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Metal Casting Technologies : December 2010
28 www.metals.rala.com.au Introduction hermal analysis of cooling curves from the solidification of cast irons has been used as a foundry process control tool for nearly 50 years [1, 2]. Originally thermal analysis was intended to provide an indication of metal composition in the form of a "carbon equivalent liquidus value" for engineering grey (flake graphite FC) irons thus enabling process control without the need for an analytical laboratory. Carbon equivalent measurement provided a ready guide to the chilling characteristics and mechanical properties of the iron before it was poured into moulds. This paper provides a short review on how this "simple" test has been continually improved to provide much more information about the solidification behaviour of cast irons and so enable casting defects to be avoided in grey, ductile and compacted graphite irons. Recently thermal analysis has made a major contribution to the successful application of compacted graphite iron for modern diesel engine blocks. Importance of carbon equivalent The use of a cooling curve to determine the carbon equivalent liquidus (CEL) of a cast iron together with information from a suitable chill test forms the basis of metallurgical control in the production of engineering grey iron castings. The CEL value provides an assessment of metal composition in terms of its suitability to pour a given grade of grey iron, i.e. an iron with a minimum given tensile strength, since CEL values can be related to mechanical properties. The higher the CEL value the greater is the resistance to chilling, the higher is the proportion of eutectic graphite in the microstructure and consequently, the lower is the tensile strength. At CEL values above that of eutectic composition primary graphite is produced. Cast irons can solidify via two eutectic reactions (Figure 1). In the grey iron reaction eutectic liquid solidifies as austenite + graphite. In the white iron reaction the eutectic liquid solidifies as austenite + iron carbide (Fe3C). The white iron reaction is encouraged by low C and Si contents (i.e. low CEL), fast cooling rates (i.e. thin sections), low degrees of nucleation in the liquid iron and the presence of any carbide forming elements such as Cr. The formation of any white iron eutectic structure is highly undesirable in castings since it is hard, brittle and difficult to machine. The foundry term for white iron formation is chilling since white iron is most likely to form at the edges of thin sections where cooling rates are highest. Most foundries use some form of chill testing. This involves the pouring of liquid iron at a standard temperature into small wedge or forced chill test pieces to indicate the tendency of the iron to solidify with a white (chilled) structure. The test piece is fractured and the extent of the white iron microstructure is revealed by its white fracture surface and is reported as depth of chill. Chill tests monitor the combined effect of composition and the degree of nucleation in the iron and are especially useful in judging the effectiveness of inoculation treatment. Inoculation is used to refine the cell size of the eutectic graphite in grey irons, thus improving tensile strength, and to prevent the formation of eutectic carbide and of very fine undercooled graphite, ensuring consistent machinability . Control of cast irons by thermal analysis By John Pearce T Figure 1. Schematic double eutectic diagram in cast irons. TECHNICAL FEATURE