped coupon #1

1095 NoScale2000 dipped coupon #2

1095 NoScale2000 brushed coupon

The 52100 coupon coated with NoScale200 also had a somewhat spotty appearance similar to the 1095 brushed coupon. The AEB-L NoScale2000 coupon looked at least as bad as the uncoated coupon, perhaps worse.

52100 NoScale2000 coupon

AEB-L NoScale2000 coupon

Looking at the cross-sections, the 52100 coupon looks very similar to the uncoated coupon. The AEB-L coupon has a decarb layer of approximately the same thickness as the uncoated coupon.

52100 NoScale2000 cross-section

AEB-L NoScale2000 cross-section

I let Daniel O’Connor know about my subpar results and he said that, “[Y]our results are not typical of my user base.” He told me about good experiences many knifemakers have had using it. If you have had better experiences with the NoScale2000 let us know in the comments.

ATP-641 Coupons

Surprisingly the 1095 coupon coated with ATP-641 had some spotty scale that formed, not looking too different than the uncoated coupon. The 52100 coupon, however, didn’t have any obvious scale just some discoloration. But the AEB-L coupon formed scale and didn’t look very different than the corresponding uncoated coupon.

1095 ATP-641 coupon

52100 ATP-641 coupon

AEB-L ATP-641 coupon

Looking at the cross-sections with metallography there is no obvious decarburization in either the 52100 or the AEB-L. This is somewhat surprising given the rough scale that formed on the AEB-L. Maybe the ATP-641 affected the surface in some way on its own while still preventing decarb.

52100 ATP-641 cross-section

AEB-L ATP-641 cross-section

Turco Coupons

The 1095 Turco coupon had some scale that formed, lining up with the grinding marks. Perhaps this was because of the coarser 60 grit finish on the 1095 coupon vs the 120 grit on the 52100 and AEB-L coupons. The 52100 coupon had some discoloration and it is hard to tell if it was scale or not but otherwise looked good. The AEB-L perhaps had some light scale that formed but otherwise looked very good.

1095 Turco coupon

52100 Turco coupon

AEB-L Turco coupon

There was no apparent decarburization visible in the metallography analysis of the 52100 coupon. The AEB-L coupon also looked good apart from one corner where presumably the Turco had rubbed off prior to heat treatment.

52100 Turco cross-section

AEB-L Turco cross-section

Condursal Z1100 Coupons

The Condursal seemed to be very similar to the Turco apart from color (green vs orange paint). The resulting coupons also looked pretty similar to the Turco coupons. For some reason there was more scale on the 52100 coupon. And the rougher 60 grit finish I did with these followup coupons also seemed to affect the amount of scale that formed somewhat. I did not do metallography with these coupons.

1095 Condursal Z1100 coupon

52100 Condursal Z1100 coupon

AEB-L Condursal Z1100 coupon

Foil

I only used foil with AEB-L. As expected the steel came out completely clean apart from some rainbow coloration from the small amount of oxygen left in the packet.

AEB-L foil coupon

Surprisingly there is what looks like a thin decarb layer visible in the cross-section. Perhaps the little bit of oxygen that interacts with the steel is enough for a small amount of decarburization.

AEB-L foil coupon cross-section

User Error?

This is my first time using many of these coatings so there can certainly be criticism of my technique. Dipping into the coating did not seem like the very best way to apply most of them. The one I thought had the best texture for dipping was the NoScale2000 which was also the one that didn’t come off when quenching. The Turco and Condursal recommend very thin coatings in the datasheets but I was concerned some of it might have been running off while drying. The coating thickness did not appear to be even. Perhaps spraying would have been better for some/all of these coatings. I attempted brushing the coatings on but didn’t like it; even letting them dry in between coats seemed to lead to me brushing off as much as I was brushing on. The surface finish also appears to matter with these coatings, a finer finish is better. And they need to be clean of course.

Scale-X

One coating I found about too late is Thermodur Scale-X. It is a paint-based coating that comes in a spray can so you don’t need a dedicated sprayer. Based on my experience with the coatings in this article I think spraying might be better so this sounds very convenient. And the two paint-based coatings I tried worked well. So I will be getting some to test it.

Summary and Conclusions

Coatings can be useful in certain instances for oil and water hardening steels. I had somewhat inconsistent results with them. I also found that the amount of decarb is relatively small at temperatures typical of water and oil hardening steels (less than 1600°F/870°C) so it depends on how much of a hassle you think applying the coatings is. If you plan to remove material through grinding after the coatings may not be necessary for low alloy steels. Thoroughly cleaning the steel and having a fine finish is helpful for the coatings to work at their potential. Learning to effectively apply them likely requires some trial and error. I wasn’t totally happy with dipping or brushing them on, may spraying is better. For steels that require high temperature, in this case AEB-L, I was able to prevent decarb with the coatings except for NoScale2000. Decarburization is quite significant without any type of protection and should definitely be avoided. The Turco and Condursal coatings led to a more scale-free surface than the ATP-641 and NoScale2000. I greatly prefer foil for heat treating air hardening steels.

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[1] https://www.toolwrap.com/pages/all-about-tool-wrap?srsltid=AfmBOoqG9o8GgfgNq8u2WT2Eh9IeyYGLYITqxhqOkq-qoc1ENjuQ2joR

[2] https://atp-europe.com/wp-content/uploads/2023/08/ATP-641.pdf

[3] https://nuclayer.twinoaksforge.com/product/noscale-2000-16oz/

[4] https://webaps.ellsworth.com/edl/Actions/GetLibraryFile.aspx?document=12319&language=en

[5] https://www.theduffycompany.com/condursal-z1100/

 

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Intro

I have many articles about all of the nitty gritty details of heat treating and the metallurgy behind every step. However, there may be some cases where knifemakers are afraid of all of the terminology and science and think heat treating is too complicated for them. When it comes down to it, the steps of heat treating are not particularly difficult. When you follow a recipe for how to make cookies you don’t need to know the science behind every step, but following them will still get you cookies at the end. An expert would know what went wrong if your cookies were too crunchy, too puffy, spread out too much, etc. And how to modify the recipe to change the flavor and texture of the cookies. However for most of us we will just follow the recipe. You can do the same thing with heat treating knife steel! So for this article I will tell you how to follow a datasheet. I will include some links to articles with more information about what happens in each step, but you can get to those when you are ready. Another great place to learn more about heat treating is my book Knife Engineering: Steel, Heat Treating, and Geometry.

Video

There is a video version of the following content, which demonstrates some of the things described in the article. The article has some more detail that the video doesn’t have.

What Are We Trying to Accomplish When Heat Treating?

When you purchase knife steel you are getting very soft steel in a condition that is easy to grind, drill, machine, or cut. This is called the “annealed” condition. However, soft steel is not good for a knife as the knife edge would easily deform. It would not “hold” a good edge. So we have to perform a series of heat treating steps to get it to high hardness for good knife performance. We are targeting a range of hardness usually between about 58 and 63 Rockwell C (Rockwell C is the unit of measurement). Sometimes the hardness unit is abbreviated as “Rc” or “HRC.” The higher end of hardness gives better edge retention but has lower “toughness.” Toughness is a measure of resistance to breaking.

Furnaces for Heat Treating

The basic piece of equipment for heat treating is a furnace. A forge can also be used but it isn’t my favorite method. I have recommendations on how to properly heat treat with a forge here. There are several manufacturers of furnaces and some knifemakers will make their own. I use EvenHeat furnaces, I have a KO 22.5, an LT 22.5, and a Salt Bath 709. The KO model is the high temperature model that can reach 2350°F. I previously had the LB model which has a somewhat larger chamber (width and height) with the tradeoff that it reaches 2200°F. The LT model is designed for tempering up to 1200°F. A regular furnace can also be used for tempering but they aren’t “tuned” and calibrated for low temperature, and you have to wait for the furnace to cool down before tempering. You can also use a conventional oven for tempering though the temperature will be more variable.

