Effects of graphite nodule count on microstructural homogeneity of austempered ductile iron (ADI)

Abstract

The microstructural homogeneity of austempered ductile iron (ADI) with different graphite counts was evaluated by microstructural observations and hardness measurement. ADI was composed of spheroidal graphite, ausferrite, and stabilized austenite. Microstructural observation and microhardness evaluation indicated that graphite nodule counts affected the microstructure significantly, in a sense that the microstructure was more homogeneous for samples with higher graphite counts. More graphite nodules provided more positions for ferrite nucleation and gave rise to a fine ausferrite microstructure. However, for samples with fewer graphite nodules, the diffusion of carbon atoms in austenite far away from graphite was inhibited, which stabilized the austenite and gave rise to the existence of large-size blocky austenite. Consequently, ADI with high graphite nodule count exhibited superior ductility. This study suggested that more graphite nodule is preferred in ADI to obtain a more homogeneous microstructure.