Upcoming Blade Show Atlanta

I will be at Blade Show in just a few days (June 5-7). I will be teaching two classes: “Factors that Affect Edge Retention” and “What is Coming Next for Knife Steel?” You can buy tickets by clicking here.

Video

Here is the video version of the following article:

Custom vs Industry Heat Treating

In a previous article I wrote about the wide series of tests we performed to dial in the heat treatment of MagnaMax using small furnaces and plate quenching. But many knife companies and knifemakers will “send out” to a larger heat treater and the process is different in several ways. There are also a handful of knife companies that have their own large-scale heat treating facilities. Big tool steel heat treaters most commonly use large vacuum furnaces that utilize a pressurized gas quench. I previously wrote about how this affects the heat treating and final properties in this article, which will have many more details than I will provide here. One of the major differences is that the pressurized gas quench is slower than a plate quench. That is a generalization, of course, if a piece of steel is very large it is certainly possible for a plate quench to be slower than a small, thin piece with a gas quench. And gas quenches can have a range of speed based on the pressure (measured in “bars”) and how loaded up with steel the furnace is. The slower quench can lead to reduced hardness and toughness, because undesirable carbides can form at grain boundaries during the slow cooling speed. The 2% molybdenum in MagnaMax helps with this because it is a very effective element for “hardenability,” which is how slow you can cool and still achieve full hardness. For example, Uddeholm compared their improved Vanadis 8 to their earlier Vanadis 10 [1], which increased Mo from 1.5 to 3.6% and showed experimentally the difference in hardness with cooling rate:

You can see that if the quench is sufficiently fast that the hardness does not further increase with even faster quenches. This is also true for toughness, though there can be cases where toughness is reduced from a slower cooling rate without that also showing up as reduced hardness. In the previous article I showed data that MagnaCut had slightly reduced toughness when heat treated with a gas quench vs a faster plate quench:

MagnaMax Experiments

Hardness

I worked with Peters’ Heat Treating to test the large scale heat treating of MagnaMax. Fortunately we had the previous data already from the plate quench heat treatments which narrows down the range of things we needed to test. Typically the “optimal” temperatures do not change with the slower cooling rate, just the resulting hardness and toughness. We used 1 x 3 inch coupons for each heat treatment to measure the hardness. 2050°F was used with and without cryo, and 2100°F and 2150°F austenitizing temperatures were used with cryo only. For each of those austenitizing temperatures they were tempered at 300, 350, 400, 500, 700, 900, and 950°F. They were quenched with a “2 bar” pressurized gas quench, which means roughly twice the pressure of the atmosphere. I measured the hardness for each of these which generated the following chart:

I also measured the hardness of the charpy coupon specimens which used 2050-350°F (with and without cryo) and 2150-350°F. These came up a bit higher in hardness then the hardness coupons, perhaps because they were smaller (0.4 x 2.17 inches). With the plate quench heat treatments the situation was reversed where the charpy coupons tested a bit lower than the hardness coupons. The hardness coupons for plate quenching were 1 x 1.5 inches and the charpy coupons were heat treated in groups of three so the overall dimensions were 1.5 x 2.17 inches. So I plotted the hardness for all of those conditions to compare the plate quench to the gas quench:

So you can see that the gas quench generally led to somewhat lower hardness than the plate quench, as expected from the slower cooling rate. This is one of the reasons why knife companies can have a harder time reaching the same hardness values of a custom knifemaker. There are other factors as well, such as the knife company heat treating many more knives, which means that there can be more variation in temperature within the large furnaces, and variation in steel composition by using steel from multiple batches over multiple years. So this explains why knife companies typically advertise hardness with a range, such as 61-63 HRC, from the factors I just mentioned plus also statistical variability in the hardness testing itself.

Toughness

The 2050-350°F coupons tested relatively similarly with and without cryo, despite the two conditions being about 1 HRC different with the hardness coupons. The non-cryo condition measured 61.3 HRC and the cryo condition measured 61.6 HRC. The 2150-350°F coupon measured 63.8 HRC. The toughness decreased linearly with higher hardness. When comparing to the plate quench, the toughness is slightly lower as expected:

New Data Sheet and Recommended Heat Treatments

So using all of the data generated so far I made a new data sheet to summarize the heat treatment and properties of the grade for knifemakers, knife manufacturers, and heat treating companies. You can read the datasheet here.

