S90V and S125V Knife Steel – History, Properties, and How to Heat Treat

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And thanks to Ken Pinnow, Ed Severson, and Al Kajinic for talking to me about the development of S90V and S125V.

High Wear Resistance Stainless Steels

Here I have another article about high wear resistance stainless steels, leading up to an eventual article on my testing of M398 (read my early analysis here). Where to set the cutoff between “high” wear resistance stainless steels and “medium” I’m not sure, but I also have articles about ZDP-189, S60V, M390, and S30V. To come will be S110V and of course M398.

S90V History

CPM-S90V was originally named CPM-420V released around 1996 [1]. The patent for the steel [1] lists a lot of studied compositions and I will not be looking at all of them so I recommend reading the patent if you want to look at all of the tests that were performed.

At that time there were several powder metallurgy stainless steels available, both from Crucible and competitors. Crucible’s first PM stainless was S60V (originally 440V) which I wrote about in this article. Crucible then patented a steel with very high wear resistance called MPL-1, the patent for which was submitted in 1986 [2]. Bohler patented M390 steel in 1988. And Uddeholm released Elmax sometime around 1990 [3]. All of these previously released PM stainless steels had high chromium (16-24%) for corrosion resistance in combination with some amount of vanadium for wear resistance. This is perhaps expected as the most common martensitic stainless steel for high wear applications was 440C with ~17% chromium. In fact both S60V and Elmax appear to be modifications of 440C with added vanadium and carbon. Increased amounts of vanadium in powder metallurgy were being used in steels like CPM-10V which achieved better combinations of toughness and wear resistance so this makes sense as the route for designing early powder metallurgy stainless steels. Vanadium carbides are very hard and contribute greatly to wear resistance but are detrimental to toughness in conventionally-produced steels. However, it was discovered that in powder metallurgy steels the carbides are very small and therefore actually are better for toughness than other carbide types.

So where S90V deviated from the previous vanadium-alloyed powder metallurgy stainless steels was the reduction in Cr down to 14%. Why reduce the Cr content and what did it accomplish? One important thing to know is that Crucible at that time used the “crossed-cylinder wear test” to measure the adhesive wear resistance of their steels. In this test a cylinder of the tool steel is worn against a cylinder of tungsten carbide. High vanadium steels like 10V had very high results in the crossed-cylinder wear test so Crucible metallurgists were also trying to get higher values in their stainless steels. In the S90V patent they point out that the major improvement in the design of the steel was improved adhesive wear as measured in the crossed-cylinder wear test.

Crossed-Cylinder Wear Test

However, the existing PM stainless steels like S60V, M390, Elmax, and even MPL-1 had very low values in the crossed-cylinder test when compared with 10V. The reason was that the high chromium in the steels would lead to the formation of chromium carbide rather than the high hardness vanadium carbide. The higher the chromium content the less vanadium carbide is formed. Small vanadium carbides are the key to high values in the crossed-cylinder test. You can read more about the decrease in vanadium carbide with chromium content in this article about carbides.

Higher chromium means less vanadium carbide

Higher chromium means more chromium carbide

So Crucible metallurgists discovered that if the chromium content were reduced that more vanadium carbide (and less chromium carbide) would be formed in the steel, and therefore increase the crossed-cylinder wear test results. They compared vanadium content vs wear resistance for a range of steels at different chromium levels and found that a 14% Cr content led to greatly increased performance. (Note: Higher is better).

And if the chromium is reduced further you have non-stainless PM tool steels which have even better adhesive wear resistance.

S90V was also found to have improved corrosion resistance and toughness when compared with the earlier S60V. And certainly better toughness than MPL-1. In the chart below higher values mean better toughness, but for corrosion resistance lower is better.

Other Steel Designs Explored in the Patent

Note: The ~0.1% nitrogen is the “baseline” nitrogen that the steel picks up from nitrogen atomization and the atmosphere. The average nitrogen content in the commercial S125V is not public information.

