Abstract: Influence of copper content on sulfide morphology and mechanical properties of 303 and 303Cu austenitic free-cutting steels was investigated by means of Thermal-Calc thermodynamic calculation software, scanning electron microscope(SEM), Phenom Particle X scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS). The results show that the proportion of sulfides with a length-width ratio ≤ 3 in the as-cast 303 steel is 94.75%, while in the as-cast 303Cu steel is 88.37%. Sulfides are predominantly spherical, ellipsoidal, spindle-shaped, or short rod-shaped and distributed along grain boundaries in clusters, characteristic of a type II sulfides. In the forged steels, sulfides are elongated along the forging deformation direction, with increased length-width ratio, dimensions, and decreased quantity per unit area. Thermal-Calc software calculations indicate that with copper content increasing, the precipitation temperature and content of sulfides increase. Additionally, the addition of copper leads to the precipitation of CuS, which reduces the mechanical properties of the material. The as-forged 303Cu steel exhibits a tensile strength of 582.5 MPa, yield strength of 363.7 MPa, and impact absorbed energy of 62.1 J, while the as-forged 303 steel shows superior comprehensive mechanical properties with a tensile strength of 821.9 MPa, yield strength of 410.4 MPa, and impact absorbed energy of 147.5 J. The tensile properties and impact absorbed energy of the as-forged steels are significantly improved compared to that of the as-cast steets, it indicates that forging deformation contributes to enhancing the mechanical properties of the 303 and 303Cu austenitic free-cutting steels.

Key words: Thermal-Calc software, copper content, 303 free-cutting steel, MnS morphology, mechanical properties