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Metal Casting Technologies : September 2005
BacktoBASICS Eutectic Carbide (Chill) Formation in Iron Castings J.F. Meredith, Casting Solutions Pty Ltd he term "cast iron" is a generic one, which covers a wide range of iron- carbon-silicon alloys the properties of which can vary greatly. Generally, the mechanical properties of cast irons are determined by the strength of the matrix and the size, form and distribution of the graphite; which in turn are primarily influenced by the cooling rate in the mould, the chemical composition of the iron and the degree of nucleation of the melt. At a given cooling rate, the mechanical properties of cast irons can be directly related to the chemical composition and degree of nucleation. Variations in cooling rate caused by section size or thermal properties of the mould material can have a profound effect on the mechanical properties of cast irons with otherwise identical chemical compositions and degrees of nucleation. In order to appreciate the significance of these variables, it is necessary to have an understanding of the solidification mechanisms. GENERAL SOLIDIFICATION MECHANISM The majority of commercial flake graphite cast irons are of hypo-eutectic composition (CEV less than 4.25). During solidification, there are two possible eutectic reactions in the system: the grey iron eutectic (Fe-C) and the white iron eutectic (Fe-Fe3C). In the pure binary system, a temperature difference of only about 6˚C separates the two eutectic reactions. These are shown in figure 1 which is a simplified version of the eutectic region of the Fe-C phase diagram. Solidification begins at the liquidus temperature with the nucleation and growth of primary austenite dendrites. Austenite growth causes the remaining liquid to be enriched with carbon until it reaches eutectic composition. If the remaining liquid is cooled below the equilibrium grey iron reaction temperature (TG1) then it solidifies as eutectic cells of austenite and graphite. If however, the remaining liquid is undercooled below the equilibrium white iron temperature (TW1) such that the degree of undercooling is sufficient for the white iron eutectic to nucleate in preference to the grey iron eutectic then the liquid will solidify as austenite plus cementite (Fe3C). Graphite and cementite have very different physical properties: graphite is soft while cementite is hard and brittle. The properties of these constituents are reflected in the overall properties of the iron. Grey cast iron tends to be relatively soft and is easily machined, whilst white cast irons are extremely hard and difficult to machine and they process poor mechanical properties as a result of the brittle nature of the cementite phase. EFFECT OF CHEMISTRY Controlling which eutectic reaction takes place is accomplished by modifying the chemistry of the liquid iron in such a way that the temperature interval between the Fe-C and the Fe-Fe3C eutectics is either widened or narrowed. In addition, additives to the melt encourage nucleation of one or the other of the phases. Carbon is the most important element in flake graphite irons since it determines the maximum strength attainable and the mechanical properties in general. Increasing the percentage of carbon in an iron also increases the likelihood of graphitisation. Most engineering cast irons contain between 3.0 and 3.5 per cent total carbon in which range there is a useful balance between cost, foundry properties and mechanical properties attainable. The effects of other elements in cast iron and of cooling rates are important mainly as they effect the amount of free graphite T Simplified version of the eutectic region of the Fe-C phase diagram. 1 continued on page 77 Who's Who of Metals 2005/6 69