One of the big decisions for a heat treating furnace is 120 or 240V. The higher voltage EvenHeat models are roughly twice as fast and also have larger chambers. This makes heat up time twice as long with the low voltage and it takes longer to “rebound” after opening the door and inserting steel. It is better to use 240V if possible, and if you don’t have the capability for doing that you should contact an electrician. Another important decision is the depth, or length, of the furnace. It is usually a good idea to get one a bit longer than you think you would need. If you ever wanted to make a longer blade such as a bowie or kitchen knife you will need a relatively long furnace. Also the temperature is usually somewhat different at the ends of the furnace, especially right next to the door. So having some space between the blade and the front/back is a good idea.

Steps of Heat Treating

The main properties you achieve after heat treating will be obtained after three main steps: austenitizing, quenching, and tempering. For stock removal makers these will be the main steps you perform, while for a forging bladesmith you will perform a couple other steps after forging to set up the steel for these three steps. You heat the steel up hot (austenitize), cool it rapidly (quench), and then reheat to a low temperature (temper). Not very complicated.

Austenitizing

Austenitizing is the high temperature step soaking step, generally 1450-2250°F depending on the steel and desired hardness. Typically a higher austenitizing temperature means higher hardness, as shown on the following table from the MagnaCut datasheet:

You can read about what happens during austenitizing in this article: https://knifesteelnerds.com/2018/02/28/austenitizing-part-1-what-it-is/

Atmospheric Protection – Foil or Coatings

One thing that a datasheet often assumes is that you know about protecting your steel from the atmosphere and oxygen. Oxygen leads to scale formation and also decarburization, which leads to a layer of soft steel under the scale. With low alloy steels austenitized under 1600°F or so the scale and decarb may not be bad enough to protect the steel as long as you plan on removing some material after heat treatment. There are also coatings that can protect the steel. I recently tested a few of them though I haven’t published those results yet.

For high alloy and stainless steels the most typical method is to use “heat treating foil.” The two common types are 309 and 321 foil, which are stainless steels. 309 is rated up to 2240°F while 321 is rated up to 2000°F, though the 309 is more expensive. You create a foil envelope to place the knife in and fold each of the sides. Some people include talcum powder to prevent sticking though I haven’t typically had issues with sticking apart from high temperatures (>2000°F) and long soak times. Some people recommend putting paper or something else to burn up in the foil packet but this isn’t necessary. Foil is most often used with high alloy and stainless steels which don’t require an oil quench, because removing the foil before quenching in oil is difficult.

Cross-section of AEB-L steel heat treated in a furnace at 1925°F without any protection. The “bright” layer is the “decarb layer” which no longer contains carbon. It is over 0.2 mm thick.

Preheating

Many datasheets recommend “preheating” steps where the steel is heated up to an intermediate temperature before increasing the temperature up to the final target. I described preheating in this article. This process can be done with multiple furnaces or by soaking at the preheating tempreature prior to a ramp to the final temperature. This process is to help the steel achieve a uniform temperature so that it doesn’t transform unevenly. However, knives are thin enough where this isn’t typically an issue. I recommend holding the furnace for 30 minutes at the target final austenitizing temperature instead. You can read why I think so in this article about small knife furnaces and temperature distributions inside them.

Soak Time

After the steel has reached the austenitizing temperature it needs to be held at that temperature for some period of time, called a “soak.” The soak time recommended in a datasheet is supposed to start after the steel has reached the temperature. Sometimes I cheat and start the timer after the furnace has “rebounded” to the temperature (it cools down from opening the door and inserting a cold piece of steel). Once the steel has reached the same color as the furnace it has reached the temperature. You are only seeing the surface but steel is highly conductive and knives are thin so once the surface has reached the temperature it is only seconds before the center has as well.

You will notice that the MagnaCut chart above shows different soak times for each austenitizing temperature, as higher temperatures will often mean a shorter required soak time. The thickness of the steel also matters, the datasheet for MagnaCut says “Min soak time” and if the steel is thicker than 1/8″ (3.3 mm) it may need a few more minutes.

Some knifemakers are afraid of soaking because they think that holding it at temperature will grow the grain size. It is true that holding steel longer will grow the grains but temperature matters much more than time. Holding a steel for 30 minutes is not going to hurt it if the temperature is correct. Undersoaking is a more common issue than oversoaking.

Quenching

The quenching step is what actually hardens the steel. The austenitize dissolves carbides so that carbon is “in solution” prior to quenching but the rapid quench transforms the steel to the hard phase of steel. If the quench is too slow the steel will not reach its full hardness and will have poor properties.

Steels are broadly categorized into three quenching groups: water hardening, oil hardening, and air hardening. Low alloy steels and simple carbon steels typically require a water or oil quench. These are steels like 1095, O1, 80CrV2, and 52100. High alloy and stainless steels are “air hardening” and do not require a water or oil quench to harden after austenitizing. These are steels like A2, D2, 440C, CPM-154, S30V, MagnaCut, and Vanadis 4 Extra.

With low alloy and carbon steels the speed of the quench necessary is controlled by the thickness of the steel (the “cross section”) and the steel. The speed of quench can be broken down into 1) water and brine, 2) fast oil, 3) medium oil, and 4) slow oil. If you get only one oil I would recommend a fast oil like Parks 50 because high hardenability oil hardening steels like O1 can still be quenched in Parks 50, but a low hardenability steel like 1095 would not work with a slow oil. You can buy Parks 50, sometimes generically called Quench 50, from a couple different places including Dubois. I have an article comparing different oils and cross-sections here. I learned that a steel like 1084 can be quenched in a range of different oils at only 1/8″, but at thicker sizes like 1/4″ the type of oil definitely matters. That article also ranks common low alloy steels by “hardenability” (how fast of a quench they need) to help you pick the oil for different steels and cross-sections. With oil and water quenching you “cut” into the liquid with the knife and move up and down rapidly to break up the “vapor jacket” that slows down cooling from the oil/water boiling on the steel surface. Moving side to side can lead to warping.

It is popular for knifemakers now to quench air hardening steels through a “plate quench,” often with aluminum plates. The steel is placed in between aluminum plates so that heat is drawn out. This is faster than sitting in air and helps keep the knife flat (if the bevels are not yet ground). This can also be performed without removing heat treating foil.

Tempering

After quenching, the steel is very high in hardness, but is also brittle. During tempering the steel is reheated to a lower temperature to increase toughness and reduce brittleness. The hardness of the steel is also lowered somewhat (see the MagnaCut chart from earlier or the chart below).

Tempering Chart for 154CM

Typically steel is tempered at least twice, and each temper is usually 1-2 hours long. Tempering at a higher temperature usually decreases hardness though there is an exception called “secondary hardening” in high alloy steels when tempered between about 750 and 1100°F (400-600°C). You can read about the mechanisms behind tempering in this article. Tempering can be done in a conventional oven or in a heat treating furnace after it cools down. I have some tips on using a heat treating furnace for tempering in this article. EvenHeat also makes a furnace designed for tempering.

More complete tempering chart for 154CM that shows “secondary hardening” above 750°F (400°C)

Cooling After Tempering

There are some people that recommend a rapid quench in between tempering steps. This can be done if you are in a hurry. Air cooling to room temperature in between is sufficient. If you are curious as to why you have to cool in between rather than holding longer you can read the article I linked to under the section labeled “retained austenite”.

Cold Treatments

There is an optional “cold treatment” step during heat treating that can increase hardness. Some datasheets recommend this in between tempering steps but I prefer it directly after quenching. In effect it is an extension of the quench down to a lower temperature. A delay between the quench and the cold treatment can lead to less effect of the cold treatment. The MagnaCut chart from before has hardness values from quenching to room temperature, performing a cold treatment in a household freezer, or a cryogenic treatment in liquid nitrogen. No real hold is necessary at the low temperature, the steel just needs to reach the temperature. Typically 30-60 minutes is enough. I have an article on cryo treatments of AEB-L where I discuss a lot more about how cryo works.

