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The manufacturing process of heavy section ductile iron castings is strongly influenced by the risk of graphite degeneration under slow cooling rates. Appearance of this kind of defect is commonly linked to significant reductions in the mechanical properties of large castings. Studies on the effect of inoculation on chunky graphite formation in heavy sections have led to contradictory results in the literature and this triggered the present work. New experimental data are presented on the effect of mould inoculation on chunky graphite appearance during solidification of nodular irons which clearly demonstrate that mould inoculation increases the risk of chunky graphite formation in heavy sections. This is in agreement with some previous works which are reviewed, and it is suggested that the contradiction with other results could relate to the fact that these latter works dealt with chill casting.

Copper is known as a pearlite promoter in cast iron and has been used as such for a long time, most often together with low amounts of manganese. Literature data, however, has shown that these two elements act differently on the ferritic and pearlitic transformations. In order to provide more insight in the role of this element on the solid-state transformation of spheroidal graphite (SG) cast iron, this paper investigates the effect of adding copper in small step increments, from 0.11 to 1 wt. % Cu, to SG irons containing about 0.15 wt. % Mn.The characteristic temperatures for the stable and metastable eutectoid transformations as recorded during cooling after solidification in standard cups and after austenitizing are presented together with microstructure information. It is found that a copper content of about 0.6 wt. % is the upper limit over which only small amounts of ferrite could be obtained except at very low cooling rates. This is tentatively related to the lowering of the temperature for ferrite and the associated decreased kinetics for austenite decomposition in the stable system.

Thermal analysis is applied to the study of the formation of chunky graphite (CHG) in heavysection castings of spheroidal graphite cast irons. To that aim, near-eutectic melts prepared in one single cast house were poured into molds containing up to four large cubic blocks 30 cm in size. Four melts have been prepared and cast that had a cerium content varying in relation with the spheroidizing alloy used. Postinoculation or addition of antimony was achieved by fixing appropriate amounts of materials in the gating system of each block. Cooling curves recorded in the center of the blocks show that solidification proceeds in three steps: a short primary deposition of graphite followed by an initial and then a bulk eutectic reaction. Formation of CHG could be unambiguously associated with increased recalescence during the bulk eutectic reaction. While antimony strongly decreases the amount of CHG, it appears that the ratio of the contents in antimony and cerium should be higher than 0.8 in order to avoid this graphite degeneracy.

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