High Nitrogen S90V

Interestingly, the Crucible metallurgists also looked at partial replacements of carbon with nitrogen in amounts of 0.32 or 0.46%. The authors of the patent stated that higher nitrogen contents were explored in an attempt to improve corrosion resistance without affecting wear resistance. And it appears that they were successful in doing that (see the previous chart for the grade marked S90VN for improvement in corrosion resistance). And interestingly the crossed-cylinder wear test also showed markedly improved results in the S90VN with 0.32% nitrogen and 2.01% carbon. This may be from nitrogen promoting the formation of vanadium carbide rather than chromium carbide as I wrote about in this article on S45VN. So I’m not sure why they didn’t further pursue a nitrogen-modified version since the stated improvement in the patent was in the area of crossed-cylinder wear. I was able to speak to one of the developers of S90V, Dr. Ken Pinnow, and he said that the benefit to using nitrogen wasn’t great enough and so they used the simpler design instead. It wasn’t until S30V a few years later than an intentional addition of 0.2% nitrogen was used in a commercial product. There was definitely some scatter in the crossed-cylinder results (a slightly different steel or heat treatment could lead to significantly different values) so maybe the difference between S90V and S90VN was not as big as it appears.

S125V and S145V

They also looked at yet higher vanadium contents than S90V, including steels I have labeled S90V-12%V, S125V, and S145V. They looked at a few different variants of the 12% vanadium steel, including one with 1% Mo and two with 2.7% Mo (with two different carbon levels). The 12% vanadium version was not immediately sold commercially but eventually we got a version of it with 2.5% Mo sometime around 2005 called S125V [4]. The final version also had even higher carbon than either version that was shown in the patent (3.3 vs 2.95% carbon). I don’t know why they went higher carbon because in the patent they show higher hardness than S90V at the lower carbon content. Reduced carbon would mean better corrosion resistance and toughness. However, the hardness of S90V and S125V is not incredibly far apart as will be seen in the hardness tests later in this article so perhaps modifications were necessary. Apparently they went with the 2.5% Mo version because it had better wear resistance as shown in the chart above, and a bit better corrosion resistance. I spoke to Al Kajinic who formerly worked for Crucible Research and he said that he doesn’t remember any details but that he and Maria Sawford would have decided on the final composition of S125V. Kajinic also patented S110V a few years later.

The 14.5% V version I have called S145V has never been commercially produced as far as I know.

Early Use of S90V in Knives

Phil Wilson wrote an article about the new steel CPM-420V in 1996 [5], and a follow-up in 1998 [6]. The initial article was at the time of the steel’s release and so his experiments with the steel were limited primarily to heat treating and measuring hardness. By his later article he had made over 30 knives out of it and he had also spoken to other makers about using it. Phil has been known for trying new steels and high alloy steels and he is one of the most extensive users of S90V, especially early on.

Phil was limited somewhat in hardness at the time because his furnace could only reach 2000°F, so he was heat treating knives in the 55-58 Rc range. Later he got a new furnace and began heat treating the knives to higher hardness. The relatively low hardness was also acceptable, in part, because that is also where most makers were heat treating their CPM-440V (S60V).

Phil reported testing two fillet knives on 22 Albacore tuna. He used a 5 inch fillet on the initial cut down the lateral line and behind the head, and then a longer 9 inch fillet knife for the rest of the cuts. This process involves the cutting of very tough skin and exposure to saltwater while cutting. After the 22 tuna the knives were still cutting leather and rope easily. And no corrosion was visible on the knives.

He also got reports back from customers who had experience with custom knives in various steels. A married couple who had previously tested Phil’s knives reported processing 75 large salmon before the knife required resharpening. And Rabbi Yurman, who performed the neck cut on 1000 chickens per day at that time. He reported that the knife went through a whole week without getting dull.

Phil Wilson also reported discussions with other knifemakers who had been using the steel including Wayne Goddard, Barry Gallagher, and P.J. Tomes. Goddard performed his standard rope cutting test and measure 64 cuts on 1/2 inch rope, and he said that most steels were around 30 cuts, and few exceeded 45 (Goddard also wrote briefly about the steel in the March 1997 Blade Magazine). Barry Gallagher, P.J. Tomes, and Phil Wilson all agreed that grinding and finishing S90V was very difficult, which we would expect based on its high wear resistance. P.J. Tomes reported that in his cardboard cutting tests that the S90V outperformed all other steels he had tested by far.

