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.     

Predictions and measurements of residual stress in repair welds in plates

This paper presents the work, from the European Union FP-5 project ELIXIR, on a series of rectangular repair welds in P275 and S690 steels to validate the numerical modelling techniques used in the determination of the residual stresses generated during the repair process. The plates were 1,000 mm by 800 mm with thicknesses of 50 and 100 mm. The repair welds were 50%, 75% and 100% through the plate thickness. The repair welds were modelled using the finite element method to make predictions of the as-welded residual stress distributions. These predictions were compared with surface-strain measurements made on the parent plates during welding and found to be in good agreement. Through-thickness residual stress measurements were obtained from the test plates through, and local to, the weld repairs using the deep hole drilling technique. Comparisons between the measurements and the finite element predictions generally showed good agreement, thus providing confidence in the method.     

FCAW process to avoid the use of post weld heat treatment

Post weld heat treatment (PWHT) is the most common technique employed for relieving residual stresses after general repair welding. Besides, the primary purpose of reducing the effect of stresses induced by welding, PWHT is also intended to temper the metallurgical structure of the heat-affected zone (HAZ). Unfortunately, there are significant difficulties in carrying out post weld heat treatment such as; the complexity of weld geometry, the possibility of distortion in the case of any mechanical loads, difficulty in heating symmetrically, and also PWHT may cause degradation of the material properties (especially creep and tensile strength in the case of multi PWHT cycles). Most of the repairs in industry are performed with manual metal arc welding (MMAW), however, the benefits of the flux cored arc welding (FCAW) process have been appreciated by industry for many years.     

Residual stresses measurement by neutron diffraction and theoretical estimation in a single weld bead

Welding residual stresses are important in pressure vessel and structural applications. However, residual stress remains the single largest unknown in industrial damage situations. They are difficult to measure or theoretically estimate and are often significant when compared with the in-service stresses on which they superimpose. High residual stresses lead to loss of performance in corrosion, fatigue and fracture.     

Effect of vibratory weld conditioning on the residual stresses and distortion in multipass girth-butt welded pipes

For a fully welded body valve, the last procedure is welding, so it is important to control the residual stress and distortion in order to assure spool rotation, valve watertightness, stress corrosion resistance and non-deformability in active service. In this study, the effects of vibratory weld conditioning (VWC) on the residual stress and distortion were studied in multipass girth-butt welded pipes through comparison between VWC and normal submerged arc welding. The results show that VWC can reduce the residual hoop stresses at the outer surface and the radial distortion significantly; but VWC has only a slight effect on the residual axial stresses at the outer surface and axial distortion. Moreover, the residual stresses decrease and are lower than the yield strength using VWC, which decreases the susceptibility of a weld to fatigue damage, stress corrosion cracking and fracture, and improves the safety of welded structures.     

Code characterisation of weld residual stress levels and the problem of innate scatter

Economic and safe management of operating nuclear power plant is increasingly dependent upon structural integrity assessments for pressure vessels and piping. The residual stress distribution assumed in defect assessments for welded joints often have a deciding influence on the analysis outcome. Guidance on characterising conservative levels of weld residual stress can be found in structural integrity codes and procedures such as R6, API 579 and BS7910. There is an increasing need to develop more realistic and reliable residual stress distributions that will deliver more accurate integrity assessments. However, future development of such distributions will have to deal convincingly with what is often termed the “innate scatter” of weld residual stresses. This paper first identifies and illustrates some of the origins of apparent innate scatter. The stability of the welding process is examined. The importance of transient weld bead starts/stops and the lay-up of passes in multi-pass welds are demonstrated. Uncertainties associated with two commonly used residual stress measurement techniques are reviewed and simple quantitative studies used to reveal the role of measurement gauge length and, more significantly, errors in spatial location on the level of measured residual stress. The final part of this paper surveys how structural integrity codes and procedures currently characterise welding residual stresses for defect-assessment purposes and discusses the development of more realistic residual stress profiles based on statistical treatment of scatter and uncertainties.     

Weld repair practices without post weld heat treatment for ferritic alloys and their consequences on residual stresses: A review

The use of the half-bead, temper bead welding (TBW), and cold repair techniques is proving to reduce the cost of repairs and extend the life of aged components in power plants, petrochemical and hydrocarbon processing industries. It has been a significant area of interest for more than twenty years. A critical factor in this context is residual stress. The presence of residual stresses can lead to cracking which ultimately results in structural failure. This paper reviews the half-bead, TBW, and cold repair technique practices and their consequences on residual stresses within the nuclear, power, refinery and petrochemical industries and some of the contributions made by our group of researchers in this area.     

Numerical prediction and experimental validation of temperature and residual stress distributions in buried‐arc welded thick plates

The paper deals with the numerical simulation and experimental investigations of a buried‐arc welding process on a butt‐welded plate sample. In the numerical investigations, the finite element analysis is carried out by applying the element birth and death technique in the thermal analysis, while the mechanical analysis is performed simultaneously in one step to reduce simulation time. The temperature history at two locations is recorded with thermocouples, while residual stresses are measured by using the hole‐drilling stress relaxation method at four points. The heat input efficiency for the buried‐arc welding process is determined by using a parametric analysis. The numerically obtained temperatures and residual stresses correspond very well with the experimental measurements. Furthermore, by using buried‐arc welding, 73% of the weld filler material is saved and 59% of energy, whereas the CO₂ emission to the atmosphere is reduced by 83% in comparison with conventional metal active gas welding for a model of the same dimensions.     

