Austempering

The material is heated and transformed from Ac3 into austenite state (see Austenite). Then the work piece is quenched in a hot bath (salt bath or metal bath, see hot bathhardening) which is kept at bainitic temperature leaving it there until the transformation of austenite is completed. The bainitic structure formed in this process contains the carbides already in the form as they are formed during tempering after hardening. Thus, it is not necessary to temper the bainitic structure subsequently. Any process can be used for cooling down the material to room temperature.

The temperatures which can be applied for bainite transformation are provided in the TTT diagram for isothermal transformation applicable to the relevant steel. The transformation process does not start until a certain warm-up time is over. If the work piece is removed from the hot bath after it has adopted the hot bath temperature, martensite is formed during the cooling process. While keeping the work piece at the martensite point, the austenite is completely transformed into the tough bainitic structure.

Since no tempering is required, there are no such processes which result in tempering brittleness. The bainitic temperature should be as low as possible so that the structure is as fine-grained as possible.

Additional references:
Hardening and tempering
Hardening
Bainitic transformation

  • Fig. 1: Rough bainitic structure of carbon steel, 1000:1, (s. Bainite)
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