Circumferential Welds Research Articles

Laser Welding of SLM-Manufactured Tubes Made of IN625 and IN718.

The advantage of selective laser melting (SLM) is its high accuracy and geometrical flexibility. Because the maximum size of the components is limited by the process chamber, possibilities must be found to combine several parts manufactured by SLM. An application where this is necessary, is, for example, the components of gas turbines, such as burners or oil return pipes, and inserts, which can be joined by circumferential welds. However, only a few investigations to date have been carried out for the welding of components produced by SLM. The object of this paper is, therefore, to investigate the feasibility of laser beam welding for joining SLM tube connections made of nickel-based alloys. For this purpose, SLM-manufactured Inconel 625 and Inconel 718 tubes were welded with a Yb:YAG disk laser and subsequently examined for residual stresses and defects. The results showed that the welds had no significant influence on the residual stresses. A good weld quality could be achieved in the seam circumference. However, pores and pore nests were found in the final overlap area, which meant that no continuous good welding quality could be accomplished. Pore formation was presumably caused by capillary instabilities when the laser power was ramped out.

Pipeline Magnetic Flux Leakage Image Detection Algorithm Based on Multiscale SSD Network

In order to solve the problem of low detection accuracy of small targets in the SSD detection algorithm, a pipeline magnetic flux leakage image detection algorithm based on multiscale SSD network is proposed in this paper. The dilated convolution and attention residual module are introduced into the SSD algorithm to fuse the low-resolution high-semantic information feature map with the high-resolution low-semantic information feature map so as to improve the resolution of the low-resolution feature map and provide detailed features for small targets. Finally, the target location and category are obtained by regression algorithm. The experimental results show that the proposed algorithm can automatically identify the location of circumferential weld, spiral weld, and defect of magnetic flux leakage data. Compared with the original SSD algorithm framework, the improved algorithm has higher detection accuracy, 97.62%, 18.01% lower false detection rate, 18.36% lower false detection rate, better robustness, and obvious effect on small target detection.

Studying rigidity of the welded liner - composite shell construction

The mechanical behaviour of an aluminium welded liner in a cylindrical metal composite vessel of high pressure made by winding on a linear carbon-filled plastic tape with tension is investigated. The resulting pressure of the wound composite shell on the welded liner is simulated by cooling the shell. It is believed that the weld is obtained by friction welding and has characteristics that are reduced in relation to the base material of the liner. The problem is solved in a geometrically and physically nonlinear formulation using a 3D solid element model, taking into account technological deviations and in real (current) time. The calculations were performed in the LS-DYNA software package in a dynamic formulation. It is shown that liner delamination from the sheath starts from the side of the circumferential weld in the area of technological deviations.

Numerical Analysis of Subcooled Water Flashing Flow From a Pressurized Water Reactor Steam Generator Through an Abruptly Broken Main Feed Water Pipe

A computational fluid dynamics (CFD) analysis was performed to investigate the hydraulic response of the flow field inside the pressurized water reactor (PWR) steam generator (SG) secondary side and the connected part of main feed water pipe to an abrupt main feed water line break (FWLB) accident. To realistically analyze the transient flow field situation, the flow field was assumed to be occupied initially by highly compressed subcooled water except that the upper part of the SG secondary side where steam occupied as in the practical case and the break was assumed to occur at the circumferential weld line between the feed water nozzle and the main feed water pipe. This would result in a subcooled water flashing flow from the SG through the short-broken pipe end to the surrounding atmosphere, which was numerically simulated in this study. Typical results of the prediction in terms of the fluid transient velocity and pressure were illustrated and discussed. To examine the physical validity of the present numerical simulation of the subcooled water flashing flow, the transient mass flow rates predicted in this study were compared with the other previous numerical predictions based on the subcooled water nonflashing (no phase change) flow or saturated water flashing flow assumptions and the prediction by a simple analysis method.

