Near-Ms Austempering of Carbide-Free Bainitic Steel: Effects on Phase Transformation, Microstructure and Mechanical Properties

The parameters of the austempering process play a crucial role in governing the microstructure and mechanical properties of carbide-free bainitic (CFB) steel. In this study, a CFB steel was austempered at temperatures close to its martensite start (Ms = 372 °C) temperature to investigate the bainitic transformation kinetics, microstructure, and mechanical properties. To identify the optimal strength–ductility combination, austempering was carried out at 360 °C, 380 °C, and 400 °C for comparison. The results show that austempering slightly below Ms (360 °C) produces the highest yield-to-tensile strength ratio and a good strength–ductility balance. Dilatometry curves indicate that the onset of bainite transformation occurs fastest when austempering slightly below Ms. The stronger transformation driving force and the presence of athermal martensite are the primary reasons. The enhanced thermodynamic driving force and increased nucleation density promote the formation of a larger amount of bainitic laths. Electron backscatter diffraction (EBSD) analysis reveals that the retained austenite blocks are finest after austempering at 360 °C, which helps alleviate the ductility loss associated with the reduction in retained austenite content.