Stress-Controlled Creep-Fatigue of an Advanced Austenitic Stainless Steel at Elevated Temperatures (original) (raw)
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High-temperature effects on creep-fatigue interaction of the Alloy 709 austenitic stainless steel
International Journal of Fatigue, 2021
High-temperature creep-fatigue behavior of the Alloy 709 is investigated by performing strain-controlled creep-fatigue tests at 650 • C and 750 • C with tensile hold times of 0, 60, 600, 1,800 and 3,600 s at 1% strain range and strain rate of 2 × 10 − 3 s − 1. Results revealed that creep-fatigue life at 650 • C fluctuates due to Dynamic Strain Aging (DSA) during hold periods. Linear damage summation (LDS) was employed to construct the creep-fatigue interaction diagram of the alloy at different loading conditions. An increased density of cracks and creep cavities is observed at 750 • C with less dislocation density compared to 650 • C.
Creep-Fatigue Interaction of an Advanced Austenitic Stainless Steel (Alloy 709)
ANS Global/Top Fuel Conference, 2019
Generation IV nuclear reactors are designed to be safer, more reliable, have a longer lifetime and more efficient than existing nuclear reactors. An example of Generation IV nuclear reactors is Sodium-Cooled Fast Reactor (SFR) that is expected to operate at about 550 o C. The structural materials for SFR should have superior mechanical properties that can withstand much harsher operating conditions of higher temperatures, radiation doses, and corrosive environments in the nuclear reactor core. Preliminary investigation on mechanical properties of a Fe-25wt.%Ni-20Cr (Alloy 709) austenitic stainless steel suggests it might be a potential candidate for SFR. Creep-fatigue interaction is expected to contribute to the degradation of many components such as reactor cladding, pressure vessels, and gas turbines operating at high temperatures in next-generation nuclear reactors. Strain-controlled low-cycle fatigue tests were performed at strain amplitudes ranging from 0.15% to 0.6% at 750 °C in air following ASTM standard E2714-13 at a constant strain rate of 2 × 10 -3 s -1 . Also, at 750 °C, different hold times of 1, 10, 30 and 60 minutes were introduced at the maximum tensile strain to investigate the effect of hold time on the fatigue life at strain amplitude of 0.5%, while 10, 30 and 60 minutes were introduced at strain amplitude of 0.3%. During low-cycle fatigue tests, fatigue life was found to decrease with increase in strain range and hold time. The fractographs of the deformed samples are characterized and presented.
Material Science and Engineering: A, 2020
To understand high temperature creep-fatigue interaction of the Alloy 709, strain-controlled low-cycle fatigue (LCF) tests were performed at strain ranges varying from 0.3% to 1.2% with fully reversible cycle of triangular waveform at 750 � C. In addition, different hold times of 60, 600, 1800 and 3600 s were introduced at the maximum tensile strain to investigate the effect of creep damage on the fatigue-life at strain range of 1% at 750 � C. The creep-fatigue life and the number of cycles to macro-crack initiation and failure are found to decrease with increasing hold time indicating higher crack initiation and growth rates. Creep-fatigue life is evaluated by a linear summation of fractions of cyclic and creep damages according to ASME code. The frac-tographs of the samples deformed at 1% strain range indicated that fatigue might have been the dominant mode of deformation whereas, for the samples deformed at the same strain range with different hold times, both fatigue and creep have contributed to the overall deformation and fracture of the alloy.
Materials at High Temperatures, 2020
Since the preliminary data suggest that Fe-25Ni-20Cr austenitic stainless steel (Alloy 709) is an excellent candidate as a structural material for high-temperature applications such as Sodiumcooled Fast Reactor (SFR), the effect of strain range on creep-fatigue interaction of the Alloy 709 is investigated by conducting strain-controlled creep-fatigue tests with tensile hold times of 0, 600, 1,800 and 3,600 s at strain ranges varying from 0.6% to 1.2% at 750°C and 2 × 10 −3 s −1 strain rate. Strain-controlled fatigue tests were performed at strain ranges from 0.3% to 2.5% at 750°C and 2 × 10 −3 s −1 strain rate. The predicted fatigue life of Alloy 709 shows a better correlation with the characteristic slopes predictive method. With increasing strain range at a given hold time, the number of failure cycles decreases until saturation. The fractography of the deformed samples exhibited increased number of cracks with strain range along with M 23 C 6 precipitates and high dislocation density.
