Volume 23, Issue 4 (July 1995)
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 magnesium-treated cast iron. Cooling rate variations were brought about by changes in the casting size or mold preheating temperature or mold wall thickness.
The results of this investigation indicate that the solidification time of magnesium-treated iron casting is always greater than that of an untreated iron casting, when the cooling rate variables are similar. Since the morphology of graphite in the two cases is different, it is thus indicated that graphite morphology affects the solidification time. Though the solidification time of magnesium-treated iron permanent mold castings shows an increase when the casting size is increased, no unilateral increase or decrease is observed when the magnitudes of the other two variables, viz., mold wall thickness and mold preheating temperature favor decrease and increase in cooling rate respectively.
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 variations in solidification times. The reason for this randomness has been discussed 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 time. The graphite shape of the castings in these experiments had good nodularity, with the size being related to the cooling rate.