Crucible S45VN Steel – Everything You Need to Know

Thanks to John Dyess, Michael Foote, Xun Fan, and Mark Vanderwest for becoming Knife Steel Nerds Patreon supporters!

Thanks to Niagara Specialty Metals for getting me a bar of S45VN for the experiments necessary for this article.

History of S45VN

S45VN is a new powder metallurgy stainless steel being produced by Crucible that has just been announced. See the datasheet here.

Crucible Industries was the first company to have Industrial production of powder metallurgy tool steels in 1970. You can read more about powder metallurgy and its history in this article. Crucible’s first major stainless PM steel was S60V, formerly called 440V, and the earliest record I can find of that steel is from 1983 [1]. Crucible had been producing steels with very high wear resistance in combination with good toughness by designing them with high vanadium additions. Vanadium forms very hard carbides which contribute to wear resistance, and the use of powder metallurgy keeps the carbide size small for good toughness. S60V was a modification of the common stainless grade 440C by adding 5.5% vanadium with a corresponding increase in carbon. However, the high chromium content of S60V (17%) reduced the amount of vanadium carbide that was formed. Instead, a complex chromium carbide is formed which does not have the same high hardness as the pure vanadium carbide. You can read more about the interactions of different element in forming carbides in this article. Crucible researchers found that if the Cr content was reduced from 17 to 14% that the amount of vanadium carbide formed is much greater and that this resulted in an increase in wear resistance for a given vanadium content:

That reduction in Cr led to S90V stainless steel which has 14% Cr and 9% V. The steel was found to have a good combination of toughness, corrosion resistance, and very high wear resistance. The patent for S90V was filed in 1995 [2]. However, knifemakers and knife companies also wanted a more balanced stainless steel as the high wear resistance of S90V can make processing of the steel and sharpening for the end customer more difficult. Crucible researchers used the same 14% Cr as S90V to best utilize the vanadium additions but also sought to improve corrosion resistance through an increase in molybdenum. A 4% Mo addition was present in the stainless bearing steel 154CM which was added for “hot hardness” but also improves corrosion resistance. Therefore a Mo addition can be used for stainless steels with a bit lower Cr for an improvement in corrosion resistance. That way the 14% Cr content can be used for the formation of vanadium carbide but make up for the lower Cr content through the Mo addition.

To maintain a better balance between wear resistance and toughness they added 4% vanadium resulting in the popular stainless PM steel S30V released in 2001 developed in close collaboration with Chris Reeve Knives. A further modification was made to S30V which became S35VN released in 2009. S35VN had a somewhat reduced vanadium content (3%) in combination with 0.5% niobium. This change led to a finer carbide structure and better toughness and machinability than S30V at the cost of a small reduction in edge retention. Read about the effect of niobium in steel in this article. I will be writing a more in-depth history of S30V and S35VN in the future.

S45VN is the latest evolution in this line of steels which has an increased Cr content (16%) for an improvement in corrosion resistance over S30V and S35VN. It seems to be a popular opinion among some enthusiasts that S30V and S35VN are now old hat, so perhaps this new grade will attract them again. The S45VN chemistry was first introduced by Niagara Specialty Metal’s Western Sales Manager and Knife Steel Specialist, Frank Cox. Final refinement of the chemistry and Mill Production methods were developed by Bob Skibitski of Crucible. Crucible and Niagara collaborated to offer a steel with improved corrosion resistance without sacrificing other properties.

Design of S45VN Steel – Nitrogen

Apart from the increase in Cr, S45VN also has a 0.15% nitrogen addition. Nitrogen can increase hardness and corrosion resistance in stainless steels, read more here. S30V also has a nitrogen addition [3], a point not often recognized since the nitrogen is not listed in the datasheet [4]. The nitrogen addition is not made to S35VN [5], but it has returned for S45VN. Nitrogen steels have seen some interest and marketing in recent years with steels such as LC200N, BD1N, Nitro-V, and 14C28N [6]. Powder metallurgy steels generally have at least 0.05% nitrogen in them from pickup from the atmosphere and the nitrogen gas atomization process [2]. However, nitrogen can also be intentionally added to the steel for higher amounts. In the article I wrote about nitrogen-alloyed steel I focused primarily on nitrogen “in solution” which increases hardness and to some extent corrosion resistance. Nitrogen is not as prone to form chromium nitrides as carbon is to form chromium carbides, so nitrogen can often be added to steel for an increase in hardness without decreasing corrosion resistance. However, with the relatively high amount of vanadium and niobium in S45VN the nitrogen is not actually in solution. Instead, the nitrogen promotes the formation of vanadium and niobium carbide which also decreases the amount of chromium carbides. That shift in favored carbide types improves wear resistance by having more of the harder vanadium/niobium carbides (MC) and also improves corrosion resistance because less chromium carbide (M₇C₃) is formed:

