Rie Sumiya

Simulation of helium bubble behavior in neutron-irradiated stainless steel during welding

A simulation model of helium bubble behavior and crack initiation at grain boundaries in the heat-affected zone (HAZ) during welding is proposed, and the effects of welding conditions and helium concentration on bubble evolution and cracking are evaluated. The model was based on the following assumptions; homogeneous bubble nucleation, bubble coalescence by random motion, bubble growth by vacancy absorption and ductile fracture. The result of calculations for different welding heat inputs reveal that the final bubble size increases with increasing the weld heat input and with decreasing bubble spacing at the grain boundary. The calculated critical heat input for cracking as a function of helium concentration is in good agreement with the results of welding experiments on neutron-irradiated stainless steels and helium-doped stainless steels.

Welding Residual Stress Analysis Using Axisymmetric Modeling for Shroud Support Structure

Recently, several cracks caused by stress corrosion cracking (SCC) have been found on welds of shroud supports in Boiling Water Reactor (BWR) plants. The major cause of SCC in a weld joint is considered due to welding residual stress generated in the fabrication processes of the components. For continuous safety operations, it is necessary to estimate the structural integrity of such shroud supports with cracks based on the distribution of residual stresses induced by welding. In order to know and to validate the numerical method of residual stresses induced by welding of large scale and complex shaped components, a BWR shroud support mock-up with a hemispherical base of reactor pressure vessel (RPV) was fabricated by Japan Nuclear Energy Safety Organization (JNES) as a national project. The mock-up has a 32° section of actual BWR shroud supports with approximately the same configurations, materials and welding conditions of an actual component. During welding in the fabrication process of the mock-up, temperature was measured and after completion of the mock-up fabrication, surface residual stress distributions for each weld were also measured by the sectioning method. In addition, through-thickness residual stress distributions were investigated. Residual stress for each weld was calculated by using axisymmetric models considering temperature dependent elastic-plastic material properties. Though the actual structure of shroud supports is essentially complex, we simplified axisymmetric models in the center of the cross section. The analysis results show a similar profile and good agreement with the measured results on the surface of all the welds and through the welds at the upper and lower joints of the shroud support leg.Copyright

A basic study for application of laser peening to large-scale steel structure

Laser peening can introduce compressive residual stress to the surface and therefore is effective in enhancing fatigue strength. In this study, we conducted laser peening in air with a water film formed by a nozzle and examined whether the distribution of residual stress along the thickness was different from that achieved by laser peening in water in our previous studies. We also assessed the resultant residual stress and fatigue life when the pulse energy was reduced for developing a simple method to conduct laser peening on large structures. Compressive residual stress equivalent to that observed after laser peening in water was obtained in nozzle-type laser peening in terms of magnitude and depth. With the reduction of pulse energy, it was observed that the depth of the compressive residual stress tended to decrease significantly and the fatigue life also tended to reduce. The results indicate that the depth of the compressive residual stress has a considerable effect on the fatigue life of welded structures as well as the magnitude of the surface residual stress.

Stability of residual stress induced by laser peening under cyclic mechanical loading

Purpose – The purpose of this paper is to investigate the behavior of compressive residual stress induced by laser peening under external loading on an age‐hardened high‐strength aluminum alloy A2024‐T3, a low‐carbon austenitic stainless steel SUS316L (Type 316L) and a nickel‐based alloy NCF600 (Alloy 600).Design/methodology/approach – The surface residual stress was measured intermittently by X‐ray diffraction during cyclic uniaxial loading.Findings – The compressive residual stress due to laser peening significantly decreased during the first few cycles at stress ratio of 0.1 with the maximum loading stress exceeding the 0.2 per cent yield stress. No remarkable decrease was observed afterward until the end of the loading cycles.Originality/value – Under symmetric loading at the stress ratio of −1 to A2024‐T3, a major decrease took place in the compression side of the first loading cycle. The surface residual stresses remained in compression within all the extent of the present experiments, even if the m...

Temper-Bead Weld by Underwater Laser Beam Welding

In repair welding for nuclear reactor vessel, low alloy steels are affected by heat input during welding process. The conventional repair welding for wall steel constructions requires post weld heat treatment (PWHT) to achieve the desired microstructure properties. However, post weld heat treatment is very difficult for some structures in operating plants. In such case, temper-bead welding technique is available as a repair welding method. Temper-bead welding employs a multi-pass deposition of welding metal. Each layer of beads provides heat for thermal treatment of the previous weld bead or layer, which lowers hardness of the heat affected zone (HAZ) and improves mechanical properties like the toughness. Toshiba has developed underwater laser cladding and laser seal welding techniques for reactor components repair welding. In this report, some experimental results of laser based underwater temper-bead welding are presented.© 2009 ASME

The Measurement and Modelling of Residual Stresses in a Full-Scale BWR Shroud-Support Mock-Up

