Hiroshige Inoue

Numerical simulation of equiaxed grain formation in weld solidification

Abstract 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.

Formation mechanism of vermicular and lacy ferrite in austenitic stainless steel weld metals

Abstract Solidification and subsequent transformation of austenitic stainless steel weld metals that solidified in the ferritic–austenitic mode were investigated from the viewpoint of crystallography. The formation mechanisms for the vermicular and lacy ferrite observed in the weld metals were clarified. The ferrite morphology is determined by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite. In particular, for the formation of continuous lacy ferrite, it is necessary that the ferrite continues to grow with the Kurdjumov–Sachs orientation relationship with austenite that is established at the stage of ferrite nucleation.

Effect of surfactant redistribution on weld pool shape during gas tungsten arc welding

Abstract In previous evaluations of GTA welding on stainless steel plates the surfactant content in the workpiece has been assumed to be in a state of equilibrium. However, surfactant concentration usually had to be adjusted in order to achieve agreement between the weld pool shapes in experiments and those in simulations. In this study a physicochemical approach to the redistribution of surfactant at the surface and in the bulk is presented for the first time. Sulphur was considered as a representative surfactant. The model allows surfactant molecular transport via convection, diffusion, and sorption. It is shown that sulphur atoms accumulate at the surface at stagnation points, affecting the coefficient of surface tension and thus the final weld pool shape. Thus, the sulphur content in the simulation approaches the nominal sulphur content in the steel plate. The theory is strengthened by both electron probe microanalysis and Auger electron spectroscopy.