M398 Steel Testing – Edge Retention, Toughness, and More

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The steel ratings table from my new book Knife Engineering has now been added as a Patreon-exclusive with M398 added to the table.

Previous M398 Article

I wrote about M398 back in May 2019 right after I learned about the steel where I covered what information was provided by Bohler and what educated guesses we can make about its performance. I predicted that the edge retention would be very good, better than S90V but not as good as S125V. I also wrote quite a bit about its toughness, Bohler in its datasheet already showed that its toughness was significantly reduced relative to M390, and comparisons with other datasheets showed that its toughness would likely be lower than any other available stainless knife steel, including S125V. Estimates also predicted a slight reduction in corrosion resistance relative to M390. I now have some M398 steel which was given to me by Alpha Knife Supply. I ran the steel through a range of standard tests to see how it compares to other steels in its category of high edge retention stainless steel.

History and Design of M398

I wrote a little bit about M398 in the previous article. It is a modification of Bohler’s M390 which was patented in the late 1980s. The carbon and vanadium of M390 was increased to provide more wear resistance. M390 was a modified version of D7 tool steel with increased chromium and reduced carbon to make it stainless. Read more about the history of M390 in this article. One disappointing thing about M398 is that it is a simple modification of a 30 year old steel. Bohler did not use any of the advances in steel design over that 30 year span to improve the steel. Bohler itself even has a patent on high niobium stainless steels which is very interesting, but has not produced any commercial products with it.

Hardness

I tested the hardness of M398 by austenitizing for 20 minutes followed by a plate quench, cryo, and temper twice for two hours each time. My hardness values look very close to M390 despite Bohler claiming an increase in hardness. Some knifemakers who have heat treated M398 have achieved about 1-2 Rc higher than me so I’m not sure why my values were lower. Knifemaker Roman Kasé tells me that he was only able to get over 64 Rc when using an oil quench. This would indicate that the “hardenability” of M398 is relatively poor, meaning a plate quench is not enough to achieve maximum hardness. It could also be that my M398 is somewhat low in carbon or has some other small variation in composition.

Microstructure

According to Bohler, M398 has a very high amount of carbide, with 25% chromium carbide and 5% vanadium carbide. More carbide means higher wear resistance and edge retention but reduced toughness. The proportion of the vanadium carbide (VC) matters as VC is harder and contributes more to wear resistance. So we expect S125V with 16% VC to have better wear resistance than M398, but ZDP-189 with 0% VC to have less even though all three of those steels have a similar overall amount of carbide. M398 has rather large carbides, likely in part from the high chromium carbide content as chromium carbides grow more rapidly than vanadium carbides. There are also many carbide “clusters” where many carbides are connected to each other leading to a larger average carbide size. The carbides are larger than stainless steels in a similar property range including S60V, ZDP-189, S90V, S125V, and S110V. Compare with micrographs of more steels in this article.

M398

S125V

S110V

S90V

S60V

ZDP-189

Toughness

We tested toughness with our standard unnotched charpy specimen, the specifications of which can be found here. I heat treated the steel at 2050°F for 20 minutes, plate quenched, liquid nitrogen cryo, then tempered at 400°F twice for two hours. The toughness of M398 is among the lowest I have ever tested, especially when hardness is taken into account. ZDP-189, for example, had slightly higher toughness despite being 3.5 Rc higher. And S125V had only slightly less toughness despite being 1 Rc higher. S125V is closer to the hardness trendline extrapolated from S60V, so S125V appears to be slightly tougher when at equal hardness. S90V and S110V are both a higher tier for toughness than M398. Also, while M398 only looks slightly lower than other high wear resistance stainless steels, the toughness differences are more significant at lower values. For example, the difference between 2.5 and 5 ft-lbs is roughly the same as between 5 and 10 ft-lbs or between 10 and 20 ft-lbs. However, none of these high edge retention stainless steels has particularly good toughness. M398 should not be used for any applications where toughness is required.

