EFFECT OF COOLING RATE VARIABLES ON THE SOLIDIFICATION TIME AND MICROSTRUCTURE OF PERMANENT-MOLD MAGNESIUM-TREATED IRON CASTINGS

This paper deals with studies on the effect of cooling rate variations on the solidification times and microstructure of permanent mold magn esium-treated cast iron. Cooling rate variations were brought about by changes in the casting size or mold preheating temperature or mold wa il thickness. The results of this investigation indicate that the soli dification time of magnesium-treated iron casting is always greater th an that of an untreated iron casting, when the cooling rate variables are similar. Since the morphology of graphite in the two cases is diff erent, it is thus indicated that graphite morphology affects the solid ification time. Though the solidification time of magnesium-treated ir on permanent mold castings shows an increase when the casting size is increased, no unilateral increase or decrease is observed when the mag nitudes of the other two variables, viz., mold wall thickness and mold preheating temperature favor decrease and increase in cooling rate re spectively. Microstructural analysis indicated that random variations in the graphite morphology, despite the residual magnesium being above the required limit, were primarily responsible for the random variati ons in solidification times. The reason for this randomness has been d iscussed in terms of heat transfer. Further experiments were conducted where the heat transfer variables were chosen based on this analysis and it was possible to achieve proper trends in the solidification tim e. The graphite shape of the castings in these experiments had good no dularity, with the size being related to the cooling rate.