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Graphite flotation in ductile iron castings: definition and influencing factors.

This article represents the first of two summaries highlighting research sponsored by AFS at BCIRA concerning carbon flotation. While the information presented was taken entirely from the BCIRA work, the format and order of presentation were changed to facilitate dissemination of the research. What is Carbon Float?

Carbon float is defined as carbon segregation at the top surfaces of ductile iron castings caused by the buoyancy of graphite particles in liquid iron. Flotation of hypereutectic nodules occurs in ductile irons having carbon equivalents greater than 4.5 as defined by the expression CE = C% + (Si% / 3) + (P% / 3).

Flotation arises because the density of the graphite nodules is less than that of the liquid iron. It results in bands at the top surfaces of castings and at the underside faces of internal cores. The graphite content is significantly higher in the float regions.

Except when amounts of flotation or section sizes are small, the band has a carbon content of just over 5% and the area of the cross section occupied by graphite is just over 20%. As flotation becomes more severe, the extent of the affected zone increases. Graphite Appearance, Form

The form of graphite in the flotation band ranges from fairly well-formed hypereutectic nodules to a variety of undesirable forms, including "cauliflowered" and exploded nodules and chunky graphite. The undesirable forms become increasingly prevalent as the extent of flotation, section size and casting temperature increases. Chunky graphite occurs with high silicon contents. Some of these undesirable forms are shown in the microstructures in Fig. 1.

Hypereutectic nodules are better shaped when nodule counts are significantly increased by bismuth treatment, by metal stream inoculation or when small amounts of flotation occur particularly in smaller section sizes.

Caution: Generally, flotation becomes visible on machined surfaces when the hypereutectic nodules are badly shaped, either in exploded or cauliflowered form. When the nodules have a good shape, the areas of flotation may not be distinct. influencing Factors

Other foundry variables-bismuth treatments, residual magnesium contents, inoculation practices and the use of ceramic filters in gating systems-do not significantly influence the extent of flotation, but may modify graphite form in the affected band. Assessment of Carbon Float

Metallographic examination of graphite structures close to the upper cast surfaces and the comparison of these structures with those in other positions within the casting are effective ways of assessing the presence of carbon float, Examining the appearance of machined surfaces is not always a reliable method of determining the presence of float.

Ultrasonic attenuation measurements can be used to detect the occurrence of more extensive areas of graphite flotation. Attenuation is increased when the hypereutectic nodules are severely misshapen, exploded or cauliflowered.

Next month: The influence of carbon flotation on mechanical properties and the control and assessment of flotation. References A. G. Fuller, T. N. Blackman, " Effects of Composition and Foundry Variables on Graphite Flotation in Hypereutectic Ductile Irons," AFS Transactions, vol 94, pp 823-862 (1986). T. N. Blackman, "Effects of Graphite Flotation on the Tensile and Impact Properties of Ductile Iron Castings," AFS Transactions, vol 97, pp 213-224 (1989).
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Title Annotation:Cast Facts
Author:Ryntz, Edward F., Jr.
Publication:Modern Casting
Date:Nov 1, 1990
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