M390 Steel – History and Properties (and 20CV and 204P)

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History of M390 Development

In the early 1900s, high carbon-high chromium steels were developed, and during World War I in England they were used as an alternative to high speed steels with high tungsten content. Tungsten was very expensive and only available to mine in certain countries and so during war it could be difficult to obtain. The earliest common high carbon-high chromium steel was approximately similar to D3 steel, which has higher carbon than the more common D2 which came a couple decades later, though both have about 12% Cr. And while it was discovered that these 12% Cr steels didn’t work nearly as well in high speed machining tasks, they were very good for “cold work” tools, particularly dies, because of excellent wear resistance from the many chromium carbides. You can read more about the development of D3 and D2 in this article. Later there were other modifications of D3 such as D4 which had higher molybdenum for greater “hardenability” meaning it could be quenched more slowly and still achieve full hardness.

High vanadium steels were patented in the late 1930s which became steels like M4 and T15 with 4-5% vanadium. It was found that vanadium carbides are extremely hard and therefore contribute greatly to wear resistance with high speed steels. You can read more about the development of high vanadium steels, M4 and T15 in this article about the metallurgist James P. Gill who developed those steels. Around 1950-1951, David Giles of Latrobe Steel patented two “cold work” high vanadium steels [1][2] which were later named A7 and D7. A7 was a modification of A2 with higher carbon and vanadium for wear resistance. D7 was a modification of D4 with higher vanadium for the same reason. These were the highest wear resistance cold work tool steels for several years before the introduction of powder metallurgy steel production in the early 1970s which allowed the design of even higher vanadium steels like 10V.

In the early 1980s, Bohler steel introduced K190 steel which is a powder metallurgy version of D7 [3]. Bohler promoted the improved toughness and machinability of K190 vs conventionally produced tool steels like D2 because of the powder metallurgy process which led to a refined carbide structure. For the plastics industry Bohler also wanted to have a high wear resistance powder metallurgy product which had high corrosion resistance (ie stainless). Despite the relatively high chromium in K190, it is not stainless, you can read why in this article on D2 corrosion resistance. As stated by Bohler, the design target for M390 was to have high wear resistance and corrosion resistance, though they stated that high toughness is not particularly necessary for plastics processing.

The patent application for M390 was submitted in 1989 [4]. The basis for designing M390 was to take K190 and modify it to have stainless levels of corrosion resistance [3]. In fact, M390 is sometimes known as “stainless K190” [5]. The reduced carbon relative to K190 means that less chromium is necessary to achieve stainless levels of corrosion resistance. It also means that the overall carbide content isn’t raised relative to K190 so that toughness is not reduced. Starting with the high carbon, 4% vanadium K190 as its basis means that M390 would have relatively high wear resistance.

It is a bit puzzling why the tungsten was added to M390 when it was not present in the K190. It is not without precedent that tungsten would be contained in similar steels, as D6 has a tungsten addition. Tungsten and molybdenum have similar effects in steel which is why they are somewhat interchangeable in high speed steels. However, tungsten is more expensive than molybdenum, you have to use twice as much tungsten to get the same effect (because tungsten is a heavier element), and tungsten does not improve hardenability like molybdenum does. So I don’t know why they added tungsten rather than increase the Mo content by 0.3%. Tungsten is not a very common element in stainless steels.

History of M390 in Knives

M390 was one of the earliest powder metallurgy stainless tool steels, coming after S60V but before SG2, Elmax, S90V, ZDP-189, or S30V. However, it was not commonly used in knives until after the release of any of those products. The general lack of M390 in knives was presumably because Bohler was more focused on selling the steel to the plastics industry. It saw limited use in some European custom knives, such as this knife by Dietmar Kressler made in 1999 [6]:

Dietmar Kressler knife in M390 made in 1999 [6]

The increase in use of M390 in knives did not come from Bohler, but from Latrobe steel in Pennsylvania, USA. They began producing a copy of M390 called Duratech 20CV, and this steel started to be used by major USA knife companies. Latrobe at the time was owned by Timken, and 20CV steel began to appear on the Timken website in 2005 [7], and advertised in Blade magazine around the same time [8]. Timken-Latrobe collaborated with SOG to test 20CV, reporting superior edge retention to S30V. Latrobe separated from Timken in 2006 [9] but continued to sell Duratech 20CV and it became popular with several major knife manufacturers. I asked SOG about how they ended up involved with 20CV and they told me that they had made Recondo and Auto Clip models with BG-42 which had very good performance but they were not happy with the manufacturability of the steel. Latrobe suggested 20CV instead and SOG had better results with the manufacturability of 20CV, and contributed knives for CATRA testing by Latrobe. SOG then released the Team Leader model in Duratech 20CV.