“As quenched” hardness of AEB-L with no cold treatment, a freezer, or liquid nitrogen from different austenitizing temperatures.

You may have noticed that hardness can drop if the austenitizing temperature was too high; Using 1900°F (1035°C) austenitizing temperature with AEB-L only resulted in around 0.5 Rc increase in hardness by using liquid nitrogen. From 1975°F (1080°C) the hardness went up from 60.5 Rc to 64 Rc.

You can use most any dewar for holding liquid nitrogen. An off-brand dewar from Amazon or Ebay works just fine. The important specs are the size of the container, usually in liters (mine is 10L), and the diameter of the neck. The most common diameter is 50 mm (~2 inches) but this can be restrictive for wider blades. However, the larger the neck the faster the dewar loses nitrogen, so typically you need to buy a relatively large dewar to get a larger diameter neck.

Extra Steps After Forging – Normalizing and Annealing

When steel is forged it is no longer in the annealed condition. It may not be soft enough for cutting, drilling, etc. And it is not in the best condition for performing the austenitize, quench, and temper. The basic process that needs to be followed is a normalize and anneal. Normalization involves heating to a relatively high temperature (usually somewhat higher than an austenitizing temperature like 1650°F), and then air cooled. This gives the steel a uniform microstructure and grain size. Annealing is done from a temperature typically a bit lower than austenitizing (like 1400°F) and then slow cooled. Annealing makes the steel soft and sets it up for the final heat treating steps. You will also notice below an optional step labeled “grain refinement” that I generally recommend skipping as I have not found an improvement with my testing. This step is relatively common with knifemakers but not in industry and is not recommended in datasheets.

I have an article that describes this process and provides recommended temperatures for normalizing and annealing different steel. For those few brave knifemakers I also have an article on how to anneal stainless steel and high alloy steel after forging it.

Heat Treating in a Forge Instead of a Furnace

The article up until this point discusses heat treating using a controlled temperature furnace. This is my preferred way of heat treating and provides consistent results every time. Some knifemakers like to heat treat with a forge with less control over the exact temperature. I developed a method for minimizing error in forge heat treating as it is very common to overheat the steel and get poor properties. You can read my recommendations in this article.

Every once in a while a knifemaker contacts me and says I am wrong about forge heat treating and that he can get it perfect every time. Below is an example of tests I performed on some of that steel. The toughness was terrible. If you follow my instructions in the linked article you will increase your chance of success.

What Temperatures Do You Choose From the Datasheet?

Datasheets will often given ranges for possible austenitizing and tempering temperatures. The first thing you should look for is a “recommended” heat treatment. You can modify from there of course but this gives a good starting point. Here are a couple examples:

From the MagnaCut Datasheet

From the Vanadis 4 Extra Datasheet

If the datasheet only gives a range for austenitizing I would start with something in the middle of the range. MagnaCut datasheet says 1950-2200°F and halfway in between that would be 2075°F (1135°C), close to the 2050 recommendation. Vanadis 4 Extra says 940-1180°C which would be 1060°C (1940°F), the same as their recommendation for “large sections.”

For tempering a good starting point is usually 400°F (200°C) or 1000°F (540°C). 400°F is good for most steels, though some high alloy and high speed steels will recommend the 1000°F temper. Often these steels can also be tempered at 400°F but the austenitizing temperature would also need to be adjusted based on that change and that would take some work to figure out.

Another thing you can do is check the back of my book Knife Engineering. I give recommended temperatures for most steels in there.

For many datasheets there are many temperature combinations that could work and it is mostly about selecting a target hardness. 58-63 Rc is a good range to target for many knives; use the higher end for thin slicing knives (and careful customers) and lower for knives that need higher toughness. 60 Rc is a good round number if you want a starting point and aren’t sure.

Elements of a Datasheet

Every datasheet is a bit different but I will break down the various parts of datasheets with the MagnaCut datasheet:

Page one of this datasheet (and often multiple pages of other datasheets) is mostly information about the steel. That top left paragraph describes the steel and its general properties. Next it shows micrographs where you can see that MagnaCut has a much finer microstructure than CPM-154. Below that is a table with information about the carbide types in MagnaCut vs other steels. Then a “Tool Steel Comparagraph” shows bar charts of properties of MagnaCut vs various comparison steels. “Typical Applications” serves to tell potential buyers in different product categories when they might want to look at MagnaCut. At the top right you find the composition. Below that is some physical properties which most knifemakers don’t need. Then some test results showing how MagnaCut stacks up in toughness, edge retention, and corrosion resistance.

On the second page we find the actual temperature recommendations that we have been discussing. First is the forging and annealing temperatures, which of course you don’t need unless you actually forge the steel.

Stress Relieving

Below that is a category of treatments called “Stress Relieving” which we have not discussed thus far. These treatments are optional but can be useful in certain scenarios. During grinding and machining various stresses are built up in steel that can lead to increased movement and warping during heat treating. Performing a stress relief can help with this, where it says, “Annealed Parts: Heat to 1100-1300°F…” Stresses can still be built up during grinding of heat treated steel which required a temperature just under tempering. This is because we don’t want to overtemper the steel, of course.

Size Change

This datasheet also lists the estimate size change after heat treating the steel. The heat treated structure is slightly larger than the annealed structure of ferrite. So typically we expect a small increase in the size of the part after the heat treatment is complete. Using a higher austenitizing temperature will reduce the size increase somewhat, and could even lead to shrinkage. Using cryo would lead to a larger size. This is from retained austenite (higher austenitizing temperature) or from converting more retained austenite to martensite (cryo). You can read about what all of that means in my cryo articles.

When Different Datasheets Contradict Each Other

I have a whole article on the heat treatment of 80CrV2 where I found virtually every datasheet to recommend something different. For example, Swiss Steel Group has what I would call a fairly standard recommendation where it says to austenitize between 800 and 830°C (1475-1525°F). Then Bestar has a recommendation for 840-880°C (1545-1615°F). And the New Jersey Steel Baron datasheet recommends 1465-1480°F. In terms of “thermal cycling,” Bestar offers no suggestions for normalizing and annealing, Swiss Steel Group recommends annealing at 680°C (1250°F) with a furnace cool, and NJSB recommends 1650°F for 10 minutes air cool, 1500°F for 10 minutes and air cool, and 1350°F for 10 minutes and air cool.

Sometimes these datasheets are generated by steel suppliers and not by metallurgists working for the steel company. This appears to have been the case for the New Jersey Steel Baron datasheet. The 1650-1500-1350 normalizing steps are not standard and cannot be found in any other datasheet. The 1350°F in particular could be skipped entirely as it wouldn’t really do anything. And the 1465-1480°F austenitizing range is on the low end for a steel like this. Many of the NJSB datasheets seem to have these same recommendations regardless of the steel – 1075, 1080, 1084, 1095, 15N20, 5160, 52100, 80CrV2, L6, O1,  W1, and W2 datasheets are all virtually identical. I would avoid these datasheets.

Bestar is also an odd case as for a couple of their steels they recommend oddly high austenitizing temperatures. Generally it looks like their datasheets are usually reliable apart from these few cases. In this case the heat treatment would “work” but would be in danger of reduced toughness from austenitizing too high.

Usually datasheets are not as different as in the case of 80CrV2. Standard steels like this one vary somewhat more when compared to a steel made by a single company. They are the only ones that make it so they are the only ones to make a datasheet. I wouldn’t pay too much attention to heat treating guides from knifemakers. Sometimes they are based on good information from datasheets or my experiments, but sometimes they aren’t.

