The influence of Ce micro-alloying on the precipitation of intermetallic sigma phase during solidification of super-austenitic stainless steels

The influence of Ce addition on the precipitation of ferrite (δ) and sigma (σ) phase in super-austenitic stainless steels during solidification was investigated via a combined experimental and theoretical approach. Thermodynamic calculations reveal that the secondary phase of δ and σ are precipitated in sequence near the end of the solidification process due to the segregation of Mo and Cr. The Ce addition improves the thermal stability of the δ, expands the range of Mo content for δ phase precipitation and increases the δ fraction in the solidified structure in the Gulliver–Scheil model. Experimental observation shows that a small addition of Ce accelerates δ nucleation in the interdendritic solute-rich liquid, refines the solidified structure and reduces Mo and Cr segregation, which suppresses precipitation of the σ phase. The σ phase precipitation is replaced by δ phase precipitation after a Ce addition of 0.016%. The change in the secondary phase from σ to δ shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase, which is significant for hot working.