Image from a 2005 Blade Magazine [8]

Carpenter steel introduced their copy of M390 as “Micromelt 20-4” in 2009 [10][11] and was renamed as “CTS-204P” by the beginning of 2010 [12]. Latrobe was purchased by Carpenter in 2011 [13] though Carpenter continued to sell their CTS-204P. Crucible began to sell the steel as CPM-20CV after Latrobe was purchased. Latrobe does not have their own powder metallurgy manufacturing facilities so presumably this means that Crucible was producing 20CV for Latrobe before the purchase of Latrobe by Carpenter.

By 2010 [14] M390 was becoming more common in production knives and its popularity (along with 20CV and 204P) has steadily grown since. I previously wrote about the performance differences (or lack thereof) between the three versions in this article. 2010 was especially the year when M390 started to make a splash in the USA, with announced knives from Benchmade [15] and Kershaw [16], and the steel was available in full sheets from Bohler-Uddeholm USA [17][18] and in smaller sizes from Alpha Knife Supply [19]. Bohler-Uddeholm USA also conducted CATRA and toughness experiments to help in the marketing of their knife steels at that time. Alpha Knife Supply tells me that they had issues with the material from the HIP “can” used in the powder metallurgy process being left on the steel, which is a low carbon stainless steel. This led to reports in odd surface behavior such as low hardness readings even though the edge was fully hard. They began surface grinding the material to avoid that issue.

M390 Properties

Microstructure

The high chromium content in M390 means that less of the high hardness vanadium carbide is present in the steel, and the majority of carbide is the lower hardness chromium carbide. The vanadium does increase the hardness of the chromium carbides, however. You can read more about the interaction between different elements for carbide formation in this article on carbides.  Chromium carbides tend to be larger than vanadium carbides in power metallurgy steels which limits toughness, but they are easier to sharpen because they are softer than common sharpening abrasives like aluminum oxide. Bohler reports that M390 has about 18% chromium carbide and 2.5% vanadium carbide [20], and in my metallography I found about 22% carbide. This high carbide volume results from the high carbon and chromium in the steel, which puts its carbide content similar to steels like Maxamet, S90V, and 15V. The high volume of chromium carbide means that M390 has a larger carbide structure than those steels, however. Stainless steels with a lower carbide content like S35VN, S45VN, or Vanax also have a finer microstructure. You can compare with many other steels in this article which contains all of my micrographs.

M390

S90V

S30V

S35VN

S45VN

Vanax

Elmax

Corrosion Resistance

M390 has relatively high “chromium in solution” which is the primary factor that controls corrosion resistance apart from molybdenum and nitrogen. In my corrosion resistance testing, M390 scored very well (I tested CTS-204P), better than S30V, S35VN, S90V, and SG2, and probably a bit better than S45VN. It did not do quite as well as S110V or LC200N. Vanax would also be expected to have superior corrosion resistance though M390 is likely better than Elmax. Overall, the corrosion resistance of M390 is quite good. Below is a summary of my corrosion resistance test where a higher score means that it rusted less in a 1% saltwater test. I also have a chart which estimates corrosion resistance based on Cr, Mo, and N in solution using the results of my corrosion resistance testing which I have shown below.

Edge Retention

The edge retention of M390 is also relatively high from its high carbide content and the 4% vanadium addition. This good edge retention has been confirmed in many tests, including CATRA results from Bohler-Uddeholm, Latrobe, and my own. The Latrobe data shows all results relative to 440C, where 440C is set to 100, and every other result is divided by 440C to show how it does in comparison. Therefore, I did the same simple math with the Bohler-Uddeholm and Knife Steel Nerds data so that they can be easily compared. You can see the rest of the Knife Steel Nerds data which includes 48 different knife steels here. The Bohler and Latrobe data show a similar result for M390 edge retention while my testing was somewhat lower. Despite the Bohler and Latrobe numbers seeming to confirm a higher result I believe the Knife Steel Nerds number is more reliable (I know, I’m biased). Edge retention estimates based on the carbide content of M390 tend to predict a value closer to what we tested. The relative edge retention I tested is also close to the values found in the dataset presented in this article which came from a major knife manufacturer. I don’t know why the Bohler and Latrobe values are higher, however. The Bohler-Uddeholm tests and mine were with steels at 61-62 Rc except for 440C which was 56-59 Rc. The Latrobe data does not state the hardness values.

Ease in Sharpening

As mentioned previously, with M390 getting its good edge retention through high carbide volume rather than the hard vanadium carbides, ease in sharpening can be somewhat better than steels with comparable edge retention. This is true when using aluminum oxide abrasives, anyway. When sharpening with diamond or CBN stones there would be no benefit.

