Creep-fatigue behaviour of an AISI stainless steel at 550 °C

In the past, a lot of experimental studies have been devoted to creep-fatigue interactions in austenitic stainless steels. Tests have been carried mainly at temperatures of at least 600 °C and at high applied strains, which are supposed to be the most damaging. The present work is dedicated to mechanical tests, TEM observations and lifetime predictions at 550 °C which corresponds to the real industrial temperature in Liquid Metal Fast Breeder Reactors. It is shown that if pure fatigue test results are close to those performed at 600 °C, some of the creep-fatigue results are different, particularly for small applied strains which correspond once more to the industrial conditions. In the 0.25–0.3% strain amplitude range, the stress is larger with hold time than without whatever is the hold time up to 5 h. The numbers of cycles to failure are greatly reduced and no saturation with the hold time is observed, contrary to higher temperature results. The stress–strain behaviour is discussed considering several high temperature mechanisms such as ageing, recovery and viscoplasticity and using TEM observations and stress partitioning into kinematic, isotropic and thermal stresses. Finally, a simple linear damage accumulation model is applied to the 550 °C and 600 °C tests, using the measured stresses. The stress dependence on hold time can partly explain the observed failure results on fatigue life.     

Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest, since the ductility and toughness of the material may change drastically after long aging times. Two kinds of materials, i.e. AISI 321 base and without Ti-stabilized weld steel and AISI 347 base with Nb-stabilized weld steel, were compared during aging at 700 °C up to 6000 h. Both materials present increased amount of precipitate and decreased impact energy as the aging time increases. The decreased extent of impact energy with aging is almost the same for both base materials. However, it presents differences for 347 and 321 weld samples. The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates. 321 weld sample precipitates more numerously than 347 weld sample due to the absence of stabilized Ti/Nb on the former. Large amount of carbides is formed on 321 weld sample immediately after welding. The carbides are transformed to sigma phase, which is mainly responsible for the much more sigma phase precipitation compared with other samples, after high-temperature aging. The fractographs showed, in general, brittle fracture mode in 321 weld impact-fractured specimens after aging at 700 °C for 6000 h. However, other samples show ductile fracture mode in general. Several approaches should be employed to control sigma phase precipitation in weld material. These approaches include: decreasing content of ferrite and M₂₃C₆ carbide in weld and selecting Nb added weld wire during welding.     

Microstructure and dry-sliding wear properties of DC plasma nitrided 17-4 PH stainless steel

An attempt that the precipitation hardening steel 17-4PH was conducted by DC plasma nitriding (DCPN) is made to develop a kind of candidate material for nuclear reactor. Nitriding process performed at temperature ? 400 °C takes effect on creation of the layers composed of S-phase (expanded austenite) and (expanded martensite). Up to the temperature of 420 °C, the S-phase peaks disappear due to the transformation occurrence (S-phase → + CrN). For the samples nitrided at temperature ? 450 °C, no evidence of is found owing to a precipitation () taking place. For the 480 °C/4 h treated sample, it is the surface microhardness that plays the lead role in the wear rate reduction but the surface roughness; while for the 400 °C/4 h treated sample, it is both of the surface roughness and the S-phase formation. Dry sliding wear of the untreated 17-4PH is mainly characterized by strong adhesion, abrasion and oxidation mechanism. Samples nitrided at 400 °C which is dominated by slight abrasion and plastic deformation exhibit the best dry sliding wear resistance compared to the samples nitrided at other temperatures.     

17-4PH马氏体不锈钢350℃长期时效脆化研究

采用光学显微镜(OM)、扫描电镜(SEM)、硬度测定以及示波冲击试验(instrumental impact test)研究了17-4PH马氏体不锈钢在350％：长期时效过程中显微组织、硬度、冲击韧性以及断口形貌的变化规律。结果表明：该马氏体不锈钢在350℃长期时效的过程中，随着时效时间的延长．其硬度升高，并在时效9000h时达到最大值；其裂纹萌生功(Ei)，裂纹扩展功(Ep)和总冲击功(Et)都随时效时间的延长而逐渐下降。根据示波冲击曲线获得了17-4PH马氏体不锈钢的动态断裂韧性Ktd，其动态断裂韧性也表现出和Ei，Ep及Et相类似的变化规律。该不锈钢的夏氏V型缺口(Charpy V-notch)冲击试样断口形貌随着时效时间的延长由韧窝断裂为主向准解理断裂和沿晶断裂为主变化。     

