Abstract: The oxidation and carburizing mechanisms of Cr35Ni45 type pyrolysis furnace tubes serviced for different time(as-cast,1.5 a and 6 a) were systematically investigated by scanning electron microscopy(SEM),electron probe and X-ray diffraction(XRD).Compared with original uniform microstructure distribution,there are three zones including an oxidation layer,a carbide depletion zone and a carbide-rich zone at the subsurface region of the Cr35Ni45 tube inner wall after long time service. The oxidation behavior of the Cr35Ni45 tubes at high temperature consists of the external oxidation of chromium and the internal oxidation of silicon,and the outer oxidation layer is greatly affected by repeated destruction and reformation in decoking. The carburizing behavior of the Cr35Ni45 tubes mainly results from coking on the inner wall surface,and repeated destruction of the outer oxidation layer can aggravate this carburization. But due to auto-remediation of the outer oxidation layer,carburization of both the serviced tubes is at a lesser degree. Also,repeated destruction and reformation of the outer oxidation layer cause the depletion of Cr,the increase in critical concentration of C in the subsurface of the inner wall and carbide participation at the below region,leading to the formation of a carbide depletion zone and a carbide-rich zone.