Carbide dissolution in thin wall ductile iron

This investigation focuses on the study of the dissolution by annealing of carbides present in thin walled ductile iron parts. Ledeburitic carbides are usually present in the microstructure as a consequence of the rapid solidification rate induced by the small thickness. The dissolution treatment is carried out by annealing in the austenitising temperature range. The study involves unalloyed ductile irons of different equivalent carbon values that show initial amounts of free carbides that reach 40%. The results show that even very large amounts of cementite can be rapidly and easily dissolved by annealing. The dissolution rate ranges between 2 and 9% per minute. This dissolution rate is much faster than that expected from the literature data and is attributed to the particular characteristics of thin wall castings, such as the low concentration of carbide stabilising elements and the small distance for diffusion of carbon from the cementite to the nodules. The short time required for carbide dissolution implies that carbides can be completely dissolved during the austenitisation stage of many practical heat treatment cycles.