Other early CPM-420V knives I found: Kit Carson in the May 1999 Blade Magazine. Ken Onion is mentioned as using some CPM-420V in August 1999. Darrel Ralph in December 1999. David Broadwell in February 2000. Roger Gamble in May 2000. Greg Lightfoot in May 2000. Brian Tighe in August 2000.

In terms of production knives, Microtech was offering knives in 420V by the middle of 2000.

420V and 440V were renamed S90V and S60V in July 2000 [7].

S90V has been used in knives basically since it was first released, but it has never really taken over as a common knife steel. This is primarily due to its very high wear resistance and difficulty in grinding and finishing. It is generally viewed as a boutique steel for customers who value very high wear resistance and edge retention, and will accept somewhat higher costs or time in sharpening. I think it was also affected by the release of S30V in 2001 which was promoted as being designed specifically for knives and easier to work with. For example, by 2002 Tony Marfione of Microtech said that S30V would be their new “standard steel” while S90V would be their “premium steel” [9].

Use of S125V in Knives

In part because of S90V’s reputation for being difficult to work with and to sharpen, the use of S125V is even more limited. It is also difficult to manufacture the steel in the first place [8], so basically from start to finish no one enjoys working with it.  It is almost exclusively used in custom knives, and not many of those are released either. The steel has seen a bit more popularity in Russian custom knives, as apparently they have more interest in very high wear resistance stainless steels there.

Properties of S90V and S125V

Hardness

S90V and S125V are capable of rather high hardness, approximately 66 Rc given an appropriate heat treatment. The two steels have relatively similar “as-quenched” hardness or with a low temper of 300°F, though S125V seems to have somewhat better tempering resistance (hardness is higher with a 400°F temper). The following hardness values came from heat treating small coupons with a 20 minute austenitize, plate quench, liquid nitrogen cryo, and tempering twice for two hours each time.

An S90V CATRA knife I heat treated with 2050-400 resulted in 61.7 Rc, within range of the above values. However, the toughness specimens measured about 1 Rc lower. Perhaps the quenching rate was somewhat slower because of the larger volume of steel. A CATRA knife and toughness specimens of S125V were about 0.5 Rc higher than the S90V. The S90V datasheet recommends austenitizing 2100-2150°F; however, I think it is clear that lower austenitizing temperatures can yield plenty of hardness, at least with cryogenic processing. A heat treatment using 2050-400 results in pretty balanced properties for either S90V or S125V.

Microstructure

S90V has a finer microstructure than M390 or ZDP-189, the average carbide size appears to be smaller than S60V though S90V has more “clumping” of carbides. The non-stainless 10V without chromium carbides has a finer overall carbide structure with less clumping of carbides. S125V has a similar carbide appearance to S90V but with larger carbides as would be expected by the higher carbon and vanadium. You can compare with more steels in this article.

S90V

S125V

M390

S60V

S110V

ZDP-189

10V

Toughness

We have tested S90V and S125V toughness with our standard test using 1/4 size unnotched charpy specimens. The specifications of these samples can be viewed here. S90V has similar toughness to other PM stainless steels like S60V, S30V, and M390. Despite Crucible finding that S90V toughness was superior to S60V I found them to be very close. I have also shown a dotted line to indicate the approximate effect of hardness on toughness to more easily compare between steels. S125V is another notch below those other steels because of its greater volume of carbide. The overall difference in terms of ft-lbs is relatively small, but differences are more significant at low values. In other words, 5 vs 2.5 ft-lbs is roughly as significant as 10 vs 5 ft-lbs. None of these powder metallurgy stainless steels is particularly tough but there are clear tiers, with S125V and M398 at the bottom, then the steels along the line with S90V and S60V, then steels like S35VN, CPM-154, XHP, and Vanax. However, non-stainless steels alloyed with high vanadium contents like 10V and Vanadis 8 have toughness similar or greater than S35VN while also having edge retention similar to S90V.