ThermoCalc at 1950°F

Design of S45VN – Niobium

In the article about niobium additions, I wrote about how niobium is a stronger carbide former than vanadium, leading to hard niobium carbides even in high chromium steels. I used ThermoCalc to compare S45VN if the niobium were replaced with vanadium, i.e. using 3.5% V instead of 3.0% V and 0.5% Nb. I also adjusted the carbon content to maintain carbon “in solution” constant so that hardness is unchanged. Replacing niobium with vanadium leads to a reduction in MC (niobium and vanadium carbides) and chromium in solution (corrosion resistance), and an increase in undesirable chromium carbide (M₇C₃), in a somewhat similar fashion to nitrogen.

ThermoCalc at 1950°F

As noted in the niobium article, small additions of niobium can also improve toughness by refining the microstructure as a whole by changing the sequence of solidification. So using niobium instead of vanadium should lead to a refined microstructure in general in part because of modifications to solidification as well as reducing the amount of chromium carbide. Chromium carbides coarsen more rapidly than vanadium and niobium carbides in powder metallurgy steels so reducing the amount of chromium carbide helps with carbide size.

Therefore, a niobium additions helps improve both toughness and corrosion resistance by increasing chromium in solution and refining the carbide size. The combination of niobium and nitrogen helps to keep the chromium carbide content as low as possible to help offset the increase in chromium so good toughness is maintained.

Design of S45VN – Nitrogen and Niobium

The reduction in chromium carbide (M₇C₃) from nitrogen and niobium helps to offset the increase in Cr relative to S30V and s35VN in terms of excessive chromium carbide formation. Chromium carbides tend to be larger than vanadium carbides, so the amount of chromium carbide should be kept as low as possible to maximize toughness. The result is that the overall carbide content is not increased much when compared with S30V, though still a bit higher than S35VN. The overall carbide content controls to a great extent the toughness of the steel. And the nitrogen addition allows S45VN to have similar MC content to S35VN despite the increase in chromium (higher Cr reduces MC), so that wear resistance and slicing edge retention is good. In the table below you can also see C, Cr, and Mo in solution. Carbon in solution controls the as-quenched hardness (more is better), and the chromium and molybdenum in solution control corrosion resistance (more is better). You can see that the carbon in solution remains the same for S45VN compared with S30V and S35VN, but the Cr in solution is higher for improved corrosion resistance. These numbers are calculated using ThermoCalc software:

Design of S45VN – Summary

The main changes we expect from S45VN vs S35VN and S30V are an improvement in corrosion resistance due to the increase in Cr of about 1%. We expect a reduction in toughness when compared with S35VN because of the increase in total carbide content. The design including niobium and nitrogen additions should help maintain toughness similar to or better than S30V. Edge retention is covered later in this article. The increase in Cr in solution puts S45VN at a similar Cr-level to Elmax, but it is still somewhat below S110V and M390. The following chart shows C and Cr in solution for a range of PM vanadium stainless steels using ThermoCalc estimates with the recommended austenitizing temperature found in each datasheet:

Heat Treatment and Hardness

I heat treated S45VN by wrapping it in heat treating foil and heating it in an Evenheat furnace. I soaked the steel for 20 minutes, plate quenched between 1″ aluminum plates, and then gave a cold treatment, either in a household freezer or liquid nitrogen. The steel was then tempered twice for 2 hours each time at the indicated temperature. The hardness measurements lined up relatively closely with those given in the S45VN datasheet. Using a freezer saw very little change in hardness when compared with using liquid nitrogen instead. I extended the austenitizing temperature above the recommended range in the datasheet (1900-2000°F) because of prior experience with heat treating S35VN.

Hardness Measurements in Datasheet:

Relatively high hardness can be achieved with a 300°F temper, in the 63-64 Rc range. Perhaps even higher hardness could be obtained by austenitizing at yet higher temperatures. To determine why the freezer led to similar hardness to liquid nitrogen, I used magnetic saturation to measure the amount of retained austenite. It was found that in both cases the retained austenite content was relatively low, so even higher austenitizing temperatures would be required to start to see a divide between freezer and liquid nitrogen. The peak in hardness is generally seen around 15% retained austenite. You can see examples of that with AEB-L in this article. I usually recommend using a freezer if no liquid nitrogen or dry ice is available for cold treatments. A freezer is not as effective at minimizing retained austenite but it is better than nothing.

Toughness Testing

To test the toughness of S45VN I did three different heat treatments, all of which utilized liquid nitrogen after the quench. I used the same subsize unnotched specimens we have used previously, the specifications of which are found on this page.

