Microstructural evolution in friction stir welding of high-strength linepipe steel

Microstructural evolution during friction stir welding (FSW) of a high-strength linepipe steel was studied. The various grain structures developed through a complex process including the rearrangement of low-angle boundaries, continuous dynamic recrystallization and phase transformation. In most parts of the stir zone (SZ), acicular-shaped bainitic ferrites were formed by the phase transformation during the FSW process. A fine-grained microstructure developed mainly in the thermomechanically affected zone (TMAZ), where continuous dynamic recrystallization occurs. The shear texture in the SZ became considerably weak due to the phase transformation during the FSW process. The hardness of the SZ was significantly higher than that of the other FSWed regions due to the bainitic ferrites.