Mechanism for the Role of Silicon on the Transition from Graphite to Cementite Eutectic in Cast Iron
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AMONG the relevant quality indices in cast iron is its chilling tendency, that is, tendency to solidify according to Fe-C-X metastable system. The chilling tendency (CT) depends on the physical-chemical state of the liquid cast iron whereas the formation of the chill (i.e., fraction of cementite eutectic in the casting) is also dependant on the casting cooling rate. 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. Figure 1 gives a comparison of the chilling tendency for two cast irons (I and II). Notice that cast iron I exhibits a lower chilling tendency than cast iron II. From these comparisons alone, the difference in the CT for various cast irons can be established, but absolute CT values cannot be determined. It is well known that the CT of cast iron determines its subsequent performance in diverse applications. In particular, cast irons possessing a high CT 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, then a relatively small CT will EDWARD FRAS´, Professor, and MARCIN GO´RNY, Associate Professor, are with the AGH-University of Science and Technology, 30-059 Cracow, Poland. HUGO F. LOPEZ, Professor, is with the Materials Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201. Contact e-mail: [email protected] Manuscript submitted January 7, 2012. Article published online May 23, 2012 4204—VOLUME 43A, NOVEMBER 2012
favor the formation of gray iron. In turn, this leads to low hardness and poor wear properties in as-cast components. Hence, considerable efforts[1–9] have been made in correlating the inoculation practice, iron composition, pouring temperature, etc. with the chill of cast iron. In contrast, there are only a few attempts aimed at elucidating the mechanisms responsible for the chill of cast iron.[5,10] W. Oldfield[10] assumed that graphitizing elements such as Si, Ni, Cu, and Co 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 DTs,c range and increase chill. However, this hypothesis cannot explain for example why the inoculation or changes to the carbon content of cast irons hardly influence the DTs,c range while reducing the chill to a great degree. Alternatively, F. Neuman,[12] assumed that elements (e.g., silicon) that raise the carbon activity lower its solubility in the melt, favoring its ability to precipitate as graphite, thus reducing its chill. This proposal is based on the thermodynamics of equilibrium processes, and in fact, it is only valid when considering the effect of various elements on the liquidus temperature in stable Fe-C-X systems. Therefore, it cannot b
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