Edge Retention

We also tested edge retention using CATRA with our standard test knife as described in this article. Shawn Houston heat treated the steel, ground the knife, and sharpened the knife to 15 dps with a 400 grit CBN metallic bonded stone. I resharpened the knife and retested 3 more times. We were shooting for 61-62 Rc as with the majority of other knives that were tested, though overshot a bit with 62.8 Rc. The dotted lines represent the estimated influence of hardness on edge retention. So even after compensating for hardness, M398 still had slightly better edge retention than S90V, though lower than S125V. This is of course excellent edge retention and should be enough for most any knife.

Toughness-Edge Retention Balance

Because the toughness of M398 is so poor, its toughness-edge retention balance is not setting any records. The values below are taken from the CATRA and toughness values given above. However, there are small differences in hardness between the two tests, so I have adjusted the edge retention to estimate where it would be at the hardness of the toughness test. Adjusting edge retention based on hardness is easier than predicting changes in toughness. So the toughness value for S125V below is lower than M398, though as I pointed out in the toughness section the hardness of S125V was higher when it was tested, and when at the same hardness S125V may be slightly better than M398. Regardless, S90V and S110V have superior toughness-edge retention balance to M398. Because this category of stainless steels already has very poor toughness, I would prefer to use one of those two steels. They have less carbide and smaller carbides than M398, but maintain similar edge retention because they have a higher ratio of harder vanadium carbide.

Corrosion Resistance

Corrosion resistance was tested on M398 using the test parameters previously described in this article. The steel was heat treated at 2050°F and tempered at 400°F. The 1 x 1.5 inch specimens were finished to 400 grit, sprayed with 1% saltwater every 8 hours, rinsing the specimens with water each time, and I photographed them after 24 hours. I compared with 20CV (same as M390), S60V, ZDP-189, S90V, S125V, and S110V to compare with other stainless steels with high edge retention. ZDP-189 was not tested as I already found it to have very poor corrosion resistance. The table below shows the austenitizing temperature that was used because that controls how much chromium carbide is dissolved, putting chromium into solution to promote corrosion resistance. I then have the Thermo-Calc estimated chromium and molybdenum in solution. Thermo-Calc expects the chromium in solution to be reduced in M398 relative to 20CV and M390. However, 20CV, S110V, and M398 had roughly comparable corrosion resistance in this test. S90V, S125V, and S60V were clearly worse. Both 20CV and S110V have very good corrosion resistance so this tells us that M398 is also very good.

20CV (same as M390)

S110V

M398

S90V

S125V

S60V

Ease in Sharpening and Grinding

The most common abrasive type in sharpening stones, sandpaper, and grinding belts is aluminum oxide, which is softer than vanadium carbide. Therefore, the higher the vanadium carbide fraction the more difficult sharpening can be when it comes to material removal. So M398 may be somewhat easier to sharpen than S90V, S110V, or S125V but more difficult than M390, S60V, and ZDP-189. When using CBN or diamond sharpening stones this difference is removed and how difficult it is to grind away material is then roughly proportional to the edge retention of each steel (higher edge retention means harder to sharpen). However, removing material is only one element of sharpenability. Deburring, for example, is primarily controlled by hardness and the amount of retained austenite, which is more controlled by heat treatment than steel. The CATRA knife austenitized at 2050°F along with cryo was relatively easy to deburr. Using higher austenitizing temperatures or skipping cryo could make deburring more difficult by increasing the amount of retained austenite.

Summary and Conclusions

M398 offers very high edge retention for a stainless steel in combination with very good corrosion resistance. However, the toughness is very low and the carbide structure quite coarse for a powder metallurgy steel. S90V and S110V offer better toughness with a similar level of edge retention. S90V has somewhat lower corrosion resistance, but S110V and M398 are roughly comparable. M398 may be a bit easier to sharpen or grind than S90V and S110V due to less vanadium carbide; however, given its high wear resistance it is not going to be great either way. I think S90V and S110V are more balanced steels in general, and S125V still offers the highest edge retention of any stainless knife steel. I think it is good that Bohler added another stainless choice to the high wear resistance category as Crucible has had a monopoly in that area in the past. But I wish that Bohler had put more effort into designing a superior steel from the ground up rather than simply adding carbon and vanadium to a 30 year old steel (M390), ignoring advancements in steel design that even Bohler has patents in.