The 2000°F austenitize led to an improvement in both hardness and toughness relative to the 1950°F austenitize recommended in the datasheet. An improvement in toughness with a higher austenitizing temperature was also seen with heat treatments of S35VN which likely behaves somewhat similarly (I do not yet have a separate article on S35VN):

The toughness of S35VN was also found to peak at a tempering temperature around 350-400°F which may also help explain why the 2000-400 heat treatment led to the best combination of hardness and toughness:

Microstructure of S45VN

To investigate the cause behind the toughness increase with higher austenitizing temperature, I looked at the microstructure for the 1950-450 and 2000-400 specimens. The previously shown retained austenite measurements showed only a small increased in RA by austenitizing at 2000°F, likely ruling out a strong effect of retained austenite. I found that the carbides were significantly finer by using the 2000°F austenitize which is a likely explanation for the improvement in toughness:

S45VN – 1950°F (20% carbide volume)

S45VN – 2000°F (17% carbide volume)

While austenitizing temperature does lead to the dissolution of more carbide, this does not always lead to an improvement in toughness because grain growth is possible and with more carbon in solution there is a chance of embrittlement. Read more about austenitizing and effects on properties here: Part 1, Part 2, Part 3. In this case with S45VN the carbon in solution is not particularly high and the high carbide content means that grain growth is minimal, so the reduction in carbide content with higher temperature led to an improvement in toughness. The carbide volume is somewhat higher than calculated in ThermoCalc, shown previously in the article. This is expected due to ThermoCalc being an equilibrium calculation (infinite hold time).

Microstructure Comparison with Other Steels

For comparison, here are micrographs with a few other stainless steels in the same general class. The 2000°F heat treatment microstructure appears to have smaller carbides than S30V and Elmax so we expect better toughness than those steels. Vanax has smaller carbides/nitrides than these other steels, however, because it is largely free of chromium carbides and because nitrides coarsen very slowly [7].

S30V – 2000°F (20% carbide volume)

S35VN – 1975°F (15% carbide volume)

Elmax – 1975°F (19% carbide volume)

M390 – 2140°F (22% carbide volume)

Vanax – 1975°F (18% carbide/nitride volume)

S90V – 2050°F (23% carbide volume)

Toughness Comparison with Other Steels

S45VN has only slightly reduced toughness when compared with S35VN, and S45VN is an improvement over S30V, particularly when hardness is factored in:

Elmax had somewhat better toughness though at lower hardness, when hardness is considered the toughness of S45VN is better. M390 and 20CV have surprisingly high toughness given the high carbide content. I suspect that those samples (austenitized at 2140°F) have relatively high retained austenite which can help with toughness but is not always desirable. Hopefully we can do more investigations on M390 toughness later. Vanax has higher toughness than S45VN, being similar to S35VN when adjusted for hardness.

Heat Treatment Recommendation

Based on the toughness testing result, I think a higher austenitizing temperature than the 1950°F recommended in the datasheet is best. The 2000°F austenitize in combination with cryo and a 400°F temper led to a good combination of hardness (61-62 Rc) and toughness along with low retained austenite. A higher austenitizing temperature also leads to a small increase in chromium in solution for better corrosion resistance. I used a 20 minutes soak time with oversize 1/8″ steel, and tempered twice for two hours each time. If liquid nitrogen is not available then use dry ice, and if neither is available then a freezer is next best. Cold treatments are less effective if there is a delay between the quench and the cold treatment, particularly when using a freezer. You can read more about cryo treatments here: Part 1, Part 2, and Part 3. If a further increase in hardness is desired for high edge retention or edge stability then temper between 300 and 350°F.

Edge Retention

The S45VN datasheet provides a number for CATRA edge retention relative to 440C, which it lists as 143%. This is in comparison with 440C which is 100%, S30V at 145%, and M390/20CV at 180%. However, the datasheet has an asterisk next to the 143% value and says that it is “based upon market feedback.” These estimates should perhaps be treated with some suspicion as the S35VN datasheet has the same asterisk for its 145% value which puts it at the same level as S30V, but independent experiments put S35VN around 132% at 61 Rc [8]. The same study found values for 61 Rc S30V at 149% and 61.5 Rc M390 at 179% [8], which are similar to those given by Crucible (though for 20CV rather than M390). However, Bob Skibitski of Crucible confirmed to me that the 143% number is based on actual CATRA experiments. Interestingly, the S45VN datasheet has a newly adjusted CATRA value of S35VN of 140%, better reflecting the somewhat reduced edge retention of S35VN when compared with S30V.