My general recommendations for heat treating do not change based on the difference in furnace type and quench rate, I still recommend 2150°F and 350°F with a cryo treatment. Of course it is possible to customize that further for higher hardness at the cost of some toughness or vice versa. I definitely encourage knife companies to contact me if they want to test the hardness-toughness balance of their heat treatments and to dial in things to where they want them to be.

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[1] Tidesten, Magnus, Anna Medvedeva, Fredrik Carlsson, and Annica Engström-Svensson. “A new cold work PM-grade combining high wear resistance with high ductility.” BHM Berg-und Hüttenmännische Monatshefte 162, no. 3 (2017): 117-121.

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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

The post What is Atlantic 33 Non-Tempering Steel? appeared first on Knife Steel Nerds.

Thank You Patreon

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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.

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.

The post How to Anneal Stainless Steel After Forging appeared first on Knife Steel Nerds.

Thank you Patreon Supporters!

Thank you to my Patreon supporters who help make these studies happen. Patreon funds were used for purchasing two ultimately useless knives, and some steel. Patreon supporter Marcus Ho even sent me some steel all the way from China which also ended up not working out. And I used Patreon funds to pay Shawn Houston (also a Patreon supporter) to do microscopy and grind two CATRA knives for me. So when I say that Patreon makes the study happen I don’t mean in an abstract sense. If you want to help fund more knife steel research please come join us on Patreon where you can get articles and videos early. Or if you sign up for the highest tier you can even get a sweet Knife Steel Nerds mug.

Video

Video version of the following information:

History of 8Cr13MoV/8Cr14MoV

To cover the history of 8Cr13MoV we have to go back to Japan, or even further back to Europe. Hitachi in Japan came out with their own stainless tool steels called “Gingami” meaning “silver paper” with three different designations. All three of these steels were copies, or nearly so, of stainless steels available in Europe. They were released somewhere in the 1933-1951 time period. In 1960 Fukami Steel Company was founded in Japan and they introduced AUS-4 and AUS-6 in 1968. AUS-4 was a 420/420HC type steel and AUS-6 was similar but with higher carbon and a small molybdenum and vanadium addition. AUS-6 was very similar to Gingami 2, and also other European steels such as Sandvik’s 12C27. AUS-8 was introduced in the 1970s which was AUS-6 with increased carbon up to ~0.8%. AUS-6 and AUS-8 became very common in Japanese factory knives, such as Kershaw’s imported knives introduced in the late 1970s. You can read more about all of this history (including citations) in my book The Story of Knife Steel.

8Cr13MoV, also called 8Cr14MoV, is a generic Chinese designation for AUS-8. They have identical composition. The naming scheme is pretty self-explanatory, I think, but maybe it makes sense to describe it explicitly. The first number designates the carbon content, so “8” refers to ~0.8% carbon, Cr13 means 13% chromium, Mo means a small molybdenum addition, and V means a small vanadium addition. So 7Cr17 means 0.7% carbon and 17% chromium, 9Cr18 means 0.9% carbon and 18% chromium, and 9Cr18MoV is the same with molybdenum and vanadium. Another steel that seems to be building in popularity is 10Cr15CoMoV which is 1% carbon, 15% Cr, plus cobalt, molybdenum, and vanadium; a copy of Takefu’s VG-10. I have heard these designations sometimes referred to in a derogatory way as “Chinese alphabet soup” though I think it should be known that these designations are a slight modification of an earlier European designation system. For example, 9Cr18MoV has existed for a long time in Europe as X90CrMoV18, also referred to by another European designation system as 1.4112. This is the European version of 440B which has some small differences with the American AISI 440B, such as higher chromium (18 vs 17%) and molybdenum (1-1.3% vs <0.75%). 7Cr17 is the same as AISI 440A. To the average consumer I’m sure all of the numbers are somewhat mystifying and steel names tend to be more memorable.

