Effect of processing parameters on microstructure and mechanical properties of Fe-Mn-Cr-N steel during friction stir processing

Objective The purpose was to study refinement mechanism of microstructure of Fe-Mn-Cr-N under high strain rate and large strain deformation conditions. Methods The surface treatment of Fe-Mn-Cr-N austenitic stainless steel was carried out by friction stir processing technology. Rotation speed was 600～800 r/min and processing speed was 50～70 mm/min. OM and EBSD were used to analyze morphology and recrystallization behavior of the processed area under different parameters, and the mechanical properties were studied by microhardness tester and universal testing machine. Results The results showed that the surface of Fe-Mn-Cr-N steel was flawless except 600 r/min-60 mm/min, and microstructure of welding nugget zone was refined from the original coarse columnar grain to about 10 μm uniform grain. Microstructure of the processed area was composed of a large number of high angle grain boundaries, abundant low angle grain boundaries and Σ3 twin grain boundaries. Fe-Mn-Cr-N steel processed at 600 r/min-50 mm/min had the best comprehensive mechanical properties, whose yield strength and tensile strength increased from 211.4 MPa and 421.2 MPa to 400.9 MPa and 649.0 MPa of the original material. Conclusion The experimental results could provide technical parameters and data support for surface strengthening technology of austenitic stainless steel containing nitrogen.