Microstructure evolution during friction stir welding of 1Cr11Ni2W2MoV martensitic stainless steel at different tool rotation rates

This study investigates the effect of tool rotation speed on the microstructure evolution of the friction stir welded 1Cr11Ni2W2MoV martensitic stainless steel. The microstructure changes in the different weld zones were examined using optical microscopy (OM), scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD). The microstructure differences suggest that the stir zone (SZ) and thermo-mechanical affected zone (TMAZ) have been stirred in the austenite phase region, resulting in a smaller martensite lath, prior austenite grain size and higher hardness than those in the base metal (BM). Grain refinement in the SZ and TMAZ occurred at all welding conditions, with the finest microstructure was produced at lowest tool rotation rate. The heat-affected zone (HAZ) has been divided into three subzones. The HAZs close to the BM consist mainly of tempered martensite lath with lower average hardness values than that in the BM, while a newly hardened martensite lath has been observed in local regions in the HAZ close to the TMAZ, which has an average hardness equal to that in the BM. The microstructure changes in the HAZ have led to the suggestion that the HAZ close to the TMAZ reached a temperature between Ac1 and Ac3, while the peak temperature in the HAZs close to the BM did not exceed the Ac1.