The internal porosity defects detected in four castings produced in two foundries were carefully observed and analyzed by using FE-EPMA, and then, the root cause of the defects was investigated. Based on the generally authorized theory that the internal porosities might be formed at late stage of the solidification, the experimental results are verified. The formation mechanism of dual layer and its characteristic folds or creases on inner surface of defects shown in the experimental results could not be explained by the authorized theory, but the features seem to be possible to explain when the basic films might originate on the surface of the liquid metal and might be entrained in turbulent flow. The reductive reaction of carbon and FeO in the solid bifilms formed in the liquid metal is considered to contribute the pore nucleation in ductile iron. Therefore, in addition to the control of the process variables already developed by many researchers, the internal porosity defects in ductile cast irons using resin-bonded sand and cores might be prevented by avoiding the entrainment of carbon and oxide films.

使用FE-EPMA仔细观察和分析了在两个铸造厂生产的四个铸件中检测到的内部孔隙缺陷，然后研究了缺陷的根本原因。基于普遍认可的理论，即在凝固后期可能会形成内部孔隙，从而验证了实验结果。实验结果表明，缺陷的内表面双层的形成机理及其特征性的褶皱或折痕不能用授权的理论来解释，但是这些特征似乎可以解释何时基膜可能起源于缺陷的表面。液态金属，可能夹带湍流。液态金属中形成的固态双膜中碳和FeO的还原反应被认为有助于球墨铸铁中的孔成核。因此，除了控制许多研究人员已经开发的工艺变量外，还可以通过避免夹带碳和氧化膜来防止使用树脂粘结砂和型芯的球墨铸铁内部孔隙缺陷。