Nobuyoshi Yanagida

Effect of Weld Overlay Repair on Residual Stress and Crack Propagation in a Welding Pipe

Intergranular stress corrosion cracking (IGSCC) has been still one of concerns at the weld zone in boiling water reactors (BWRs). Therefore, Weld Overlay (WOL) process has been developed and applied to repair of BWR pipe joints. To understand residual stress and crack growth behavior is important to evaluate the reliability of pipe joints with WOL. In this paper, the residual stresses were calculated by using thermal elasto-plastic analysis by finite element method (FEM). The analytical model was assumed the primary loop recirculation (PLR) pipe joint with WOL followed by the Japanese guideline. The tensile hoop residual stress was changed to compressive stress on the inner surface of the pipe. On the other hand, tensile axial residual stress was occurred on the inner surface of the pipe by butt-welding and some WOL cases increased the tensile axial stress. In addition, the stress intensity factor for fully circumferential cracks was evaluated using calculated residual stress distributions. As a result, the effect of application of WOL on the crack growth behavior is insignificant in PLR pipe joint.Copyright

Residual Stress Analysis of Bead Welded Low Alloy Steel Plate Specimens Subjected to Post Weld Heat Treatment

We developed a residual stress analysis method for bead welded low alloy steel JIS SQV2A (equivalent to ASTM A533B cl. 1) plates subjected to post weld heat treatment (PWHT). Two specimens were fabricated; each was a bead welded low alloy steel plate. One was in the as-welded condition (as-welded specimen) and the other was subjected to PWHT at 625°C (PWHT specimen). Strain gauges were used to measure the distributions of the residual stress in these specimens. The measurement data showed that the longitudinal stress at the center of a bead was 0 MPa and that in the heat-affected zone was 100 MPa. The transverse stress at the center of a bead was 200 MPa in the as-welded specimen. The absolute residual stress was decreased to less than 50 MPa for the PWHT specimen. We conducted finite element analyses to predict the distributions of welding residual stress in these specimens. The amount of phase transformation strain in low alloy steel was taken into account in the welding residual stress analysis, and creep strain was taken into account in the stress mitigation analysis. The results from the analyses agree well with the experimental results. These findings prove that welding residual stress can be simulated during a thermal elastic plastic (TEP) analysis by conducting a phase transformation and taking the generation of creep strain in the PWHT samples into consideration can be used to simulate that stress mitigation.Copyright

Study on Residual-Stress Redistributions During the Process of Manufacture of a Vessel Penetration Set-On Joint

The stress-redistribution phenomenon in a vessel penetration set-on joint due to post-weld heat treatment (PWHT) was studied using finite element (FE) analyses and mocked-up experiments. The mocked-up consisted of a nickel-based alloy (NCF600) tube welded onto an alloy-82 cladded, low-alloy steel plate (SQV2A) using an alloy-182 butt weld. The angle of the tube to the plate surface was 45 degrees, simulating a side hill, a control rod drive (CRD), and a stub-tube nozzle attachment used in boiling-water reactor (BWR) plants. PWHT at a temperature of 625 °C was conducted after welding and then the inner surface of the tube was machined. Three-dimensional FE modeling was performed to simulate the cladding, the butt weld, the PWHT, and the inner-surface machining of the tube. Thermal elasto-plastic and thermal elasto-plastic creep analyses were conducted to simulate the process of residual-stress build up and its redistribution by PWHT. To validate the FE analysis, the residual stresses in the mocked-up specimen were experimentally measured using the deep-hole-drilling (DHD) and sectioning methods. The analytical and experimental results revealed that residual-stress redistributions in the mocked-up specimen were different in circumferential positions. High-residual stresses in the low-alloy steel plate were particularly mitigated during the PWHT. The stress relief in the low-alloy steel plate primarily controlled the global stress balance between the cladding, the weld metal, and the stub tube.Copyright

Study on Stress-Strain Relation for Type 316L Stainless Steel Using Mixed Hardening Law

To determine stress-strain diagrams for a pipe butt joint of type 316L stainless steel, stress-strain diagrams for pipe specimens subjected to monotonic uniaxial tensile load were measured. Tensile-test specimens were extracted from the deposited metal area, heat-affected zone, and parent-material area of the pipe butt joint. The specimens of the deposited-metal area and the heat-affected zone were extracted from positions at the pipe inner surface, mid-thickness, and the pipe outer surface. The measurement temperatures were 20, 300, 600, and 800°C. The measured stress-strain diagrams show that measured stress at the same given strain increases from measurement point on the outer surfer of the pipe to that on the inner surface. This stress increase is thought to be related to the number of thermal-load cycles used for the weld. The number of cycles at the pipe inner surface was greater than that at the pipe outer surface. To use the measured stress-strain diagrams in a thermal elasto-plastic analysis of welding residual stress and distortion, the measured diagrams for the deposited-metal-area pipe specimen and the parent-material-area pipe specimen were fitted to calculated diagrams by using an isotropic/kinematic mixed hardening law. Material constants for approximating the stress-strain diagrams for the parent-material specimen and deposited-metal specimen were determined. The calculated stress-strain diagrams derived from the isotropic/kinematic mixed hardening law show good agreement with the measured stress-strain diagrams.Copyright

