Microstructure evolution during remelting of 9Cr18 semisolid billet prepared by sloping plate method

Semisolid processing is a new near-net-shape manufacturing process suitable for fabricating components with complex shapes. Semisolid billet remelting is a key process performed prior to the subsequent semisolid thixoforming. Understanding the remelting behavior will provide significant theoretical guidance for the semisolid thixoforming process. In this study, during remelting, we investigated the microstructural evolution of a 9Cr18 semisolid billet prepared by the sloping plate method. The microstructures of 9Cr18 specimens for a semisolid billet and traditional casting ingot were discussed. The effects of the initial microstructure and remelting temperature on the remelting behavior were also clarified. The microstructural evolution and remelting behavior were observed via optical microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. The results show that an excellent 9Cr18 semisolid billet can be obtained via the sloping plate method. Forced convection through the sloping plate was found to play a key role in breaking the continuous distribution of dendrites. The typical microstructure was found to exhibit a primary solid austenite globular grain (γ₁) and a eutectic structure of secondary austenite (γ₂) and M₇C₃ carbide. The average size of a globular grain was determined to be 93.5 μm and the shape factor was 0.69. The globular grains showed a smooth boundaries. The Fe, C, and Cr elements showed obviously different contents in the solid and liquid phases. The Cr and C elements were enriched in the liquid phase, whereas the Fe content was higher in the solid phase. Compared with the traditional casting ingot, the remelting microstructure of the 9Cr18 semisolid billet prepared by the sloping plate method was more suitable for subsequent semisolid thixoforming. A more even distribution of the chemical composition and more globular grains can be obtained after semisolid remelting. The solid/liquid interface was smooth on the remelted specimen. The width of the M₇C₃ carbide decreased to 0.5 μm after remelting and the morphology became nearly granular. The observed microstructural evolution during remelting of the 9Cr18 semisolid billet contributes to our understanding of the thixotropic behavior in the subsequent semisolid forming process.