Determination of the columnar to equiaxed transition in hypoeutectic lamellar cast iron

Abstract

Shrinkage porosity as a volume change related casting defect in lamellar cast iron was reported in the literature to form during solidification in connection to the dendrite coherency. The present work includes an experimental study on dendrite coherency - also called columnar-to-equiaxed transition in lamellar cast iron using thermal analysis and expansion force measurements. Investigation was carried out in order to study the mechanism of dendrite coherency formation. Cylindrical test bars were cast from the same alloy with different pouring temperature, amount of inoculant and time between the addition of inoculant and start of pouring the samples. Cooling rate and expansion force was recorded as a function of time. A numerical algorithm based on temperature differences measured under solidification was used to interpret the solidification process. Three different methods have been compared to determine the columnar to equiaxed transition. The compared methods were based on registered temperature differences, based on registered expansion forces during the volume change of the solidifying samples and based on the calculated released latent heat of crystallization. The obtained results indicate a considerable influence on the formation and progress of coherency due to variation of casting parameters. It has been shown that the coherency is not a single event at a defined time moment rather a process progressing during a time interval. © 2014 ISIJ.