I don’t know when 8Cr13MoV was first used, but it was first advertised in knives by Spyderco in 2005 with their new “byrd” line of knives produced in China. Spyderco had requested their new knives be produced in 440C and tests of the knives were positive. However, Sal Glesser requested they check the composition of the steel and found it to be identical to AUS-8, not 440C as the factory told them. They learned the steel was designated as 8Cr13MoV in China and advertised the knives as such [1]. There is no distinction between 8Cr13MoV and 8Cr14MoV, all published compositions I have been able to find are the same, apart from Spyderco’s composition table which lists the chromium content only as 13.0%, rather than other published ranges I have found which show 13-14.5% chromium.

My History with 8Cr13MoV/8Cr14MoV

Because of how common AUS-8 and 8Cr13MoV knives are, I was interested in testing the steel for some time. However, despite how common it is in factory knives, obtaining individual bars of it was actually pretty difficult. Japanese and Chinese steels are pretty difficult to obtain in the USA to begin with, as most of it is sold directly to knife companies in their respective companies. Furthermore, because they are known as “budget” or even “low end” steels custom knifemakers in the USA are not exactly clamoring for it so the knife steel supply companies were not trying to get any.

My first solution was to purchase two “Schrade Leroy” choppers made in 8Cr13MoV which have large flat areas. This would allow me to anneal (soften back to the steel factory state) the steel and then heat treat it how I want. The flat areas mean I could make steel coupons for testing such as toughness coupons and potentially CATRA knives. That gives me control over the heat treating and geometry of the steel so I am not limited to available knives.

The knife had easily removable handles so I annealed one of them and posted to my Patreon that I would soon be testing the steel. One of my Patreon supporters, Marcus Ho, contacted me and offered to purchase some 8Cr13MoV bar stock so I wouldn’t have to use my more complicated method for obtaining the steel. Marcus is a knifemaker in Hong Kong so he had experience with sourcing steel in China. You can follow him on instagram under his company name HK.Knifeworks. He sent me a bar of steel along with a very official composition certification.

So I set the Schrade knife aside and started working with the new bar of steel. I started with developing hardness curves with different temperatures, which looked about where I expected them to be. But then I tested the toughness and it was worse than I expected, being more in line with 1% carbon stainless steels like 154CM or 440C. I then looked at the microstructure and it had too much carbide in it, looking again like 154CM or 440C. So I tested the composition of the bar and it was indeed 440C. This was an ironic reversal of the situation that Spyderco saw with their byrd knives. I was so disgusted with this whole debacle and all of the time I had wasted on a bar of 440C that I stopped working on the steel altogether.

More recently I learned that Aus Maker Supplies in Australia had obtained some 8Cr14MoV to sell to knifemakers and I asked if they could send me some. This time I actually got a bar of real 8Cr13MoV/8Cr14MoV and I am happy to report that I have actually tested the real thing.

Hardness

I tested the heat treatment of 8Cr14MoV with a range of austenitizing temperatures after soaking for 15 minutes, plate quenched, and tempered at 300 or 350°F. For half of them I used a cryo step in liquid nitrogen after the quench but before the temper. I didn’t find any published information on heat treating the steel, but Aichi has a recommendation for AUS-8 of 1050°C austenitize and 180°C temper (1925°F/350°F).

As with any steel, cryo led to higher hardness, and a higher austenitizing temperature from which peak hardness was obtained. Without cryo peak hardness was from 1950°F, so the recommendation for AUS-8 of 1925°F is good I think. It is dangerous to heat treat close to the peak hardness, because austenitizing above the peak means that there is excess “retained austenite” dropping the hardness which leads to worse performance. These are small coupons so in an industrial environment with slower quenching speeds the hardness may be somewhat lower. 61 Rc with the 1925-350 heat treatment is pretty good. Also the steel can achieve relatively high hardness, up to 64.5 Rc with a low 300°F temper. A more conservative 350°F you can still get into the 62.5-63.5 Rc range.

Microstructure

8Cr13MoV has a generally fine carbide structure but has many larger carbides mixed in. Sandvik and Uddeholm razor steels 13C26, 14C28N, and AEB-L seem to have about the limit in terms of carbon and chromium content before large carbides become unavoidable without more expensive manufacturing such as powder metallurgy. I have AEB-L below as a comparison to see what I am talking about, along with the higher carbon 440C and VG10 which have even larger carbides.

