EFFECT OF Ce ON GRAPHITE NODULE COUNT AND SIZE DISTRIBUTION IN DUCTILE IRON

Abstract

The graphite nodule density and size distribution are two very important parameters in the characterization of the microstructures of ductile iron. Various amount of cerium element ( Ce ) (from 0.005% to 0.020%) were added into ductile iron cast in sand moulds to investigate the effect of Ce concentration on graphite nodule count and size distribution. The addition of Ce has a significant effect on solidified microstructures of ductile iron. Increasing of Ce concentration from 0.005% to 0.014%, an obvious increase in nodule graphite amount is achieved and graphite size becomes finer. When the addition level of Ce reaches 0.018% or 0.020%, spheroidal graphite begins to coarsen, resulting in the decrease in graphite amount and formation of the some non-spherical graphite. An evident nucleus and faceted crystals inside a nodule are clearly distinguished and the thin flakes that constitute the nodule are observed from the SEM photograph. EDX-analysis of nucleus of the spheroidal graphite shows that it is composed of sulfur, cerium, magnesium and trace calcium. The 3-D nodule count of ductile iron is 11.7 × 103/ mm 3 with 0.005% Ce addition. The value is 45.8 × 103/ mm 3 with Ce addition up to 0.014% and the majority of diameter is less than 30 µm. However, with 0.018% Ce residual, the nodule count decreases to 29.0 × 103/ mm 3. The value is close to that of 0.018% Ce when Ce concentration is 0.020%, and the majority of diameter is less than 40 µm. The results indicate that the graphite nodule count reaches the maximum and mean diameter reaches the minimum with 0.014% Ce residual existing in ductile iron.