Simultaneous thermal and contraction/expansion analyses of cast iron solidification process

Abstract

Simultaneous thermal and contraction/expansion analysis of hypoeutectic grey (lamellar graphite) and ductile (nodular graphite) cast irons solidification is recorded, using an equipment with a special designed ceramic cup, incorporating a thermocouple and contraction/expansion measuring device. Both cooling and contraction/expansion curves and their specific parameter values are displayed in a real time. The two compared cast irons occupy different and opposite positions during solidification process evolution: lower level of temperatures at the beginning and especially at the end of solidification, but higher level during the eutectic reaction for ductile cast irons. In the tested conditions, solidification of hypoeutectic cast irons with lamellar graphite is characterized by a greater undercooling during the eutectic reaction (lower ΔT1 and ΔT2, referring to the metastable eutectic temperature), with higher value for eutectic recalescence (ΔTr) and the maximum recalescence rate. Iron castings, including nodular graphite (ductile irons), show an end of solidification at higher undercooling (ΔT3), with less negative pick level of the first derivative of cooling curve. More graphitic initial expansion [edi(gr)], which favours the shrinkage formation, characterized nodular graphite iron castings. There is a good relationship between some parameters on the cooling curves and corresponding events on the contraction/expansion curves, such as solidification undercooling degrees, comparing to metastable eutectic temperature, and graphitic expansion [edi(gr)] for both tested cast irons: higher the level of ΔTr, and of ΔT1, ΔT2, ΔT3 (less negative) and GRF1 (graphitic factor), higher the [edi(gr)] level is.