Influence of Chemical Composition and Cooling Rate on Chunky Graphite Formation in Thick-Walled Ductile Iron Castings

In this paper, the influence of the addition of different levels of bismuth on the amount of the area affected by chunky graphite in thick-walled ductile iron castings was investigated. Ductile iron with increased silicon content and ductile iron with nickel addition were investigated. In order to obtain different cooling rates, cone blocks with diameter of 300 mm and a height of 350 mm were cast. Three groups of test melts were used, EN-GJS-400-15, EN-GJS-500-14 with high silicon level and EN-GJS-400-18 with 0.7 wt% Ni. Cerium was added as component of the inoculant and nodularizer. In order to determine the effect of cooling rate on the graphite morphology in macro- and microstructure, three samples from different positions in casting were prepared; one from the thermal center of casting where the cooling rate is the lowest, one from the edge where the cooling rate is the highest and one from the area between these two zones. Based on the results, a mathematical model was developed to estimate the amount of the area affected by chunky graphite depending on the cooling rate and the chemical composition of the melt. The effect of higher silicon level and the addition of nickel have the least effect at the highest cooling rate, i.e., at the edge of the cone block. The addition of bismuth to a melt with a high silicon level has proven to be very effective in preventing the formation of chunky graphite. This investigation confirmed that chunky graphite is a local defect and that the sample size and sampling location should be taken into account when interpreting the results.