This paper presents results from a programme of residual stress measurements and modelling carried out in Japan on a full-scale mock-up of the hemi-spherical base of a Boiling Water Reactor (BWR) pressure vessel. The shroud support mock-up consisted of four main parts: pressure vessel, support plate, support cylinder and support legs. The mock-up was manufactured using a combination of ferritic steel and nickel base alloy (i.e. alloys: 82, 182 and 600) in a similar manner to that of the actual component. Overall the mock-up had an outer diameter of 6.6m and a height of 3.4m. The residual stresses generated by the nickel alloy welds during manufacture were measured using the Deep-Hole Drilling (DHD) [1–3] and Sectioning [4, 5] techniques, and modelled using ABAQUS. Presented here are measurement and modelling results from three weld locations within the mock-up: at one location through the “double-bevel” butt-weld joining the top of the support leg to the support cylinder (named H10) and at two locations through the “asymmetric double-V” weld joining the bottom of the support leg to the cladded pressure vessel (named H11a and H11b). The semi-destructive DHD technique was carried out first at all three locations on-site in Japan before the fully destructive Sectioning technique was used. Both techniques measured the biaxial (i.e. mock-up-hoop and -axial) residual stresses. The DHD and Sectioning techniques were not carried out at the exact same locations, rather similar locations due to the axisymmetry of the mock-up. Modelling of the residual stresses generated was undertaken for each weld location using a 2D axisymmetric finite element analysis containing between 40–50 discrete weld beads. The modelled residual stresses were generated using thermal load modelling followed by elastic-plastic mechanical analysis under kinematic hardening rules. Overall there is excellent agreement between the measured and modelled residual stresses at all locations. At all locations the measured peak tensile residual stresses (i.e. H10 = 410MPa, H11a = 260MPa and H11b = 230MPa) were found to be in the hoop direction just below the inner weld cap surface. The modelled peak tensile residual stresses were again found in the hoop direction near the inner weld capped surface, however, they were found to be approximately 155MPa greater than the measured residual stresses, and for locations H10 and H11a similar peaks were found near the outer weld cap surface as well.Copyright

Laser desensitization treatment for inside surface of SUS304 stainless steel pipe welds

The purpose of this study was to develop a technology for preventing the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power YAG laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. By irradiating a laser beam of the appropriate power density, a laser de-sensitization heat treatment (LDT) process was realized that formed both a molten layer of approximately 0.2 mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test to evaluate IGSCC showed that no cracks had appeared on the surface of the LDT parts. Also, after LDT was applied at a width of 40 mm in the vicinity of welding joints in the inside surface of pipes (thickness: 8 mm), approximately 250 MPa of tensile stress was measured as a residual stress on this LDT- processed surface. On the other hand, the tensile stress on the outside surface of these pipes decreased to the compression stress.

Decreasing Waste of Laser Cutting by Metal Fume Capturing With Water

Decommissioning of aged nuclear reactors is planned, and cutting technologies for thick structure are necessary to reduce storage space of radioactive wastes. Though thermal cutting technology is suitable for cutting thick materials, radioactive fume is one of the problems due to increase the environmental dose. A water jet-guided laser cutting technology is one of the solutions for cutting irradiated materials, because radioactive fume is confined in the water and doze level won’t be increased. However, this technology was developed for precision machining like dicing and slotting of silicon wafers, cutting thick materials by using this process is very diffcult. In this study, cutting technology for thick material with a water jet-guided laser was discussed. Phenomenon during cutting thick stainless steel was observed by using high speed camera and optimum conditions for both water jet and laser cutting were derived. Finally, 50mm thick stainless steel plate was successfully cut by using this technology.© 2014 ASME

Effects of Environments and Shot Peening on Creep and Creep-Fatigue Behaviors of Ni-Fe-Base Superalloy Inconel®Alloy 706

Fleet leader machines with non-shot-peened discs made of Inconel® alloy 706 were experienced to have cracks in the first stage gas turbine wheels. These were inter-granular cracking and observed to be highly stressed locations with less potential for oxidation, which is thought to be quasi-brittle inter-granular cracking due to stress induced atmospheric oxygen penetration, so called, hold-time cracking. To recognize this phenomenon, creep and creep-fatigue tests with smooth and notched specimens were conducted at 600 and 650°C in air and vacuum and confirmed the environmental effects on those lives and fracture modes.The effectiveness of shot peening which was used as one of the countermeasures for this phenomenon was verified by using the creep-fatigue tests. The durability was also evaluated by thermal and stress aging tests at 450 and 500°C up to around 104 hours. Little relaxations were observed during the thermal agings after the initial rapid relaxation of the surface residual stress, but the effects of the loading stresses were observed above the yielding stress at each temperature.Copyright

ICONE19-43979 The Effect of Mechanical Loading on Residual Stress Induced by Laser Peening

The stress corrosion cracking(SCC) of austenitic stainless steel and nickel-based alloy is a primary concern for stable operation in Boiling Water Reactors(BWRs) and Pressurized Water Reactors (PWRs). Laser peening is one of the promising technologies for prevention of SCC initiation to induce compressive residual stress near the surface region. However, the compressive residual stress by laser peening is likely affected due to applied stress. In this study, the effect of mechanical loading on the compressive residual stress by laser peening was investigated. The specimens of low carbon austenitic stainless steel (Type316L), nickel-based alloy (Alloy600) and their weldments were prepared, and several levels of cyclic loading were applied to their specimens. The surface residual stress at surface was measured by X-ray diffraction method before and after loading. The relaxation behaviour of the compressive surface residual stress by laser peening was evaluated at the base metal and heat affected zone (HAZ) of the weldment. It was confirmed that compressive surface residual stress remained after stress loading which corresponds to 0.2% proof stress, and that the relaxation of the surface residual stress remarkably observed after initial loading during cyclic loading test. These relaxation behaviour were similar in Type316L, Alloy600 and their HAZ.