Device and Methods for Measuring of Acoustic Anisotropy and the Residual Stress in the Main Gas Pipelines’ Metal

One of the main conditions of safe operation of gas pipelines is the use of non-destructive diagnostic methods. Particularly important problem is the earlier operational diagnosis of pipes’ material of main gas pipelines based on the evaluation of the stress-strain state, elastic moduli and properties anisotropy by acoustic methods. The aim of the work is to develop methods for acoustic assessment of the stress-strain state, the elastic moduli and the properties anisotropy of pipeline material and to study these characteristics in different sections of main pipelines using a device based on contactlessEMAtransducers.Methods are implemented using specialized equipment (the structurescopeSEMA) and non-contact electromagnetic acoustic transducers. As an object of research, we used fragments – the cuttings of gas pipelines with circumferential welds both after fabrication and removed from service with stress corrosion cracking, including ones with corrosion damage and without visible damage.The method of determining the plane stress-strain state of pipeline elements is based on the phenomenon of acoustoelasticity – the dependence of the propagation velocity of ultrasonic waves on mechanical stresses. The method for determining the elastic characteristics of materials and the anisotropy of their properties is based on the relationship between the velocities of ultrasonic waves and the elastic properties of the medium. Both techniques are implemented by sounding the sample in one section using a longitudinal wave and two shear waves with mutually perpendicular polarization planes coinciding with the main stresses, and measuring their propagation times.It is shown that the stress state distribution both in the circumferential direction and along the generatrix is uneven which is caused by the peculiarities of samples (presence of a welded joint, stress corrosion cracking, long-term operation). The smallest acoustic anisotropy is observed for the Young's modulus. The anisotropy of the properties of most samples is in the range of 12–14 % for shear modulus, 9–10 % for Young’s modulus, 13–15 % for Poisson’s ratio. For samples with stress corrosion cracking a sharp decrease in the anisotropy coefficient is observed which makes it possible to use the indicated characteristics as informative parameters in detecting stress corrosion cracking.A feature of the proposed methods is high accuracy, due to the absence of necessity to determine the material density and precision measurement of its thickness, the measurement error of which is significant by known methods.Keywords:main gas pipeline, elastic moduli, acoustic anisotropy of properties, the biaxial stress-strain state, longitudinal and transversal waves.

Determination of stress concentration factors in offshore wind welded structures through a hybrid experimental and numerical approach

Offshore wind turbine (OWT) monopile support structures generally consist of steel cans connected together through circumferential welding joints. One critical factor to evaluate the localised increase in stresses is the stress concentration factor (SCF) which depends on the welding quality. The complex welding profiles in OWT monopiles makes the accurate calculation of SCF quite challenging. In this work, an innovative approach for the calculation of SCFs in offshore welded structures is proposed based on combined 3D (three-dimensional) laser scanning technology (LST) and 3D finite element analysis (FEA). The precise geometry of the welded specimens is captured using 3D LST, and then imported into a finite element software to perform 3D FEA modelling to accurately calculate SCFs. A 2D (two-dimensional) FEA model of a typical offshore welded structure with ideal geometry is also developed in this work. In addition to numerically calculate SCFs, the 2D FEA model is further combined with non-linear RSM (response surface method) to derive analytical equations, expressing SCFs of offshore welded structures in terms of key welding parameters. Both LST-FEA3D and RSM-FEA2D models are applied to calculate SCFs in large-scale S-N fatigue welded specimens. The results indicate that the LST-FEA3D approach is capable of capturing the variation of SCFs along the width of the welded specimens and identifying the critical points where fatigue crack is most likely to initiate; and the RSM-FEA2D is valuable and efficient in deriving analytical parametric equations for SCFs.

Failure analysis of a superheater tube ruptured in a power plant boiler: Main causes and preventive strategies

The present study concentrated on the metallurgical, micro- and macroscopic observations, hardness evaluation, as well as X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) analysis induced during the high-temperature operation of a pipeline that resulted in its relatively short-time failure. An ASTM A213-T12 superheater pipeline is used in a fossil fuel power-plant unit and is constructed by an array of steel tubes joined together by circumferential weldments. Macroscopic observations indicated a thinning and prolonged deformation of the superheater tube in the region close to the fire zone besides thick corrosion products on the other side of the pipe. Hardness measurements showed a remarkable decrease in hardness values of superheater metal near to the failure zone, indicating that grain coarsening or any other microstructure weakening must happen. Scanning electron microscopy (SEM) observations proved the ductile nature of cracking and justified the prolonged failure of the tube. The observed voids in the microstructure of the failed section demonstrated the occurrence of creep phenomenon. XRD and EDS analyses demonstrated the nature of sediments formed on the tube and described how the formation of these corrosion products contribute in the removal of any possible protective passive layer, exposing the tube to further corrosive deterioration and creating loop stresses which, in turn, will induce creep phenomenon and accelerate the occurrence of a failure in the tube.

