Effect of Prior Martensite on the Tempering Stability of Microstructure and Hardness for Low‐Temperature Bainite Steel

This study systematically investigates the effect of prior martensite on the tempering stability of the microstructure and hardness of low‐temperature bainite steel using scanning electron microscopy, electron back‐scattered diffraction, transmission electron microscopy, X‐ray diffraction analysis, and hardness testing. Microstructural characterization indicates that the steel with and without quenching at a certain temperature before the isothermal bainitic transformation exhibits two kinds of microstructures, one consisting of bainitic ferrite and retained austenite with martensite and another without martensite. Furthermore, the tempered steel with prior martensite exhibits finer bainitic laths (60–160 nm) and maintains stable hardness (500 HV1) even when tempered up to 500 °C, whereas the tempered steel without prior martensite shows a considerable hardness degradation when tempered above 400 °C. It is demonstrated that the finer bainitic laths and the more thermally stable mechanical behavior are related to the prior martensite, which not only provides more potential nucleation sites for the transformation of bainite by introducing the α‐γ interface but also delays the decomposition of retained austenite by exerting compressive stress on adjacent retained austenite to prevent the carbon precipitation.