The Role of Nucleation Behavior in Phase-Field Simulations of the Austenite to Ferrite Transformation

Three-dimensional (3-D) phase-field simulations of the austenite (γ) to ferrite (α) transformation during continuous cooling at different cooling rates were performed for an Fe-0.10C-0.49Mn (wt pct) steel, with the aim of studying the interaction between the assumed nucleation temperature range and the effective interfacial mobility when fitting transformation kinetics curves. Ferrite nuclei are assumed to form continuously over a temperature range of δT. An effective interfacial mobility is assumed with an activation energy of 140 kJ/mol and a pre-exponential factor, μ ₀. The pre-exponential factor and the nucleation temperature range are used as the only two adjustable parameters to match an experimental reference transformation curve for a particular cooling rate. The initial austenitic microstructure and the nuclei-density input data are based on experimental observations. A number of combinations of values (μ ₀, δT) are found to represent the experimental reference curve equally well when related to the accuracy of experimental measurements. The comparison between the simulated and the experimental ferrite grain-size distribution is used as an additional criterion to establish the best estimate of nucleation temperature range and interface mobility.