Microstructure evolution and residual life assessment of service exposed Cr35Ni45 radiant tube alloy

Abstract The degradation of material microstructure was frequently encountered in radiant tubes during high temperature service, resulting in their premature failure. Effect of service time on the phase transformation and oxidation behavior of the Cr35Ni45 radiant tubes was analyzed using scanning electron microscopy (SEM), electronic probe coupled with wavelength dispersive spectrometer, X-Ray diffraction (XRD) and transmission electron microscopy (TEM). The orthorhombic M 7 C 3 and the face-centered cubic NbC primary carbides in the as-cast tube transformed into the face-centered cubic M 23 C 6 and the face-centered cubic G phase, respectively, during service at 1000 °C. Metallographic observation showed that a continuous Cr 2 O 3 layer and a discontinuous SiO 2 layer formed on the inner surface of the radiant tubes due to the oxidizing environments, and a precipitate free region was found beneath the oxide layers. Mechanical properties of the tubes decreased with the increasing degree of microstructure degradation as service time prolongs. Residual life assessment of the serviced tubes was conducted by nonlinear fitting Larson–Miller parameter versus rupture testing stress. In spite of lower mechanical properties for the serviced tubes compared to as-cast tube, residual life assessment indicates that they have considerable residual life at the service temperature.