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YouTube Video

There is also a video for the following information:

Salt Pots and What I Used

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

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

Why Use Salt Pots?

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

Salt Pots and Safety

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

Time to Heat Steel in Molten Salt vs Air

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

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

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

Micrographs from [1]

Time to Heat vs Time to Soak

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

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

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

Data from [2]

Rapid Austenitizing for Better Performance?

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

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

Prior experiments with 8670 steel in a conventional furnace

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

Data adapted from [3]

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

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

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

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

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

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

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

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

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

Single vs Triple quench of CruForgeV

Prior Triple Quench Salt Pot Experiments by Dr. John Verhoeven

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

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

Do I Recommend Salt Pots for Austenitizing?

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

Summary and Conclusions

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

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

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

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

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

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

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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.

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

Buck Knife

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

The post Knife Steel Nerds at Blade Show 2024 appeared first on Knife Steel Nerds.

It gets expensive to buy knives just to cut them up, and metallography costs even more. Support more knife steel research by going to Patreon.com/KnifeSteelNerds You get to read articles and watch videos before anyone else, participate in the Patreon community, and even get a free mug if you sign up for the highest support level.

Update: Artisan Cutlery posted to their Instagram confirming our findings:

View this post on Instagram

A post shared by ArtisanCutlery (@artisancutlery)

Video

There is also a video of the following information:

AR-RPM9 Announcement and Name

AR-RPM9 was first announced in June 2020 on the CJRB Instagram, though oddly their Instagram doesn’t seem to go back that far now. I have a screenshot of it here:

CJRB is another brand owned by Artisan Cutlery. AR-RPM9 stands for “Artisan-Rare Powdered Material 9 Element Steel.” That is a very odd name so let’s break it down one element at a time. Artisan makes sense, it refers to Artisan Cutlery. But why “Artisan-Rare”? Just to have an oblique reference to AR rifles? I honestly don’t know. Then “Powdered Material” is odd, generally when we say “PM” steel that means “Powder Metallurgy” or “Particle Metallurgy” but not “powdered material.” In my original Patreon post about this steel I hypothesized that perhaps they were using weasel words for some other process that isn’t powder metallurgy but then they released a video in September 2020 where they specifically called it “powdered metallurgy” process, but we will come back to that. Then “9 Element Steel” which is a good lead-in to the composition discussion.

Composition of AR-RPM9 (Why it should actually be called AR-RPM7)

Artisan Cutlery revealed the composition of the steel in that September 2020 video. I was excited to see what it might be since they claimed to have designed a “new exclusive steel” designed specifically “with our…knives in mind.” Plus nine elements is quite a bit, so they must have gotten very creative in the design of the steel. Unfortunately, the reveal was quite underwhelming. The composition they specified was identical to the common 9Cr18MoV, the same steel as the European 1.4112, and very similar to American 440B. There were a few oddities in the description. One of the elements the video said was part of the 9 was “less than 0.4% nickel.” That is the same maximum allowable nickel content in 9Cr18MoV. Having a maximum allowable content means that it isn’t intentionally added; there is no minimum requirement. This is because nickel is present in a lot of recycled stainless steel. For example, I measured between 0.16 and 0.28% nickel in stainless powder metallurgy steels from Crucible, Carpenter, Bohler, and Uddeholm. So we are already down to 8 elements not nine.

Measurements of nitrogen, copper, nickel, cobalt, and tungsten in a range of powder metallurgy steels

Another oddity in the Instagram video was that he said there were “rare earth elements” in the steel, with elements plural so potentially we are back up to 9 or maybe even more. Before saying what those rare earth elements were, the Instagram video cut off. Rare earth elements are used in some steels and include elements like lanthanum and cerium, used for a couple different reasons in the steelmaking process. So a pretty unusual addition, especially to announce publicly, as often they are used without advertising to the public, in part because it is a trade secret and in part because they are used in very small amounts. Fortunately, Artisan Cutlery later clarified to Knife Newsroom shortly after what these elements are, and even later in December they uploaded a full version of the cutoff instagram video to YouTube. The answer was baffling: phosphorus and sulfur. Phosphorus and sulfur are not rare earth elements. I have never heard anyone say they are rare earth elements. Those are common impurity elements that are often specified with a maximum allowable content. They are not desirable, and are not considered an addition to the steel. So now we are down to 7 elements. We need to rename the steel AR-RPM7.

The only potentially relevant change is cobalt, which they said is 0.3%. 9Cr18MoV doesn’t have a cobalt specification, and 0.3% is a very small amount, so an argument could be made that it would still fit under the 9Cr18MoV spec. Cobalt is also present in a lot of recycled steel, so there is generally at least a small amount of it, though usually less than 0.3%. Furthermore, an addition of 0.3% cobalt to a steel would have a very minor effect on properties. Even with controlled testing of properties with and without 0.3% cobalt I’m not sure a metallurgist could come up with which is which. So maybe this is the change to 9Cr18MoV that they thought was enough to be a “new steel.”

I measured the composition of the steel with Optical Emission Spectroscopy (OES) and the elements were within expected ranges. Of course other trace elements were present which is also expected but I have not included them so as to not confuse anyone.

Powder Metallurgy? (Why we can’t call it AR-RPM7 either)

In my original Patreon article in June 2020 I expressed skepticism about AR-RPM9 being powder metallurgy. Artisan Cutlery already misunderstood how many elements were in their steel and apparently have no idea what a rare earth element is, so the chances they know what powder metallurgy is seemed small. Furthermore, there are several steels made in China using “Sprayform” which is a different technology than “Powder Metallurgy.” One example that has come out since AR-RPM9 is Twosun Knives YJ01-V1 which is a sprayform version of a slightly modified M390. This steel is made by Foshan Fenghe PSF (Precision Spray Form).

What is Spray Form?

Sprayform is an intermediate technology in between conventional casting and powder metallurgy. In conventional casting, the liquid steel is slowly cooled in a mold to form an ingot. The slow cooling leads to alloy segregation and large carbides. With powder metallurgy the liquid steel passes through a nozzle and is sprayed with nitrogen gas to rapidly solidify (gas atomization) the steel into fine particles (powder), which is then placed in a canister under high pressure and temperature called hot isostatic pressing (HIP) to create a solid ingot. The steel is then forged and rolled like normal. I previously wrote about powder metallurgy in this article.

Sprayform is somewhat similar to powder metallurgy in that the liquid iron is sprayed with nitrogen gas, but instead of fully solidifying, the partially solidified steel is sprayed onto a table which lowers during processing to create the entire ingot. This leads to an intermediate microstructure which is not as fine as powder metallurgy but also not as coarse as conventional steelmaking. I have an article comparing D2, sprayform D2, and powder metallurgy D2. Below are micrographs showing the difference in D2 microstructure with the three different technologies:

Conventional D2

Sprayform D2

Powder Metallurgy D2

As one more example, here are micrographs from Fenghe PSF showing their ripoff of M390 made with sprayform (right) compared with the powder metallurgy version (left):

Powder Metallurgy M390 (left) and Sprayform M390 (right)

You can see that the carbides in a sprayform steel are in between powder metallurgy and conventional. Sprayforming is done because it is lower cost vs powder metallurgy, eliminating the HIP step.

What Technology is Used for AR-RPM9?

So Shawn Houston of Triple B Handmade and I bought a couple knives from Artisan Cutlery to investigate the microstructure.

High Magnification (same as prior D2 micrographs)

AR-RPM9 Medium Magnification

Here is Sprayform D2 for comparison:

Sprayform D2 Medium Magnification

It is evident immediately that this is certainly not a powder metallurgy steel. There are many large carbides that I have never seen in any PM steel. I don’t believe that it would be possible to make a PM steel with carbides that large. With the largest carbides being around 10 microns I think sprayform is the most likely, though I could almost be talked into it being a conventionally produced steel. To compare the carbide size with 9Cr18MoV, the conventional version of this steel, we looked at edges of knives in both steels:

AR-RPM9 knife with relatively small carbides visible

9Cr18MoV knife with relatively large carbides visible

The carbides appear larger in the edge of the 9Cr18MoV knife when compared with the AR-RPM9. And the carbides in the AR-RPM9 look much too large to be powder metallurgy. So I think sprayform is the most likely production technology.

Changing the Name of AR-RPM9 to ASF7

We already discussed how the “9” should actually be a “7” in AR-RPM9, but let’s analyze the rest of the name: “Artisan-Rare Powdered Material 9 Element Steel.” We can leave the “Artisan” since that is the company selling the knives. The “rare” part I take some issue with, so we can drop that from the name. And I have discussed how “Powdered Material” is misleading so we can drop that. So we will rename the steel “Artisan Spray Form 7 Element Steel” or ASF7 for short. There, we fixed it. No more false advertising.

