Effect of Cryo on Toughness

Skillgannon, you asked about how that changed how the edge behaves. Do you mean the difference between low vs high temper treatments? If so, my understanding is that the high tempers (secondary hardening) cause carbides to form. While this increases wear resistance, it decreases toughness, while robbing the martensite matrix of carbon to form those tempering carbides. Nathan Carothers thread(s) on 3v go into more detail and a more accurate description. His impact tests show a better “apex stability “ with the low tempers vs the high tempers, because with the lower tempers the martensite matrix is slightly more carbon “rich”, which makes the edge stronger. A pocket knife, not being an impact tool, probably won’t show the difference between the 2 heat treatments as well as a competition chopper. Same with a kitchen knife, probably won’t notice too much of a difference. If i was making a knife that would see a lot of impact use, I would try the low temper protocols for sure. A knife made to cut rope/carpet/abrasive stuff, I might go with the high tempers, because of the slight increase in wear resistance the secondary hardening carbides provide.
 
samuraistuart said:
No advice really. Just go direct from quench to cryo, no snap tempers. And I have also heard that a faster quench on these air quench steels is said to help minimize RA as well. So a medium speed oil for the initial quench, then into plates for straightening, then into a 24-36 hour cryo hold, then triple tempers 2 hours each ~400f. To get fancy, prequench the M4 and then respheroidize it. I haven’t tried that approach with M4 tho. There is a tech paper available online about CPMM4 heat treating. I forget the name of it, but a google or Bladeforums search will bring it up. It’s a good read.
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I’ve been thinking a lot about oil quenches with these steels lately, rather than platequenching at profile stage. I’m going through a lot of belts grinding higher alloy steels after plate quenching prior to grinding.
 
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