球墨铸铁皮下气孔形成过程的试验研究

Experimental Study on Formation of Blowholes in Nodular Cast Iron

通过对球墨铸铁件表面及横截面上的镁、硫百分含量的光谱测定,发现铸件表面上的镁、硫呈现富集状态,铸件表面到心部,镁含量梯度增加;同时,对铸件内部的缩松及缩孔进行了电镜扫描,结果表明,孔壁基本上由石墨组成,镁及硫的含量极低。对试验结果进行了充分讨论,认为浮力并不构成球墨铸铁液中气泡迁移的主要动力,球墨铸铁液的体积收缩及其中的石墨化膨胀共同构成对气泡的挤压力。气泡在挤压力的作用下,向铸件各个方向迁移,伴随气泡一并迁移的高熔点非金属夹杂物在金属液表面氧化结膜,阻碍了后续气泡的向外迁移而被滞留在表皮以下形成皮下气孔。

Through spectrum testing the percent content of magnesium and sulfur on the surface and cross section of a nodular iron casting, it was found that the surface of the casting had magnesium and sulfur enrichment on the surface and magnesium gradient increased from the surface to the center of the casting. At the same time, by means of electron microscope scanning ibr the shrinkage and the shrinkage porosity, it was shown that the wall of shrinkage or shrinkage porosity was basically composed of graphite and almost had not magnesium and sulfur. The experimental results have been fully discussed, and then, it has been put forward that buoyancy is not the main driving force to migrate the bubble in the nodular iron liquid. Liquid volume shrinkage and the graphite expansion commonly constitute extrusion pressure to the bubble, and the bubbles are focused to migrate to all direction by this extrusion pressure. Metal oxides with high melting point which migrate with the bubbles form oxide films on the surface of metal liquid, meanwhile, the oxide films hinder the bubbles to migrate outward, and the stranded bubbles under the oxide film form blowholes.