Intergranular damage in AISI 316L(N) austenitic stainless steel at 600 °C: Pre-strain and multiaxial effects

This study is devoted to the effect of a multiaxial stress state and of pre-straining on the creep properties of an austenitic stainless steel. Creep tests on both smooth and notched specimens have been carried out on type 316L(N) steel at 600 °C. In comparison to the annealed state, pre-straining caused a substantial increase in creep lifetime but also a dramatic drop in intergranular damage resistance. The effect of a pre-strain on creep ductility was so strong that compact tension specimens in pre-strained state tested under relaxation conditions cracked, whereas specimens in annealed state were not prone to cracking. A model taking into account both pre-strain and multiaxial effects was developed and identified on the basis of local intergranular micro-cracks measurements on notched specimens. It satisfactorily predicts the results of relaxation crack propagation tests. This model may also provide a useful estimation of the relaxation cracking risk of 316L(N) as a function of pre-strain level and stress triaxiality ratio.     

350℃下长期时效对17-4PH不锈钢动态断裂韧性的影响

利用示波冲击试验系统研究反应堆用17-4PH马氏体不锈钢在使用温度（350℃）下长期(约11000h)时效过程冲击性能和动态断裂韧度的变化规律，并用扫描电镜观察分析不同时效时间的CharpyⅤ型缺口试样（CVN）的断口形貌。结果表明：该马氏体不锈钢在350℃长期时效的过程中，随着时效时间的延长,其塑性变形能E_PL和撕裂能E_TE以及冲击功E_t均随时效时间的延长而逐渐下降。根据示波冲击曲线获得了该钢的动态断裂韧度K_Id，其动态断裂韧度也随时效时间的延长而逐渐下降，并在试验的初始阶段下降很快，在试验的中后期下降较为缓慢。另外，该不锈钢的CVN冲击试样断口形貌随着时效时间的延长由韧性断裂机制的韧窝断裂为主向脆性断裂机制的准解理断裂和穿晶断裂为主变化。这些均说明，随着时效时间的延长，该材料的韧性降低，发生了脆化，且脆化主要发生在试验的初始阶段。     

Pitting corrosion in CVD SiC at 300 °C in deoxygenated high-purity water

SiC is a candidate for nuclear applications at elevated temperatures but has not been fully studied under typical light-water reactor operating conditions, such as moderate temperatures and high pressures. Coupons of high-purity chemical vapor deposited SiC were exposed to deoxygenated, pressurized water at 573 K and 10 MPa for up to 5400 h. Ceramographic examination of the exposed SiC surfaces revealed both embryonic and large, d > 300 μm, pits on the surface after initial exposure for 4000 h. The pits were characterized using scanning electron microscopy for structure and chemistry analysis. Pit densities were also determined by standard counting methods. The chemical analysis revealed that the pits are associated with the formation of silica and subsequent loss of Si, which is expected due to several suggested reactions between SiC and water. Subsequent exposure under nominally identical water chemistry conditions for an additional 1400 h removed the pits and the samples exhibited general corrosion with measurable loss of Si from the surface.     

Evaluation of inelastic constitutive models under plasticity-creep interaction for steel at 600°C

Uniaxial tests to identify plasticity-creep interaction in steel at 600°C were carried out as the Benchmark Project by the Subcommittee on Inelastic Analysis and Life Prediction, JSMS. The purpose of this paper is to present recent experimental data and predictions of constitutive models obtained in the project. Ten types of constitutive models were utilized to compare analytical predictions to sixteen benchmark experiments which are grouped into four categories: (I) tensile and creep tests under monotonic loading, (II) mixed mode tests under plastic and creep loading, (III) ratcheting deformation tests under program loads, and (IV) cyclic deformation tests under the combination of different strain rates. The benchmark tests in Group IV are used to estimate the creep-fatigue life of steel; the results will be published in a separate paper.     

