An ultrafine grained 304L stainless steel with bimodal structure was produced by martensitic thermomechanical processing and joined by applying friction stir welding (FSW). The thermomechanical processing comprised a cold roll procedure up to 80% reduction followed by annealing at 700 0 C for 300 min. The results showed that the martensite morphology changed from lath type in the 60% rolled sample to a mixture of lath and dislocation-cell types in the higher rolling reductions. Calculation of the Gi free energy change during the reversion treatment showed that the reversion mechanism is shear controlled at the annealing temperature and so the existence of bimodal structure was attributed to the existence of two different kind of martensite prior to annealing. After FSW, different grain structures in different regions of the weld nugget were observed due to the asymmetry in the heat generation during the welding process. Grain growth was found to be the most predominant phenomena in the region just ahead of the rotating tool during the thermal cycle of FSW. A banded structure was observed in the advancing side of the weld nugget. TEM observations revealed that nanometric sigma phase precipitates were present both in the grain boundaries and inside the grains of this region. Shear textures were clearly identified in the weld center. The lack of rotated cube texture shows that the discontinuous dynamic recrystallization (DDRX) is not active in the final microstructure. Increasing the welding speed can reduce the final grain size of the weld nugget leading to higher hardness. Hardness is found to increase in the weld and this is not just a grain refinement effect, but also due to the presence of sub-boundaries and a high density of dislocations. Keywords : Deformation induced martensite, Friction stir welding, Recrystallization mechanism, ThermoCalc analysis, Sigma phase