Properties of AR-RPM9

So now that we have discussed what AR-RPM9 likely is (and what it isn’t), what are the properties of the steel? Sprayform technology does improve toughness, though not to the extent of powder metallurgy. You can see an example with D2, sprayform D2 (PSF27), and powder metallurgy D2 (CPM-D2):

So we would expect a similar improvement over 9Cr18MoV, which would have similar toughness to 440C which I have already tested:

With 440C being around 6 ft-lbs at 59 Rc, we would expect AR-RPM9 to be in the 8-12 ft-lbs range. Nothing too spectacular but that’s what the main improvement would be over 9Cr18MoV. In terms of hardness, edge retention, and corrosion resistance, any differences would be minimal. You can read an article where I compared the edge retention of conventional and powder metallurgy 154CM to see why I think so (they performed the same). If you want to estimate the edge retention of AR-RPM9 vs other steels in my big CATRA chart you can look at 440C, or possibly a bit less.

Summary and Conclusions

AR-RPM9 is very poorly named, having only 7 elements and likely being made with sprayform but not powder metallurgy. It is essentially a sprayform version of 9Cr18MoV. Despite their claims, this steel isn’t “new” nor is it particularly exciting. The sprayform technology likely improves the toughness somewhat but wouldn’t impact much else.

The post AR-RPM9 Knife Steel is False Advertising appeared first on Knife Steel Nerds.

Thanks to Spencer Sandison for providing the Wootz steel for this new study! Thanks to Shawn Houston for performing the microscopy. And thanks to the Knife Steel Nerds Patreon supporters who allow me to do these studies through providing funds for all the experiments. All Patreon dollars go towards knife steel experiments.

YouTube Video

I have a video of the following content as well:

Wootz Background and History

The history of Wootz is almost as old as steel itself so we cannot do an exhaustive history of the material here. Unlike pattern-welded Damascus, Wootz gets its pattern through alloying and thermal cycling. It is not more than one steel forge welded together, but a single material. The history of its “rediscovery” is also very interesting. I previously reviewed Dr. John Verhoeven’s book Damascus Steel Swords: Solving the Mystery of How to Make Them, and the review also has a link to an interesting YouTube video on the same subject. I have also covered studies by Dr. Verhoeven in this article on Damascus myths, and studies on Wootz by Juha Perttula in this article. More about their studies is covered in my book Knife Engineering, and more on the history of Wootz and its rediscovery is in my book The Story of Knife Steel.

New Study on Wootz

Spencer Sandison Wootz knife

I worked with bladesmith Spencer Sandison on this study as he has made Wootz knives in the past. But most importantly he volunteered. Spencer uses electrolytic iron flake (high purity iron), pig iron, and ferroniobium. Pig iron is created during steelmaking when iron ore is smelted with coke, which introduces a very high carbon content. Pig iron is relatively high in impurities, however. The pig iron acts as the carbon source in Spencer’s Wootz. Dr. Verhoeven determined that a small amount of vanadium was in the ancient iron ore used in Wootz production. This carbide former allowed segregation to occur for the carbide banding after thermal cycling. Spencer, however, adds a small amount of niobium to act as a carbide former.

Evolution of carbide bands through thermal cycling by Verhoeven and Pendray

So Spencer adds those ingredients to a crucible and tops it with green glass to form a protective slag and a sealed lid to prevent oxidation. The crucible goes into the furnace to 2900°F and is held until melting, after which the crucible slowly cools and the steel solidifies. The steel is next held at high temperature called a “roast,” followed by forging down the steel. The thermal cycles the steel undergoes during the forging steps sets the final microstructure. It is important that the steel is not overheated, as that would dissolve all of the carbides. When they re-precipitate, they are evenly distributed again and would require more thermal cycles to re-segregate. This is one of the reasons that the carbon content of ancient Wootz is very high, 1.2-2.0%, averaging around 1.5%. Notice how in the iron-carbon phase diagram below, the temperature range of the “Austenite+Cementite” region gets wider as the carbon content goes up.

Carbides that are in the “rich” region of the steel (high in carbide formers) will coarsen during thermal cycling, while the lean region carbides will shrink with further cycles, leading to carbide bands. These rich and lean regions come from segregation during casting.

Schematic diagram of carbide coarsening by Verhoeven

Composition

I had the composition of Spencer’s Wootz measured with optical emission spectroscopy (OES) and Leco combustion. OES is a very good technique though its carbon and sulfur measurement is not as good as Leco because they are very light elements. We got the following result:

The carbon turned out well, with a value close to ancient blades. Spencer added a little bit of manganese for hardenability and for avoiding iron sulfides (manganese sulfides form instead). The phosphorus is high though that was also true of ancient Wootz. And of course that small niobium addition showed up in the final composition. Silicon was not intentionally added but pig iron typically has silicates in it (silicon oxides) which is the likely source of the silicon.

Heat Treating and Microscopy

This was actually the second Wootz blank that Spencer sent me, the first he did not add any manganese and I did not successfully harden the steel with his recommended heat treatment. Typically he uses 1425°F for 6 mintues with a quench in Parks 50. I copied this heat treatment and I got inconsistent hardness readings, some high and some low, which typically indicates that some pearlite formed during the quench. The toughness coupons confirmed this as they broke in a ductile manner indicating they were not fully martensitic. Unfortunately it was a small piece of steel and I used all of the steel with these coupons. The low austenitize and short soak may be working for Spencer’s knives due to the edge being relatively thin prior to heat treatment but did not work for heat treating toughness coupons.

I don’t normally austenitize from only 1425°F, especially with only a 6 minute austenitize. Using 1475°F and 10 minutes should help with issues of insufficient soaking and ensuring full austenitization. But we also decided we would have a small manganese addition with the second one. While the heat treating was more consistent than the original steel, it was clear that there was still some pearlite forming with the Parks 50 quench. I went as thin as I could on the steel for the toughness coupons (just oversize for the final dimensions 2.5mm thick) but it wasn’t happening. I was still getting some hardness values below 60 Rc after quenching. I had to use a water quench; finally I got consistent hardness after quenching. Even coupons with Parks 50 that seemed to be testing ok for hardness we could see black pearlite colonies in the fracture surface and the metallography:

Parts of the microstructure are visible in the fracture surface including carbides (white particles) and pearlite colonies (black)

Dark pearlite colonies are visible in the microstructure

With a water quench we finally avoided pearlite. Why was the steel so difficult to harden? One reason is the very low manganese (even lower in the original). Manganese is an element that increases “hardenability,” the rate required to quench to avoid pearlite or other transformation products that aren’t martensite. A very high hardenability steel will achieve full martensite even with an air cool. Hardenability is not a measure of how hard the steel will become, but how slowly you can cool and still achieve full martensite. Another factor is the high carbon content, which means the steel “wants” to form pearlite more, reducing hardenability. You can read more about hardenability in this article, and more about quenching oils vs water in this article.

Fracture surface of water-quenched Wootz

Hardness

With the water quench the hardness results were relatively predictable, I got around 67.5 Rc as-quenched, and after a 450°F temper it was around 60.5-61 Rc. We used a relatively high tempering temperature (while trying to avoid tempered martensite embrittlement) to give the Wootz a better chance in the toughness test.

Microstructure

Looking at the heat treated microstructure at medium magnification we can see the characteristic bands of carbide that gives Wootz its macroscopic pattern:

You can also see some grey elongated features in the steel which are impurities, also called inclusions. This steel has a relatively high content of them. One way of analyzing the impurities without the rest of the microstructure is to observe it as-polished rather than etched:

So now the impurities are visible as black features in the micrograph. There were enough of them that they could also be seen in the sharpened edge under magnification:

The likely source of the impurities was the pig iron which isn’t particularly clean material. Of course ancient blades also typically had relatively high impurity contents, and “backyard” steelmaking methods typically result in higher impurity content than commercial steel. However, one thing that Spencer noted was that he took a different approach to how he manipulated the “junk” material that results in the top of the ingot. Normally he forges that to one side of the billet and grinds it away. In this case he kept it in the center of the billet to try and grind it away later, but perhaps some of this was forged into the rest of the piece.

Toughness

I measured the toughness of the Wootz with my standard chapy impact test, and compared it with other low alloy and simple carbon steels:

When controlling for hardness, Wootz had the worst toughness of the low alloy steels. This was somewhat expected. This Wootz had a relatively high impurity content, which did not help. Juha Perttula looked at the effect of phosphorus content on Wootz steel [1] and as expected higher phosphorus leads to poorer toughness. However, this was not the only reason the Wootz tested somewhat poorly.

