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K390 Video
Here is the video version of the following information:
What is K390?
K390 was developed as an alternative to Crucible’s CPM-10V, which has had success since the late 1970s as a steel with very high wear resistance yet also moderate toughness. CPM-10V achieved these properties by having a relatively high volume of vanadium carbides, very hard carbides that contribute greatly to wear resistance. Because the carbides are so hard, it requires less of them for a certain level of wear resistance. Carbides are brittle, so having a lower volume of carbide for the same wear resistance means greater toughness. The patent application for K390 was submitted in Austria by Bohler in 2002 [1], and the steel was announced in 2004 [2]. The main modifications to CPM-10V were increased Mo, W, and Co. This gives K390 better “secondary hardening” and therefore “hot hardness” so that it can maintain its hardness at high temperatures. The higher Mo also gives it greater “hardenability” so that large dies can still harden all the way through.
Another comparison is Vanadis 8, which wasn’t released until 2017, which I have previously tested and written about. It is similar to K390 but with even lower vanadium (8%) and a simpler overall design without W and Co. With K390 having vanadium in between 10V and Vanadis 8 we would expect its wear resistance and toughness to also be between those two steels. However, the extra alloying elements could potentially throw some curveballs in there. In Bohler’s literature [3], they have charts showing K390 to have slightly worse wear resistance than CPM-10V but a substantial improvement in toughness, we will be testing that.
Bohler data on K390 vs CPM-10V (listed as 10% Vanadium) [3]
K390 in Knives
K390 has seen relatively limited use in custom knives, such as Phil Wilson who used some early on [4]. Phil was working closely with Bohler-Uddeholm when they were ramping up their advertising to the knife industry around 2009-2011, so he was given samples to test. The recent usage by Spyderco has created more interest in the steel, starting with a “Mule” in 2013 [5] and a Police 4 in 2017 [6].
Microstructure
K390 (1975°F)
CPM-10V (1800°F)
Vanadis 8 (1900°F)
My micrographs of the three steels look quite similar. Perhaps Vanadis 8 has a little less carbide than the other two. The approximate carbide volumes are 17.5% for 10V, 17% for K390, and 16% for Vanadis 8. None has a clear advantage in terms of carbide size, despite claims from Uddeholm and Bohler of “3rd Generation” powder metallurgy technology. I previously tested differences between the different PM sources in this article. Each is made up of only vanadium carbide without extra types such as chromium carbide, molybdenum carbide, etc.
Wear Resistance and Edge Retention
K390, Vanadis 8, and CPM-10V are all in the upper echelons for wear resistance and slicing edge retention. The only non-stainless steels with higher edge retention either have higher vanadium (CPM-15V) or are ~70 Rc PM high speed steels (Z-Max, Maxamet, Rex 121). When comparing K390 to Vanadis 8 and CPM-10V it is important to note the dotted grey lines on the chart, those represent the approximate effect of hardness on CATRA edge retention. Note for example that CPM-10V with its two different heat treatments approximately follows that slope. So while K390 is substantially higher in edge retention on the chart, if compensated for hardness K390 is much closer to Vanadis 8. This is somewhat surprising just because K390 has 1% higher V along with W and Co. CPM-10V, however, does show measurably higher slicing edge retention than K390 and Vanadis 8. However, all three are in the same general “category.”
Toughness
The new testing I did for this article was for toughness. I had previously tested the edge retention of K390 but the material I used was just a bit too thin for using for my standard toughness coupons. Pavlo, who is currently serving in the Ukranian military, sent me some thicker K390 for testing. After a few months he emailed me asking if I had a chance to test it yet which made me start to feel some guilt about not getting to it sooner for him but better late than never.
