Nitro-V – Its Properties and How to Heat Treat It

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Nitro-V Steel

Nitro-V is a stainless steel sold by New Jersey Steel Baron which was first released in 2017 [1]. The steel was designed and produced in collaboration with Buderus Steel as a version of Uddeholm AEB-L modified with nitrogen and vanadium. Another obvious comparison is with 14C28N which was designed as a version of 13C26 modified for improved corrosion resistance. 13C26 is nearly identical to AEB-L but produced by Sandvik. You can read more about the history of AEB-L, 13C26, and 14C28N in this article.

Peter Bruno of New Jersey Steel Baron provided me the following information on the steel: Nitro-V Data

Composition of Nitro-V

Nitro-V does indeed look very similar to AEB-L, having the same C, Cr, and Si along with a minor difference in Mn. The nitrogen content is the same as 14C28N, which may simply be due to limits in nitrogen additions with conventional steel production. You can read about those limits in this article on nitrogen additions. The vanadium addition is very small, likely too small to make a contribution to wear resistance and edge retention. Typically such small additions of vanadium are for the purpose of grain refinement in low alloy steels. Low alloy steels see all of their carbide dissolve at forging temperatures and high heat treating temperatures which allows the grain size to grow rapidly. However, the high amounts of chromium in stainless steels means that more vanadium is required to contribute to grain refinement. You can read more about carbides and interactions between different elements in this article on carbides. Both ThermoCalc and JMatPro predict that no vanadium carbide is present at typical heat treating and forging temperatures. Therefore the vanadium is likely doing very little in Nitro-V.

Hardness of Nitro-V

Like other stainless steels Nitro-V has high hardenability so it can be cooled in air or plate quenched for hardness. Oil quenching is also possible though plate quenching is good for helping to maintain flatness. I recommend cryogenic processing for maximizing strength when heat treating. You can read more about cryo here: Part 1, Part 2, and Part 3. For heat treatment experiments on Nitro-V I austenitized for 15 minutes at 1850, 1900, 1950, and 2000°F followed by plate quenching and then immediately going into liquid nitrogen for an hour. The steel was then tempered at 300, 350, 400, or 450°F. Peak hardness was achieved at 1950°F, and remained flat to 2000°F. Therefore I recommend austenitizing no higher than 1950°F. The higher 2000°F has the same hardness despite the increase in hardness of the martensite because of an increase in retained austenite. That means that the 2000°F austenitize likely leads to a decrease in strength and toughness. Heat treating without cryo would mean a reduction in hardness and a reduction in the peak austenitizing temperature. I recommend at least going into a household freezer directly after quenching to reduce the retained austenite as much as possible. To check your heat treatment against mine, it is best to perform heat treatments from a range of austenitizing temperatures and going straight into the cold treatment used, whether freezer, dry ice, or liquid nitrogen. Check hardness and austenitize no higher than the peak hardness.

When using a freezer instead of liquid nitrogen, the peak austenitizing temperature is usually reduced relative to liquid nitrogen treatments because the freezer is not as effective at reducing retained austenite. Knifemaker Mike Pierce reported to me that he achieved peak hardness of 60.5 Rc from 1900°F without a cold treatment, which was increased to 63 Rc when going directly into the freezer after the quench.

The peak hardness of Nitro-V is relatively similar to AEB-L and 14C28N. Both AEB-L and Nitro-V reach about 64 Rc, while 14C28N reached 63.5 Rc, though I didn’t austenitize high enough with 14C28N to see where the hardness begins to decline. The peak austenitizing temperature of Nitro-V is a bit reduced relative to AEB-L.

Effect of Nitrogen on Hardness

Nitrogen is typically added to steel in an attempt to improve hardness, corrosion resistance, or both. 14C28N, for example, was designed to have a reduction in carbon along with an increase in nitrogen to make up for the carbon in terms of hardness and an increase in chromium for improved corrosion resistance. Nitrogen is not as prone to form nitrides with chromium as carbon is to form carbides. So nitrogen can sometimes be added for improved hardness without a sacrifice in corrosion resistance. However, when comparing with AEB-L above, the hardness was not enhanced with Nitro-V. The peak hardness is limited by retained austenite rather than necessarily by how much carbon and nitrogen can be put in solution. You can read about why hardness is limited by retained austenite in this article on Vanax steel.

Chromium and Corrosion Resistance

In stainless steels, the amount of retained austenite for a given amount of carbon in solution (hardness) is how much chromium is in solution (corrosion resistance). The similar peak hardness of Nitro-V and AEB-L may therefore indicate that the amount of chromium in solution is similar. Furhtermore, since AEB-L and Nitro-V have the same bulk Cr, to put more Cr in solution relative to AEB-L it would require a reduction in carbide content which would reduce wear resistance and slicing edge retention.

Calculations of chromium in solution from Thermo-Calc confirms that the two are similar, especially when considering that Nitro-V needs to be austenitized at a somewhat lower temperature. I have cut off the lines where peak hardness was achieved in each. 14C28N, however, has significantly more chromium in solution, which likely gives it better corrosion resistance than Nitro-V and AEB-L.