As I said previously, another thing you can do is check the back of my book Knife Engineering. I give recommended temperatures for most steels in there. Usually I found a trustworthy datasheet to have charts on tempering. And in many cases I have hardness-toughness data from my own tests to confirm what a good heat treatment range is. I can’t always promise to have the 100% perfect heat treatment, but I know it is a good one. And in some cases like with 80CrV2 I have heat treatment guides for specific steels on my website.

Where to Find Datasheets

Datasheets can be found in a variety of places. The first and most obvious is from the manufacturer itself. Here are a few websites:

Crucible – https://www.crucible.com/products.aspx

CPM steels S30V, S90V, 10V, etc. Standard steels like O1, A2, D2, M2, M4, 440C, 154CM

Some of the standard tool steels are hidden in the old Tool Steel Selector – https://www.crucible.com/eSelector.htm

Niagara Specialty Metals also has a collection of Crucible datasheets.

Carpenter – https://www.carpentertechnology.com/blog/blade-alloys-101

CTS-XHP, 204P, BD1, standard steels similar to the Crucible lineup

This website is kind of big and difficult to navigate so I linked to an old “blade alloys” page with links to many of them. Carpenter used to have more old datasheets for various standard steels that seem to be gone now, unfortunately. Some of those were replaced with Latrobe Steel datasheets.

Bohler – https://www.bohler-edelstahl.com/en/

M390, N690, S390, etc. They also have many standard grades but each is given a meaningless Bohler designation like K110 is D2.

Another big and ungainly website. You will want to look at “Cold Work Tool Steels” and “High Speed Steels.” Stainless steels are on two different pages under “Corrosion resistant and non magnetic steel” and also “Plastic Mould Steels.”

Uddeholm – https://www.uddeholm.com/us/en-us/

Elmax, Vanax, Vanadis 4 Extra. They also have many standard grades but each is given a meaningless Uddeholm name like Sverker 21 is D2.

Uddeholm strip steels like AEB-L, 26C3, and 15N20 are on a different website with limited heat treating information – https://www.uddeholmstrip.com/

Alleima (formerly Sandvik) – https://www.alleima.com/en/products/strip-steel/strip-products/knife-steel/hardening-guide/hardening-programs/

13C26, 12C27, 14C28N

Alleima has recommendations for a “belt furnace” or a “batch furnace.” A belt furnace is literally a conveyer belt furnace which most knifemakers do not have. This is why those heat treatments have relatively short soak times that must be changed based on stock thickness. I prefer to use the batch furnace recommendations, since that is the type most knifemakers are using and the longer recommended soak time is less sensitive to changes in stock thickness.

More Obscure Sources for Heat Treating Information

The ASM Heat Treater’s Guide is an awesome expensive book that has heat treating information on all of the old standard grades. However, there is a free app (used to be on iOS but now only on Android) which has the recommended temperatures from the book, though it is missing all of the charts and micrographs: https://heat-treater-s-guide-companion.en.softonic.com/android An older edition of the book is also available to borrow on archive.org: https://archive.org/details/heattreatersguid0000unse/mode/2up

Tool Steels by Gill and Roberts – An old awesome book, the best edition is the 1980 4th edition but only the 3rd edition is online – https://babel.hathitrust.org/cgi/pt?id=wu.89089662902&seq=6 Unfortunately in 1944 the standard names for steels like D2, A2, M2, etc. did not exist yet so you have to know how to read steel compositions.

Tool Steel Simplified by Palmer and Luerssen – another old book by Carpenter metallurgists which has some good information on a few old standard grades. It can be borrowed for free online – https://archive.org/details/toolsteelsimplif0000fran

There are a lot of other old books but this is enough for now.

Summary

The three major steps of heat treating are austenitizing, quenching, and tempering. These steps will control the properties of the steel.

After forging there are steps needed to soften the steel and set it up for the three main final steps. The main two steps are normalizing and annealing. Some knifemakers put too much emphasis on the “thermal cycling” steps to try to reduce grain size. The final three steps, especially austenitizing, will largely control the properties. Many steel datasheets do not include normalizing or even annealing temperature recommendations; they are available on my website in the “thermal cycling” article or in my book Knife Engineering.

Follow manufacturer datasheets to get good temperatures. There are also heat treatment guides for several steels on my website. Also the back of the book Knife Engineering has recommended austenitizing and tempering temperatures.

 

The post Introduction to Knife Steel Heat Treating from a Metallurgist appeared first on Knife Steel Nerds.

Thanks to my Patreon supporters I have a salt pot furnace and was able to get some high temperature salts to test out austenitizing in the salt pot for this article. Patreon dollars also went towards metallography for this study. If you want to support further knife steel research become a Patreon supporter today!

YouTube Video

There is also a video for the following information:

Salt Pots and What I Used

“Salt bath” or “salt pot” heat treating refers to the use of molten salts for heating steel. Specific salts are used for different temperature ranges, so that they melt under the necessary temperature without certain bad things happening by overheating them. For these experiments I used “Neutral Salt B” by Hubbard-Hall which I purchased from High Temp Tools: https://hightemptools.com/products/high-temperature-salt  They list the working range as 1300-1700°F but also says 1700 can only be used for short periods of time. I have also used low temperature salts for austempering which is a totally different use of salt pots; you can read those experiments here. Of course I used a different low temperature salt for those austempering experiments, which you can also buy from High Temp Tools. If you are using molten salts to heat treat stainless or high speed steels you will also need different salts as that would need higher temperatures. These salts are certainly available; salt pot heat treating of high speed steel is actually pretty common, but I don’t know where to buy small amounts of them. If you know a good source let us all know in the comments.

The furnace I used was the EvenHeat Salt Bath 709 which accomodates a 9″ long pot (plus extra that sticks out), though they are also available in larger sizes. There is a control thermocouple within the furnace as well as within the pot itself. So when operating the furnace you heat to the target temperature using the furnace thermocouple first and wait for the salt to melt. Then you insert the second thermocouple into the molten salt and tell the controller to switch modes to the second thermocouple.

Why Use Salt Pots?

Salt pots have a few advantages. One is that you don’t need to protect the steel from oxygen. The steel is present within the molten salt so there is no atmosphere at all. No need for coatings or foil. Just stick the steel into the molten salt and pull it out to quench. Another is the rapid heat transfer. Liquid is much better at transferring heat than air. This is why sticking your hand into boiling water at 212°F (100°C) is much, much scarier than sticking your hand in an oven at the same temperature, or even 500°F (260°C) for that matter.

Salt Pots and Safety

Probably the number one thing you hear about operating salt pots is how dangerous they are. This is true; if you drop significant amounts of water into the salt bath it could explode and you would be in significant danger. You have to take it seriously, wear protective equipment such as a face shield and long sleeves, and avoid letting any water contact the salt. However, I have also heard some knifemakers take this to extremes, saying things like not to even leave any sharpie markings on the steel and this is exaggerating the level of danger. Don’t take it lightly, but if you follow proper procedures and take safety precautions you can use these furnaces without issues.

Time to Heat Steel in Molten Salt vs Air

As I mentioned, one of the big benefits of salt pots is the shorter time required for heating the steel. To test this I used 1/8 x 1-1/2 x 2-1/4 inch (3.3 x 40 x 55 mm) coupons of 1084 and used 1 minute increments (separate coupons for the different hold times). This is total time in the furnace, not soak time at temperature. Of course, larger coupons would take longer to heat up. I then did the same experiments by using my normal furnace where the coupon was heated in air:

Unfortunately I was a bit late on the 2 minute coupon and it was actually 2 minutes and 20 seconds, so we can’t see exactly how long it took to heat up the steel with the salt pot. But certainly by 3 minutes the steel had reached the target temperature. There was then a small increase in hardness up to 4-5 minutes in the salt as more carbide dissolved. With a 3 minute hold in the conventional furnace the hardness didn’t change at all, meaning it still hadn’t formed any austenite (somewhere under 1335°F/723°C or so). It wasn’t until 5-6 minutes that the steel reached the target temperature. So you can save a few minutes of heat treating time when using salt instead of air. How much this matters will depend on the individual, of course.