8Cr13MoV 1925°F-350°F Area 1

8Cr13MoV 1925°F-350°F Area 2

8Cr13MoV 1950°F-350°F Area 1

8Cr13MoV 1950°F-350°F Area 2

AEB-L

440C

VG10

In terms of grain size we also did an etch to reveal prior austenite grain boundaries. It is notoriously difficult to reveal grain boundaries so they are somewhat difficult to see. However, it is apparent that there was some grain growth with 1950°F when compared with 1925°F.

8Cr13MoV 1925°F-350°F

8Cr13MoV 1950°F-350°F

We also observed broader “segregation” and carbide stringers on a macro scale, especially in the 1925°F CATRA knife, which is shown below with two different angles. We also had a surface defect from the carbide stringers, which is seen as a shadow in the darker of the two. This happens when a carbide stringer is big enough that it breaks out during grinding. Segregation and carbide stringers are not unique to 8Cr13MoV, they are relatively common with ingot cast steels (as opposed to powder metallurgy). This is especially the case with higher carbon conventional steels like 440C and 154CM. The carbide stringers are parallel to the edge, perpendicular to the “grind lines.”

Toughness

For toughness tests I heat treated two conditions: 1925-350°F without cryo (roughly matching factory knife heat treatments), and 1950-350°F with cryo (higher hardness and closer to a custom heat treatment). These resulted in about 60 Rc and 62 Rc, respectively.

As expected based on the presence of some larger carbides, the toughness of 8Cr13MoV is significantly lower than AEB-L, 14C28N, LC200N, and Nitro-V. However, the toughness of 8Cr13MoV is still better than most other conventional high carbon stainless steels like 440C, VG10, and 154CM. It is also better toughness than most powder metallurgy stainless steels, even Vanax, XHP, and S35VN (though not MagnaCut). So overall 8Cr13MoV/8Cr14MoV has pretty good toughness despite its reputation as a low end steel. AUS-8 would also be similar.

CATRA Edge Retention

For CATRA edge retention coupons I used the same heat treatments, though they both came out slightly harder at about 61 and 62.5 Rc. However, one interesting thing is that the 1925-350°F heat treatment, the softer of the two, measured higher for edge retention, with an average after four tests of 395 mm of cardstock cut in the CATRA test. The 1950-350°F instead measured 376 mm. The 1950 coupon tested very consistently with values of 370, 382, and 377 mm. The 1925, however, seemed to do better the more I retested it with 357, 399, 389, and 429 mm. After puzzling over this I think the reason is that we had more carbide stringers near the edge with the 1925-350°F CATRA coupon, as discussed in the microstructure section. So perhaps what happened is that I was sharpening into a more carbide-rich region leading to the higher values. This is part of the inconsistency of conventionally cast steels, leading to more variable microstructure and therefore properties. Powder metallurgy steels are typically more consistent. To be kind to 8Cr13MoV I plotted only the higher 1925 coupon on the chart below:

The steel tested about where it was expected to fall, a bit above the lower carbon AEB-L and Nitro-V, but below higher carbon steels like 440C, VG10, and 154CM. When compensating for hardness it did very similar to LC200N and 14C28N. Both of those steels (LC200N and 14C28N) tested a bit better than I expected based on their composition, as I expected them to be closer to AEB-L and Nitro-V. So I am curious if I were to test three knives if they would still end up where they did (as opposed to resharpening the same knife), or if there are other features of those steels that cause them to test a bit higher. But we are talking relatively small differences, it’s not as if the steels were competing with S30V.

Comparison with My Original Ratings

I had been running Knife Steel Nerds for a few years before I finally did my ratings of knife steels. I wanted to have experimental results before rating them. One of the few steels I gave a rating for without testing it was 8Cr13MoV/AUS-8, in part because it is so common and many people asked me to rate it. So I gave my best guess in the table which I reproduced below:

I guessed that the toughness would be a “6,” in between 5’s like Vanax and S35VN and the 7 of MagnaCut. And indeed the steel ended up in between those. The edge retention I gave a “3,” the same as AEB-L, LC200N, and 14C28N, and it tested similarly to those steels. So I pointed all that out to pat myself on the back for my predictive abilities based on educated guesses and everything I’ve learned about steels over the years. It’s fun to brag sometimes. I didn’t test corrosion resistance of the steel, it gets a 7 because it is largely a carbon-chromium steel (with a little Mo and V), and so should fall in line with other steels of similar carbon-chromium ratio. If everyone is really clamoring for corrosion resistance tests of these types of steels I will think about doing another round of salt spray tests.

