倾斜板法制备9Cr18半固态坯料二次重熔的微观组织演变

研究利用倾斜板法制备的9Cr18半固态坯料在二次重熔过程中微观组织演变，以期对后续触变成形提供必要的理论基础。研究结果表明，采用倾斜板浇注法可以得到优质的9Cr18半固态坯料，其典型组织为初生固相奥氏体（γ₁）球状晶粒和晶界网状组织构成，网状组织为二次奥氏体（γ₂）和M₇C₃碳化物液相共晶组织。球状晶粒的平均直径为93.5 μm，形状因子0.69，半固态坯料球状晶粒边界光滑，大小相对均匀。固液两相中Fe、C、Cr存在着明显的元素差异。液相组织中Cr、C元素富集，固相中Fe含量较高。采用波浪形倾斜板法制备的9Cr18合金半固态坯料重熔组织形貌优于传统铸锭重熔组织。半固态坯料重熔后的组织化学成分更为均匀，晶粒也更圆整，固液界面平滑，且加热温度越高，晶粒越圆整，液相率越高。二次重熔后碳化物尺寸明显减小，平均宽度仅为0.5 μm，长度大大减小使得其形态接近粒状。 

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.