Numerical simulation of equiaxed grain formation in weld solidification

Numerical simulation is developed on grain structure development during weld solidification of steel. Monte Carlo (MC) method is applied to the simulation of nucleation and growth of solid, being combined with finite difference calculation of heat conduction and solute diffusion. Based on the experimental result that titanium nitride (TiN) works as a nucleating agent of equiaxed grain formation, given number density of TiN is allocated to MC cells randomly and instantaneous nucleation is assumed to occur when the melt undercooling of the cell exceeds a given critical level. It is found that the simulation can reproduce the weld solidification by reflecting the effect of TiN as a nucleating agent and the effect of soluble titanium that increases melt undercooling. Those effects have been recognized only as combined effects in previous experimental studies, but through the present simulation, they are first investigated in a separate manner. The simulation results indicate that equiaxed grain formation is promoted not only by a nucleating agent, TiN, but also by melt undercooling increased by soluble titanium and that both are requisite to bring about the columnar-to-equiaxed transition (CET). The results are discussed in connection with theoretical models on CET and confirm the rationality of the models.© 2003 Elsevier Science Ltd. All rights reserved.