Failure Analysis on Leakage of the Circumferential Weld of Steel 20 Buried Pipeline

Nowadays, the welding quality of circumferntial welds is the main influencing factor of the service behaviors of long-distance transport pipeline, and the accidents caused by the circumferential welding defects have get extensive attention. To figure out the leaking reason of circumferential weld of the steel 20 buried pipeline, with macro inspection, nondestructive testing, physical and chemical properties, methods of medium leakage and fracture analysis, we found that the welding defects in the weld toe are mainly responsible for pipeline leaking. The welding defects of circumferential weld destroy the continuity of welding, which result in the stress concentration and cracks. Under the combined action of corrosion, erosion, working stress and welding residual stress, the cracks extend along the welding weak areas and penetrates the welding metal, finally leading to the leakage failure of the welds, also some suggestions are given to prevent similar accidents.

Fatigue damage analysis of offshore wind turbine monopile weldments

Offshore wind turbines (OWT) are subjected to harsh environmental conditions in addition to the variable service loads. The present study is aimed at performing a realistic fatigue life estimation of the monopile structure using operational service loads recorded by online monitoring systems. Fatigue damage analysis has been conducted at the circumferential weld joints using finite element (FE) method by considering geometrical and material property discontinuities. Global-local modelling of the OWT was performed in as-welded condition to capture the local stress range at the weld toe, which acts as the critical site where cracks are most likely to initiate and propagate. The S-N fatigue design approach and maximum stress range at the weld toe have been used to determine the fatigue crack initiation life in monopiles. The results from the proposed approach show that a realistic life assessment can be made on monopile structures by accounting for the geometrical effects at the circumferential welds.

Mechanical Properties for Circumferential Welding Applied to UNS S32750 Super Duplex Stainless Steel Using the GMAW Process with CMT Control

Super Duplex Stainless Steel has been studied for oil and gas industry applications since the 90 s, but their welding always is a technical issue. Then different methods were developed to meet all requirements presents in ASME standards, where most of are applied for lower production, when compared to other materials such carbon steel. This study accomplished circumferential welding experiments in base material (BM)—UNS S32750 Super Duplex Stainless Steel with a thickness of 3.68 mm. Welding was performed using GMAW (Gas Metal Arc Welding) process with CMT (Cold Metal Transfer) control and the aid of a Motoman robot and a turntable device, which were used to ensure the accurate positioning of the torch and that the welding has been fully implemented in the 1 G position, respectively. This process achieves higher production in relation other welding process usually applied for this material class. The joints which presented adequate dimensional results were, then, subjected to tensile and hardness Vickers tests. To avoid the lack of penetration problem, the welding was performed using the CMT process combined with pulsed arc, thus, resulting in full penetration and improved surface finish. The results have shown that the CMT procedure, combined with pulsed arc, led to an adequate superficial finishing, mechanical properties and corrosion resistance in accomplishment with the requirements presented in applicable standards.

Electron Beam Welding of Large Components for The Nuclear Industry

The nuclear industry requires rapid and high quality joining of large scale components. Electron beam welding (EBW) has the potential to respond to these requirements. The aim of Nuclear Advanced Manufacturing Research Centre (Nuclear AMRC) is to develop solutions for the future application of this technology. One example is the research on deep penetration EBW for joining large scale pressure vessels for small modular nuclear reactors. This will require several circumferential welds of ~ 6 metres length each. In addition joining of sections of the upper and lower vessel heads and of HIP sections with varying wall thickness must be developed. In collaboration with the US Electric Power Research Institute (EPRI) the Nuclear AMRC is working to produce two-thirds scaled demonstrators of the lower and the upper pressure vessel assembly (based on a generic NuScale model). 100 mm deep single track, full penetration welds of pressure vessel steel have been demonstrated. In addition, within 26 minutes joining of shells was achieved with 6 metres long circumferential welds (78 mm full penetration). In future the joining of complex sections and sections with variable thickness will be investigate.

Effect of Residual Stress on the Destruction of Field Joints of Gas Pipelines Operating in Conditions of the North

Welding residual stresses are determined by algebraic summation of stresses by the effect of external loading and can have a significant impact on the performance of welded joints. The present article examines the effect of residual welding stress on the destruction of circumferential butt welds of the gas pipeline sections. The features of the distribution of the welding residual stresses of annular welded pipe joints are considered. It has been established that thin-walled tubes are formed in the inner layers of the sealing elements with growing tension pins forming inside. Analysis of destruction of ring welded joints of the underground and underwater sections of the pipeline was carried out. The welding residual stresses were investigated with X-ray diffraction technique. It is shown that the initiation and growth of macrocracks in the field joints of thin-walled main gas pipelines occur in the heat-affected zone from the interior face of the pipe wall in the zone of high tensile residual welding stress. Analysis of the destruction of real welded joints of gas pipelines has proved that the residual welding stress is the main factor that influences the development of brittle and fatigue cracks in them.

