Precipitation and dissolution of carbides in low alloy ductile iron plates of varied thickness

Abstract

The present work has investigated the feasibility of achieving carbide free microstructures in low alloy thin wall ductile iron (TWDI) after dissolution heat treatments. Studies were conducted using low alloy and unalloyed hypereutectic irons, obtained from thin and regular thickness blocks, cast using sand moulds. The amount of carbides present in the microstructure was measured before and after annealing heat treatments carried out at temperatures ranging between 870 and 930°C. As cast specimens contained up to 30 vol.-% of carbides. The results show that annealing heat treatments dissolved more than 90% of as cast carbides present in thinner wall specimens, while dissolution was below 30% for 13 and 25 mm Y block specimens. Chemical microanalysis demonstrated that the content of carbide stabilising elements in the carbides increased as the cooling rate during solidification diminished. Therefore, carbides present in irons solidified at a slower cooling rate are more difficult to dissolve by annealing.