Residual Stress Prediction for Non-Axisymmetric Vessel Penetration Welds

Residual stress distribution in an oblique nozzle jointed to a vessel with J-groove welds was analyzed using a three-dimensional finite element method. All welding passes were considered in a 180-degree finite element (FE) model with symmetry. Temperature and stress were modeled for simultaneous bead laying. To determine residual stress distributions at the welds experimentally, a mock-up specimen was manufactured. The analytical results show good agreement with the experimental measurement data, indicating that FE modeling is valid.Copyright

Allowable sizes of axial flaws in pressurized pipes made of moderate toughness materials

Failure stresses for axially part-through flawed pipes made of moderately tough materials are predicted by several fracture mechanics. However, allowable flaw sizes using these fracture mechanics cannot be simply described because there are many effective parameters such as pipe diameter, wall thickness, material properties, etc. To establish codes and standards to evaluate flaws for piping of light water reactors, we determine unified allowable sizes for axial flaws in pipes subjected to internal pressure from J-integral based fracture mechanics. The allowable sizes are simply tabulated using a single parameter which consists of pipe geometry and material properties.

Finite Element Analyses of Residual Stresses in Typical Welded Joints Used in Nuclear Power Plants and Comparisons With Experiments

Recent discoveries of stress corrosion cracking (SCC) at nickel-based metals in pressurized water reactors (PWRs) and boiling water reactors (BWRs) have raised concerns about safety and integrity of plant components. It has been recognized that welding residual stress is an important factor causing the issue of SCC in a weldment. In this study, both numerical simulation technology and experimental method were employed to investigate the characteristics of welding residual stress distribution in several typical welded joints, which are used in nuclear power plants. These joints include a thick plate butt-welded Alloy 600 joint, a dissimilar metal J-groove set-in joint and a dissimilar metal girth-butt joint. First of all, numerical simulation technology was used to predict welding residual stresses in these three joints, and the influence of heat source model on welding residual stress was examined. Meanwhile, the influence of other thermal processes such as cladding, buttering and heat treatment on the final residual stresses in the dissimilar metal girth-butt joint was also clarified. Secondly, we also measured the residual stresses in three corresponding mock-ups. Finally, the comparisons of the simulation results and the measured data have shed light on how to effectively simulate welding residual stress in these typical joints.Copyright

Residual Stress Improvement in Repair Welded Plates Using Water-Shower Cooling During Welding Process

To reduce tensile residual stress at a repair-welded area, a method that applies water-shower cooling behind a torch was developed. The width of the repair-welded area in this study was 27 mm, while the maximum width of welded areas in previous studies was 18 mm. To examine how much the welding method reduced residual stress at large-width welds, we first applied the method to multipass bead on plate specimens. The first layer consisted of a five-pass bead. A single large-width weld pass was applied with water-shower cooling. To cover the five-pass bead on plate area with a large-width weld, the torch was moved in a weaving motion. Residual stresses were measured. The measurements showed that tensile stresses remained in the five-pass bead on plate specimen. The tensile residual stresses were improved to compressive when our welding method was applied at the large-width weld pass. Then, we applied our method to a repair-welded specimen. The specimen was a butt-welded joint with an X-shaped groove. Repair welding was performed around the boundary of the welded area. A single large-width weld pass with water-shower cooling was applied at the surface. Residual stresses were measured. The measurements showed that tensile stresses remained on the surface of the X-shaped groove welding. Tensile residual stresses increased around the repair-welded area. When our welding method was applied, residual stresses were improved to compressive. Therefore, our welding method can reduce tensile stress in a repair-welded plate.© 2007 ASME

Effect of water-shower cooling during welding on tensile residual stress improvement in multi-layer welding of austenitic stainless steel plates

Abstract A new welding method that uses a water shower behind the welding torch has been developed in order to reduce tensile residual stress in a welded region. When this method is applied to the welding of austenitic stainless steel, the welding and cooling conditions mainly determine how much the residual stress can be reduced. To optimize these conditions, we first used the robust design technique to determine the effects of the interpass temperature, the heat input quantity and the water-shower area on the residual stress distribution of bead-on-plate. We found that, to decrease the tensile residual stress, the interpass temperature should be high, the heat input low, and the water-shower area large. Effect of the water-shower cooling on multi-layer welding was examined analytically and experimentally. It was found that the residual stresses were tensile without water-shower cooling, but compressive with water-shower cooling under the optimized conditions. It can therefore be concluded that the new welding method is appropriate for reducing tensile residual stress in multi-layer welding of austenitic stainless steel.

Optimized Conditions for Tensile Residual Stress Reducing Weld Using Water-Shower Cooling

To reduce tensile residual stress in a welded region, we developed a new cooling method that applies a water shower behind the welding torch. When this method is applied to the welding of austenitic stainless-steel, the welding and cooling conditions mainly determine how much the residual stress can be reduced. To optimize these conditions, we first used FEM to determine the effects of preheating temperature, heat input quantity, and water-shower area on the residual stress, and found that, to decrease tensile residual stress, preheating temperature should be high, heat input low, and the water-shower large. To confirm the effectiveness of these optimized conditions, residual stresses under optimized or non-optimized conditions were experimentally measured. It was found that the residual stresses were tensile under the non-optimized conditions, but compressive under the optimized ones. These measurements agree well with the FEM analysis. It can therefore be concluded that the optimized conditions are valid and appropriate for reducing residual stress in an austenitic stainless-steel weld.Copyright