Welding Residual Stress Solutions for Dissimilar Metal Surge Line Nozzle Welds

2010 ◽  
Vol 132 (2) ◽  
Author(s):  
D. Rudland ◽  
A. Csontos ◽  
T. Zhang ◽  
G. Wilkowski
Keyword(s):  
Residual Stress ◽  
Nuclear Power ◽  
Power Plants ◽  
Nuclear Industry ◽  
Welding Residual Stress ◽  
Nozzle Geometry ◽  
Material Strength ◽  
Piping System ◽  
Dissimilar Metal ◽  
Weld Material

At the end of 2006, defects were identified using ultrasonic testing in three of the pressurizer nozzle dissimilar metal (DM) welds at the Wolf Creek nuclear power plant. Understanding welding residual stress is important in the evaluation of why and how these defects occur, which in turn helps to determine the reliability of nuclear power plants. This paper presents analytical predictions of welding residual stress in the surge nozzle geometry identified at Wolf Creek. The analysis procedure in this paper includes not only the pass-by-pass welding steps, but also other essential fabrication steps of pressurizer surge nozzles. Detailed welding simulation analyses have been conducted to predict the magnitude of these stresses in the weld material. Case studies were carried out to investigate the change in the DM main weld stress fields resulting from different boundary conditions, material strength, weld sequencing, as well as simulation of the remaining piping system stiffness. A direct comparison of these analysis methodologies and results has been made in this paper. Weld residual stress results are compared directly to those calculated by the nuclear industry.

Welding Residual Stress Solutions for Dissimilar Metal Surge Line Nozzles Welds

2008 ◽  
Author(s):  
D. Rudland ◽  
T. Zhang ◽  
G. Wilkowski ◽  
A. Csontos
Keyword(s):  
Residual Stress ◽  
Nuclear Power ◽  
Power Plants ◽  
Nuclear Industry ◽  
Welding Residual Stress ◽  
Material Strength ◽  
Piping System ◽  
Dissimilar Metal ◽  
Weld Residual Stress ◽  
Weld Material

During the last year, defects had been located by ultrasonic testing in three of the pressurizer nozzle dissimilar metal (DM) welds at the Wolf Creek nuclear power plant. Understanding welding residual stress is important in the evaluation of why and how these defects occur, which in turn helps to determine the reliability of nuclear power plants. The analysis procedure in this paper included not only the pass-by-pass welding steps, but also other essential fabrication steps of pressurizer surge nozzles. Detailed welding simulation analyses have been conducted to predict the magnitude of these stresses in the weld material. Case studies were carried out to investigate the influences to main weld stress fields with different boundary conditions, material strength, weld sequencing, as well as simulation of the remaining piping system stiffness. A direct comparison of these analysis methodologies and results has been made in this paper. Weld residual stress results from nuclear industry (conducted by Dominion Engineering, Inc.) and the US NRC (conducted by Engineering Mechanics Corporation) are also compared.

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

2010 ◽  
Author(s):  
Dean Deng ◽  
Kazuo Ogawa ◽  
Nobuyoshi Yanagida ◽  
Koichi Saito
Keyword(s):  
Numerical Simulation ◽  
Residual Stress ◽  
Residual Stresses ◽  
Welded Joints ◽  
Nuclear Power ◽  
Power Plants ◽  
Butt Joint ◽  
Welding Residual Stress ◽  
Dissimilar Metal ◽  
Water Reactors

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.

Part Repair and Start/Stop Effects and Three Dimensional Residual Stress in Dissimilar Metal Welds

2015 ◽  
Author(s):  
F. W. Brust ◽  
E. Punch ◽  
E. Kurth
Keyword(s):  
Residual Stress ◽  
Stress State ◽  
Nuclear Power ◽  
Power Plants ◽  
Three Dimensional ◽  
Filler Materials ◽  
Piping System ◽  
Dissimilar Metal ◽  
Residual Stress State ◽  
Weld Residual Stress

