石墨砂在精铸型壳中应用机理探讨

Application Mechanism of Graphite Sand in Shell Mold by Investment Casting

利用石墨在高温下氧化的特性,将颗粒状的石墨砂替代耐火砂粒应用到型壳的某一层或某两层,可改变型壳的潜热、热导率、蓄热系数等指标,使浇注的铸件温度场改变,延长金属液的补缩时间和扩大补缩面积;石墨砂氧化后在铸件的界面形成还原性气氛,使铸件表面粗糙度降低,麻点和脱碳层减少;石墨的烧蚀和烧失过程可减少铸件的收缩阻力,形成的空隙有助于解决清砂难的问题。在实际的硅溶胶型壳精密铸造生产中,通过对前板、套筒、双流道泵体等典型铸件制壳时采用的石墨砂工艺,均达到了预期的效果,体现了石墨砂的应用优势。

Owing to the characteristics of graphite oxidation at high temperature,granular graphite sand instead of refractory sand was applied to one or two layers of shell mold to change the thermal performance indicators such as latent heat,thermal conductivity,heat storage coefficient,and pouring temperature field,thus extending the feeding time and distance of metal liquid.A reducing atmosphere is formed on the interface of casting after graphite sand oxidization,which improves the surface roughness and reduces oxygen pitting and decarburization layer.The graphite ablation and loss process can reduce the shrinkage resistance of the casting,helping to solve the problem of sand removal.In the actual investment casting production of silica sol shell,graphite sand was adopted during the shell making of typical castings such as slide plate,sleeve,double channel pump body,etc,and the expected effect was achieved,reflecting the advantages of graphite sand.