The required time to dissolve carbides also depends on the temperature. Below shows published micrographs of 1080 steel [1] with different hold times at only 730°C (1346°F) where it took 5 minutes just to transform the steel to austenite (g), and then another ten minutes to dissolve most of the carbide (h). You can also see how the transformation happened, as the white ferrite was replaced by dark martensite (austenite at high temperature before quenching) with increasing hold time. Then more carbide (white particles) were dissolved with further hold time. You can read more about how this happens in this article on austenitizing.

Micrographs from [1]

Time to Heat vs Time to Soak

1084 is a simple steel with only carbon, manganese, and silicon added to it. The main element that needs to diffuse for transforming the steel to austenite is carbon. Carbon is a very small element and therefore it diffuses very fast. However, when other alloying elements are added it can lead to significantly longer times required for austenitizing. I used 52100 with the factory-annealed condition to illustrate this. 52100 has 1.5% chromium which significantly slows down the time for carbides to dissolve, even at a higher temperature of 1525°F (830°C):

While the 1084 was nearly 66 Rc after only 3 minutes, 52100 with the same hold time at 50°F higher temperature was only 59 Rc. This isn’t because the steel didn’t heat up as fast but because the carbides in the steel did not dissolve as quickly from the chromium addition. The hardness didn’t “level off” until 7+ minutes in the molten salt, and was still climbing after 11 minutes. This is partly why I recommend having a sufficiently long soak time to ensure that you reach a steady state before quenching. Sometimes knifemakers in my comments will ask why I am soaking for so long (10-30 minutes) and say that they only use 5 minutes or even shorter. That results in poor consistency because the hardness can change significantly with a small change in soak time. This also means that the heat treatment becomes very sensitive to the size of the piece as well. Maintaining the same time but with different sized knives would yield very different results.

Another thing to note is that this was with the factory-annealed condition, with relatively coarse spheroidized (round) carbides. The austenitizing time could be shorter with a different starting microstructure. I wrote about this difference and how to use it with forge heat treating here: How to Thermal Cycle Knife Steel. Below shows the difference between pearlite and spheroidized carbides for austenitizing 52100:

Data from [2]

Rapid Austenitizing for Better Performance?

Rapid heating and short soak times are also used in some cases to achieve a finer grain size, and a finer grain size can result in higher toughness. I tested this by heat treating 1084 coupons with a total of 3 minutes in the salt at 1475°F. A true “soak time” includes only the time actually at temperature, so the true soak time is something like 1-2 minutes. I also tried a “triple quench” with another coupon by repeating this procedure two more times. The coupons were quenched in Parks 50 and tempered at 400°F twice. I compared with another coupon that was given a more conventional soak for ten minutes in my normal EvenHeat furnace in air with the same 1475°F, Parks 50 quench, and 400°F tempering.

The conventional heat treatment with the ten minute soak resulted in the same toughness but slightly higher hardness. So despite the potential for finer grain size this did not result in improved toughness. This is relatively similar to a result I saw with 8670 heat treatments where using a 5 minute soak resulted in lower hardness and toughness than a 10 or 15 minute soak:

Prior experiments with 8670 steel in a conventional furnace

Temperature matters much more than soak time for grain size, and in general you don’t need to be afraid of grain growth with a longer soak. Here is a study on 52100 [3] that illustrates this:

Data adapted from [3]

The big surprise was the triple quench which resulted in much lower toughness than the other two heat treatments. The ~11 ft-lbs of the triple quench makes the steel much closer to 1095 and O1, significantly higher carbon steels (~0.95% carbon), which were around 8 ft-lbs at the same hardness:

The older 1084 datapoints on this chart are slightly lower than these new values. That 1084 was from a different steel manufacturer.

The fracture grain of the single and triple quenched 1084 was very fine, so I knew the drop in toughness was not from significant grain growth. So I decided to look at the microstructure of the two conditions:

Single Quench 1084 (3 minutes in 1475°F salt)

“Triple Quenched” 1084 (3 minutes in 1475°F and quenched, repeated)

The main thing I noticed is that there was much more carbide in the single quench version. This makes some sense given that it had 1/3 the total soak time. However, the ten minute in air condition had a similar soak time and did not have a significant drop in toughness. It appears that performing the austenitize and soak repeatedly led to more rapid carbide dissolution than a single soak. Heat treating from a martensitic condition (after quenching the first time) is known to lead to more rapid austenite formation, so I believe this is what happened with the multiple quench. With more carbide dissolved, this put more carbon in solution which also reduces toughness for a given hardness. This is the same reason why O1 and 1095 have lower toughness, because they had high carbon in solution with the same 1475°F ten minute soak. You can read more about this (and some things that can be done to improve toughness with high carbon steels), in this article about austempering.

With O1 I was able to improve toughness by austenitizing at a lower temperature of 1425°F. It is quite likely that I could improve the toughness of the triple quenched 1084 by using a lower austenitizing temperature as well. However, I think this result is still a good warning to those that think that any “special heat treatment” they perform could potentially help and certainly wouldn’t hurt. With any specialized heat treatment, the potential improvement is usually small but the potential for being detrimental is always there, and probably more likely.

O1 toughness was improved by using a lower austenitizing temperature (1425 vs 1475°F)

We did a triple quench experiment with CruForgeV in a conventional furnace back in 2018. In this case we did each soak for 10 minutes in the furnace, the first two at 1450°F and the final at 1500°F. With this experiment we also saw an increase in hardness and a drop in toughness by triple quenching, though the drop in toughness was not as extreme as the salt pot triple quench of 1084. The smaller drop is likely due to 1) the chromium addition to CruForgeV so its carbides dissolve more slowly and it is not as sensitive to overheating, and 2) the lower temperature used in the first two quenches.

Single vs Triple quench of CruForgeV

Prior Triple Quench Salt Pot Experiments by Dr. John Verhoeven

There is an experiment reported by Dr. John Verhoeven in his book Steel Metallurgy for the Non-Metallurgist [4] that he wrote for knifemakers and bladesmiths. He tried it with 1045, 1086, and 5150 low alloy steels. First he heated the steels to 1650°F for 15 minutes and quenched in oil. This was the starting condition where he measured the grain size. Then he heated each steel to 1450°F for 4 minutes in a salt pot and quenched in oil, repeating the procedure two more times (triple quench). He found a decrease in grain size relative to the initial condition after the 1650°F and quench (larger ASTM number means finer grain size):

In this case his results make sense given that the 1650°F austenitize would definitely result in a larger grain size. The repeated cycles at 1450°F were beneficial in refining the grain size, though the result from one or two cycles was not tested. So I think it makes sense for this particular experiment to show a finer grain size from a rapid salt pot “triple quench” while my toughness experiments did not show an improvement. It is also possible that my triple quench actually did improve the grain size but the higher carbon in solution was a more important factor. Measuring very fine grain sizes is difficult.

Do I Recommend Salt Pots for Austenitizing?

I heat treat a wide range of steels with a wide range of austenitizing temperatures so having to use multiple salts and pots doesn’t really work for me. Cleaning out a pot to switch to a different type of salt is definitely not an exciting prospect. So I would probably only recommend salt bath furnaces for those that are using a narrow range of steel types and benefit from shorter soak times and the clean surface coming out, both of which lead to saving them time. But there are several knifemakers out there that swear by salt pots so there might be some benefits I am not thinking of.

Summary and Conclusions

Salt bath furnaces have much faster heating rates than conventional furnaces and also do not require any protection of the steel furnace (there is no oxygen present). Toughness experiments did not reveal an improvement to properties despite the potential for grain refinement with rapid heating and short soak times.

---

[1] Samuels, Leonard Ernest. Light microscopy of carbon steels. ASM International, 1999.