My “High alloy non-stainless” toughness chart has gotten crazy so I have shown a reduced chart for clarity. The rest of the steel toughness charts can be seen in the chart below for comparison. K390 is again quite close to Vanadis 8. It appears there might be a bit more separation at lower hardness but I didn’t have a coupon any lower than 62.6 Rc so we can’t see that for sure. Both do have an advantage over CPM-10V, which lines up approximately with the claimed properties from Bohler. So kudos to Bohler for having accurate values in their datasheet. Once above about 65 Rc, however, CPM-10V and K390 have similar toughness. This gives CPM-10V the advantage at high hardness because it has an advantage in slicing edge retention while having the same toughness. Comparing with some common PM stainless steels, K390 is similar in toughness to S35VN, CPM-154, Vanax, and XHP. This is significant because K390 has much higher wear resistance than those steels, so if the toughness of one of those steel is sufficient for the task, you can get much greater edge retention if sacrificing corrosion resistance is acceptable. K390 does have a significant advantage in toughness over the popular M390/20CV/204P stainless steel, so K390 has both better toughness and edge retention, though of course with much worse corrosion resistance. There aren’t any stainless knife steels available that can match the combination of toughness and wear resistance that K390, Vanadis 8, and CPM-10V offer.
How to Heat Treat
For knifemakers working with K390 I also have recommendations for how to heat treat the steel. The K390 datasheet recommends the “high temper” regime around 1025°F. However, in steels I have tested with low vs high temper I have found superior toughness with the low temper options. You can read two examples with CPM-CruWear and CPM-10V. Below shows prior coupons I tested with a range of austenitizing temperatures along with a tempering temperature of 300°F:
Surprisingly, the toughness coupons I tested, which had a 400°F temper, were only about 0.5 Rc lower than the 300°F temper shown above. I would anticipate 1-2 Rc lower by tempering at 400°F instead, so perhaps the K390 I used for the toughness tests was higher in carbon or some other composition difference. Given that the steel was still 62.5 Rc with 1800°F and 300°F, it may be possible to austenitize even lower than 1800°F for applications that work better in the 60 Rc range. When using cryo you can austenitize up to 2150°F or so for very high hardness. I did not measure coupons without cryo, so the top end austenitizing temperature is probably lower than 2150°F without it. Use a dry ice or a freezer if liquid nitrogen is not available. You would want to make your own “austenitize vs hardness” chart to ensure that the chosen austenitizing temperature isn’t too high. So to summarize the recommended heat treatment:
Austenitize 1800-2150°F for 20 minutes (thicker steel may need longer, the top end is likely too high without cryo)
Quench – I prefer a plate quench but the steel has pretty high hardenability so many different quenches would work
Cryo – Liquid nitrogen is best, dry ice is second best and freezer last
Temper – 400°F gives a good balance of hardness and toughness. Temper twice for two hours each time
Steel Ratings
Below I have the steel ratings for K390, which I hadn’t previously rated given I was missing the toughness data. Also I have corrected the toughness rating for CPM-15V. It was listed as a “3” but looking at my Excel spreadsheet that converts the toughness results to a rating it was 3.5. So I don’t know why it was down as 3. It is fixed now.
Summary and Conclusions
The properties of K390 are quite excellent with “good” toughness along with very high wear resistance and edge retention. Vanadis 8 and CPM-10V offer a similar set of properties though none of these three have been used in many knives. The combination of toughness and wear resistance is unmatched by any available stainless steel. The large volume of very hard vanadium carbides means that both knife companies and custom knifemakers tend to complain about the grinding and polishing of these steels. So perhaps K390, Vanadis 8, and CPM-10V will never reach “mainstream” status but I will maintain some hope.
[1] https://patents.google.com/patent/AT411534B/en
[2] Schemmel, Ingrid, Werner Liebfahrt, Andreas Bärnthaler, and Stefan Marsoner. “PM Tool Materials: Böhler K390 Microclean-A New Powder Metallurgy Cold Work Tool Steel for Highly Demanding Applications.” In European Congress and Exhibition on Powder Metallurgy. European PM Conference Proceedings, vol. 3, p. 1. The European Powder Metallurgy Association, 2004.
