Graphite Nodule and Eutectic Cell Count in Cast Iron: Theoretical Model Based on Weibull Statistics and Experimental Ver
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FLAKE graphite cast iron is the most frequently used in foundry practice. Hence, there are numerous reports related to the production of cast iron, some of which are of essential importance as they are linked to the solidification of graphite eutectic. In flake graphite cast iron, the austenite-graphite eutectic solidification process is concomitant with the formation of eutectic cells that are more or less spherical (Figures 1(a) and (b)). These eutectic cells consist of interconnected graphite plates surrounded by austenite. Because each eutectic cell is the product of a graphite nucleation event, cell count measurements can be used to establish the graphite nucleation susceptibility of a given cast iron. In general, increasing the eutectic cell count in a given cast iron leads to the following: (1) increasing strength of cast iron (through a reduction in ferrite and an increase in graphite type A );[1] (2) reducing the chill of cast iron[2] and, as a consequence, the making of possible production machinable castings, free from the high hardness carbide eutectic; (3) increasing preshrinkage expansion[3,4]—if the mold lacks sufficient rigidity, expansion of the
E. FRAS´, Professor and Head, K. WIENCEK and M. GO´RNY, Assistant Professor, are with the AGH University of Science and Technology, Mickiewicza 30, Cracow, Poland. Contact e-mail: [email protected] H.F. LO´PEZ, Professor and Chair, is with the University of Wisconsin–Milwaukee, Milwaukee, WI 53201, USA. Manuscript submitted June 22, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
casting during solidification can cause unsoundness in the form of internal porosity or surface sinking defects, particularly in those parts of the casting last to solidify (the probability of developing unsoundness increases with the cell count). Ductile cast iron is a modern engineering material whose production is continually increasing. In this cast iron, each nucleus of graphite gives rise to a single graphite nodule (Figure 1(d)); thus, nucleation establishes the final nodule count. Increasing the nodule count in a given ductile cast iron leads to the following: (1) increasing strength and ductility in ADI iron,[5] (2) reducing microsegregation of alloying elements[6,7] and improving microstructural homogeneity (here, the type of eutectic transformation, stable or metastable, is also influenced due to the redistribution of alloying elements); (3) reducing the chilling tendency of cast iron;[8,9] (4) increasing preshrinkage expansion;[3] and (5) increasing the fraction of ferrite in the microstructure.[10] Accordingly, it can be stated that graphite eutectic cell and nodule count influenced some important factors for foundry practice. It is well known that the nodule count in ductile iron or eutectic cell count in flake graphite cast iron can be significantly influenced by the cooling rates,[1,11–13] chemical composition,[1,11–13] and bath superheat temperature and time.[12] Nevertheless, a drastic increase in VOLUME 38A, FEBRUARY 2007—385
Fig. 1—Illustration of (a) the developm
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