A Microscale Model for Ausferritic Transformation of Austempered Ductile Irons
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INTRODUCTION
DUCTILE cast iron, also known as nodular cast iron, is a C-Si-Fe-based alloy with a microstructure formed by graphite nodules and a metal matrix. In a continuous cooling process, the resulting matrix could be ferritic, pearlitic, or ferritic-pearlitic, whereas other microstructures, such as ausferritic and martensitic, may be formed by following specific cooling processes. In austempered ductile iron (ADI), the graphite nodules are embedded in an ausferritic matrix formed by ferrite subunits (lamellae, platelets, or disks) and carbon-rich austenite.[1,2] ADI is employed in several industrial applications, such as the fabrication of shafts, cams, and gears, due to its high strength, wear resistant, adequate fatigue response, and good toughness properties.[3] The ausferritic matrix is currently obtained by means of two different heat treatments. The first one is a three-step heat treatment,[4–7] in which a ductile cast iron is heated from ambient temperature up to the austenitization temperature Tc in order to obtain a full austenitic matrix with the appropriate carbon content
ADRIA´N D. BOCCARDO, Ph.D. Student, and PATRICIA M. DARDATI, Professor, are with the Department of Mechanical Engineering-GIDMA, CONICET, Facultad Regional Co´rdoba, Universidad Tecnolo´gica Nacional, Maestro M. Lopez esq. Cruz Roja Argentina, Co´rdoba, Argentina. Contact e-mail: [email protected] DIEGO J. CELENTANO, Professor, is with the Department of Mechanical and Metallurgical Engineering, Research Center for Nanotechnology and Advanced Materials (CIEN-UC), Pontificia Universidad Cato´lica de Chile, Vicun˜a Mackenna 4860, Santiago de Chile, Chile. LUIS A. GODOY, Director, is with the Institute for Advanced Studies in Engineering and Technology, FCEFyN, CONICET-Universidad Nacional de Co´rdoba, Av. Ve´lez Sarsfield 1611, Co´rdoba, Argentina. Manuscript submitted January 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
(austenitizing step). Then, the material is suddenly cooled down and kept at the austempering temperature TA to initiate the austempering process (quenching and austempering steps). The three-step heat treatment is used, for example, for ductile cast iron with a ferritic, pearlitic, or ferritic-pearlitic initial matrix. The second procedure employed in practice is an in situ heat treatment,[3,8] in which the austenitizing step is absent because after solidification the material is cooled down and kept at the desired austenitization temperature in order to obtain austenite with an appropriate carbon content. As in the three-step heat treatment, the material is then suddenly cooled down and kept at the austempering temperature to initiate the austempering process. In both heat treatments discussed above, the cooling rate from Tc to TA and the chemical composition play a key role. If the cooling rate is not high enough, the reconstructive ferritic and pearlitic transformations are not avoided. In this situation, the hardenability of the ductile cast iron could be improved with the addition of some
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