Effect of Austempering below and above Ms on the Microstructure and Wear Performance of a Low-Carbon Bainitic Steel

Abstract

The microstructure and wear performance of a low-carbon steel treated by austempering below and above martensite start temperature (Ms) were investigated. The results show that the bainite, fresh martensite (FM) and retained austenite (RA) were observed in samples austempered above Ms. Except for the three above phases, the athermal martensite (AM) was also observed in samples austempered below Ms. The bainite transformation was accelerated and finer bainite was obtained due to the AM formation in samples austempered below Ms. In addition, the strength and hardness were improved with the decrease of the isothermal temperature and time, whereas the total elongation decreased with the increasing isothermal time and the decreasing isothermal temperature. Moreover, the materials austempered below Ms exhibited better wear performance than the ones treated above Ms, which is attributed to the improved impact toughness by the finer bainite and the enhanced hardness by AM. The best wear resistance was obtained in the samples austempered at 300 °C below Ms for 200 s, due to the highest hardness and considerable impact toughness.