Vancron and the Problem with Nitrogen Knife Steels

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YouTube Video

There is a video that also presents this material which I recommend you watch:

New Tests on Vancron Steel

I recently got some Vancron steel from Malachi of Troopah Knives and we were able to do some toughness and CATRA edge wear testing. I heat treated all of the coupons and knifemaker Shawn Houston of Triple B Handmade ground the CATRA blades. I had previously a very small amount of Vancron that I was able to do some hardness coupons with but that was the extent of my previous investigations. Vancron is a somewhat interesting steel as it has a very large nitrogen addition (1.8%) though it is non-stainless, along with a high vanadium content of 10%. It is a modification of an earlier steel called Vancron 40 which had higher Mo and W but this resulted in Mo/W carbides that they wanted to avoid. I have a previous article about nitrogen-alloyed knife steels in terms of how they are made and the effects of nitrogen.

Nitrogen and Adhesive Wear

The high nitrogen of the steel is intended to improve its adhesive wear resistance. Adhesive wear is when two metals in contact with each other will rub and weld to each other which pulls out material from the mating surface. Galling is one type of adhesive wear. Vancron is reported to have very excellent adhesive wear due to having vanadium carbonitrides. Carbonitride is a combination of the terms carbide (carbon) and nitride (nitrogen) because the hard particles in Vancron are formed with both carbon and nitrogen along with the vanadium. The superior adhesive wear resistance is found even when comparing with steels that have large amounts of vanadium carbides like Vanadis 10 or Vanadis 8, which have a similar vanadium content but only carbon rather than the combination of carbon and nitrogen. One proposed mechanism is that vanadium nitride leads to the formation of vanadium oxides more easily that act as a sort of lubricant [2]. This occurs because of the low thermal conductivity of vanadium nitride which leads to more frictional heat being generated and therefore more oxide formation. Another proposed mechanism is that the protruding carbonitrides have less tendency to bond with steel than carbides [3]. And perhaps the simplest explanation is that the carbonitrides are smaller and therefore at a higher density than the vanadium carbides, which has often been shown to lead to superior adhesive wear resistance [4].

Image of adhesive wear from [1]

Microstructure of Vancron

It has been found previously that vanadium nitrides coarsen more slowly than vanadium carbides which is why the nitrides are smaller in size [5]. With powder metallurgy steels the carbide size starts out extremely small in powder form and they slowly coarsen during the high temperature processing of hot isostatic pressing (HIP) which fuses the powder into a solid ingot, and then hot forging and rolling of the ingot into sheet.  And indeed when we compare the heat treated microstructures of Vancron against the carbon-only CPM-10V and Vanadis 8, we do see that Vancron has significantly smaller carbides/nitrides:

Vancron 1975°F – 16% vanadium carbonitride

CPM-10V 1800°F 17% vanadium carbide

Vanadis 8 1900°F – 16% vanadium carbide

Vanax 1975°F – 10-12% chromium carbonitride, 4% vanadium carbonitride

Toughness

We would expect those small carbonitrides to lead to an improvement in toughness vs CPM-10V and Vanadis 8, which have a similar volume of hard particles. I heat treated my coupons with a “high temper” of 1000°F after austenitizing at 1900°F, and a “low temper” of 350°F after austenitizing at 1800°F. This resulted in 61.7 Rc for the high temper and 66.4 Rc for the low temper. Vancron can go to even lower or higher hardness but these heat treatments should lead to good comparisons with other steels. Vancron toughness is shown with black squares below. The toughness change with hardness was quite flat despite the ~5 Rc difference in hardness. Perhaps this was related to lower toughness with the high temper regime as was also found with CPM-CruWear and some other grades. The toughness was good, somewhat better than 10V, and similar to Vanadis 8. The toughness was similar to Vanadis 8 despite the difference in carbonitride size. There is a significant difference in toughness between D2, PSF27, and CPM-D2, for example, which have the same composition but different carbide size. Perhaps once we are down to a certain carbide size the impact toughness doesn’t improve much by going even smaller.

CATRA Edge Wear

The big surprise for me, however, was that the edge retention of Vancron was significantly lower than Vanadis 8 and CPM-10V. I thought that the high vanadium content would lead to good edge wear resistance despite the change to carbonitrides. In retrospect this is perhaps expected, as the Vancron datasheet shows its abrasive wear resistance as being only marginally better than Vanadis 4 Extra and significantly worse than Vanadis 8, which was also seen in the CATRA test (compare with CPM-4V, an equivalent grade to V4E).

