Circumferential Butt Welds Research Articles

Different fatigue design stress methods for common constructional details in cylindrical shells under axial loading – Influence of the r/t‐ratio and the shell thickness

AbstractCylindrical shells are frequently used in industrial constructions, for example in pipes, towers and masts and are often subjected to fatigue loading. Two common welded constructional details were studied: i) detail 1, the wall thickness transition with a circumferential butt weld and ii) detail 2, the welded ring stiffener. Both details are loaded with axial stresses due to N and/or M. In the new Eurocode prEN 1993‐1‐9:2021 three different fatigue design stress methods are provided in general. For these two constructional details, beside the modified nominal stress method, the hot spot stresses and effective notch stresses are determined with Finite‐Element‐Analyses, with variation of geometric parameters, like shell thickness t, radius‐over‐thickness‐ratio, t2/t1‐ratio at the thickness transition and thickness ts of the ring stiffener. Stress concentration factors for the different fatigue design stress methods are presented and the influence of the geometric parameters are investigated. Especially the difference between a plate with an infinite radius‐over‐thickness‐ratio and the pipe with a finite radius‐over‐thickness‐ratio is shown. Also, the influence of the shell thickness is investigated. For comparison of the three different methods the utilization factors of the hotspot stress method and the effective notch stress method are compared with those of the modified nominal stress method. A recommendation is given, which design stress method is appropriate for the two studied details, depending on the geometric parameters.

Numerical investigation of residual stresses in welded joints of cylindrical shell

Welded cylinder structures such as pressure vessels and pipes for transportation have been applied in power stations, aerospace, and shipping industries. The study of weld-induced residual stress is vital in predicting the life of welded cylinder vessels. Depending upon the required diameter and length, they are welded by either circumferential welding or longitudinal welding. In the present work, a sequentially coupled thermal, structural analysis is carried out on circumferential and longitudinal butt weld joints of AH-36 cylinder components. The thermal field distribution and subsequent residual stresses during Gas Tungsten Arc Welding (GTAW) are studied. The moving heat source considered for analysis is based upon Goldak’s double ellipsoidal model. The weld-induced axial and hoop stresses are evaluated on both the outer and inner surfaces of the cylinder. The results for circumferential and longitudinal butt weld joints are compared. The magnitude of peak hoop and axial stresses in longitudinal butt weld joints are 45% and 95% higher than in circumferential butt weld joints. The developed analysis model, used to evaluate the thermal histories and residual stresses, is validated with experimental measurements.

Tantalum Alloy Welding: Does the Thermal Cycle Influence the Microstructure?

The aim of the present work is to verify the microstructural behavior of a B 521 tantalum alloy UNS Grade R05200 after welding, in relation to the welding thermal cycle. The joint design was a 1.5 mm thickness circumferential butt welding, on a 32 mm outside diameter pipe, welded in 1 G position (horizontal, flat, and rotating). The chosen welding process was gas tungsten arc welding (GTAW). The microstructural analysis showed the presence of coarse, dendritic-columnar structures, as well as a hexagonal cell, with no cracks noted. Hardness tests showed an increase in hardness, from 120 HV to 425 HV, in the heat-affected zone. Through finite element methods, the behavior of the temperature field was estimated and compared.

Failure analysis of a welded stainless-steel piping system with premature pitting

A failure analysis of welded pipes (ASTM A312 Grade TP 316L) in a water treatment plant was done and it is presented in this paper. The premature failure was detected by several water leakages caused by pitting corrosion three months after hydrostatic tests and before transferring the plant to the operator. The leakages were located near the bottom of the circumferential butt welds of the entire piping system. The samples were analyzed using visual inspection, radiographic examination, optical emission spectrometry, ferrite number (FN) measurements, metallographic examination and Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS). The results showed that the corrosion rate was extremely high compared to other studies available in the literature for austenitic stainless steel and the pits were in the weld metal and the Heat Affected Zone (HAZ) of the circumferential welds. The lack or deficiency of a purging gas during the field welding generated heat-tinted zones around the welds. Visual inspection showed that the pits were preferentially located at heat-tinted zones and they have large sub-surface cavities. The SEM inspection confirmed the presence of structures compatible with sulfate-reducing bacteria and iron-oxidizing bacteria and revealed that the failure was caused by microbiologically-influenced corrosion (MIC). The review of standards and codes used to perform hydrostatic testing of austenitic stainless steel piping systems showed that the water used for hydrostatic tests should have been treated before testing or the surfaces should have been deeply drained and dried. The failure was caused by the water used during the hydrostatic testing since it was not fully drained from the piping system and remained stagnant leading to MIC.

