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Metal Casting Technologies : September 2008
Figure 3. Microstructure of grey iron showing eutectic graphite flakes in a matrix of pearlite with some phosphide eutec- tic (light coloured islands) In plain carbon steels, increasing carbon content up to 0.84% and the formation of pearlite, cause the yield and tensile strengths to rise rapidly. This rise, however, is accompanied by a corresponding decrease in ductility and malleability. A wide range of properties can be obtained with plain carbon steels by varying the carbon content and giving appropriate heat treatment. Carbon in cast iron Cast iron is an iron-carbon-silicon alloy having a micro- structure which consists of either carbide eutectic, graphite eutectic, or both and usually containing 2 to 4% carbon and 0.25 to 3.0% silicon along with varying amounts of manganese, sulphur, phosphorus and other elements. The carbon content of cast iron is a very important determining factor in the type of microstructure and properties obtained. There are two main categories of cast iron, namely, grey iron and white iron, which are characterised by the type of fracture they exhibit. The type of fracture and the hardness obtained depends upon the form in which the carbon is present which may be as eutectic graphite or eutectic cementite together with pearlite or ferrite. In the case of heat-treated irons, however, the matrix may be composed of austenite, tempered martensite or other intermediate structures. Figure 3. During solidification and subsequent cooling down in the solid state, cast iron is austenitic above the critical temperature and the solubility of carbon in austenite decreases with decreasing temperature. In grey cast iron the carbon which is rejected from austenite during normal cooling is precipitated upon the already existing graphite flakes. In white cast iron, such rejected carbon precipitates upon the already existing cementite or as needles within grains or as grain boundary filaments of carbide. At the eutectoid temperature, the carbon which is precipitated, together with ferrite, may form pearlite or, with high silicon contents and sufficiently slow cooling rates, may form graphite which precipitates upon the already existing graphite. When carbon is precipitated as graphite, the matrix is partially or wholly ferritic. METAL Casting Technologies September 2008 91