Effect of Tool Rotation Speed on Microstructure and Hardness of Friction-Stir-Welded 9Cr-1Mo Steel

In this study, friction stir welding (FSW) of 9Cr-1Mo steel was carried out with tungsten lanthanum oxide (W-La2O3) tool to study the influence of tool rotation speeds (200 rpm and 500 rpm) on microstructure and hardness. A smooth defect-free weld joint with complete penetration was accomplished by both rotational speeds. In stir zone (SZ), a fine and lath martensitic microstructure was explored for the 200 rpm and the 500 rpm welds, respectively. The tangible increment in the average hardness of both SZ and thermomechanical affected zone (TMAZ) compared to the base metal (stir zone/TMAZ: ~ 420 VHN, base metal: ~ 212 VHN) was observed due to the effect of phase transformation.