Critical temperature for massive transformation in ultra-low-carbon Fe–C alloys

Abstract The kinetics of the isochronal austenite (γ) → ferrite (α) transformation for a series of ultra-low-carbon Fe – C alloys (with a carbon concentration from 0.001 to 0.05 at.%) were investigated for cooling rates in the range of 5 – 20 K min − 1 using high-resolution dilatometry. The rate of formation of the product (ferrite) phase as a function of time exhibits maxima. The first transformation-rate maxima pertain to diffusion-controlled ferrite growth, whereas the last, main transformation-rate maximum corresponds to massive transformation. The thus determined critical temperatures, at and below which massive transformation takes place, are always below the T 0 temperatures, where the Gibbs energy of the (metastable) austenite equals that of (metastable) ferrite of the same carbon concentration, and are also below the ferrite solvus temperatures for carbon concentrations below about 0.01 at.% C and above the ferrite solvus temperatures for carbon concentrations above about 0.01 at.% C.