Solidification, Macrostructure and Shrinkage Formation of Ductile and Compacted Irons
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00444-z
Abstract This manuscript summarizes recent investigation of the authors into the solidification and shrinkage defect formation of ductile and compacted graphite cast irons. The study makes use of the DAAS (direct austempering after solidification) technique, developed earlier by some of the authors, to reveal the solidification grains. In addition, color metallography is used to reveal the microsegregation pattern. The formation of shrinkage defects is related to both the solidification macrostructure and the microsegregation pattern. The examination of the macrostructure shows that large solidification grains, composed by dendritic austenite and graphite, characterize the solidification of compacted graphite irons of carbon equivalent values ranging from hypoeutectic to hypereutectic. Similar results are observed for ductile iron, but in this case, the hypereutectic irons display a finer macrostructure. The
characteristics of the primary austenite dendrites are revealed by the microsegregation pattern, which is exposed by the color etching reagent. This pattern also clarifies the nature of the interaction between the growing austenite and the compact and spheroidal graphite particles. The microshrinkage cavities in both ductile and compacted graphite irons form at the interior of the austenite grains, where fluid feeding cannot compensate the contraction of the last-to-freeze portions of the melt. Schematics of the solidification of ductile and compacted irons are proposed, accounting for the experimental observations.
Introduction
a thorough understanding of its origin; nevertheless, in most of the published works, the study of the influence of different factors on porosity follows an empirical treatment. Extensive work is found in the literature regarding factors affecting shrinkage porosity in cast iron. Among others, the influence of the mold media and inoculant type,1 phosphorous content,2 dissolved gas in the liquid phase3 and rare earth content4 on the shrinkage characteristics has been studied. Microshrinkage forms during solidification, and its location is governed by the distribution of the last remaining liquid phase at the end of solidification. Therefore, the shape of the solid dictates the location and distribution of the microshrinkage cavities. Nevertheless, there is not a complete agreement on the features of the solidification of DI and particularly of CGI.
Both ductile iron (DI) and compacted graphite cast iron (CGI) are very competitive cast materials that are used in many industrial applications. However, the increasing demand on the properties of the finished products, along with the pressure to reduce costs, creates a great need for consistently producing better-quality castings. A major problem affecting DI and CGI castings is the presence of microshrinkage defects that can affect the mechanical properties of the irons and often lead to the rejection of the parts. In particular, when high-strength pieces
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