Vanadis 8 – Better than CPM-10V

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History of Vanadis 8

The history of Vanadis 8 goes back a lot further than just the steel itself, really we have to know the history of 10V to know all of the background that is relevant. You can read about CPM-10V in this article. CPM-10V is important because Vanadis 8 is intended as a replacement for CPM-10V, or at least for similar applications. CPM-10V was developed by Crucible in the late 70s, as the first powder metallurgy steel to utilize a microstructure made up of only vanadium carbide, as opposed to chromium and molybdenum/tungsten carbides. This gave 10V a combination of good toughness along with very high wear resistance from the ~10% vanadium and therefore ~17% vanadium carbide.

Vanadis 10

But Vanadis 8 was not Uddeholm’s first attempt at a competitor for CPM-10V. The first was Vanadis 10, which appears to have existed from the late 80s or perhaps the early 90s. I have not found a patent for Vanadis 10 but the composition of the steel was shown in a patent for Vanadis 4 which was filed in 1987 [1]. Vanadis 4 was not the same as the current Vanadis 4 Extra. Both Vanadis 4 and Vanadis 10 used a base composition of 8% Cr and 1.5% Mo plus vanadium and carbon for hard vanadium carbides. This 8% Cr-Mo basis started with Vasco Die in 1964, which led to other steels like CPM-3V, CPM-CruWear, Sleipner, and Z-Tuff/CD#1. CPM-3V is a powder metallurgy version of Vasco Die, which is why CPM-3V, Vanadis 4 (original), and Vanadis 10 all look like they are from the same series of steels, just with different levels of vanadium to control the balance of wear resistance and toughness.

Vanadis 4 Extra

The problem with Vanadis 4 and Vanadis 10 is the high chromium content led to a significant amount of chromium carbides. Chromium carbides grow to larger sizes than vanadium carbides in powder metallurgy steels which reduces toughness. Because chromium carbides are softer than vanadium carbides, they also provide less of an increase in wear resistance for a given amount of carbide. So a microstructure of only vanadium carbides is best for the highest combination of toughness and wear resistance. CPM-10V had only vanadium carbide and less overall carbide than Vanadis 10 so it had significantly better toughness than Vanadis 10. This put Uddeholm at a disadvantage with the prior product.

Uddeholm first redesigned their Vanadis 4, however, with the patent being filed around 2002 [2] and the first articles about the steel around 2004 [3]. Vanadis 4 had about 6% each of vanadium and chromium carbide for a total of 12%, while Vanadis 4 Extra has about 8% vanadium carbide. The overall carbide size was reduced by eliminating the chromium carbides. The toughness was improved by a decrease in the total carbide and a reduction in carbide size. The abrasive wear resistance was only slightly reduced compared with Vanadis 4 by the reduction in carbide volume, because the Extra version is entirely made up of the harder vanadium carbides.

The increase in molybdenum of Vanadis 4 Extra also provided some advantages. More molybdenum increases “hardenability” or the quench rate that is required to achieve full hardness in heat treatment. This allows larger pieces to be heat treated for tools, or for furnaces with low quench rates to be utilized in heat treatment. Molybdenum also improves “secondary hardening” where the steel is tempered at high temperatures to achieve its final hardness. The high temperature tempering provides more resistance to softening during machining operations, and the steel can be coated with high temperature coatings without losing hardness.

Vanadis 8

So Vanadis 8 is essentially a higher vanadium version of Vanadis 4 Extra, using a similar base composition of 4.8Cr-3.6Mo, but with higher vanadium and carbon to have more vanadium carbide for higher wear resistance. This also decreases toughness relative to Vanadis 4 Extra, of course, but these are unavoidable tradeoffs. The patent was filed around 2015 [4], and articles about the steel appeared around 2017 [5].

The new composition avoids chromium carbides which means that toughness is improved compared with the earlier Vanadis 10 as shown in the micrographs below where Vanadis 8 is top and Vanadis 10 is bottom. In Vanadis 10 many chromium carbides are visible (light grey) which are somewhat larger than the vanadium carbides. Vanadis 8 was calculated to have about 15.8% vanadium carbide by Uddeholm, and 17.2% vanadium carbide in CPM-10V. Therefore we would expect Vanadis 8 to have a bit better toughness than 10V at the cost of a little wear resistance.

Micrographs are from [5]

In the patent they state that CPM-10V had only 11 joules in an unnotched Izod toughness test, compared with 41J with Vanadis 8, nearly a 4x increase in toughness. And in their article about Vanadis 8 they show Vanadis 10 having about 15J, which again is a large increase in relation to the prior product. Better toughness for Vanadis 10 vs 10V sounds a bit unrealistic, and the very large increase in toughness of Vanadis 8 over 10V also sounds unrealistic, but I will discuss that more in the toughness measurements section later on.

