Austempering is an isothermal heat treatment process which is carried out on ferrous materials to produce improved mechanical properties and reduced distortion.
Austempering process is used for medium to high carbon steels as well as for gray iron and ductile iron castings. Austempering produces bainitic microstructure in case of steels where as in case of cast iron the microstructure consists of ausferrite (mixture of acicular ferrite and high carbon stabilised austenite).
Austempering consists of quenching from the austenitising temperature into a molten salt bath (usually nitrate – nitrite salt bath) which is held at a constant temperature called the austempering temperature. The temperature ranges from 240 to 400 degree C depending upon the material being austempered and the properties desired. The quenched parts are then kept immersed in the molten salt bath for a certain amount of time which again depends upon the material being austempered and the properties desired. The holding times can vary from few minutes to few hours. During the holding period, an isothermal reaction takes place and the desired microstructure is obtained. Austempering usually produces an uniform microstructure.
Applications of austempering of steel include clips and coil springs where maximum flexibility or resilience and toughness is required.
Austempered ductile iron (ADI) castings are becoming increasing popular in the automotive industry for parts such as crank shafts, connecting rods, Cam shafts, wheel cassette assemblies, engine mounts, differential casings, suspension components and so on. ADI castings are also used for pneumatic tool applications and wind turbine applications. ADI castings offer cost and weight competitive advantages over the other conventionally used competing materials. A lot of work is still going on in finding and developing newer applications for ADI castings.
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