Wow that's cold! Freezing high-alloy steels and why it's important.
There has been a long myth that high alloy, high carbon stainless steels don't make good blades, and this has been true probably until the early 90's. Back when a knife forged from a bar of simple steel and heat treated in a home forge was the standard for performance, there was no way of knowing that freezing a knife in really cold temperatures would greatly improve the way it functions. There have been many amazing studies in metallurgy since the 1960's, and it's been shown that adding a cryogenic treatment to the heat treatment protocol, if done right, can greatly improve the performance of a knife.
Recently I heat treated a batch of high carbon stainless steel chef knives made of Uddeholm AEB-L, a steel I use for almost all my chef knives. Right out of the quench, we tested it on a Rockwell hardness tester, and it tested at 50HRC. It's as if it didn't harden. Comparatively, a simple low alloy steel like a leaf spring would test at about 60-62HRC, and that's very brittle considering the steel. If you drop it it will break like glass.
We submerged the batch of blades in a dry ice bath for 15 hours at -85 degrees Celcius. Water freezes at 0 so that's incredibly cold. When the blades were done freezing, we tested it and found that it shot up all the way to a very hard 65HRC! After tempering the blades it's now a very hard and flexible 60HRC, and I'm still amazed at how well this steel performs. Think of it like a stainless version of AISI 52100 steel, and don't just take it from me! Knifemaker Devin Thomas said it in one of his articles detailing why he loves using AEB-L.
This jump in hardness happens to many high alloy steels, and for a quality knife, I think cryogenic treatment is a must. All this information about freezing and working with high alloy steels are available online, and in most cases, the steel supplier will have very specific instructions on how to work with the metal. There's no secret quenching technique passed down through generations of smiths, just thousands of dollars invested into studying how to make these steels perform the best possible way.
There's something I'd like to point out though, and I've seen many people do it in the past. It's the fact that high alloy steels should not be forged. I've seen blades advertised as "hand-forged 440C" or "hand-forged D2 blended 5160", and these would sound pretty silly to anyone who knows anything about the way these steels are treated. If there's anything I want to leave you with, it's the idea that when you purchase knives, please get them from people who know what they're doing.