The effect of air-cooling heat treatment on the microstructure and phase transformations of structural steels

The aim of this study was to investigate the microstructure, phase transformations, and hardness changes of S235JR, S275JR, and S355JR structural steels after air-cooled heat treatment. The steel samples were austenitized at 1000°C and then cooled in air, and the sample temperatures were recorded with a thermocouple during cooling. The microstructure and phase transformations of the samples were analyzed both experimentally and with the Computerized Combining Phase Diagrams and Thermochemistry (CALPHAD) material property software. It was observed that higher carbon and manganese levels in structural steels lower the phase transformation temperatures, promote bainite formation, reduce the average ferrite grain size, and consequently lead to an increase in hardness. The phase ratios and hardness values were determined by the predicted experimental results with a deviation of less than 3%. These results show that a balanced ferrite and bainite phase distribution is achieved air cooling and the software effectively models these transformations.