austempering

Austempering is a heat treatment process used to produce a bainitic microstructure in steel or ductile iron. The metal is first austenitized, meaning heated into the austenite range, and then rapidly transferred to a bath or controlled quench medium held at a lower, isothermal temperature. The material is kept at this temperature until the bainitic transformation is complete, after which it is cooled to room temperature. The goal is to obtain a uniform, tough microstructure with reduced distortion and residual stresses compared with conventional quench-and-temper treatments.

In steel, austempering involves austenitizing at or above the appropriate temperature for the alloy, followed by quenching into a salt bath, polymer bath, or mineral oil bath typically maintained in the bainitic temperature range, often around 250 to 350°C. The transformation to bainite occurs gradually, and the portion may contain retained austenite depending on the alloy and temperature. The result is a bainitic ferrite plus cementite or carbide, sometimes described as lower or upper bainite depending on the transformation temperature.

In ductile iron, austempering produces an ausferritic microstructure, a bainitic ferrite with stabilized carbides, which can yield high toughness and good wear resistance with relatively low distortion and residual stress compared with conventional quenched and tempered iron.

Advantages of austempering include improved toughness, better fatigue resistance, reduced distortion, and lower residual stresses. Disadvantages include longer processing times and the need for specialized equipment such as controlled-temperature baths. Applications span gears, shafts, cam followers, wear parts, and other components where a favorable strength-toughness balance and dimensional stability are important.