Study on the SCC Growth of Alloy 182 Under Constant Displacement Condition in High Temperature Oxygenated Water

SCC initiates and propagates along the fusion line or in the weld metal in BWR and many SCC initiation & propagation studies have been performed so far (Saito, et al. (1997), Kikuchi, et al. (1997), Itow, et al. (1997, 2000), Suzuki (1999), Namatame, et al. (2001)). SCC growth behavior can be evaluated by conjunction of SCC growth rates and the residual stress of the welded component, which consists of tension/compression region. Especially, thick components such as core shrouds have increasing and decreasing tensile stress profile under constant displacement. In general, SCC growth rates are obtained from CT specimens under constant loading condition. This study shows that SCC growth rates depend on dK/dt as well as on K and that their growth rates under constant displacement with decreasing K are lower than those under constant loading condition with increasing K.

On the Mechanics of Residual Stresses in Girth Welds

In this paper, some of the important controlling parameters in governing weld residual stress distributions are presented for girth welds in pipe and vessel components, based on a large number of residual stress solutions available to date. The focus is placed upon the understanding of some of the overall characteristics in through-wall residual stress distributions and their generalization for vessel and pipe girth welds. In doing so, a unified framework for prescribing residual stress distributions is then outlined for fitness-for-service assessment of vessel and pipe girth welds. The effects of various joint geometry and welding procedure parameters on through thickness residual stress distributions are also demonstrated in the order of their relative importance.

Modeling and Quantitative Prediction of Environmentally Assisted Cracking Based Upon a Deformation-Oxidation Mechanism

In order to provide a quantitative tool for predicting EAC growth rates for austenitic alloys such as austenitic stainless steels, Alloys 600 and 182 in simulated LWR environments, a model based upon the synergistic effects of deformation and oxidation at the crack tip has been proposed. In this paper, the effects of hardening, variation of K with crack growth and oxidation kinetics on EAC growth rate are described with special emphasis on field applications. The results imply recent cracking incidences in BWR core shroud and Primary Loop Recirculation (PLR) piping and PWR Vessel Penetrations (VP). In particular, the significance of oxide analysis at the crack tip and also of a kinetic approach to oxidation under stress or strain condition is described. Potential future directions for mitigation of such cracking are discussed.

Prediction and Measurement of Residual Stresses Arising From Quenching of Stainless Steels

This paper presents results from an experimental and numerical study examining the creation of highly triaxial residual stresses in stainless steel. This was motivated by a need to model and understand creep in aged power plant. The residual stresses were introduced by rapid spray water quenching of heated solid stainless steel spheres and cylinders. Finite element (FE) simulations predicted high compressive residual stresses around the surface of the specimens and tensile residual stresses near the centre. Surface residual stresses were measured using the incremental centre-hole drilling (ICHD) technique. Neutron diffraction (ND) was used to measure the interior residual stresses. The measurements were in good agreement with FE predictions. The ND measurements confirmed that a highly triaxial residual stress state existed in the core of the specimens.

A Study on Fatigue Analysis Procedure of Nuclear Welded Structures Based on Structural Stress and Fracture Mechanics Approach

The mesh-insensitive structural stress procedure by Dong is modified to apply to the welded joints with local thickness variation and inignorable shear/normal stresses along local discontinuity surface. Validity of the modified mesh-insensitive structural stress procedure is identified comparing the structural stresses calculated for various FE models. Fatigue crack initiation cycles are determined by using the structural stresses and the various fatigue crack growth models. Fatigue test is performed to identify the validity of the fatigue analysis results. Finally, as a result of comparison between test and analysis results, it is found that the structural stress/fracture mechanics approach is valid for fatigue analysis and the requirements for development of a new fatigue crack initiation analysis procedure are presented.

Factors Influencing SCC and IASCC of Stainless Steels in High Temperature Water

SCC growth studies were performed in high temperature, high purity water on various grades and various conditions of stainless steel. The synergistic effects of corrosion potential, sensitization, cold work (yield strength), temperature and irradiation were evaluated, and their implications to interpreting and modeling SCC in unirradiated and irradiated structures are discussed.

Fracture Assessment of Through-Wall and Surface Cracked Pipes by BS 7910 and API 579 Assessment Procedures: A Comparative Study

Fracture assessment procedures such as BS 7910 and API 579 are formulated based on the Fracture Mechanics concept for assessing integrity of structures such as pipelines, pressure vessels, etc. In the current study those procedures are applied to through-wall and surface cracked pipe geometry under four-point bending. The predicted maximum tolerable applied loads are then compared with pipe full-scale fracture testing results published by Miura et al (2002). Other assessment schemes namely, GE/EPRI, Net-section plastic collapse, LBB.NRC and finally LBB.ENG2, as reported in the same publication are also included in the current paper for sake of comparison. The comparative study showed that BS 7910 and API 579 predict similar maximum tolerable load for through-wall pipes but different value for surface-cracked pipes. Difference in predictions for the latter geometry is owing to the use of different stress intensity factor/reference stress solution by BS 7910 than API 579. However, both procedures provided conservative results compared with the experimental data as well as other engineering routes mentioned in Miura et al (2002).

Strategy of Considering Microstructural Effect on Weld Residual Stress Analysis

Thermal distortion and residual stress are essentially generated by welding and it is well known that they affect the performance of welded structures such as brittle fracture, fatigue, buckling deformation, and stress-corrosion cracking. Welding distortions and residual stresses can be possible controlled and reduced by using some countermeasures. Not only thermal stress behavior but also prediction of microstructural phase during weld heat cycles are very important. High strength steels or martensitic stainless steels are used in a lot of power plant components, and the effect of phase transformation on mechanical behavior during welding in these steels becomes much larger than that of mild steels and austenitic stainless steels. Simultaneous simulation between thermal stress and microstructure during welding should be necessary in a precise evaluation. Analytical method and several applications to actual components are introduced in order to emphasize the effect considering microstructure on weld residual stress analysis.

Residual Stress Redcution by Using Weld Metal With Property of Low-Temperature Phase Transformation

Residual stress in welded joints by using a new weld metal with the property of low-temperature phase transformation is numerically analyzed. 10 % Nickel and 10 % Chromium are involved in the developed welding material for producing the property of martensitic phase transformation at a low temperature and for generating compressive residual stress. A fillet-welded joint between plate and stiffener is used for the numerical simulation of the thermal elastic-plastic finite-element analysis with coupling phase transformation effect. Moving heat source is considered by using the gradual deposition of the finite-element during welding. Distribution of the computed residual stress mostly agrees with the measuring values by strain gauge. Compressive residual stress distributes in the weld metal for both longitudinal and transverse directions with weld line. The effects of the material of weld metal and welding pass sequence on residual stress are investigated. Residual stresses on the weld toe and root are improved lower by using the low-temperature transformation weld wire than the conventional one, regardless of the sequence of welding deposition. It is found that the weld metal with property of low-temperature phase transformation is effective to reduce residual stress near weld metal.

SCC Growth Rates and Reference Curves for Low Carbon Stainless Steels in BWR Environment

Nuclear Plant Operation and Maintenance Code has been developed and is going to be applied for nuclear power system components in Japan. If a crack is detected in a component, the evaluation of crack growth due to stress corrosion cracking (SCC) is required. In recent years, the components in BWR primary systems made of low carbon stainless steel, such as core shroud and PLR piping, have suffered from SCC and it is necessary to prepare the crack growth rate reference curves for the materials. In this paper, the development of the SCC growth rate database for low carbon stainless steel in BWR water and the proposed reference curves in Japan are described.