Their article on Vanadis 8 also pointed out the hardenability improvement in Vanadis 8 over the earlier 10 due to the increased Mo. And this would also be true in comparison with 10V which has only 1.3% Mo. So the steel can be cooled more slowly and still achieve full hardness.

Chart is from [5]

Microstructure of Vanadis 8

Below I have a micrograph of Vanadis 8 in comparison with CPM-10V and Bohler K390, the other two steels which are the main alternatives. Vanadis 8 has slightly less carbide volume vs the other two, and is perhaps a bit finer. 10V and K390 appear more similar. The small reduction in carbide volume was estimated by Uddeholm in the patent, and fits with the lower vanadium content. However, the 10V was austenitized at a lower temperature so less carbide would be dissolved when compared with the other two.

Vanadis 8 (1900°F)

CPM-10V (1800°F)

K390 (1975°F)

Heat Treatment of Vanadis 8

The Vanadis 8 datasheet recommends austenitizing between 1870 and 2150°F (1020-1180°C) and tempering at 1000°F or higher, though it also notes that you can get a little higher hardness at 970°F but says not to go lower than that.

Those are recommendations for the secondary hardening range, however. That range is preferred by steel companies because of it reduces retained austenite without cryo, and improves hot hardness to help prevent softening when grinding or applying high temperature coatings. I prefer tempering in the low temperature range (<750°F) because it has resulted in higher toughness for several steels including CPM-CruWear and CPM-10V. I did a range of hardness coupons with Vanadis 8 to characterize its hardness with low temperature tempering because it is not available in the datasheet, the results are below. I austenitized each piece for 30 minutes, plate quenched, then liquid nitrogen, followed by two tempers 2 hours each.

The steel is capable of very high hardness, as expected, easily exceeding 64 Rc even when tempering at 400°F. So the steel can be heat treated to a range of hardness values depending on the desired properties.The low temper plus cryo also leads to higher potential hardness than the upper temper shown in the datasheet.

I also tested the toughness of Vanadis 8 using our standard subsize unnotched charpy specimens. Using 1850-500 resulted in 60.9 Rc and 1950-400 resulted in 64.2 Rc. Those hardness values are consistent with the hardness curves I generated. The toughness results are shown in the following section.

Toughness

The toughness of Vanadis 8 compared with other high alloy tool steels is shown in the following charts, one chart showing all steels and the other cutoff at 15 ft-lbs to show the comparison better with other steels in a similar toughness range. The toughness of Vanadis 8 is confirmed to be superior to CPM-10V. At ~61 Rc the Vanadis 8 is an impressive 39% better than 10V, though this is much less than the 3.7x increase claimed in the patent.

Why is Vanadis 8 Tougher than CPM-10V?

This is a difficult question to answer though we can go through some possibilities. In the patent the inventors state, “The reason for this improvement is not fully clarified but it would appear that the low Si-content in combination with a high Mo-content improve the strength of the grain boundaries.” This is an interesting conjecture but I’m not sure it is any more than that. Many tool steels have ~1% Si like CPM-10V, including Vanadis 10, but also steels with excellent toughness like CPM-3V and CPM-CruWear. Even Crucible CPM-4V which is nearly identical to Vanadis 4 Extra has higher Si (0.8 vs 0.4%) and if anything it has tested higher for toughness than Vanadis 4 Extra. I have found studies on hot work tool steels like H11 that found better toughness with low Si steels when using high tempering temperatures, however [6].

I also have not seen any studies showing an improvement in toughness from increased Mo. I did a literature search again to make sure, but all I found were studies comparing Mo-alloying with W-alloying in high speed steels, which doesn’t really translate to what we are discussing here. I did find one study on hot work tool steels that reported that the effect of Mo on toughness was highly dependent on selected austenitizing and tempering temperatures, and recommended 1.5% Mo [7]. In Uddeholm’s own hot work steels, they rate the toughness of the 1.3% Mo Vidar the same as the 2.3% Mo Dievar, and the steels are mostly identical other than Mo. However, the lower Mo in 10V would mean that it has to have more carbon in solution, or be tempered lower, to achieve the same hardness as Vanadis 8 when tempered in the high temperature range (~1000°F). Perhaps that leads to a bigger gulf in toughness when tempering in that range, as reported in the patent.

However, that still leaves the improved toughness of Vanadis 8 measured by myself even with the low temper. One obvious difference is the slightly lower carbide volume with Vanadis 8 which would be expected to improve toughness, though a 39% improvement would seem to be too much. CPM-10V does have a slight sulfur addition of 0.07%. Crucible seems make these small sulfur additions to CPM steels developed in the 60s and 70s, including CPM-M4, CPM-T15, and CPM Rex 76, all of which have 0.06% sulfur. The sulfur is intended to make the steels easier to machine, and increased sulfur versions of those high speed steels are also available (~0.22%) for even better machinability. With powder metallurgy steels the sulfur has less of a deleterious effect on toughness, and 0.06-0.07% is a pretty small addition. I am not sure if this is enough sulfur to make a difference but perhaps it is a contributor here. Powder metallurgy steels produced by Uddeholm and Bohler have somewhat lower oxygen content than Crucible steels, though in other tests I’ve performed I haven’t found this to lead to a toughness difference.

