Influence of Carbide Morphology and Microstructure on the Kinetics of Superficial Decarburization of C-Mn Steels
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DECARBURIZATION is an important process during heat treatment of steels. It occurs at high temperatures in contact with substances that promote the oxidation of carbon, typically water vapor and hydrogen-containing atmospheres,[1,2] through the reactions, respectively: C þ H2 O $ CO þ H2
½1
C þ 2H2 $ CH4
½2
HENRIQUE DUARTE ALVARENGA, Ph.D. Student, is with the Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium, and also with the Department of Metallurgy and Materials Science, Universiteit Gent University, Technologiepark 903, 9052 Ghent, Belgium. Contact e-mail: [email protected] TOM VAN DE PUTTE and NELE VAN STEENBERGE, Researchers, are with ArcelorMittal Global R&D Gent - OCAS, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium. JILT SIETSMA, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands, and also with the Department of Metallurgy and Materials Science, Universiteit Gent University. HERMAN TERRYN, Professor, is with the Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, and also with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted January 16, 2014. Article published online October 8, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
Decarburization takes place during hot-rolling processes, especially in the production of springs, rods, and bars, and it changes the surface properties which may lead to failure of the component.[3,4] In other applications such as electrical steels and enameling, decarburization is intentionally induced during annealing in a reducing atmosphere, producing steel strip with very low-carbon concentration.[5–8] Because of its common occurrence, a good understanding of the mechanisms and the kinetics of decarburization is of essential importance, to improve quality and reduce costs of the processes. For an initial microstructure that contains ferrite and carbides, and depending on the temperature, the physical mechanism that leads to decarburization is rather complex and can be divided into three different stages: dissolution of the carbides, the carbon diffusion through the ferrite and austenite phases, and the surface reaction. Surface reaction and carbon diffusion determine the decarburization width under atmospheres with low and high humidity, respectively.[7,9–15] The overall diffusion of carbon in steel depends on the phases present at given temperatures, as shown in Figure 1 for hypoeutectoid steels. At a temperature above A3 for pure iron, or 1185 K (912 C), ferrite is not thermodynamically stable at lowcarbon concentrations and the only phase present is austenite. In this case, the carbon can be assumed to be completely dissolved in austenite and the effect of the ferrite (a) to austenite (c) transformation during decarburization can be ignored. Under this condition, the VOLUME 46A, JANUARY 2015—123
decarburi
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