Solidification of Ductile and Compacted Irons, Macrostructure and Shrinkage Formation

NICOLÁS TENAGLIA; MARCOS LÓPEZ; ROBERTO BOERI; JUAN MASSONE

Simposio; Keith Millis Symposium; 2018

Ductile Iron Society

The manuscript summarizes recent investigation of the authors about the solidification and shrinkage defects formation of ductile and compacted graphite cast irons. The study makes use of the DAAS 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 hypo 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.