Numerical simulation of residual stresses induced by laser beam welding in a SUS316 stainless steel pipe with considering initial residual stress influences

With the development of computer hardware and software, numerical simulation technology has been widely used to predict welding temperature field, residual stresses and distortion. However, till now the influences of initial stresses induced by the manufacturing process before welding on the welding-induced residual stresses are rarely investigated experimentally and numerically. In the present work, we have developed a computational approach based on thermal elastic plastic FEM to clarify how the initial stresses due to heat treatment affect the welding-induced residual stresses in an austenitic stainless steel pipe. A heat treatment process, which is similar to solution heat treatment, is employed to produce initial stresses in the pipe before welding. After the heat treatment, the laser beam welding is used to perform a girth weld in the middle of the pipe. Through comparing the residual stress distributions after heat treatment and laser beam welding, we have investigated the influence of the initial residual stresses on the welding-induced residual stresses. The numerical results suggest that the initial residual stresses prior to welding have significant effects on the residual stresses after welding in the pipe model.     

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.     

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。      

Modeling of weld residual stresses in core shroud structures

This paper presents a computational model to predict residual stresses in a girth weld (H4) of a BWR core shroud. The H4 weld is a multi-pass submerged-arc weld that joins two type 304 austenitic stainless steel cylinders. An axisymmetric solid element model was used to characterize the detailed evolution of residual stresses in the H4 weld. In the analysis, a series of advanced weld modeling techniques were used to address some specific welding-related issues, such as material melting/re-melting and history annihilation. In addition, a 3-D shell element analysis was performed to quantify specimen removal effects on residual stress measurements based on a sub-structural specimen from a core shroud. The predicted residual stresses in the H4 weld were used as the crack driving force for the subsequent analysis of stress corrosion cracking in the H4 weld. The crack growth behavior was investigated using an advanced finite element alternating method (FEAM). Stress intensity factors were calculated for both axisymmetric circumferential (360°) and circumferential surface cracks. The analysis results obtained from these studies shed light on the residual stress characteristics in core shroud weldments and the effects of residual stresses on stress corrosion cracking behavior.     

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.     

Effects of thermal load and cooling condition on weld residual stress in a core shroud with numerical simulation

Stress corrosion cracking (SCC) in the heat affected zone is the primary damage form due to weld residual stress, corrosion and neutron irradiation environment in the core shroud of a boiling water reactor. The distribution of weld residual stress around a weld is necessary to be clarified to evaluate the structural integrity of core shroud for SCC. Moreover, studying the effects of welding parameters on residual stress on reducing the residual stress is very important to suppress the initiation and propagation of SCC.In this paper, we used a finite element method (FEM) to clarify the distribution of weld residual stress around the sixth horizontal weld (H6a) between the lower ring and the cylinder in the core shroud. The simulation results of axial stress were consistent with the experimental results at the inside and outside surfaces of the core shroud, respectively. The effects of thermal loads and cooling conditions were also investigated with the same model. We simulated the welding progress with water cooling on the inside and outside surfaces of the core shroud in order to study the influence of cooling conditions on the residual axial stress around the weld. The simulation results indicated that water cooling decreased the residual axial stress at the same side due to changing the temperature-affected fields. Moreover, with fixing the peak temperatures of weld passes, the simulation results of the distribution of residual axial stress by the thermal loads with different heating time were compared. The simulation results suggested that the heating time was expected to be longer and the heat flux to be smaller for reaching the small tension residual axial stress or even compression stress around the H6a weld.     

