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Investigations on carburizing mechanisms of Cr35Ni45Nb subjected to different service conditions in a high-temperature vacuum environment

The carburizing behaviors and mechanisms for Cr35Ni45Nb alloy subjected to different service conditions were studied in a high-temperature vacuum environment. Generally, the carburizing process of an alloy is always accompanied by diffusional heterogeneous reactions regardless of the service condition of the alloy. For a carburized original tube, there is a layered structure at the inner wall of the tube, which is comprised of a M 7 C 3 zone, a M 7 C 3 –M 23 C 6 mixed zone, and a M 23 C 6 zone with different morphologies. However, for a 6-year tube (short for a tube serviced for 6 years), the composite oxide layers formed previously act as effective barriers to carbon infiltration. Moreover, the Cr 2 O 3 scale tended to be carbonized to form carbide scale to spall from the surface in a reducing environment, while the SiO 2 kept stable all along. Once the oxide layers were removed or carbonized enough, inconceivable internal carburization occurred widely.