Circumferential Butt Welds Research Articles

Three-dimensional finite element simulation of residual stresses in circumferential welds of steel pipe including pipe diameter effects

In circumferential welding of a pipe component, it is often considered that axisymmetric model can provide a reasonable prediction of the residual stress distributions. However, in general, the axisymmetric model cannot reproduce the traveling arc along circumferential welds and rapid change of residual stresses that can be observed in the overlapping region. Moreover, it tends to overestimate the hoop residual stresses in circumferential welds. Therefore, three-dimensional finite element (FE) model is essential for the accurate simulation of circumferential welding which can incorporate the three-dimensional effects. This paper presents the three-dimensional FE simulation of circumferential butt welding of a steel pipe. The thermo-mechanical model used as well as the simulation methodology is detailed, and the results are discussed. In addition, parametric studies with inside radius to wall thickness ratio ranging from 10.0 to 100.0 have been presented to investigate the effects of pipe diameter on residual stresses. Axial and hoop residual stresses are plotted for the considered range of pipe diameters, and the differences are discussed.

Robust sensing and control of the weld pool surface

Gas tungsten arc welding is the primary process for precision joining. To develop a control system for this process, a novel sensor is proposed to measure the depth of the weld pool surface using a non-transferred arc. A flat surface is periodically established to provide a real-time reference. The main-arc-on period is selected as the system's input. Because of possible large variations/ranges in manufacturing conditions, an interval model control algorithm with updated intervals is adopted. To demonstrate the use of the proposed sensor and control algorithm, closed-loop experiments under different variations have been conducted. Circumferential butt welding of steel pipe has also been included as an application example.

Introduction of the Element Interaction Technique for Welding Analysis and Simulation

The residual stresses generated due to welding in pressure components may have several harmful effects, such as decrease in the resistance to cycling load and corrosive environments. The analysis of the welding process has been developed extensively in two and three dimensions. The element movement technique has been shown to be very effective in simulating the filler material deposition leading to a reduction in the analysis time. However, when attempted for wider fields of applications, it had some limitations, especially when moving the elements toward the base-plate. In this paper, the element interaction technique is introduced utilizing the concepts of both the element movement and element birth techniques. The new technique is verified versus the currently developed procedures. In this technique, the elements of the weld pool are held in place in contact with the elements of the base plate, and the interaction is made to be a function of time. This gave several flexibilities in modeling the welding process. Hence, the technique is then used to analyze simple fillet welding of a plate and circumferential butt welding of a pipe.

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.

A parametric study of residual stresses in multi-pass butt-welded stainless steel pipes

Multipass circumferential butt-welding of stainless steel pipes is simulated numerically in a non-linear thermo-mechanical FE-analysis. In particular, the through-thickness variation at the weld and heat affected zone, of the axial and hoop stresses and their sensitivity to variation in weld parameters are studied. Recommendations are given for the through thickness variation of the axial and hoop stresses to be used when assessing the growth of surface flaws at circumferential butt welds in nuclear piping systems.

Feasibility study on the application of laser butt welding of tubes to a pipe coil production line

Summary This paper describes a feasibility study on the integration of a laser source as an automatic unit for circumferential butt welding of tubes on pipe coil production lines, immediately prior to the cold-bending station. Using a 6 kW CO2 source, carbon low-alloy Cr-Mo and austenitic stainless steel tube, in thicknesses ranging from 3.4–11.2 mm, were successfully welded. Cr-Mo steels require on-line preheating treatment which can be achieved by laser defocused passes immediately prior to welding. The results of the preliminary qualification tests performed on laser-welded joints on the types of joint configurations involved (materials, diameters and thicknesses) are described, together with the technological tests required for final approval of the process. Laser circumferential butt welding of tube has proven to be effective, giving satisfactory, repeatable results and good joint performance. An exhaustive comparison with current welding techniques (TIG and MIG) is then made, together with a detailed analysis of the potential advantages and benefits which may be expected from using the laser-welding technique, as well as a preliminary estimate of investments and running costs. An accurate analysis of other possible applications and a possible layout of a laser working cell integrated into factory production lines is presented. Thus maximum production is obtained while benefitting fully from the potential of laser technology.

