Effect of welding methods on the magnitude of residual stresses in welding up cracks in grey cast iron casing components*

It is shown that the magnitude of residual stresses in welding up cracks in casing components is not sufficient to distort them and depends on the welding method. The behaviour of the residual stresses is the same for all methods investigated in these investigations. The deformation work of a welded joint in tensile or shear loading depends on the welding method and determines the probability of formation of weld zone cracks.     

Mechanism of formation of welded cruciform joints in conductors with coating by resistance microwelding

The first part of this work presented the results of the experimental determination of residual stresses along top joints in pipes. The purpose of the second part will be to present a critical evaluation of the microstructure and hardness of the welded joints for pipes with small diameter used in oil refineries, aiming to correlate the results with the residual stresses. The methodology consists of welding pipes with a diameter of 2 in., using a manual TIG process. AWS ER70-S3 addition metal with a 2.5 mm diameter was used. An electronic welding source and a data acquisition system for control were used to control the welding parameters. Metallographic samples were conventionally prepared and electropolished using chloride acid solution. Optical microscopy and scanning electron microscopy were used to evaluate the microstructure. Microhardness tests were made along the weld. The results show that the combination of heat cycles between the root and the finish with low heat support caused significant reining of grains in the welded area and in the heat affected zone, as well as promoting a reduction in the hardness values. The same behaviour was not reached for the samples welded with higher welding heat input. None of the welded samples presented hardness values above the maximum established in the standards (248 VH).     

Influence of Material Model on Prediction Accuracy of Welding Residual Stress in an Austenitic Stainless Steel Multi-pass Butt-Welded Joint

Both experimental method and numerical simulation technology were employed to investigate welding residual stress distribution in a SUS304 steel multi-pass butt-welded joint in the current study. The main objective is to clarify the influence of strain hardening model and the yield strength of weld metal on prediction accuracy of welding residual stress. In the experiment, a SUS304 steel butt-welded joint with 17 passes was fabricated, and the welding residual stresses on both the upper and bottom surfaces of the middle cross section were measured. Meanwhile, based on ABAQUS Code, an advanced computational approach considering different plastic models as well as annealing effect was developed to simulate welding residual stress. In the simulations, the perfect plastic model, the isotropic strain hardening model, the kinematic strain hardening model and the mixed isotropic-kinematic strain hardening model were employed to calculate the welding residual stress distributions in the multi-pass butt-welded joint. In all plastic models with the consideration of strain hardening, the annealing effect was also taken into account. In addition, the influence of the yield strength of weld metal on the simulation result of residual stress was also investigated numerically. The conclusions drawn by this work will be helpful in predicting welding residual stresses of austenitic stainless steel welded structures used in nuclear power plants.     

低碳钢管道焊接残余应力有限元分析

利用ＡＤＩＮＡ非线性分析有限元程序,对低碳钢管道环焊缝接头焊接残余应力进行有限元分析。在热弹塑性分析中考虑了材料热物理和力学性能依赖于温度变化。结果表明：在管道接头内表面焊缝中心及近缝区轴向和环向残余应力均为拉应力,随离开焊缝距离的增加,逐渐过渡为压应力。在管道接头外表面焊缝中心处的轴向残余力为压应力,而环向残余应力为拉应力。计算预测值和实侧值基本一致,表明有限元法能够经济而有效地预测管道环焊缝接头的焊接残余应力。     

焊接残余应力对焊接接头蠕变性能的影响

应用大型有限元分析软件ABAQUS及其RESTART功能,建立了焊接温度场模型、残余应力场模型和蠕变分析模型.使用焊接残余应力作用下蠕变的顺次耦合有限元计算方法,对Cr5Mo加热炉炉管焊接接头残余应力和蠕变进行了数值模拟.计算方法为掌握复杂的焊接残余应力对高温炉管焊接接头的蠕变影响奠定了基础.比较了考虑焊接残余应力和仅承受内压两种工况下的炉管接头蠕变情况.结果表明,虽然焊接残余应力在短时间内松弛,但焊接残余应力决定了炉管的蠕变变形,焊接残余应力是影响炉管蠕变的重要因素.     

机械喷丸对7A52铝合金焊接接头残余应力改善的有限元模拟

分别建立了7A52铝合金双丝MIG焊热力耦合计算模型及GCr15钢弹丸撞击7A52铝合金试板的三维简化模型.在不影响计算结果的前提下,适当增大了焊接试板弹丸撞击区域网格尺寸.获得了焊接接头的残余应力场计算结果,并分析了弹丸大小、弹丸速度对喷丸残余应力场的影响规律,进而优化了喷丸参数.在此基础上利用隐式求解器与显式求解器之间的数据传递方法,将铝合金试板焊后残余应力场与喷丸处理过程进行耦合计算.结果表明,焊后试板喷丸处理对焊缝及近缝区表面残余应力、试板厚度方向残余应力分布状态均有较明显改善.     

