Still working on getting an update to everyone about the status of knife steel after Crucible’s bankruptcy and the sale of many of the assets to Erasteel. Still more i’s to dot and t’s to cross.
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Video
Here is the video version of the following information:
History of CPM-3V
The thing most don’t know about CPM-3V is that it is a powder metallurgy version of an older conventional steel called Vasco Die. Vasco Die was patented by Harry Johnstin of VASCO in 1964 [1]. This was the first popular “8% Cr die steel” which created a new category. The most popular cold work die steels were (and are) A2 and D2. D2 is used for higher wear resistance and A2 is used for higher toughness. The 8% Cr is right in the middle between D2 (12%) and A2 (5%). And with the 2.5% vanadium for wear resistance and the relatively low carbon of 0.8% it has an interesting combination of properties, better overall than either A2 or D2. The steel was advertised as twice as tough as D2 while have ten times the wear resistance of A2. This created a new category of cold work die steel which is still seen today in steels like DC53 and Sleipner. You can read more about the history of Vasco Die and all of the developments that led up to it in my book The Story of Knife Steel. Vasco Die was never really used in knives, though the higher carbon version, Vasco Wear, did get used by some knifemakers, and by Gerber in some production knives. And of course CPM CruWear, the PM version of Vasco Wear, has also developed some popularity in recent years.
In the mid-1990s, Kenneth Pinnow and William Stasko of Crucible were looking to make a high toughness powder metallurgy tool steel. Up until that point, powder metallurgy steels had focused on high hardness high speed steels and creating new steels with higher wear resistance than was possible with conventional steelmaking like CPM-10V. Crucible’s highest toughness PM steel was CPM-M4 and that was basically an accident. Uddeholm had released Vanadis 4 in the late 1980’s to be a more balanced steel and its toughness didn’t even match CPM-M4. Uddeholm would later redesign this steel and call it Vanadis 4 Extra. Pinnow and Stasko realized they needed a steel with lower carbide volume. Powder metallurgy leads to higher toughness because it keeps the carbide size small, but to achieve the highest levels of toughness they needed to have less overall carbide. They experimented with a few compositions and found that a PM version of Vasco Die had excellent toughness and wear resistance.
It is somewhat surprising to me that they managed to patent this steel in 1997 [2] since Vasco Die had already previously been patented (though the patent had expired). It seems their claim was that the powder metallurgy process significantly changed the steel, and in more ways than just making the carbides smaller. With conventional casting of steel there is significant segregation of elements, and this led to relatively large chromium carbides which couldn’t be dissolved in Vasco Die. With powder metallurgy, the heat treated microstructure is free from chromium carbide and has only vanadium carbide instead.
Images from 3V patent showing CPM-3V (Fig. 1) and Vasco Die (Fig. 2). The larger carbides labeled “C” are chromium carbides. 3V is only small vanadium carbides.
CPM-3V became Crucible’s go-to steel for applications requiring high toughness; they released it around 1999. Surprisingly this steel didn’t really take off in knives. And I would argue it never really has. Jerry Hossom was the first custom knifemaker I noticed using it regularly. In more recent years perhaps the best known user of the steel is Nathan Carothers with his “Delta” heat treatment. I don’t mean to suggest no one is using 3V, I see currently available knives from a range of manufacturers including Bark River, ESEE, Cold Steel, and others, but it never seems like there are as many 3V knives as there could be. I think it being non-stainless has held it back as knife manufacturers tend to stick with stainless steels.
Heat Treating and Hardness
I did a new set of heat treating coupons for CPM-3V as I wanted to go a bit higher on austenitizing temperature than coupons I had done for the book Knife Engineering. The hold time at 1850°F was 45 minutes, 1950°F was 30 minutes, 2050°F was 20 minutes, and 2150°F was 10 minutes. While CPM-3V is typically heat treated in the 58-62 Rc range because of its excellent toughness, it can be heat treated to higher hardness if desired. This is significantly higher than the 61 Rc the datasheet shows the steel maxing out at. This is because I used the low temper range (300-500°F), rather than the datasheet recommended high temper range (1000-1050°F).
Microstructure
CPM-3V has about 5% vanadium carbide, and is free of chromium carbide despite the relatively high chromium content. This is in contrast to Z-Tuff which has about 3% chromium carbide and only 1% vanadium carbide. CPM-1V also has about 1% vanadium carbide but about 2% M6C (molybdenum/tungsten carbides found in high speed steels). So 3V has only a bit higher carbide volume than those two steels but it is all the hard vanadium carbide type which should mean more balanced properties.
3V (1925°F) – 5% carbide volume
Z-Tuff (1925°F) = 4% carbide volume
CPM-1V (2050°F) – 3% carbide volume
Toughness
One mystery about CPM-3V toughness comes from a patented Crucible steel that was never released commercially, a modified version of 3V. The steel was patented in 2006 by Alojz Kajinin and Andrzej Wojciesczynski. I actually wrote about this modified steel in my very first article on Knife Steel Nerds in February 2018. I didn’t even advertise the article because it was a test before I wrote a series on austenitizing that I did promote with the announcement of the website. The modified 3V was designed by partially replacing vanadium with niobium, which made the carbides smaller. They showed experiments in the patent that the longitudinal toughness of 3V was much higher than the transverse toughness. With transverse toughness the crack grows along the rolling direction, and carbides and impurities are aligned along the rolling direction giving easier paths for crack growth. However, with the modified 3V they found the transverse toughness to be roughly equal to the longitudinal toughness.
