Cooling quenched parts to roughly minus 75 to minus 85 °C with dry ice, mechanical freezers or nitrogen vapor, so austenite that did not transform at room temperature continues to martensite. It raises hardness and stabilizes dimensions in gauges, bearings and carburized parts. It should follow the quench promptly and be followed by a temper, because the newly formed martensite is untempered and brittle.
Gauge blanks go quench, sub-zero treatment at minus 120 F, then temper twice, no shortcuts.
Go cold right after the quench; letting parts sit at room temperature first stabilizes the austenite and weakens the effect.
Also heard as
- cold treat
- freezer
- deep freeze
- subzero
Do not mix it up with
- Cryo: Sub-zero treatment stops around minus 80 °C to convert retained austenite, while deep cryogenic treatment goes near liquid nitrogen temperature with a slow ramp.
Related terms
Retained austenite
Austenite that fails to transform during the quench and survives at room temperature inside hardened steel.
Cryo
Shop name for deep cryogenic treatment, a slow cool to near liquid nitrogen temperature after hardening.
Tempering
Reheating hardened steel below its lower critical temperature to trade some hardness for toughness.
Growth
The increase in a part's size after hardening, caused mainly by austenite turning into bulkier martensite.