Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

The austempering of steel was first pioneered in the 1930s by Edgar C. Bain and Edmund S. Davenport, who were working for the United States Steel Corporation at that time. Bainite must have been present in steels long before its acknowledged discovery date, but was not identified because of the limited metallographic techniques available and the mixed microstructures formed by the heat treatment practices of the time. Coincidental circumstances inspired Bain to study isothermal phase transformations. Austenite and the higher temperature phases of steel were becoming more and more understood and it was already known that austenite could be retained at room temperature. Through his contacts at the American Steel and Wire Company, Bain was aware of isothermal transformations being used in industry and he began to conceive new experiments

Further research into the isothermal transformation of steels was a result of Bain and Davenport's discovery of a new microstructure consisting of an "acicular, dark etching aggregate." This microstructure was found to be "tougher for the same hardness than tempered Martensite". Commercial exploitation of bainitic steel did not become common overnight. Common heat treating practices at the time featured continuous cooling methods and were not capable, in practice, of producing fully Bainitic microstructures. The range of alloys available produced either mixed microstructures or excessive amounts of Martensite. The advent of low-carbon steels containing boron and molybdenum in 1958 allowed fully Bainitic steel to be produced by continuous cooling. Commercial use of bainitic steel thus came about as a result of the development of new heat treating methods, those that involve a step holding the work piece at a fixed temperature for a period of time sufficient to allow transformation became collectively known as austempering.

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