A Hypothesis for Cast Iron Microstructures
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aphite in cast irons have been the subject of intense interest and huge research efforts mainly since the 1950s, but a full understanding has been elusive. Readers are referred to the review by Loper[1] for a wideranging synopsis covering many details not included in this study. Here, a different review is made of the literature, exploring the possibility of a unifying approach based on the hypothesis that oxide films (as doubled-over ‘‘bifilms’’) are present in liquid irons. Recently, a comprehensive understanding of the microstructure of Al-Si alloys has been proposed in terms of bifilms, explaining both the mechanism of modification and the structures of hypoeutectic and hypereutectic alloys.[2] Bifilms (double films), usually oxides, are introduced into molten metals at every pour or stirring event. These surface films, which are doubled up during the process of entrainment into the bulk, seem to be of major significance for the development of solidification structure.[3,4] Their outer surfaces seem to
be favored substrates for the precipitation of many, if not all, second phases during solidification, whereas the inner unbonded interface acts as a crack. The films are generally so thin they are effectively invisible to casual observation, which leads to a liquid that is invisibly cracked and, on solidification, to a solid whose cracks may or may not be visible. Nakae and Shin,[5] among many others, have drawn attention to the analogous features of Al-Si and Fe-C alloys. This article is an extension of the bifilm hypothesis, which is apparently valuable to an understanding of the Al-Si system as well as to a possible understanding of the various morphologies of carbon in the form of graphite and carbides in the Fe-C alloy system. Naturally, the presence of invisible defects in opaque liquids is not easily confirmed directly, although it will not be impossible in the longer term. In the meantime, this article surveys the experimental evidence from the literature to ascertain whether indirect evidence is supportive of this hypothesis.
II. JOHN CAMPBELL, Emeritus Professor, is with the Department of Materials and Metallurgy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Contact e-mail: [email protected]. This article is based on a presentation given in the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online September 1, 2009. 786—VOLUME 40B, DECEMBER 2009
THE EVIDENCE FOR OXIDE BIFILMS IN CAST IRON
De Sy[6] has shown that liquid cast iron generally contains significant quantities of oxygen in solution in excess of its solubility. He concluded, on the basis of careful and rigorous experiments, that the undissolved fraction of oxygen was present as SiO2 particles. Interestingly, by heating to 1550 °C, he confirmed the METALLURGICAL AND MATERIALS TRANSACTIONS B
expectation that the SiO2 s
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