It is well known that ductile iron is more prone to the formation of internal defects and to the swelling-out of the mold than grey iron. This is likely related to differences in the way the volume changes due to solidification and graphitization occur for these two cast irons. Direct study of these phenomena is difficult because interdendritic feeding and graphite expansion are intimately related.
A simple physical model is proposed to simulate the effects of a few variables on the tendency for a S.G. cast iron to lead to a casting with internal defects: its chemical composition, inoculation and the rate of heat extraction.
The aim of the model is to calculate the pressure of the residual liquid at any point of the casting during its solidification. A special attention is payed to the pressure drop due to the movement of the residual liquid in the mushy zone. Therefore the evolution of the fractions of liquid, austenite and graphite are taken into account for calculating the local volume changes which are assumed to be the main driving forces for the liquid flow. In the same time, the evolution of the permeability of the mushy zone is estimated for calculating the liquid flow pattern and the pressure drop pattern.
The numerical values for the quantities which describe the solidification of the cast iron are issued from quantitative image analysis of samples quenched during directional solidification (Q.D.S.).