海洋構造物の溶接変形の予測と制御に関する基礎的研究 第１報 海洋構造物の溶接部の局部溶接変形量とその影響因子

Most of offshore structures for oil exploitation are composed of a deck and legs which are framed by welding many pipes. The prediction and control of the welding deformation is very important for the efficient and economical building of offshore structures. This is a basic study of the prediction and control, of which object is the offshore structure of jacket type.Almost all studies on the welding deformations of pipes (cylindrical drums, cylindrical shells) have been performed in relation to the local deformation by the circumferential butt welding. In this study, the welding deformation when a pipe (brace) is welded on the wall of another pipe (chord), like the welding of T-joint or Y-joint, was investigated in the actual building process of a jacket. The local deformations of the braces by welding were measured near the joints. At the same time, the influential factors, such as brace sizes, groove shapes, welding sequences, heat inputs, etc., on the local deformations were investigated.The correlations between the deformations and the factors were examined. As a result, it was made clear that the total heat input (the summation of heat inputs per unit length, of all welding passes) for each joint had influence on the local deformations most. The relations between the local deformations, namely, axial shrinkage, in-plane rotation, out-of-plane rotation, of the brace and the total heat input were formulated by the regression analysis. Also, the mechanisms of the increase and decrease of the deformations were discussed in detail, related to the total heat input, the welding sequence, etc. In the next report, the welding deformation of the whole structure will be simulated by the theoretical analysis, using the above relations for the local deformations.

Local shrinkage and rotation of welds in offshore structures: Prediction and control of welding deformation in offshore structures (1st Report)

Summary Most offshore structures for oil exploitation are composed of a deck and legs produced as a framework by many welded tubular components. The prediction and control of welding deformation is extremely important for the efficient and economic construction of offshore structures. This paper describes a fundamental investigation of the prediction and control of welding deformation in jacket-type offshore structures. Most studies examining the welding deformation of tubular components (cylindrical drums, cylindrical shells) concern the local deformation occurring during circumferential butt welding. This study investigates the welding deformation which occurs when a tube (brace) is welded on the wall on another tube (chord), as during the welding of T or Y joints, in the actual jacket construction process. The local deformations of the braces are measured near the joints. The influencing factors, such as e.g. brace sizes, groove shapes, welding sequences, heat inputs, etc, affecting the local deformation are also simultaneously investigated. The correlations between the deformation and controlling factors are examined. The results show that the total heat input (obtained by summing the heat inputs per unit length of all welding passes) for each joint most affects the local deformation. Relations between the local deformation, i.e. the axial shrinkage, in-plane rotation, and out-of-plane rotation of the brace, are derived by regression analysis. The mechanisms of deformation increase and and decrease are discussed in detail in relation to the total heat input, welding sequence, etc. In the next report, the welding deformation of the structure as a whole will be simulated in a theoretical analysis using the above relations derived for the local deformation.

The measurement of the distribution of residual stresses through the thickness of a welded joint

A new method for the measurement of residual stresses in welded joints of complex cross section is described. The method involves the measurement of strain changes in the body as a narrow slot is cut in the plane of interest, in small increments of depth. A finite element model of the cross section is used to relate the strain changes at the measurement points to the residual stresses across the slot plane.The accuracy of the method is demonstrated by using it to measure a known residual stress field in a cold bent bar. The method has been used in combination with a block removal technique to measure the through wall distribution of axial residual stresses at a circumferential butt weld in a cylinder with a protruding root bead. It could also be applied to the measurement of residual stresses in other joint configurations, such as fillet welds or T‐butt welds.

Applications of a fracture mechanics model of structural reliability to the effects of seismic events on reactor piping

A fracture mechanics model of structural reliability is described. The model assumes that failure occurs due to the subcritical and catastrophic growth of as-fabricated defects. The material properties, stress history, number and dimensions of the initial cracks are treated as random variables. Crack growth is calculated using fracture mechanics principles. The capability of modeling two-dimensional cracks and thickness gradients of the applied stresses represents a significant advance over previous work in this field. The model has been used to estimate the influence of earthquakes on the integrity of circumferential girth butt welds in the large (diameter greater than 30 in.) primary coolant system pipes of a commercial pressurized water reactor. In the absence of earthquakes, the probability of leaks and catastrophic double-ended guillotine breaks is estimated to be 10 −6 and 10 −12 per plant lifetime, respectively. These probabilities were only slightly increased by the occurrence of earthquakes. The cyclic stresses in the heatup-cooldown cycle had the greatest effect on the crack growth. Radial gradient thermal stresses due to temperature fluctuation of the coolant during transients have only a small effect on the amount of crack growth. Sensitivity studies show that significant changes in modeling assumptions are needed before the calculated failure probabilities are raised to the level of current estimates. This suggests that perhaps factors such as design and construction errors or stress corrosion cracking may be significant contributors to the probabilities.