PHASE AND MICROSTRUCTURE SELECTION IN CAST IRON SOLIDIFICATION

摘要

The mechanical and thermal properties of cast iron strongly 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 lamellae, noduli 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, or inoculants and spheroidizing agents. The microstructures are reported as a junction of specimen shape and size (i. e. cooling rate). Additionally, alloys but tons are made by arc melting in clean atmosphere. The set of results provide information on metastable micro-structures 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. The main highlights of this work are: -The occurrence of the ferrite-cementite eutectic has been experimentally documented. Up to now the literature reported the formation of microstructures, defined generally as bainitic, in thin section of rapidly solidified Fe-C alloys. Eutectic temperature and composition have been obtained from a computer calculation of the phase diagram. The hierarchy of undercooling needed for producing different microstructures in Fe-C, Fe-C-Si and in industrial cast irons have been evaluated. -The lamellar form results as the normal mode of growth of graphite in high purity Fe-C melts. -Spheroid formation depends on heterogeneous nucleation of graphite on substrate particles mostly deriving from reaction of Mg with Si and/or impurity elements.