Austempering
In austempering, the microstructural end product of the gray iron matrix formed below the pearlite range but above the martensite range is an acicular or bainitic ferrite, plus varying amounts of austenitic depending on the transformation temperature. The iron is quenching from a temperature above the transformation range in a hot quenching bath and is maintained in the bath at constant temperature until the austempering transformation is complete.
In all hot quenching processes, the temperatures to which castings must be heated for austenitizing and the required holding times at temperature prior to quenching in the hot bath correspond to the temperatures and times used in conventional hardening, that is, temperatures between 840 and 900°C (1550 and 1650°F). The holding time depends on the size and chemical composition of the casting.
Gray iron is usually quenched in salt, oil, or lead baths at 230 to 425°C for austempering. When high hardness and wear resistance are the ultimate aim of this heat treatment, the temperature of the quench bath is usually held between 230 and 290°C. The effect of iron composition on the holding time may be considerable. Alloy additions, such as nickel, chromium, and molybdenum, increase the time required for transformation.
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