Simulation of Nucleation of Proeutectoid Ferrite at Austenite Grain Boundaries during Continuous Cooling

The nucleation kinetics of proeutectoid ferrite during continuous cooling in three Fe-C-Mn-Si steels, measured in-situ by three-dimensional X-ray diffraction microscope, are compared with numerical simulation that takes into account differences in the activation energy of nucleation among grain boundary faces, edges, and corners. The essential feature of ferrite nucleation in the 0.21 pct C steel, i.e., nucleation occurred just below Ae3 and ceased at a small undercooling, is reproduced taking into account the site consumption, primarily at grain corners and overlap of solute diffusion fields in the grain boundary region or the matrix and assuming a very small or almost null activation energy of nucleation. In the 0.35 and 0.45 pct C steels, small activation energy, as reported by Offerman et al., was not unequivocally obtained because ferrite nucleation occurred at considerably large undercoolings, even below the paraequilibrium Ae3 in these steels. The increasing rate of the observed particle number with decreasing temperature is considerably smaller than calculation.