Just FYI to everyone, from the Maxim oil website and tech data sheet, P50 is to be used from room temp (ambient) to 120°F. They also say, "it is as effective at 50F as it is at 120F because of its low viscosity." To those of you heating P50, keep that in mind. I would never go past 120°F. I keep mine in the garage in central TX during the summer, and pull it inside during cooler months. Notice is said "cooler", not "cool", certainly not "cold"!!! LOL! Canola oil should be warmed up to about 130°F, but not P50. Not really sure that this is a quench oil problem, tho.
I go to 220 grit scratch pattern lengthwise, but totally agree that 120 grit is PLENTY good. The idea is to not have any scratch pattern going straight up from the edge (or spine for that matter).
I'm curious as to exactly HOW the cracks happened. You said "not much pressure". I'll put it this way, I recall a few years ago I had a 52100 blade that took a warp immediately post quench. Normally I would run one temper cycle, then use the 3 point clamp method during the 2nd cycle to take it out. But I got big headed and decided I would see if I could just slightly bend that 52100 blade, which was about 67HRC, and it snapped in two "without much pressure". Grain structure was great! I'm suggesting that NO pressure is applied to the blade once the martensite finish has happened, which is significantly warmer than ambient with simple steels.
Without being there and carefully noting every exact procedure, I would say you are good to go, just no more "not much pressure"!
