High Temperature Low-Cycle Fatigue and Creep-Fatigue Behavior of Fe-25Ni-20Cr Austenitic Stainless Steel

摘要

Preliminary investigation on mechanical properties of a Nb-strengthened and nitrogen-stabilized Fe-25wt.%Ni-20Cr (Alloy 709) advanced austenitic stainless steel suggests that it might be a potential candidate for Sodium-Cooled Fast Reactor (SFR), which has higher technology readiness level for deployment. However, the creep-fatigue deformation behaviour is unknown for this alloy. To understand high temperature creep-fatigue interaction of the Alloy 709, strain-controlled low-cycle fatigue (LCF) tests were performed at strain amplitudes ranging from 0.15% to 0.6% with fully reversible cycle of triangular waveform at 750°C in air following ASTM standard E2714-13. In addition, different hold times of 1, 10, 30 and 60 min were introduced at the maximum tensile strain to investigate the effect of the creep damage on the fatigue-life at strain amplitude of 0.5% at 750°C. During continuous cyclic loading, fatigue life is found to decrease with increase in strain amplitude. The creep-fatigue life and the number of cycles to crack initiation are found to decrease with increasing hold time indicating the rapid initiation and propagation of cracks. The fractographs of the samples deformed at 0.5% strain amplitude indicated that fatigue might have been the dominant mode of deformation whereas, for the sample deformed at the same strain amplitude with different hold times, both fatigue and creep have contributed to the overall deformation of the alloy. Further studies are underway to carry out creep-fatigue tests at different hold times, strain ranges, and temperatures as well as microstructural characterization of the samples following deformation.