Effect of corrosion potential on the corrosion fatigue crack growth behaviour of low-alloy steels in high-temperature water

The low-frequency corrosion fatigue (CF) crack growth behaviour of different low-alloy reactor pressure vessel steels was characterized under simulated boiling water reactor conditions by cyclic fatigue tests with pre-cracked fracture mechanics specimens. The experiments were performed in the temperature range of 240-288 °C with different loading parameters at different electrochemical corrosion potentials (ECPs). Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by SEM were used to quantify the cracking response. In this paper the effect of ECP on the CF crack growth behaviour is discussed and compared with the crack growth model of General Electric (GE). The ECP mainly affected the transition from fast (‘high-sulphur’) to slow (‘low-sulphur’) CF crack growth, which appeared as critical frequencies ν_crit = f(ΔK, R, ECP) and ΔK-thresholds ΔK_EAC = f(ν, R, ECP) in the cycle-based form and as a critical air fatigue crack growth rate da/dt_Air,crit in the time-domain form. The critical crack growth rates, frequencies, and ΔK_EAC-thresholds were shifted to lower values with increasing ECP. The CF crack growth rates of all materials were conservatively covered by the ‘high-sulphur’ CF line of the GE-model for all investigated temperatures and frequencies. Under most system conditions, the model seems to reasonably well predict the experimentally observed parameter trends. Only under highly oxidizing conditions (ECP ? 0 mV_SHE) and slow strain rates/low loading frequencies the GE-model does not conservatively cover the experimentally gathered crack growth rate data. Based on the GE-model and the observed cracking behaviour a simple time-domain superposition-model could be used to develop improved reference CF crack growth curves for codes.     

Corrosion behaviour of stainless steels in flowing LBE at low and high oxygen concentration

The corrosion behaviours of austenitic steel AISI 316L and martensitic steel T91 were investigated in flowing lead-bismuth eutectic (LBE) at 400 °C. The tests were performed in the LECOR and CHEOPE III loops, which stood for the low oxygen concentration and high oxygen concentration in LBE, respectively. The results obtained shows that steels were affected by dissolution at the condition of low oxygen concentration (C_[O2] = 10⁻⁸-10⁻¹⁰ wt%) and were oxidized at the condition of high oxygen concentration (C_[O2] = 10⁻⁵-10⁻⁶ wt%). The oxide layers detected are able to protect the steels from dissolution in LBE. Under the test condition adopted, the austenitic steel behaved more resistant to corrosion induced by LBE than the martensitic steel.     

Characterization of creep-fatigue in ferritic 9Cr-1Mo-V-Nb steel using ultrasonic velocity

The microstructural evolution of ferritic 9Cr-1Mo-V-Nb steel, subjected to creep-fatigue at 550 °C, was evaluated nondestructively by measuring the ultrasonic velocity. The ultrasonic velocity was strongly depended on the microstructural changes during creep-fatigue. The variation in the ultrasonic velocity with the fatigue life fraction exhibited three regions. In the first region (within 0.2 N_f), a significant increase in the velocity was observed, followed by a slight increase between the fatigue life fractions of 0.2 N_f and 0.8 N_f and a decrease in the final region. The change of the ultrasonic velocity during creep-fatigue was interpreted in relation to the microstructural properties. This study proposes an ultrasonic nondestructive evaluation method of quantifying the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.     

Effects of dissolved oxygen on electrochemical and semiconductor properties of 316L stainless steel

The effects of dissolved oxygen on the electrochemical behavior and semiconductor properties of passive film formed on 316L SS in three solutions with different dissolved oxygen were studied by using polarization curve, Mott-Schottky analysis and the point defect model (PDM). The results show that higher dissolved oxygen accelerates both anodic and cathodic process. Based on Mott-Schottky analysis and PDM, the key parameters for passive film, donor density N_d, flat-band potential E_fb and diffusivity of defects D₀ were calculated. The results display that N_d(1−7 × 10²⁷ m⁻³) and D₀(1−18 × 10⁻¹⁶ cm²/s) increase and E_fb value reduces with the dissolved oxygen in solution.     