Edge Retention

The edge retention of S90V and S125V are the highest of any stainless knife steels that we have tested with CATRA so far. The high vanadium and 14% chromium results in very high wear resistance which in turn leads to excellent performance in edge wear tests. This chart also has dotted lines which indicate the approximate effect of hardness for comparing between different steels.

Toughness-Edge Retention Balance

Toughness and edge retention are two major properties that conflict with each other in knife steel. Increasing one tends to decrease the other. So I like to plot the two properties against each other to get an idea of the performance category where a steel sits. For this chart I have adjusted the CATRA value based same hardness as the toughness coupon for each steel, as predicting the effect of hardness on edge retention is easier than with toughness. You can see that S90V has a very good toughness-edge retention balance compared to other stainless steels at a similar edge retention range. It has both higher edge retention and toughness than M390, ZDP-189, and S110V, for example. This is expected based on our discussion above where S90V had similar toughness to many steels with lower edge retention. The higher proportion of vanadium carbide gives S90V a better property balance. S125V has higher edge retention and lower toughness, of course.

You can see these against the superior properties of the non-stainless vanadium-alloyed PM steels in the following chart:

Ease in Sharpening, Grinding, Polishing

S90V and S125V get their superior toughness-edge retention balance through vanadium carbide. Vanadium carbides are harder than aluminum oxide which is the most common abrasive in sharpening stones, belts, sandpaper, etc. That can make sharpening, grinding, and polishing more difficult. Sharpening is much easier when using diamond or CBN stones as those abrasives are harder than vanadium carbide.

Corrosion Resistance

I previously tested the corrosion resistance of S90V and it was basically middle of the road. I have not tested S125V; it has about 10.3% Cr in solution along with 1.6% Mo. So it may have slightly better corrosion resistance than S90V, in a similar range to S30V/S35VN. You can read more about corrosion testing and predicting corrosion resistance based on composition in this article. S90V has lower corrosion resistance than S110V and 204P (M390), though it is adequate for most knives.

Summary and Conclusions

S90V and S125V were developed to offer very high levels of wear resistance while also being stainless and having the potential for relatively high hardness. Reduced Cr (14%) relative to earlier PM stainless steels (16%+) led to better wear resistance without sacrificing toughness or corrosion resistance. S90V has seen use in knives over the years though its use remains relatively limited because of its high wear resistance. S125V is used even less because of the difficulty in working with it. The hardness, corrosion resistance, and toughness-wear resistance balance are quite good with S90V. In particular, S90V has similar toughness to many other PM stainless steels while also having significantly better edge retention. S125V has the highest edge retention of any available stainless steel though it gives up toughness and workability vs S90V to get it.

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[1] Pinnow, Kenneth E., William Stasko, and John Hauser. “Corrosion resistant, high vanadium, powder metallurgy tool steel articles with improved metal to metal wear resistance and a method for producing the same.” U.S. Patent 5,936,169, issued August 10, 1999.

[2] Hauser, John J., William Stasko, and Kenneth E. Pinnow. “Wear and corrosion resistant articles made from pm alloyed irons.” U.S. Patent 4,765,836, issued August 23, 1988.

[3] Modern Plastics, vol. 67, no. 7-12, p. 16, 1990.

[4] https://forum.spyderco.com/viewtopic.php?t=18915

[5] http://www.seamountknifeworks.com/js/web/viewer.html?file=articles/pdf/Newest_Particle_Steel_CPM420V.pdf

[6] http://www.seamountknifeworks.com/js/web/viewer.html?file=articles/pdf/CPM_S90V_Update.pdf

[7] “2000 sharp.” Blade Magazine. March 2001, p. 69.

[8] https://www.bladeforums.com/threads/cpm-s125v.711116/#post-7800595

[9] Kertzman, Joe. “Stellar Steels.” Blade Magazine. September 2002, p. 49.