thanks doug,
aldo's site has nothing listed about the 15n20. cryo for 52100 is recommended by carpenter, crucible, and latrobe. can i get the blades cold enough in dry ice/acetone bath?
Carpenter, Crucible and certainly Latrobe make the steel for bearing manufacturers, all of the heat treating specs are geared towards making bearings. Retained austenite in a perfect sphere, designed to handle compressive loads, will produce a different set of variables, and even opportunities, in dealing with it. Almost all the specs for 52100 for its intended use as bearings show an austenitizing temp of around 1550F., this will result in a spike in retained austenite that when later converted, in a sphere, can create an outward force which could be handy in compressive strength, in a knife blade it is just a problem that needs to be fixed. I'm not saying that RA is desirable in a bearing either, I am just saying when and how it is dealt with can be.
If you want to pursue the other claims of lattice improvement or special precipitates that are claimed by cryo folks, or if you are dealing with steels having Cr above 12% or significant amounts of nickel then, freezing is the way to go. Folks really into cryo have a lot of curious ideas about it, but if you are seeing any significant increase in HRC hardness from freezing a blade made from a simple alloy or carbon steel, something is not right in your initial heat treatment; and simply correcting your temperatures is far easier than setting up an unnecessary cryo system.
Following the bearing making specs, I found that starting at 1500F you could expect as much as a full point Rockwell drop in as-quenched hardness for every 25 degrees due to the pro-eutectoid carbon brought into play. If these samples were subjected to liquid nitrogen with no other treatments that lost Rockwell hardness was regained; only two phases account for that- austenite making up its mind about being martensite. The same steel heated from 1425F to 1500F in 25 degree increments saw a climb in HRC with a possible 67 HRC in the middle of that range, until it leveled off at 1500F.
Scott, if you have a 25+/- window to work in I would set your heat for 1475F and not sweat it too much, it should work out in that sweet spot. The 15n20 will respond well to heating to around 1500F with an oil quench.