Kinetic Model of γ to α Phase Transformation at Grain Boundaries in B-Bearing Low Alloy Steel

The effect of Boron (B) on nucleation and growth of ferrite from austenite grain boundaries is examined theoretically assuming the junction of 4-austenite grain boundaries as dominant nucleation sites of ferrite. B segregates to the austenite grain boundaries and reduces the grain boundary energy and thereby retards the ferrite nucleation at the grain boundary. The retardation is expressed as a decrease of nucleation frequency due to an increase of activation energy for nucleation and the calculated value of the fraction of active nucleation sites is in satisfactory agreed with experimental results. The reduction of the austenite grain boundary energy, obtained by applying the Gibbs isotherm for adsorption to the B segregation, is of the same order of as the one which is deduced from results of calculation for decrease in the nucleation frequency based on experimental result. The growth of ferrite is calculated using DICTRA yielding both the volume fraction and the grain size of transformed ferrite as a function of time, which agreed with the experimental results. This suggests the slight influence of B on growth rate. However, the increase of the diffusion cell size due to B addition is considered to be the main reason for the subtle larger grain size of ferrite as compared with B-free steel, which is in good accordance with the experimental fact.