Fig. 5. Holes drilled into cores become chill pins when the mold is filled with iron (left). Clusters of chill pins cast inside the part force those areas to freeze sooner so they are shrinkage-free.


prone. Ladle and stream inoculation may not eliminate the carbides. This effect was observed in a metalcast- ing facility using medium frequency furnaces, with one furnace melting and the other furnace delivering iron for tundish treatments. Six tundish treatments were made over one hour to drain the furnace, before switching to the next furnace. As a consequence, the first three treatments were conducted as desired and the last three treatments produced carbides within the structure, despite the use of powerful, continuous stream inoculation with high potency inoculants. Te casting facility learned some of the lost nucleation


effect could be restored by adding crystalline graphite to the iron while replacing carbon losses during holding, but that one hundred percent graphite electrode turnings were reported to be the best type of carbon replacement material to eliminate the carbide tendency. Te rate of loss in carbon and nucleation effect from holding base iron is believed to be different for various holding furnaces and temperatures. Holding magnesium-treated iron can induce shrink-


age. After 25 to 30 minutes of holding iron in an autopour without any freshly treated iron additions, the state of nucleation changes and it becomes shrinkage prone. Tis change can be detected using thermal analysis. Te effect can be corrected by additions of proprietary sulfur and oxygen-coated inoculant.


5. Use Special Nodulizers to Avoid Shrinkage Historically, MgFeSi alloys have been alloyed with


rare earth metals, which are used to neutralize tramp element effects to avoid edge carbides at low pouring temperatures and to optimize nodule count. For many years, mischmetal was the most common type of rare


图5、在砂芯上钻 孔，然后在充型时 铸成有冷却作用的 销。把多个这样的 销置于零件内部以 便它快速冷却、收 缩自由。


造厂，另外一台炉子向一台炉子熔炼盖包处理供给 供液。转换到下一炉之前，要经6次盖包处理才能 使炉内的供液完全用完，需1小时以上。因此，前 三包处理得到符合要求的结果，后三包处理尽管使 用高效的孕育剂连续随流孕育，组织中仍然有碳化 物产生。


铸造厂知道，向铁液中添加晶态石墨，弥补保温 时烧损的碳，可以部分恢复失去的生核作用，但是 据报道只有百分之百的石墨电极屑，是最佳类型的 碳元素替代材料，可以消除生成碳化物的倾向。 原铁液保温以致碳元素烧损和生核作用减退的速 度有所不同。由于保温炉和保温温度不同。 经球化处理过的铁液保温可能引起收缩缺陷。铁 液在自动浇注机中停留25至30分钟，没有再添加 经过处理的铁液，生核发生了变化，易发生收缩缺 陷。这种变化可以用热分析方法检测到。这种状况 可由加入利用包覆了硫和氯的孕育剂解决。


5.采用特种球化剂避免收缩


从历史上看，早就有在MgFeSi合金中加入稀土 金属的情况，其作用是中和杂质元素的影响，避免 浇注温度低时铸件边缘白口倾向和优化石墨球数 量。多年来的添加稀土金属最常用的类型是混合稀


Fig. 6. A metal spring inserted into the core to produce a cylinder head casting serves to provide a high surface chilling device (left). A sectioned cylinder head reveals the remnants of a spring that did not totally melt but fused to the surrounding iron.


图6、将一个金属 弹簧插入到砂芯， 产生铸造用的缸 盖，它是高比表面 冷却装置。一部分 气缸盖揭示了剩余 的弹簧没有完全融 化,融合到周围的 铁里。


34 | FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION May 2013


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