Numerical Simulation of Austempering Heat Treatment of a Ductile Cast Iron
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NODULAR cast iron is currently employed in industry because of its improved mechanical properties and facility to cast. Its microstructure is formed by graphite nodules which are embedded in a metal matrix. In an as-cast nodular cast iron, the matrix may be ferritic, ferritic-pearlitic, or pearlitic. The mechanical properties of nodular cast iron, such as yield strength and hardness, may be improved by means of an austempering heat treatment. In an austempered ductile iron, the nodules are embedded in an ausferrite matrix formed by ferrite platelets and carbon-enriched austenite. To perform an austempering treatment, the material is initially heated up and kept at the austenitizing temperature Tc between 1123 K and 1223 K (850 °C and 950 °C) in order to transform the initial matrix into one which is completely austenitic and with the
ADRIA´N D. BOCCARDO, Ph.D. Student, and PATRICIA M. DARDATI, Professor, are with the Department of Mechanical Engineering-GIDMA, Facultad Regional Co´rdoba, Universidad Tecnolo´gica Nacional, Maestro M. Lopez esq. Cruz Roja Argentina, Co´rdoba, Argentina. Contact e-mail: [email protected]. edu.ar DIEGO J. CELENTANO, Associate Professor, is with the Department of Mechanical and Metallurgical Engineering, Pontificia Universidad Cato´lica de Chile, Research Center for Nanotechnology and Advanced Materials (CIEN-UC), Vicun˜a Mackenna 4860, Santiago de Chile, Chile. LUIS A. GODOY, Director, is with the Institute for Advanced Studies in Engineering and Technology, CONICET-Universidad Nacional de Co´rdoba, FCEFyN, Ve´lez Sarsfield 1611, Co´rdoba, Argentina. MARCIN GO´RNY, Associate Professor, and EDWARD TYRAŁA, Adjunct, are with the Department of Engineering of Cast Alloys and Composites, Faculty of Foundry Engineering, AGH University of Science and Technology, Reymonta 23, Krakow, Poland. Manuscript submitted July 17, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS B
appropriate carbon content;[1,2] see Figure 1. Next, the material is suddenly cooled down to the austempering temperature TA between 523 K and 723 K (250 °C and 450 °C) to initiate the austempering process. The chosen temperature TA depends on the desired microstructure (lower or upper ausferrite). Finally, the part is cooled down up to the ambient temperature Tamb . The austempering process has two stages.[3,4] In the first stage, the ausferritic transformation takes place. At the end of the first stage, the ferrite platelets volume fraction and austenite carbon concentration are at a maximum. Then, the second stage begins with carbide precipitation from austenite. The time interval between these two stages is called processing window, as shown in Figure 1. The extent of the processing window changes due to the presence of chemical segregation. Because the transformations occur at different times in different regions of the sample, the effect of heat treatment can be optimized within the processing window. In industrial parts with large chemical segregation, it is difficult to define an ideal austempering time for the wh
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