Chunky graphite formation in small section ductile iron castings

Chunky graphite is a degenerated graphite form which can be found in the thermal centre of ductile iron heavysection castings. Previous studies made on cubic blocks (300 and 180 mm in side) manufactured using alloys withfully ferritic matrix structures show that low cooling rates, excessive post-inoculation and high silicon and/orcerium contents in the melts are the most important factors that promote this kind of defect. The enhancement ofthese critical factors led to obtain chunky graphite in sections lower than 50 mm. Different experimentalconditions have been used in order to establish the main parameters that affect this graphite malformation. Theuse of cutting-edge techniques in the analysis of chemical compositions has revealed that no significant differencescan be found when comparing chunky areas and well-formed spheroidal graphite areas. On the other hand, it hasnot been possible to establish any correlation between the oxygen contents and the scale of the defect. However, itis noteworthy that the oxygen content is related to the use of magnesium or cerium as nodulizer agent.