Microstructure Refinement Strategies in Carburized Steel

The fatigue behavior of carburized steels improves with refinement of prior austenite grains during heat treatment. This improvement may be related to intergranular crack nucleation or refinement of the martensite-retained austenite microstructure in the case of carburized steels. Microstructural refinement strategies are being evaluated in this study. Commercial 52100 steel is used to simulate the high carbon contents in the case. Specimens are being subjected to various thermal treatments in a TA Instruments DIL805 quenching dilatometer. Reheating cycles to austenitization temperatures are evaluated with respect to both prior austenite grain size and associated martensite and retained austenite refinement. Additionally, a novel thermal processing technique, step-quenching, is also being evaluated. The heat treatment process consists of two athermal martensite transformation steps separated by isothermal holding at a temperature below the M_S temperature. Dilatometry curves show a drop in the M_S temperature following the primary martensite transformation step, indicating austenite stabilization during isothermal holding. There is evidence that both chemical and mechanical stabilization of austenite may be responsible for depressing the M_s temperature. The microstructure refinement associated with step quenching is compared to the microstructure generated from a conventional quenching process.