A novel nitrogen-free austenitic stainless steel with a hardness of >200 HV was developed using metal injection molding (MIM), and the effects of graphite addition on the sintering behavior, mechanical properties, and corrosion resistance of heat-treated samples were investigated. The results show that a certain amount of graphite addition increases the relative density to >98%. In samples with the addition of 0–500 ppm graphite, large grain-boundary precipitates reduced corrosion resistance and ductility. In contrast, when graphite addition was increased to 750–1500 ppm, fine precipitates, which exhibited coherent lattice relationships with the matrix, were uniformly distributed within the grain and grain boundaries; this significantly improved the mechanical properties and corrosion resistance. The tensile strength and elongation intervals were 546.94–608.62 MPa and 29.68–24.63%, respectively. To prevent overburning, samples with a graphite content higher than 3000 ppm were sintered at a lower temperature, resulting in a higher porosity and lower performance.

This study investigated the evolution of density, grain size, and pore characteristics during the sintering of metal injection molding (MIM) 420, 420 + 0.3C and pre-alloyed 420Nb stainless steel powders. The results show that C promotes the reduction of oxides on the surface of stainless steel, thereby accelerating sintering at 1330 °C, which is the initial sintering stage of MIM 420. MIM 420Nb showed the slowest sintering rate due to the strong binding force between Nb and C. At 1350 °C, the sintering densities of MIM 420 and 420 + 0.3C slightly improved, whereas their grain sizes grew significantly. Scanning electron microscopy images show grain boundary-pore separation, which significantly retarded the grain boundary diffusion mechanism and hence reduced the densification rate. The addition of C accelerated the pore-grain boundary separation; thus MIM 420 + 0.3C showed the lowest density at this temperature among the materials analyzed in this study. Nb suppressed the grain growth rate; thus, MIM 420Nb exhibited the highest density among the three materials. At 1370 °C, MIM 420 + 0.3C reached the highest density owing to the creation of a liquid phase. Theoretical calculations proved that there is a linear relationship between the grain boundary area per unit volume and the interfacial pore area per unit volume. Furthermore, when the ratio of grain size to pore size is 28, the contact probability between the grain boundaries and pores is significantly reduced to approximately 10%, leading to an extremely slow densification rate and a rapid grain growth rate, which is consistent with the experimental results.