Solidification Macrostructure of Compacted Graphite Cast Iron and Its Relationship with Shrinkage Porosity

Abstract

This work focuses on the study of the solidification macrostructure of compacted graphite iron and its relationship with shrinkage porosity formation. Cast samples especially developed to present shrinkage porosity were examined after applying the DAAS technique, which involves hot shake out of the castings at temperatures above the eutectoid, followed by an austempering. The samples were then sectioned, polished and etched with Nital 2%. The results were compared with those obtained earlier by Rivera et al on spheroidal and flake graphite cast irons. The results show that, similarly to spheroidal and flake graphite irons, the solidification of compacted graphite iron is dominated by the presence of relatively large grains of austenite that can be observed by the naked eye. Differences in the grain size were detected as a function of the position into the different volumes analyzed, observing a typical ingot structure. The dispersed shrinkage cavities formed were found inside the grains and located between the austenite dendrite arms. The results allow the authors to propose a novel explanation of the solidification of compacted graphite irons that shares common features with spheroidal and lamellar graphite irons.