Toughness and Heat Treating Response

For a while I had a toughness chart which showed a relatively high value of about 8-9 ft-lbs for M390 at ~62 Rc in our toughness testing which put it close to steels like S35VN which have significantly less carbide and therefore would be expected to have superior toughness. This was somewhat puzzling behavior because I don’t know of anything about M390 in particular that would lead to higher than expected toughness for its given amount of carbide. However, I had a suspicion that the samples were heat treated at a temperature which was too high, leading to excess “retained austenite.” Excess retained austenite drops the hardness by a small amount but lowers the “yield strength” relative to the hardness, and also can transform to brittle, untempered martensite during use. Excess retained austenite also makes deburring in sharpening much more challenging. Therefore, even if a higher impact toughness value is measured I would not recommend heat treating the steel in that way. To see if I was right, I enlisted the help of Shawn Houston to heat treat some M390 for me because I was unable to obtain liquid nitrogen because of COVID-19. The previous specimens were heat treated at 2140°F, so we tested temperatures between 2000 and 2150°F:

The chart shows that M390 is capable of rather high hardness, particularly for its level of corrosion resistance. You can also see that there is a peak in hardness when austenitizing at 2100°F, which drops when raising the temperature up to 2150°F, which means that excess retained austenite is in the steel, despite the use of cryo after quenching. To confirm that it was excess retained austenite which was leading to the high toughness, I tested samples using 2100°F and tempered at 500°F. The higher tempering temperature was necessary to achieve a similar hardness to the original samples that were tempered at only 365°F. The toughness of the new specimens was significantly lower, and more in line with expectation based on the carbide content and relatively large carbides:

So when plotting all of the steels for toughness together you can see that M390 has relatively low toughness. This can be acceptable in knives as long as they do not have thin edge geometry and knives that are not subjected to high stresses or impacts.

Legacy of M390

As has been described in the history section, M390 as a steel design has been successful enough that it has been copied by at least two other companies, and has become popular in many knives. And the steel has been modified for yet higher edge retention with the 7% vanadium M398, which I wrote about in this article. The longevity of M390 is relatively impressive given how early it was in terms of a powder metallurgy stainless steel.

Summary

M390 was developed by Bohler in the late 1980s as a modification to K190, which in turn was a powder metallurgy version of D7 tool steel. The steel saw little use in knives until Latrobe steel copied M390 and sold it as 20CV, which then became popular with several production knife companies. Carpenter began producing CTS-204P, and all three versions have been steadily building in popularity with knives ever since. M390 has excellent corrosion resistance, potential hardness, and edge retention. Its ease in sharpening is also good with soft abrasives, at least relative to its level of wear resistance. The microstructure is relatively coarse for a powder metallurgy steel and it has a high volume of carbide, which means that the toughness of the steel is relatively low.

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[1] Giles, David J. “Ferrous alloys and abrasive-resistant articles made therefrom.” U.S. Patent 2,575,218, issued November 13, 1951.

[2] Giles, David J. “Ferrous alloys and abrasive-resistant articles made therefrom.” U.S. Patent 2,575,219, issued November 13, 1951.

[3] Kulmburg, A., and B. Hribernik. “The Heat Treatment of P/M Tool Steels.” In Materials Science Forum, vol. 102, pp. 31-42. Trans Tech Publications Ltd, 1992.

[4] Kulmburg, A., J. Stamberger, and H. Lenger. “Use of an Iron-Base Alloy in the Manufacture of Sintered Parts With a High Corrosion Resistance, a High Wear Resistance as Well as a High Toughness and Compression Strength, Especially for Use in the Processing of Synthetic Materials.” European patent 348,380, issued November 19, 1992.

[5] Huth, S., H. Hill, and W. Theisen. “Corrosion of stainless PM tool steels: Interpretation of current density potential curves in acid environments: Dedicated to Prof. em. Dr.-Ing. Hans Berns on the occasion of his 75th birthday.” HTM Journal of Heat Treatment and Materials 65, no. 4 (2010): 195-200.

[6] http://www.kresslerknives.com/Messermagazin.pdf

[7] https://web.archive.org/web/20051127135836/http://www.timken.com:80/products/specialtysteel/engineering/tech_info/knifesteels.asp

[8] July 2005 issue of Blade Magazine

[9] https://news.timken.com/2006-12-08-Timken-Completes-Sale-of-Timken-Latrobe-Steel-to-The-Watermill-Group-and-Hicks-Holdings

[10] https://web.archive.org/web/20090318072938/http://www.cartech.com:80/ssalloys.aspx

[11] https://web.archive.org/web/20090724045148/http://www.cartech.com:80/ssalloys.aspx

[12] https://web.archive.org/web/20100107063714/http://www.cartech.com:80/ssalloys.aspx

[13] https://www.businesswire.com/news/home/20110620005821/en/Carpenter-Technology-Acquire-Latrobe

[14] https://www.bladeforums.com/threads/was-m390-used-in-knives-later-than-20cv.1725021/#post-19703919

[15] June 2010 issue of Blade Magazine

[16] July 2010 issue of Blade Magazine

[17] https://bladeforums.com/threads/m390-super-steel.704854/page-2#post-7949980

[18] https://bladeforums.com/threads/m390-super-steel.704854/page-3#post-8368188

[19] https://bladeforums.com/threads/m390-vs-zdp-189-rope-cutting-informal-testing.774510/page-9#post-8692335

[20] https://www.bohler-edelstahl.com/app/uploads/sites/92/2019/09/M398En.pdf