Untransformed austenite left in a hardened part because the quench stopped above the martensite finish temperature, which sits below room temperature in high-carbon and high-alloy steels. Carburized cases commonly carry 10 to 30 percent. It lowers hardness and can transform later in service, causing growth in gauges and bearings. Shops reduce it with sub-zero treatment and multiple tempers and measure it by X-ray diffraction.
XRD came back 28 percent retained austenite on the case, those pins go in the freezer before temper.
Some retained austenite is welcome in carburized gears and bearings for contact fatigue life, so the argument is over how much, not whether.
Also heard as
- RA
Related terms
Sub-zero treatment
Chilling hardened steel well below room temperature to turn retained austenite into martensite.
Double tempering
Tempering a part twice, cooling to room temperature in between, to temper martensite formed after the first.
Growth
The increase in a part's size after hardening, caused mainly by austenite turning into bulkier martensite.
Carburizing
Diffusing carbon into the surface of low-carbon steel at high heat, then quenching for a hard case.
Martensite
The hard, needle-like phase that forms when austenite is quenched fast enough to trap its carbon.