Inelastic stress-strain response for notched specimen of Cr-1Mo steel at 600°C

Following a series of cooperative studies A-I and A-II (phase III) concerning the inelastic behaviour of high temperature materials under uniform state of stress, finite element analyses were carried out on circumferential notched cylinders subjected to plasticity-creep interaction conditions. Using an electric capacitance type extensometer “Strain-Pecker”, which is capable of measuring a local strain response with a gauge length of 0.5 mm under high temperature conditions, stress-strain responses for both global and local regions near the notch root were evaluated. Ten kinds of inelastic constitutive model were introduced into a finite element code, and the responses for four kinds of loading pattern were examined for two types of notch shape.     

Fatigue-creep life prediction of Cr-1Mo steel under combined tension-torsion at 600°C

The results of fatigue-creep life prediction for Cr-1Mo steel under multiaxial stress conditions of combined tension-compression and cyclic torsion are summarized. This work was performed as the second task A-II of the cooperative project of phase 3 by the Subcommittee on Inelastic Analysis and Life Prediction of High Temperature Materials, JSMS. The ratios of axial to torsional strain range, Δε/Δ(γ/3^1/2), were set to be non-unity in these phase 3 tests for both in-phase and out-of-phase (with 90° phase difference) strain-controlled wave patterns, including pure torsion, while the ratios were unity in phase 2. By comparing the evaluated failure lives with the corresponding data of phase 2, some discussion on the effect of the wave patterns is presented and the validity of the life prediction methods is evaluated.     

Structure and optical properties of sapphire implanted with boron at room temperature and 1000 °C

Many previous studies of ion-implanted sapphire have used gas-forming light ions or heavier metallic cations. In this study, boron (10¹⁷ cm⁻², 150 keV) was implanted in c-axis crystals at room temperature, 500 and 1000 °C as part of a continuing study of cascade density and “chemical” effects on the structure of sapphire. Rutherford backscattering-ion channeling (RBS-C) of the RT samples indicated little residual disorder in the Al-sublattice to a depth of 50–75 nm but almost random scattering at the depth of peak damage energy deposition. The transmission electron micrographs contain “black-spot” damage features. The residual disorder is much less at all depths for samples implanted at 1000 °C. The TEM photographs show a coarse “black-spot damage” microstructure. The optical absorption at 205 nm is much greater than for samples implanted with C, N, or Fe under similar conditions.     

Transformation of Al2O3 to LiAlO2 in Pb–17Li at 800 °C

A FeCrAl substrate was pre-oxidized for 2 h at 1000 °C to thermally grow an external Al₂O₃ scale and then isothermally exposed to Pb–17 at.% Li for 1000 h at 800 °C to determine if this layer would protect the underlying alloy from dissolution. After exposure, a small mass gain was measured, indicating that the layer did inhibit dissolution. However, characterization of the external layer determined that it had transformed to LiAlO₂ with an increased thickness and a much larger grain size than the original layer. This observation has implications for the use of Al₂O₃ as a permeation barrier in Pb–Li cooled fusion blanket systems.     

Recovery of neutron-induced damage of SiC by thermal annealing up to 1400 °C

Recovery of the specimen length of neutron-irradiated SiC was observed using a precision dilatometer. The specimens were heated isothermally and isochronally. The accuracy of length measurement at high temperature using the dilatometer was compared with that of length measurement at room temperature using a micrometer. It was clarified that the dilatometer method showed high accuracy and stability. The dilatometer method was applied to observe length recovery by isothermal annealing at 1200 °C of the neutron-irradiated SiC, and at least two recovery rates were clarified.     

An XPS study on the evolution of type 304 stainless steel surface during routine TCSU operation

A surface analysis study was carried out to monitor the first-wall evolution in the Translation, Confinement, and Sustainment Upgrade (TCSU) experiment. A type 304 stainless steel sample was exposed to processes including the standard ex-situ surface preparation, helium glow discharge cleaning (He-GDC), plasma discharges, and backfilling the vacuum chamber with filtered N₂. After each process, the sample was carried to a surface analysis chamber for X-ray photoelectron spectroscopy (XPS), using a custom designed in-vacuum transfer device. Results indicated that He-GDC was effective in removing both physically and chemically bound carbon and oxygen on the stainless steel surfaces due to the physical impact of the glow. The plasma discharges resulted in oxidation on the surface. The use of filtered nitrogen during vacuum breaks was verified as an effective method for minimizing carbon and oxygen contamination.     