One issue is that the Wootz has a very high carbon content, resulting in a high carbide content and potentially higher carbon in solution. Higher carbon steels usually have lower toughness, as I have described in earlier articles. You can see this trend in the following chart:

However, the other thing working against the Wootz is the banded carbide structure, the structure that we want for the beautiful look that Wootz provides. In a prior study performed by metallurgist Juha Perttula [1], he compared a 0.75% carbon steel with a 1.58% carbon Wootz in two interesting conditions. In one he processed the steel to have an even distribution of carbides, and in the other he left it in the typical banded structure of Wootz. Of course the lower carbon 1075 had significantly better toughness. But even when comparing the two conditions of the Wootz steel, a uniform carbide distribution led to better toughness:

Edge Retention

I tested the Wootz in the CATRA test, which you can read about here. The Wootz did pretty well compared with other low alloy steels, though not as well as high wear resistance steels with high chromium and/or vanadium contents.

The Wootz approximately matched 52100, which is among the best of the low alloy steels thanks to its 1.5% chromium that increases the hardness of the cementite (iron carbide). This good performance is likely from the relatively high carbide content and the carbide banding. Banded microstructures have previously been found to help with edge retention in 8Cr13MoV as well as in pattern-welded Damascus. One thing to note is the 0.03% niboium is unlikely to have contributed to wear resistance as the volume fraction of niobium carbide is extremely small (as is the size of those carbides).

Dr. Verhoeven previously did his own CATRA comparison of Wootz, AEB-L, 52100, and 1086 steels [2]. This test was done along with knifemakers Howard Clark and Al Pendray. At 60 Rc the 52100 and 1086 both did better than the Wootz, but at 40 Rc the Wootz was better than the others at the same hardness. Dr. Verhoeven proposed that perhaps the carbides were not crossing the edge in a manner that would increase edge retention. Perhaps they tested a section that was light in carbides and I tested in a region that was rich in carbides. Another proposed reason is that in the past I have not found cementite (iron carbide) to significantly help in the CATRA test, likely because cementite is softer than the 5% silica in the CATRA cardstock used for testing. In fact I have found previously that more iron carbide can potentially be detrimental in the CATRA test:

52100, with its chromium enriched cementite, has carbide harder than the silica and does significantly better. So this may also partially explain the poor result of Wootz in Verhoeven’s study. However, that does not explain why the Wootz in my test did better. Perhaps there was poor sharpening of the Wootz blade for Verhoeven’s test, as in the 60 Rc test the Wootz blade was worse from the first cut, which was not the case in the 40 Rc test:

Verhoeven Wootz edge retention study with 60 Rc steels

The first cut is not purely a test of sharpness, as each “cut” is actually a stroke in one direction and then back again, so there is some edge wear even in “cut 1.” However, the difference shown above may be big enough to indicate a sharpening issue rather than inferior performance of the Wootz. This is also partially supported by the better result with the 40 Rc heat treatments:

In this test, comparing the (I) edge geometry (see the Verhoeven article for more details), the Damascus with the pearlite microstructure at 40 Rc labeled (P), it was outperforming from the first cut (back and forth stroke) and did the best over the course of the test. The 1086 was just below it and the 52100 was even below the 1086. I cannot think of any reasonable explanation for the poor performance of the 52100 at the lower hardness apart from poor sharpening. Or at least inconsistent sharpening.

Modern vs Ancient Wootz

A major reason for the comparison of Wootz performance at low hardness values by both Juha Perttula and John Verhoeven is that ancient blades that have been measured typically have a pearlite microstructure and therefore low hardness compared with quench and tempered blades. This was in part because quench and tempering was not yet a standard practice, and also because the low hardenability of the steel would make it difficult to achieve a martensitic microstructure anyway (see the heat treating section). So the edge retention and edge strength would be relatively low when compared with what we can achieve with modern blades with clean steel of lower carbon content heat treated to 56-62 Rc or so.

Another item for discussion is whether these modern recreations of Wootz match the properties of the ancient material. I believe the answer to that is yes, the microstructures analyzed by Verhoeven, Perttula, and others, are very similar. And compositions of “modern” Wootz often mimic the ancient blades, with high carbon content and few alloying additions. Whether the methods used to achieve the microstructures are exactly the same is somewhat irrelevant when compared with the final result.

However, there can be some differences with the ancient steel, such as the fact that Spencer Sandison used a small niobium addition while at least some of the ancient material seems to have relied on a very small amount of vanadium in their ore source. Spencer and some other smiths also rely on chromium or manganese additions to achieve a similar effect.

Definition of Wootz

And that brings up the question: What is Wootz? If there is a banded carbide structure that results in a pattern at the surface, is that Wootz? Does it only count if it has a similar composition to ancient blades? What if the composition is similar but it did not begin with a small crucible and ingot of steel? Sometimes people prefer to use the term “Crucible Damascus” for the material rather than Wootz. However, this does not automatically discount modern steel, as melting steel in crucibles was the most common method for making tool steel until well into the early 1900s.

Ed Fowler began seeing a banded structure in his 52100 from the many thermal cycles that he put the steel through, and he began calling his steel “52100 Wootz” [3]. The high chromium in 52100 also increases the size of the “austenite + cementite” region making 52100 better for developing carbide banding in a similar way to ancient Wootz. Can this 52100 properly be called “Wootz”?

Some knifemakers now etch their commercial steel knives because they develop carbide banding due to alloy segregation and carbides that are difficult to dissolve during forging. Therefore the banding is difficult to avoid in commercial steel, resulting in a visible pattern. Of course this also occurs in stainless steels. I don’t typically see this referred to as “Wootz,” but perhaps could be due called Wootz due to a similar result. This would make high carbon stainless steel the original “stainless Damascus.” Though perhaps we are straining too much at definitions.

Dan Bidinger knife in A2 tool steel lightly etched to highlight the carbide banding

Wootz vs Pattern-Welded Damascus and Modern Steels

This comparison is hard to make since Wootz can be multiple things as discussed above, and pattern-welded Damascus can be a combination of many different types of steel. I have an earlier study on pattern-welded Damascus here as well as a follow-up with Damasteel. In our pattern-welded Damascus study we tested combinations which had both better edge retention and toughness than the Wootz tested in this study. We had the benefit of being able to select any two (or more) steels that we wanted. Extra alloy additions allow us to have harder carbides that add more to wear resistance, and having better distributed carbides helps with toughness, as well as steels with less carbide overall. A modern powder metallurgy steel with a smaller amount of very hard carbides such as vanadium or niobium carbides, has much better wear resistance and toughness.

If we compare simple Wootz compositions with roughly similar ultra high carbon steels such as 26C3, the toughness of the 26C3 is much greater. The 26C3 has smaller, more evenly distributed carbides and lower impurities and thus better toughness.

Oil quenched Wootz within a carbide band

26C3 steel

However, the carbide bands appear to have helped the edge retention of Wootz when compared with 26C3, as its CATRA edge retention measured higher than the 26C3. The extra carbon and therefore carbide content of the Wootz likely also helped. However, if we use a looser interpretation of what Wootz is we could potentially use compositions that would have better overall properties, such as lower carbon with high chromium (ie 52100). In which case the question of Wootz performance becomes pretty big, with many potential compositions.

Summary and Conclusions

Ancient Wootz Damascus is a high carbon steel with carbide banding, resulting in a macroscopic pattern, especially when etched. The high carbon plus the banding helps with slicing edge retention but is bad for toughness. The low hardenability of simple Wootz compositions makes achieving full hardness difficult without a water quench. What counts as “Wootz” is not clear to me and the definitions are murky. Overall it was fun to test this ancient steel. Maybe in the future we can compare with other compositions like high chromium Wootz.

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[1] Perttula, Juha. “Wootz Damascus steel of ancient orient.” Scandinavian Journal of Metallurgy 33, no. 2 (2004): 92-97.

[2] Verhoeven, John D., Alfred H. Pendray, and Howard F. Clark. “Wear tests of steel knife blades.” Wear 265, no. 7-8 (2008): 1093-1099.

[3] https://www.edfowler.com/index.php/the-steel

 

 

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Thanks to Roman Kasé for donating the K888 and Niagara Specialty Metals for donating the CPM-1V. Thanks to my Patreon supporters for funding this study, those dollars went toward metallography and CATRA blade grinding. You can support knife steel research by visiting Patreon.com/KnifeSteelNerds

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History of Matrix Steels

Matrix high speed steels were developed in the 1960’s by VASCO. I previously wrote about historical and current matrix steels in this article. The original steels came out of improved techniques for measuring the composition of the “matrix” of the steel. Some of the carbon and alloying elements go toward forming carbides, what remains is in solution in the “matrix” of the steel. Carbides help with wear resistance but are detrimental to toughness, so the metallurgists decided to make high toughness steels by taking the measured matrix composition of the best high speed steels and using that composition instead. The first was a matrix version of M2 high speed steel called VASCO-MA, and they followed that up with Matrix II, a matrix version of M42 high speed steel.

