Damage law of HP-Nb alloy reformer tube in service process

The microstructure damage and mechanical properties degradation of HP-Nb alloy reformer tubes during service were studied. The microstructure evolution and oxidation damage of reformer tubes with different service time were analyzed by optical microscope and X-ray diffractometer. At the same time, its mechanical properties at room temperature, high temperature creep properties and high temperature relaxation resistance were studied. The results show that the primary carbides of HP-Nb alloy furnace tubes were coarsened by long-term high temperature service, and a continuous network structure was formed at the grain boundary. A large number of secondary carbides were also precipitated in the crystal, and grow with the extension of service time. The initial carbide was transformed from M₇C₃ carbide to M₂₃C₆ carbide. Internal oxidation occurred during the service of HP-Nb alloy, and the oxide film and Cr-depleted zone under the oxide film were formed on the inner and outer surfaces of the furnace tube. After long-term service, the room temperature tensile strength, plasticity and toughness, high temperature creep performance and high temperature relaxation resistance of the furnace tube were significantly reduced.