Effect of Heat Treatment Process on Mechanical Properties of a Medium Carbon Low Alloy Steel

Effect of heat treatment processes on the microstructure and mechanical properties of a medium carbon low alloy steel was studied by means of color metallography, XRD and mechanical tests.The adopted heat treatment processes included air quenching and then austempering in salt bath, watercooling and then austempering in salt bath, as well as directly austempering in salt bath. The results show that after treatments according to the above three processes the steel may exhibited microstructure composed of different amount of bainite and martensite, and better mechanical properties in comparison with the cast ones, i.e. its impact toughness and hardness were increased by 92%-183% and 31%-55% respectively. For the case of air cooling and then austempering in salt bath, the amount of bainite decreased gradually with the increase of air cooling time while the amount of martensite progressively increased, correspondingly its hardness and impact toughness showed a tendency of increase and decrease respectively.The mechanical performance of the medium carbon low alloy steel is closely related to the ratio of bainite to martinsite in the microstructure. It is noted that the steel with a duplex microstructure of 50%-60% bainite and 30%-40% martensite exhibited an optimal comprehensive mechanical performance.