Sensitivity of Austempering Heat Treatment of Ductile Irons to Changes in Process Parameters
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NTRODUCTION
AUSTEMPERED ductile iron (ADI) is an metallic alloy with a microstructure at ambient temperature formed by graphite nodules inserted in a metallic matrix called ausferrite, the latter being a microconstituent formed by ferrite subunits embedded in austenite. Because of several advantages identified with the use of ADI, such as a wide range of mechanical properties, casting and machining simplicity, there is an increasing
A.D. BOCCARDO is with the Instituto de Estudios Avanzados en Ingenierı´ a y Tecnologı´ a, IDIT, CONICET-Universidad Nacional de Co´rdoba, Ve´lez Sarsfield 1611, Co´rdoba, X5000, Argentina and also with the Grupo de Investigacio´n y Desarrollo en Meca´nica Aplicada, GIDMA, Facultad Regional Co´rdoba, Universidad Tecnolo´gica Nacional, Maestro M. Lopez esq. Cruz Roja Argentina, Co´rdoba, X5000, Argentina. Contact e-mail: [email protected] P.M. DARDATI is with the Grupo de Investigacio´n y Desarrollo en Meca´nica Aplicada, GIDMA, Facultad Regional Co´rdoba, Universidad Tecnolo´gica Nacional. L.A. GODOY is with the Instituto de Estudios Avanzados en Ingenierı´ a y Tecnologı´ a, IDIT, CONICET-Universidad Nacional de Co´rdoba and also with the Facultad de Ciencias Exactas, Fı´ sicas y Naturales, Universidad Nacional de Co´rdoba, Ve´lez Sarsfield 1611, Co´rdoba, X5000, Argentina. D.J. CELENTANO 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, 7820436, Chile. Manuscript submitted July 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
use of this material in the automotive and in the agricultural industries. ADI can be obtained from a conventional ductile iron (DI) which is subsequently subjected to the so-called three-step austempering heat treatment.[1–3] The steps of this heat treatment are as follows: In the first step, the austenitization takes place in which the part is heated up and kept at the austenitizing temperature Tc (1123 K (850 C) Tc 1223 K (950 C)) in order to transform the initial ferritic-pearlitic matrix into a completely austenitic matrix and with the appropriate carbon content. In the second step, the part is cooled down and kept at the austempering temperature TA ( 523 K (250 C) TA 723 K (450 C)) to develop the austempering process. In the third step, the part is cooled down to the ambient temperature Tamb . The durations of the first and second steps are, respectively, called the austenitization (tc ) and austempering (tA ) times, as shown in Figure 1. In the three-step heat treatment, it is possible to identify parameters of different nature related to (a) the thermal cycle (austenitizing and austempering temperatures, austenitization and austempering times, cooling rate when the part is cooled down from Tc to TA ), (b) the DI initial microstructure (including the graphite nodule count, type of initial matrix, and chemical composition), and (c) the part geometry; all of them having
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