M2N nitride phases of 9% chromium steels for nuclear applications

M₂N nitride phases of 9% chromium steels with an extra-low carbon content have been investigated using a transmission electron microscope and an energy-dispersive X-ray (EDX) spectroscopy. The steel samples were normalized for 1 h at 1050 °C and then tempered at 600-780 °C for 30 min to 5 h followed by an air cooling. Through the analyses of the electron micro-diffraction patterns and EDX data for the precipitate particles on the extracted carbon replica, two types of Cr-rich M₂N nitride phases with the same hexagonal structure but totally different lattice parameters, a = 2.80 Å/c = 4.45 Å and a = 7.76 Å/c = 4.438 Å, were determined in the steels. Four types of Cr-rich M₂N phases with different lattice parameters probably existed in the steels. The M₂N phase revealed a decrease in its Cr content, an increase in its V content as the tempering temperature was increased, and no obvious change in its content for the metal fraction with an increasing tempering time.     

Elastic and plastic properties of βZr at room temperature

Room temperature elastic and plastic properties of a single phase β_Zr have been studied by in-situ neutron diffraction compression testing. The measured macroscopic Young’s modulus is ∼60 GPa and the yield strength is ∼500 MPa. Dislocation slip is the major mode of plastic deformation. An Elasto-Plastic Self-Consistent (EPSC) model was used to interpret the experimental results and was shown to be effective in extracting the single crystal properties from the polycrystalline data. The single crystal elastic constants of the β-phase are determined as: C₁₁ = 145.9 ± 2.6 GPa, C₁₂ = 117.4 ± 2.5 GPa and C₄₄ = 29.8 ± 0.2 GPa. The calculated elastic modulus of 〈1 0 0〉, 〈1 1 0〉, 〈1 1 1〉, 〈2 1 1〉 and 〈3 1 0〉 directions was ∼41.2, 66.2, 82.9, 66.2 and 47.7 GPa, respectively. Pencil glide on the {110}, {112} and {123} planes was used in the EPSC model and gave a good simulation to the early part of the plastic deformation. The average β-phase strain is best represented by the peak average method, while in cases where only a limited number of diffraction peaks are available, the {211} grain family is a good candidate for estimation of the average β-phase strain.     

Intensely irradiated steel components: Plastic and fracture properties, and a new concept of structural design criteria for assuring the structural integrity

In order to develop a systematic and reasonable concept assuring the structural integrity of components under intense neutron irradiation, two basic tensile properties, true stress-true strain (TS-TS) curves and fracture strain, were investigated on an austenitic stainless steel and martensitic steel. Application of Swift equation is confirmed to a large plastic strain range of TS-TS curves. Fracture strain ?_f data were well correlated as ?_f + ?₀ = const. where ?₀ is the pre-strain representing the irradiation hardening.Based on those formulations and available experimental information, several critical issues to be dealt with in developing the concept were identified possible reduction in ductility, significant change in mechanical properties, remarkable cyclic softening and other unique cyclic properties observed during a high-cycle fatigue testing, and the redundancy of the plastic collapse concept to bending. Existing structural codes are all based on the assumption that there will be no significant changes in mechanical properties during operation, and of high ductility. Therefore, a new concept for assuring structural integrity is required for application not only to components with high ductility but also components with reduced ductility. First, potential failure modes were identified, and a new and systematic concept was proposed for preventing these modes of failure, introducing a new concept of categorizing the loadings by stability of deformation process to fracture (as type F and M loadings). Based on the basic concept, a detailed concept of how to protect against ductile fracture was given, and loading type-dependent limiting parameters were set.Finally, application of the detailed concept was presented, especially on determination of loading type (in numerical approach, the formulation of TS-TS curves and fracture strain derived above are needed), and on how to determine the limiting parameters as allowable limits. Experiments were done to identify the loading type of a tensile loading acting on a structure with a discontinuity. Tensile loadings acting on an intensely neutron-irradiated flat plate with a hole in the center cause plastic tensile instability and necking at the minimum ligament section but do not initiate any surface crack at the initiation of necking.     

