Small side note, largely unrelated:
A commonly recognized Mf for D2 that I have seen is frequently listed as -100F. Obviously that value would depend a lot on the way it was treated, but for arguments sake, lets say you gotta get it -100 to complete your quench. Is that a fair statement?
It is also commonly recognized that a significant delay during your quench can cause the austenite to stabilize or form into a more stable structure that is not martensite. I'm not a metallurgy expert, is that an accurate statement?
In a steel like D2 that already has a tendency to retain significant amounts of RA (I have seen larger values in recent literature suggesting it may be understated in older texts) it seems logical to me that going directly into sub zero before a temper will maximize martensite conversion, at the risk of causing cracks. risk -> reward. Someone please tell me what I'm missing here.
I'm not a metallurgy expert, but it was a subject in my materials classes at NCSU (RIP Armand Cook) and, being an interest of mine I took full advantage to discuss it with my professors, and I spent plenty of hours reading about it in the library (5th floor - M, I still remember). So, while I don't know nearly as much about the subject as some of the people here, I have seen it from the industrial and academic points of view and practiced it later in my tool and die work and something I have learned from all of this is that in industry a steel designed for a ball bearing or stamping die will have more research focused on things like predictable dimensional changes and reduced risk of cracking than fine edge stability.
This leads me back to the point I was making about freezing before temper, which generally goes against most accepted industry practice. My own HT experiments showed that D2 works better if you go all the way to Mf before fiddling around with any kind of snap temper. Perhaps I was doing something wrong? But then the results were duplicated when I got Bos to skip his snap temper (which goes against his standard HT for reasons already listed). Did he also do something wrong?
So, based on both my (admittedly weak) understanding of metallurgy and imperial observations, it is my opinion that some steels should be frozen before tempering when used as a knife rather than a stamping or forming tool because cracking is less likely in a thin object like a blade and issues like toughness in impact and predictable dimensional changes may be less important.
So based on this, I don't think it can be said that all knives should receive a temper immediately. I think it depends on their Mf and their intended use.
For reasons that I do not understand, transformation at Mf that occurs at very low temperatures appears to be less than instantaneous. Perhaps someone could shed light on that for me. Why is cryo a multi hour prospect?
I'm not trying to argue with anybody, I have a lot of respect for everyone here. My eyes and ears are wide open. Please explain what I'm missing here.
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