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BACKGROUND

S P H E R O I D A L graphite (SG) cast iron is a competitive engineering material. The spheroidal shape of the graphite provides higher mechanical properties than those of gray (lamellar graphite) iron. Machinability is better than that of steel. Under certain circumstances, the probability of shrinkage formation can be decreased because of the expansion caused by graphite precipitation during solidification.t1] At the present time, computer modeling of phase transformations and computer-aided design are perceived by the metal casting industry as major tools in improving the quality of SG iron castings through process control and product design. Manufacturing times and costs will also be saved when these modern tools are fully implemented. One of the main goals of computer modeling of the liquid/solid and the solid-state phase transformations of SG cast iron is to predict the mechanical properties of the material. If the microstructure of an SG iron casting can be related quantitatively, through computer simulation, to the processing conditions and the configuration of the casting and, ultimately, to the mechanical properties in various locations in the casting, it would be possible to improve, or at least to control, casting properties by altering either the production process or the heat treatment. Accordingly, the development of an appropriate computer model for predicting the fractions and length

DONGKAI SHANGGUAN, previously Assistant Research Engineer, The University of Alabama, is Manufacturing Engineer, Electronics Division, Ford Motor Co, Dearborn, MI 48121. SULI CHANG, Graduate Research Assistant, and DORU M. STEFANESCU, University Research Professor and Director, are with the Solidification Laboratory, Department of Metallurgical and Materials Engineering, The University of Alabama, Box 870202, Tuscaloosa, AL 35487-0202. Manuscript submitted May 25, 1991. METALLURGICAL TRANSACTIONS A

scales of the final phases can be a significant contribution to the production of quality SG cast iron. Figure 1 shows a typical temperature-time curve (cooling curve) for an SG iron continuously cooled from the liquid state down to room temperature. Schematic microstructures of transformation products are superimposed on the curve. The transformation process includes three distinct stages: solidification of primary dendrites, the eutectic solidification, and the eutectoid reaction. The computer model should therefore describe all of these three transformations. Examination of the phenomenology of solidification of SG iron is conducive to the following sequence o f solidification, t2] (1) For a hypoeutectic alloy, primary austenite dendrites begin to form as the temperature decreases below the liquidus temperature. Even for an alloy of eutectic composition, solidification will start with the formation of austenite dendrites due to nonequilibrium solidification. (2) At the eutectic temperature, austenite dendrites and graphite spheroids nucleate independently in the liquid. (3) Very limited growth of spheroidal