Jumping ahead to the year 2002, Crucible released CPM-1V, which is a powder metallurgy version of VASCO-MA. The steel was released to offer the highest toughness of any of their available powder metallurgy steels, even CPM-3V. In 2005, Bohler released K890, advertised as a competitor to CPM-3V, it sort of looks like a cross between CPM-1V and CPM-3V, with elevated carbon, vanadium, and cobalt when compared with CPM-1V. K888 was released at the beginning of 2024, and it is essentially CPM-1V with a cobalt addition. Cobalt is added to many high speed steels for higher hardness (with a decrease in toughness). You can read more about the effects of cobalt on steel in this article.

One intriguing thing about K888 is that Bohler is advertising that it has higher toughness at 64 Rc than their prior K890 has at 62 Rc, and that K888 sees only a small drop in toughness between 62 and 64 Rc.

Bohler’s chart of toughness for K888

Toughness drops significantly with higher hardness so the claim that K888 maintains its high toughness at such a high hardness value is exciting. For example, in testing by Hitachi of their tool steels they found that above 62 Rc the toughness of their steels drop significantly:

Hitachi toughness chart for their steels

Experiments

So I was interested to compare K888 and CPM-1V to see the effect of the cobalt addition, and I had not yet tested CPM-1V so this was a good excuse. Knifemaker Roman Kasé was able to obtain a couple small pieces of K888 for me and I got CPM-1V from Niagara Specialty Metals. CPM-1V is also now available at Alpha Knife Supply. I tested the steel for hardness, toughness, and edge retention.

Microstructure

The big benefit of matrix steels is the lower volume of carbide. Carbides are brittle particles so the less carbide found in the steel the greater the toughness. Shawn Houston did the metallography for the two steels:

CPM-1V 2050°F austenitize – 3% carbide volume

K888 2050°F austenitize – 4% carbide volume

K888 has somewhat more carbide as expected, ThermoCalc predicts 1.15% MC (vanadium carbide) and 1.84% M6C (tungsten/molybdenum carbides) in K888; CPM-1V it predicts 0.87% MC and 1.34% M6C. But the big surprise is how much bigger the carbides are in the K888. We will see if this affects the measured toughness later in the article. I have some comparable grades below such as Z-Tuff and CPM-3V, two high toughness powder metallurgy steels, as well as the conventionally produced matrix steel Caldie. CPM-1V has the finest carbides of any powder metallurgy steel I have looked at so far. You can see even more micrographs here.

Z-Tuff

CPM-3V

Caldie

Hardness

Bohler shows that K888 can be heat treated as high as 64 Rc if you use the high end of the austenitizing temperature and temper within the recommended window (blue shaded region):

I made full tempering curves for both K888 and CPM-1V. I don’t typically do full tempering curves (it takes forever), but I thought it would be useful for comparing the effect of the slightly higher carbon in the K888 as well as the cobalt addition.

The hardness I measured for K888 is roughly equal to what was shown in Bohler’s datasheet. You can see that the potential hardness is higher for K888, which is due to the cobalt addition and slightly higher carbon. However, CPM-1V is still capable of relatively high hardness. To get a better feel for the two grades, I have a comparison between the two steels on one plot:

The K888 curve is for an austenitizing temperature of 2000°F and CPM-1V is 2050°F. With the lower carbon in CPM-1V it needed a higher austenitizing temperature to have equivalent as-quenched hardness, and here the CPM-1V was slightly higher as-quenched. However, K888 saw a bigger jump in hardness with a 300°F (150°C) temper than CPM-1V did. This is “precipitation strengthening” due to small carbides coming out during tempering. The same effect happens in the high temperature range around 900°F. You can read more about precipitation strengthening, also called secondary hardening, in this article on tempering. The higher hardness after tempering is then maintained across the tempering range in K888. This is a result of the cobalt addition, which is an alloy that slows the kinetics of tempering. This is what makes cobalt useful for high speed steels, which are designed to hold their hardness at high temperature.

Toughness

I used a few different combinations of austenitizing and tempering to compare CPM-1V and K888. For K888 I compared the low and high temperature tempering with 2050°F (1120°C) along with 1000°F (540°C) and 350°F (175°C). Based on the K888 tempering chart from Bohler, the 2050-1000 combination is presumably the one they used for the 64 Rc toughness datapoint, so it was an important one to test. I also tried a lower austenitizing temperature, 2000°F (1095°C) with 400°F (200°C). For CPM-1V I used all low temperature tempering with 2000-400, 2050-300, and 2050-350. The resulting hardness and toughness is below:

The first thing that sticks out is that the CPM-1V tested significantly better than the K888. The 2050-300 treatment led to 64 Rc and 22 ft-lbs, an excellent combination of hardness and toughness. 2050-350 led to 62.5 Rc with 36 ft-lbs, again an impressive combination. The 2000-400 heat treatment, however, did not lead to an improvement in toughness when compared with 2050-350 despite the lower hardness.

For K888 the 2050-1000 heat treatment, which is presumably the heat treatment used for the 64 Rc toughness datapoint in the datasheet, had the best toughness, but it was still below the result for CPM-1V at comparable hardness. The other interesting thing was that the low temperature tempering at 350-400°F did not seem to be high enough to give the steel good toughness. This may be from the cobalt addition delaying tempering.

K888 does not look particularly impressive on the chart, though it did manage to have somewhat higher toughness than CPM-CruWear at comparable hardness. The CPM-1V was again somewhat higher than K888 around 64 Rc, showing a clearer improvement in toughness at that high hardness. At 62.5 Rc, the CPM-1V continues to look impressive, having comparable toughness to high toughness steels like Caldie, CPM-3V, and A8 mod when they are at significantly lower hardness. Z-Tuff still remains alone at the top of the chart since the 2000-400 heat treatment of CPM-1V did not lead to an improvement in toughness. However, if the CPM-1V is extrapolated to lower hardness we would expect similar toughness to Z-Tuff:

It would be interesting to test other heat treatments to see if this extrapolation to lower hardness could be achieved. Such as a 2050°F austenitize with a higher tempering temperature such as 400-500 or 1000°F to see if the toughness could be further improved in the 60-62 Rc range. Because the low tempering range was not successful for K888, modified heat treatments for lower hardness would explore lower austenitizing temperatures with the same 1000°F, or increased tempering temperatures beyond 1000°F (540°C). Presumably the toughness would remain below CPM-1V due to the larger carbides. Alternatively it would also be fun to test Z-Tuff with a lower tempering temperature of 300-350°F and see what its toughness is like at higher hardness. So far I have only tested it after tempering at 400°F.

To show how good the toughness of CPM-1V is compared to low alloy steels and stainless steels I created charts with 1V overlaid on those:

Edge Retention

I tested one CATRA knife each, Shawn Houston ground the blades after I heat treated them. Each were heat treated using an austenitizing temperature of 2050°F (1120°C) and a tempering temperature of 350°F (175°C). This resulted in 63.1 Rc for CPM-1V and 63.9 Rc for K888.

As expected the two steels are near the bottom of the chart, following the approximate edge retention of AEB-L and ApexUltra. If compensating for hardness, the two steels also approximately match CD#1 (nearly identical to Z-Tuff) and A8 Mod. So I think overall the two steels did about as well as could be hoped for high toughness powder metallurgy steels with low carbide volume and only 1% vanadium.

Summary and Conclusions

These are two matrix high speed steels based on the original matrix steel VASCO-MA, though K888 has slightly higher carbon plus a cobalt addition. The cobalt addition led to somewhat lower toughness in the K888 when compared with CPM-1V. The cobalt did lead to more tempering resistance and higher hardness, though that tempering resistance also affected its toughness, especially in the low temperature tempering range (350-400°F). CPM-1V has an excellent combination of hardness and toughness in the 62-64 Rc range, though it may be possible to achieve higher toughness in the 60-62 Rc range if we try higher tempering temperatures. K888 and CPM-1V have similar edge retention, close to AEB-L, ApexUltra, and Z-Tuff/CD#1. Overall after testing these steels CPM-1V is my preference for a steel that maintains very good toughness at high hardness. I don’t know of any knife supply companies offering K888 yet, and is apparently difficult to obtain from Bohler right now, though CPM-1V is currently available at Alpha Knife Supply.

 

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The following is a collaboration project between myself and Malachi Chou-Green. He obtained the Vanadis 60, did some heat treating experiments, and also did metallography including some fancy electron microscopy. He also heat treated and machined the CATRA knife that I tested. His original experiments were published some time ago on his Patreon, so if you want to see what he is up to I recommend visiting his Patreon page and becoming a supporter.

