Microstructure–Property Correlation of Heat-Treated SUP9 Spring Steel Using Different Cooling Media

This study investigates the effect of different cooling media during heat treatment on the microstructure and mechanical properties of SUP9 spring steel. The study focused on correlating microstructural evolution with microhardness and analytically predicting tensile properties. The specimens of the SUP9 spring steel were austenitized at 1000 °C for 1 hour and cooled with various media such air, oil, water, brine solution, and ice. The normalized specimens showed lower microhardness and mechanical strength because of stress relief and relatively coarse microstructures. On the other hand, increasing the cooling rate promoted the formation of refined lath martensite, which resulted in significant improvements in microhardness and estimated tensile properties. The maximum microhardness of 340 HV and 286 HV was observed in pearlitic and ferritic regions, respectively, due to the effect of ice quenching, which also resulted in the maximum yield strength ( YS ) and ultimate tensile strength ( UTS ) values of 967.49 MPa and 1086.37 MPa. Also, the properties of brine quenching were better than the standard water quenching. The results gave detailed microstructure-property correlation for different quenching conditions. This comprehensive microstructure-property correlation can be exploited to optimize the mechanical performance of spring steels for advanced engineering applications.