CPM-S60V – The Forgotten Super Steel

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History of S60V

Around 1970 Crucible steel started production of industrial-scale powder metallurgy tool steels. It was recognized early on that high vanadium tool steels had much greater toughness when produced with powder metallurgy. The vanadium leads to the formation of very hard vanadium carbides that contribute greatly to wear resistance. However, the vanadium carbides become very large with conventional production which limits toughness. Powder metallurgy greatly reduces carbide size. In fact with powder metallurgy production, vanadium carbides are usually smaller than the common chromium carbides or molybdenum/tungsten carbides since they are more stable and coarsen more slowly. Crucible Steel metallurgists took advantage of this fact with the development of CPM-10V which had high amounts of vanadium carbide, achieving very high levels of wear resistance along with good toughness. Read more about CPM-10V in this article.

It wasn’t long until Crucible metallurgists attempted to also develop a stainless steel with increased vanadium. The first that we have a record for was a steel called CPM-440V, which later came to be known CPM-S60V. The history of 440V development, in terms of who developed it and when it was released, are a bit difficult to nail down since the steel was not patented. However, there are a few things we can piece together.

Crucible metallurgists were not the first to explore increased vanadium stainless steels. A patent [1] from David Giles of Latrobe Steel in 1955 had compositions very similar to S60V, though I’m not sure that it ever became a commercial product. This steel, as well as the final S60V, was essentially the common stainless steel 440C but with a healthy dose of vanadium and the necessary carbon to go with it. The composition ranges in the Giles patent are relatively wide so it’s not completely clear which composition he was most excited about but the S60V-like steel had a good combination of wear resistance and corrosion resistance according to the patent. As I wrote about in the article on 10V, there were connections between David Giles and Crucible metallurgists but I can’t say for sure if the David Giles patent was an inspiration for S60V. The Giles patent had expired by the time 440V/S60V was released so that was not a problem.

The earliest reference I have found to CPM-440V (sometimes listed as CPM-T440V) is from 1983 [2], and it seems to have been released around that time as a 1984 article was describing it as a new product [3]. Interestingly I am seeing at least two different compositions for 440V [4][5], and I’m not sure if they switched at some point or if two versions were sold, the primary difference being the carbon content but there also appears to have been a small deviation in Cr, Mo, and V as well. The only place I see both listed together is in the S90V (then CPM-420V) patent where the lower carbon version is listed as CPM-440VM [6]. In other datasheets or articles it seems almost random whether the composition is the lower or higher carbon version until the mid-1990s when they appear to have settled on the higher carbon version. The lower carbon version looks particularly close to the Giles patented steel.

Use in Knives

The potential for using CPM-440V in knives was recognized pretty early on. A 1987 Popular Mechanics article by Butch Winter [7] described the steel as solving the lack of corrosion resistance of 10V while still offering “outstanding edge-holding” though specific makers using it were not mentioned.

Blade Magazine

I can’t find a mention of 440V in Blade Magazine until the June 1990 issue in connection with Michael Walker patenting a type of composite construction for knives with all titanium except for a steel edge, such as CPM-440V. Then in the December 1991 issue Terry LaBorde advertised “CPM-T 440V all stainless Damascus.” Wayne Goddard wrote an article about CPM steels in the September 1992 issue of Blade Magazine. Goddard said that, “My tests show it to have two-to-three times more edge-holding ability than 440C, both being tested at 57-58 Rc.” Goddard predicted that if Crucible succeeded in their plan to make the steel more available and at cheaper prices, “CPM 440V may be come the most used steel of the ’90s.” And the article ended with an editor’s note stating that Texas Knifemakers Supply was beginning to stock CPM 440V. The July 1996 issue says that Spyderco introduced the Military model that year in CPM-440V, the first major knife manufacturer to use it.

Failure to Launch

Other custom knifemakers I see used 440V in the 90s included Leon Treiber and Phil Wilson. However, it seems that the use of 440V in knives never really took off. Perhaps it was the difficulty in grinding and finishing compared to the common alloys of the time like 440C and ATS-34/154CM. Or the higher cost of using powder metallurgy steels compared to those steels. CPM-440V also required a higher austenitizing/hardening temperature to reach 60 Rc than 440C and 154CM. Phil Wilson reported that with his furnace having a max temperature of 2000°F he was limited to about 58-59 Rc with 440V. Because of toughness concerns some knife makers and companies were hardening 440V to relatively low levels like 56 Rc where its strength was relatively low and its edge retention was not as high as the steel was capable of. The steel was also perceived as being difficult to sharpen, again especially compared to the common 440C and 154CM. Sales of steel to the knife industry took off much more for Crucible once S30V was introduced in 2001, giving the knife industry a steel they seemed to be much more interested in. CPM-440V was rebranded to CPM-S60V some time around the year 2000 [8], and CPM-420V was renamed to CPM-S90V.

