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

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 Cr7C3 and NbC have transformed into chromium-rich Cr23C6 and niobium-rich carbides G phase in the middle section of the aged pyrolysis furnace tube. The Cr2O3 and SiO2 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 M23C6 were transformed into M7C3 type carbides in the carburization zone under the Cr-depleted zone of inner surface, while the M7C3 type carbides also existed under the Cr-depleted zone of outer wall.