[2] Stickels, C. A. “Carbide refining heat treatments for 52100 bearing steel.” Metallurgical Transactions 5, no. 4 (1974): 865-874.

[3] Khzouz, Erik Ryan. “Grain growth kinetics in steels.” (2011).

[4] Verhoeven, John D. Steel metallurgy for the non-metallurgist. ASM International, 2007.

The post Rapid Heat Treating with Salt Pots appeared first on Knife Steel Nerds.

Another rather large heat treating study! This one took quite a bit of time, effort, and money. If you want to support further research visit Patreon.com/KnifeSteelNerds and become a Patreon supporter. All of the money I receive that way goes to knife steel research. And you get some perks like seeing articles and videos early, and at a high enough tier you get a free Knife Steel Nerds mug!

Video 

Here is the video version of the following information:

Should Stainless Steel Be Forged?

Stainless steels are generally more difficult to move under the hammer than simple carbon and low alloy steels. They are typically more expensive. The forging range is usually narrower; you have to stop forging at a higher temperature or it may fracture. But of course stainless steel has the major advantage that it is corrosion resistant. There are many myths around forging stainless and carbon steels that are used to justify the use of only simple steels. These myths scare some knifemakers away from using stainless steels that might otherwise try them.

Is Stainless Steel Improved by Forging?

There is an old tradition in the knife world that says that only carbon steels should be forged. Some have gone so far to say that stainless steels do not “benefit” from forging in the same way that carbon steels do. This is somewhat difficult to refute, as the benefits of forging are often overblown to begin with. I have an older article on forging vs stock removal you can read here. However, I argue that high alloy tool steels and stainless steels have more potential benefits to forging than low alloy steels. The reason is because with simple steels all of the carbides are dissolved at forging temperatures and re-precipitated later; this makes the carbide structure easier to control with thermal cycling alone. High alloy steels have carbides that do not dissolve without melting the steel itself so there is more possibility of improving that structure through further working. Of course all steel purchased by knifemakers has already been forged and/or rolled from an ingot, so we are often talking about a relatively small amount of further forging.

D3 tool steel forged to different degrees thickness starting from a 10″ round ingot [1]

And the carbide structure has directionality to it, leading to different properties in the longitudinal and transverse directions (along the rolling direction and perpendicular to it). In my forged vs stock removal article I gave reasons for why forging blades to shape rarely leads to superior toughness, but if such a benefit was to be gained it would be more pronounced in high alloy and stainless steels.

M7 high speed steel with different degrees of reduction [2]. You can see that the carbide bands are elongated along the rolling direction.

What is a High Alloy Tool Steel?

Most of the information in this article will relate to not only stainless steels but also high alloy tool steels like A2, D2, CPM 3V, etc. Stainless steels used in knives are simply a subcategory of high alloy tool steels. Low alloy tool steels and simple carbon steels behave somewhat differently like 1095, O1, 52100, 80CrV2, and others. The line between low and high alloy can be somewhat fuzzy, some give it as 5% total alloy content. So if the chromium, tungsten, molybdenum, etc. add up to more than 5% it is high alloy. For our purposes a high alloy tool steel is any with at least 3% chromium. These steels are air hardening and their carbides dissolve at higher temperatures. This does not mean all of these steels will be annealed with exactly the same temperatures, hold times, cooling rates, etc. but similar ideas will apply to them.

How Hot to Forge Stainless and High Alloy Tool Steel

A common mistake with forging of stainless and high alloy tool steels is heating the steel too hot. It is a common misconception that because the steel is more difficult to move under the hammer that more temperature is necessary. When steel is overheated, the grain boundaries melt first, leading to steel breaking apart when it is forged. Some knifemakers mistakenly assume they must not have been hot enough, and try even hotter! The temperature at which grain boundaries melt is roughly the same with stainless steel as it is with simple carbon steels, and sometimes lower. Datasheets typically recommend 2100°F (1150°C), though some will recommend lower like 1900-2000°F (1035-1100°C). 2100°F/1150°C seems to work for most knifemakers I speak to. However, many knifemakers are used to lower carbon steels like 1084 or 80CrV2 which can handle higher temperatures and they are not used to dialing the forge down for higher carbon steels. This is why “cast iron” has very high carbon content (>2%); higher carbon means lower melting temperature so it is easier to melt the cast iron before casting. High carbon steels that are more commonly used in forging like 26C3, White #1, Blue Super, and ApexUltra are also more sensitive to overheating since they have relatively high carbon contents (>1.2%).

How to Normalize Stainless Steel

Stainless and high alloy steels are not normalized. The goal of normalization is not grain refinement but rather dissolving all of the carbides before air cooling. As I noted above, with most stainless steels the carbides do not dissolve until melting. Perhaps we could come up with some kind of creative treatment that could improve/change the microstructure prior to annealing but this is not common in the steel industry. Normalizing is not necessary and we will be skipping it with stainless and high alloy tool steels.

What Are We Trying to Accomplish During Annealing?

Annealing is the step we perform between forging and the final austenitize and quench. The steel is annealed by the manufacturer before you get it. For stock removal makers, annealing is not necessary except in rare circumstances. The purposes of annealing are multi-fold:

1. Soften the steel so it is ready for machining, drilling, bandsaw cutting, etc.
2. Prepare the steel for good response to austenitizing so that we don’t need excessive hold time or temperature.
3. Maximize the final properties after heat treatment including hardness, toughness, etc.

Problems with Annealing Stainless Steel – Long Times, Scale, and Decarburization

One issue with annealing stainless steel is that the recommended annealing procedures often require cooling from 1600F+ down to 1000F at 25 degrees Fahrenheit per hour (~15°C/hr). That takes over 24 hours! Not only does this take a long time but most knifemakers do not have furnaces equipped with inert gas or vacuum so this means that scale and decarburization are major concerns. Without any protection you could end up removing a significant amount of carbon from the steel. To mitigate this it is best to wrap the steel in foil during annealing. I found double wrapping to help some as well. This is not a surefire way to prevent any scale or decarb as the times are very long. Therefore it is best to leave some material to remove after heat treating to ensure all of the scale and decarb is removed. I have not experimented with coatings for annealing but they may also work.

Why AEB-L is a Good Stainless to Start With

The first steel I experimented with was AEB-L. It checks a lot of boxes for forging bladesmiths as it is a very fine carbide steel with properties that can be similar to low alloy and simple carbon steels. Some have called it “stainless 52100” for its fine carbide size and excellent toughness. It is also relatively low cost, as some bladesmiths have sticker shock buying expensive stainless steels when they are used to buying 1084 or 80CrV2 for less than $5 per pound. AEB-L’s relatively low carbide content also means it is somewhat easier to forge than other stainless steels; I have an article on which steels are most difficult to forge here. Having only chromium carbides means it is easier to grind and finish than some of the more exotic stainless and high alloy tool steels with very hard vanadium carbides. One potential downside to AEB-L is it is not typically available thicker than about 1/4″ (6 mm), so bladesmiths that like to forge from heavy stock, round bar, etc. will be out of luck.

My New Experiment with AEB-L

We started with 1/4″ AEB-L which my father, Devin Thomas, hot rolled down to 0.130″ (3.3 mm), which is a bit less than a 50% reduction. The temperature used for rolling was around 2100°F. I then annealed it in different ways and measured the annealed hardness, hardness after quench and temper, and toughness after quench and temper. The different annealing procedures I tried will be given in the sections below. The final austenitize I used was 1925°F (1050°C) for 15 minutes, plate quench, cryo in liquid nitrogen, then double temper at 350°F (175°C).

Traditional Slow Cool Annealing

I have an earlier article that discusses the mechanisms within steel that occur during annealing. The typical method is for heating the steel to a temperature where the steel is austenitic (the high temperature, nonmagnetic phase of steel), but typically lower than austenitizing before quenching. For a simple carbon or low alloy steel this is somewhere in the range of 1350-1450°F (730-790°C) while austenitizing before quenching is typically 1475-1550°F (800-845°C). This lower temperature means more carbide is present, then during slow cooling the soft ferrite forms while feeding carbon to those carbides and growing them, resulting in a “spheroidized” structure (round carbides).