Corrosion behavior of CLAM steel weldment in flowing liquid Pb-17Li at 480 °C

CLAM (China Low Activation Martensitic) steel is considered as one of the candidate structural materials in liquid LiPb blanket concepts. Welding is one of the essential technologies for its practical application, CLAM steel weldment shows a great difference with base metal due to the effect of welding thermal cycle. In order to investigate the corrosion behavior and mechanism of CLAM weldments in liquid Pb-17Li, the experiments were performed by exposing the TIG weldment samples in flowing LiPb at 480 °C. The weight loss test of exposed specimens show that in 500 h, 1000 h dynamic conditions, corrosion resistance of CLAM steel weldment is poor, SEM analysis shows that the thicker martensite lath in weld area lead to higher corrosion amount, EDS results show that the influence of corrosion on surface elements is small, and surface corrosion is even, EDX analysis shows that the penetration of Pb-17Li does not exist in the joint. With the increasing of exposure time, the corrosion rate decreases. Metallographic analysis shows that the presence of Cr has great influence on the corrosion resistance of the steel matrix. The area short of Cr in thick martensite lath of CLAM steel weldment is easily corroded. After a series of theoretical and experimental analysis, a basic presumably corrosion behavior model is established, which makes contributions to the in-depth understanding of the corrosion mechanism of CLAM weldments.     

Progressive dissolution of molybdenum foils in liquid uranium at 1160°C

Immersion experiments have been performed to investigate the progressive dissolution of 0.3-mm-thick foils of molybdenum metal in liquid uranium at 1160 ° C, for immersion times of 3, 6, 10, and 60 min, and 20 h, in a zarconia crucible. The original foils, as-received and heat-treated at 1160 ° C, and the uranium-reacted foils have been studied microscopically (SEM-EDAX) and the internal morphology has been determined. The flow lines in as-received molybdenum disappear under heat-treatment, while the undistinguishable grains recrystallize upon heating into grains with average size of 20 μm, persisting in samples immersed in liquid uranium. The recrystallization is not uranium-assisted, as uranium does not penetrate into intergranular regions. After incubation time of 3–4 min, outer molybdenum grains dissolve in liquid uranium, thereby reducing the foil thickness progressively. Full dissolution occurs for about 15-min immersion.     

Effect of flowing sodium on corrosion and tensile properties of AISI type 316LN stainless steel at 823 K

AISI type 316LN stainless steel was exposed to flowing sodium in mass transfer loop (MTL) at 823 K for 16 000 h and then examined for changes in the tensile properties due to the mass transfer and corrosion effects. Comparisons in microstructural and mechanical properties were made between annealed, thermally aged and sodium exposed materials. Microstructural examination of thermally aged and sodium exposed materials revealed precipitation of carbides at the grain boundaries. The sodium exposed samples contained a degraded layer at the surface up to a depth of around 10 μm and a surface carburized layer of about 30 μm. There was about 15% increase in yield strength and a decrease of about 20% in ductility for the sodium exposed material vis-a-vis thermally aged material and this was attributed to carburization effects and microstructural changes.     

In-reactor uniaxial tensile testing of pure copper at a constant strain rate at 90 °C

Recently, annealed specimens of pure copper have been tensile tested in a fission reactor at a damage rate of 6 × 10⁻⁸ dpa/s with a constant strain rate of 1.3 × 10⁻⁷ s⁻¹. The specimen temperature during the test was about 90 °C. The stress response was continuously recorded as a function of irradiation time (i.e. displacement dose and strain). The experiment lasted for 308 h. During the dynamic in-reactor test, the specimen deformed and hardened homogeneously without showing any sign of yield drop and plastic instability. However, the specimen yielded a uniform elongation of only about 12%. The preliminary results are briefly described and discussed in the present note.     

Some studies of molecular ions obtained from a U + C target at > 2100°C

The OSIRIS ISOL-facility at Studsvik is used for studies of short lived products from reactor neutron induced fission. Several species of molecular ions are produced in the target, forming sidebands to the fission mass distribution. Other species are obtained when reactive gases are introduced into the target chamber. Observations of the temperature stability of molecular ions and their formation probabilities under different conditions are reported.