贯穿件J形坡口焊接残余应力分析

核电站反应堆压力容器(RPV)顶盖控制棒驱动机构(CRDM)管座J形坡口焊缝在一回路高温高压水环境下存在应力腐蚀开裂(SCC)的风险,而焊接残余应力是SCC的主要驱动力。使用二维轴对称模型有限元方法对CRDM中心管座J形坡口进行焊接残余应力分析。为了探索一种简单、高效和保守的方法,研究了热源简化、焊缝形状简化、屈服强度、相变和强化行为对焊接残余应力的影响。结果表明:双椭球热源与均匀热源得到的残余应力结果基本一致;焊缝形状由鱼鳞状简化为方块模型对焊接残余应力结果影响不大,但是与合并焊道的结果相差较大;采用低屈服强度得到的残余应力结果并不保守;在ANSYS软件中,固液相变对残余应力结果影响不大;等向强化模型的结果比随动强化模型的结果保守;在工程上,建议采用均匀热源、方块焊道模型和等向强化模型进行焊接模拟。     

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.     

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.     

核电仪表罐焊接工艺研究

在防城港核电厂二期工程3、4号机组中,部分产品选用了控氮奥氏体不锈钢作为仪表罐的主体材料,仪表罐需要按RCC-M 2007版标准进行制造.本文对控氮奥氏体不锈钢焊接的相关问题进行了总结,经过一系列的焊接工艺评定和焊接性试验,解决了诸多焊接方面的难题,同时收获了应用RCC-M 2007版S篇标准的一些心得.     

Toughness investigation on simulated weld HAZs of SQV-2A pressure vessel steel

In order to get detailed information about weld HAZs toughness of SQV-2A steel and determine the optimum welding and heat treatment parameters, the toughness of simulated CGHAZs (coarse grained heat affected zone) and CGHAZs (intercritically reheated CGHAZ) were systematically investigated. The influence of tempering thermal cycles on weld ICCGHAZs toughness was clarified. The effect of post weld heat treatments (PWHT) on weld CGHAZs toughness was also determined. The results showed that high toughness (absorbed energy >200 J) of weld HAZs could be achieved by selecting the optimum welding and PWHT parameters (cooling time Δt_8/5: 6–40 s, PWHT: 893 K, 3.6–7.2 ks). Tempering thermal cycles with peak temperature of above 573 K could remarkably improve the toughness of deteriorated ICCGHAZs and reduce the hardness, when cooling time Δt_8/5(2) of the reheating thermal cycle was 6 s, which implies that welding of SQV-2A without PWHT is possible, provided that low heat input welding is adopted and welding procedure is correctly arranged. Metallography and fractography revealed that M–A constituents in weld HAZs played an important role in controlling weld HAZ toughness.     

Development of new Z-factors for the evaluation of the circumferential surface crack in nuclear pipes

The purpose of this study is to develop new Z-factors to evaluate the behavior of a circumferential surface crack in nuclear pipe. Z-factor is a load multiplier used in the Z-factor method, which is one of the ASME Code Sec. XI's recommendations for the estimation of a surface crack in nuclear pipe. It has been reported that the load carrying capacities predicted from the current ASME Code Z-factors, are not well in agreement with the experimental results for nuclear pipes with a surface crack. In this study, new Z-factors for ferritic base metal, ferritic submerged arc welding (SAW) weld metal, austenitic base metal, and austenitic SAW weld metal are obtained by use of the surface crack for thin pipe (SC.TNP) method based on GE/EPRI method. The desirability of both the SC.TNP method and the new Z-factors is examined using the results from 48 pipe fracture experiments for nuclear pipes with a circumferential surface crack. The results show that the SC.TNP method is good for describing the circumferential surface crack behavior and the new Z-factors are well in agreement with the measured Z-factors for both ferritic and austenitic pipes.     

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.     

Thermal and mechanical analysis of welded structures

The prediction of the residual state of stress and deformation in a welded structure is one of the most interesting, challenging, and complex problems in structural mechanics. The wide spectrum of physical phenomena provides the interest; economic and safety considerations provide the challenge; and the essential nonlinearity of the analytical models provides the complexity. Even if it is assumed that reasonable prediction of tje transient temperature field in the structure is possible, determining the residual mechanical state is both a difficult and an expensive task. The analysis inevitably involves temperature-dependent mechanical properties and, in addition, the severe thermal gradients and high temperatures generic to the welding process induce irrecoverable inelastic creep and plasticity in the structure.In spite of this, the stress analysis is now considered to be a straightforward application for general purpose, nonlinear finite element structural programs. A few special features of such analyses, however, will be discussed: (1) the legitimacy of time-dependent plasticity theories for treating the residual stress problem; (2) criteria for choosing plane stress, plane strain, generalized plane strain, or fully three-dimensional models; (3) methods for coping with possible floating solid regions during cooling; and (4) the use of linear constraints to treat weld metal deposition and intermittent contact.Since the most important parameters in the welding process that pertain to the stress analysis are the cooling rate and the welding torch efficiency, the heat transfer problem seems to require a critical look. The dominant features of this problem are: (1) source (torch) characterization; (2) radiation from surfaces that are heated to high temperatures; (3) latent heat effects; and (4) subsidiary considerations, such as enforced convection heat transfer modes that are designed to control the cooling rate. Motion of the welding torch, even at low speeds, is not usually a critical factor in determining the residual mechanical state. Again, finite element analysis is applicable, provided that the solution accuracy can be adequately estimated. Several alternative, two-step, implicit time integration schemes will be compared, especially with regard to accuracy and numerical stability for welding-type problems. The efficacy of ‘flux correction’ will also be discussed and the application of these ideas to typical industrial welding problems will then be outlined.     

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。     

BWR pipe crack and weld clad overlay studies

Several topics pertaining to the problem of stress corrosion cracking (SCC) of piping in boiling water reactors are addressed in this paper: (1) the effects of impurities, dissolved oxygen content, and strain rate on susceptibility of SCC of “Nuclear Grade” Type 316NG and sensitized Type 304 stainless steel, (2) finite-element analyses and experimental measurement of residual stresses in weldments with weld overlays, and (3) analysis of field components to assess effectiveness of in-service inspection techniques and the in-reactor performance of weld overlays. Several anion impurities including sulfates, chlorides, nitrates, borates, and carbonates were studied under both near neutral and slightly acidic conditions. At the low impurity concentrations expected in reactor coolant systems (<0.1 ppm), the sulfur species appear to be the most deleterious. They promote intergranular SCC in sensitized stainless steel and transgranular SCC in the low-carbon “Nuclear Grade” stainless steel. Correlations between experimental data and a phenomenological model that describes the effect of strain rate on SCC are presented. Measurements of the residual stresses produced by weld overlays confirm that the process is very effective in producing compressive stresses on the inner surface of the weldment. Examination of a weld overlay removed from service suggests that no additional throughwall crack growth had occurred after application of the overlay.     

Design and Analysis of the Laser Robotic Welding System for ITER Correction Coil Case

ITER correction coil (CC) cases have characteristics of small cross section, large dimensions, and complex structure. The cases are made of heavy thick (20 mm), high strength and high toughness 316LN austenitic stainless steel. The laser welding is used for the case closure welding, due to its low heat input and deformation. According to the structural size and feature of the two types of cases, a set of laser welding workstation for the ITER CC case closure welding is designed. A slip plate module for the welding robot is designed to increase its workspace. According to the result of the movement simulation of the welding robots, the workstation is successful to cover all weld seams on the cases. Also, the welding platform and fixtures (includes the special welding tilter of the SCC) of the BTCC case and the SCC cases are designed. To verify the design structural feasibility of the welding tilter of SCC, the structural analysis for the rotating process has been formed in detail by using ANSYS. The simulating results show that the stress of the welding tilter can meet both static and fatigue criteria, and thus the basic structure is valid.     

T91/15-15Ti异材焊接接头微观组织和力学性能研究

T91和15-15Ti是第4代核能钠冷快堆和铅铋快堆候选结构材料,国内外对铁素体/马氏体钢和奥氏体钢焊接性能、焊后热处理、焊接应力等进行了广泛的研究。本文对T91/15-15Ti试板焊态和热处理态焊接接头的焊缝、热影响区和母材微观组织和力学性能变化进行了研究,为T91和15-15Ti异材焊接参数选用和材料应用提供实验数据。