Video

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Background of Super High Speed Steels and Vanadis 60

High speed steels with greater than 2% vanadium for wear resistance were developed in the late 1930s  and early 1940s leading to M4 high speed steel and T15 high speed steel. I have written a short article on the history of M4 steel, and my recent book The Story of Knife Steel has a more etensive history if you are interested in reading further. Previously vanadium was found to lead to very hard carbides for very high wear resistance but when the additions exceed ~2% the steel would not harden. The breakthrough in the 1930s was that they learned that more carbon was necessary in conjunction with the vanadium, to compensate for the carbon that was being tied up in the vanadium carbides. However, vanadium was still limited to about 4-5%, otherwise the carbides would become too large and the steel would fracture during forging and/or rolling.

Another development occurred in the early 1960s when it was discovered that greater control over carbon content and cobalt addition could be used to make high speed steels that could be heat treated to 70 Rc. Several steels were patented within only a few years by different companies, with the most common being VASCO Hypercut patented in 1963, given the standard designation M42. You can read more about the history of these steels in this article. The compositions of these steels are shown in bold, and the older lower carbon versions of each are shown below each of the bold compositions in the “M40” series.

Around the same time Crucible developed powder metallurgy technology which was capable of creating finer microstructure in tool steels by avoiding the slow solidification associated with conventionally cast alloys. They began producing these steels for commercial scale in 1970. You can read more about how powder metallurgy works and its history in this article. At the beginning Crucible used this technology to improve performance of existing steels like M2 or T15, but they also began designing steels specifically for the technology. The first of these was CPM Rex 76, patented in 1972, which was a high hardness high speed steel with 3% vanadium.

Shortly after Crucible introduced their powder metallurgy technology, Stora in Sweden introduced their own version of the technology. This led to a legal battle that Crucible would eventually win, requiring Stora and Uddeholm to pay fines. Stora developed a higher vanadium 70 Rc high speed steel called ASP 60 which was introduced by 1975. It had 6.5% vanadium for higher wear resistance than Rex 76. Uddeholm purchased Stora in 1976. In 1978 Crucible published results of their experiments [1] comparing Rex 76 and ASP 60 which they claimed showed that higher than 3% vanadium did not lead to better performance in high speed tools. They also made a 6% vanadium version of Rex 76 and they demonstrated that it had no better tool life than the standard 3% vanadium version.

J.H.G. Stake of Uddeholm was not happy with these claims. He said [1], “I represent the firm that sells the steel mentioned which is available commercially, the ASP steels by the ASEA-STORA process. It is very difficult to comment on the slides shown here regarding the comparison between ASP 60, Rex 76, and Rex 76 with 6% vanadium. The main reason is that the trials have been done without any coolant and that is very unusual in commercial application in general practice. We prefer to go to the toolmaking industry and have the trials made there.”

Uddeholm would eventually begin selling ASP 60 under the name Vanadis 60 instead, and Erasteel which also had historical connections to Stora, would sell it under the name ASP 2060. Both have the same composition. Despite Crucible’s earlier claims about 3% vanadium being as much as is necessary, they would later release higher vanadium versions of Rex 76. The 9.5% vanadium version called Rex 121 was released in 1998 [2]. The 5% vanadium version is known as Rex 86 (Zapp sells it as Z-Max) which was released in 2005 [2]. Other high vanadium 70 Rc powder metallurgy high speed steels would be released by other companies, many of which are shown below:

For more exciting knife steel and tool steel history, plus a history of the modern knife industry including custom knives, high-end production knives, and Damascus, see my book The Story of Knife Steel.

Composition and Microstructure

As mentioned above ASP 2060 (Erasteel) and Vanadis 60 (Uddeholm) are identical in composition. They have molybdenum and tungsten (and to a lesser extent chromium adn vanadium) added for “hot hardness,” the ability for a steel to maintain its hardness at high temperatures. This is important for machining operations where the tools are run at “high speed” and thus heat up due to friction. Hot hardness is created through a phenomenon called “secondary hardening” where the precipitation of very tiny carbides causes the steel to further harden when tempering at high temperatures, which you can read about in this article. The vanadium content of Vanadis 60 is relatively high at 6.5% for wear resistance. Cobalt is added to further improve hot hardness by affecting how the carbides form in secondary hardening (as opposed to forming carbides such as molybdenum and tungsten). You can read more about cobalt in this article.

The toughness and wear resistance of high speed steels is largely controlled by the carbides. Where increasing carbide volumes and sizes tend to decrease toughness, increased volumes of harder carbides increase wear resistance. Each of the above mentioned steels forms some M6C carbides (tungsten/molybdenum carbides) and MC carbides (vanadium carbides). With this in mind, vanadium carbides are generally more desirable because they are harder and smaller than the tungsten/molybdenum carbides. Therefore the vanadium carbides are less detrimental for toughness while also contributing more to wear resistance.

Below I have calculated carbide contents using Thermo-Calc for Vanadis 60, Rex 86, Maxamet, and Rex 121. Importantly these are “equilibrium” calculations, meaning they assume an infinite hold time. Each is calculated for the austenitizing temperature that was used for the CATRA knife coupons I tested.

So based on total carbide volume alone we would expect Maxamet to have the highest toughness (it has the lowest carbide volume), followed by Rex 86, Vanadis 60, and finally Rex 121. Maxamet and Vanadis 60 have similar MC content but Vanadis 60 has higher M6C, so we would expect Vanadis 60 to have higher wear resistance than Rex 76, Rex 86, and Maxamet, but less than Rex 121.

Below are the micrographs for these steels to compare with Malachi’s Vanadis 60 micrograph.

Maxamet austenitized from 1975°F

Z-Max/Rex 86 austenitized from 2150°F

Rex 121 austenitized from 1925°F

Looking at the micrographs above, the carbide volume of Vanadis 60 appears to be similar to that of Maxamet, but the carbides are a bit smaller. Z-Max has a similar carbide size to Maxamet but a lower carbide volume. Rex 121 has the most carbide and the largest carbides as expected based on its composition.

We measured the carbide volumes of each steel using a simple method called “point counting” to compare with the Thermo-Calc estimates. The total carbide volume measured was significantly lower in Vanadis 60 and Rex 86/Z-Max than the equilibrium estimate. This is relatively unusual as equilibrium carbide volume estimates ten to underpredict the true carbide volume because it assumes an infinite hold time at austenitization temperature. Maxamet and Rex 121, both had carbide volume a few points above the Thermo-Calc equilibrium estimate which is more typical.

Malachi also took scanning electron microscope (SEM) micrographs of Vanadis 60 austenitized at 1900 and 2125˚F using the “backscatter” imaging technique where contrast is partially generated from composition differences within the sample. Heavier elements scatter more electrons back into the detector leading to bright areas. Thus vanadium carbides show up as grey while the tungsten/molybdenum carbides are white, due to the relative atomic weights of the carbide forming elements (V < Mo < W).

Vanadis 60 austenitized from 1900°F

Vanadis 60 austenitized from 2125°F

Using different colors he point counted the volume of M6C, and total carbides and compared them against the Thermo-Calc predictions. The measured M6C (tungsten/molybdenum) carbide volume was lower than Thermo-Calc, while the measured MC (vanadium) carbide volume was high. So it appears that Thermo-Calc is overpredicting M6C carbide and underpredicting MC carbide stability in Vanadis 60. Perhaps similar differences are leading to the overprediction of carbide volume in the Z-Max, and imply that the MC carbide volume of Z-Max may also be greater then Thermo-Calc predicts. This could partially explain why Z-Max preformed better in CATRA testing than my equation suggested it would.

Hardness and Heat Treating of Test Coupons

The ASP 2060 datasheet has a better heat treatment hardness chart than the Vanadis 60 datasheet so I have included it below:

Malachi reported that he got 70.5 Rc with 2125-1000°F (1160-540°C) and 68.8 Rc with 1900-1000 (1040-540°C). He also austenitized the CATRA coupon from 2025°F (1100°C) which I tempered at 1000°F (540°C) for a hardness of 68.4 Rc. Malachi reported to me he later measured the temperature of the furnace he was using and found it to be reading too high, (thus the furnace temperature was lower than 2025°F), which may explain why the hardness was lower than his previous 1900-1000 coupon. I also heat treated toughness coupons using 2025-1000°F and the resulting hardness was 69.0 Rc. As expected Vanadis 60 is capable of very high hardness.

Edge Retention

To test the edge retention, Malachi heat treated and machined the CATRA knife for testing. I performed the final sharpening and tested for slicing edge retention:

Somewhat surprisingly the resulting edge retention was only slightly higher than Z-Max after compensating for hardness, and was significantly below Maxamet. This was puzzling because Maxamet and Vanadis 60 have similar vanadium and carbon, while Z-Max is somewhat lower in both elements.

