New Steel Bohler M398 – The New King of Edge Retention?

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M398 Steel

Edit 11/19/2020: Since the release of this article I have experimentally evaluated M398, which you can read about here.

Bohler has recently released a new stainless powder metallurgy steel – M398, I first learned about it here. This is a modification of their older PM stainless M390 which was released in the late 1980’s. The change in composition is primarily an increase in vanadium, from 4% to 7.2%, with a corresponding increase in carbon to maintain the same level of hardness after heat treatment. The steel is only available in Russia right now as far as I know, and the datasheet currently available is in Russian [1]. However, quite a bit of information is available in the datasheet, allowing us to have a pretty good understanding of the properties that the steel has. Here is the composition of the new steel and several steels in a similar “class” to compare to:

S145V was patented along with S90V (now expired) but has not seen a commercial release as far as I know. Roughly comparable steels include S60V, MPL-1 (also called Supracor), and “Elmax2” which is an unreleased, patented stainless steel from Uddeholm that I have named Elmax2 because they haven’t named it. The patent is over 10 years old and it doesn’t look like the steel is going to see a wide release. Those comparable grades are similar, along with M390, because they combine a high Cr content (17% or greater) along with high vanadium (4% or greater). The high Cr is significant in part because it reduces the amount of “vanadium carbide” that is present in the steel as opposed to “chromium carbide.” The vanadium carbide is harder and therefore contributes more to wear resistance and edge retention. The more carbide a steel has the better the wear resistance and the worse the toughness is. To see how the different carbide types and carbide fraction affects edge retention read this article. Here is a chart of Crucible steels showing the effect of carbide volume on toughness:

Adapted from [2][3][4]

So the carbide volume fraction is very important for steel design and predicting steel properties. The other steels in the previous chart such as S110V, S90V, and S125V are relatively different from M398 but are similar when it comes to total carbide volume fraction and level of wear resistance. Fortunately for us, the carbide fractions of M398 and M390 are shown in the datasheet so we don’t have to estimate:

The original M390 has a reported carbide volume of 20.5% and the new M398 has about 30%, so there has been an increase in carbide of almost 50%. We would expect this to lead to a significant reduction in toughness along with an increase in wear resistance. And the datasheet shows that the toughness of M398 is only about a third of M390, though at somewhat higher hardness:

Comparing with other Bohler steels, you can see that the toughness of M398 is quite low. M390 itself is a little less tough than K390, which is a high wear resistance non-stainless steel comparable to CPM 10V. And M398 has quite a bit less toughness than that:

Adapted from [1][5][6][7][8]

So it is apparent that the design of M398 is all about wear resistance and essentially ignores toughness. M390 has only slightly lower toughness than K390 with similar carbide volume (though K390 was tested at 62 Rc). M398, on the other hand, has significantly less toughness than the similar carbide volume S290. However, M398 does have some positive traits, such as increased hardness and reduced retained austenite relative to M390:

An increase in hardness and reduction in retained austenite makes me believe that the amount of chromium in solution has been reduced, learn why chromium in solution matters for hardness in this article on Vanax steel. The bulk Cr content in M398 is the same as M390 but the amount of chromium carbide increased from 18% to 25%, which also tells us that chromium in solution has been reduced. Less chromium in solution means lower corrosion resistance, which is confirmed by the corrosion test that they did:

Evaluating the Steel Design

So overall, M398 has higher wear resistance and hardness than M390, but lower toughness and corrosion resistance. Is it a “good” design? One way we evaluate powder metallurgy steels is with the carbide volume numbers. The design goal for Uddeholm, Bohler, and Crucible is generally the same for cold work tool steels and stainless steels: maximize vanadium carbide content. More carbide means less toughness, so the lower the volume of carbide the better the toughness is. For a given volume of carbide, wear resistance is maximized by having the hardest carbides possible, which are usually vanadium carbides. Uddeholm recently replaced their Vanadis 6 and Vanadis 10 steels with a new Vanadis 8 which was designed to eliminate chromium carbides present in the older versions to improve overall properties. CPM 15V, known for its excessive amount of wear resistance, has 23-25% carbide volume of all vanadium carbide, while M398 has a full 5% more than 15V. And M398 has a relatively poor ratio of vanadium carbide to chromium carbide because of its high chromium content. So in terms of maximizing the wear resistance-toughness balance we don’t expect M398 to break any new ground. However, within any carbide volume range there is a range of properties that result based on mostly unknown factors such as carbon and alloy in solution and how large the carbides are relative to other designs with a similar carbide volume. Minor surprises are usually in the area of toughness, in my previous comparison of CATRA edge retention steels usually end up where you expect them to be based on their carbide content.

Comparing to Other Steels

But how does it compare to other knife steels in its same class? Below I will use the available information to come up with as good a comparison we can with other stainless steels in its class. These comparisons will have a range of accuracy. I recommend seeing them as “fun” educated guesses. Since the steel is brand new and largely not available we couldn’t do any experimental comparisons even if we had the money and time to do so. But we can do some comparisons based on available information and see where we think things will end up.

Edge Retention

Using the reported carbide numbers from M398 along with other steels we can use a simple equation to predict the approximate edge retention of different steels. You can read about how that equation was derived in this article. For all of these steels I assumed that the chromium carbide is “vanadium-enriched” which increases the hardness of the carbide somewhat. For MPL-1 the breakdown of its carbide types is not known, only the total volume, so I assumed all of it is the vanadium-enriched chromium carbide. And for Vanax I showed only the total carbide volume because I didn’t want to add another column for its vanadium nitrides, which are different than vanadium carbides. For each steel I used 61 Rc for the estimated CATRA. All of the values are relative to 440C at 58-59 Rc which would be “100” on the chart:

Carbide values from [1][2][3][9][10][11][12][13]

For the steels that we have experimental values for, these predictions are close. S30V is reported to be about 145%, Elmax about 142%, and M390 about 179%: Bohler-Uddeholm CATRA results. The predicted CATRA is quite high for M398, only S125V, S145V, and MPL-1 have clearly better predicted edge retention. S90V and S110V are close, however, but with significantly lower carbide volume which could mean that they have superior toughness to M398.

Toughness

To estimate how the toughness compares between Bohler (M390 and M398), Crucible (S30V, S90V, S125V, S145V, S60V, MPL-1, and S110V) and Uddeholm (Elmax, Elmax2, and Vanax) we need to see how the different toughness tests compare. We could simply predict based on carbide volume alone, but that would be too easy. Both Bohler and Uddeholm use an unnotched Izod toughness test with a 7 x 10 x 55 mm specimen. However, Bohler does this test in the “longitudinal direction” while Uddeholm does the test in the “transverse direction.” Those directions are relative to the direction the steel is rolled. You can read about why those directions are important in this article on toughness. The end result is that Uddeholm toughness values are somewhat lower than those for Bohler, particularly for low carbide volumes:

Uddeholm Toughness from [11][13][14][15]

You can also see that Elmax and Vanax steels underperform the non-stainless trendline of Vanadis 4 Extra (V4E) and Vanadis 8. However, the Elmax2 steel seems to do as well as the high carbide Vanadis 60 high speed steel. The end result is that Elmax2 has similar toughness to the original Elmax, a fact that surprised the designers of the steel [11].

The Bohler toughness values can perhaps be compared to Uddeholm if shifted down to compensate for the difference in direction. The comparison isn’t perfect but I’m trying to do some educated guesswork here. Somewhat surprisingly, however, if the izod toughness of Uddeholm in joules is converted to the ft-lbs of Crucible c-notch charpy toughness test, the values are basically right on top of each other:

So if we shift down the Bohler values, and convert everything to ft-lbs, we get a rough comparison of toughness for all of these stainless steels. The toughness of S110V was estimated using its reported carbide volume as no toughness numbers are available. Estimated toughness by comparing manufacturer’s testing, in combination with the estimated CATRA values, allows us an educated guess for the toughness-edge retention combination for the steels allowing for easy comparison:

There is also a trendline, which is simply an averaged line drawn through all of the steels. It shows generically whether steels are better or worse than average. Steels that are above the trendline show a superior combination of properties and those below it have a somewhat lower toughness-edge retention balance. M398 is low because it has a very large volume of carbide of mostly chromium carbide. Vanadium carbide gives more wear resistance for a given amount of carbide, and the total carbide content controls toughness to an extent. S90V, S110V, and S125V appear to offer a superior combination of properties to the new M398. The lower chromium content of those steels means they have more vanadium carbide rather than chromium carbide, so they are expected to have similar edge retention with lower carbide volume. If we get a chance to test the toughness of these steels ourselves we can check to see if this is born out with independent testing as opposed to mixing and matching manufacturer provided toughness values.

All of These Steels Have a Lot of Carbide

However, after all of these comparisons I can’t help but be disappointed that the steel companies appear to still be stuck on very high carbide volume stainless steels when using powder metallurgy. All of these powder metallurgy stainless steels in the chart are in a fairly narrow band of properties. There is no 3V or Vanadis 4 Extra equivalent stainless steel with lower carbide volume for better toughness. To show the stark difference here are some non-stainless Crucible PM steels overlaid on the above chart:

Conclusions and Summary

I’m personally not all that interested in yet another high edge retention steel that sacrifices toughness, ease in sharpening, and workability. We had other options and this initial look doesn’t make M398 appear to be an improvement over the other available steels. M398 was a relatively simple modification of M390 and I can’t help but think that a more optimized version could be made. M390 did offer similar toughness to the cold-work tool steels like K390 with a similar carbide volume, but M398 has significantly lower toughness than the similar carbide volume S290. So M398 doesn’t appear to be over-performing relative to its composition and carbide content. I wrote about some Bohler-patented high wear resistance steels using niobium instead that look very interesting but haven’t seen a commercial release. Those steels would be breaking new ground and potentially offering new property combinations not previously available. But if Bohler pushes out M398 more broadly it will be interesting to see how knifemakers utilize the steel for applications that benefit from high wear resistance and edge retention.

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[1] http://www.bohlernn.ru/media/Bohler_M398.pdf

[2] 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.

[3] http://www.crucible.com/PDFs/DataSheets2010/dsS30Vv1%202010.pdf

[4] http://www.crucible.com/PDFs/DataSheets2010/Data%20Sheet%204V.pdf

[5] http://www.bohler-bleche.com/media/productdb/downloads/K890DE.pdf

[6] https://www.bohler-edelstahl.com/media/K490DE.pdf

[7] https://www.bohler-edelstahl.com/media/productdb/downloads/K390DE.pdf

[8] Maili, I., R. Rabitsch, W. Liebfahrt, H. Makovec, and E. Putzgruber. “New Bohler powder metallurgy high speed steel with excellent hot hardness.” In Proc. of the 6 th International Tooling Conference, Karlstad, Sweden. 2002.

[9] Kajinic, Alqjz, Robert B. Dixon, and Brian A. Hann. “Wear and corrosion resistant PM tool steels for advanced bearing applications.” In Bearing steel technology. ASTM International, 2002.

[10] Kajinic, Alojz, Andrzej Wojcieszynski, and Maria Sawford. “Corrosion and wear resistant alloy.” U.S. Patent Application 11/124,350, filed November 9, 2006.

[11] Sadberg, Odd and Lennart Jonsson. “High chromium and carbide rich tool steel made by powder metallurgi and tool made of the steel.” French Patent filed January 1, 2003.

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

[13] https://www.uddeholm.com/app/uploads/sites/36/2017/08/VANAX-Superclean-eng-1705-e1.pdf

[14] https://www.uddeholm.com/files/PB_Uddeholm_vanadis_8_english.pdf

[15] https://www.uddeholm.com/files/vanadis_60-english.pdf