One caution about these data sheets, they generally refer to pieces of steel thicker than knife blades. Also knife blades qualify as irregular and probably the intricate shapes referred to above. Also be aware that the best Rockwell hardness testers only have a precision of ± 2 HRc so any improvement in hardness is marginal...
NIST type specs list a +/- of 1 point variance in Rockwell calibrations, however the test blocks often will have a 2 point swing. I have found this to be a general average since one can be very fastidious on the upkeep and calibration of a good tester with top of the line test block
s (just one will probably not get you tighter than 2, but triangulating off several will) and get some pretty precise and consistent readings, but you have to be fairly OCD for it. I proabaly have close to half a dozen test blocks and never take less than 5 readings if it really counts, but then there is that OCD thing again:3:
I was thinking pretty much the same thing on the shapes considered in other industries Doug. I am not usually of the line of thinking that knife shapes negate industrial steel specs, but in matters of heating and cooling rates it cannot be ignored entirely. Freezing the blades probably can’t hurt, but I have never found it necessary with O-1. Barring over-soaking or over-heating there should be no problem achieving 65-66 HRC with knife shaped cross sections. Industry will have much differently shaped parts and thickness, and cooling rates will be, well, industrial. With perhaps in the range of 6-8% retained austenite, you are really pushing the point of diminishing returns. If one has the room for 2 or more points gain in HRC, I would adjust the soak temp and time for the quickest fix, as it strongly suggests greater than usual retained austenite in O-1. And if one does get such a gain from cold treatments it confirms the conversion of a problematic amount of retained austenite that would lead me to review my soak times and temps for a simple little knife blade.
Ausbrooks, I very much like that PDF you linked to, I saved it away in my library for future use. It takes a very refreshing objective approach to the subject and sticks with verifiable facts as much as possible, while pointing out the large amounts of unsubstantiated claims regarding cryo. Very fair and concise, a rare gem in the literature on the topic.