Solidification experiments for the study of phase selection in cast iron

Abstract

The mechanical and thermal properties of cast iron depend on microstructure. There is a wealth of studies in the literature on metastable and stable eutectics and on graphite growth under various experimental conditions aimed at elucidating the cause of the formation of either lamellar, nodular or compact grains. However, the early stages of nucleation are difficult to isolate and the models for graphite nucleation are even controversial.

In this work the problem is tackled with experiments of solidification at various rates and with different equipments: conventional cooling, copper moulding and melt spinning, with both Fe-C-Si alloys made from pure elements and industrial cast irons. Bulk ingots, cone shaped samples (with diameter ranging from 1 to 6 mm) and ribbons are obtained respectively. Experiments are made using either no additives or inoculants and spheroidizing agents. The microstructures are reported as a function of specimen shape and size (i. e. cooling rate). Additionally, alloys buttons are made by arc melting in clean atmosphere.

The set of results provide information on metastable microstructures and the extent of undercooling of the alloys showing graphite shapes which can be related to the level of contaminants in the melt. Unusual microstructures were obtained which are explained in terms of ferrite-cementite metastable eutectic. Computer calculation of phase diagrams helps in explaining phase selection.

Keywords:

• Phase selection
• rapid solidification
• metastable eutectic
• graphite shape