Tensile-shear correlations obtained from shear punch test technique using a modified experimental approach

Shear punch testing has been a very useful technique for evaluating mechanical properties of irradiated alloys using a very small volume of material. The load-displacement data is influenced by the compliance of the fixture components. This paper describes a modified experimental approach where the compliances of the punch and die components are eliminated. The analysis of the load-displacement data using the modified setup for various alloys like low carbon steel, SS316, modified 9Cr-1Mo, 2.25Cr-1Mo indicate that the shear yield strength evaluated at 0.2% offset of normalized displacement relates to the tensile YS as per the Von Mises yield relation (σ_ys = 1.73τ_ys). A universal correlation of type UTS = mτ_max where m is a function of strain hardening exponent, is seen to be obeyed for all the materials in this study. The use of analytical models developed for blanking process are explored for evaluating strain hardening exponent from the load-displacement data. This study is directed towards rationalizing the tensile-shear empirical correlations for a more reliable prediction of tensile properties from shear punch tests.     

Conducting thermomechanical fatigue test in air at light water reactor relevant temperature intervals

In Light Water Reactors (LWR), many structural components are made of austenitic stainless steels (SS). These components are subject to extreme conditions, such as large temperature gradients and pressure loads during service. Hence, the fatigue and fracture behavior of austenitic SS under these conditions has evoked consistent interest over the years. Most studies dealing with this problem in the past, investigated the isothermal fatigue (IF) condition, which is not the case in the service, and less attention has been paid to thermomechanical fatigue (TMF). Moreover, the existing codes of practice and standards for TMF testing are mainly derived from the high temperature TMF tests (T_mean > 400 °C). This work presents the development of a facility to perform TMF tests under LWR relevant temperature interval in air. The realized testing parameters and tolerances are compared with the recommendations of existing codes of practice and standards from high temperature tests. The effectiveness of the testing facility was verified with series of TMF and IF tests performed on specimens made out of a commercial austenitic SS TP347 pipe material. The results revealed that the existing tolerances in standards are quite strict for the application of lower temperature ranges TMF tests. It was found that the synchronous, in-phase (IP) TMF tested specimens possess a higher lifetime than those subjected to the asynchronous, out-of-phase (OP) TMF and IF at T_max in the investigated strain range for austenitic SS. Nevertheless, the fatigue lifetime of all the test conditions was similar in the engineering scale.     

The ratchetting behavior of pressurized plain pipework subjected to cyclic bending moment with the combined hardening model

A series of six tests have been conducted using carbon steel and stainless steel cylindrical specimens having mean diameter/thickness ratios in the range 8 ≤ D_m/t ≤ 28. Each cylinder is pressurized up to its calculated design pressure and is loaded with an alternative bending moment at frequencies typical of seismic events simultaneously. Ratchetting of the cylinder wall has been observed and recorded in the hoop direction. A finite element analysis with the nonlinear isotropic/kinematic (combined) hardening model has been used to evaluate ratchetting behavior of the cylinder under mentioned loading condition. Stress-strain data and material parameters have been obtained from several stabilized cycles of specimens that are subjected to symmetric strain cycles. The finite element results are compared with those obtained from experimental set-up. The results show that initial the rate of ratchetting is large and then it decreases with the increasing cycles. The FE model predicts the hoop strain ratchetting rate to be near that found experimentally in all cases that M/M_P0.2 ≤ 1. Also, M/M_y ratios for the onset of ratchetting in stainless steel specimens are less than carbon steel specimens with same D_m/t ratios.     