Materials Research
Stainless steels are well known by their corrosion resistance. The austenitic types, in particular, are also applied as structural components in engineering systems operating at high temperatures such as nuclear reactors, petrochemical furnaces and turbines. For these applications operational temperatures may go up to 800ºC. Under constant load applications the main mechanism of failure, which would limit the material's life, is creep. In the present work creep parameters were evaluated in the high temperature interval of 600 to 800ºC for an AISI 316 austenitic stainless steel. Dislocation substructures were observed by transmission electron microscopy in creep ruptured specimens. Two distinct mechanisms of dynamic strain aging and dynamic recovery associated with different values for the power law exponent n and the Arrhenius activation energy Q for creep were verified below and above 700ºC, respectively.
Challenges in Mechanics of Time Dependent Materials, Fracture, Fatigue, Failure and Damage Evolution, Volume 2”, proceedings of the 2019 Annual Conference on Experimental and Applied Mechanics, 2019
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, straincontrolled 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.
High Temperature Cyclic Deformation Behavior of an Advanced Austenitic Stainless Steel (Alloy 709)
19th International Conference on Environmental Degradation of Materials in Nuclear Power Systems –Water Reactors, 2019
Advanced structural materials are needed to improve the efficiency, safety and reliability of next-generation nuclear reactors. Preliminary investigation on mechanical properties of a Fe-25wt.%Ni-20Cr austenitic stainless steel (Alloy 709) suggests it might be a potential candidate for Sodium-Cooled Fast Reactor. High temperature cyclic deformation behavior including fatigue and creep-fatigue for the Alloy 709 is considered for design and safety purposes. Strain-controlled lowcycle fatigue tests were performed at strain amplitudes ranging from 0.15% to 0.6% at 750 °C in air following ASTM standard E2714-13, with a constant strain rate of 2 × 10-3 s-1. In addition, different hold times of 1, 10, 30 and 60 minutes 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, while 30 and 60 minutes were introduced at the maximum tensile strain at strain amplitude of 0.3% at 750 °C. During continuous cyclic loading, fatigue life is found to decrease with increase in strain amplitude, while the creep-fatigue life is found to decrease with increasing hold time indicating rapid initiation and propagation of cracks. The fractographs of the deformed samples are presented and discussed.
Evaluation of the creep–fatigue damage mechanism of Type 316L and Type 316LN stainless steel
International Journal of Pressure Vessels and Piping, 2008
Low-cycle fatigue tests with continuous cycling and creep-fatigue tests with 10 min hold times at tensile maximum strain were conducted at 600 1C in air for Type 316L and Type 316LN stainless steels containing nitrogen contents of 0.04% and 0.10%. The creep-fatigue life was less than the fatigue life for both alloys. The fatigue and creep-fatigue life and saturation stress were increased with the addition of nitrogen. The fracture mode was transgranular for fatigue and intergranular for creep-fatigue regardless of the nitrogen content. The dislocation structure was cellular for Type 316L and planar for Type 316LN after fatigue and creep-fatigue tests. Carbides were precipitated at grain boundaries after creep-fatigue tests and nitrogen decreased the precipitation. Creep-fatigue life was well predicted by a model based on cavity nucleation and growth at grain boundaries. The increase of creep-fatigue life with the addition of nitrogen was due to the decrease of precipitation and stress relaxation by the change in dislocation structure.
E n g i n e e r i n g S t r u c t u r a l I n t e g r i t y A s s e s s m e n t : f r o m p l a n t a n d s t r u c t u r e d e s i g n , m a i n t e n a n c e The R5 procedure uses a ductility exhaustion approach to calculate the creep damage observed in the heat affected zone (HAZ) of thick section 316H austenitic stainless steel weldments. The present work considers the influence of thermo-mechanical history on the creep ductility and creep damage accumulation under a condition of multiaxial stress state. A systematically designed pre-treatment procedure was used to introduce a range of microstructures, representative of those observed in the HAZ of these weldments. Double notched bar creep specimens were manufactured and used to investigate creep behaviour of 316H stainless steel at a temperature of 550°C. The rate of creep strain accumulation is shown to be a function of the thermo-mechanical pre-treatment. The accumulation of creep damage is correlated with the microstructure and stress state. Finally, the results are discussed with respect to the sensitivity of the currently used ductility exhaustion model to the key material inputs, in terms of the uniaxial creep ductility, multiaxial stress state and stress relaxation rate.