Integrated System of Thermal/Dimensional Analysis for Quality Control of Gray and Ductile Iron Castings Solidification

Abstract

The main objective of the present work is to introduce a specific experimental instrument and technique for simultaneously evaluating cooling curves and expansion/contraction of cast metals during solidification, adapted for commercial foundry use. The recorded data are processed using specialized software, which conveniently displays both cooling and contraction/expansion curves and their specific parameter values. Experiments compared hypoeutectic gray (GI) and ductile (DI) irons with white (WI) irons. Three important moments were found on the expansion/contraction-cooling curves: the start of eutectic freezing, the point of maximum expansion and the end of solidification. All of the tested irons have similar values for initial expansion up to the start of eutectic freezing (0.44%) due to the ferrostatic pressure, silica sand mold expansion, mold movement, etc. The maximum expansion, reached between the temperature of eutectic recalescence and the end of solidification, depends on the carbides/graphite ratio and graphite morphology: WI-0.465%, GI-0.552%, DI-1.032%, as averages. Graphitic expansion, absent for WI, increased to 0.109% (GI) and up to 0.596% (DI). For both GI and DI, the expansion at the end of solidification is only 6% lower compared to the maximum level, while for WI it decreased more than 50%. Specifically, the acceleration rate of the graphitic expansion up to the maximum level (Kgr1) is different compared with its deceleration to the end of solidification (Kgr2), and also are different for irons being tested, GI versus DI. Nodular graphite led to the (Kgr1) factor being 2.5 times higher, compared to GI, whereas only a slight difference was observed between GI and DI for the Kgr2 factor. Higher graphitic expansion in DI led to higher shrinkage sensitivity, compared to GI, as measured in furan resin mold test castings.