Influence of Copper Addition and Temperature on the Kinetics of Austempering in Ductile Iron
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I.
INTRODUCTION
AUSTEMPERED ductile iron (ADI) is a highly attractive material for technical applications.[1–4] The ADI, which is a nodular cast iron that undergoes a specially designed heat treatment, offers a good alternative to steels and aluminum alloys when high strength, good ductility, and low wear are required. The properties of ADI depend on the weight fractions and the morphology of ferrite and austenite in the microstructure formed during austempering. The final microstructure strongly depends on the initial composition and phase transformation kinetics during austempering. The typical chemical composition of the ductile iron is Fe-3.5 pct C-2.5 pct Si-0.4 pct Mn-0.05 pct Mg, but Cu might also be used to control the pearlitic transformation during the heat treatment. The heat treatment consists of the following two stages: (1) austenization at high temperature ranging between 1123 K (850 C) to 1223 K (950 C) for the period YOGEV AMRAN, formerly Graduate Student, Department of Materials Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel, is now Engineer, Israel Institute of Metals, Technion R&D Foundation, Haifa 32000, Israel. Contact e-mail: yogev1@ technion.ac.il ALEXANDER KATSMAN, Senior Researcher, and MENACHEM BAMBERGER, Professor, are with the Department of Materials Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel. PETER SCHAAF, Professor, is with the Department of Materials for Electronics, Institute of Materials Engineering and Institute of Micro- and Nanotechnologies, Ilmenau University of Technology, 98684 Ilmenau, Germany, and with the University of Go¨ttingen, II. Institute of Physics, 37077 Go¨ttingen, Germany. Dedicated to the memory of Professor B.Z. Weiss, Technion, Israel, and Professor U. Draugelates, TU Clausthal, Germany. Manuscript submitted November 18, 2009. Article published online June 8, 2010. 1052—VOLUME 41B, OCTOBER 2010
required to ensure that the matrix is fully austenitic, followed by rapid cooling to the austempering temperature; and (2) austempering—holding at a temperature TA below the bainite start temperature, in the range of 523 ‚ 723 K (250 ‚ 450 C), for a long time until the required microstructure in achieved. Finally, the samples are cooled by water quenching. At the austempering temperature, the following two reactions may take place in a sequential order[1]: (a) Ausferrite transformation, in which the homogenous austenitic matrix transforms to ausferrite—a mixture of ferrite (a, bcc structure) and high carbon austenite (hc-c, fcc structure). (b) After a long enough dwell time, the hc-c decomposes to ferrite and iron carbides, the microstructure known as bainite. The carbides reduce the mechanical properties[1–3] of ADI, and therefore, the second reaction should be eliminated. The final ADI microstructure is composed of spheroidal graphite embedded in the ausferrite matrix. Small amounts of martensite and/or carbides may also be present in the microstructure. The weight fractions of ferrite and high-carbon c in the
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