Microstructural analysis of Cr35Ni45Nb heat-resistant steel after a five-year service in pyrolysis furnace

Abstract To analyze the microstructure across thickness exposed to various environment of the pyrolysis furnace tubes, the sample of Cr35Ni45Nb heat-resistant steel from a five-year service tube was sliced into slices from inner to outer wall. Moreover, microhardness was examined to investigate the influence of the microstructure on the property of the heat-resistant steel. The results show that the primary carbides Cr 7 C 3 and NbC have transformed into chromium-rich Cr 23 C 6 and niobium-rich carbides G phase in the middle section of the aged pyrolysis furnace tube. The Cr 2 O 3 and SiO 2 layer covered the outer and inner surfaces, and the Cr-depleted zones were formed under the oxide layer of the inner and outer walls. The carbides of M 23 C 6 were transformed into M 7 C 3 type carbides in the carburization zone under the Cr-depleted zone of inner surface, while the M 7 C 3 type carbides also existed under the Cr-depleted zone of outer wall.