The Microstructure Refinement and Mechanical Properties Improving of Friction Stir Processed Fe–Mn–Cr–N Steel

Fe–Mn–Cr–N austenitic stainless steel is widely used in engineering fields because of its good ductility, but its relatively low yield strength limits its application range. In this study, Fe–Mn–Cr–N steel was fabricated with different parameters by friction stir processing (FSP). To explain the effect of the rotation rate and feed speed on the microstructure evolution and mechanical properties of Fe–Mn–Cr–N steel, four sets of parameters were selected. The numerical simulation results reveal that the maximum temperature and strain at the tool-workpiece interface on the advancing side (AS) are 948.8 °C and 39.3 mm/mm, respectively. Under the action of severe strain and frictional heat production during processing, the original coarse columnar grains were refined into uniform equiaxed grains with an average grain size of approximately 10 μm in the stir zone (SZ). Moreover, under the experimental conditions, the δ-ferrites gradually transformed into austenite, and all the δ-ferrites disappeared in the SZ with the greatest strain. In addition, a large number of Σ3 twins formed in this region. Consequently, microstructure refinement is dominated by dynamic recrystallization (DRX), while nucleation is accelerated by dissolution and spheroidization of δ ferrites and evolution of Σ3 twin boundaries. The Fe–Mn–Cr–N steel fabricated at 600 rpm-50 mm/min had the best comprehensive mechanical properties, i.e., its YS, UTS and El changed to 400.9, 649.0 MPa and 60.7 pct from 211.4, 421.2 MPa and 62.4 pct of the original as-casted Fe–Mn–Cr–N steel, respectively. The improvement in the YS was mainly attributed to solution strengthening (σSS = 114.4 MPa) and boundary strengthening (σGB = 189.5–238.1 MPa).