Microstructural and dilatational changes during tempering and tempering kinetics in martensitic medium-carbon steel

The microstructural and linear strain changes of martensitic medium-carbon steel were investigated during continuous heating to 600 °C at different rates using dilatometry and transmission electron microscopy (TEM). The precipitation of transition ε -carbides between 70 °C to 240 °C and the precipitation of cementite between 200 °C to 450 °C were observed depending on the heating rate. The measured strain changes during tempering stages 1 and 3 were converted to the fraction of tempered martensite based on the theoretical iron atomic volume change between martensite and tempered martensite. The presegregation amount of carbon before tempering was calculated to be about 0.16 wt pct by comparing the theoretical strain change with the measured strain change. Tempering kinetic models were developed using the tempered martensite fractions converted from the measured strain changes during tempering stages 1 and 3. The kinetics models exhibit a good correlation with the experimentally measured tempering kinetics.