Friction-stir welding of ultrafine grained austenitic 304L stainless steel produced by martensitic thermomechanical processing

•An ultrafine grain 304L stainless steel was successfully welded by Friction Stir Welding (FSW).•Grain structure in the weld nugget is not homogenous due to asymmetry in the heat generation and heat transfer during process.•Sigma phase precipitation was found as the reason of the banded area formation in the weld nugget advancing side.•A mixture of B/B‾ and A∗ shear texture was identified as the dominant weld nugget texture.

An ultrafine grained 304L austenitic stainless steel 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. 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide