CPM-15V and the Lost CPM-20V – How Much Vanadium Can you Add?

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History of CPM-15V

CPM-15V is essentially a higher vanadium extension of CPM-10V developed by Crucible steel in the late 1970s. I wrote about the history of CPM-10V in this article so I won’t copy-paste all of that here. Prior to powder metallurgy technology it was known that adding higher vanadium would lead to greater wear resistance due to the very high hardness of vanadium carbide. However, once the vanadium content exceeded 4-5% the carbides would be large enough that the steel would fail in forging, and toughness would also be reduced. So the highest wear resistance steel for many years was T15 high speed steel with ~5% vanadium. With CPM-10V they saw what the limit was for vanadium additions and found that once they reached about 11% vanadium the carbide size would be increased because the vanadium carbides would form in the liquid steel before the steel could be gas atomized into powder. Read about the powder metallurgy process in this article. The more vanadium that is added the higher the temperature where the vanadium carbides form. When the formation temperature is higher than the temperature of the liquid steel then they form prior to atomization giving the large carbides in the 11% vanadium PM steel below (labeled CPM 11V):

Therefore the vanadium content for CPM-10V was limited to just under 10% to avoid issues with large vanadium carbides. The large carbides lead to poor toughness and grindability even if the steel is still forgeable. However, in the early 1990s Crucible metallurgists William Staska and Kenneth Pinnow looked at increased liquid steel temperatures to allow higher vanadium contents. This is the temperature the steel is “atomized” from in the powder metallurgy process so it is often called the atomization temperature. Higher temperatures are not necessarily desirable because it leads to shorter life of the ceramic refractory holding the liquid steel, but is necessary for the higher vanadium content. Interestingly, Stasko and Pinnow did not only look at 15V, however, but also higher vanadium versions called 18V and 20V in the patent and published papers on the research [1][2]. The patent reports that there is a limit of about 3250°F for production before the refractories see major issues.

CPM-15V with 2910°F atomization temperature – large carbides [1]

CPM-15V 3020°F atomization temperature – small carbides [2]

Micrographs from [2]

While the carbides were kept relatively small all the way up to CPM 20V, the high carbide content means that the toughness is reduced simply by the high volume of brittle carbide. However, this does lead to higher wear resistance. The published papers and patents don’t indicate why CPM 18V and CPM 20V were not pursued commercially. Perhaps they were deemed more extreme than necessary, either for the customers or for production.

The table above compares properties of the tested steels along with CPM 10V. The higher vanadium steels show better wear resistance but reduced toughness, as expected. Pin abrasion is a type of abrasive wear resistance (hence the name) where lower is better. The steel is rubbed against a 150 mesh garnet cloth. The crossed cylinder wear test is a type of adhesive wear resistance where two cylinders are rotated perpendicular to each other. One cylinder is the tested steel and the other is made of cemented carbide. With the crossed cylinder test a higher number is better. Toughness is a charpy c-notch test where higher is better. The charpy c-notch results cannot be directly compared to the subsize unnotched charpy tests that we do for Knife Steel Nerds. One odd thing I noticed is that the datasheet for CPM-15V shows the crossed cylinder wear as being 124 even though the paper/patents show that as being for CPM 20V. I’m not sure if that was a mistake or an attempt to make 15V look better than it is. The 10V patent puts its crossed cylinder wear at 90 which is also what they put in the 15V datasheet, but that value is higher than they report in the 15V paper (see the table below). Maybe the test is highly variable and they tried to adjust in the datasheets to make everything consistent.

Heat Treating CPM 15V

Crucible recommends a high temper for 15V (1000-1050°F) as they do with other non-stainless tool steels. The high temper gives the steel hot hardness to prevent softening in grinding or high temperature operation or coatings. And also reduced retained austenite without cryogenic processing which means no size changes or warping during operation which is a big concern for tool manufacturers. The reported hardness results are below:

However, I generally recommend a low temper (300-500°F) because of the increased toughness we have found with several different steels, such as CPM-CruWear. Below is the measured hardness I found for CPM-15V when using a low temper. The hardness with cryo peaks at 1975°F so the optimal austenitizing range is significantly lower than when using the high temper range; the datasheet recommends 1950-2150°F. Without cryo the maximum temperature would be even lower though I’m not sure by how much. The 15V patent says the as-quenched hardness without cryo was 66.7 Rc with 1950°F and reduced to 66.0 Rc with 2050°F. So the peak without cryo is probably in the range of 1900-1950°F somewhere.

Heat Treatment Recommendation

So to do my recommended low temper I would use 1850-1950°F for 30 minutes followed by a plate quench and then a cold treatment, the coldest you have available (freezer, dry ice, liquid nitrogen, etc.). Temper at 300-500°F twice for 2 hours. If I didn’t have liquid nitrogen I might stick to the 1850-1900°F range. Doing your own hardness vs austenitizing temperature measurements would show you where the hardness drop is. Using temperatures above the hardness drop means excess retained austenite which is bad.

Microstructure

The carbide structure of CPM-15V is relatively fine as was shown by Crucible. The carbides are somewhat larger than 10V because of the increase in carbide content. 15V has about 23-25% vanadium carbide as opposed to the 16-17% in 10V. I have also shown 10V and 4V below for comparison. More steels can be seen in this article.

CPM-15V

CPM-10V

CPM-4V

Toughness

The toughness I measured for CPM-15V is similar to what Crucible reported; it is lower than 10V. The heat treatment I used was 1900°F along with 500°F temper, using a plate quench and cryo. That resulted in about 65 Rc and about 4 ft-lbs. If you backed off on hardness down to 60-62 Rc you could probably get another 1-2 ft-lbs or so. However, if you compare with some popular stainless PM grades like 20CV/M390 you may be surprised that 15V toughness is not lower than it is. This makes sense given that both 15V and M390 have a similar volume of carbide, so their toughness is also similar.

Edge Retention

The very high vanadium of 15V gives it very high wear resistance and therefore high slicing edge retention. This knife was also 1900-500°F but resulted in 63 Rc.

Grinding, Finishing, and Sharpening

The very high wear resistance of 15V means that grinding or other methods for steel removal are more difficult and time consuming than most other steels. The relatively fine carbide structure from PM helps but this is a very extreme steel. Finishing and polishing high vanadium steels is especially difficult because the vanadium carbides are harder than common abrasives like aluminum oxide and silicon carbide.

Summary and Conclusions

CPM 15V is one of the most extreme steels available with its 14.5% vanadium giving the steel very high wear resistance. The steel was only possible in production by utilizing higher atomization temperatures that are hard on ceramic refractory. This makes 15V more difficult to produce, more expensive to buy, and more expensive and time consuming to grind and polish. The high carbide content also means the toughness is not especially high, but surprisingly it is not any worse than some popular steels like 20CV/M390.

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[1] Stasko, William, and Kenneth E. Pinnow. “Prealloyed high-vanadium, cold work tool steel particles and methods for producing the same.” U.S. Patent 5,238,482, issued August 24, 1993.

[2] Stasko, W., K. E. Pinnow, and W. B. Eisen. “Development of ultra-high vanadium wear resistant cold work tool steels.” Advances in Powder Metallurgy and Particulate Materials–1996. 5 (1996): 17.