Microstructure Evolution and Nitridation in an As-Cast 25Cr-35Ni-1Mo Radiant Tube After Long-Term Service

The microstructure evolution of 25Cr-35Ni-1Mo radiant tubes was investigated after approximately two years of service in a continuous annealing furnace. The inner and outer tube walls were exposed to N-2-containing combustion environment and protective atmosphere, respectively. The decarburization feature analysis indicated that carbon content decreased toward the tube surface. However, precipitates at the inner and outer walls were coarser, their content increased and the microhardness value also increased compared to that in the central area. X-ray diffraction illustrated that M-2(C,N) and M-6(C,N) were the dominant carbide at the inner and outer walls, and precipitates in the central area were mainly M23C6 and M-6(C,N) phases. The results were assumed to be associated with the nitridation phenomenon that occurred in the N-2-containing environment at elevated temperatures.