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.

► The microstructural evolution of API X100 grade steel during FSW was examined.
► The rearrangement of LABs occurs in the BM + HAZ by significant heat.
► Fine grains are mainly developed in the TMAZ by continuous dynamic recrystallization.
► Acicular-shaped bainitic ferrites are observed in most parts of the SZ.
► The fraction of these could be a distinct criterion to distinguish the FSWed regions.