Porosity reduction in CO2 laser welding of aluminium alloys ‐ Influence of penetration,joint, oxygen gas and oxide films

This study aims to ensure the safety of nuclear power plants. The accidents involving leaks from the welded zones at the pipe penetration part of a reactor vessel or at a coolant pipe are reported at home and abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for maintaining high safety of the plants, the estimation of plant's life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with finite element method is used to measure the welding residual stresses accurately.

The mock-up is idealized for the welded joint at the pipe penetration part of the actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in a three-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analysed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.     

双相不锈钢管道全位置焊接残余应力三维有限元数值模拟

以热弹塑理论为基础，利用ANSYS非线性分析有限元程序，对双相不锈钢管道接头环焊缝残余应力进行三维数值模拟。建立了管道全位置焊接瞬态温度场和应力场三维移动热源模型，获得了环焊缝焊接接头轴向和环向残余应力的分布规律：在管道接头内表面的焊缝及近缝区的轴向和环向残余应力均为拉应力，随着离开焊缝距离的增加，由拉应力逐渐过渡为压应力。在管道接头外表面焊缝中心处的轴向残余应力为压应力，而环向残余应力为拉应力。从环向位置上的应力变化规律可以看出正半周和负半周的应力分布具有明显的对称性。研究结果为优化生产工艺，控制残余应力提供了理论依据。     

Residual welding stresses in laser beam and tungsten inert gas weldments of titanium alloy

Abstract

The residual welding stresses in laser beam (LB) and tungsten inert gas (TIG) weldments of a titanium alloy in thin plate form were investigated experimentally in the present work. A hole drilling technique was used to measure the residual stresses in the weldments. The effects of the welding method and post-weld heat treatment (PWHT) on the residual stresses were analysed. The results show that (i) the residual stress distribution in the LB welded joints is similar to that obtained for traditional fusion welding processes, although the distribution zone is much narrower in LB welding, (ii) the residual stress in the heat affected zone for LB welding is about 100 MPa lower than that for TIG welding, and (iii) PWHT in vacuum greatly relieves the welding residual stress.     

T型焊接试件焊接残余应力分布的测定

为了掌握钢结构中焊接残余应力的具体分布状态,采用基于逆磁致伸缩效应原理的无损检测方法,利用磁测应力仪对T型焊接试件焊缝附近不同层深处的焊接残余应力进行了实际测量,得到了距焊件表面1、1.5和2 mm三个不同层深的焊接残余应力分布规律,并和基于有限元法的数值模拟计算结果进行了对比分析.结果表明,焊缝附近区域残余应力较大,随着层深的增加,横向和纵向残余应力均由压应力逐渐过渡为拉应力,而纵向残余应力在远离焊缝中心的区域内,则改由拉应力过渡为压应力.     

Experimental Investigation into the Effects of Weld Sequence and Fixture on Residual Stresses in Arc Welding Process

This study concentrates on the effects of weld sequence and welding fixtures on distribution and magnitude of induced arc welding residual stresses built up in butt-joint of Gas Tungsten Arc Welding (GTAW) AA5251 plates. Aluminum plates have been welded under different welding conditions and then, longitudinal and transverse residual stresses were measured in different points of the welded plates employing hole-drilling technique. The results indicate that welding sequence significantly alters the distributions of both longitudinal and transverse residual stresses while the changing in the weld sequence leads to 44% decrease in longitudinal residual stress. Besides, both the geometry of weld pool and distribution of residual stresses are affected by the welding fixtures while implementation of fixture causes about 21 and 76% reductions in the depth of weld pool and transverse residual stress, respectively, for the material and welding conditions used in this research.     

改善焊接圆管抗应力腐蚀性能的水冷法研究

采用数值模拟和残余应力实测方法,研究水冷法改善１Ｃｒ１８Ｎｉ９Ｔｉ奥氏体不锈钢管和２０钢圆管的多层对焊残余应力分布的有效性,结果表明水冷法可效调整圆管的焊接残余应力分布,使圆管内表面的焊接缝附近区域获得双向压缩残余应力,应力腐蚀敏感性实验结果表明,水冷法可显著提高焊接圆管的抗应力腐蚀性能。     

耐热钢厚壁管环焊残余应力的有限元分析

以热-弹塑性理论为基础，建立了厚壁管环焊残余应力的二维轴对称有限元模型，利用ANSYS有限元分析软件，模拟了耐热钢厚壁管环焊对接的应力分布。结果表明，焊缝内表面及其附近处的轴向应力和环向应力均是拉应力，而外表面是压应力，接头处内表面应力水平高于外表面；径向应力数值远低于环向应力和轴向应力；各方向残余应力的最大值均位于距管道外表层一定距离处，其数值大小接近于材料的屈服点应力。     

Surface residual stresses in multipass welds produced using low transformation temperature filler alloys

Tensile residual stresses at the surface of welded components are known to compromise fatigue resistance through the accelerated initiation of microcracks, especially at the weld toe. Inducement of compression in these regions is a common technique employed to enhance fatigue performance. Transformation plasticity has been established as a viable method to generate such compressive residual stresses in steel welds and exploits the phase transformation in welding filler alloys that transform at low temperature to compensate for accumulated thermal contraction strains. Neutron and X-ray diffraction have been used to determine the stress profiles that exist across the surface of plates welded with low transformation temperature welding alloys, with a particular focus on the stress at the weld toe. For the first time, near surface neutron diffraction data have shown the extent of local stress variation at the critical, fusion boundary location. Compression was evident for the three measurement orientations at the fusion boundaries. Compressive longitudinal residual stresses and tensile transverse stresses were measured in the weld metal.     