Residual stress and plastic strain analysis in the brazed joint of bonded compliant seal design in planar solid oxide fuel cell

This paper uses finite element method (FEM) to predict the residual stress and plastic strain in the brazed joint of sealing foil-to-window frame in bonded compliant seal (BCS) design in a planar solid oxide fuel cell (PSOFC). The effects of window frame material type, sealing foil thickness, filler metal thickness and window frame thickness on residual stress and plastic strain are discussed. Large residual stress is generated in the joint, and the stress and strain are concentrated around the fillet. It is proved that the BCS design can mitigate and trap some residual stress by plastic deformation within the sealing foil. The residual stress and the ability of trapping stress of sealing foil are affected by window frame material and structure thickness. Based on the comprehensive considerations of the impact of residual stress and plastic strain, Alloy 625 as a window frame material is found to be better than Haynes 214, Hastelloy X and SUS 316L. The optimum thickness of sealing foil and filler metal BNi2 are found to be 150 μm and 75 μm, respectively. The residual stress and plastic strain are increased with the increase of window frame thickness.     

Finite element analysis of metallurgical phase transformations in AA 6056-T4 and their effects upon the residual stress and distortion states of a laser welded T-joint

Aircraft industry makes extensive use of aluminium alloy AA 6056-T4 in the fabrication of fuselage panels using laser beam welding technique. Since high temperatures are involved in the manufacturing process, the precipitation/dissolution occurrences are expected as solid state phase transformations. These transformations are likely to affect the residual distortion and stress states of the component. The present work investigates the effect of metallurgical phase transformations upon the residual stresses and distortions induced by laser beam welding in a T-joint configuration using the finite element method. Two separate models were studied using different finite element codes, where the first one describes a thermo-mechanical analysis using Abaqus; while the second one discusses a thermo-metallo-mechanical analysis using Sysweld. A comparative analysis of experimentally validated finite element models has been performed and the residual stress states with and without the metallurgical phase transformations are predicted. The results show that the inclusion of phase transformations has a negligible effect on predicted distortions, which are in agreement with the experimental data, but an effect on predicted residual stresses, although the experimentally measured residual stresses are not available to support the analyses.     

In-situ heat generation measurement of the anode and cathode in a single-layer lithium ion battery cell

Considering that the anode and cathode in batteries have different heat generation behaviors and that there is almost no technique for measuring the heat generation of the anode and cathode under nondestructive conditions, we proposed a novel in-situ nondestructive temperature measurement technique for acquiring the heat generated by the anode and cathode. To this end, a Swagelok Li-ion battery cell is designed to visualize the temperature of the anode and cathode by using an infrared camera. Compared with the anode's heat generation, the cathode generates more heat at a 0.5 C current. The reversible heat generation of the electrode has an exothermic effect, which could be transformed into an endothermic effect within 0% to 100% depth of discharge (DoD). The irreversible heat generation always has an exothermic effect and decreases during the delithiation process. Therefore, it can be concluded that it is likely that variations in the heat generated by the anode and cathode can be measured by using the proposed nondestructive method. Finally, it is meaningful that the effects of the anode/cathode chemistry and other factors such as C-rate and temperature on the local heat generation will be investigated in future works.     

Monitoring of transient 3D temperature distribution and thermal stress in pressure elements based on the wall temperature measurement

Abstract

Two methods for monitoring the thermal stresses in pressure components of thermal power plants are presented. In the first method, the transient temperature distribution in the pressure component is determined by measuring the transient wall temperature at several points located on the outer insulated surface of the component. The transient temperature distribution in the pressure component, including the temperature of the inner surface is determined from the solution of the inverse heat conduction problem (IHCP). In the first method, there is no need to know the temperature of the fluid and the heat transfer coefficient. In the second method, thermal stresses in a pressure component with a complicated shape are computed using the finite element method (FEM) based on experimentally estimated fluid temperature and known heat transfer coefficient. A new thermometer with good dynamic properties has been developed and applied in practice, providing a much more accurate measurement of the temperature of the flowing fluid in comparison with standard thermometers. The heat transfer coefficient on the inner surface of a pressure element can be determined from the empirical relationships available in the literature. A numerical-experimental method of determination of the transient heat transfer coefficient based on the solution of the 3D-inverse heat conduction problem has also been proposed. The heat transfer coefficient on the internal surface of a pressure element is determined based on an experimentally determined local transient temperature distribution on the external surface of the element or the basis of wall temperature measurement at six points located near the internal surface if fluid temperature changes are fast. Examples of determining thermal and pressure stresses in the thick-walled horizontal superheater header and the horizontal header of the steam cooler in a power boiler with the use of real measurement data are presented.