Abstract
The formation of isothermal martensite from the retained austenite in an AISI 52100 ball bearing steel was investigated. Optical microscopy reveals that there are mainly two types of isothermal martensite formation: the growth of the athermal martensite and the nucleation and growth of new martensite in the retained austenite. X-ray diffraction shows that during the isothermal transformation, the ratio of lattice constantsc/a decreases, and TEM verifies the precipitation of Fe, Cr)3C in martensite. The kinetics of the isothermal transformation in the quenched steel also shows “C” shape characteristic. At the first stage of the isothermal formation, recovery of the athermal martensite occurs with an activation energy of 91.8 kJ mol-1, implying that the diffusion of carbon in athermal martensite results in the precipitation of carbide and the relaxation of the strain energy at the martensite/matrix boundary. In the second stage, the activation energy for the isothermal formation is 130 kJ mol-1; that may be the energy required for the rearrangement of the configuration of dislocations, forming preferred sites for nucleation.
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