Insight into the induction hardening behavior of a new 0.40% C microalloyed steel: Effects of initial microstructure and thermal cycles

The induction hardening behavior of a new, hot-rolled 0.4 wt% carbon steel with the two different starting microstructures of upper and lower bainite has been simulated using a Gleeble 3800. The effect of heating rate in the range 1–500 °C/s on austenite grain size distribution has been rationalized. Dilatometry together with Scanning Electron Microscopy combined with Electron Backscatter Diffraction analyses and thermodynamic simulations provide insight into the austenite formation mechanisms that operate at different heating rates. Two main mechanisms of austenite formation during re-austenitization were identified: diffusional and diffusionless (massive). At conventional (1–5 °C/s) and fast (10–50 °C/s) heating rates the austenite formation mechanism and kinetics are controlled by diffusion, whereas at ultrafast heating rates (100–500 °C/s) the formation of austenite starts by diffusion control, but is later overtaken by a massive transformation mechanism. Comprehensive thermodynamic descriptions of the influence of cementite on austenite formation are discussed. The finest austenite grain size and the highest final hardness are achieved with a lower bainite starting microstructure processed with a heating rate of 50 °C/s to an austenitization temperature of 850 °C followed by cooling at 60 °C/s.