Voodoo, thy name is Cryo
Steel gets hard because it wants to be in a different crystal structure, depending on the alloy and temperature.
If we say a typical heat treatable steel, say 4140:
1. At room temp, it wants to be in a ferrite structure, provided of course it was previously slow cooled (annealed, and soft).
2. Heat it up into red heat ~1300°F the microstructure changes to austenite.
3. Cool it very slowly, it changes back into ferrite.
4. Cool it rapidly (quench it) it changes into martensite, which is hard and brittle.
5. Temper the martensite, and the material is reduced in hardness, but gains toughness. Tempering for most steels is anywhere between 400°F and and 1000°F.
6. The finished hardness is stronger than mild steel, but is not brittle, and is tough.
In most cases, when material is quenched, not all of the material changes from austenite to martensite. Some % remains, and this "retained austenite" is the subject of cryo treatments. The theory is that a cryo treatment (over 100 degrees below zero) can cause this retained austenite to transform into martensite, improving the metal. Most cryo treaters will then re-run tempering afterwards, to ensure this newly formed martensite gets tempered the same way the rest of the part did previously.
I've experimented with cryo in the past, to see if it would improve the durability of some tools. I never saw enough benefit to justify the hassle. And once or twice, the tool cracked.
But I knew a heat treater in the Bay Area who was happy to cryo treat high-end speakers. His customer swore it made them sound better. Dat be Voodoo...
