Isothermal treatment in which parts are quenched from austenitizing into molten salt, usually at 260 to 400 °C, and held until transformation to bainite is complete, then cooled. No tempering is needed. Austempered parts are tougher and distort far less than quenched-and-tempered parts at the same hardness. It suits thin carbon steel sections such as springs, clips and blades, and austempered ductile iron uses the same route.
Mower blades get austempered to 48 HRC, they bend on a rock instead of snapping.
Section size is the limit: thick plain carbon parts cannot lose heat fast enough in salt and make pearlite before they reach bath temperature.
Also heard as
- austemper
Related terms
Bainite
A tough mix of ferrite and fine carbides that forms between the pearlite and martensite temperature ranges.
Marquenching
Quenching steel into hot oil or salt just above Ms, holding to even out temperature, then air cooling.
Salt bath
A pot of molten salt used to heat, quench or case harden parts by quick, even immersion.
TTT diagram
Time-temperature-transformation chart showing how long austenite takes to transform when held at a fixed temperature.