Based on our carbide volumes measurements, both the Maxamet and Vanadis 60 CATRA knives should have carbide volumes around 22%. However, as discussed previously Thermo-Calc seems to underpredict vanadium carbide and overpredict tungsten/molybdenum carbides in Vanadis 60 so perhaps the same thing is happening in Maxamet. This could be compounded by the difference in “tungsten equivalent” contents between the steels. Tungsten equivalent is calculated by multiplying the molybdenum content by two and adding it with the tungsten content, to account for the difference in atomic weight between the two. Since tungsten and molybdenum form M6C carbide, steels with higher tungsten equivalent contents are more prone to M6C carbide formation. The tungsten equivalent of Maxamet is 13% and Vanadis 60 is 20.5%, so perhaps the structure of Maxamet is mostly MC type carbides. We would need to do backscatter imaging with the Maxamet to confirm if there is indeed more MC carbide in the Maxamet than Thermo-Calc predicts.

Toughness

I heat treated and machined the Vanadis 60 charpy toughness coupons using the same 2025-1000°F heat treatment as the CATRA coupon, resulting in 69 Rc:

As expected by the very high hardness (69 Rc) and the relatively high carbide volume (~22%), the toughness was fairly low at 2.7 ft-lbs. Zooming in on the high hardness steels I added a trendline showing the approximate change in toughness with hardness for CPM T15, CPM Rex 45, CPM Rex 76, and Z-Max. Vanadis 60 looks like it has toughness a notch down from those steels while being more in line with Maxamet. This makes sense given that Maxamet has about the same carbide volume while those other steels have have lower carbide volumes ranging from 14-18%. Carbides are very hard particles and so they are detrimental to toughness. You can also see that as hardness goes up the difference between the highest and lowest toughness steels decreases. So at 61.5 Rc we have measured values all the way from ~4 ft-lbs to 45 ft-lbs. But at 67 Rc it only ranges from 2.6 to 5.9 ft-lbs. Thus it seems the higher the hardness the less influence the carbides have on the measured toughness.

One interesting thing to note is that the spread in toughness was particularly high for the Vanadis 60 coupons, coming out at 3.4, 2.1, and 2.6. This spread is especially apparent when we look at the 95% confidence intervals of the toughness measurements of other high hardness steels shown below. I am not sure what led to the variation, perhaps it was merely chance.

While it is hard to predict toughness trends, it seems that at the same hardness we would expect Vanadis 60 to have toughness between that of Z-Max and Maxamet, leaning more towards the Maxamet end of the spectrum. As mentioned above, the Vanadis 60 and Maxamet samples tested have similar carbide volumes but Vanadis 60 has finer carbides. Usually, we would expect finer carbides to lead to better toughness, however in this case the improvement seems somewhere between quite small and non-existent. Toughness in steels is determined by two factors, crack initiation and crack growth. In tool steels, the ductility is low enough that usually, crack initiation is the controlling factor, in these cases smaller carbides are harder for cracks to initiate on, leading to better toughness. However, crack growth is generally easier with a smaller interparticle distance (the distance between carbides). So while crack initiation is usually the controlling factor, maybe as the carbide volume increases, improvements to toughness caused by finer carbides are mitigated by their risk of increasing crack propagation.

Summary and Conclusions

Our testing of Vanadis 60 found toughness about where you would expect given its high carbide volume and very high hardness, and as expected it is capable of very high hardness and has excellent wear resistance. However, the slicing-edge retention was lower than we predicted based on the composition, testing more similarly (but still a bit higher) than Z-Max rather than the more compositionally similar Maxamet. We also explored the interesting history of Vanadis 60; while several other 5%+ vanadium 70 Rc powder metallurgy steels have come since then, Vanadis 60 was the first.

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[1] Kasak, A., and E. J. Dulis. “Powder-metallurgy tool steels.” Powder Metallurgy 21, no. 2 (1978): 114-123.

[2] https://www.crucible.com/Products.aspx?c=7

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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.

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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

The post Testing Chinese Knife Steel 8Cr13MoV/8Cr14MoV appeared first on Knife Steel Nerds.

Thank you to all the new Knife Steel Nerds Patreon supporters! Your support allows me to do new exciting experiments on knife steel. Some exciting ones coming up in the coming months!

Video

I also have a video which covers the same information (more or less) in this article.

Intro

Jim Sutton of Zapp Tooling Alloys contacted me recently to talk about testing their Z-Wear. Zapp has previously purchased their powder steel from a USA company, and they are adding a European steel company as another source. They wanted to compare the properties between the two suppliers to ensure they are equivalent, and/or to determine if one is better than another. Some companies have claimed that their powder metallurgy process is better than other companies, notably Bohler and Uddeholm (of the parent company voestalpine) claim their process is “3rd generation” for a finer powder size and lower “inclusions” in the steel [1]. I previously compared the inclusion content and properties of Bohler, Uddeholm, Crucible, and Carpenter powder metallurgy steels in this article. Bohler-Uddeholm has previously claimed that Erasteel uses “2nd generation” technology, though Erasteel has since made improvements to their process which they call Dvalin which makes similar claims to reduction in inclusions to Bohler-Uddeholm:

Common impurities in steel include sulfur (S), phosphorus (P), and oxygen (O). My testing found P and S to be similar among the manufacturers, but oxygen content to be somewhat lower for Bohler and Uddeholm, Carpenter to be the worst, and Crucible in the middle:

That prior article also has micrographs of polished specimens of each where you can look at the impurities themselves. None of them looked particularly “clean” but you can look at the micrographs and decide for yourself.

However, these differences did not lead to a difference in measured toughness, as M390 and 20CV had the same toughness:

As a side note, I found this austenitizing temperature of 2140°F to be too high leading to inflated toughness, which you can read about here. Also the reported finer powder size did not lead to an obvious difference in carbide size:

M390

20CV

A much bigger effect than powder size is carbide type. The similar CPM-4V (Crucible) and Vanadis 4 Extra (Uddeholm) have only vanadium carbides, and their average carbide size is much smaller than either M390 or 20CV:

Vanadis 4 Extra

CPM-4V

And when testing the toughness of those two grades there was no advantage to the Vanadis 4 Extra. In fact the CPM-4V had greater toughness in the one condition that was the same between the two (1975°F austenitize, 400°F temper, transverse testing direction). However, this was due to small composition differences between the two grades. The CPM-4V had lower carbon so resulted in slightly lower hardness but better toughness.

The only comparison between manufacturers involving Erasteel I have performed was of RWL34 (Erasteel) and CPM-154 (Crucible) which are basically identical. In this case the Erasteel had slightly higher toughness. This was part of a study of Damasteel stainless Damascus which you can read here.

However, all of these tests were done with my normal average of 3 or 4 toughness coupons. Perhaps the difference in toughness would only show up with more coupons tested, looking for one-off coupons with a large oxygen inclusions, so I suggested to Zapp that we test 9 coupons from each manufacturer and determine if there were any clear differences between the two.

Z-Wear

I previously tested Z-Wear, which you can read here. That study was done with knifemaker Warren Krywko, and we found a few key things:

1. A “low” temper of 400°F led to a better balance of hardness and toughness than a “high” temper of 1000°F.

2. Cryo led to an increase in hardness with little or no change to toughness. Increased cryo time beyond 1hr did not affect the properties.

3. The powder metallurgy versions, Z-Wear and CPM-CruWear, have significantly better toughness than the conventional version, CruWear. This is due to the finer microstructure from powder metallurgy.

CruWear (ingot, not the CPM version)

Z-Wear (Powder Metallurgy)

4. When tempering at 400°F, there was no change in properties with more than 2 tempers:

New Z-Wear Testing

So the new test used a 1950°F austenitize for 30 minutes, plate quench, cryo in liquid nitrogen, and a double temper at 400°F for two hours each time. 9 coupons were produced and tested from each of the two manufacturers.

The USA version ended up 1 Rc harder but one ft-lb lower in toughness. I didn’t get any one-off low toughness coupons that would suggest that one version or the other has large carbides or inclusions leading to lower toughness. The standard deviation of the toughness of the European version was slightly higher, but that could just be random, plus higher toughness coupons tend to have a bigger spread than low toughness coupons, so I wouldn’t read anything into that. To determine the reason for the higher hardness of the USA version I measured the composition of the two using OES (optical emission spectroscopy) along with combustion. Combustion tested carbon, sulfur, nitrogen, and oxygen. OES cannot test oxygen, and the carbon and sulfur values from combustion is somewhat better than OES, so both are presented in the table (listed as LECO C or LECO S at the bottom).