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.

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.

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.

Experimental study of weld fatigue strength reduction for a stainless steel piping component

An experimental mean curve and a design fatigue curve corresponding to 95% survival probability were derived from realistic fatigue experiments on a non-welded water pressurized piping component with primarily focus on high cycle fatigue. The components were subjected to a synthetic variable amplitude bending deformation. Comparison with the results obtained for a similar piping component with a circumferential butt weld allowed the determination of an experimental fatigue strength reduction factor. Comparison with the fatigue procedure and design curve in ASME BPVC Section III allowed to quantify its conservatism with regards to accounting for the presence of a weldment and more generally transferability.

Light Band Zone Formation and its Influence on Properties of Magnetically Impelled Arc Butt (MIAB) Welded Carbon Steel Tubes

Magnetically Impelled Arc Butt (MIAB) welding is a solid state welding process especially used in circumferential butt welding of tubes in which heat is generated prior to upsetting by an electric arc moving along the peripheral edges of the weldment with the aid of an external magnetic field. This paper aims at studying the formation of Light Band (LB) zone and its effect on the weld properties of MIAB welding of carbon steel tubes. A detailed microstructural analysis has been carried out to understand the microstructural transformations taking place in Thermo-Mechanically Affected Zone leading to formation of LB zone. Welded samples show better tensile properties with higher upset current as it eliminates LB zone formation at weld interface. For lower upset current, width of the LB zone increases with increase in arc rotation current resulting in lower weld tensile strength.

Comparison of weld characteristics between longitudinal seam and circumferential butt weld joints of cylindrical components

The control of weld induced residual stresses and distortions are very important in longitudinal seam (L-Seam) and circumferential (C-Seam) butt joints of cylindrical components. In this study, a three-dimensional, sequentially coupled thermo-metallurgical-mechanical analysis was performed for L-Seam and C-Seam butt weld joints of AISI 304 cylindrical components to evaluate weld characteristics during gas tungsten arc welding (GTAW) process. Initially, weld experiments were carried out to obtain the heat input and the macrographs which were further validated with the predicted weld molten pool sizes during heat source fitting (HSF) analysis. The Goldak's double ellipsoid heat source function obtained in HSF analysis was employed in the thermal and mechanical analysis. The results reveal that there are significant differences in residual stress distributions between L-Seam and C-Seam butt joints. Full scale shop floor welding experiments were performed to verify the effectiveness of the proposed numerical models and these are in good agreement with the experimental measurements.

Analysis preheat effect on thermal cycle and residual stress in a welded connection by FE simulation

Preheat effect on thermal cycle and residual stress was studied by employing a 3D finite-element method. A circumferential multi-pass butt weld of pipes based on ASME section IX was investigated and the experiment for thermal cycle validation was carried out. A spherical heat source density with a Gaussian distribution was employed and by an appropriate coordinate system, the rotation of electrode around the pipe was modeled reasonably. Sequentially coupled, thermal–mechanical analysis was applied using temperature-dependent thermo-mechanical behavior for material properties and the variation in peak temperature, cooling rate, and the axial residual stress due to the preheat effect were evaluated.The simulation results revealed that the pattern of final residual stress was influenced by the preheat temperature. However the thermal cycle experienced in the welding zone seems to be significantly affected by the preheat temperature.

Finite element simulation of welding residual stresses in martensitic steel pipes

Finite element (FE) simulations of the welding of two high grade steel pipes are described. The first is a P91 steel pipe welded with a similar P91 weld consumable, and the second is a P92 steel pipe welded with dissimilar nickel–chromium based weld consumables. Both welds are multipass circumferential butt welds, having 73 weld beads in the P91 pipe and 36 beads in the P92 pipe. Since the pipes and welds are symmetric around their axes, the FE simulations are axisymmetric, allowing high FE mesh refinement and residual stress prediction accuracy. The FE simulations of the welding of the P91 and P92 pipes comprise thermal and sequentially coupled structural analyses. The thermal analyses model the heat evolution produced by the welding arc, determining the temperature history throughout the FE models. Structural analyses use the computed temperature history as input data to predict the residual stress fields throughout the models. Post-weld heat treatment (PWHT) of both pipes has also been numerically simulated by assuming that the FE models obey the Norton creep law during the hold time period at 760°C. The residual stresses presented here have all been validated by corresponding experimental measurements. Before PWHT, it has been found that, at certain locations in the weld region and heat affected zone (HAZ) in the pipes, tensile hoop and axial residual stresses approach the tensile strength of the material, presenting a high risk of failure. It has also been found that PWHT substantially reduces the magnitude of residual stresses by varying degrees depending on the material.