Using the ThermoCalc estimated carbide content we can use a previously devised edge retention prediction equation to determine its approximate CATRA edge retention. S45VN has a higher content of chromium carbide compared with S35VN and S30V, with equivalent MC to S35VN but lower than S30V. So we expect better edge retention than S35VN but where it sits compared to S30V is hard to predict with certainty. The equation puts S45VN at about 142% at 61 Rc, very close to the value provided by Crucible. This puts the steel in line with Elmax and S30V, as shown on the following chart:

Hopefully we will see some independent CATRA testing to confirm but it appears that S45VN has very similar edge retention to S30V and Elmax. Here are a few more steels in a table for comparison:

Corrosion Resistance

As discussed in the design section, S45VN has higher Cr in solution compared to S30V and S35VN, but each has a similar Mo in solution. Therefore the corrosion resistance of S45VN should be improved relative to the earlier S30-series steels. With similar Cr in solution to Elmax but higher Mo it should also be a small improvement in corrosion resistance over Elmax. S110V and M390/20CV/204P do have higher Cr in solution than S45VN. S110V has a similar Mo content as well so it should still have better corrosion resistance than S45VN. The Mo content of M390 is lower, however, so it is difficult to predict how things balance out when comparing higher Cr vs higher Mo. I tested S45VN in my 1% saltwater spray corrosion test. The results of these experiments (minus S45VN) were previously shown in this article. I austenitized the S45VN at 1950°F as recommended in the datasheet but a 2000°F austenitize would lead to slightly better corrosion resistance. Below is a comparison between S45VN and S35VN where 24 hours is on the left and 48 hours on the right. S45VN is somewhat better, especially after 24 hours.

S45VN

S35VN

204P (same as M390 and 20CV)

This puts S45VN more on par with 204P/M390/20CV as shown in this comparison chart of my previous corrosion experiments. The “corrosion rating” is based on how much rust was visible on the specimens after 24 hours of 1% saltwater exposure.

The above rating is an experimental result and is not the same as the “corrosion resistance rating” that I assigned to each knife steel in table form at the bottom of the corrosion article. In that table S45VN would be an 8.4 based on the criteria specified, or 8.6 using the 2000°F austenitize I am recommending. When comparing against steels in the same class, that is somewhat below the rating for Vanax (10), M390 (9.1) and S110V (9.1), but above the ratings for Elmax (7.8), S35VN (7.8), S30V (7.7), and S90V (7.4). The ratings are approximate and the actual corrosion resistance performance will vary based on the corrosive media, heat treatment, and surface finish.

Grinding, Polishing, and Sharpening

One of the main limiting factors for polishing and sharpening is the amount of hard MC (vanadium or niobium carbides). These carbides are harder than standard abrasives which is what makes polishing more difficult. With the similar MC content to S35VN, S45VN is expected to sharpen and polish similarly as well. And it should be somewhat easier to polish and sharpen than S30V. Therefore the steel may be worth trying by those that prefer sharpening Elmax to S30V. It will not be as easy to finish as something like CPM-154 which is a favorite among custom knifemakers putting on hand-rubbed finishes.

Summary and Conclusions

S45VN shows improved toughness and corrosion resistance relative to S30V along with similar edge retention. Or slightly reduced toughness relative to S35VN but with improved corrosion resistance and edge retention. It shows a better overall set of properties, I think, making S45VN a good upgrade to either S30V or S35VN. Alternatively S35VN could be used for slightly better toughness but if toughness is a limiting factor there are steels with significantly higher values. S45VN is somewhat of an incremental advance but it does seem to be an upgrade overall vs the earlier S30-series steels. I also think that S45VN is an upgrade over Elmax as it has similar edge retention but somewhat better potential hardness, corrosion resistance, toughness, and a finer carbide structure. My recommended heat treatment for S45VN is 2000°F austenitize along with a fast quench and a cold treatment followed by a 300-400°F temper depending on desired hardness.

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[1] Chilton’s iron age 226 (1983): p. vi.

[2] Pinnow, Kenneth, 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 5,679,908, issued October 21, 1997.

[3] http://www.crucible.com/PDFs/DataSheets2010/dsS30Vv1%202010.pdf

[4] http://staging.njsteelbaron.com/wp-content/uploads/2019/05/CPM-S30-V-1-8-HT-NR66769.pdf

[5] http://staging.njsteelbaron.com/wp-content/uploads/2019/05/CPM-S35.-3-32-HT-NR67798.pdf

[6] https://knifenews.com/are-we-entering-the-era-of-nitrogen-based-blade-steels/

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

[8] https://knifesteelnerds.com/wp-content/uploads/2018/08/Bohler-Uddeholm-CATRA.pdf