Nodule Evolution of Ductile Cast Iron During Solidification
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DUCTILE iron (DI) castings are ferrous alloys in which precipitations of graphite are embedded in a metal matrix in the form of spherical nodules. These are industrially used in parts that require moderate ductility without sacrificing mechanical resistance, as in valves, hardware elements, and auto parts. Studies focusing on DI have become important in recent years due to its extensive use and a growing trend to replace forged steels due to their lower comparative costs. Efforts have been made to improve DI mechanical properties by means of alloying, allowing it to replace cast and forged steels in different applications.[1] In order to achieve further improvements in the quality of DI casting parts, it is important to understand the physical mechanisms involved in the solidification process, since its mechanical properties are influenced by the microstructure of the matrix and the morphology of the graphite nodules present.[2–5] The solidification process of DI begins at high temperatures [~1438 K (~1165 C)], which makes it difficult to have experimental evidence of the transformation sequence from liquid to solid state. Assuming that the cooling process from the liquid state of a wellinoculated casting occurs under equilibrium conditions (cooling rate @ 0), the first solid to precipitate in a hypereutectic DI (carbon equivalent CE = 4.5 pct) is the graphite phase in the form of particles. These
S.C. MURCIA, formerly M.Sc. Student with the Engineering Materials Research Group, School of Engineering, Eafit University, Cra 49 No. 7 Sur 50, Medellı´ n, Colombia, is now Ph.D. Student with the University of Maryland Baltimore, Baltimore, MD. E.A. OSSA, Professor, is with the Engineering Materials Research Group, School of Engineering, Eafit University. Contact e-mail: eossa@eafit.edu.co D.J. CELENTANO, Associate Professor, is with the Mechanical and Metallurgical Engineering Department, Pontificia Universidad Cato´lica de Chile. Vicun˜a Mackenna # 4860, Macul, Santiago, Chile. Manuscript submitted June 18, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
graphite particles grow through the depletion of carbon atoms in the liquid iron until the temperature reaches the range of eutectic transformation. Austenite then nucleates in areas of low C concentration; therefore, austenite surrounds graphite nodules. During eutectic solidification, only austenite is in contact with liquid, and carbon diffusion through austenite is the mechanism that controls the nodules’ growth.[6–10] After solidification, carbon diffusion continues toward the preexisting graphite nodules since the solubility of carbon in austenite decreases with temperature.[6] Subsequently, eutectoid transformation occurs, where the austenite transforms into pearlite and more carbon atoms diffuse into the nodules. Finally, depending on the amount of carbon (CE), DI metal matrix can be a mixture of ferrite and pearlite. Some evidence of the solidification process can be found thanks to the microstructures obtained through quenching DI at different temperatures,[7,11,12] givi
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