Carbides increasing in size during slow cooling as the ferrite grows into the austenite

There are a couple differences with high alloy and stainless steels. For one, the temperatures for annealing are typically higher, with 1600-1650°F (870-900°C) being most typical. The steels do not transform to austenite until higher temperatures, necessitating the higher temperatures. Typically the hold time is longer at this temperature as well, such as two hours. Following that hold time, the cooling rates are also slower. These are “air hardening” steels so they are more prone to hardening if the cooling rates are not sufficiently slow. This is partially why low cooling rates like 25°F/hr are recommend in datasheets, though that is also true in many datasheets for low alloy steels. So for AEB-L I wanted to try 50°F/hr and 100°F/hr and see how the resulting properties compared with the as-received steel from the manufacturer. The temperature you must cool to for ensuring full transformation depends on the cooling rate and the steel. However, most annealing procedures will recommend a temperature below which no more transformation is likely to occur, such as 1000°F. I used 1100°F for the 50°F/hr anneal and 1000°F for the 100°F/hr anneal. If the steel is fully transformed it doesn’t particularly matter what cooling rate is used below that temperature.

The hardness after annealing was at first a bit surprising because even with the fast 100°F/hr the hardness was significantly lower than what is delivered by Uddeholm (As-received). Perhaps Uddeholm uses an even more rapid form of annealing.

AEB-L As-Received by Uddeholm

50°F/hr anneal

100°F/hr anneal

This hardness difference appears to be confirmed by the metallogaphy, as the carbides in the two anneals I performed have somewhat larger carbides. I next compared the resulting hardness and toughness with prior toughness testing done with the as-received steel.

The same 1925-350 heat treatment with the as-received material (labeled “Stock Removal”) is the 60 Rc point. So the 50°F/hr anneal resulted in slightly lower hardness with equivalent toughness, and the 100°F/hr anneal resulted in slightly better hardness and toughness. This is mildly surprising based on the annealed hardness and somewhat larger carbides in the annealed condition. But overall this is a good result, with the 100°F/hr anneal being relatively fast, having very low annealed hardness for ease in working, and then excellent hardness-toughness after final heat treatment.

Isothermal Hold Annealing

Another similar way to anneal is to do a hold at a lower temperature rather than a slow cool. After heating to 1600°F like with the prior annealing, I held at either 1300°F (700°C) or 1200°F (650°C). Holding at a higher temperature is roughly equivalent to a slow cooling rate, while holding at a lower temperature is equivalent to a faster cooling rate. I held at 1300°F for 4 hours and 1200°F for 6 hours, as I was concerned it would take longer at the lower temperature (based on published transformation curves for stainless steels).

I also decided to try a technique that is common with low alloy steels which is to do “grain refining” cycles prior to the 1300°F anneal. I heated to 1600°F for 30 minutes and air cooled, which I did twice before the same anneal. I did not find grain refining cycles to improve properties in 1084 in a prior experiment, but I thought it wouldn’t hurt to try it again. I labeled this condition “cycled.”

These were somewhat closer in hardness to the as-received condition. However, oddly the hardness of the 1200°F condition  was lower than the 1300°F. Yes I held it longer at that temperature but the transformation was completed in both cases (as will be shown in the metallography), so that should not have been a factor.

Isothermal Anneal 1300°F

1200°F Isothermal Anneal

The 1200°F anneal looks like it resulted in somewhat finer carbides thought they don’t look that different. And both look relatively similar to our slow cool anneals. There are probably subtle differences if we did a full statistical analysis, but the real differences are probably more apparent through the annealed hardness and the final heat treated hardness and toughness.

The properties look pretty similar between the three and I’m not sure there are any “real” differences between the three. They are all around 60.5 Rc and ~35 ft-lbs. Maybe the grain refining cycles or the lower 1200°F led to a slight improvement but those small improvements may disappear if we did the experiment multiple times and averaged the results.

Temper Annealing

Another type of annealing is quite different than the others which is temper annealing, which I have written about before. In fact this is the general type of annealing that I recommended for high alloy steels in Knife Engineering. With normal tempering of martensitic steel, the higher you temper the softer the steel gets. If you temper hot enough you get annealed, soft steel. However, normally we do an austenitizing and quench step from a lower than normal temperature prior to a very hot temper. I recommended this annealing treatment because of older studies that showed a superior grain size and toughness with high speed steels after a temper anneal rather than a more conventional anneal. But the tempering times for optimal properties were very long, 12-24 hours depending on steel and tempering temperature. Also I began to be worried that perhaps the anneal was better only for high speed steels, since they use very high austenitizing temperatures where grain growth is more common. Perhaps the temper anneal resulted in more stable carbides that don’t dissolve as readily at the high temps, therefore they still pinned the grain boundaries. For most steels where grain size is not as difficult to control, this benefit would be lost.

So based on those prior reported experiments I austenitized all of them at 1600°F for two hours prior to plate quenching. This also has the benefit of maintaining the same austenitize as the prior annealing experiments. I then tried three different tempering conditions: 1300°F 4 hours (Q13), 1400°F 4 hours (Q14), and 1400°F 24 hours (Q24). Long anneals were optimal for the high speed steels but I wanted to have shorter anneals which is why I also tried the 4 hour tempers.

The 1300°F anneal (Q13) is definitely harder than we would desire. The 1400°F for 4 hours (Q14) is better, and the 24 hour anneal (Q24) is definitely soft enough. Maybe we could do something in between 4 and 24 hours with 1400°F instead.

Q13 – 1300°F 4 hour temper anneal

Q14 – 1400°F 4 hour temper anneal

Q24 – 1400°F 24 hour temper anneal

Only the Q24 looks properly annealed with medium size spheroidized carbides throughout. The Q13 condition specifically has relatively large regions which still look like martensite, or only very small spheroidized carbides.

Comparing the toughness, Q24 and Q14 are very similar, while the Q13 was significantly worse. The average hardness I measured with the Q24 was around 60 but I got several readings that were lower; overall it seemed to test less consistently. Maybe that means the carbides were too coarse, again pointing to an optimal hold time being somewhere between the 4 and 24 hours I tested with 1400°F. The Q13 ended up being slightly lower in hardness which is just randomness. I did another set of small hardness coupons and it was more similar to the Q14. My hypothesis for why the Q13 had worse toughness is related to the inconsistent microstructure. Maybe those martensite regions led to large grains because there weren’t carbides available to pin the grain boundaries.

AEB-L Annealing Summarized

When looking at all of the conditions as a whole, the overall finding was that if we have properly annealed the steel the properties are relatively similar. We got good (low) annealed hardness with most of them, and the resulting hardness and toughness were also pretty similar. My favorite of the conditions was the 100°F/hr anneal. The overall time to anneal is around 8 hours, meaning you can anneal it overnight after forging the steel. And it had the best combination of hardness and toughness after the final heat treatment of 1925°F austenitize and 350°F temper.