PWR nuclear power plants have dissimilar metal (DM) welds at many junctions between the vessels and the piping. The DM welds are made with Alloy 82 filler materials between carbon steel and stainless steel. These are potentially susceptible to Primary Water Stress Corrosion Cracking (PWSCC). PWSCC is mainly driven by the tensile weld residual stresses (WRS) that develop during fabrication of the piping system. In particular, weld repairs that often occur during the weld fabrication process also play a strong role in the development of the weld residual stress state in and near the DM welds. Most weld residual stress analyses performed to date in order to characterize the weld residual stress state in DM welds for PWSCC crack growth, leakage, and subsequent failure used axis-symmetric assessments. The purpose of this work is to provide direct assessment of the appropriateness of this axis-symmetric assumption on the WRS by comparison with full three dimensional analyses of several nozzles. In particular, weld start stop effects on the original weld will be assessed. In addition, the effect of partial arc weld repairs will be included. Repair cases considered include 15% and 50% deep repairs of length 48-degree and 96-degree of the circumference, along with the baseline case with no repair. The more complex three dimensional WRS state from the three dimensional analyses are compared to the corresponding axis-symmetric solutions and guidelines regarding the appropriateness of 2D solutions are discussed. Finally, some limited calculations of stress intensity factors at locations along the repair are presented.

An Analysis of Residual Stress Improvement for a Dissimilar Metal Nozzle-to-Pipe Weld by Using the Heat Sink Method

2008 ◽  
Author(s):  
J.-S. Park ◽  
J.-M. Kim ◽  
G.-H. Sohn ◽  
Y.-H. Kim
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Heat Sink ◽  
Nuclear Power ◽  
Power Plants ◽  
Base Alloy ◽  
Fatigue Cracking ◽  
Hole Drilling ◽  
Fe Model ◽  
Dissimilar Metal

This study is concerned with the mechanics analysis of residual stress improvement by the heat sink method applied to a dissimilar metal weld (DMW) for the use in nuclear power plants. The DMW joint considered here is composed of ferritic low-alloy steel nozzle, austenitic stainless steel safe-end, and nickel-base alloy A52 weld metal. To prepare the DMW joint with a narrow-gap, the gas tungsten arc welding (GTAW) process is utilized, and the heat sink method is employed to control thermal gradients developed in the critical region of work pieces during welding. Weld residual stresses are computed by the non-linear thermal elasto-plastic analysis using the axisymmetric finite element (FE) model, for which temperature-dependent thermal and mechanical properties of the materials are considered. A full-scale mock-up test is conducted to validate analytical solution for the DMW joint, and residual stresses are measured by using the hole-drilling method. Results of the FE modeling and mock-up test for the DMW joint are compared and effects of the heat sink method are discussed. It is found that a significant amount of residual compressive stresses can be developed on the inner surface of the DMW joint by using the heat sink method, which can effectively reduce the susceptibility of the welded materials to stress corrosion or fatigue cracking.

Korean Regulatory Experience on PWSCC in Korean NPP

2010 ◽  
Author(s):  
Kyung-Cho Kim ◽  
Sung-bu Choi ◽  
Koo-Kab Chung ◽  
Hae-Dong Chung
Keyword(s):  
Residual Stress ◽  
Boric Acid ◽  
Nuclear Power ◽  
Power Plants ◽  
Reactor Vessel ◽  
Inside Surface ◽  
Alloy 600 ◽  
Root Cause ◽  
Weld Material ◽  
Alloy 690

The degradation of alloy 600 and its weld material (alloy 82/182) has been reported in many nuclear power plants. In Korea, the crack induced by PWSCC was discovered in the drain nozzle of Yongkwang units 3 & 4 in 2006∼2008 and SG plug weld of Yongkwang unit 3 in 2007. In July 2007, during visual inspections of SG tube plugs at Yonggwang unit 4, boric acid deposits were observed around five Alloy 600 welded plugs. The root cause of the cracking in alloy 600 plugs was revealed to be due to the fact that the cracks were mainly caused by residual stress induced from the welding, expanding and tight-fitting. Younggwang unit 3 found the white small deposits on the drain nozzle on the 10th RFO in 2007. The root cause of the cracking in drain nozzle was revealed to be due to the initiation of a crack on the inside surface of drain nozzle and propagated to through wall cracks in the axial and circumferential direction. Younggwang unit 3 found the white widespread deposits on the upper head of a reactor vessel on the 12th RFO in 2010. Utility is trying to reveal the root cause of the cracking in the vent line of the reactor head according the KINS requirement. In this article, Korean regulatory experiences for PWSCC are introduced. After these PWSCC experiences, all SG tubes welded by Alloy 600 were replaced and all SG drain and instrumentation nozzles with Alloy 600 have been replaced into Alloy 690 material.