Numerical analysis of deformation in multi-pass circumferential TIG welding with narrow gap

As one of the practical issues for narrow gap tungsten inert gas welding, the mechanism of gap width shrinkage in multi-pass circumferential butt welding of thick-walled pipes was examined experimentally and numerically. The experimental result using 600-mm outer diameter pipes with 200-mm wall thickness revealed that the shrinkage of gap at the beginning of the welding process of 174 pass welding for 73 layers would be independent of the groove shape. Also, from the results of the thermal elastic–plastic finite element analyses, it was found that a 2D axisymmetric model was inadequate in predicting the initial shrinkage of gap width. Moreover, the computational results indicate that the shrinkage of gap width could be estimated quantitatively using a 3D model of whole pipe and defining the appropriate heat input area, while the results of the 3D partial model of pipe with small degree was insufficient to predict the initial shrinkage precisely.

1307 アルミニウムパイプの全周突き合せ溶接における溶け込み監視システム(OS18-2 金属材料の生産加工II,OS18 金属材料の生産加工)

1307 アルミニウムパイプの全周突き合せ溶接における溶け込み監視システム(OS18-2 金属材料の生産加工II,OS18 金属材料の生産加工)

Stochastic Fatigue Assessment for Berthing Monopiles in Inland Waterways

As an unsupported structure, a berthing monopile is often expected to work under harsh conditions. Approaching vessels may introduce considerable berthing energy, resulting in substantial tensile stress on the pile. This vessel impact occurs frequently as a primary load over the service life of the berthing monopile. In this situation, the fatigue safety of circumferential butt welds on piles becomes a crucial aspect in monopile design. This paper evaluates linear elastic fracture mechanics (LEFM)-based fatigue reliability for circumferential splice welds on steel monopiles. Studied monopiles are those installed in inland waterways, for which the predominant fatigue loading is vessel berthing. Important factors involved in the fatigue reliability assessment are addressed. A geometry function for determining the range of stress intensity factor is identified, an approximate stress concentration factor for piles under bending moment is mathematically formulated, and the beta distribution is applied to characterize the doubly bound hot-spot stress range caused by operational water-level constraints. A detailed case study is presented for illustrative purposes. This paper provides a practical approach to assessing LEFM-based fatigue reliability for steel berthing monopiles. DOI: 10.1061/(ASCE)WW.1943-5460.0000063. © 2011 American Society of Civil Engineers. CE Database subject headings: Piles; Marine terminals; Welds; Cracking; Fatigue life; Joints; Reliability; Waterways. Author keywords: Steel piles; Marine terminals; Welds; Cracking; Fatigue life; Tube joints; Reliability; Inland waterways.

Intergranular Strains in Pre-Strained and Welded Pipes

Neutron diffraction has been used to investigate the weld residual stresses and the intergranular residual strains in butt-welded 316H pipes. Measurements have been made on pipes subjected to varying degrees of plastic pre-straining before welding, in order to assess the effects of plastic strain on the weld residual stresses and the intergranular strains in the material. The intergranular strains following plastic deformation will also be affected by the annealing effect of the welding. Pipes were initially prepared with plastic strain of 0, 10, 15, 20 and 25% plastic deformation. Thereafter, the pipes were cut in half and welded with a circumferential butt-weld. Bar specimens were extracted from the remote end of the 0, 10, 15, 20 and 25% pre-strained and welded pipes. Cross-weld bar specimens were also machined from the 0 and 20% pre-strained and welded pipes. Neutron diffraction measurements were made at ENGIN-X, ISIS and FRM-II, Munich. The aim of this paper is to evaluate the intergranular strains developed after pre-straining from measurements made in remote bar specimens from the remote-end of the pipes. The annealing effect of the welding cycle on the intergranular strains is also studied, with measurements done at several points on cross-weld bar specimens, obtaining the strain response of different hkl lattice planes. The results show that the {200} and {220} planes are at the extremes of response during loading. Furthermore, the welding thermal cycling relaxed the intergranular strains from the prior plastic deformation.