Resurgence?

Now it would seem knife makers, knife companies, and knife buyers are more used to the idea of higher cost powder metallurgy steels. And many of the knife makers and companies have experience with high wear resistance steels. Niagara Specialty Metals re-introduced CPM-S60V in 2015 but I haven’t seen many knives coming out in the steel even with its newly improved availability. Perhaps the steel is seen as outdated or the fact that it never took off originally means that it still has a lingering poor reputation. Because of all of this I have been curious about the properties of S60V. Should knife companies and makers be jumping on this steel or are newer steels just better?

Properties of S60V

Microstructure and Ease in Sharpening and Finishing

All of the early powder metallurgy stainless steels used a combination of high chromium (17+%) in combination with vanadium. These steels included the first (440V/S60V), and Bohler’s M390 and Uddeholm’s Elmax. The high chromium reduces the amount of “pure” vanadium carbide that forms, instead making a vanadium-enriched chromium carbide which is higher in hardness than typical chromium carbide but not to the level of vanadium carbide. Therefore these three grades all have around 2% vanadium carbide [6][9][10] which is lower than steels with ~14% chromium like S30V which has 4% vanadium carbide, more than S60V despite having less vanadium. Read about the changes in carbides with different compositions in this article.

M₇C₃ is a chromium-rich carbide and MC is vanadium carbide

The amount of vanadium carbide controls wear resistance and difficulty in polishing and sharpening. Vanadium carbide is harder than the common abrasive aluminum oxide used in many grinding belts and sharpening stones so removing steel is more difficult with high levels of vanadium carbide. Many people report having an easier time sharpening Elmax and M390 than S30V and this is likely due to lower amounts of vanadium carbide. S60V also probably has relatively good ease in sharpening and finishing for its level of wear resistance when compared to high vanadium steels like S90V or 10V.

However, S60V also has a surprisingly fine carbide structure, a bit finer than Elmax, M390, or CPM-154. Perhaps its better ratio of V:Cr helped with that as vanadium carbides are less prone to coarsening in powder metallurgy steels. Compare with more steels in this article of micrographs.

S60V 2050°F

M390 2140°F

Elmax 1975°F

CPM-154 1950°F

S30V 2000°F

Hardness and Heat Treating

Because of some reports that S60V cannot reach high levels of hardness it is probably worthwhile to discuss heat treating. I have not done a full set of hardness coupons with S60V. Below is the hardness data provided by Crucible [11], they included air cooling and oil quenching but did not test the effect of cryogenic processing.

The datasheet recommends 1850-2050°F for austenitizing but I think this range is probably too low. For one, using 1850°F only results in 54-56 Rc which is quite low for most applications. Also, the hardness has not dropped from excessive retained austenite even by 2050°F. This also appears to be acknowledged by Crucible as their “Recommended heat treatment” uses 2050°F. A better austenitizing range might be 1950-2150°F. I have heat treated a few different coupons using the current S60V material, all with cryo after the quench. Using 1950°F and 400°F resulted in 58.5 Rc and 2050°F and 400°F resulted in 61.0 with a toughness coupon and 61.7 Rc with a CATRA knife. So I think the 2050-400°F combination is a good starting point, resulting in 61+ Rc with cryo and around 60 Rc without cryo, which has a good combination of strength and edge retention along with decent toughness. A 20-30 minute austenitize seems to work fine. To see the steel’s potential for high hardness I also tried heat treatments with 2100°F for 15 minutes and 2150°F for 10 minutes, each followed by a plate quench, cryo, and temper at 300°F. The 2100°F heat treatment led to 64.3 Rc and 2150°F resulted in 65.0 Rc. Therefore very high hardness levels are possible for those knifemakers that are interested.

Edge Retention

With its high vanadium and relatively high carbide content S60V has very good edge retention. Higher than steels like S30V, Elmax, and M390/20CV/204P, though lower than the higher vanadium 10V or S90V. See this article for a description of my edge retention experiments.

