Microstructure Evolution and Nitridation in an As-Cast 25Cr-35Ni-1Mo Radiant Tube After Long-Term Service
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INTRODUCTION
IN the manufacturing process for the cold-rolled sheet of steels, steel strips are submitted to an annealing furnace section after cold rolling in order to obtain required mechanical properties. The continuous annealing furnace is now commonly used as reheating equipment, in which the annealing process involves passing steel strips continuously through a heating furnace filled with classical N2-H2 protective atmosphere. In the continuous annealing process, radiant tubes are used as indirect heating equipment. Heat is transferred from combustion gas to the radiant tube, and then it radiates energy to the load, without any direct flame or combustion exhaust coming in contact with the load. Centrifugally cast Fe-Ni-Cr based heat-resistant steels[1] are widely used in heat treatment applications, such as radiant tubes, for their good exhibition of mechanical property and corrosion resistance at high temperatures. The microstructure of these centrifugally cast tubular materials consists of austenite dendrites delineated by a network of eutectic carbides.[2–4] As a consequence of the high Cr content in this class of steels, various types of those carbides can be precipitated. For example, precipitation of Cr-rich carbide M7C3 and M23C6 is very common, and the addition of stabilizing elements such as Ti, Nb, and V usually results in MC type carbides,[5,6] and so on. Detailed mechanical properties of these heat-resistant steels are dependent on the stability of the microstructure, particularly the formation, coarsening,
ZHICHAO ZHU and CHUNHUI LIU, Master Students, CONGQIAN CHENG, Postdoctor, and JIE ZHAO, Professor, are with the School of Material Science and Engineering, Dalian University of Technology, Dalian 116085, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted November 1, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
and transformation of these precipitates. However, precipitates initially formed in as-cast microstructures undergo morphological and chemical changes under service conditions such as high-temperature aging, corrosion, and creep. Then coarsening and phase transformation of these precipitates occur. Those changes, with few exceptions, are undesirable and they can be detrimental to the corrosion resistance and mechanical properties of heat-resistant steels. Precipitation behaviors in austenitic stainless steels were studied extensively, and detailed information of such phases can be followed in the literature of other workers.[7–9] Despite the long history of the use and research of these tubular materials based on this wellknown ternary alloy system, small changes in the chemical composition or thermomechanical processing can profoundly influence the microstructure evolution. The presence of multiple phases derived from the addition of small quantities of components in these alloys introduces more variables, and the study of hightemperature behavior including microstructural evolution of the multiphase alloy that occur after prolonged exposure to serv
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