Effect of Strain Range on High Temperature Creep-Fatigue Behaviour of Fe-25Ni-20Cr (wt.%) Austenitic Stainless Steel (Alloy 709 (original) (raw)
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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.
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.
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.
Stress-Controlled Creep-Fatigue of an Advanced Austenitic Stainless Steel at Elevated Temperatures
Materials, 2022
Creep–fatigue interaction occurs in many structural components of high-temperature systems operating under cyclic and steady-state service conditions, such as in nuclear power plants, aerospace, naval, and other industrial applications. Thus, understanding micromechanisms governing high-temperature creep–fatigue behavior is essential for safety and design considerations. In this work, stress-controlled creep–fatigue tests of advanced austenitic stainless steel (Alloy 709) were performed at a 400 MPa stress range and 750 C with tensile hold times of 0, 60, 600, 1800, and 3600 s, followed by microstructural examinations. The creep–fatigue lifetime of the Alloy 709 was found to decrease with increasing hold time until reaching a saturation level where the number of cycles to failure did not exhibit a significant decrease. Softening behavior was observed at the beginning of the test, possibly due to the recovery of entangled dislocations and de-twining. In addition, hysteresis loops showed ratcheting behavior, although the mean stress was zero during creep fatigue cycling, which was attributed to activity of partial dislocations. Microstructural examination of the fracture surfaces showed that fatigue failure dominated at small hold times where the cracks initiated at the surface of the sample. Larger creep cracks were found for longer hold times with a lower probability of dimpled cavities, indicating the dominance of creep deformation. The results were compared with other commonly used stainless steels, and plausible reasons for the observed responses were described.
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.
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.
Effect of dynamic strain aging on high temperature properties of austenitic stainless steel
Materials Science and Engineering: A, 2004
18-8 type austenitic stainless steel has been subjected to tension test at a range of strain rates (5 × 10 −4 -5 × 10 −2 s −1 ) and temperatures (298-973 K) to investigate the effect of temperature and strain rate on its mechanical properties. It was found that the serrated flow behavior of this material, one of the evident features of dynamic strain aging (DSA) phenomenon can be divided into two sections according to the temperatures, i.e. the range of 523-673 K and 723-873 K at the strain rate of 5 × 10 −4 s −1 . Within these two temperature ranges, one can find some similar rules, for example, the critical strain ε c for the onset of serrations reduces with increasing temperature. By calculating the activation energy of the DSA process of the material, it is found that the DSA at lower temperature range is caused by the interaction between (C, Ni) solute atom atmosphere and dislocation, and the DSA at higher temperature range is caused by the interaction between (C, Cr) solute atom atmosphere and dislocations. In order to further investigate the influence of DSA on high temperature strength of the material, the static tensile tests at temperatures of 573, 723, and 873 K, creep tests were also carried out, respectively, after various DSA pre-treatments and cold-working processes. The results show that the high temperature short-time strength and 873 K creep rupture strength of the specimens increase with increasing pre-strain temperature and pre-strain, and the strengthening effect of DSA is higher than that of cold-working at the same pre-strain.
Creep and tensile behaviour of austenitic Fe–Cr–Ni stainless steels
Materials Science and Engineering: A, 2009
The control of creep behaviour during service of reformer tubes made of HP-40 austenitic stainless steels is still limited by the knowledge of creep mechanisms in these alloys. Two different HP-40 alloys modified with a low-level addition of Nb were studied. Creep tests were carried out at 980 and 1050 • C with different stress levels, in the range of 20-50 MPa, and their results were plotted in a Norton-type diagram. Also, low strain rate tensile tests were performed at temperature of 950, 980 or 1000 • C. As low strain rate tensile tests showed a plateau at nearly constant stress for a given strain rate, they could be somehow linked with creep tests. Accordingly, tensile and creep results were plotted together on a Larson-Miller (LMP) diagram. The fracture modes of tensile and creep samples were investigated and the effect of different parameters such as sample dimensions, temperature and atmosphere, was also studied.
Dynamic strain aging of austenitic stainless steels and Ni-base alloys
2010
Dynamic strain aging (DSA) affects mechanical properties of materials and promotes strain localization. DSA results in serrated plastic flow, which was observed by means of constant extension rate tensile (CERT) tests in different grades of AISI 316 austenitic stainless steel and Ni-base alloys Alloy 600 and Alloy 690 in the temperature range of 200-600 o C and values of strain rate from 10-6 to 10-3 s-1. Negative strain rate sensitivity was reported for the studied materials at temperatures and strain rates where serrated plastic flow appeared. The map for the occurrence of serrated flow as a function of strain rate and temperature was built for the materials. The activation enthalpies of dynamic strain aging appearance were found to be 120 kJ/mol for the austenitic stainless steel and 159 kJ/mol for both Ni-base alloys. The internal friction (IF) peak associated with the interstitial atoms of nitrogen and carbon in the solid solution of AISI 316NG austenitic stainless steel and Alloy 600 was reported. The activation enthalpies of nitrogen diffusion in AISI 316NG steel (140 kJ/mol) and carbon diffusion in Alloy 600 (162 kJ/mol), obtained by means of internal friction, correspond well to the activation enthalpies of DSA appearance. The height of the internal friction peak increases depending on the pre-straining conditions in the similar way for AISI 316NG steel and Alloy 600. Annealing in situ at the IF peak temperature results in the decay of the IF peak enhanced by pre-straining for both materials. At the initial stage, the peak annealing processes can be described as an exponential decay function with characteristic times of 0.95 and 0.92 ks for AISI 316NG steel and Alloy 600, respectively. Transmission electron microscopy was performed on the specimens of AISI 316NG steel after CERT tests at temperatures of 400 and 200 o C, where serrated and smooth plastic flow was observed, respectively. Long-range planarity was observed in the dislocation structures of the specimen tested at 400 o C. The microstructure of the specimen strained at 200 o C exhibited cellular dislocation structure. It was concluded, that the diffusive redistribution of interstitial atoms in the DSA regime affected the deformation behavior of the material by restricting dislocation cross-slip, which in turn promotes strain localization, affecting the mechanical performance of the material.