Microstructures and tensile properties of friction stir welded 0.2%C–Si–Mn steel

The 0.2 wt%C steels with the addition of 2 wt%Si and 0–4 wt%Mn were prepared by hot rolling followed by subsequent annealing for normalization. The materials were subjected to friction stir welding (FSW) conducted above A₃ temperature. Microstructures and tensile properties using small tensile specimens were investigated for both base materials and stir zones. The base materials with the addition of over 2 wt% Mn showed a relatively good balance of strength and ductility when Si content was 2 wt% due to the transformation induced plasticity effect of retained austenite. The tensile properties of stir zones were substantially enhanced by FSW, and the stir zones of the 0.2C–2Si–3Mn (wt%) and 0.2C–2Si–4Mn joints exhibited the high tensile strength of approximately 1700 MPa and the high elongation of about 30%, in particular excellent local elongation of 25%. This is due to the refinement of packet and block sizes in the lath martensite induced by the grain refinement of the prior austenite grains by FSW.