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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 A distributed file system for large, widely dispersed Unix and Windows networks from Transarc Corporation, now part of IBM. It is noted for its ease of administration and expandability and stems from Carnegie-Mellon's Andrew File System.

AFS - Andrew File System
 at BCIRA BCIRA British Cast Iron Research Association  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 hy·per·eu·tec·tic  
adj.
Having the minor component present in a larger amount than in the eutectic composition of the same components.
 nodules Nodules
A small mass of tissue in the form of a protuberance or a knot that is solid and can be detected by touch.

Mentioned in: Leprosy
 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 nodule: see concretion.
nodule

In geology, a rounded mineral concretion that is distinct from, and may be separated from, the formation in which it occurs.
 counts are significantly increased by bismuth bismuth (bĭz`məth) [Ger. Weisse Masse=white mass], metallic chemical element; symbol Bi; at. no. 83; at. wt. 208.9804; m.p. 271.3°C;; b.p. about 1,560°C;; sp. gr. 9.75 at 20°C;; valence +3 or +5.  treatment, by metal stream inoculation inoculation, in medicine, introduction of a preparation into the tissues or fluids of the body for the purpose of preventing or curing certain diseases. The preparation is usually a weakened culture of the agent causing the disease, as in vaccination against  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 met·al·log·ra·phy  
n.
The study of the structure of metals and alloys, especially by optical and electron microscopy and x-ray diffraction.



met
 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 Loss of signal power in a transmission.
Attenuation

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities.
 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 mis·shape  
tr.v. mis·shaped, mis·shaped or mis·shap·en , mis·shap·ing, mis·shapes
To shape badly; deform.



mis·shap
, 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).
COPYRIGHT 1990 American Foundry Society, Inc.
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Title Annotation:Cast Facts
Author:Ryntz, Edward F., Jr.
Publication:Modern Casting
Date:Nov 1, 1990
Words:514
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