[3] https://www.us.bohler.com/app/uploads/sites/92/2020/12/productdb/api/k390en.pdf
[4] https://forum.spyderco.com/viewtopic.php?t=72669#p1122789
[5] https://www.spyderco.com/edge-u-cation/mule-team-project/
[6] https://www.spyderco.com/catalog/2017_Spyderco_Product_Guide.pdf
I just don`t understand why not always reaching max hardness for a given knive steel, especially for those secondary hardening steels. If I need something else for assumed use and therefore geometry ( toughness/stain-resistance) I`d rather stay with a different steel. E.g. M390, S30V or the likes with a HRC of 60 or even less just doesn`t make sense to me. I`m in bad luck with mediocre heat-treated S30V from “renowned” companies and never jumped on the bandwagon S45VN …
Well, for one it’s hard to max out the hardness without cryo, which is expensive on a larger scale, and even really good room temp quench is hard to do on scale, so lower hardness values by vacuum furnace heat treating is accepted. Plus, most companies would much prefer an edge roll than chip when there’s going to always be that one person using it like a screwdriver.
And there’s just some general stigma of “60hrc is just what we use” from historical steels. Stuff like 14c28n is usually way under hardened for how tough the stuff is, and it could hold much finer geometries with great stability if it were so, the razors it was designed for are probably the only thing that uses that steel correctly. Best to stick to smaller makers if you want to ensure you get a heat treatment done with care. Even plain steels seem to follow this trend though, take a less hard steel and harden it as far as you can go and it’s be much tougher than a steel not taken to it’s max tempered hardness. 1095 vs 1084 is a big, and surprising comparison.
Great work, enjoy your articles and research. Amazing how the knife industry has advanced in recent years. As a materials scientist it is rewarding to be able to actually experience the difference in various materials of construction in these tools.
Noticed a minor error in the following line, wanted to let you know:
“CPM-10V, however, does show measurably higher slicing edge retention than K390 and CPM-10V.”
I think that S90V is the superior steel for everyone who needs (or wants) a steel that has superior edge retention while still not being too brittle.
Of course it’s not a steel you’d use for a machete or a sword, but for an blade up to 20 cm it should be not only tough enpough but also provide more edge retention than you’ll ever need.
Might be a tad harder to sharpen, but yeah, there’s diamonds for that.
Agreed, S90V has been the best of all worlds for me, CTS-XHP and Magnacut have also been really enjoyable
there were some dragonfly2’s in k390, not really a design in need of high toughness. i’d have thought it would be much more similar to m390; quite a surprise.
Talk to Dulo Knives (Svetlozar) in Bulgaria about their practical experience making knives with these high end steels. They have made thousands of custom knives with these materials and have learned something in the process. They believe K390 is less likely to chip than Vanadis 8 which is contrary to the toughness findings.
I have a quick question that I can’t seem to find a straight answer to: the sum of both toughness and edge retention in K390 and CruWear is the same (13). K390 is 5.5/7.5, and CruWear 8/5. Wouldn’t these steels be pretty much the same in practical use? Let’s take the same knife, with the same thickness and the same blade geometry. K390 should cut longer, but I could sharpen CruWear at lower angles without sacrificing toughness too much. Could I get similar results by simply manipulating blade angles? If so: does it apply to more extreme cases (Maxamet or even 420HC)?
Changing the edge angle also changes cutting ability, not only edge retention. So you can’t turn the performance of one steel into another with edge geometry.
I would also add that toughness is not the only factor that dictates what edge angle you can use for a given cutting task. edge stability is critical also. in the case of 420HC it is very tough but it maxes out in hardness at around 58rockwell ie low edge stability. so if you try and take advantage of its toughness by reducing the edge angle such that it outcuts a more ware resistant steel you will end up with the edge rolling before you realize any improvement in edge retention.
I would love to see a k390 core damascus. With a h2 sandwhich.
I just got the link to your home page from colleague. Very interresting information. Will follow your page in the future.