Evolution of microstructure during double-sided friction stir welding of microalloyed steel

Microstructures observed by analytical scanning and transmission electron microscopy in the overlap region of a double-sided friction stir welded microalloyed steel (EH46) were recorded in detail. They are compared with microstructure in the thermo-mechanically affected region of the weld and with the base material. The differences suggest that the overlap region has been stirred in the single phase ferrite, and consists mainly of small equiaxed ferrite grains with strain induced precipitates, while the thermo-mechanically affected zone was processed in the austenite-ferrite phase field, resulting in a mixture of bainite lath packets and ferrite grains. The almost complete absence of pearlite or cementite in the overlap region has led to the suggestion that it dissolved during friction stir welding, providing carbon for strain induced precipitation. Also, in the complex microstructures of the overlap region, ferrite grains containing a high density of cell-like structures were observed, some having precipitates nucleated on their intersections. This implies that strain induced continuous dynamic recrystallization has occurred.