Stress corrosion cracking and fatigue crack growth of an API 5L X70 welded joint in an ethanol environment

Environment-induced cracking may occur in oil and gas pipelines, especially in welded joints, where fatigue cracks may originate at points of high stress concentration. This work evaluates the susceptibility of welded joints to stress corrosion cracking (SCC) in a simulated fuel-grade ethanol (SFGE) environment, based on slow strain rate (SSR) and fatigue crack growth (FCG) tests. The tests were designed to promote crack propagation through the heat-affected zone (HAZ) or through the weld metal (WM) of a circumferential weld on an API X70 steel pipe. The SSR tests in SFGE showed reduction of elongation to failure and mixed fracture, with clear signs of SCC in both the regions of the weld. The FCG tests demonstrated that the time-dependence of corrosion fatigue interactions may involve alternating periods of increasing and decreasing crack growth rates throughout the test. Acceleration of crack propagation due to SCC predominated in the WM and deceleration predominated in the HAZ.

T-junction experiments to investigate thermal-mixing pipe flow with combined measurement techniques

In context with thermal fatigue of T-junction pipelines caused by thermal mixing of flows, experiments are carried out at the Fluid-Structure-Interaction (FSI) test facility at the University of Stuttgart with a horizontal T-junction, in which a cold water flow (20° C) in the branch pipe (DN40) and a hot water flow (max. 280° C) in the main pipe (DN80) are mixed at pressure up to 75 bar. The focus of present investigation is on the flow mixing behavior close to the T-junction with presence of a weld root model inside the pipe, which is representing a typical pipeline weld connection with internal circumferential weld root.In this work, the experimental investigation is performed with multiple measurement techniques. Temperature measurement in the mixing region and inlet flow is performed with micro-thermocouple technique. The Near-Wall LED Induced Fluorescence (NW-LED-IF) method is applied to investigate the influence of the rimmed weld root in the mixing region. Due to the operation safety, the experiments are performed with 30 bar. The experiments are conducted with different flow temperatures in the main pipe line of 150 °C, 120 °C and 90 °C. The experiments are performed with periodical and continuous mixing processes. This paper presents the results of the experiments with two different measurement techniques. Fatigue assessment for the piping materials in the mixing region is discussed according to the thermocouple measurements. The impact of the rimmed weld root on the nearby flow field is shown in the NW-LED-IF measurement. The results show mixing behavior in the T-junction can be changed with temperature difference in the inlet flow and the rimmed weld root can increase the possibility of thermal fatigue damage significantly.

Assessment of yield strength mismatch in X80 pipeline steel welds using instrumented indentation

Strain-based design (SBD) of pipelines allows stress owing to displacement-controlled loads originating from landslides, seismic motions, or frost heaves, to exceed yield stress. In such cases, the distribution of the strain over a large area (pipe) rather than local area (weld) is preferred. Accordingly, in SBD, yield strength of the circumferential girth weld is over matched when compared to the base metal. In this work, API-X80 pipeline steel sections were joined together using the robotic Gas-Metal Arc Welding (GMAW) process; filler metals and shielding gases were varied to achieve three different levels of strength (even, over and under match) between the weld and base metal. Weld metal and HAZ cross-section microstructures were investigated and correlated with micro-hardness maps to ensure that different mismatch levels were achieved. Yield strengths of the welds in the hoop direction were measured using both instrumented indentation and conventional tensile testing. The instrumented indentation technique used a 100-μm diameter nearly-flat indenter to estimate yield strengths for the welds and heat-affected zones from the load-displacement response for each zone. The measured yield strengths were compared with 0.2% offset tensile yield strength obtained through conventional tensile testing of all-weld and base metal specimens. Strength measurements, in the hoop direction, for weld metal from both of the techniques agreed to within 4.6%. The results showed that the nearly-flat tip indentation method can be used as a tool to directly estimate strength mismatch of pipeline girth welds. Measurements of yield strength for narrow coarse and fine grained heat-affected zones, which cannot be measured through any other method, can also be assessed using the techniques described herein.

