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Metal Casting Technologies : September 2007
64 www.metals.rala.com.au TECHNICAL FEATURE the molten metal correction. Table 2 shows the chemistries of the base iron samples taken from the furnace prior to tapping and the after correction samples taken from the ladle prior to pouring. Note the similarities of the base iron chemistries. RESULTS AND DISCUSSION The three chemically distinct heats generated a large number of samples for metallographic evaluation. The surface examined was the interface between molten metal and the bonded sand core, where mold metal reactions can influence graphite degeneration to disrupt the bulk microstructure development and form a skin of a different morphology. Reactions that may take place at the surface can be associated with the presence of elements, such as, oxygen, nitrogen and sulfur that preferentially combine with graphite spheroidizing elements present in the melt and influence the shape of graphite particles. The key is to be able to control such reactions and retain the desired casting microstructure throughout. Just as any other phase change controlled by diffusion, graphite formation occurs by the mechanisms of nucleation and growth. The driving force is a delta-temperature, or undercooling, described by the difference between the growth arrest temperature in the thermal analysis curve and the equilibrium phase boundary temperature. Fast cooling rate and resulting large kinetic undercooling promote nodular graphite growth; while slow cooling rate and resulting small kinetic undercooling favor lamellar graphite development. Compacted graphite is a transition between these two morphologies. Graphite crystallizes in a hexagonal structure. The unit cell is bound by six prism faces and two basal faces that form the sheets in the layered structure. The basal sheets consist of a continuous hexagonal network of strongly bonded carbon atoms and the prism plane represents a high energy plane where impurities adsorb preferentially. Spheroidal or nodular graphite grows primarily by the addition of carbon atoms to the basal plane, while flake graphite growth occurs along the prism plane.8 The presence of impurities increases the mobility of the prism face and the graphite morphology can be related to the residual concentrations of surface-active impurities. In the experiments the ratio of temperature gradient to growth rate promote conditions of small kinetic undercooling, which influenced by the presence of graphite spheroidizing elements within the controlled CGI window, have a tendency for vermicular graphite growth morphology. Although other researchers9 suggest that nucleation is similar among all types of cast irons, differences in their growth behaviors are undoubtedly evident. Figure 4a shows a typical non-etched uniform CGI microstructure of the bulk of the material, whereas Figure 4b depicts a microstructure transition from compacted to flake graphite at the surface of the casting. The latter confirms flake graphite presence as a result of surface-active impurity elements absorption onto the prism face, causing it to act as BASE IRON AFTER CORRECTION heat 1 heat 2 heat 3 heat 1 heat 2 heat 3 %C 3.80 3.84 3.78 3.75 3.80 3.73 %Mg <0.001 0.001 0.001 0.013 0.019 0.046 %S 0.013 0.012 0.014 0.011 0.012 0.016 %Si 1.85 1.81 1.86 2.10 2.50 2.83 C,S: Leco furnace; Mg: ICP/MS; Si: Optical Emission Spectroscopy Table 2. Chemical analysis of heats studied Figure 4. CGI optical microscopy. a) Interior, b) Surface. (50X) Figure 5. Photomicrographs of samples from a) heat 1, b) heat 2 and c) heat 3. (50X) b)