Is There a New Bohler Knife Steel Coming?

Thanks to our new Knife Steel Nerds Patreon supporters! Bård Nordang, Distal Steel, Just_A_Fan, Donald Clarkson, Joel Kuh, and Manuel Meyer.

YouTube Video

The video version of the following information is available here:

New Journal Article with Bohler Involvement

While looking at newly published journal articles in the major Materials Engineering journals I noticed a new one which has been “accepted” but not yet published in the final journal [1]. The title of this article is “Novel development of an NbC-containing powder-metallurgical martensitic steel with outstanding tribocorrosion resistance.” Most of the researchers on the paper are from Ruhr-University Bochum with one coauthor from Bohler steel, the company behind steels like M390. Professor Werner Theisen has previously published about niobium powder metallurgy stainless steels and he has a couple patents in conjunction with Bohler steel. I have written briefly before about his niobium steels in this article on niobium alloying of knife steel. Niobium additions are desirable in powder metallurgy stainless steels because niobium carbides are very hard and contribute greatly to wear resistance. But unlike vanadium, niobium has basically no effect on the chromium, allowing it to stay in solution and contribute to corrosion resistance. His previous research was on high niobium steels, from 3-15%, and the patent mostly discussed ~9% niobium steels [2]. With typical processing of powder metallurgy steels, adding this much niobium leads to production issues because niobium carbides will form in the liquid steel and clog the “nozzle” where the liquid steel is sprayed by nitrogen to “atomize” it into a powder. You can read more about how powder metallurgy works in this article. To get around this they patented a method where no carbon is added so no niobium carbides are formed, and then carbon is introduced to the powder by mixing it with graphite before the powder is HIPed (Hot Isostatic Pressed) into a solid ingot. None of these steels have been produced commercially to my knowledge. Perhaps it was the extra step of adding graphite that prevented commercialization. This is supported by a statement in the paper which says, “However, the diffusion alloying process is very complex and not very economical, so that no large-scale industrial implementation is carried out.” So for the steel presented in this new paper they limited the niobium content to 2.0%. I have its composition below along with a few steels for comparison:

The new steel looks sort of like a “matrix” version of the original patented S110V before it was modified to be the final version that was commercialized. Both have 13-14% Cr in combination with ~3.5-4% Mo and 1.5-2% Co. And both were designed to limit the amount of niobium so that no nozzle clogging issues occur. Cobalt is a relatively uncommon element. Both S110V and this Bohler niobium product were designed to have relatively high amounts of chromium and molybdenum for corrosion resistance but if you have too much of these elements you can’t harden the steel anymore because they are “ferrite stabilizers” meaning you cannot transform the steel at high temperature to austenite which is necessary for quenching the steel to hard martensite. Cobalt is an “austenite stabilizer” so it can help counteract this issue. You can read more about S110V in this article.

Properties of the New Steel

For the paper they made their main comparisons to Bohler M390 and K190. M390 is of course a common knife steel and K190 is a powder metallurgy version of an old tool steel called D7, which is similar to D2 but with 4% vanadium along with increased carbon. K190 is not a stainless steel.

Hardness

The new steel was austenitized at 1130°C/2065°F and quenched in oil, cold treated at -80°C and tempered twice at 180°C/350°F. The K190 was austenitized at 1070°C/1960°F and the M390 from 1150°C/2100°F, but otherwise heat treated the same. The resulting macrohardness was 750 HV for the Nb Steel, 839 HV for K190, and 836 HV for M390. Using a conversion equation from Paul Beiss [3], this gives roughly 60.5 Rc for Nb Steel, 63.9 Rc for M390, and 64.0 Rc for K190. Because only niobium carbides were left in the microstructure of the niobium steel, which do not dissolve until very high temperatures, this is likely the approximate maximum hardness of the steel.

Microstructure

The Nb Steel has a very fine microstructure since it has only ~2.5% niobium carbide vs M390 and K190 which have greater than 20% carbide volume. Also those two steels have chromium carbides which are typically larger in powder metallurgy tool steels. The images are pretty low-res and have part of a watermark on them but you can still get the gist. X230CrVMo13-4 is K190 and X190CrVMo20-4 is M390.

Corrosion Resistance

Tested corrosion resistance showed excellent results due to the lack of chromium carbide in combination with the high chromium and molybdenum in solution. The measured “pitting potential” in 0.9% saltwater (higher is better) was 778 mV for the Nb Steel, 205 mV for M390, and -367 mV for K190. This steel would be a good one for saltwater knives.

Toughness

This finer microstructure, lower volume of carbide, and lower hardness means that the measured toughness was much better than M390 or K190, 130 joules for the Nb Steel, 35 J for K190 and 40 J for M390. This was tested with an unnotched Izod impact test. Below I have a chart of Bohler steel toughness to get an idea of comparisons. K490 is a 4V/Vanadis 4 Extra type steel, and K890 is in the range of CPM-3V, and even K890 wasn’t reaching 130 J, so this Nb Steel should be very impressive for toughness.

If we estimate the Nb Steel toughness when compared with other steels I have tested, we would expect it to be quite high, in the range of steels like AEB-L and Z-Tuff.

Wear Resistance

The only wear resistance numbers given were for a sliding wear test where the Nb Steel predictably had much lower wear resistance than M390 and K190. We can estimate the wear resistance of the steel using equations I have previously developed using a database of CATRA edge wear test results. With the relatively low volume of hard niobium carbides I would estimate the steel to be in the range of steels like A2, Z-Tuff/CD#1, and AEB-L when at equivalent hardness.

Properties Summarized

This Nb Steel would be an interesting alternative to LC200N with a similar combination of properties. Perhaps it would even have somewhat better toughness though it is unlikely to be able to match LC200N in terms of cost. The Nb Steel would have excellent toughness and corrosion resistance though moderate maximum hardness and wear resistance.

What Was the Steel Designed For?

In the paper they say the steel would be ideally suited for steels that need to be resistant to both corrosion and wear (tribocorrosion). So they proposed bearings in corrosive environments, plastic mold steels, industrial cutting tools, and tools in the medical sector. Pocket knives and kitchen knives were not mentioned though they didn’t have to have knives in mind for it to work in them, of course.

Will the Steel Actually be Released?

The previous patents on 9% niobium PM stainless steels (that were never commercialized) perhaps gives us some skepticism about the steel actually seeing the light of day. However, the paper addressed the issues with commercialization of those previous products and this steel was apparently designed to overcome them, so perhaps there is some hope.

---

[1] Hahn, Isabel, Sabine Siebert, Rainer Fluch, Werner Theisen, and Sebastian Weber. “Novel development of an NbC‐containing powder‐metallurgical martensitic steel with outstanding tribocorrosion resistance.” steel research international.

[2] Theisen, Werner, H. U. T. H. Stephan, Jochen Perko, and Herbert Schweiger. “Wear-resistant material.” U.S. Patent 8,623,108, issued January 7, 2014.

[3] Beiss, P. “Properties of Powder Metallurgy Tool Steels.” Powder metallurgy-ASM Handbook, ASM International, Ohio, USA (2015): 491-514.