Microstructure gradient characteristics and mechanical properties of friction stir welded high strength QP980 steel

Abstract

Friction stir welding (FSW) was carried out on 1.2 mm thick QP980 steel plate and W-Re stirring tool was used for FSW. This study focused on the microstructure gradient characteristics and mechanical properties of QP980 FSW joints. The FSW joints without defect could be obtained under welding parameter of 400 rev min−1–400 mm min−1. The martensite was the mainly microstructure in the stir zone (SZ) and the microhardness of the SZ was increased to about 480 HV. The martensite of the subcritical heat affected zone (SCHAZ) underwent tempering transformation and decomposition, small amount of carbides were precipitated from the martensite, so the microhardness of the SCHAZ was slightly decreased. The microstructure in the SZ had the highest dislocation density and the highest fraction of low angle boundaries. The tensile test results indicated that the joints were broken in the base metal (BM), which indicated that the FSW joints had excellent performance.