Toughness

I used my standard toughness test of subsize unnotched charpy specimens with S60V. The heat treatments used were given in the previous heat treatment section. I was a bit surprised by the toughness as it was in line with many other popular stainless steels including Elmax, 154CM, VG10, 154CM, S30V, and M390. It is a step down from S35VN, Vanax, and CPM-154. However, many people report good performance of knives in VG10, S30V, and M390; and S60V matches or exceeds the toughness of those grades. I don’t know if Crucible steel production is better now than it was in the past, or if my heat treating is better than what was being done in the past, or if perceived toughness issues with S60V were related to other things like low strength (rolling rather than chipping). However, all I can say is that the toughness of S60V looks comparable to many other common steels. Its carbide structure, as pointed out before, looks at least as good as many other powder metallurgy grades as well so I think these toughness values are reasonable.

Corrosion Resistance

I have not tested the corrosion resistance of S60V but I have tested other steels and created a system for predicting corrosion resistance as explained here. I re-ran the chromium/molybdenum in solution numbers using the composition of the recently produced S60V. The conclusion is that the corrosion resistance of S60V is only average, having about 10-11% chromium in solution along with only a small amount of molybdenum to help improve corrosion resistance with that relatively low chromium. This predicted level of corrosion resistance is consistent with results given in the S90V patent [6] where S60V was found to have poorer corrosion resistance than S90V, Elmax, and M390. That means S60V is pretty close to the dividing line between what I consider stainless and non-stainless. This is another reason I think 1850-2050°F is a low range to recommend for heat treating as 1850°F is likely to lead to relatively poor corrosion resistance compared to other stainless steels. Using higher austenitizing temperatures will lead to somewhat better corrosion resistance. See this article to learn more about the effect of heat treatment on corrosion resistance.

S60V vs M390 and S90V

M390 appears to be the hot steel these days for folders, with people seeming to like it for its relatively high edge retention along with good ease in sharpening for that level of edge retention. S60V has yet higher edge retention and probably is not much more difficult to sharpen. And toughness measurements put it in a similar range to M390 as well. So if people like M390/20CV/204P and want a bit more edge retention I think S60V is a good place to look. The main downside relative to M390 is lower corrosion resistance. S90V has higher edge retention and corrosion resistance than S60V but is more difficult to sharpen. For those comfortable with CBN and diamond sharpening stones S90V probably has a better combination of properties than S60V. However, with how common aluminum oxide-based sharpening stones are, and with knife makers and companies generally avoiding S90V for its very high vanadium carbide content, S60V seems like an interesting steel for certain applications. M390 has never been my favorite steel, but with so many people liking it there is no reason why S60V couldn’t fill a similar niche but with a bit better edge retention.

Summary and Conclusions

Crucible CPM-S60V started out as CPM-440V introduced around 1983. It experienced slow growth in knives, ultimately being more or less replaced by S30V as a more attractive option to knife companies. However, S60V has very high edge retention in combination with decent toughness and should be easier to sharpen than steels like S90V. I recommend heat treating it with a 2050°F austenitize along with a 400°F temper. A cryo treatment after the quench leads to a bit higher hardness. S60V seems like an interesting alternative to M390 for a bit higher edge retention and should work well in certain applications.

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[1] Giles, David J. “Ferrous alloys and corrosion and wear resisting articles made therefrom.” U.S. Patent 2,709,132, issued May 24, 1955.

[2] Chilton’s iron age 226, no. 21 (1983): 53.

[3] Gear Processing & Manufacturing (1984): 4-24.

[4] 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.

[5] “CRUCIBLE CPM 10V, CPM 9V, MPL-1 ,CPM 440 V, CPM REX M4.” Alloy Digest. June 1993.

[6] 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.

[7] Winter, Butch. “Sharpen Up.” Popular Mechanics. April 1987.

[8] http://zknives.com/knives/folding/hiend/kboa.shtml

[9] Almström, Linda, and Camilla Söderström. “Alternative materials for high-temperature and high-pressure valves.” (2010).

[10] Gornik, Christian, and Jochen Perko. “Comprehensive wear study on powder metallurgical steels for the plastics industry, especially injection moulding machines.” In Materials science forum, vol. 534, pp. 657-660. Trans Tech Publications, 2007.

[11] http://www.crucible.com/PDFs/DataSheets2010/Datasheet%20CPM%20S60V.pdf