So for a new experiment on CPM-3V I decided to test both the longitudinal and transverse toughness and see if the transverse toughness was really as bad as it was shown in the 3V mod patent. I used an austenitizing temperature of 2000°F, plate quench, cryo, and a temper at 400°F. This resulted in about 61.5 Rc, which is significantly higher in hardness than our original 3V toughness tests. Those coupons were heat treated by Warren Krywko who heat treated a whole bunch of toughness coupons that I tested early on. I think maybe the cooling rate in quenching he was performing was somewhat slower than what I did, maybe because he was doing many specimens at the same time in the plate quench. That is significant because a slower cooling rate leads to both lower hardness and lower toughness, as I wrote about in last month’s article. All of the coupons we heat treated were tempered at 400°F rather than the 1000°F temper the datasheet recommends, because of our earlier findings with CPM-CruWear/Z-Wear.
Comparing the hardness-toughness to the previous CPM-3V tests I found significantly better properties. The transverse toughness I measured was only ~7% worse than the longitudinal, which is quite impressive. The transverse toughness reported in the 3V mod patent looks like an anomaly, I have never seen such a large difference between longitudinal and transverse toughness. Though I have to say, the very small drop between longitudinal and transverse toughness I measured here is quite impressive.
With the improved toughness measurements 3V nearly rivals the results of Z-Tuff and CPM-1V which both have lower carbon and vanadium. This actually does make some sense, as the carbide volume is similar between the three steels. To me the composition designs of Z-Tuff and CPM-1V are not totally optimal because they have a significant amount of non-vanadium carbides. The best toughness-wear resistance balance is generally achieved by having all vanadium carbides. Perhaps in the future we could explore higher hardness heat treatments like in the 62-64 Rc range.
Wear Resistance and Edge Retention
I tested the CATRA edge retention of 3V way back in my giant CATRA study. It did exceptionally well for its level of toughness at 463 mm cardstock cut, significantly higher than CPM-1V and CD1/Z-Tuff. Looking back I do think the heat treatment used was cheating a bit, as the tempering temperature was only 300°F. If the tempering temperature was a more appropriate 400°F, the austenitizing temperature for the same hardness would be higher and thus there would be a bit less carbide in the steel. However, this would only marginally affect the final result.
Edge Retention-Toughness Balance
The combination of wear resistance and toughness of 3V is quite impressive. I added a new datapoint for 3V with its newly higher toughness test. I had to adjust up the CATRA edge retention based on the predicted change with hardness for the chart since the CATRA knife was 1.4 Rc lower than the toughness coupon. CPM-3V is really in a class of its own for the level of toughness it has for the level of edge retention. Of course it doesn’t do anything too crazy when it comes to edge retention, but it is sufficient for many knives and has excellent toughness to go with it.
Corrosion Resistance
I have never tested the corrosion resistance of CPM-3V. With its 7.5% Cr and no chromium carbide its corrosion resistance can be quite good for a non-stainless. Some people talk about “semi-stainless” steels and if any steel were to fit in this category it would be 3V. It actually has more chromium in solution than D2, which is perhaps the best known “semi-stainless” steel. Z-Tuff and CD#1 also have a similar chromium content but because they have some chromium carbide in the heat treated condition we would expect 3V to have somewhat better corrosion resistance. In a corrosion test performed by Scott Larimore and Kelly W., CPM-3V was found to survive “in the elements” without rust. However, this was only the case with the low temper condition (<750°F) but not the high temper condition (>750°F). This is expected, though the stark difference never fails to surprise me. The low temper condition is labeled “3VL” and the high temper condition is labeled “3VH.”
Corrosion test by Scott Larimore and Kelly W.
Summary and Conclusions
I think CPM-3V has an excellent set of properties with very high toughness and decent wear resistance and edge retention for that level of toughness. With the 7.5% Cr and lack of chromium carbides its level of corrosion resistance is also quite good for a non-stainless steel. I don’t see anyone out there criticizing the steel, but somehow it seems to never quite get the recognition and popularity it probably deserves.
[1] Johnstin, Harry G. “Alloy steels and articles thereof.” U.S. Patent 3,219,442, issued November 23, 1965.
[2] Pinnow, Kenneth E., and William Stasko. “Wear resistant, powder metallurgy cold work tool steel articles having high impact toughness and a method for producing the same.” U.S. Patent 5,830,287, issued November 3, 1998.
Not so much of a knife person as tools, but 3V did make some inroads in custom plane irons and it’s probably being used by a strange agent toolmaker who likes to give artificial names to the steels they use to make it sound like they are proprietary. Woo in the tool world.
It has a big list of attributes and impressive numbers, but nothing about it in use really draws you to it, at least that’s been the case for us. A snafu caused one batch destined for group buy customers to land at 59 instead of 61, and 59 hardness in a woodworking chisel is a dead end unless it’s being used to pry open bucket lids and paint cans.
It has a very persistent burr at 59, too, if a fine edge is needed and the only thing that makes it usable at that hardness is buffing the apex off to prevent deflection.
Not trying to toot your horn, and I don’t even have a magnacut knife, but I can’t think of any reasonable application where I’d choose 3V over magnacut, and 3V is probably harder to get someone to heat treat right, and I doubt if it’s any cheaper- and if so, not much.
What is 3V’s beat applications in a knife? What about as a 8″ chef knife? Does toughness matter there? Would likely have to wipe it dry. How hard would it be to resharpen?
Best application would be hard use knives like choppers or even swords. Chef’s knives are not typically going to be used forcefully against tough objects. Better to have more wear resistance so that you don’t have to sharpen them as often.