Influence of the Mn content on the kinetics of austempering transformation in compacted graphite cast iron

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TRODUCTION

IN the last decade, many studies have been performed which extend to compacted graphite (CG) cast iron the austempering heat treatment widely used to improve the mechanical properties of nodular cast iron, also called austempered ductile iron (ADI). However, the dependence of the kinetics parameters on the alloy concentration remains an open question in that cast iron. The austempering transformation in cast irons involves three stages. In stage I, parent austenite transforms into acicular ferrite and high-carbon austenite (g → aFe 1 ghc), forming a microstructure called ausferrite. The mechanism of the reaction consists of the nucleation and growth of ferrite needles that reject carbon atoms into the surrounding austenite, which becomes more stable due to the increase in the carbon content, which, on its own, causes the martensitic starting point to become lower than room temperature. The fraction of austenite that has not become rich enough in carbon transforms into martensite during the cooling to room temperature. The transformation progresses until the phases reach a metastable equilibrium. At this point, the transformation stops temporarily and begins a steady-state period (stage II ), in which the amounts of acicular ferrite and high-carbon austenite remain approximately constant. Stage II is also called the processing window, where optimum mechanical properties are obtained. When the metastable high-carbon austenite eventually decomposes into the more thermodynamically stable ferrite and carbides, stage III starts, with the consequent reduction of the mechanical properties, particularly ductility and toughness. ¨ Mossbauer spectroscopy has been proven to be a powerful [1–5]

J. DESIMONI and R.C. MERCADER, Professor of Departamento de ´ Fısica, Facultad de Ciencias Exactas—U.N.L.P., CC. No67 (1900) La Plata, Argentina, and Researchers of CONICET; K. LANERI, Teaching Assistant ´ of the Facultad de Ingenierıa U.N.L.P. and Fellow of ANCyT; R. GREGORUTTI, Research Assistant of LEMIT-CICPBA and Teaching ´ Assistant of the Facultad de Ingenierıa—U.N.L.P.; and J.L. SARUTTI, Research Assistant of the LEMIT-CICPBA and Professor of Facultad de ´ Ingenierıa U.N.L.P. Manuscript submitted February 22, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A

analytical technique for quantitative studies of metallurgical ¨ phenomena.[6] Because of its microscopic character, Mossbauer spectroscopy is particularly suitable to accounting for the mechanism of transformations that involve nucleation and growth processes. Several articles have determined the kinetics of austempering transformation in cast irons, using an X-ray diffrac¨ tion technique.[4,5,7–9] Mossbauer spectroscopy can be used advantageously toward this end, due to its ability to identify ferrite, austenite, martensite, and intermetallic phases. The characterization is based on the magnetic and hyperfine properties of the different phases, even for the complex phase diagram of the Fe-C system. Problems appear only when the phase under investigation contribut

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