Summary and Conclusions

8Cr13MoV started as a copy of the Japanese steel AUS-8. I told the complicated and boring story of why I have had such a hard time getting this steel so I could actually test it. I think the obtainable hardness of the steel is pretty good, it can be heat treated up to 64 Rc or so, and is very easy to heat treat into the 60-61 region without cryo and 62-63 Rc region with cryo. The 1925-350°F (1050-180°C) heat treatment recommended by Aichi for AUS-8 is a good starting point for a cryo-free heat treatment. With cryo you can austenitize up to the 1975-2025°F range for high hardness if desired. 300°F is as low as I typically recommend tempering, though 350°F gives more balanced properties. The microstructure is disappointing if you are comparing to AEB-L and 14C28N though looks pretty good when compared with higher carbon conventional steels like 440C, VG10, and 154CM. The toughness was better than those 1% carbon steels but not as good as the fine AEB-L and 14C28N, but quite respectable either way. The edge retention wasn’t much better than AEB-L for the toughness deficit, but with the good hardness 8Cr13MoV can have it isn’t too bad. Luckily for me I didn’t have to modify the ratings I had already given the steel in each category because my predictions were pretty good. If you are in Australia 8Cr14MoV/8Cr13MoV is a good low cost steel to purchase from Aus Maker Supplies. And if you are buying knives in the steel you don’t need to worry about the steel itself being “junk,” there are a lot worse choices. Of course that assumes the knife company is heat treating it properly.

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[1] https://forum.spyderco.com/viewtopic.php?f=2&t=14579&p=140419

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K390 Steel and Heat Treating Background

I already have a video and article about the history and properties of K390, which you can see here. K390 is a powder metallurgy non-stainless steel with high wear resistance due to very high vanadium, 9%. It is in a similar category to CPM-10V and Vanadis 8.

Since that article and video were published, new samples were heat treated by Svetlozar Chaushev; funding and logistical help was provided by K390 enthusiasts Manuel Meyer and Othman Fahim. I finished machining the samples and tested them for hardness and toughness. The main additions to the previous information we had is a comparison of tempering temperatures. My samples were tempered at 400°F (205°C) but these new ones were tempered at 500°F (260°C) or 1000°F (538°C). Some steels that we have tested in the past showed a reduction in toughness by tempering at 500 rather than 400°F, despite the reduction in hardness. An example of this behavior was seen with CPM S35VN. This is referred to as tempered martensite embrittlement. However, the temperature range of tempered martensite embrittlement is different for different steels and can be suppressed to higher temperatures than 500°F, so it is worthwhile to test.

Datasheets for many non-stainless tool steels recommend a “high temper” in the range of 1000-1050°F. This high temper gives the steel more heat resistance so that it doesn’t lose hardness during grinding, applying coatings, or high temperature operation. High temperature tempering also helps with reducing retained austenite even without cryo. However, in previous comparisons we have made between tempering at 400 and 1000°F we found higher toughness with the 400°F temper. You can read about these comparisons in articles about heat treating CPM CruWear/Z-Wear and CPM-10V.

Heat Treating Experiments

Below is a table of the heat treating experiments that were performed:

The time at the high austenitizing temperature was fixed at 20 minutes for the tests that I did (bottom of the table), but Chaushev used 15 minutes for 2050°F/1120°C and 10 minutes for 2100-2150°F (1150-1175°C). The K390 datasheet recommends 20-30 minutes for 1030-1150°C (1885-2100°F) and 10 minutes for 1180°C (2150°F). Shorter hold times are used at high temperature because carbides dissolve more rapidly at high temperature, and grain growth can be a concern. Chaushev used an oil quench while I did a plate quench in my tests; both are relatively fast quenching methods.

