Fig. 3. Higher silicon leads directly to higher nodule count, more ferrite and more early carbon precipitation. Keep the percent silicon steady at the minimum level, using just enough to avoid carbides.
Excessive inoculant addition rate or the use of bismuth to
increase nodule count can lead to high nodule counts with uniform sizes. High nodule counts can be useful to turn off shrinkage, but only if a wide nodule size distribution can be produced. Tis implies graphite precipitation proceeds at a steadier pace from the start to the end of freezing and not too fast during the first part of freezing. Magnesium content also should be controlled. Enough magnesium should be used to produce good nodules, but an excess of magnesium can lead to slag defects or spiky graph- ite formation, in addition to shrinkage problems.
3. Keep Base Sulfur Content Consistent Te base sulfur content of iron can have a large impact
on nodule count and size distribution. For very thin castings prone to carbides, some metalcasting facilities will intention- ally operate with a higher base sulfur. Tese nodules appear to be similar in size and may lead to shrinkage problems if the nodule count becomes too high, especially in heavier sections. For reproducible nodule count, the base sulfur content must be uniform from one treatment to the next. Large variations in the base sulfur, such as when converting between gray and ductile iron, could lead to variable nodule counts and nodule size distributions and shrinkage propensity.
4. Avoid Long Hold Periods As base iron is held, carbon is lost and the state
of nucleation changes over time. After holding for 30 minutes at tapping temperature, the subsequently magnesium-treated and inoculated iron will become slightly more shrinkage prone and strongly more carbide
图3、更高的硅含 量直接导致更多的 球状石墨粒数、铁 素体和早期碳沉 淀。保持硅的含量 在最低水平,合理 使用就能避免碳 化物。
益的，但只有在石墨球粒度分布广时才有效。这意味 着从凝固开始到终了石墨析出的步调比较一致，而且 在凝固初期不能太快。
镁的含量也应加以控制。应有一定的镁含量以产生 良好的石墨球，但镁含量过高会导致夹渣缺陷或形成 开花状石墨，还有增加收缩倾向。
3.保持初始的硫含量稳定一致
初始硫含量变化大，如铸铁的初始硫含量对石墨球 的数量和粒度分布有很大的影响。由于薄壁铸件容易 产生渗碳体，一些铸造厂会有意提高铁液中的初始硫 含量。如果石墨球数量太多并且粒度相近，特别是厚 大铸件，就可能会导致收缩缺陷。为保持石墨球数量 稳定，那样每次处理的铁液的初始硫含量必须一致。 在灰铸铁和球墨铸铁之间进行转换时，就可能会引起 石墨球数量、粒度分布和收缩倾向的变化。
4.避免长时间保温
随着原铁液保温时间延长，碳元素烧损，成核情况 也发生变化。铁液在出炉温度下保温30分钟之后，随 后进行球化和孕育处理的续液，发生收缩的倾向稍有 增强，而产生碳化物的倾向大为增强。包内孕育和随 流孕育可能无法消除碳化物。在一家采用中频炉的铸
Fig. 4. Reduced shrinkage is observed in the MgFeSi alloy treated with lan- thanum (right) vs. mischmetal.
图4、观察比较MgFeSi合金与镧混 合和MgFeSi合金与稀土混合产生 的收缩减少。
May 2013
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 33
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