Grain boundary segregation and toughness of friction-stir-welded high-phosphorus weathering steel

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A3 and below A1. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A1 (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A3. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A1 was larger than that in the stir zone formed above A3 is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.