Quantitative Analysis of Microstructural Refinement in Simulated Carburized Microstructures

Microstructure refinement strategies in simulated carburized microstructures were evaluated because of their potential for improving the fatigue performance of case-carburized components. Commercial 52100 steel was used to simulate the high-carbon content in the case. Specimens were subjected to various thermal treatments in a quenching dilatometer. Reheating cycles to austenitizing temperatures were evaluated with respect to both prior austenite grain size (PAGS) and associated martensite and retained austenite (RA) refinement. Quantitative stereological measurements were performed to evaluate the microgeometry of plate martensite and the size distribution of RA regions. Decreasing the reheating temperature resulted in finer PAGS, and multiple reheating cycles resulted in a narrower PAGS distribution. Refinement in PAGS led to a reduction in martensite plate size and finer distribution of RA. Additionally, interrupted quenching below martensite start (M_S) temperature was evaluated. This processing route results in a refinement of martensite plates and more stable RA. The stabilization of austenite may be mechanical or chemical in nature, owing to the deformation of austenite during primary transformation, or due to partitioning of carbon into austenite similar to quenching and partitioning steels.