7 Ways to Avoid Shrinkage Defects
Examine your metallurgy, tooling and molding practices to improve the quality of your castings.
DOUG WHITE, ELKEM METALS INC., BOCA RATON, FLA.
避免收缩缺陷的7种方法 考核您的冶金过程，工模具和造型作业以提高铸件质量
Doug White, 埃肯金属公司，美国佛罗里达州伯克莱屯
these methods can help reduce shrinkage defects and lead to more consistent casting quality.
S
1. Maximize the Graphite Precipitation Expansion Effect Without Nodule Flotation
One of the first steps in avoiding shrinkage problems in ductile iron melting is to select a suitable carbon equivalent (CE) that avoids the flotation of graphite nodules leading to shrinkage. The chart in Figure 1 advises the total carbon plus one-third silicon should not exceed 4.55. The diagram was developed as a general rule for sections varying from 0.5 in. to 1.5 in. For very thin sections, such as in manifolds, the CE may be higher. For thicker sections, it must be lower to avoid nodule flota- tion and an increased risk of shrinkage. When carbon precipitates from liquid iron during freezing, an expan- sion effect occurs. Shrinkage will be minimized at the highest possible carbon content, where the iron freezes in the eutectic mode, just below the content where pri- mary graphite precipitates and nodule flotation occurs.
everal factors in ductile iron melting and pour- ing can be controlled to maximize yield and reduce shrinkage. Whether dealing with a prob- lem part or a need to improve current yields,
可
以控制球墨铸铁熔炼和浇注过程中的几个因 素，以最大限度地提高工艺出品率和减少收 缩缺陷。无论是处理有问题的部件或需要提
高目前的工艺出品率，这些方法可能有助于减少收缩缺 陷，从而获得比较稳定的铸件质量。
1.使析出石墨产生膨胀的作用最大而不产生石墨 漂浮
要避免球墨铸铁产生收缩缺陷，首先要在熔炼时选 定合适的碳当量（CE），避免石墨漂浮导致的收缩缺 陷。按图1的图表，总碳含量加三分之一硅含量之和不 宜超过4.55。对于壁厚0.5英寸到1.5英寸之间的铸件， 该图表可作为通用的规则。对于薄壁铸件,例如歧管铸 件，CE可以比较对于壁较厚的铸件，它必须较低，以避 免石墨漂浮增加收缩缺陷的风险。在凝固过程中，碳自 铁液中析出产生膨胀效应。当铁液在共晶模式下凝固， 碳含量刚好低于初生析出石墨，石墨球发生漂浮的条件 下，碳含量越高，收缩倾向越小。作为为美国铸造学会
Fig. 1. Use this chart to determine the preferred carbon equivalent in your ductile iron.
图1、显示球墨铸铁中最 合适的碳当量。
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION May 2013
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