Enhanced mechanical properties of medium carbon steel casting via friction stir processing and subsequent annealing

This study provides an effective surface processing technology to increase the local mechanical properties of medium carbon steel castings. Ultrafine dual-phase structure of the ferrite and the martensite was obtained in the processed zone with a depth of 1 mm via submerged friction stir processing (FSP). Significantly enhanced yield strength (YS) of 2070 MPa was achieved in the FSP steel compared to that of the base material (BM) with a relatively low YS of 590 MPa, but a low uniform elongation (UE) of 3.0% was achieved compared to that of the BM (9.4%). After annealing, obvious carbide precipitation was observed in the original quenched martensite phases. Therefore, good strength-ductility synergies with high YS of 1020 MPa and 925 MPa, and acceptable UE of 5.9% and 9.3% were achieved in the FSP steel after annealed for 2 h at 500 °C and 600 °C, respectively.