The LECO carbon values were a little lower than the OES, but both tests showed the USA version having higher carbon, which is probably the primary reason that steel came out with higher hardness. The P and S content were similar between the two, phosphorus a little lower in the USA version but sulfur a little lower in the European version. The oxygen content of the European version was similar to the Bohler and Uddeholm stainless steels I previously tested. However, somewhat surprisingly the oxygen content of the USA version was the lowest result I have yet had for any PM steel. Perhaps something about the composition of the stainless steels led to somewhat higher oxygen content such as the high chromium content. I am not sure if the USA oxygen content was so low in this case due to variability between heats of steel or some improvement to their process since my last test. Either way I think the results of both of these steels look good.

Because the hardness was different between the steels I wanted to compare against the previous Z-Wear results to see the “normalized” toughness for a given hardness. To see if 1 ft-lb toughness is as much improvement as we would expect based on roughly 1Rc lower hardness. So I plotted them all on the same chart:

The new USA material tests roughly fall on the same trendline as the previous Z-Wear testing, while the new European material falls a bit below the trendline. I believe this is likely due to the small composition differences, or perhaps a difference in annealing (resulting in a different response to the same heat treatment). I would consider these materials to be basically equivalent.

Summary and Conclusions

While there has been significant marketing from the European powder metallurgy companies that they have superior powder metallurgy technology, I have not found a difference when it comes to impact toughness testing. Whether other types of tests would reveal a difference I can’t say, but in general I think performance-wise the composition of the steel matters more than the company making the powder.

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[1] Tornberg, C., and A. Fölzer. “New optimised manufacturing route for PM tool steels and High Speed Steels.” In Proceedings of the 6th International Tooling Conference: The Use of Tool Steels: Experience and Research, vol. 1, pp. 10-13. Karlstadt Sweden, 2002.

The post USA vs Europe – Powder Metallurgy Knife Steels appeared first on Knife Steel Nerds.

I drove my family all the way out to Bethlehem, PA to film a video about steel history so I hope you watch it. Read about my knife steel history book here: https://knifesteelnerds.com/2023/05/09/new-book-the-story-of-knife-steel-innovators-behind-modern-damascus-and-super-steels/

The post Innovators that Changed Knife Steel appeared first on Knife Steel Nerds.

New Q&A video with questions from Patreon supporters. Go to Patreon.com/KnifeSteelNerds if you want to support knife steel research.

00:00 Ultra high toughness with ferrite/martensite steels?

05:09 Normalize 80CrV2 with stock removal?

09:18 How much forging to eliminate carbide stringers?

11:16 Will CruForgeV ever come back?

12:25 What is the most important property for knife steel?

15:04 Different CATRA test for cooking knives?

17:22 Does bainite have a place in knives?

20:22 Austenitizing soak time?

23:40 High temperature tempering is bad?

 

The post Most Important Property for Knife Steel? Q&A appeared first on Knife Steel Nerds.

You can help support more knife steel research like this by going to Patreon.com/KnifeSteelNerds

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.

Thanks to the new Knife Steel Nerds Patreon supporters! The newly performed research in this article is all paid for through Patreon funds, such as the purchase of expensive Damasteel. Patreon money is only used for knife steel research. Sara Rempelos, Theo N, Christopher Kuehl, Dirk Hoogenbosch, Nick, nats spawnee, Krisztián Szegi, Todd Ellner, Seth V, Jonathan Graham, Ben Secrist, Drew Certain, Scott Armstuts, Ronald Justin Agee, Jim Darnall, Chris G, Farrell K., Bill Behnke, Paige, Luis Hernandez, clockworkfish, Barry Gordon, KarRawr, Ming Lin, Theodore Loach, Matt Bobchin, Flo, J.T. Pouland, Mark Watt, 愷麒 王, and Nickolay Matershev.

Video

Here is the video version of the following information:

Damasteel History

Damasteel was developed and patented by Pelle Billgren and Kay Embretsen [1]. At the time, Billgren worked for the steel company Erasteel in Sweden and he was looking for new uses of their powder metallurgy steel. He connected with bladesmith and Damascus steel maker Embretsen and they worked together to make what eventually was branded as Damasteel. The original material they patented was a combination of Elmax steel and 304L, but by the time it was advertised in the USA in 1996 it had their final combination of RWL34 and PMC27. Damasteel was unique in that it was two different powders which were layered up with no “solid” sheet component. RWL34 is the same composition as ATS34/154CM, and it the name is a tribute to Robert Waldorf “Bob” Loveless who famously introduced those steels to the knife industry. PMC27 is a powder metallurgy version of Sandvik’s 12C27. You can read more about the history of Damasteel, Kay Embretsen, Damascus steels, and knife steel in general in my new book The Story of Knife Steel: Innovators Behind Modern Damascus and Super Steels. Around 2016, the RWL34/PMC27 combination was renamed DS93X. They also have low carbon austenitic stainless products for non-knife applications, and “san-mai” products with a core steel called Damacore. I previously wrote about Damacore in this article. The rest of this article focuses on the DS93X product with RWL34 and PMC27.

Composition and Microstructure

As shown on the chart, the two steels, RWL34 and PMC27, are very similar to their inspirations, ATS34 and 12C27. I get mixed information on whether 154CM or CPM-154 have a small vanadium addition, it has a very limited effect on properties, as there is so much chromium carbide in RWL34 that those act to keep the grain size small. The high chromium content also prevents significant content of vanadium carbides from forming; instead, the vanadium is found in the chromium carbides. There also are no molybdenum carbides to be found (Mo2C or Mo6C). Instead the molybdenum is found in the chromium carbides (M7C3 or M23C6, where M can be chromium, molybdenum, etc.).

RWL34 is the “bright” layer and PMC27 is the “dark” layer in the micrograph above. Because Damasteel is constructured with layering of powder, there can be somewhat diffuse transitions between the two steels, which on a small scale are not as sharp as in pattern-welded Damascus made with solid sheet steels. This does not appear to have any deleterious effect on the “macro” pattern observed in the final material.

12C27 has a very find microstructure without powder metallurgy due to careful composition design and processing. You can read more about how those steels were designed in this article on AEB-L. With powder metallurgy, the PMC27 remains very fine with very small carbides (the bottom material in the micrograph above). RWL34 has much more carbide than PMC27 (roughly 16 vs 4%) with its higher carbon and alloy content but the carbides remain relatively fine due to the powder metallurgy process. You can compare with the microstructure of many other knife steels in this article with many micrographs.

Heat Treatment

There are a surprising number of things to talk about with Damasteel heat treatment, and I have gotten quite a few questions about the datasheet. Until recently the (old) datasheet recommended an 8 minute hold time for 3.2 mm (1/8″), with an increase or decrease of one minute per 0.5 mm. However, a new datasheet was released this year (2023) which recommends a 15 minute hold time instead. The 15 minute hold is more in line with my recommendations.

Heat treatment instructions from the current Damasteel datasheet.

Adjusting by the minute based on thickness makes sense when using a conveyer belt furnace, but for conventional furnaces this is riding the line too close where different sizes of knives (even at the same thickness) can affect the time for the steel to heat up to temperature. Some knifemakers have tried to ride this line in hopes of a finer grain size; however, grain size is much more greatly controlled by temperature than by time. Instead it is more important that the steel sees enough time, to ensure that carbides are dissolved, sufficient carbon is in solution for hardness, and sufficient chromium is in solution for corrosion resistance. The danger of an insufficient soak is much worse, leading to poor hardness, toughness, and corrosion resistance. While the chances of an overly long soak time are quite low. The changes to the steel in terms of transformation and carbides dissolving will “level off” after a certain amount of time and then changes are very slow after that. So I recommend that the soak is “long enough” rather than trying to make it as short as possible.

Example of change in carbon and chromium in solution with austenitizing time for 52100. With short soak times the changes are great, while after sufficient soaking the behavior is more flat.

Another change between the old and new datasheet is the cold treatment recommendation. The old datasheet said to use a cold treatment for heat treatments IV and V, while the new datasheet does not include this instruction, and says only that “deep freezing is not necessary but completes the martensite transformation and increases hardness.” I do not recommend skipping a cold treatment when using the 1080°C/1975°F austenitze, for reasons I will explain shortly. I recommend sticking to 1050°C/1925°F if only quenching to room temperature.

Hardness values are listed for RWL34 and PMC27 for each of the five heat treatment conditions but it does not specify whether this assumes a cold treatment or a room temperature quench. Whether the cold treatment is included has a strong effect on these final values. Another interesting aspect is that rather than giving a final hardness for the “DS93X” Damasteel combination of the two steels, instead it gives hardness values separately for RWL34 and PMC27, presumably so that the knifemaker can select a heat treatment that works for both alloys. Also the PMC27 look rather low when compared with the data on 12C27 from Sandvik [2][3] as shown on the chart I created below. The RWL34 values are much more in line with reported heat treatment data from Hitachi ATS-34, Latrobe 14-4CrMo, and my own experiments with CPM-154.

