I would certainly be interested in a CPK 4V knife!
It's interesting, Nathan, that your optimized 4v was able to fare better than the D3V in your nail cutting test. If asked, without knowledge of the result, which steel would perfom better, I know a lot of folks would (wrongly) guess 3v. I guess what I mean is that it's sometimes easy to regard "toughness" and "edge stability" as two steel characteristics that are positively correlated to each other. IOW, it's tempting to conclude that the steel with greater toughness will inherently have better edge stability than a steel known to be less tough. It is easy to blend them together (in your head like how you might rate a certain steel's performance as compared to another. I've certainly done this!)
As you've demonstrated, that is not always true. Toughness and stability of the cutting edge are distinct and seperate aspects of a steel. (Why that is true, I don't know)
Un hardened mild steel is pretty tough, as defined by joules of energy absorbed in an impact fracture such as a Charpy V notch test.
S30V has good wear resistance, as defined by micro grams of material eroded in a sliding abrasion test or cards cut in CATRA testing.
It's funny that people can take the values from tests like that at face value but cutting a nail is hype because it doesn't generate a quantifiable number, even if the result is repeatable and comparatively useful.
Normal use is cutting wire, stripping insulation, cutting dryway tape, cutting carpet, prying staples, opening boxes, shaving burrs, cutting tent stakes etc. These all apply varying degrees of lateral stress inducing chipping and edge roll that would be impossible to measure with some of the more conventional tests. But what we have found through cut testing and experience is too much carbide is a bad thing, but you want some. And too hard is a bad thing, but it needs to be pretty stinking hard to support a fine edge. If you start with a good steel and get your microstructure right you can go pretty hard. Much harder than people realize.
3V was designed to be tough but its out-of-the-box edge stability is mediocre. It took real work to get good fine edge stability in Delta 3V, and it's still only approaching the edge stability of really good W2. But it does it with a level of toughness and wear resistance that W2 can't begin to touch.
One of the things we did to 3V was push the aust time and temp to force some plate martensite for more of a crisp edge (it's designed to be lath). Our tweaks were never about toughness, always about edge durability in rough use. These tweaks caused issues with RA which require tweaks of their own while avoiding the secondary hardening hump (again, in order to achieve that crisp edge).
4V is plate martensite out-of-the-box. And the larger carbide volume fraction actually increase compressive yield strength. But it's still 85% iron, compared to 75% for something like S30V. It achieves a lot of its impressive wear resistance through old-fashioned
very hard martensite, not tons of carbide dumped in. This approach gives both higher toughness (in the context of a knife edge), and the ability to support a thin edge at higher hardness without being chippy. They didn't add nickel for toughness meaning it can work without the secondary hardening hump (with some other tweaks). The end result was a great deal of potential for a steel that is a stripped out race car next to 3V. I'll always like 3V better, but 4V is pretty amazing in some applications (if given the right adjustments, which are pretty straight forward).
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