Effect of Welding Parameters on Properties of Friction Stir Welded High Nitrogen Steel

Recently, high nitrogen steels (HNSs) containing nitrogen more than 1 wt%, having both high mechanical properties and good corrosion resistance, have been developed. However, they exhibit very poor fusion-weldability, because they experience desorption of nitrogen and porosity formation during fusion welding processes. On the other hand, friction stir welding (FSW) would be an effective welding method to alleviate problems associated with fusion welding processes in HNSs due to a solid-state joining process. Several studies have already examined feasibility of FSW for HNSs, and microstructure and properties of the welds, but effect of welding parameters on mechanical and corrosion properties of the welds has not been systematically examined. The objective of this study is to clarify effect of welding parameters on these properties of friction stir welded 1.3%N containing steel. FSW could produce defect-free welds in the HNS at various welding parameters using polycrystalline cubic boron nitride (PCBN) tool. FSW refined the grain structure in the stir zone, which resulted in higher hardness, strength and toughness than the base material. The grain size increased with increasing rotational speed and decreasing travel speed. Simultaneously, FSW resulted in rapid precipitation of chromium nitrides on the grain boundaries in the stir zone, which caused reduction of the corrosion resistance, and the amount of the chromium nitrides decreased with increasing rotational speed and increasing travel speed. This study showed that FSW was an effective method to produce a defect-free weld in the HNS, and that combination of higher rotational speed and higher travel speed resulted in better mechanical and corrosion properties of the stir zone.