The Vancron datasheet shows it having much less abrasive wear resistance than Vanadis 8 [6]

I assumed this low abrasive wear resistance was due to whatever test Uddeholm uses, as certain types of abrasive wear tests use a large abrasive size that leads to an advantage for steels with large carbides. For example, in tests by Crucible [7] they found D2 to slightly outperform S90V in abrasive wear though in the CATRA test S90V was much better than D2. So I thought that a similar effect would be seen with Vancron. I also had some hope that vanadium carbonitrides would be similar to vanadium carbides because of the performance of LC200N and Vanax in the test. LC200N has a similar amount of chromium carbonitride as AEB-L has chromium carbide, but the LC200N slightly outperformed the AEB-L at similar hardness, indicating that chromium carbonitride may be higher in hardness than chromium carbides. And Vanax had only slightly less edge wear resistance than S30V and Elmax despite its carbonitrides being signficantly richer in nitrogen with its 0.35% C and 1.55% N. The edge wear resistance for Vancron was in fact similar to Vanax if controlled for steel hardness. And Vancron and Vanax have a similar carbonitride size and volume so if the carbonitrides in Vancron were higher in hardness we would expect a significant improvement in edge wear resistance. So the best conclusion I can see from this is that the vanadium carbonitrides in Vancron are significantly softer than plain vanadium carbides found in CPM-10V and Vanadis 8.

Toughness-Edge Retention Balance

With lower hardness of vanadium carbonitrides than vanadium carbides this is limiting in terms of how high these nitrogen-vanadium steels can be. The toughness is largely controlled by the volume of the carbides/nitrides, and so if the carbonitrides are contributing less to wear resistance then the toughness-edge retention balance is reduced. That makes the carbon-vanadium grades like Vanadis 8, K390, and CPM-10V a better choice in terms of toughness-edge retention balance. This is somewhat disappointing because I had heard that Vancron was significantly easier to grind and sharpen than other 10% vanadium grades like CPM-10V and I thought Vancron might offer similar edge wear resistance to those grades but while being easier to work with; certainly that would be an attractive combination of properties.

Corrosion Resistance

Nitrogen can help with corrosion resistance by improving the pitting resistance. However, the nitrogen works by strengthening the “chromium oxide film” created by chromium, not in replacing it. The chromium is insufficient in Vancron to be stainless despite the high nitrogen. Shawn Houston found it to easily rust if he wasn’t careful during water-cooled grinding of the CATRA blades.

Summary and Conclusions

None of this is to say that Vancron is a bad steel, necessarily, I was just surprised at how low its edge wear resistance was when compared to the other 10% vanadium steels. And of course the steel does very well for its intended application – metal forming applications which typically fail by adhesive wear. The steel does offer high potential hardness and a toughness-edge retention balance in the same ballpark as powder metallurgy high speed steels and stainless steels. But it remains expensive and difficult to obtain, and at this point I am not seeing advantages in using it vs other less expensive grades.

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[1] Terwey, J. Torben, Mohamed Ali Fourati, Florian Pape, and Gerhard Poll. “Energy-based modelling of adhesive wear in the mixed lubrication regime.” Lubricants 8, no. 2 (2020): 16.

[2] Hatami, Sepehr, Alexandra Nafari, Lars Nyborg, and Urban Jelvestam. “Galling related surface properties of powder metallurgical tool steels alloyed with and without nitrogen.” Wear 269, no. 3-4 (2010): 229-240.

[3] Heikkilä, Irma. “Influence of tool steel microstructure on galling resistance against stainless steel.” In Tribology Series, vol. 43, pp. 641-649. Elsevier, 2003.

[4] Mussa, Abdulbaset, Pavel Krakhmalev, Aydın Şelte, and Jens Bergström. “Development of a new PM tool steel for optimization of cold working of advanced high-strength steels.” Metals 10, no. 10 (2020): 1326.

[5] Lindwall, Greta. “Multicomponent diffusional reactions in tool steels: Experiment and Theory.” PhD diss., KTH Royal Institute of Technology, 2012.

[6] https://www.uddeholm.com/app/uploads/sites/43/2017/09/vancron-eng_p_1904-e3.pdf

[7] 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.

References only in the YouTube video:

Mesquita, R. A., and C. A. Barbosa. “Full Density and Alternative Consolidation: Powder Metallurgy and Spray Formed 10% V Cold Work Tool Steels, A Comparison.” In European Congress and Exhibition on Powder Metallurgy. European PM Conference Proceedings, vol. 5, p. 1. The European Powder Metallurgy Association, 2004.

Randelius, Mats. “Influence of microstructure on fatigue and ductility properties of tool steels.” PhD diss., KTH, 2008.