Analysis of tensile deformation and failure in austenitic stainless steels: Part I - Temperature dependence

This paper describes the temperature dependence of deformation and failure behaviors in the austenitic stainless steels (annealed 304, 316, 316LN, and 20% cold-worked 316LN) in terms of equivalent true stress-true strain curves. The true stress-true strain curves up to the final fracture were calculated from tensile test data obtained at −150 to 450 °C using an iterative finite element method. Analysis was largely focused on the necking and fracture: key parameters such as the strain hardening rate, equivalent fracture stress, fracture strain, and tensile fracture energy were evaluated, and their temperature dependencies were investigated. It was shown that a significantly high strain hardening rate was retained during unstable deformation although overall strain hardening rate beyond the onset of necking was lower than that of the uniform deformation. The fracture stress and energy decreased with temperature up to 200 °C and were nearly saturated as the temperature came close to the maximum test temperature 450 °C. The fracture strain had a maximum at −50 to 20 °C before decreasing with temperature. It was explained that these temperature dependencies of fracture properties were associated with a change in the dominant strain hardening mechanism with test temperature. Also, it was seen that the pre-straining of material has little effect on the strain hardening rate during necking deformation and on fracture properties.     

The CN method applied to the stationary problem of particle reflection by a purely scattering or weakly absorbing half-space; extension to the asymptotic time-dependent case

The application of the classical C_N method fails for a pure scattering or weakly absorbing medium. The asymptotic C_N method, in stationary mode, is the way to solve the transport equation in this limiting case, when c is equal or very close to 1. The asymptotic method allows us to obtain the asymptotic time-dependent emergent angular distribution for a given impinging angular intensity at t = 0 whatever c may be. The numerical results for the classical and asymptotic methods are consistent in the overlapping range.     

Experimental study on the density wave oscillation in parallel rectangular channels with narrow gap

An experimental investigation was performed on the density wave oscillation (DWO) with two parallel rectangular channels, which have a cross section of 25 mm × 2 mm and a heated length of 1000 mm. Test parameters are 1 MPa to 10 MPa for pressure, 200–800 kg/m² s for mass velocity, and 10–50 °C for inlet subcooling. The results show that in general the flow becomes more stable while mass velocity, pressure, and inlet subcooling are increased. The period of oscillation becomes shorter if mass velocity is increased or inlet subcooling is decreased. Pressure has little effect on period of the DWO in this research. The dimensionless subcooling number N_sub and phase change number N_pch were adopted to compare results from rectangular channels with those from round tubes. The comparison indicates that the data from rectangular channels agree with those from the round tubes. The RELAP5 software was used to simulate the DWO in rectangular channels. The prediction show good consistency with experimental phenomenon. However, different two-phase flow model behaves differently when pressure changes in prediction.     

Determination of plasma parameters in rf glow-discharge plasma

The Langmuir probe measurements were carried out in a planar rf discharge of N₂ and Ar gases. The dc characteristic I-U curves are calculated from the measured rf characteristic I-U curves for frequencies 10-60 MHz. The measured parameters such as electron temperature and electron density are compared with the simulated results. At gas pressures 10-40 Pa, the sheath thickness at the powered electrode was proportional to f^−0.5 for simulation and f^−2/3 by using the electrical parameters of the probe measurements and to p^−0.5 for both cases.     

Atom probe tomography characterization of radiation-sensitive KS-01 weld

The microstructure of a radiation-sensitive KS-01 test weld has been characterized by atom probe tomography. The levels of copper, manganese, nickel and chromium in this weld were amongst the highest of all the steels used in Western reactor pressure vessels. After neutron irradiation to a fluence of 0.8 × 10²³ n m⁻² (E>1 MeV) at a temperature of 288 °C, this weld exhibited a large Charpy T_41J shift of 169 K, a large shift of the fracture toughness transition temperature of 160 K, a decrease in upper shelf energy from 118 to ∼78 J, and an increase in the yield strength from 600 to 826 MPa. However, the mechanical properties data conformed to the master curve. Atom probe tomography revealed a high number density (∼3 × 10²⁴ m⁻³) of Cu-, Mn-, Ni-, Si- and P-enriched precipitates and a lower number density (∼1  × 10²³ m⁻³) of P clusters.     

