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Metal Casting Technologies : September 2005
BacktoBASICS which will occur in a given casting, its size and distribution, and the distribution of any iron carbide present. Graphitising elements such as silicon have the effect of increasing the temperature difference between the Fe-C and Fe-Fe3C eutectics, lowering the white iron reaction temperature and slightly raising the grey iron reaction temperature. Under equilibrium cooling conditions, an addition of 2%Si has the effect of widening the temperature interval of the two eutectics from about 6˚C to 30˚C. Carbide stabilising elements such as chromium, narrow the temperature interval between the Fe-C and Fe-Fe3C eutectics thus making the formation of graphite less likely. THE EFFECT OF COOLING RATE The cooling rate through solidification and subsequent cooling has a direct and profound effect on the mechanical properties of cast irons. With high cooling rates, such as might occur on casting edges adjacent to the mould wall, or in very thin casting sections, there may not be sufficient time for graphitisation to take place. This is particularly so if the silicon is at low levels in which case the iron may be undercooled below the equilibrium white iron reaction temperature, so that the white iron eutectic will nucleate in preference to the grey iron eutectic. In this case, the matrix will consist of primary cementite and pearlite resulting in a very hard and brittle iron. At slightly lower cooling rates, or with increased silicon content, some graphitisation may take place and the matrix is then a mixture of primary cementite, pearlite and graphite. Figure 2 shows an example of this where the high cooling rate in the corners of this thin casting has resulted in the formation of eutectic carbides. With further reduction to the cooling rate, all primary cementite decomposes during, or shortly after solidification, and the structure will consist of graphite flakes distributed in a pearlitic matrix. Very slow cooling, such as may be experienced at the centre of heavy casting sections, can result in a matrix with large amounts of ferrite and a soft, low strength iron. Cooling rate will also have a marked effect on the size, shape and distribution of the graphite flakes. Very high cooling rates will result in fine graphite flakes often with an undesirable dendritic distribution with preferred orientation. Medium cooling rates will tend to form a more desirable graphite size and shape with random distribution, whilst slow cooling rates will tend to form large and coarse flakes. DEGREE OF NUCLEATION As cast iron solidifies, the eutectic cells begin to grow on nuclei present in the melt. The more nuclei present, the more eutectic cells formed and the smaller they will be. Higher eutectic cell counts promote the formation of finer graphite clusters within the cells, which in turn increases the final strength of the iron. Fewer and larger eutectic cells result in coarser graphite clusters thus increasing the weakening effect of the graphite. Eutectic carbide formation is associated with low melt nucleation. The degree of nucleation of a melt is influenced by the way the metal is melted and treated. High superheat temperatures tend to destroy nuclei, as does long holding times. Ladle inoculation increases the number of nuclei present during solidification thus reducing eutectic cell size and increasing the strength of the iron compared to an uninoculated iron of similar chemistry and cooling rate. Chill in corner of thin section grey iron casting 2 REFERENCES 1. Microstructure Development During Metalcasting, John E Gruzleski -- American Foundrymen's Society 2000 2. Fundamentals of Foundry Technology, P. D. Webster -- Portcullis Press 1980 continued from page 69 Who's Who of Metals 2005/6 77