Estimation of the maximum allowable lack of penetration defects in circumferential butt welds of structural tubular towers

This paper analyses the structural integrity of structural tubular towers (i.e., towers of wind turbines and floodlight towers) with lack of penetration defects on their circumferential butt welds. The methodology presented is particularised to the analysis of the lack of penetration defects detected in certain sections of several wind towers after the construction process. It is also analysed how such defects affect the fitness for service of the towers during their theoretical lifespan (20 years). The methodology (based on the use of Failure Assessment Diagrams, FAD) can easily be extrapolated to the assessment of other types of defects or towers. Its main hypotheses consist of establishing that the defects behave as internal cracks with certain geometries and also that fracture and fatigue are the key processes affecting the structural integrity of the towers. Then, the resulting allowable crack size, corresponding to the lifespan of the tower, for the different sections analysed and for the different crack geometries considered is determined.

Behaviour of stresses in circumferential butt welds of steel pipe under superimposed axial tension loading

A computational procedure is presented for analyzing behaviour of stresses in circumferential butt welds of carbon steel pipe subject to superimposed mechanical loading. Three-dimensional uncoupled thermo-mechanical finite element (FE) analysis method is developed in order to predict the weld residual stress states in circumferentially butt-welded steel pipe. The FE method is verified through the experimental work. Then, three-dimensional elastic–plastic FE analysis is carried out to investigate the behaviour of stresses in steel pipe circumferential welds undergoing superimposed axial tension loading using the weld residual stresses and plastic strains obtained from the thermo-mechanical FE method. The simulated results show that spatial variations of the weld residual stresses are present along the circumference and a rapid change of the residual stresses exists at the weld start/stop position, therefore three-dimensional FE analysis is essential to accurately simulate the circumferential welding of steel pipe. Moreover, when axial tension loading is applied to the circumferentially welded steel pipe, bending moment is generated at the weld area caused by the circumferential shrinkage of the weld during welding, thus affecting the axial and hoop stress evolutions in the course of mechanical loading.

Stress concentration factors at welds in pipelines and tanks subjected to internal pressure and axial force

In this paper, analytical expressions for stress concentration factors in pipes subjected to internal pressure and axial force are derived for a number of design cases based on classical shell theory. The effect of fabrication tolerances in simple butt welds is assessed. Analyses based on classical mechanics are compared with results from axisymmetric finite element analyses for verification of the presented methodology. Stress concentration factors are presented for circumferential butt welds in pipes welded together from pipes with different thicknesses, welds at buckling arrestors, welds at flanged connections in pipelines, and welds at ring stiffeners on the inside and the outside of the pipes. It also includes stress concentration factors at end closures in pipes for gas storage. Larger pipes are fabricated from plates with a longitudinal weld. This fabrication process introduces out-of-roundness in the pipes. The actual out-of-roundness is a function of internal pressure. An analytical expression for the bending stress in the pipe wall due to this out-of-roundness is presented. The derived stress concentration factors can be used together with a hot spot stress S– N curve for calculation of fatigue damage.

Residual stresses in welded structures

The nature of residual stresses in welded structures is discussed in terms of their magnitude, directionality, spatial distribution, range and variability. The effects of the following factors on the residual stresses are considered: material properties, material manufacture, structural geometry, fabrication procedure, welding procedure, post-weld treatments and service conditions. Examples are given of residual stress distributions in plate butt welds, circumferential butt welds and weld cladding. These illustrate the different magnitudes and distributions of residual stress that can be found in different joint geometries, and demonstrate the effects of the mechanical, thermal and metallurgical properties of the constituent materials and the sensitivity of residual stresses to pass sequence and to the restraints applied during welding. Further examples for the common case of circumferential butt welds in pipes and pressure vessels are used to illustrate the extent of residual stresses as a function of distance from the weld and the effects of post-weld heat treatment. Measurements or analytical predictions of residual stresses are often subject to significant scatter or variability. This scatter may be due to systematic factors such as variability in measurement location or material properties, or to experimental error in measured data, erroneous assumptions in analytical modelling or unknown factors such as pre-existing residual stresses, inadequately documented welding or fabrication procedures or unrecorded local repairs. Improved prediction and reduction of uncertainty of residual stresses will require better recording of the whole manufacturing and service history of the welded structure and its component materials and better understanding and analysis of the many processes that may affect the residual stresses.