Microstructure Evolution and Nitridation in an As-Cast 25Cr-35Ni-1Mo Radiant Tube After Long-Term Service

Abstract

The microstructure evolution of 25Cr-35Ni-1Mo radiant tubes was investigated after approx- imately two years of service in a continuous annealing furnace. The inner and outer tube walls were exposed to N2-containing combustion environment and protective atmosphere, respec- tively. The decarburization feature analysis indicated that carbon content decreased toward the tube surface. However, precipitates at the inner and outer walls were coarser, their content increased and the microhardness value also increased compared to that in the central area. X-ray diffraction illustrated that M2(C,N) and M6(C,N) were the dominant carbide at the inner and outer walls, and precipitates in the central area were mainly M23C6 and M6(C,N) phases. The results were assumed to be associated with the nitridation phenomenon that occurred in the N2-containing environment at elevated temperatures. 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 anneal- ing furnace is now commonly used as reheating equip- ment, 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 com- bustion 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, and transformation of these precipitates. However, precip- itates initially formed in as-cast microstructures undergo morphological and chemical changes under service condi- tions such as high-temperature aging, corrosion, and creep. Then coarsening and phase transformation of these pre- cipitates 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