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

2010 ◽  
Author(s):  
Tao Zhang ◽  
Bud Brust ◽  
Gery Wilkowski ◽  
Sam Ranganath ◽  
Lihua Wang ◽  
...  
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 (UT) 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 nozzle welds in the plant.

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.

Weld-Overlay Analyses: An Investigation of the Effect of Weld Sequencing

2008 ◽  
Author(s):  
T. Zhang ◽  
G. Wilkowski ◽  
D. Rudland ◽  
F. Brust ◽  
H. S. Mehta ◽  
...  
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Nuclear Reactor ◽  
Internal Cooling ◽  
Element Analysis ◽  
Weld Overlay ◽  
Weld Material ◽  
Diameter Pipe ◽  
Pipe Joints ◽  
Girth Welds

The weld overlay process has been developed and applied to repair of nuclear reactor pipe girth welds for many years in BWR plants. The objectives of such repairs were 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. In this paper, a six-inch diameter Schedule 120 stainless steel pipe with an overlay thickness of 7.87 mm (0.31 inch) was picked as a validation case. Weld sequencing effects were thoroughly studied. The residual stresses were calculated by using thermal elasto-plastic finite-element analysis (FEA). After comparing results using different weld sequences, it was found that the calculated weld residual stresses on ID surface were very sensitive to weld sequencing in FE analyses as well as internal cooling rate. The influence of the weld sequencing was relatively secondary to the pipe distortion. An optimum (producing compressive residual stress on the ID surface) weld sequencing was obtained and applied to a 711.2 mm (28-inch) diameter pipe-to-elbow girth weld with an overlay thickness of 24.9 mm (0.98 inch) and a pipe thickness of 29.5 mm (1.16 inch).

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.

Weld Residual Stress in Large Diameter Nuclear Nozzles

2011 ◽  
Author(s):  
Tao Zhang ◽  
F. W. Brust ◽  
Gery Wilkowski
Keyword(s):  
Residual Stress ◽  
Residual Stresses ◽  
Nuclear Power ◽  
Kinematic Hardening ◽  
Large Diameter ◽  
Thick Wall ◽  
Isotropic Hardening ◽  
Residual Stress Field ◽  
Mixed Hardening ◽  
Weld Residual Stress

Weld residual stresses in nuclear power plant can lead to cracking concerns caused by stress corrosion. These are large diameter thick wall pipe and nozzles. Many factors can lead to the development of the weld residual stresses and the distributions of the stress through the wall thickness can vary markedly. Hence, understanding the residual stress distribution is important to evaluate the reliability of pipe and nozzle joints with welds. This paper represents an examination of the weld residual stress distributions which occur in various different size nozzles. The detailed weld residual stress predictions for these nozzles are summarized. Many such weld residual stress solutions have been developed by the authors in the last five years. These distributions will be categorized and organized in this paper and general trends for the causes of the distributions will be established. The residual stress field can therefore feed into a crack growth analysis. The solutions are made using several different constitutive models such as kinematic hardening, isotropic hardening, and mixed hardening model. Necessary fabrication procedures such as repair, overlay and post weld heat treatment are also considered. Some general discussions and comments will conclude the paper.

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.

High-Temperature Constitutive Behavior of Austenitic Stainless Steel for Weld Residual Stress Modeling

2012 ◽  
Author(s):  
Dongxiao Qiao ◽  
Wei Zhang ◽  
Zhili Feng
Keyword(s):  
Residual Stress ◽  
Finite Element ◽  
Stainless Steel ◽  
High Temperature ◽  
Residual Stresses ◽  
Constitutive Rule ◽  
Dissimilar Metal ◽  
Weld Residual Stress ◽  
Dissimilar Metal Weld ◽  
Metal Weld

Weld residual stress is a major driving force for initiation and growth of primary water stress corrosion cracking (PWSCC), which is a critical challenge for weld integrity of reactor pressure vessel nozzles in nuclear industry. Predicting weld residual stresses for the purpose of understanding and mitigating PWSCC requires the knowledge of material constitutive rule especially strain hardening behavior over a wide range of temperatures. Though it is adequate for describing deformation at low temperature, the conventional, rate-independent, elastic-plastic constitutive rule falls short in predicting the strong microstructure-mechanical interaction such as the softening due to recovery (dislocation annihilation and realignment) and recrystallization at elevated temperature in welding. To quantify the extent of softening under temperature and strain conditions relevant to welding, a framework has been developed by combining advanced experimental techniques and finite element modeling. First, physical simulation in a Gleeble testing machine is used to simulate the temperature transients typical of dissimilar metal weld by subjecting round tensile bar shaped specimens to rapid heating and cooling. Second, the digital image correlation (DIC) technique is used to map the non-uniform strain field and extract local strain history needed for accurately determining the true stress vs. true strain curve of softened material. Third, the thermally-mechanically processed specimens are characterized metallographically to correlate the microstructure changes to the measured stress-strain behavior. Finally, a thermal-stress finite element model of three-bar frame is used to study the effect of softening on the predicted weld residual stresses. As a first step toward developing the much-needed, comprehensive material constitutive relation database for dissimilar metal weld, the framework has been applied to study AISI 304L austenitic stainless steel. The extent of softening due to different duration of high-temperature exposure is studied and its influence on final residual stresses is discussed.

