I told myself I was going to stop getting into technical discussions on metallurgy on this forum. Mete and Kevin don't need me bumbling things up...
But one last time...
John,
A quench can be said to have been successful if you got under the pearlite nose. Eh? Well, in more complex steels there can be more to it than that. The rate of quench can effect where that carbon goes. A fast quench in D2 gives the carbon less time to interact with carbide formers on the way down. This can be good - giving you better corrosion resistance. But it can also lower Ms and Mf - leading to RA, leading to crappy edge stability. So if you're not going to use cryo (which is used to address RA which can be 15% or more), you may want to air quench without plates. You also want to limit the carbon you put into solution to only what you need for your martensite, also to reduce RA. So I'd soak it at 1,500 for 15 min, then ramp quickly to 1,800 for 30 min, then quench in still air. And I'd temper at 475 or higher, to decompose RA. You should experiment with your temper to find the highest hardness that doesn't suffer the edge roll that comes with RA. Edge stability is king - and with D2 high HRC and low RA are key.
That's a lot of talk about RA huh? Well, there are a lot of cool things you can do with D2, but many of them promote RA, which has to be addressed, which is why I use cryo.
I have thought about experimenting with a high temperature HT to get the carbon where I want it, followed by a low temperature HT to reaustenitize, but not free up as much carbon to raise Mf for the second quench and do away with cryo. Probably a dumb idea though....
Steve, there is a chart somewhere that shows a notched impact strength peak at something like 500 F and HRC 60, and several years ago when I started with D2 knives, that is where I started. Then I found another chart that had a different toughness peak at 800. And one that had a wear resistance peak at something like 400, and another at 850, and on and on and on. But how hot were they austenitized? Did they get cryo? A snap temper first? Then I found you can have two D2 knives at exactly identical HRC 62, and one has bad edge roll and one doesn't, depending on austenitizing temp, timing of cryo and tempers. Same hardness - way different knives. This is because of RA. Retained austenite is where that high impact strength comes from - it goes down from there because you're decomposing RA, and your increasing carbide volume. Well, that RA is good in a die, but really really bad in an edge. I finally came to realize that the "peaks" on those charts had no bearing on blade performance. The HT is different, so it is apples and oranges - and what they're measuring isn't relevant. Higher tempers decompose RA, but I think lower tempers with cryo are better.
Okay, I'm done this time. Really.
