A new theory for aluminium-induced gas defects in iron castings

A critical review of the various theories which have been proposed to explain the occurrence of aluminium-induced sub-surface gas defects (“pinholes”) in iron castings is presented. These theories do not adequately explain the occurrence of the defect, and also do not seem to point to effective solutions to the pinholing problem. A new mechanism for pinhole formation is presented here which involves the reaction of aluminium nitride with iron oxide to produce nitrogen, alumina, and iron. It is suggested that the nitrogen gas bubble produced by this reaction remains trapped beneath the surface of the casting and constitutes the pinhole defect. This new mechanism appears to be consistent with most of the experimental observations on pinholing.