Induction Hardening 5150 Steel: Effects of Initial Microstructure and Heating Rate

Induction heating has permitted great progress in the surface hardening of a wide variety of steels, but results in a wide range of local thermal cycles. The metallurgical changes during rapid heating and cooling have not been sufficiently studied with respect to heating rate and prior microstructure. In the present investigation, induction dilatometry was performed on 5150 steel with ferrite-pearlite and tempered martensite initial microstructures to assess effects of experimentally controlled prior microstructure and heating rate on austenitization kinetics. Heating rates were varied from 0.3 to 300 °C/s to simulate industrial processes, and post-hardening metallography and hardness testing were performed. Results show that the transformation kinetics for prior ferrite-pearlite microstructures are significantly slower than for prior tempered martensite microstructures, although hardness is equivalent for a given thermal cycle. Metallographic evidence suggests significant remnant segregation of chromium in regions of pearlitic cementite (enriched); evidence of segregation was not observed metallographically for prior tempered martensite. Diffusion-based transformation simulations support observed ferrite-pearlite alloy segregation, suggest residual alloy segregation is possible for prior tempered martensite, and can be used to tailor austenitization thermal cycles to process requirements. Detailed time and temperature-dependent local microstructure development results from this study are directly applicable to practical induction hardening simulations.