Systematic experiments were conducted on carburized 4320 steel to study thermalinduced transformation of austenite to bainite by measuring the volumetric transformation strains. Up to about 210°C, thermal-induced transformation involved a diffusion-controlled reaction and therefore was time dependent. The mechanism is similar to that described by the second stage of tempering. An Arhennius-type relation was used to model the kinetics of the reaction leading to a volume increase. A positive hydrostatic stress tended to increase the rate of transformation. The anisotropy of the transformation strains were proportional to the deviatoric stress.

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