Tempering chart for Damasteel from their datasheet.

Comparison of Sandvik 12C27 and Damasteel PMC27 heat treatment data.

In my heat treatment experiments I used the same 15 minute austenitize specified in the current datasheet, a plate quench, and tested either with a room temperature quench (no cryo), or a dip in liquid nitrogen (cryo). Rockwell hardness is affected by both steels, and would not only reflect the harder of the two steels. However, when comparing the resulting “composite” hardness of the Damasteel product, the values are basically the same as RWL34 cryo heat treatments and are not similar to PMC27. For example, in a combination of ~62 Rc 1095 and very soft nickel (approximately 8%), the composite hardness value was only ~51.5 Rc. You can read more in the previous article I wrote on experiments of many Damascus steels.

Note also that the hardness drops without cryo above 1925°F, which is why I recommend limiting the austenitize to 1925°F when cryo is not available. This is in line with other heat treatment data from Hitachi and Latrobe on ATS-34, as shown below. When “overaustenitizing,” the hardness drops from excessive retained austenite. Cryo can transform some or all of this retained austenite so that hardness will climb with even higher austenitizing temperatures. Excess retained austenite, even if the final hardness is acceptable (ie 60 Rc), will lead to poor performance, as the retained austenite is very soft and will lead to deformation at lower stresses than would be seen at the same hardness without the retained austenite. The steel will deform as if it is at a significantly lower hardness. You can read more in this article on cryo.

So why does the steel heat treat like RWL34 if roughly half of the steel is PMC27? The reason is because the two steels do not behave the same as when the two are heat treated separately. When combining two steels in Damascus, contrast is achieved after etching because the alloy contents are different, in this case the biggest difference is in the 4% Mo in the RWL34 which makes that steel resist etching better than the PMC27 which creates the bright layer. However, carbon is a very small “interstitial” element (between the iron atoms), which means it diffuses very rapidly. During the forge welding and forging process the carbon equalizes between the two steels, changing the heat treatment behavior of the two steels. It is slightly more complicated than this because some carbide will still be present in the CPM-154 even at a high forging temperatures such as 2150°F. So the carbon that will equalize will be the carbon “in solution” for the two steels at the forge welding and forging temperature. Using ThermoCalc for that temperature I estimate that RWL34 loses about 0.1% carbon while PMC27 gains about the same amount, resulting in the RWL34 having 0.95% carbon and PMC27 having 0.7% carbon. In this case, the resulting “carbon in solution” of the two steels during austenitizing ends up being very similar to the original RWL34, and thus the hardness values of PMC27 on its own are basically irrelevant to the heat treatment. This would make the PMC27 roughly the same composition as AEB-L, which also has its peak austenitizing temperature at 1925°F without cryo and 1975-2000°F with cryo (see this article). Instead it would be better if the datasheet just showed the resulting hardness data for heat treating the “DS93X” combination of the two steels. Below I have shown a heat treatment summary from my own experiments:

So with 1/8″ material, a 15 minute austenitize works well, thicker material may necessitate longer hold times. Using 1900-1925°F is good if not using cryo, with cryo you can austenitize as high as about 2000°F for higher hardness. Temper to desired hardness.

Edge Retention

The edge retention of Damasteel compared with other Damascus patterns was previously shown in this article on Damascus performance. The Damasteel did very well, comparable to “mid-tier” wear resistance steels such as CPM-3V or CPM-154. We used a heat treatment of 1950°F for 15 minutes, plate quench, liquid nitrogen cryo, and temper at 400°F. Read more about CATRA edge retention testing in this article.

High alloy Damascus combinations tested for edge retention with CATRA

A giant chart of other steels to compare against

The edge retention was tested with the “Hugin” pattern which is essentially a “ladder” pattern which puts a wave into the steel. We found ladder patterned Damascus to have better edge retention than “random” straight layers as shown on the ApexUltra/L6 chart.

Damasteel “Hugin” pattern CATRA coupon

The wavy appearance created in the “edge” of a ladder pattern bar (this is not Damasteel)

ApexUltra/L6 Damascus showed better edge retention with a ladder pattern

However, you will notice that the layers of the Hugin pattern flatten out as they approach the edge, which is not because the patterning doesn’t reach the center of the bar, but rather because of the shape of the initial billet which are layered with powder prior to forging flat (notice how the layers also flatten out as they approach the spine of the test knife). This results in layers parallel to the edges while looking more typical of layered steel in the center. Thus we were somewhat surprised that the edge retention was identical to the ladder patterned steel we tested of AEB-L/154CM, which are nearly identical steels. Could the Damasteel have even better edge retention if the layers “crisscrossed” the edge like traditional pattern-welded Damascus? To test this we made a new coupon using a “dense twist” pattern which does have layers crossing the edge with a higher density.

Damasteel “dense twist” CATRA coupon

Damasteel “dense twist” cross-section

However, when I tested the dense twist the result was largely the same as Hugin. If there was a true increase in performance it is very small. In either case, the edge retention of the Damasteel is very close to CPM-154 (the same as RWL34), despite the presence of a large amount of PMC27 with much less wear resistance (similar to AEB-L on the chart below). Therefore the ladder patterning leads to a significant boost in edge retention.

Thus it appears that the effect of the layers is minimal, and most of the effect comes from the change in shape of the microstructure instead. There is still directionality to the microstructure, especially carbide banding, which would have its shape changed through the ladder patterning. Below is an image of carbide banding:

Carbide banding in high speed steel (rolling direction is vertical) [4]

This carbide banding theory could be confirmed by testing CATRA edge retention of a single steel which was given ladder patterning. We effectively did this test already with the 3125 layer AEB-L/154CM where layers were no longer visible. We also tested layer counts of 25, 125, and 625. However, regardless of the layer count the edge retention was the same, as shown in the chart below:

The reason the layers are not visible is not simply because the layers got so thin they cannot be viewed. When the layers become thin enough, the larger alloying elements (not only carbon) will equalize between the steel leading to a roughly consistent composition throughout the steel in terms of molybdenum and chromium. No pattern was apparent in the 3125 layer steel.

3125 layer AEB-L/154CM steel with no apparent layers.

Toughness

I also tested the toughness of the Hugin pattern Damasteel with two heat treatments:

1. 1950°F for 15 minutes, plate quench, liquid nitrogen cryo, 400°F temper (~61 Rc)
2. 1975°F for 15 minutes, plate quench, liquid nitrogen cryo, 300°F temper (~64.0 Rc)

The toughness of Damasteel was close to the RWL34 on its own, with only a small boost due to the tougher PMC27 (compare with AEB-L on the chart). This was similar to other Damascus steels that we tested, where toughness was largely controlled by the “less tough” of the two steels. In our initial tests of the dense twist patter, we got a surprising amount of variability when compared with the Hugin pattern:

Because of this unexpected behavior, I made four more coupons and retested, and this time the behavior was more consistent. The average toughness did not change that much, however. It appears that when compensated for hardness the toughness was roughly the same between the dense twist and the Hugin pattern.

Summary and Conclusions

Damasteel DS93X is a high quality powder metallurgy stainless Damascus. It showed good toughness and edge retention in our testing. There are a couple issues with the datasheet and I have slightly different recommendations for heat treating, but their most recent datasheet did change their austenitizing time recommendation in what I think is a positive change. For 1/8″ material and thinner I recommend a 15 minute austenitize from 1900-2000°F (1035-1095°C), with a max of 1925°F (1050°C) if not using cryo after the quench. Temper to desired hardness with a minimum of 300°F/150°C. I didn’t test tempering temperatures higher than 400°F to see when tempered martensite embrittlement happens. Generally a 350-400°F (175-205°C) temper gives more balanced properties than maxing out the hardness with 300°F/150°C.

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[1] Billgren, Per, and Kaj Embretsen. “Method relating to the manufacturing of a composite metal product.” U.S. Patent 5,815,790, issued September 29, 1998.

[2] https://www.alleima.com/en/products/strip-steel/strip-products/knife-steel/hardening-guide/hardening-programs/alleima-12c27-batch-hardening/

[3] https://www.alleima.com/en/products/strip-steel/strip-products/knife-steel/hardening-guide/hardening-programs/alleima-12c27-batch-hardening-deep-freezing-70c-95f/

[4] Mesquita, Rafael Agnelli, and Celso Antonio Barbosa. “High-speed steels produced by conventional casting, spray forming and powder metallurgy.” In Materials science forum, vol. 498, pp. 244-250. Trans Tech Publications Ltd, 2005.

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