Nitrogen and Corrosion Resistance

Nitrogen can also help improve corrosion resistance, when it comes to pitting it has 16 times the contribution of chromium. So 0.11% nitrogen is equivalent to about 1.75% Cr for pitting resistance. Therefore the corrosion resistance of Nitro-V may be somewhat better than AEB-L depending on the environment. Pitting is particularly a problem in salt water. Because 14C28N has a similar nitrogen content plus higher chromium in solution it is expected to have superior corrosion resistance when compared with Nitro-V. Perhaps in the future we can test the corrosion resistance of Nitro-V to see how much of an improvement there is relative to AEB-L.

Microstructure

I have previously shared a micrograph of Nitro-V that I took along with 41 other steels. I also have micrographs of AEB-L and 14C28N. Here are all three below:

Nitro-V (1950°F austenitize)

AEB-L (1975°F austenitize)

14C28N (1950°F austenitize)

The white particles are the carbides and the surrounding “matrix” is primarily martensite. Nitro-V has a very fine microstructure, with AEB-L possibly being a touch finer. AEB-L and Nitro-V have a similar volume of carbide, approximately 4-6%. 14C28N appears to have somewhat larger carbides than either but is still significantly finer than even many powder metallurgy steels. That would put the three steels in a similar class in terms of wear resistance and slicing edge retention.

Toughness Optimization

For toughness testing I austenitized Nitro-V at 1850, 1900, and 1950°F for 15 minutes, followed by plate quenching and cryo, and then tempered twice at 350°F for two hours each time. The toughness is tested by averaging three specimens which are unnotched subsize charpy specimens. You can read more about the charpy specimens we use here. I heat treated the specimens and Mike Pierce kindly machined them for me.

The 1850°F austenitized led to reduced toughness relative to the 1900 and 1950°F conditions. This could be due to insufficient dissolution of carbide (carbides are bad for toughness), or possibly because increased austenitizing temperatures led to more retained austenite despite the use of cryogenic processing. Retained austenite in the right amounts can sometimes improve toughness. Therefore the optimal austenitizing range for Nitro-V is 1900-1950°F because 1850°F leads to a reduction in both hardness and toughness and 2000°F led to excessive amounts of retained austenite.

I next produced toughness specimens using the 1950°F austenitizing temperature plus cryo but tempered at 300 and 450°F to test the effect of tempering temperature. I used 4 specimens for each condition and these were machined by me. The 300°F temper led to increased hardness with a reduction in toughness, as expected. The 450°F temper led to a reduction in hardness and toughness, so it is not recommended. This reduction in toughness was somewhat unexpected because tempered martensite embrittlement is not expected with this tempering temperature in high alloy steels. I don’t know if a 400°F temper would see similar results. But I can’t recommend tempering higher than 350°F right now because I don’t know where the reduction in toughness takes place.

Toughness Comparisons

Nitro-V has very good toughness compared with other steels. It has slightly lower toughness than AEB-L for unknown reasons. It is significantly tougher than the powder metallurgy stainless steels like M390, Elmax, and S35VN. Therefore Nitro-V would be well suited to knives requiring good toughness.

Edge Retention

Nitro-V is expected to have very similar wear resistance and edge retention to AEB-L, but there has not been any testing to confirm that. The compositions of the two are similar and the micrographs also look very similar in terms of carbide content. AEB-L has edge retention similar to 52100, which is significantly lower than higher carbide content stainless steels like 440C, CPM-154, S30V, etc. The tradeoff is that those steels have much lower toughness than AEB-L and Nitro-V, of course. Here is a chart showing the relative position of AEB-L to get an idea of where Nitro-V likely sits:

Summary and Conclusions

Nitro-V looks very similar to AEB-L with a nitrogen and vanadium addition. The nitrogen addition, however, did not increase chromium in solution for enhanced corrosion resistance and the vanadium addition was insufficient for grain refinement according to simulation software. The nitrogen may itself increase the corrosion resistance somewhat relative to AEB-L. Nitro-V is capable of approaching 64 Rc when heat treated for maximum hardness, and can of course be heat treated for lower hardness as well depending on the desired properties. Nitro-V has a very fine microstructure and very good toughness when compared with other steels. Overall the properties of Nitro-V are expected to be similar to AEB-L with perhaps an improvement in corrosion resistance. I got good results in heat treating by austenitizing between 1900 and 1950°F for 15 minutes followed by a plate quench and a cryo treatment. Use 1900°F if using a freezer instead of liquid nitrogen. Temper between 300 and 350°F depending on desired hardness. A 450°F temper led to an unexpected reduction in toughness. More testing needs to be done in terms of corrosion resistance and edge retention to confirm the steel behaves in the expected range I proposed based on its composition and microstructure.

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[1] https://www.bladeforums.com/threads/something-new-from-the-baron.1469387/