The Cementite Spheroidization Process in High-Carbon Steels with Different Chromium Contents
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INTRODUCTION
THE spheroidization annealing, which results in a microstructure of fine spherical cementite particles in a soft ferritic matrix,[1] is of significant interest for industrial applications. The main purpose of this process is to reduce the hardness of the material and thus provide a good machinability prior to further processing.[2] Two types of spheroidization annealing are often used. (1) Subcritical spheroidization below the A1 temperature, which is mainly applied for hypoeutectoid steels. During subcritical annealing of steels with an initial pearlite structure, the cementite lamellae break up into spheroids driven by the reduction in surface energy.[3,4] (2) Intercritical spheroidization above the A1 temperature, which is mainly applied for hypereutectoid steels in order to spheroidize and partially dissolve the grain boundary cementite. Therefore, the final microstructure of spheroidized hypereutectoid steels usually contains a bimodal distribution of cementite particles,[5,6] where large particles are located mainly on austenite grain boundaries. Numerical simulations of the intercritical spheroidization process in a high-carbon steel have shown that the rate of dissolution of the cementite lamellae also depends on the surrounding conditions.[7] This research will focus on the intercritical spheroidization annealing. The spheroidization process of hypereutectoid steels, for instance SAE 52100 bearing steel with 1 wt pct C,[5–9] typically consists of the following three steps: (1) N.V. LUZGINOVA, Postdoctoral Researcher, and L. ZHAO, Research Fellow, are with the Netherlands Institute for Metals Research and the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected] J. SIETSMA, Associate Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, Delft, The Netherlands. Manuscript submitted June 20, 2007. Article published online January 30, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
austenitization in the intercritical region at 1070 to 1100 K for at least 1 hour; (2) cooling from the intercritical temperature with a rate of 15 to 25 K/h to 1020 K, and further slow cooling through the A1 temperature to 950 K at cooling rates not faster than 5 to 10 K/h; and (3) air cooling from 1020 K to room temperature. Annealing above the A1 temperature but below the Acm temperature accomplishes an incomplete dissolution of cementite, then austenite with fine cementite particles normally transforms into a mixture of ferrite and cementite by either the pearlitic reaction[10] or by the divorced eutectoid transformation reaction (DET)[5,8,9,11] upon slow cooling. The latter leads to a soft microstructure of spherical cementite particles in a ferritic matrix. The main difference between these two reactions is that during the pearlitic reaction the product structure (pearlite) grows as a coupled pair of ferrite and cementite phases, whereas the DET is not a coupled reactio
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