Welding of ASTM A106 Gr. B steel pipes for high-temperature service – part 1 – residual stress analysis

Welding is the principal process of manufacture used in the fitting and repair of tube sections. However, there is a lack of information about the alterations mechanically/metallurgically caused by the welding heat cycle, especially about the behaviour of the residual stresses. The objective of the first part of this work was to evaluate the welding residual stress in small diameter pipes used in oil refineries. Two-inch diameter pipes were welded using the manual GTAW process. AWS ER 70 S3 filler rods with diameters of 2.5 and 3.25 mm were employed. An electronic power supply was used, together with data acquisition systems to control the welding parameters. Stress measurements were carried out with an X-ray mini diffractometer. The axial residual stress profiles determined in the outer surface of the pipes were formed by compressive stresses in the weld region (the fusion zone and heat affected zone) and for tension stresses in the areas more distant from the weld bead. The evidence suggested that on the inner surface of the pipes, the stress profile is the opposite from that observed for the outer surface, with tension stress in the welding zone and compressive stress in the region further from the weld bead.     

SS400钢对接接头表面纳米化及其对疲劳强度的影响

采用高能喷丸技术对SS4 0 0钢对接接头的表面进行处理 ,利用X射线衍射和透射电镜对表层进行结构表征 ,并测量了对接接头表层硬度、残余应力和疲劳强度的变化。结果表明 ,高能喷丸处理可以在对接接头的表面形成尺寸均匀、晶粒取向呈随机分布的纳米晶组织 ,从而消除了对接接头表层组织的不均匀性 ;纳米结构表层的硬度明显高于心部 ,其内部残余应力为压应力。这种压应力纳米强化层可以明显地提高对接接头的疲劳性能。     

拉剪载荷下超高强度钢点焊残余应力试验

点焊后焊接残余应力叠加工作载荷后,内部应力会重新分配.文中选用B1500HS超高强度钢薄板,制备焊接构件,并通过X射线衍射法测量焊核区、热影响区及母材区的表面残余应力.在施加拉剪载荷的情况下,分别从电极端面直径、焊接电流两方面考察点焊构件在施加拉剪载荷前后残余应力的分布情况.结果表明,点焊时电流变化对试件的残余应力有一定影响;电极端面直径增加使得焊核区的残余应力增大.施加拉剪载荷后,残余应力在焊核与热影响区有所释放,远离焊核区的释放较小.     

X80管线钢环焊缝接头残余应力的数值模拟

利用SYSWELD有限元分析软件,以热弹塑性理论为基础,采用双椭球焊接热源模型,对X80管线钢环焊缝接头的焊接温度场和应力场进行了模拟仿真.得到了焊接残余应力的分布规律,即焊缝及近缝区的残余应力值较大,远离焊缝中心残余应力值逐渐减小;由于表面和心部散热条件不同,造成了管道表面和心部的残余应力方向上的差异或数值大小的不同.研究了焊接工艺参数对残余应力的影响规律.结果表明,随着热输入的增大和预热温度的提高残余应力值逐渐降低;为了减小焊接残余应力,应尽量采用较大热输入和较高预热温度进行焊接.     

钛合金激光焊接大梯度残余应力特征分析

选用可精确描述小孔和熔池形貌的旋转高斯曲面体热源,建立了一个局部细化的钛合金激光深熔焊有限元数值计算模型,研究了激光能量密度与热输入对焊缝、小孔形貌以及焊缝及近缝区大梯度残余应力的分布特征.结果表明,钛合金激光焊接高残余应力位于小孔区域,而焊缝其余区域形成了大梯度残余应力状态;残余应力梯度在能量密度过高或过低,以及热输入过小时均较大;为避免大梯度残余应力的产生,应使焊接时激光能量密度处于中低状态.     

Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets

Abstract

This paper investigates a trailing heat sink, which was designed and applied to friction stir welding (FSW) in order to control the residual stresses and welding distortion. Residual stresses, residual plastic strains and welding distortion of 2024-T3 and 5083-H321 Al sheets welded by FSW with and without the trailing heat sink were compared. The optimal placement of the heat sink was discussed. The results revealed that the reductions in peak tensile stresses were 66% for 2024-T3 and 58% for 5083-H321 by application of the trailing heat sink in FSW. In addition, the welding distortion could be reduced drastically by this method. The 5083-H321 sheet with a size of 1000×100×3·5 mm welded by this method was very flat and had almost no distortion. This method achieved in-process control of stresses and welding distortion, without additional complicated work before or after welding operation.