ISSUE 4/2014

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Alonso, G.; Stefanescu, D. M.; Suarez, R.; Loizaga, A.; Zarrabeitia, G. Understanding graphite expansion during the eutectic solidification of cast iron through combined Linear Displacement and Thermal Analysis

The expansion and contraction processes that occur during the cooling of liquid iron and its subsequent solidification are the primary cause of microshrinkage formation in castings. Of particular interest is graphite expansion which can compensate the shrinkage produced by the solidification of austenite. Thus, a better understanding of these processes is a key objective in the engineering of castings free of shrinkage defects. The paper introduces a combined Linear Displacement Analysis (LDA)/Thermal Analysis (TA) experimental device for measuring the linear displacement during the solidification of cast iron. The experimental device comprises a steel shell surrounding the test mold. In one approach the shell is rigid and prevents mold wall movements. In this way, only the linear displacement caused by the shrinkage or expansion of the metal is recorded by the transducers. In a second approach, the shell can freely move in to horizontal directions, allowing mold expansion. Two quartz rods introduced into the liquid metal and connected to transducers record the linear displacement during the liquid-solid transformation and subsequent cooling. Thermocouples positioned at the same height with the quartz rods allow for the concomitant LDA and TA, and thus for the direct correlation between expansion/contraction and the temperature change during solidification events such as graphite formation. The LDA device was used to study the differences in the solidification mechanisms of irons with different graphite morphologies (lamellar, compacted/vermicular and spheroidal), at three different levels of carbon equivalent, to give hypoeutectic, eutectic and hypereutectic composition. It was found that graphite expansion increases with the carbon content and the magnesium residual. Spheroidal graphite iron had the highest expansion. Compacted graphite iron had slightly higher expansion than lamellar graphite iron. A method was developed to evaluate the expansion as a function of fraction solid evolved. The unrestrained mold experiments proved that to insure results consistency, experiments should be conducted with restrained molds.

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