Effect of tempering temperature on isothermal decomposition product formed below Ms

Recent papers have shown that bainitic ferrite plates can be produced by transformation at low temperatures, resulting in enhanced mechanical properties of martensitic steels and increased resistance to tempering. To that end, experimental investigations were performed on the formation of the microstructure in SAE 9254 steel during isothermal heat-treatment below the martensite start (Ms) temperature. After the isothermal heat-treatment, the same steel was subjected to tempering at various temperatures. Vickers microhardness measurements were performed on the isothermally heat-treated and tempered samples, and microhardness values for the samples were compared with those for quenched and tempered samples of the same steel. All the isothermally treated samples contained the martensite (αμ), bainitic ferrite (αb), and retained austenite (γr) phases. The retained austenite blocks (γb) decomposed in the samples tempered at 400 °C. A significant decrease in the microhardness value for the sample heat-treated at 270 °C can only be observed after the sample was tempered at 450 °C, because bainite was the predominant phase in the microstructure, which self-tempers as it forms.