What is M390MK Knife Steel?

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M390MK Knife Steel

Microtech announced that they would begin using M390MK steel in an Instagram post on January 14, 2023 [1]. This is the first I had heard about this new steel as well. No official composition of the steel has been released but they have given enough information from various sources that we can talk about it. In the comments of that instagram post they said: “Microtech worked closely with the engineers at Böhler to tweak the properties of M390 for better edge retention, corrosion resistance, and polishing. This is an exclusive steel for Microtech through Böhler and you will see it used quite extensively in the future!” In a BladeHQ interview Tony Marfione of Microtech gave the following answer [2]:

“What we did is, uh, we’ve been enjoying the, you know, the Böhler products now for a good long while. And um, you know we’ve been buying it billet at a time so our our our steel consumption has been when we place our orders we, we’re already planning for 2025. So, as we’re making our billets, you know, the engineers came back to us and said, ‘Hey would you like to, you know, if we were to tweak this just a little bit is there anything you’d like to see a little bit better in the steel.’ And it’s like oh my God the the M390 is as good as it gets as far as I’m concerned, but maybe a little bit better edge dexterity, or edge holding ability, maybe a little bit better utility, and some other little attributes. So, they were able to tweak the chemistry enough for us to just give us a little bit of extra icing, or a little bit of extra, you know, you know, a little extra spice. I don’t know how else to say it, but you know it’s not like out in left field; it’s still M390, but it is a tweak modification to what we’re currently using, and it’s called the M390MK and it is exclusive to Microtech.”

Also Microtech employee Ray Helms has a Youtube channel called X-Ring and in comments he has given on a couple videos he said this about the composition: “Internally at Böhler is (sic) has another name. It has a more controlled and higher carbon content which lends itself to more consistent heat treat and a few other advantages.” [3] And when asked what the difference was between M390MK and M390 he said, “Just a more regulated and controlled carbon content.” [2]

As far as I can find they have not said what the “MK” is supposed to stand for but I think “Microtech Knives” is more likely than “Mortal Kombat” or “Michael Kors.”

OK, What is it?

I find that reading or listening to marketing material can be misleading on the actual properties of steels so first let us analyze what they said about the composition of the steel. Helms said that the difference betrween M390MK and M390 was “Just…carbon content” and in the other statement he said that it was “higher” carbon. He also said that the carbon was “more regulated and controlled.” So we should discuss why the carbon content would need to be “regulated” or “controlled” in the first place. When you look up the datasheet for a steel and read off the composition you will see a single number for each element, such as the following for M390 [4]:

You’ll notice that above the composition chart for M390 here (not M390MK), it does clarify that this is the “average” composition. For a steel with a standard specification, such as D2, there is a required composition range for each element. So even though D2 might have an average composition of 1.5% carbon and 12% chromium, there are allowable ranges, for D2 they are 1.4-1.6% carbon and 11-13% chromium. In addition to these ranges which are required by the official specification, steel companies also have internal specifications. So perhaps the company might determine they can have a tighter range for D2 and so their carbon range is 1.5-1.6% instead. For a proprietary steel, however, the ranges are set only by the steel company internally. These ranges are usually not public, though they may be listed on composition certification sheets in some cases. It is not possible to have every heat of M390 be exactly 1.9% carbon, 20% chromium, etc. so there is a range that the company will shoot for like 1.85-1.95% carbon and 19.5-20.5%, though I do not know what the actual values are. The range they shoot for will be based partly on what the steel company can achieve on a regular basis as they obviously don’t want to be scrapping steel all the time. In some cases minimums or maximums may be set because they know the steel will not achieve the desired properties if it falls out of a certain range. Oftentimes multiple measurements of the steel composition are taken from the molten steel while they are making additions, so if a measurement comes back low then they will continue to add more until they get into their desired range. While trying to avoid overshooting, of course. As a further complication, the composition of a steel will not be perfectly consistent throughout and steel that is in the top, bottom, sides, or center of an ingot may be slightly different than a measurement at another location, for example.