Welding Residual Stress and Flaw Evaluation for Dissimilar Metal Welds With Alloy 52 Inlays

2009 ◽  
Author(s):  
D. Rudland ◽  
A. Csontos ◽  
F. Brust ◽  
T. Zhang
Keyword(s):  
Residual Stress ◽  
Nickel Alloy ◽  
Nuclear Power ◽  
Mitigation Strategies ◽  
Welding Residual Stress ◽  
Residual Stress Field ◽  
Pressurized Water ◽  
Dissimilar Metal ◽  
Water Reactors ◽  
Resistant Layer

With the recent occurrences of primary water stress corrosion cracking (PWSCC) at nickel-based dissimilar metal welds (specifically Alloy 82/182 welds) in the nation’s pressurized water reactors (PWRs), the commercial nuclear power industry has been proposing a number of mitigation strategies for dealing with the problem. Some of these methods include Mechanical Stress Improvement Process (MSIP), Full and Optimized Structural Weld Overlay (FSWOL, OWOL) and Inlay and Onlay welds. All of these methods provide either a reduction in the ID residual stress field, (MSIP and WOL) and/or apply a corrosion resistant layer to stop or retard a leak path from forming (WOL, Inlay, Onlay). For the larger bore pipe, i.e. hot leg outlet nozzle, methods such as FSWOL become cost prohibitive due to the amount of weld metal that must be deposited. Therefore, inlay welds are being proposed since only a small layer (3 weld beads) needs to be deposited on the inside surface of the pipe. Currently the ASME code is developing Code Case N-766 ‘Nickel Alloy Reactor Coolant Inlay and Cladding for Repair or Mitigation of PWR Full Penetration Circumferential Nickel Alloy Welds in Class 1 Items.’ This code case is documenting the procedures for applying these inlay welds. As part of a confirmatory analysis, the US NRC staff and its contractor, Engineering Mechanics Corporation of Columbus, (Emc2) have conducted both welding residual stress and flaw evaluation analyses to determine the effectiveness of inlay welds as a mitigative technique. This paper presents the ongoing results from this effort. Using several large bore geometries, detailed welding simulation analyses were conducted on the procedures set forth in draft Code Case N-766. Effects of weld repairs and temper bead welding are included. Using these residual stress results, PWSCC growth analyses were conducted using simulated crack growth rates as a function of chromium content to estimate the time to leakage and rupture for small initial flaws in the inlay. The paper concludes with discussions on the effectiveness of inlays based on these analyses.

Weld Residual Stress Analysis and the Effects of Structural Overlay on Various Nuclear Power Plant Nozzles

2012 ◽  
Vol 134 (6) ◽  
Author(s):  
Tao Zhang ◽  
Frederick W. Brust ◽  
Gery Wilkowski ◽  
Chin-Cheng Huang ◽  
Ru-Feng Liu ◽  
...  
Keyword(s):  
Residual Stress ◽  
Stress Distribution ◽  
Residual Stresses ◽  
Stress Corrosion Cracking ◽  
Nuclear Power ◽  
Weld Overlay ◽  
Dissimilar Metal ◽  
Weld Residual Stress ◽  
Weld Material ◽  
Pipe Joints

Welding is a commonly used and one of the most important material-joining processes in industry. The incidences of defects had been located by ultrasonic testing in various pressurizer nozzle dissimilar metal welds (DMW) at nuclear power plants. In order to evaluate the crack propagation, it is required to calculate the stress distribution including weld residual stress and operational stress through the wall thickness in the weld region. The analysis procedure in this paper included not only the pass-by-pass welding steps but also other essential fabrication steps of surge, safety/relief, and spray nozzles. In this paper, detailed welding simulation analyses have been conducted to predict the magnitude of these stresses in the weld material. To prevent primary water stress corrosion cracking (PWSCC) in pressurized water reactors (PWR) on susceptible welded pipes with dissimilar metal welds, the weld overlay process has been applied to repair nuclear reactor pipe joints in plants. The objectives of such repairs are to induce compressive axial residual stresses on the pipe inside surface, as well as increase the pipe thickness with a weld material that is not susceptible to stress corrosion cracking. Hence, understanding the residual stress distribution is important to evaluate the reliability of pipe joints with weld overlay repairs. The finite element results in this paper showed that, after deposition of the DMW nozzle and stainless steel welds, tensile weld residual stresses still exist at regions of the DMW through the thickness. This tensile weld residual stress region was significantly reduced after welding the overlay. The overlay weld also provides a more uniform and large compressive region through the thickness, which has a beneficial effect on the structural integrity of the DMW in the plant.

