Analysis of the growth of austenite grains in carbon steels during laser hardening

A comprehensive theoretical analysis is made of the kinetics of the α →γ transformation in carbon steels. It is shown that the α →γ transformation process is of the same relaxation nature during the stages of nucleation of grains as well as during the grain-growth stage, and can be described by the relaxation time of the lattice transformation process. Estimates are obtained of the radius of a critical nucleus and of the width of the lattice modification zone. A theoretical estimate is made of the size of austenite grains in carbon steels at laser heating rates in agreement with metallographic data. It is suggested that the acceleration of the diffusion processes during fast heating of steel is due to a moving lattice modification zone. A simple expression, suitable for technological estimates, is proposed for the calculation of the size of austenitic grains as a function of the heating rate and temperature.