Crystal structure and physical properties of PuPd5Al2

We report on the crystallographic aspects and the basic properties of the plutonium based compound PuPd₅Al₂. This material is antiferromagnetic at T_N = 5.6 K and does not present any hint of superconductivity down to 2 K. This material crystallizes in the ZrNi₂Al₅-type of structure with lattice parameters: a = 4.1302 Å and c = 14.8428 Å. The magnetization, heat capacity and electrical resistivity measurements indicate clearly antiferromagnetic order at T_N = 5.6 K. This material is compared to the structurally related cerium based material CePd₅Al₂ presenting superconductivity induced by pressure.     

Low cycle fatigue tests of surface modified T91 steel in 10 wt% oxygen containing Pb45Bi55 at 550 °C

Low cycle fatigue tests in air and LBE containing 10⁻⁶ wt% dissolved oxygen were conducted with T91 steel at 550 °C. T91 was employed in two modifications, one in the as-received state, and the other after alloying FeCrAlY into the surface by pulsed electron beam treatment (GESA process). Tests were carried out with symmetrical cycling (R = −1) with a frequency of 0.5 Hz and a total elongation Δε_t/2 between 0.3% and 2%. No influence from LBE on fatigue could be detected. Results in air and LBE showed similar behaviour. Additionally, no difference was observed between surface treated and none treated T91 specimens.     

Photon interaction and energy absorption in glass: A transparent gamma ray shield

The effective atomic number, Z_eff, the effective electron density, N_e,eff, and the energy dependence, ED, have been calculated at photon energies from 1 keV to 1 GeV for CaO-SrO-B₂O₃, PbO-B₂O₃, Bi₂O₃-B₂O₃, and PbO-Bi₂O₃-B₂O₃ glasses with potential applications as gamma ray shielding materials. For medium-Z glasses, Z_eff is about constant and equal to the mean atomic number in a wide energy range, typically 0.3 < E < 4 MeV, where Compton scattering is the main photon interaction process. In contrast, for high-Z glasses there is no energy region where Compton scattering is truly dominating. Heavy-metal oxide glasses containing PbO and/or Bi₂O₃ are promising gamma ray shielding materials due to their high effective atomic number and strong absorption of gamma rays. They compare well with concrete and other standard shielding materials and have the additional advantage of being transparent to visible light. The single-valued effective atomic number calculated by XMuDat is approximately valid at low energies where photoelectric absorption is dominating.     

Assessment of toughness in long term service CrMo low alloy steel by fracture toughness and small punch test

Small punch test (SPT) is a miniature sample test technique which can evaluate in-service material properties with an almost non-destructive method. In this paper, the 2.25Cr1Mo steel samples serviced for 10 years in hydrogenation reactor (with temper embrittlement), 1.25Cr0.5Mo supper-pressure vapor pipe serviced for 14 years at 520 °C and several other low alloy steels have been studied by J_IC fracture toughness and SPT. The linear relationship between the small punch (SP) equivalent fracture strain and the fracture toughness of J_IC was created. The correlations applied to the experimental data indicated advantages of using SPT for the determining fracture toughness of in-serviced low alloy steels. Additionally, size affects the fracture pattern. Small punch samples of small size show dimple fractures whereas large fracture toughness samples show quasi-cleavage fractures.     

Study of change in the efficiency of CR-39 after storage for different product companies by using TRACK_TEST program

The main relation between efficiency of CR-39 nuclear track detector from different produced companies, critical angle for track revelation (θ_C) and bulk etch rate (V_B) have been stayed.Computer program TRACK_TEST was used for calculating track parameters and plotting profiles for etch pits in nuclear track materials.The results showed that for any application of CR-39 detector should be calibrated before used it. The detectors older than 3 years seemed to show odd behaviors of V_B with detector efficiency and the critical angle (θ_C).For age = 3 years the efficiency decreases exponentially for different alpha particle energy, and the bulk etch rate increases with decreasing age.This behavior may be important in applications of this detector; for example, the calibration factor for radon measurements should be established by taking into account the age of the detector.