Role of Sulfur on the Transition from Graphite to Cementite Eutectic in Cast Iron
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ONE of the important quality indices in cast iron is its chilling tendency, that is, its tendency to solidify in the austenite–cementite metastable system. The chilling tendency depends on the physical–chemical state of the liquid cast iron while the formation of the chill (fraction of cementite eutectic in the casting), and depends additionally on the cooling rate of the casting. In the foundry practice, the chilling tendency is determined from comparisons of the exhibited fraction of cementite eutectic (chill) in castings solidified under similar cooling rates. Based only on these comparisons, differences in the chilling tendencies for various cast irons can be assessed, but absolute chilling tendency values for given cast irons cannot be attained. It is well known that the chilling tendency of cast iron determines its subsequent performance in diverse applications. In particular, cast irons possessing a high chilling tendency tend to develop zones of white or mottled iron. Considering that these regions can be extremely hard, their machinability can be severely impaired. Alternatively, if white iron is the desired structure a relatively, then low chilling tendency will EDWARD FRAS´, Professor, MARCIN GO´RNY, Assistant Professor, and MAGDALENA KAWALEC, Post-doctoral Fellow and Lecturer, are with the Department of Engineering of Cast Alloys and Composites, Faculty of Foundry Engineering, AGH-University of Science and Technology, Reymonta 23, 30-059, Cracow, Poland. HUGO F. LOPEZ, Professor, is with the University of WisconsinMilwaukee, P.O. Box 784, Milwaukee, WI, 53201. Contact e-mail: [email protected] Manuscript submitted July 25, 2012. Article published online January 15, 2013 2512—VOLUME 44A, JUNE 2013
favor the formation of gray iron. This leads to low hardness and poor wear properties in the as-cast components. The transition from graphite to cementite eutectic is also important in thin-walled shaped castings. Hence, considerable efforts[1–9] have been made in correlating the inoculation practice, iron composition, pouring temperature, etc. with the chilling tendency of cast iron. Moreover, there have been limited attempts aimed at elucidating the mechanisms responsible for the chill of cast iron.[10–13] Yet, none of these studies considers the complexity as a whole during the solidification process. Most of the proposed hypotheses assume that a single factor is the determinant in establishing the solidification structure, while the remaining factors are neglected. Oldfield[10] assumed that graphitizing elements such as Si, Ni, Cu, Co, etc., all increase the temperature range DTsc = Ts Tc between the graphite eutectic equilibrium temperature, Ts and the cementite eutectic formation temperature, Tc, thus promoting the development of a gray structure. In contrast, carbide-promoting elements such as Cr and V[11] reduce the DTsc range and increase the chill. However, this hypothesis is unable to explain why inoculation or changes in the carbon content does not seem to influence the DTsc range, while the exhibited chil
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