Aging-Induced Microstructural Evolution and Fracture Mechanisms of 35Cr45NiNb Alloy Under High-Temperature Tensile Deformation

Centrifugally cast 35Cr45NiNb alloy has been widely employed in ethylene-cracking furnace tubes owing to its excellent carburization and creep resistance. However, the influence of microstructural degradation and temperature on its high-temperature tensile behavior remains poorly investigated. In this study, an accelerated aging method at 1200 °C for 230 h (A1) and 430 h (A2) was employed to simulate approximately 4 and 8 years of service at 1050 °C, based on the Larson-Miller parameter. The equivalence was validated by the nearly identical precipitate area fractions of the A1 specimen (16.6%) and an ex-service specimen (14.8%). Combined with SEM and EBSD characterization, tensile tests at 950, 1000, and 1050 °C were conducted to elucidate the relationship between microstructure and high-temperature tensile properties. During aging, the skeletal interdendritic M7C3 carbides transformed into blocky M23C6, NbC evolved into the brittle G-phase (Ni16Nb6Si7), fine secondary M23C6 precipitates formed, and the initially continuous primary-carbide network progressively coarsened. Yield and ultimate tensile strengths decreased monotonically with increasing temperature, whereas aging produced pronounced hardening at the expense of ductility, as secondary-carbide precipitation strengthening outweighed the weakening of the primary carbide network. The fracture mode transitioned from mixed quasi-cleavage fracture at 950 °C, initiated by stress concentration at coarse phase interfaces, to ductile rupture at 1000 and 1050 °C. GND analysis further revealed an aging-dependent transition in the dominant deformation mechanism, from dislocation pile-up at the carbide network, to recrystallization after prolonged aging.