Annealing MagnaCut

I also wanted to look at a steel that may require a slower cooling rate than AEB-L so I also tested MagnaCut. Of course I developed MagnaCut so I was more interested in it. But the reason why MagnaCut was a good steel to look at is because of its 2% Mo. Molybdenum increases “hardenability” of steel so that larger sizes can be air cooled and still fully harden. This also affects annealing. Below I have Time-Temperature-Transformation (TTT) curves for two steels after they were austenitized at a low temperature for annealing:

D2 Annealing TTT after austenitizing at 1600°F

M2 annealing TTT after austenitizing at 1625°F

It can take a minute to understand the charts if you have never looked at them before. The curve to the left shows how long it took for the steel to start transforming, and the curve to the right shows how long it took for the transformation to end. For D2 with 0.74% Mo, at the fastest transformation temperature (~1400°F) it took less than 30 minutes to fully transform. For M2 with 5% Mo (and 6.5% W), it took over 3 hours to fully transform at the same temperature. This is largely cause by the Mo difference. So I was concerned that a 100°F/hr anneal may not work with a steel like MagnaCut with 2% Mo. This should then translate to the many stainless steels with less than 2% Mo (440C, Elmax, M390, S90V, etc.), and to the several stainless steels that also have around 2% Mo (S30V, S35VN, S45VN, S110V). The most common steel with even more Mo is 154CM/CPM-154 with 4% Mo though its datasheet says it can fully transform at 1300°F after 4 hours so there is a decent chance it can also use an intermediate cooling rate.

We did the same experiment of hot rolling MagnaCut from 1/4″ down to 0.130″, I performed a few different annealing cycles from 50°F/hr or 100°F/hr and measured the annealed hardness. However, I also wanted to experiment with the annealing temperature prior to cooling so we looked at that for this steel. We will get to that after we take a small detour to discuss how those annealing temperatures are chosen.

How to Select an Annealing Temperature

Most steels have an available datasheet that lists a recommended annealing procedure. For example, the MagnaCut datasheet recommends the following for annealing: Heat to 1650°F (900°C), hold 2 hours, slow cool no faster than 25°F (15°C) per hour to 1100°F (595°C), then furnace cool or cool in still air to room temperature. So if you follow my recommendation and use the faster cooling rate of 100°F/hr you can simply use that recommended temperature (1650°F/900°C), and then cool at the faster rate.

But what if the steel does not have a datasheet, or the datasheet doesn’t give an annealing temperature? At that point we need to find the temperature at which the steel has transformed to austenite. With a simple carbon or low alloy steel that can be done with a magnet because austenite is non-magnetic. However, there is another point where steel becomes nonmagnetic called the “Curie point,” and high alloy steels reach that point before they transform to austenite. So another way we can try to determine it is to heat the steel to different temperatures and quench and see the point where hardness increases. I did this with both AEB-L and MagnaCut:

You can see that AEB-L shows a rapid increase in hardness between 1400 and 1475°F which is the region over which it transforms from ferrite to austenite. After it transforms to austenite, carbon goes in solution, then you can quench and the hardness is much higher after quenching. However, if you quench after heating to 1400°F, you didn’t transform to austenite and the hardness stayed low. It looks like we could anneal AEB-L from as low as around 1500°F, and the 1600°F I chose for my study should be pretty safe.

With MagnaCut, however, the transition is more difficult to see because carbon doesn’t go into solution until a higher temperature. It looks like the steel may have started transforming to austenite around 1550°F but this actually led to the steel slightly decreasing in hardness. The steel started increasing again around 1650°F and I think this means it is likely where austenite finished transforming and is probably a reasonable temperature to anneal from. But I wanted to see how the annealing temperature would affect the annealed hardness and the final heat treated properties.

There were several surprises in this experiment to me. One is that the temperature mattered much more than the cooling rate, at least for those two cooling rates. The other surprise is that the change in annealed hardness was quite linear. There was no big jump in hardness from annealing at too low of a temperature, even at 1550°F. The “as-received” hardness for annealed MagnaCut is around 22 Rc which is similar to the hardness measured for the 1600°F anneal.

The next experiment I wanted to do was ensure that we were getting good properties from annealing at 1650°F since the annealed hardness continued lower up to 1750°F annealing temperature. And the as-quenched hardness we measured from MagnaCut before didn’t really go up significantly until 1700°F. So I heat treated coupons from the 1650, 1700, and 1750°F anneals with both cooling rates. This also allowed me to compare properties with the two different cooling rates:

The hardness after heat treating was slightly higher with the faster annealing rate, which we would expect to see from having somewhat finer carbides. There wasn’t much difference between 1650 and 1700°F though there was a small drop by increasing to 1750°F. So I would probably recommend sticking with the 1650-1700°F range for annealing MagnaCut.

The toughness was slightly higher for the 50°F/hr anneal in line with its slightly lower hardness. Both the 50°F/hr and 100°F/hr anneals resulted in somewhat higher hardness than the as-received condition. You could austenitize somewhat lower or temper somewhat hotter to reduce the hardness if desired. The forged and annealed MagnaCut had a slightly higher hardness-toughness balance than the as-received material. However, toughness testing can be somewhat variable and I have managed to get ~16 ft-lbs with 62 Rc as-received before. I think the toughness may be slightly better or slightly more consistent with the forging and annealing. Probably not enough to justify forging blades instead of doing stock removal, but I have also heard some bladesmiths claim that you will “ruin” stainless steel by forging it yourself and that is certainly not the case if you do it right. I also don’t think that toughness results of forged low alloy and simple carbon steels are enough to justify forging. Knifemakers should choose forging or stock removal based on other factors.

Summary – General Stainless and High Alloy Tool Steel Recommendations

Don’t forge too hot – 2100°F is a good target temperature. A common mistake is thinking that stainless steels need higher temperatures to forge. They do not.

Don’t forge too cold. Stainless and high alloy tool steels need higher minimum forging temperatures. They become “hot short” (brittle) at higher temperatures than simple carbon steels. Depending on the steel and the datasheet, this minimum is given in the range of 1650-1750°F (900-950°C).

Protect the steel from atmosphere during annealing – Vacuum or inert gas is best but for many knifemakers this means double wrapping in foil (not double folding but double wrapping), and leave some extra steel to grind away.

Choose the right annealing temperature – Typically in the range of 1600-1650°F for most stainless and high alloy steels but can sometimes vary. Check for a datasheet if a temperature recommendation is given and that temperature should work fine. A 2 hour hold at the temperature is typical. If no datasheet temperature is available you may be able to use the method shown above with AEB-L and MagnaCut where I found the temperature at which the hardness goes up, indicating that austenite formed.

Cool at 100°F/hr – This is significantly faster than recommendations given in most datasheets but gives a better balance of speed (~8 hours instead of 24+, can anneal overnight) and final properties (higher final hardness and toughness). There may be some steels that cannot handle this faster cooling rate if they have very high Mo contents. If the steel ends up higher than 25 Rc at the end it may need slower. As a side note, 100°F/hr was also found to be optimal with ApexUltra, a low alloy steel, so maybe we are on to something here.

Cool to 1000-1100°F, after that the cooling rate can be faster. You can experiment with whether the final cooling point can be higher than 1000 or 1100°F by annealing both ways and seeing if the hardness is the same. Once the transformation is done, further cooling doesn’t matter. I probably wouldn’t go any higher than 1200°F in any case. This really only matters if that extra hour or two will significantly impact your workflow.

So to summarize, anneal at 1600-1650°F for two hours, slow cool at 100°F/hr to 1000°F. Before that don’t forge too hot or too cold, and protect it from atmosphere during annealing.

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[1] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.

[2] Roberts, George Adam, Richard Kennedy, and George Krauss. Tool steels. ASM international, 1998.

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I will be at Blade Show Atlanta again this year. The big thing on my schedule is a Blade University class on June 7th, Friday morning. I have done different Blade University classes the past few years and it has been fun each time. This is a general class on thermal cycling using a lot of past studies that we have done which have been published to the website and in recent years to YouTube. However, we did a new study on AEB-L and MagnaCut to have better data on how to “thermal cycle” stainless and high alloy steels and we had some very interesting and exciting results. I am hoping that we can get some more bladesmiths to start forging stainless steels instead of the nearly-universal choice of carbon and low alloy steel. Buy tickets here: https://bladeshow.com/buy-tickets/

Buck Knife

The Buck knife design I worked on, 501 “The Larrin” will have a small number available in a new combination – elk horn with a brass bolster. I am hearing there will be 25 available so I wouldn’t wait too long to get one if you are at the show.

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