S35VN Steel – Properties and How to Heat Treat

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S35VN Steel History

S35VN is a steel developed and produced by Crucible Industries, released in 2009. The steel was developed by Maria Sawford as a modification of S30V by reducing the vanadium content, adding 0.5% niobium, and removing the nitrogen addition (incidental nitrogen is still present). Powder metallurgy steels typically have at least 0.03% nitrogen [1][2]. These changes led to a steel with improved toughness and machinability relative to S30V at the cost of some edge retention. Heat treatment response and corrosion resistance remained approximately the same as S30V. More information on the history leading up to S35VN can be seen in this article on the newer S45VN.

S35VN Microstructure

Reducing vanadium to 3% and replacing it with only 0.5% niobium means that that MC carbide content (VC + NbC) is also reduced relative to S30V which is why wear resistance and edge retention is reduced somewhat in S35VN. In the S35VN datasheet Crucible lists the MC content as 3.5% compared with S30V’s 4%. However, Thermo-Calc predicts 2.6% MC for S35VN compared with 3.9% MC for S30V. Therefore I think the MC content is somewhat less than Crucible has estimated. MC is the hardest carbide type and contributes the most to wear resistance. It also makes grinding, machining, polishing, and sharpening more difficult due to the increase in wear resistance. And particularly when using common abrasives such as alumina which are softer than MC. You can read more about different carbide types in this article.

A niobium addition can help refine the microstructure of steels, you can read about the effects of niobium in this article. Comparing the microstructure of S30V and S35VN, however, the carbide size looks pretty similar. You can compare with other steels in this article which collects micrographs of many knife steels.

S35VN – 1975°F Austentiize

S30V – 2000°F Austenitize

S35VN Edge Retention

In the S35VN datasheet the edge retention of S35VN and S30V are both listed as 145% (of 58-59 Rc 440C) with an asterisk next to S35VN that says, “Estimate based upon market feedback.” A hardness value is not provided for either steel. A CATRA study performed by Bohler-Uddeholm found lower edge retention in S35VN, approximately 132% compared with 149% for S30V, both tested at 61 Rc. The S45VN datasheet shows a revised value for S35VN which is 140%. Because of the shift in the value in the datasheet and the lower MC estimate in Thermo-Calc relative to S30V, I think the 132% value is most accurate for S35VN. Here is a chart comparing it with other steels:

S35VN Corrosion Resistance

I previously tested the corrosion resistance of S35VN, the results of which are shown in this article, and a comparison with S45VN in this article. S35VN did well in corrosion testing, almost as well as S45VN which was designed to have improved corrosion resistance with its increased chromium content. I did not test S30V but S35VN is likely slightly superior to S30V assuming the same heat treatment. Using my corrosion resistance testing I rate S35VN a 7.8 out of 10, similar to Elmax (7.8) but not as good as S45VN (8.4), M390 (9.1), or Vanax (10).

S35VN Hardening Response

I heat treated a range of S35VN specimens using a 30 minute austenitize for 1925 and 1950°F, and 15 minutes for higher temperatures. The samples were then plate quenched, given a cryo treatment in liquid nitrogen, and then tempered twice for 2 hours each time. My heat treatment tests resulted in similar hardness to the datasheet, shown below:

S35VN has good potential hardness; it is capable of reaching at least 64 Rc. That should provide a good range of hardness values to the knifemaker.

S35VN Toughness Optimization

Knifemaker Michael Drinkwine heat treated and machined a set of S35VN specimens for toughness specimens for me. Each condition was tested with three specimens and averaged. One odd thing was that the specimens were consistently about 1 Rc lower in hardness than mine with the same heat treatment. Hardness was measured by me in both cases so it was not a difference in hardness tester. I am not sure what the reason for the offset was. Perhaps it was the result of a different heat of steel, or a difference in furnace, or cooling rate in quenching. It’s hard to know for sure without investigating it specifically.

Looking at the trend of hardness vs toughness, the results are oddly flat, or perhaps with a peak around 61 Rc. This is somewhat unexpected because typically lower hardness means better toughness:

However, analyzing the trends of toughness shows us why the toughness ended up flat. First of all, increasing austenitizing temperature led to both increased hardness (see previous hardness comparison chart) and toughness. Similar behavior was also seen with S45VN where a refinement in the carbide structure was found to be the reason.

One specimen was tempered at 500°F which also led to a reduction in both hardness and toughness, possibly due to tempered martensite embrittlement, though it could be simply that the 500°F temper did not improve toughness relative to 400°F and statistical variation in toughness testing led to a slightly lower value.

So austenitizing in the upper end led to higher toughness. Tempering in the range of 300-400°F led to the best properties depending on desired hardness (lower tempering for higher hardness).

S35VN Heat Treatment Recommendation

Therefore I recommend heat treating S35VN by austenitizing for 15 minutes at 2025°F, plate quenching, a liquid nitrogen treatment for at least 30 minutes, and tempering between 300 and 400°F twice for 2 hours. Choose the tempering temperature depending on the desired hardness. This should result in at least 60 Rc along with good toughness, corrosion resistance, fine microstructure, and toughness.

S35VN Toughness Comparisons

Note: The S35VN values below are slightly different than in previous charts as I re-measured the hardness of all of the toughness specimens after observing the hardness offset between Michael and myself.

Comparing with other PM stainless steels S35VN does very well, being similar to Vanax at a similar hardness. Vanax has an advantage for corrosion resistance while S35VN has the advantage in terms of potential hardness. It also has better toughness than S30V, S45VN, and Elmax. M390 and CPM-154 both have higher than expected toughness which is very intriguing. We will be looking into that more in the future.

Summary and Conclusions

S35VN was released in 2009 as a modification of S30V to have superior toughness and machinability. S35VN has good potential hardness, toughness, edge retention, and corrosion resistance. It does not particularly excel in any one category though it is not terrible in any of them either. Optimum heat treatment was found to be 2025°F for 15 minutes with a cryo treatment and tempering between 300 and 400°F.

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[1] https://patents.google.com/patent/US5830287A/en

[2] https://patents.google.com/patent/US5679908A/en