Mechanical properties of friction stir welded high nitrogen containing austenitic stainless steel

Abstract

In this study, the friction stir butt welding of 2mm thick high nitrogen–containing stainless steel (HNS; Ni-free austenitic stainless steel containing 1 mass% nitrogen) plates was performed using a load controlled friction stir welding (FSW) machine with a Si3N4-based tool at various welding speeds, i.e., 50mm/min, 100mm/min/, 200mm/min and 300mm/min, and a constant tool rotating speed of 400rpm. To determine the optimum welding conditions to create reliable HNS FSW joints, the effect of the heat input on the mechanical properties of the HNS FSW joints was studied. The mechanical properties were evaluated by the Vickers hardness test and the tensile strength test. Full-penetrated and defect-free butt welded joints were successfully produced, under all the applied welding conditions. The stir zones consisted of very fine grained structures and showed an increase in the Vickers hardness. These joints also showed a higher tensile strength and yield strength than the base metal. In particular, the FSW welds obtained at a welding speed of 100mm/min, which showed the best mechanical properties, had a relatively higher Vickers hardness, which indicates a good relationship between the welding parameter (heat input) and the hardness profile due to the microstructure refinements. It was estimated that these welding conditions were optimal, and under these conditions both grain growth and α-phase formation were prevented.