Tailoring Carbon Distribution in α/γ Phase of Ductile Iron and Its Effects on Thermal Conductivity

The effects of carbon distribution on the microstructure and thermal conductivity of ductile iron were investigated in the present study. The microstructure of as-cast and quenched ductile iron were characterized by OM and SEM. Results showed that the microstructure of as-cast ductile iron was composed of spheroidal graphite, ferrite with the volume of 80%, and a small amount of pearlite, and quenched ductile iron was composed of spheroidal graphite, coarse/fine acicular martensite ( α M phase) and high-carbon retained austenite ( γ phase). The volume fraction of retained austensite and its carbon content for direct quenched ductile iron and tepmered ductile iron were quantitatively analysed by XRD. Results revealed that carbon atoms diffused from α M phase to γ phase during tempering at low temperatures, which resulted in carbon content in retained γ phase increasing from 1.2 wt% for the direct quenched sample to about 1.9 wt% for the tempered samples. Consequently, the lattice distortion was significantly reduced and gave rise to an increase of thermal conductivity for ductile iron.