Control of Solidification Pattern of Cast Irons by Simultaneous Thermal and Contraction / Expansion Analysis

Abstract

An experimental device conducts thermal analysis and volume change measurements in a single ceramic cup with cast iron quality as the variable. The recorded data are processed using specialized software. Experiments compare solidification patterns for white [WI], grey [GI] and ductile [DI] irons, to correlate the most important events between the cooling curves and contraction curves, to evaluate the sensitivity to shrinkage formation. All of the irons have similar values for initial expansion up to the start of eutectic freezing [0.437 - 0.443%]. Graphite formation promotes expansion [WI-0.002%, GI-0.109%, DI-0.596%], resulting a difference in the reached maximum expansion [WI-0.465%, GI-0.552%, DI-1.032%], placed between the end of eutectic recalescence and the end of solidification. Higher graphite expansion, greater the shrinkage sensitivity: open shrinkage increased while the density of a casting when considering total shrinkage and micro-shrinkage formation decreased. Prolonged graphitization at the beginning of eutectic reaction increased the expansion and, consequently, shrinkage sensitiveness. More graphite formation at the end of this stage also increased expansion, but this phenomena contributed to reduce of shrinkage level, due to the better access of liquid iron to compensate contraction holes. Special metallurgical treatments can favour a strong graphitization process at the end of solidification, with beneficial effects on the castings soundness.