Isothermal austenite–ferrite transformation of Fe–0.04 at.% C alloy: Dilatometric measurement and kinetic analysis

The kinetics of the total γ → α transformation of an ultra-low-carbon Fe–0.04 at.% C alloy was investigated isothermally at temperatures of 1106.4 and 1107.7 K after rapidly quenching from the austenite-phase region and by using high-resolution dilatometry. As a function of transformation time, two stages of phase transformation can be clearly distinguished in the γ → α transformation of the interstitial Fe–C alloy: a first, very fast stage and a second, sluggish one. The kinetics of the isothermal γ → α transformation has been described by a phase-transformation model involving site saturation (pre-existing nuclei), two separate growth stages and a hard impingement model for randomly distributed particles. The first, fast stage pertains to massive transformation with interface-controlled growth, whereas the second, sluggish stage involves diffusion-controlled growth of the product ferrite phase.