Friction stir processing of 316L stainless steel plate

316L stainless steel plates were friction stir processed using polycrystalline cubic boron nitride tools. Mechanical properties and microstructure evolution of the friction stir processing (FSP) zone were investigated. Tensile test results showed that the tensile strengths of the defect free FSP zones were equal to those of base metals. All the samples fractured at the base metal side. Microstructural observation results showed that the grains in the FSP zone were refined by the tool. The FSP zone was a roughly equiaxed grain structure and the grain size was in the range of 10-30 mu m, which was smaller than that in the base metal (30-80 mu m). Moreover, transmission electron microscopy observation revealed that sigma phases formed in this zone during FSP.