Predicting welding residual stresses in a dissimilar metal girth welded pipe using 3D finite element model with a simplified heat source

Dissimilar metal welds are commonly used in nuclear power plants to connect low alloy steel components and austenitic stainless steel piping systems. The integrity assessment and life estimation for such welded structures require consideration of residual stresses induced by manufacturing processes. Because the fabrication process of dissimilar metal weld joints is considerably complex, it is very difficult to accurately predict residual stresses. In this study, both numerical simulation technology and experimental method were used to investigate welding residual stress distribution in a dissimilar metal pipe joint with a medium diameter, which were performed by a multi-pass welding process. Firstly, an experimental mock-up was fabricated to measure the residual stress distributions on the inside and the outside surfaces. Then, a time-effective 3-D finite element model was developed to simulate welding residual stresses through using a simplified moving heat source. The simplified heat source method could complete the thermo-mechanical analysis in an acceptable time, and the simulation results generally matched the measured data near the weld zone. Through comparing the simulation results and the experimental measurements, we can infer that besides the multi-pass welding process other key manufacturing processes such as cladding, buttering and heat treatment should also be taken into account to accurately predict residual stresses in the whole range of the dissimilar metal pipe.     

Numerical weld modeling — a method for calculating weld-induced residual stresses

In the past, weld-induced residual stresses caused damage to numerous (power) plant parts, components and systems (Erve, M., Wesseling, U., Kilian, R., Hardt, R., Brümmer, G., Maier, V., Ilg, U., 1994. Cracking in Stabilized Austenitic Stainless Steel Piping of German Boiling Water Reactors — Characteristic Features and Root Causes. 20. MPA-Seminar 1994, vol. 2, paper 29, pp.29.1–29.21). In the case of BWR nuclear power plants, this damage can be caused by the mechanism of intergranular stress corrosion cracking in austenitic piping or the core shroud in the reactor pressure vessel and is triggered chiefly by weld-induced residual stresses. One solution of this problem that has been used in the past involves experimental measurements of residual stresses in conjunction with weld optimization testing. However, the experimental analysis of all relevant parameters is an extremely tedious process. Numerical simulation using the finite element method (FEM) not only supplements this method but, in view of modern computer capacities, is also an equally valid alternative in its own right. This paper will demonstrate that the technique developed for numerical simulation of the welding process has not only been properly verified and validated on austenitic pipe welds, but that it also permits making selective statements on improvements to the welding process. For instance, numerical simulation can provide information on the starting point of welding for every weld bead, the effect of interpass cooling as far as a possible sensitization of the heat affected zone (HAZ) is concerned, the effect of gap width on the resultant weld residual stresses, or the effect of the ‘last pass heat sink welding’ (welding of the final passes while simultaneously cooling the inner surface with water) producing compressive stresses in the root area of a circumferential weld in an austenitic pipe. The computer program (finite element residual stress analysis) was based on a commercially available code (Hibbitt, Karlsson, Sorensen, Inc, 1997. user's manual, version 5.6), and can be used as a 2-D or 3-D FEM analysis; depending on task definition it can provide a starting point for a fracture mechanics safety analysis with acceptable computing times.     

TC4钛合金焊后电子束局部热处理及焊接残余应力测试研究

针对TC4钛合金通过电子束焊后局部热处理工艺来改善接头的性能,采用X射线法对电子束局部热处理后焊接残余应力的分布趋势进行了测试研究.试验结果表明:经电子束局部热处理后,TC4钛合金焊缝区马氏体晶粒组织得到细化;同时上表面接头残余应力的分布得到改善.     

Reduction method for residual stress of welded joint using harmonic vibrational load

Welding is widely used for construction of many structures. Since welding is a process using locally given heat, residual stress is generated near the bead. Tensile residual stress degrades fatigue strength. Some reduction methods of residual stress have been presented and, for example, heat treatment and shot peening are practically used. However, those methods need special tools and are time consuming. In this paper, a new method for reduction of residual stress using harmonic vibrational load during welding is proposed. The proposed method is examined experimentally for some conditions. Two thin plates are supported on the supporting device and butt-welded using an automatic CO₂ gas shielded arc welding machine. Residual stress in the direction of the bead is measured by using a paralleled beam X-ray diffractometer with scintillation counter after removing quenched scale chemically. First, the welding of rolled steel for general structure for some excitation frequencies is examined. Specimens are welded along the groove on both sides. For all frequencies, tensile residual stress near the bead is significantly reduced. Second, welding of the specimen made of high tensile strength steel is examined. In this case, tensile residual stress near the bead is also reduced. Finally, the proposed method is examined by an analytical method. An analytical model which consists of mass and preloaded springs with elasto-plastic characteristic is used. Reduction of residual stress is demonstrated using this model.     

Effect of geometric construction on residual stress distribution in designing a nuclear rotor joined by multipass narrow gap welding

The purpose of this study is to investigate the effect of geometric construction on the distribution of residual stresses before and after heat treatment in designing a nuclear welded rotor. The local material removal method was used to measure internal residual stress of the experimental pipe after post weld heat treatment. Three finite element models were employed as follows: a model of experimental pipe, a model with a bottom protrusion existed at the weld region, and a model of two rotor discs butt-welded with a bottom protrusion at the weld region. Investigated results showed that the bottom protrusion existed at the weld region can decrease the residual stress and mitigate the stress evolution significantly on the inner surface. Under the binding effect of the rotor discs, the axial stress of inner surface region is compressive stress; the through-wall axial stress at the weld center line can be deemed to a bending type; both the hoop stress and axial stress at the weld center line on the inner surface are compressive. The impact of geometric construction on the stress evolution at the root bead begins after pass 15 deposited.     

Reduction method for residual stress of welded joint using random vibration

Welded joints are used for construction of many structures. Residual stress is induced near the bead caused by locally given heat. Tensile residual stress on the surface may reduce fatigue strength. In this paper, a new method for reduction of residual stress using vibration during welding is proposed. As vibrational load, random vibration, white noise and filtered white noise are used. Two thin plates are butt-welded. Residual stress is measured with a paralleled beam X-ray diffractometer with scintillation counter after removing quenched scale chemically. It is concluded that tensile residual stress near the bead is reduced by using random vibration during welding.     

Fatigue crack growth under residual stress field in low-carbon steel

The influence of residual stress on fatigue crack growth was experimentally and analytically investigated for surface crack. Fatigue tests were performed on straight pipe components of low-carbon steel having a circumferential inner surface crack in laboratory air environment. Some of the test pipes had been subjected to special heat treatments so as to have compressive or tensile residual stresses along the inner surface.The results show that the compressive residual stress remarkably suppresses the surface crack growth while the tensile residual stress doesn't accelerate the crack growth very much.The crack growth analyses were conducted by the application of power relationship between ΔK and . The stress intensity factors due to the non-linear stress field were calculated by the weight function method. The analyses resulted in a confirmation of the behavior of the crack growth observed in the experiments.     

Prediction of residual stress distributions due to surface machining and welding and crack growth simulation under residual stress distribution

In nuclear power plants, stress corrosion cracking (SCC) has been observed near the weld zone of the core shroud and primary loop recirculation (PLR) pipes made of low-carbon austenitic stainless steel Type 316L. The joining process of pipes usually includes surface machining and welding. Both processes induce residual stresses, and residual stresses are thus important factors in the occurrence and propagation of SCC. In this study, the finite element method (FEM) was used to estimate residual stress distributions generated by butt welding and surface machining. The thermoelastic-plastic analysis was performed for the welding simulation, and the thermo-mechanical coupled analysis based on the Johnson-Cook material model was performed for the surface machining simulation. In addition, a crack growth analysis based on the stress intensity factor (SIF) calculation was performed using the calculated residual stress distributions that are generated by welding and surface machining. The surface machining analysis showed that tensile residual stress due to surface machining only exists approximately 0.2 mm from the machined surface, and the surface residual stress increases with cutting speed. The crack growth analysis showed that the crack depth is affected by both surface machining and welding, and the crack length is more affected by surface machining than by welding.     

Alternative methods for postweld treatment of austenitic pipe welds to increase the operational safety of BWR plants

Experience has shown that austenitic piping is susceptible to IGSCC in the weld root area under BWR service conditions. Besides non-optimized materials, the residual stresses which are an inherent result of conventional welding processes are also responsible for this susceptibility. In the past, mechanical, thermal and welding post-treatment processes were developed and used with the objective of reducing tensile stresses in the root area. This paper discusses past experience and more recent developments, in particular the latest results with pipe welds treated by means of welding processes (last pass heat sink welding). These measures are suitable for producing compressive stresses in the medium-swept ID-HAZ of austenitic welds, or to at least significantly reduce the tensile stresses and thus practically eliminate the risk of IGSCC.     

控制棒驱动机构上部Ω焊缝堆焊修复结构完整性分析

针对核电厂控制棒驱动机构（CRDM）上部Ω焊缝堆焊修复（WOR）技术,采用数值模拟方法进行了修复结构完整性评估。根据堆焊修复参数制定二维轴对称高斯热源等效输入,并采用ANSYS程序的单元生死技术模拟焊接过程,得到了结构的焊接残余应力。考虑电厂运行的全部瞬态,计算了结构的瞬态应力,并开展了疲劳分析。结合焊接残余应力分析和瞬态应力分析的结果,开展了断裂力学分析。结果表明,WOR结构的疲劳结果、应力强度因子及裂纹扩展等方面均能满足相应的规范要求。      

Reduced stress welding process for nuclear plant piping

This paper discusses the development and application of FineLine™ Welding (FLW). FLW is a modified Gas-Tungsten-Arc Welding mechanized process developed by GE Nuclear Energy. The FLW process offers significant improvements over standard and narrow-gap welding. For piping, FLW reduces the weld groove width and volume of weld metal compared to standard ‘V’ or J-Bevel welds by approximately 30% for smaller pipe and 70% or more for larger sizes. Also, extremely low heat inputs are achieved, and typically result in: (1) significant reductions in welding elapsed time, particularly for thicker walls, (2) compressive/very low tensile residual stresses on the pipe inside surface, and (3) improved heat-affected-zone microstructure, with reduced shrinkage/distortion. The improved residual stress and metallurgical states provide increased resistance to stress corrosion cracking in high temperature, oxidizing water. The FLW process is successful in eliminating the need for secondary stress improvement process such as induction-heating stress improvement. Verifications of the stress improvement were performed on standard V-groove and narrow-gap welds, which were compared to thin and thick-wall stainless steel pipe FLW welds. Testing included strain-gage, X-ray diffraction, magnesium chloride and metallographic evaluations. The results show significant improvement in the residual stress level and uniformity of the FLW welds.     

Prediction of residual stress in the welding zone of dissimilar metals using data-based models and uncertainty analysis

Since welding residual stress is one of the major factors in the generation of primary water stress-corrosion cracking (PWSCC), it is essential to examine the welding residual stress to prevent PWSCC. Therefore, several artificial intelligence methods have been developed and studied to predict these residual stresses. In this study, three data-based models, support vector regression (SVR), fuzzy neural network (FNN), and their combined (FNN + SVR) models were used to predict the residual stress for dissimilar metal welding under a variety of welding conditions. By using a subtractive clustering (SC) method, informative data that demonstrate the characteristic behavior of the system were selected to train the models from the numerical data obtained from finite element analysis under a range of welding conditions. The FNN model was optimized using a genetic algorithm. The statistical and analytical uncertainty analysis methods of the models were applied, and their uncertainties were evaluated using 60 sampled training and optimization data sets, as well as a fixed test data set.     

Effect of geometric conditions on residual stress of brazed stainless steel plate-fin structure

Plate-fin heat exchanger (PFHE) is the delegate of the miniaturization of heat exchanging equipment. As the core of PFHE, plate-fin structure is fabricated by vacuum brazing. Owing to the material mismatching between filler metal and base metal, residual stresses can arise during the brazing process and decrease the structure strength greatly. Therefore, estimating the magnitude and distribution of residual stress and discussing its influence factors for the early design and fabrication of plate-fin structure are deemed necessary. This paper presented a finite element analysis (FEA) of brazed residual stresses for a counter-flow stainless steel plate-fin structure. The effects of geometric conditions including brazing gap, plate thickness, fin thickness, fin pitch, fin height and fin layers on residual stress were investigated. This work provides a reference for design and decreasing residual stress for stainless steel plate-fin structure.     

Residual stress prediction of dissimilar metals welding at NPPs using support vector regression

Residual stresses are an important factor in the component integrity and life assessment of welded structures. In this paper, a support vector regression (SVR) method is presented to predict the residual stress for dissimilar metal welding according to various welding conditions. Dissimilar welding joint between nozzle and pipe is regarded in the analyses since it has been known to be highly susceptible to Primary Water Stress Corrosion Cracking (PWSCC) in the primary system of a nuclear power plant (NPP). The residual stress distributions are predicted along two straight paths of a weld zone: a pipe flow path on the inner weld surface and a path connecting two centers of the inner and outer surfaces of a weld zone of a pipe. Four SVR models are developed for four numerical data groups which are split according to the two end section constraints and the two prediction paths and the SVR models are optimized by a genetic algorithm. The SVR models are trained by using a data set prepared for training, optimized by using an optimization data set, and verified by using a test data set independent of the training data and the optimization data. It is known that the SVR models are sufficiently accurate to be used in the integrity evaluation by predicting the residual stress of dissimilar metal welding zones.     

Control of welding residual stress for ensuring integrity against fatigue and stress–corrosion cracking

The availability of several techniques for residual stress control is discussed in this paper. The effectiveness of these techniques in protecting from fatigue and stress–corrosion cracking is verified by numerical analysis and actual experiment. In-process control during welding for residual stress reduction is easier to apply than using post-weld treatment. As an example, control of the welding pass sequence for multi-pass welding is applied to cruciform joints and butt-joints with an X-shaped groove. However, residual stress improvement is confirmed for post-weld processes. Water jet peening is useful for obtaining a compressive residual stress on the surface, and the tolerance against both fatigue and stress–corrosion cracking is verified. Because cladding with a corrosion-resistant material is also effective for preventing stress–corrosion cracking from a metallurgical perspective, the residual stress at the interface of the base metal is carefully considered. The residual stress of the base metal near the clad edge is confirmed to be within the tolerance of crack generation. Controlling methods both during and after welding processes are found to be effective for ensuring the integrity of welded components.     

Transient behavior of the sodium–potassium alloy heat pipe in passive residual heat removal system of molten salt reactor

High temperature heat pipes, as highly-effective heat transfer elements, have been extensively employed in thermal management for their remarkable advantages in conductivity, isothermality and self-actuating. It is of significance to apply heat pipes to new concept passive residual heat removal system (PRHRS) of molten salt reactor (MSR). In this paper, the new concept PRHRS of MSR using sodium–potassium alloy (NaK) heat pipes is proposed in detail, and then the transient behavior of high temperature NaK heat pipe is numerically investigated using the Finite Element Method (FEM) in the case of MSR accident. The two-dimensional transient conduction model for the heat pipe wall and wick structure is coupled with the one-dimensional quasi-steady model for the vapor flow when vaporization and condensation occur at the liquid–vapor interface. The governing equations coupled with boundary conditions are solved by FORTRAN code to obtain the distributions of the temperature, velocity and pressure for the heat pipe transient operation. Numerical results indicated that high temperature NaK heat pipe had a good operating performance and removed the residual heat of fuel salt significantly for the accident of MSR.     

Simplified analysis of shrinkage in pipe to pipe butt welds

Generally some shrinkage is typical of butt welding of pipes. Shrinkage due to butt welding could be more pronounced and significant in thin wall stainless steel pipes because the thermal expansion coefficient is roughly one and half times that of carbon steel. An axisymmetric finite element evaluation of hoop shrinkage associated with circumferential butt welds in thin wall stainless steel pipes was performed. Actual shrinkage data for a larger (24 in. diameter, 0.375 in. wall thickness) pipe and a smaller (4 in. diameter, 0.237 in. wall thickness) pipe were utilized. The results indicate that very localized residual stresses in excess of yield strength produced during cooldown of metal in the weld and heat affected zones cause redistribution of the stresses. A simplified elastic–plastic analysis approach was developed with adjustments for section modulus and Poisson’s ratio, and the strains due to radial shrinkage were calculated for inside and outside surfaces of the pipe at the weld center line. From the strain point of view, the strain values in the circumferential direction were about 1.4% for the larger size pipe and 3.4% for the smaller size pipe. The strain values in the axial direction were 2.5% for the larger pipe and 5.9% for the smaller pipe. It is concluded that these levels of strains are not detrimental in nature. However, for the smaller pipe they are on the high side and it is recommended not to use the pipe for elevated temperature service. Residual stresses were also calculated for inside and outside surfaces of the pipe at weld center line using a simplified elastic–plastic approach and a bilinear stress–strain curve and compared with published data indicating a general agreement.     

焊后热处理对16MND5钢焊接组织结构与残余应力的影响

16MND5钢广泛应用于核岛承压容器构件,其焊接接头不可避免地会引入高的残余应力,而焊后热处理可有效消减焊接残余应力以克服应力腐蚀裂纹的影响。本工作利用轮廓法和中子衍射技术研究了焊后热处理对16MND5钢焊接残余应力的影响。结果表明,轮廓法与中子衍射测试结果在趋势和数值上取得了较好的一致性,焊后热处理使焊接态的残余应力峰值从约420 MPa降低至约210 MPa。同时,利用金相法和SEM研究了焊后热处理对焊缝区域组织结构的影响。结果表明,焊后热处理主要表现为贝氏体和少量自回火马氏体的焊缝中心组织转变为回火贝氏体和回火马氏体,热处理后的焊缝区晶粒明显长大。     

Use of a new methodology for prediction of weld distortion and residual stresses using FE simulation applied to ITER vacuum vessel manufacture

The ITER VV Sectors have to be manufactured to extremely tight tolerances, without the production of a prototype to establish the welding distortions. It is thus necessary to develop a (computationally) successful methodology for modelling weld process in order to predict welding distortion and residual stresses. Further, in order to optimise the manufacture sequence and method, the model should be able to rapidly assess the effect of using different sequences. Once the optimal sequence has been decided, then the method has also to be able to accurately predict the final shape.     

Remove Welding Residual Stress for CFETR Vacuum Vessel by Trailing Ultrasonic Impact Treatment

The China fusion engineering test reactor (CFETR) vacuum vessel is welded by narrow gap TIG (NG-TIG) welding, and the welding residual stress of the CFETR vacuum vessel can be redistributed by trailing welding ultrasonic impact treatment. In order to investigate the feasibility of the residual stress removing scheme, and to obtain the optimal trailing ultrasonic impact treatment technological parameters in the process of removing welding residual stress, a welding model that similar to vacuum vessel welding seam is established by using ABAQUS software, a NG-TIG welding heat source subroutine which is written in FORTRAN language used to simulate NG-TIG welding. According to the welding simulation results, a trailing welding ultrasonic impact treatment model is established, and the effects of the impact pin number, the impact method, the impact pin diameter and the impact frequency on welding residual stress are studied. The results show that the longitudinal residual stress in welding seam and its adjacent area and the lateral residual stress in the whole region have been obviously decreased by different trailing welding ultrasonic impact processes, and have made the tensile stress in the welding seam and its adjacent area has been changed into compressive stress, which can provide theoretical guidance and reference for actual production.