Hardness

I combined the hardness data from the toughness coupons with some small hardness coupons I heat treated a couple years ago with a 300°F temper. Perhaps the two different datasets helps explain why the hardness of the 300 and 400°F specimens are quite close together, only about 0.5 Rc where I would normally expect about 1 Rc. Or maybe K390 just has good tempering resistance at that low temperature range, as evidenced by there only being a ~0.5 Rc difference between 400 and 500°F when austenitizing at 1950 and 2100°F. Different “heats” of steel can have somewhat different ranges of carbon and other alloying elements which can lead to small differences in hardness, and my two different heat treatment studies were with two different bars that would likely have been different heats. You can also see in the chart that the 1000°F leads to significantly lower hardness for a given austenitizing temperature.

K390 tempering chart from the datasheet

I pulled hardness points off the datasheet and compared against the results of these newly heat treated coupons and found that we had higher hardness than the datasheet showed:

With the datasheet they used a nitrogen quench with 5 bar of pressure, which is likely slower than the quench Chaushev performed. The datasheet also did not use cryo which may explain the hardness difference. The difference in hardness increased with austenitizing temperature, which would be expected from cryo because there would be more retained austenite after quenching from high austenitizing temperatures.

Toughness

Below shows the resulting toughness values for different austenitizing and tempering temperature combinations:

400-500°F tempering led to similar toughness with a small edge given to the 500°F temper, especially at the lower austenitizing temperature of ~1800°F (980°C). The 1000°F temper led to similar toughness for a given austenitizing temperature, though there was a significant drop in toughness up to the 2150°F austenitize. This is despite the reduced holding time of 10 minutes used with 2150°F. With the 400-500°F temper, there was a significant drop in toughness from 1800 to 1950°F, but the toughness stayed basically flat between 1950 and 2100°F. So for high toughness heat treatments, sticking to a relatively low temperature of 1800°F or perhaps even lower leads to the best toughness. If going to higher hardness it appears best to go up even higher than 1950°F for more hardness as the toughness largely didn’t change up to 2100°F.

We must remember, however, that the 1000°F temper led to lower hardness for a given austenitizing temperature, so it can be better to plot toughness against hardness instead so that we can see which heat treatments led to the best balance of hardness and toughness:

The 400-500°F tempering led to a similar hardness-toughness balance (blue dots and line), while the 1000°F temper led to a lower hardness-toughness balance. So we found a similar result to the prior comparisons we have made between low and high tempering with CPM-CruWear and CPM-10V where a low temper at 400°F led to better toughness for a given hardness.

Summary and Conclusions

The low tempering temperature from 400-500°F (205-260°C) led to superior toughness to using a high temperature temper from 1000°F (538°C). Given that the high temperature range such as 1000°F also leads to a reduction in corrosion resistance I think it is safe to say that 400-500°F is preferred. No evidence of tempered martensite embrittlement was observed by tempering at 500°F and doing so can give somewhat improved toughness at the cost of 0.5-1 Rc. Good toughness was observed across the range of 1800-2150°F austenitizing (980-1175°C).

How to heat treat K390:

Austenitize for 20 minutes at 1800-2100°F (980-1150°C). Use 10 minutes for 2150°F/1175°C. Large pieces of steel may need a longer soak time.

K390 should be relatively insensitive to quenching, especially with the thin cross section of knives. However, a plate quench or rapid gas quench would be recommended.

We didn’t test without cryo treatments, the maximum recommended austenitizing temperature is likely lower without cryo (when using the recommended 400-500°F temper).

Temper at 400-500°F.

Set the desired hardness by selecting the appropriate combination of austenitizing and tempering temperature using the chart earlier in this article. Of course your batch of K390 and your furnace, cooling rate, etc. may yield slightly different results.

 

The post How to Heat Treat K390 appeared first on Knife Steel Nerds.

New YouTube video is a Q&A from Knife Steel Nerds Patreon supporters. The following topics were covered:

Nitro-V vs 14C28N

Carbide pullout during sharpening

What is difficult about high hardness heat treating of MagnaCut

What steels are coming next after MagnaCut

Does a fine grain mean a good heat treatment?

What are good pocket sharpeners?

The post What is Next After MagnaCut? Q&A Video appeared first on Knife Steel Nerds.