The Optimized Homogenization Process of Cast 7Mo Super Austenitic Stainless Steel

Super austenitic stainless steels are expected to replace expensive alloys in harsh environments due to their superior corrosion resistance and mechanical properties. However, the ultra-high alloy contents drive serious segregation in cast steels, where the sigma phase is difficult to eliminate. In this study, the microstructural evolution of 7Mo super austenitic stainless steels under different homogenization methods was investigated. The results showed that after isothermal treatment for 30 h at 1250 degrees C, the sigma phase in steels dissolved, while the remelting morphologies appeared at the phase boundaries. Therefore, the stepped solution heat treatment was further conducted to optimize the homogenized microstructure. The samples were heated up to 1220 degrees C, 1235 degrees C and 1250 degrees C with a slow heating rate, and held at these temperatures for 2 h, respectively. The elemental segregation was greatly reduced without incipient remelting and the sigma phase was eventually reduced to less than 0.6%. A prolonged incubation below the dissolution temperature will lead to a spontaneous compositional adjustment of the eutectic sigma phase, resulting in uphill diffusion of Cr and Mn, and reducing the homogenization efficiency of ISHT, which is avoided by SSHT. The hardness reduced from 228 similar to 236 Hv to 220 similar to 232 Hv by adopting the cooling process of furnace cooling + water quench. In addition, the study noticed that increasing the Ce content or decreasing the Mn content can both refine the homogenized grain size and accelerate diffusion processes. This study provides a theoretical and experimental basis for the process and composition optimization of super austenitic stainless steels.

Automated summary

Super austenitic stainless steels with superior corrosion resistance and mechanical properties are expected to replace expensive alloys in harsh environments. However, serious segregation in cast steels due to ultra-high alloy contents makes it difficult to eliminate the sigma phase. This study investigated the microstructural evolution of 7Mo super austenitic stainless steels under different homogenization methods. The results showed that the sigma phase dissolved and remelting morphologies appeared at the phase boundaries after isothermal treatment, and stepped solution heat treatment further optimized the homogenized microstructure.