Simulation of the growth of austenite during continuous heating in low carbon iron alloys

The growth of austenite from martensite or carbon-supersaturated ferrite matrix during continuous heating, which accompanies carbon diffusion in the growing austenite, was studied by DICTRA and linearized gradient approximation extended to include soft impingement of diffusion fields in the matrix. While the austenite growth is controlled by carbon diffusion in ferrite at an early stage, it is controlled by diffusion in austenite at the intermediate and late stages. At a low heating rate, the austenite-finish temperature A_f is almost equal to the Ae₃ temperature of the alloy, whereas at a high heating rate, A_f exceeds the Ae₃ to a progressively larger extent with the increasing heating rate and matrix grain size. At a very high heating rate, i.e., 10⁵–10⁶ °C/s, the mobility of α/γ interface is likely to have a significant influence on the growth of austenite, and the untransformed ferrite matrix is transformed in a massive mode. These results are in accordance with the earlier observation of austenite formation during rapid heating in a low carbon iron alloy.