Sodium-cooled Fast Reactor (SFR) possesses highest technology readiness level for deployment among six Gen-IV nuclear reactor designs intended to provide a low-carbon energy option and endure higher operating temperatures for longer service life (60-80 years). Thus, advanced materials developed for Gen-IV reactors should be able to withstand the harsh operating conditions allowing for safety improvement, efficiency enhancement and cost reduction. The advanced austenitic stainless-steel Alloy 709 (Fe-25wt.%Ni-20%Cr) is of current interest for structural applications in the SFRs owing to its desired set of properties including mechanical properties relative to conventional austenitic stainless steels. SFRs are subjected to on-load periods at elevated temperatures and thermal transients during startups and shutdowns, resulting in creep, fatigue, and creep-fatigue interaction as major considerations in the design of such high-temperature systems. In this work, high-temperature creep, fatigue and creep-fatigue behavior of the Alloy 709 are characterized along with microstructural examinations before and after mechanical testing. Creep tests were carried out at temperatures and stresses from 700 – 800 °C and 40 – 275 MPa, respectively, and the creep data were found to follow creep-power law with true stress exponent and activation energy of 4.9 ± 0.2 and 299 ± 15 kJ/mole, respectively. The microstructural observations of the crept specimens revealed different types of precipitates including Z-phases and the evidence of dislocation-precipitate interactions together with subgrain boundary formation. This suggests that high-temperature dislocation climb deformation is the rate-controlling creep mechanism in the alloy. Additionally, Larson-Miller Parameter (LMP) and Monkman–Grant relationships were developed using the creep rupture data.
Furthermore, strain-controlled cyclic loading tests were performed under constant strain rate of 2 × 10^-3 s^-1 at strain ranges varying from 0.3% to 1.2% at temperatures of 650 °C and 750 °C with tensile hold times of 0, 60, 600, 1,800, and 3,600 seconds. Introducing hold times at the maximum tensile strain resulted in creep regimes characterized by stress relaxation. The creep-fatigue life is found to decrease with increasing hold time, strain range, and temperature. According to ASME code, Linear Damage Summation rule was employed to predict the creep-fatigue life where creep damage was calculated using LMP and integration of the stress relaxation curve. Microstructural characterization of the deformed samples with no hold time indicated that fatigue is the dominant mode of deformation whereas with the hold times, both creep and fatigue contributed to the deformation of the alloy.
Financial support through DOE/NEUP program and KACST are gratefully acknowledged.
|Country/Int. organization||Saudi Arabia|
|Affiliation/Organization||King Abdulaziz City for Scince and Technology|
|Speaker's email firstname.lastname@example.org|