Cooling rate and carbon content effect on the fraction of secondary phases precipitate in as-cast microstructure of ASTM F75 alloy

Simple test castings were used to study the effect of cooling rate and carbon content in as-cast microstructure of alloy ASTM F75, Co–26 wt.% Cr–5.7 wt.% Mo. Alloys with four C content (0.45, 0.33, 0.36 and 0.25 wt.%) were poured into investment ceramic molds. In order to obtain different cooling rates, the castings were constituted of three axisymmetrical cylinders of different diameters (12, 16 and 24 mm). Cooling curves were obtained from each cylinder and the fraction of secondary phases in as-cast microstructure was measured by image analysis. Average cooling rates of 100, 60 and 20 °C/min, were obtained in the 12, 16 and 24 mm diameter cylinders respectively, at the first solidification step occurring in the temperature range from 1390 to 1350 °C. A significant effect of this cooling rate range on the fraction of secondary phase was not observed. It was observed that the fraction of blocky carbides increased proportionally as the C content increased, whereas the amount of the eutectoid constituent showed a significant increase only in the sample containing 0.45 wt.%, as compared with the samples of other carbon contents. It was also found that the initial solidification undercooling affects the temperature of sigma phase precipitation with which the solidification of the alloy is completed.