The austenite–ferrite transformation of ultralow-carbon Fe–C alloy; transition from diffusion- to interface-controlled growth

The kinetics of the isochronal austenite (γ) → ferrite (α) transformation of ultralow-carbon Fe–C alloy were investigated for cooling rates in the range 5–20 K min−1 by high-resolution dilatometry and differential thermal analysis. The starting temperature of the transformation decreases with increasing cooling rate. The transformation kinetics, as characterized by the formation rate of product (ferrite) phase, varies greatly with cooling rate. At a small cooling rate of 5 K min−1 the transformation rate curve has a multiple-peaked nature. Upon increasing the cooling rate from 5 to 20 K min−1, the first transformation rate maxima vanish gradually, and finally so-called normal transformation behavior, characterized by the (remaining) one maximum in the transformation rate curve, is observed. On the basis of microstructural analysis and thermodynamic and kinetic reasoning, it is shown that the first transformation rate maxima pertain to diffusion-controlled ferrite growth, whereas the later single, main transformation rate maximum is due to interface-controlled, massive transformation.