So when Helms mentioned that the steel would have a more controlled and higher carbon content, that presumably means that the range they are asking for is tighter than the previous internal range by Böhler, and also the average is higher. Based on the statements it is not clear if the actual upper end has been raised or only if a narrower range is aimed for in the upper end. So if the internal carbon range were 1.85-1.95%, for example, they could be targeting 1.9-1.95% instead, or if the upper limit were increased it could be 1.9-1.98% instead. These are only possibilities, as they have not told us what the actual numbers are. I was considering purchasing one of the M390MK knives to measure the composition directly, but due to the change being described as a “tweak,” along with the statistical nature of composition variation, measuring the composition may very well lead to misleading information about the actual composition. For example, I could have a knife with steel that is on the upper end or lower end of a certain element like chromium and therefore cause me to think that the chromium has changed when in fact it has not. Or the equipment I use may have a slightly different calibration than Böhler’s, again leading to a misunderstanding on what has changed. So I think taking the statements from Microtech that the difference is a small change in carbon makes more sense than trying to measure the change. For example, a few years ago I measured the composition of some M390 using a standard technique called Optical Emission Spectroscopy (OES) along with a combustion measurement for carbon and nitrogen. For the composition I measured I got a relatively low value for chromium (<19%), but this could very well be a calibration issue or a relatively low chromium heat (or both). But if this had been a measurement of M390MK I might think that they had reduced the chromium when in fact they had not done so.

A measured composition of typical M390 from a few years ago.

How Does Increased Carbon Affect Properties?

The basic changes from higher carbon include an increase in hardness and wear resistance and a decrease in corrosion resistance. If the carbon is only being changed by a small amount, a “tweak” as Marfione said, we would expect the difference to be relatively minor. A “statistical” difference, perhaps, though there very well could be M390 heats of steel out there already with the same or at least extremely similar properties. If we imagine a bell curve of heat treating response, for example, the “tweaked” version hardness distribution would be bumped up by some small amount, likely less than 1Rc, or even 0.5 Rc. And the distribution would be slightly “tighter” due to the smaller distribution of carbon. Below shows a hypothetical example of how they might end up:

Hypothetical change in hardness distribution of heat treated knives by using M390MK instead of M390 (not based on any experimental data).

As the change in carbon is reported to be relatively small, I don’t expect the actual change in measured hardness, corrosion resistance, or wear resistance to be particularly large. It might even be difficult to measure without testing a large number of specimens to get some kind of bell curve such as the hypothetical one shown above. But this effect of carbon in stainless steels is well understood. Such as 440A, 440B, and 440C, which all have the same composition except they have different carbon contents. Each has ~17% chromium and an optional Mo addition along with 0.7% carbon (440A), 0.85% carbon (440B), or 1.05% carbon (440C). From what Microtech has told us I think the carbon change in M390MK is smaller than this example, but it is well known that using 440C gives you higher hardness than 440A at the cost of some corrosion resistance and toughness.

Hardness from Carpenter datasheets

How Does This Line Up with What Microtech Said?

The statement from Microtech said that M390MK would have, “better edge retention, corrosion resistance, and polishing.” An improvement in edge retention may result from a slightly higher hardness and slightly higher fraction of carbide. This improvement may be difficult to measure given this is a “tweak” to the composition but that is ok. A claimed improvement in corrosion resistance is very strange, however, as there is no situation I can think of where an increase in carbon would improve corrosion resistance. Even if they also changed the chromium content without mentioning that in the prior comments, we would expect that to then balance out for the same corrosion resistance, not an increase. Perhaps they were simply mistaken about the effect of carbon on corrosion resistance. The claimed improvement in polishing is also strange as I’m not sure how the increase in carbon would help with that. The only thing I can think of is if their particular polishing practice has been found to work better when the hardness is slightly higher. Though that would be easier to achieve by a change to the heat treatment rather than the carbon content. Typically an increase in hardness or carbide content and wear resistance would make polishing more challenging, not easier. So I am also confused by that claim. Perhaps if there are other changes to the composition that could explain those claims, Microtech might clarify on that point and we can discuss it again when that happens. A change in heat treatment would likely lead to a greater difference in edge retention and corrosion resistance than a small change in carbon.

Summary

While discussing these potential changes to M390 to make M390MK I think it is important to remember what Tony said about the new steel: “it’s not like out in left field; it’s still M390, but it is a tweak modification.” The steel is “still M390” but slightly changed in a way that Microtech believed would be an improvement for their process and their customers. And the M390MK branding was created by the company to differentiate from “normal” M390 and therefore have an “exclusive” product which has been known in recent years to drive certain collectors which want knives in new or exclusive steels to try. The slightly higher carbon in M390MK is expected to lead to a small increase in hardness when using the same heat treatment (likely less than 0.5 Rc), and a small decrease in corrosion resistance. It will be interesting to see if this leads to increased excitement for Microtech knives with their tweaked M390 and if end users feel there is a difference in properties.

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[1] https://www.instagram.com/p/CnZzpcjrEFD/

[2] https://www.youtube.com/watch?v=5UOf4aRuArY

[3] https://www.youtube.com/watch?v=5UOf4aRuArY

[4] https://www.bohler-edelstahl.com/app/uploads/sites/92/2018/10/M390EN.pdf