Important Residual Stress Features in Reactor Nozzle Dissimilar Metal Welds

2011 ◽  
Author(s):  
Jinmiao Zhang ◽  
Shaopin Song ◽  
Pingsha Dong
Keyword(s):  
Residual Stress ◽  
Stress Distribution ◽  
Nuclear Reactor ◽  
Residual Stress Distribution ◽  
Stress Distributions ◽  
Weld Overlay ◽  
Dissimilar Metal ◽  
Nozzle Length ◽  
Dissimilar Metal Weld ◽  
Metal Weld

This paper is focused on the study of residual stress distribution at a dissimilar metal weld (DMW) of nuclear reactor nozzle. The paper extends some of the recent research on this subject by investigating the effect of weld sequence and nozzle length design on the residual stress distributions. It also investigates the effect of a partial excavation repair and a weld overlay on the residual stress distribution. As a result, some of the important residual stress features at DMW are revealed and these features are discussed and summarized in the paper.

An Analytical Evaluation of the Effect of Weld Material Thickness on Residual Stresses Produced by Structural Weld Overlays on Pressurized Water Reactor Primary Cooling Piping

2009 ◽  
Author(s):  
L. F. Fredette ◽  
Paul M. Scott ◽  
F. W. Brust ◽  
A. Csontos
Keyword(s):  
Residual Stresses ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
Intergranular Stress Corrosion ◽  
Pressurized Water ◽  
Weld Overlay ◽  
Water Reactor ◽  
Dissimilar Metal ◽  
Structural Reinforcement

Full Structural Weld Overlay (FSWOL) has been used successfully to mitigate intergranular stress corrosion cracking in boiling water reactor (BWR) welded stainless steel piping for many years. The FSWOL technique adds structural reinforcement, can add crack resistant material, and can create compressive residual stresses at the inside surface of the welded joint which reduces the possibility of further stress corrosion cracking. Recently, the FSWOL has been applied as a preemptive measure to prevent primary water stress corrosion cracking (PWSCC) in pressurized water reactors (PWR) on susceptible welded pipes with dissimilar metal welds common to PWR primary cooling piping. This study uses finite element models to evaluate the likely residual and operating stress profiles remaining after FSWOL and describes the results of sensitivity studies which were performed to examine the effect of weld overlay thickness on the residual stresses for typical dissimilar metal weld configurations.

Summary of Weld Residual Stress Analyses for Dissimilar Metal Weld Nozzles

2010 ◽  
Author(s):  
F. W. Brust ◽  
Tao Zhang ◽  
Do-Jun Shim ◽  
Sureshkumar Kalyanam ◽  
Gery Wilkowski ◽  
...  
Keyword(s):  
Residual Stress ◽  
Stainless Steel ◽  
Residual Stresses ◽  
Weld Metal ◽  
Crack Growth ◽  
Steam Generator ◽  
Deep Hole ◽  
Pressurized Water ◽  
Dissimilar Metal ◽  
Weld Residual Stress

Flaw indications have been found in some dissimilar metal nozzle to stainless steel piping welds in pressurized water reactors (PWR) throughout the world. The nozzle welds usually involve welding ferritic (often A508) nozzles to 304/316 stainless steel pipe using Alloy 182/82 weld metal. Due to an unexpected aging issue with the weld metal, the weld becomes susceptible to a form of corrosion cracking referred to as primary water stress corrosion cracking (PWSCC). It can occur if the temperature is high enough (usually >300C) and the water chemistry in the PWR is typical of operating plants. This paper represents one of a series of papers which examine the propensity for cracking in a particular operating PWR in the UK. This paper represents an examination of the weld residual stress distributions which occur in four different size nozzles in the plant. Companion papers in this conference examine crack growth and PWSCC mitigation efforts related to this plant. British Energy (BE) has developed a work program to assess the possible impact of PWSCC on dissimilar metal welds in the primary circuit of the Sizewell ‘B’ pressurized water reactor. This effort has included the design and manufacture of representative PWR safety/relief valve nozzle welds both with and without a full structural weld overlay, multiple residual stress measurements on both mock-ups using the deep hole and incremental deep hole methods, and a number of finite element weld residual stress simulations of both the mock-ups and equivalent plant welds. This work is summarized in companion papers [1–3]. Here, the detailed weld residual stress predictions for these nozzles are summarized. The weld residual stresses in a PWR spray nozzle, safety/relief nozzle, surge nozzle, and finally a steam generator hot-leg nozzle are predicted here using an axis-symmetric computational weld solution process. The residual stresses are documented and these feed into a natural crack growth analysis provided in a companion PVP 2010-25162 paper [1]. The solutions are made using several different constitutive models: kinematic hardening, isotropic hardening, and a mixed hardening model. Discussion will be provided as to the appropriateness of the constitutive model for multi-pass DM weld modeling. In addition, the effect of including or neglecting the post-weld heat treatment process, which typically occurs after the buttering process in a DM weld, is presented. During operation the DM welds in a PWR experience temperatures in excess of 300°C. The coefficient of thermal expansion (CTE) mismatch between the three materials, particularly the higher CTE in the stainless steel, affects the stresses at operating temperature. The K-weld geometry used in the steam generator nozzles in this plant combines with CTE mis-match effects to result in service stresses somewhat different from V-weld groove cases.

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