It could be that Uddeholm simply landed on a somewhat optimal composition for toughness with Vanadis 8. There are always surprises in research and development. We may not know why it has somewhat better toughness than CPM-10V, at least without a PhD thesis worth of work that I’m not sure anyone is willing to do.

Edge Retention

I tested Vanadis 8 for edge retention in my large CATRA study which you can read here. This knife used the 1850-500 heat treatment used in toughness testing described earlier, and the knife had a similar level of hardness at 60.7 Rc. Vanadis 8 is a small notch below CPM-10V in edge retention as expected, though it is very close to K390 if compensated for hardness (the dotted lines estimate the difference in edge retention with hardness). The edge retention is plenty high for most knives, and I’m not sure if the slight disadvantage to CPM-10V is enough to worry about. Obviously the steel could be heat treated to higher hardness, such as 64-66 Rc for somewhat more edge retention.

Toughness-Edge Retention Balance

Because of its very good toughness for its wear resistance class, Vanadis 8 looks very good for toughness-edge retention balance. It has better toughness than any PM stainless steel (blue circles), including S35VN, Vanax, or CPM-154, but with significantly better edge retention than those steels. And better toughness and edge retention than popular steels M390 and S30V. So if stainless levels of corrosion resistance are not required it offers an attractive combination of properties. For higher levels of toughness, a steel like CPM-M4, Vanadis 4 Extra, CPM-CruWear, or CPM-3V would be better choices. In my book Knife Engineering, and in my previous article on recommended steel choices, I put CPM-10V as the recommendation for high alloy non-stainless with high edge retention. However, that was before I had completed toughness experiments with Vanadis 8. Because of its superior toughness with only a small decrease in edge retention, Vanadis 8 probably takes that spot for me now. The main downside for Vanadis 8 is availability; it is not widely available from knife steel suppliers currently. The two suppliers I know of are USA Knifemaker and Eurotechni, but even at those it is only available in a single thickness. So I would still recommend CPM-10V, it is still a good steel, but if Vanadis 8 becomes more available I say buy it.

Ease of Grinding, Finishing, and Sharpening

The high wear resistance of Vanadis 8 means that removing steel in grinding is more challenging. The hard vanadium carbides means that sharpening and finishing can be more difficult as vanadium carbide is harder than aluminum oxide, the most common abrasive in sharpening stones, grinding belts, and sandpaper. Diamond or CBN abrasives eliminates this issue and then it is simply the higher wear resistance of Vanadis 8 that provides difficulty.

Summary and Conclusions

Vanadis 8 is a relatively new steel which was developed to be an improvement over previously available steels CPM-10V and Vanadis 10. It offers improved toughness compared with those steels with only a small decrease in edge retention and wear resistance. It provides a toughness improvement over Vanadis 10 by eliminating chromium carbides, done by decreasing Cr and replacing it with Mo. The reasons for improved toughness over CPM-10V are not completely clear. Vanadis 8 has a combination of pretty good toughness along with excellent edge retention. The high wear resistance can provide difficulties in grinding and finishing as with other similar steels. I think it has a better property balance than CPM-10V and I hope it has increased availability in the future.

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[1] https://patents.google.com/patent/US4863515A/

[2] https://patents.google.com/patent/US7297177B2/

[3] Tidesten, Magnus, Odd Sandberg, and Lennart Jönson. “PM Tool Materials: An Optimised PM Produced Cold Work Tool Steel.” In European Congress and Exhibition on Powder Metallurgy. European PM Conference Proceedings, vol. 3, p. 1. The European Powder Metallurgy Association, 2004.

[4] https://patents.google.com/patent/US10472704B2/

[5] Tidesten, Magnus, Anna Medvedeva, Fredrik Carlsson, and Annica Engström-Svensson. “A New Cold Work PM-Grade Combining High Wear Resistance with High Ductility.” BHM Berg-und Hüttenmännische Monatshefte 162, no. 3 (2017): 117-121.

[6] Mesquita, R. A., and H-J. Kestenbach. “On the effect of silicon on toughness in recent high quality hot work steels.” Materials Science and Engineering: A 528, no. 13-14 (2011): 4856-4859.

[7] Podgornik, Bojan, Igor Belič, Vojteh Leskovšek, and Matjaz Godec. “Tool Steel Heat Treatment Optimization Using Neural Network Modeling.” Metallurgical and Materials Transactions A 47, no. 11 (2016): 5650-5659.