Analysis of welding residual stress formation behavior during circumferential TIG welding of a pipe

The objective of this study is to analyze the formation behavior of welding residual stresses during the circumferential TIG butt welding of pipes. A three-dimensional thermo-elastic-plastic numerical model has been developed by using MSC-Marc software to predict welding residual stress distributions during single-pass welding of pipes. Using a numerical computational approach, the mechanisms of the changes in stress and strain throughout the pipe welding process is described. Compared with the features of butt welded plate joints, the distribution of welding residual stress in butt welded pipe structures is obviously different. Considering the deformation effect on the plastic strain distribution, the calculation results indicate that the axial and hoop compressive plastic strain at the inner surface of welded pipes is much larger than that at the outer surface. As a result, the tensile axial and hoop residual stress is generated at the inner surface due to the thermal contracting deformation. Consequently, compressive axial residual stress occurs at the outer surface because of the inward contracting deformation and internal stress self-balance. Moreover, it is investigated and explained the phenomenon in which both axial and hoop residual stresses have sharp gradients near the weld stop location at the overlapped weld line of pipe. Because of redistribution of axial residual stresses and hoop plastic flow behaviors, residual stresses near the weld stop location could be changed.

In-Line Inspection Tool with Eddy Current Instrumentation for Fatigue Crack Detection.

Eddy current transducer with sensing coils placed orthogonally and connected in differential mode was introduced to evaluate fatigue cracks in clad pipeline circumferential welds. A dedicated embedded electronic hardware was developed to drive the transducer and measure the electrical complex impedance of the coils, and was specifically designed for operation under autonomous in-line inspection tool. In the laboratory experiments, an automated inspection was performed with the goal to evaluate transducer’s detectability, and different scanning speed was tested to reproduce in service situation. The results have confirmed that the introduced eddy current transducer is a potential solution for fatigue crack detection in clad circumferential weld root, while the hardware developed presented a reasonable SNR reaching the data rate required to be incorporated in an autonomous in-line inspection tool.

Residual stress measurements in offshore wind monopile weldments using neutron diffraction technique and contour method

Reliable assessment of the fatigue life of offshore wind monopiles operating in harsh offshore environments relies on quantifying the level of residual stresses locked-in at circumferential weld regions. This study presents, for the first time, residual stress characterisation, using the contour method, on a large structural welded mock-up, typical of the weldment used in offshore wind monopiles. The contour method and neutron diffraction measurements were also conducted on a compact tension specimen extracted from the large mock-up. The extracted compact tension sample, typically used for fracture and fatigue crack growth tests, showed notably significant remnant residual stresses that could impact fracture and fatigue test results. In addition the measured 2D map of transverse residual stresses, acting normal to the crack plane, playing a key role in fatigue crack opening/closure, exhibited variations through the thickness of the compact tension sample. The key conclusion was that the residual stresses in small laboratory samples extracted from large scale weldments should be carefully characterised and taken into account in structural integrity tests. Besides, the measurement results on the welded mock-up showed that the level of damaging tensile residual stress in large-scale mock-ups and hence real size structural welded monopiles is considerably larger than residual stresses in extracted laboratory samples; hence will have more significant influence on structural integrity of offshore wind assets.

Thermal stress analysis of a damaged waste heat boiler

Waste heat boilers are widely used in the petroleum refining industry to improve energy efficiency. However, improper design, operation or maintenance of these boilers may result in costly failures. Cracking was found repeatedly at the same location on a circumferential weld of a certain waste heat boiler in CPC refinery. Hence a special designed fixture was used to avoid crack extension. Root cause mainly due to improper arrangement of internal tubes and external piping was then mainly found by thermal stress analysis. Recommendations was proposed to shut down the boiler immediately and to rearrange the tubes and piping without changing basic design of the waste heat boiler.

An Experimental Investigation of Magnetically Impelled Arc Butt Welding of Pipes: A Review

This paper describes a new development of circumferential welding of pipes. Circumferential butt welds are commonly used to join pipes in various industries, including power plants and automobile industries. Magnetically impelled arc butt welding process is a hybrid welding technique. It uses a rotating electric arc as its heat source and is known as efficient method for pipe welding. In this process heat is evolved prior to forging by an arc generated between two coaxially aligned pipes, this arc rapidly rotates along the circumferential edges of the pipes to be welded due to the electromagnetic force exerted by the interaction of arc current and magnetic field generated by the external magnetic system. The entire weld over the full joint thickness is made in one single operation, instead of using several passes as in conventional welding. The main emphasis of this review is to describe the different works carried out in the past which help full for providing the information for the future development of research work. Present study exposes the different works that has been done in the past for improving the weld quality.