Anelasticity in austenitic stainless steel (original) (raw)
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Internal strains between grains during creep deformation of an austenitic stainless steel
Journal of Materials Science
Internal strains that develop between grains during creep of an austenitic stainless steel were measured using in situ neutron diffraction. The secondary creep prestrained test specimens were considered. Measurements were undertaken before, during and post creep deformation at 550 �C. There was no measurable change of internal strains between grains during in situ creep for 4 h at 550 �C. In addition, the effect of increasing/reducing temperatures in a range from 470 to 550 �C on the internal strains was measured and interpreted with respect to contributions from thermal expansion/contraction. No further internal misfit strains between grains were created when specimen crept during the dwell time at 530, 510, 490 and 470 �C. Results are discussed with respect to (i) the general structure of self-consistent models and (ii) the optimised use of neutron sources for creep studies.
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.
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.
Microstructure Evolution During Creep of Cold Worked Austenitic Stainless Steel
IOP Conference Series: Materials Science and Engineering, 2018
The 14Cr-15Ni austenitic stainless steel (SS) with additions of Ti, Si, and P has been developed for their superior creep strength and better resistance to void swelling during service as nuclear fuel clad and wrapper material. Cold working induces defects such as dislocations that interact with point defects generated by neutron irradiation and facilitates recombination to make the material more resistant to void swelling. In present investigation, creep properties of the SS in mill annealed condition (CW0) and 40 % cold worked (CW4) condition were studied. D9I stainless steel was solution treated at 1333 K for 30 minutes followed by cold rolling. Uniaxial creep tests were performed at 973 K for various stress levels ranging from 175-225 MPa. CW4 samples exhibited better creep resistance as compared to CW0 samples. During creep exposure, cold worked material exhibited phenomena of recovery and recrystallization wherein new strain free grains were observed with lesser dislocation network. In contrast CW0 samples showed no signs of recovery and recrystallization after creep exposure. Partial recrystallization on creep exposure led to higher drop in hardness in cold worked sample as compared to that in mill annealed sample. Accelerated precipitation of carbides at the grain boundaries was observed during creep exposure and this phenomenon was more pronounced in cold worked sample.
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.
Creep behavior of a new precipitation-strengthened 15Cr–15Ni austenitic stainless steel with optimized content of Nb, C, N and Mo, subjected to a special multicycled aging-quenching heat treatment process, was investigated at a temperature of 750 1C and stress range of 78–200 MPa for up to 10,000 h. The steel exhibited excellent creep rupture strength which exceeds even that of the commercial NF709 alloy. The value of the creep exponent n at the applied testing temperature/stress conditions was found to be of around 5.6, demonstrating that dislocation creep is the dominant deformation mechanism during the performed creep tests. The crept microstructures showed the presence of a high number of copper precipitates, and of fine dispersion of densely distributed intragranular nano-sized (Nb,Cr)N nitride precipitates which transformed to more coarsening-resistant Z-phase precipitates with the prolonging creep exposure time. The nitrides, occurring in dominant quantities in the microstructures, showed superior coarsening-resistance during creep exposure while copper precipitates exhibited a relatively high coarsening rate. A comparison between creep rupture strength of the studied steel and of its commercial equivalent grade SUS 304 JI HTB revealed that the nano-sized (Nb,Cr)N nitrides and/or Z-phase precipitates essentially played the key role in the observed improved resistance to dislocation creep of the material. Considerations over the observed creep behavior and aspects of high-temperature microstructural stability such as coarsening of strengthening precipitates, issues related to phase transformation processes suggested that the steel in the present study exhibits capability for applications in USC fossil power plants with steam parameters of around 700 1C/35–40 MPa.
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.
Analysis of creep deformation and damage behaviour of 304HCu austenitic stainless steel
Materials at High Temperatures, 2019
Creep deformation, damage and rupture behaviour of 304HCu austenitic stainless steel have been studied at temperatures 923, 973 and 1023 K over the stress range 100-240 MPa. The material exhibited relatively short primary stage of creep deformation followed by secondary (steady-state creep) stage and relatively extensive tertiary stage of creep deformation. The transient creep deformation is analysed based on the Garofalo relationship, ε ¼ ε 0 þ ε T 1 À exp Àr 0 :t ð Þþ_ ε s :t: The variations of (i) rate of exhaustion of transient creep ðr 0 Þwithsteadystate creep rate _ ε s ð Þ and time to onset of secondary creep and (ii) initial creep rate with _ ε s were found to obey power-law relationship with powers close to unity, thereby facilitating the development of master transient creep curve. The variation of _ ε s with stress and temperature obeyed Dorn's equation modified with back stress concept. The time to onset of tertiary creep is found to be proportional to rupture life (t r) while the damage tolerance factor λ ¼ ε f _ εs:tr decreased with increase in t r. In view of the prolonged tertiary creep, associated with microstructural change and intergranular creep cavitation, Kachanov-Rabotnov model has been used to assess the creep damage behaviour of the steel. The damage assessment coupled with finite-element analysis closely predicted the creep deformation and rupture life of the steel.
Exposure of stainless steel test specimens and components to loads at high temperature can result in both time dependent and time independent permanent deformation. As well as permanent deformation there are subtle changes to internal state of the material. These changes contribute to the mechanical behaviour of the material for subsequent loading cycles. In this research neutron diffraction has been undertaken to measure the internal state of a Type 316H austenitic stainless steel that had been subjected to prior tensile loading at 550 ºC. A set of specimens were subjected to tensile loading followed by straining to various stages of creep and finally unloaded. These tensile pre-loaded specimens were then subjected to room temperature in-situ tensile and compressive loading and neutron diffraction measurements were made at ENGIN-X at Rutherford Appleton Laboratory, UK and POLDI at the Paul Scherrer Institut, Switzerland. The experiments measured the internal misfit strains in the s...
Journal of Materials Engineering and Performance, 2016
In the present investigation, the effect of triaxial state of stress on creep rupture life and ductility of 316 LN stainless steel has been assessed. The creep tests were carried out on both smooth and notched specimens of the steel at 873 K in the stress range of 270-340 MPa. The notched specimens had root radius ranging from 0.83 mm to 5 mm. The detailed finite element analysis has been carried out to assess the triaxial state of stress across the notch incorporating NortonÕs law as creep deformation governing mechanism. The creep rupture life of the steel increased in presence of triaxial stresses and extent of which was more at lower net applied stresses and higher triaxiality (sharper notch). The reduction in effective stress in presence of notch resulted in higher creep rupture life of the steel under triaxial stresses. The fracture surfaces revealed mixed mode failure consisting of dimple ductile and intergranular creep cavitation for all testing conditions, however, extent of cavitation was higher for relatively higher triaxialities and lower net applied stresses. The creep ductility of the steel was found to decrease drastically under triaxial state of stress. The triaxial rupture life and creep ductility of the steel have been assessed based on different models on incorporating different components of stresses at the skeletal point.