Continuing Development of PRO-LOCA for the Prediction of Break Probabilities for Loss-of-Coolant Accidents

2009 ◽  
Author(s):  
D. Rudland ◽  
P. Scott ◽  
R. Kurth ◽  
A. Cox
Keyword(s):  
Residual Stress ◽  
Crack Initiation ◽  
Nuclear Power ◽  
Power Plants ◽  
Growth Models ◽  
Welding Residual Stress ◽  
Sensitivity Analyses ◽  
Pressurized Water ◽  
Initial Development ◽  
Dissimilar Welds

As part of a possible risk-informed revision of the design-basis break size requirements for operating commercial nuclear power plants as specified in the Code of Federal Regulations (CFR), the NRC began development of a probabilistic piping fracture mechanics code called PRO-LOCA. The initial development of this code and its background was published at a prior PVP conference. Since that time, the development of the PRO-LOCA code has continued through an international group program entitled Maximizing Enhancements in Risk Informed Technology (MERIT). The MERIT program includes participation from Canada, Korea, Sweden, UK, and the US (NRC and EPRI). The PRO-LOCA code, which aides in predicting piping break frequencies as a function of break size, incorporates many enhancements in technology since some of the earlier probabilistic codes (e.g., PRAISE) were developed. These enhancements include improved crack stability analyses, leak rate models, crack initiation and growth models, and material property data. In addition, degradation mechanisms such as primary water stress corrosion cracking (PWSCC) for dissimilar welds in pressurized water reactors (PWRs) are included in the PRO-LOCA code. This paper reviews the ongoing development of the PRO-LOCA code by giving a brief description of the recent updates made to the models embedded in the code. Some of these capabilities include improvements to crack initiation and growth models, welding residual stress distribution inputs, the addition of weld overlays, past and future inspections, the addition of importance sampling, and bootstrap methods for predicting confidence limits on output. The current version of the PRO-LOCA code was used for a sensitivity analyses in order to demonstrate the effects of welding residual stress uncertainty on the probability of leak and rupture. Plans for the continuing development of the PRO-LOCA code conclude this paper.

Analytical Study of the Relaxation of Welding Residual Stress by Excessive Loading for Austenitic Stainless Steel Piping Welds

2009 ◽  
Author(s):  
Jinya Katsuyama ◽  
Kunio Onizawa
Keyword(s):  
Residual Stress ◽  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Nuclear Power ◽  
Power Plants ◽  
Three Dimensional ◽  
Welding Residual Stress ◽  
Low Carbon ◽  
Three Dimensional Model ◽  
Butt Weld

Welding residual stress is one of the most important factors of stress corrosion cracking (SCC) for austenitic stainless steel in pressure boundary piping in nuclear power plants. The effect of excessive loading, such as an earthquake, on the residual stress was evaluated by three-dimensional analyses based on finite element method (FEM). The FEM analyses were performed using three-dimensional model for a 250A piping butt weld of low carbon stainless steel of Type 316L. A welding simulation method used in this work is based on the moving heat source with the double ellipsoid model and was confirmed by comparing with the experimental measurements. After conducting welding residual stress simulation, several loading patterns of bending moment and uni-axial displacements have been applied to a model by varying amount of moment and displacement. The analyses indicated that higher loading to bending and axial stresses caused higher relaxation of welding residual stress near piping welds. The difference in the effect of loading direction was observed for both cases. It is concluded that the SCC growth rate might be decreased as loading level increased.

Sign in / Sign up

Export Citation Format

Share Document