Metal Composition
The type and composition of the castings poured had a significant effect on porosity formation. Porosity potential was greatest for the low-CE iron and least for ductile iron. Porosity defects in all gray iron castings formed readily with unbalanced binder ratios favoring excess polyisocyanate. Porosity in low-CE irons formed predominantly as fissure-type defects, although some rounded and irregular holes also formed.
Although it is commonly accepted that ductile iron is more susceptible to porosity defects, the tests show ductile iron castings were less susceptible to defects than the gray iron. However, most of these previous findings have been with ductile iron containing aluminum poured in green sand molds. It also is generally held that ductile irons are more prone to H defects arising from interactions with water vapor and magnesium (Mg). This is probably related to residual Mg influencing H solubility or reducing water vapor.
Since the chemistry and gaseous thermal decomposition products for PUNB binders are more complex than interactions with green sand molds, the performance of ductile iron with these binders may differ considerably. However, porosity in ductile iron should be more difficult due to the higher melt interfacial surface energy. In addition, the bubbling of Mg vapor through the metal during the nodularizing process effectively purges most dissolved gases from the metal, allowing for possible absorption of core gases without supersaturation.
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