Austenite formation in 0.2% C and 0.45% C steels under conventional and ultrafast heating

The austenite formation in 0.2% C and 0.45% C steels with the initial microstructure of ferrite and pearlite has been studied. The effect of conventional (10 °C/s), fast (50 °C/s–100 °C/s) and ultrafast heating rates (> 100 °C/s) on the austenite nucleation and growth mechanisms is rationalized. Scanning Electron Microscopy (SEM), and Electron BackScatter Diffraction (EBSD) analyses provide novel experimental evidence of the austenite nucleation and growth mechanisms operating at ultrafast heating rates. Two mechanisms of austenite formation are identified: diffusional and massive. It is demonstrated that at conventional heating rates the austenite formation kinetics are determined by carbon diffusion, whereas at ultrafast heating rates formation of austenite starts by carbon diffusion control, which is later overtaken by a massive mechanism. Comprehensive thermodynamic and kinetic descriptions of austenite nucleation and growth are developed based on experimental results.