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 M7C3zone, a M7C3–M23C6mixed zone, and a M23C6zone 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 Cr2O3scale tended to be carbonized to form carbide scale to spall from the surface in a reducing environment, while the SiO2kept stable all along. Once the oxide layers were removed or carbonized enough, inconceivable internal carburization occurred widely.