Effect of As-cast Surface on Oxidation Behavior of High-Si Ductile Irons

The as-cast components in ductile iron have external and internal surfaces formed during solidification in the mold, and a significant portion of the original or modified as-cast surface is present during casting service. In foundry practice, the as-cast surface is referred to as "casting skin." A casting skin has specific features which principally differ from the casting bulk: (i) finer micro-structure, determined by higher cooling rate, (ii) internally formed defects (crack, porosity), (iii) incorporated phases from metal-mold-gas interactions, (iv) near surface chemistry depletion, and much more. The role of the casting skin on mechanical integrity of the entire cast component at room temperature has been determined previously for many cases. However, the effect of the casting skin in ductile iron on high-temperature performance has not been determined because, as a rule, high-temperature oxidation tests are performed using specimens with a perfect surface after removing casting skin by machining. Considering the specific structure of the casting skin, its high-temperature oxidation behavior could be significantly different when compared to the main casting bulk. In this article, the effect of the casting skin was evaluated during high-temperature oxidation tests of several high-Si-alloyed ductile irons that were designed for high-temperature applications. This study sheds light on high-temperature performance of ductile iron components that are used in service with a high proportion of the as-cast surface intact.