[fitzo]

the cryo is a debatable phenomenon. there are supposedly two aspects to it: conversion of retained austenite to martensite, and secondly carbide rearrangments in high alloy steel. the first is a fact, the second still a debate.

i have HT'd stainless at home for years, and it is notorious for retained austenite. i also have a rockwell tester to allow me to follow hardness as a measure of austenite conversion.

what i do know is that a SS blade coming out of an oil quench with a Rc of 58 will show a hardness increase of 3-5 points if cryo'd. my experiments on ATS and 440C have shown that it would continue to climb for a week or so if using a home deep freeze, before it leveled off. 12-16 hours at -78C in a dry ice acetone bath would do the same, as would 6-8 hours in liquid nitrogen. in my experience, all hardened to about the same rockwell. thus, i think it is a RATE phenomenon to some extent, so the lower the temp the higher the rate of conversion.

now, metallurgical literature speaks of things going on with the carbides that may be beneficial. this seems to take the ultralow temps of LN to achieve.

thus, my unstudied conclusion would be that, for a simple steel, home freezing for some length of time (I use a week) will suffice to transform unretained austenite, but lower temps are necessary for high-alloy steels.
BTW, simple steels should be snap tempered or tempered once before chilling to prevent cracking. stainless is apparently cryo before temper. i've never had a SS blade crack.

hope this helps