Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (alpha): austenite (gamma) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (alpha + gamma) two-phase field and the alpha single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (greater than or equal to 10(3) degreesC/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the alpha solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T-o temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae(3), T-o, or alpha-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the alpha: gamma boundary may play a significant role at an extremely high cooling rate (greater than or equal to10(5) degreesC/s).