Investigation of Cr34Ni45 Ethylene Cracking Furnace Tube in Service

The structure and mechanical property changes of Cr34Ni45 ethylene cracking furnace tube serviced for different time (as-cast, 1.5 years and 6 years) are systematically investigated by using SEM, EMPA, XRD and mechanics performance test methods. The microstructure of the as-cast alloy consists of austenitic matrix, NbC and M7C3 primary carbides. Eutectic M7C3 transform into M23C6, and the NbC transform into Nb3Ni2Si which is so-called η-phase during service, accompany with precipitation of secondary M23C6. Furthermore, comparing to original uniform microstructure distribution, there are three zones including oxidation layer, carbides depletion zone and carbides-rich zone at the subsurface region of the inner-wall of Cr35Ni45 tubes after long time service. Oxidation behaviors at high temperature consist of external oxidation of chromium and internal oxidation of silicon. The carburizing behavior of Cr35Ni45 tube is mainly caused by the coking on the surface of inner-wall, but carburization of the serviced tubes are both at lesser-degree because of the auto-remediation of outer oxidation layer. High temperature service heavily reduced mechanical performance of the tubes, and fracture mode transform from a mixed mode to a brittle one during service.