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An extensive finite element simulation campaign was undertaken to examine the complete manufacturing history and high temperature thermal ageing of thick-walled girth-welded austenitic steel pipes fabricated from Esshete 1250 austenitic steel. The simulations examined the impacts of prior quenching of pipe material, fabrication of closely adjacent welds, and axial restraint during welding. The simulations considered both simple isotropic and kinematic hardening behaviour, and a large matrix of Lemaitre-Chaboche mixed isotropic-kinematic hardening material constitutive models, with a focus on examining the most accurate evolutionary hardening behaviour for weld metal. High temperature (650 °C) service exposure was modelled using an RCC-MR type creep model, and the sensitivity of the predicted relaxation to variability in the model parameters was assessed. The predicted residual stresses were validated using measurements made with the deep hole and incremental deep hole drilling techniques and the contour method. • Extensive finite element simulations are performed to predict residual stresses in carefully characterised austenitic pipe girth welds with diverse residual stress measurements in the as-welded state and after high temperature ageing. • The simulations use mixed hardening models fitted using data from an extensive test programme on both weld metal and parent material. • Residual stresses in both the as-welded state and after creep relaxation at high temperature are predicted with high accuracy. [ABSTRACT FROM AUTHOR]

In this study we aim to determine how the choice of welding process might impact on the through-life performance of critical nuclear components such as the reactor pressure vessel, steam generators and pressuriser in a pressurised water reactor. Attention is devoted to technologies that are currently employed in the fabrication of such components, i.e. narrow-gap variants of gas-tungsten arc welding (GTAW) and submerged arc welding (SAW), as well as a technology that might be applied in the future (electron beam welding). The residual stresses that are introduced by welding operations will have an influence on the integrity of critical components over a design lifetime that exceeds 60 years. With a view to making an assessment based on residual stress as pertinent as possible, weld test pieces were manufactured with each process at a thickness that is representative for such components, i.e. 130 mm. Residual stress measurements were made in the as-welded state using both incremental deep hole drilling and the contour method, and after post-weld heat treatment using deep hole drilling. Part 1 of this study documents weld manufacture, while Part 2 presents the residual stress measurements and discusses their significance. • 130mm thick welds were made in a nuclear grade steel using gas-tungsten arc, submerged-arc and electron beam welding. • Residual stresses were measured using both the contour method and incremental deep-hole drilling. • Measurements were made in both the as-welded and post-weld heat treated conditions. • The results reveal that the effectiveness of PWHT is best assessed on weldments larger than normally used in universities. • Final levels of stress after post-weld heat treatment were found to be higher than comparable studies on smaller test pieces. • This is likely to be significant for safety assessments on nuclear components. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of stress–strain properties and residual stress in purpose-designed one and three-pass groove-weld specimens are used to optimise a mixed hardening constitutive model for simulating residual stresses in austenitic Type 316 stainless steel weldments. It is demonstrated that isotropic hardening over-predicts the tensile magnitude of welding residual stress in the benchmark specimens, while pure kinematic hardening gives an under-prediction of longitudinal stresses in parent material close to the weld. The most accurate predictions are those based on optimised mixed isotropic–kinematic formulations combined with a multi-pass moving heat source welding analysis. [Copyright &y& Elsevier]

Abstract: In general, the tensile residual stresses typically associated with weldments, and the presence of corrosive environments, are a major concern due to the possibility of Stress Corrosion Cracking (SCC). In the case of the dissimilar metal welds typically used in the primary circuits of pressurised light water nuclear reactors the presence of tensile residual stress and corrosive environment leads to so-called Primary Water Stress Corrosion Cracking (PWSCC). It is thus of technological importance to know the residual stresses associated with the dissimilar metal welds in order to assess the susceptibility of components to PWSCC. The current paper presents finite element simulations of the full manufacturing process of dissimilar metal weld mock-ups with and without a full structural overlay weld. The present simulations are validated by comparison with deep hole drilling (DHD) and incremental deep hole drilling (iDHD) residual stress measurements. A dedicated welding heat source modelling tool was used in order to reduce the uncertainties in the thermal solution by calibrating ellipsoidal Gaussian volumetric heat sources using the detailed welding records. In the mechanical analysis a mixed isotropic–kinematic (Lemaitre-Chaboche) material hardening model was employed to produce the most representative material response to the cyclic thermo-mechanical loading imposed during welding. [Copyright &y& Elsevier]

Abstract: The members of the European network NeT have undertaken parallel round robin activities measuring and simulating the residual stresses generated by laying a single Tungsten Inert Gas (TIG) weld bead on an AISI Type 316L austenitic stainless steel flat plate. This is a strongly three-dimensional configuration with many of the characteristics of a repair weld. The round robin finite element predictions of weld residual stresses are compared with each other in order to identify the effects on the predicted residual stresses of material hardening model, global heat input, mechanical and thermal boundary conditions, and the handling of high temperature inelastic strains. Comparison with the residual stress measurements then leads to the optimum choices for these variables. [Copyright &y& Elsevier]

Abstract: Constitutive plasticity theory is commonly applied to the numerical analysis of welds in one of three ways: using an isotropic hardening model, a kinematic hardening model, or a mixed isotropic–kinematic hardening model. The choice of model is not entirely dependent on its numerical accuracy, however, as a lack of empirical data will often necessitate the use of a specific approach. The present paper seeks to identify the accuracy of each formalism through direct comparison of the predicted and actual post-weld residual stress field developed in a three-pass 316LN stainless steel slot weldment. From these comparisons, it is clear that while the isotropic hardening model tends to noticeably over-predict and the kinematic hardening model slightly under-predict the residual stress field, the results using a mixed hardening model are quantitatively accurate. The level of inaccuracy in isotropic models is shown to be largely dependent on the extent of thermal cycling experienced by the material. Even though the kinematic hardening model generally provides more accurate results when compared to an isotropic hardening formalism, the latter might be a more appealing choice to engineers requiring a conservative design regarding weld residual stress. [Copyright &y& Elsevier]

Weld-induced in-process tempering of martensite/bainite was studied through characterisation and modelling. Three-pass gas tungsten arc (GTA) and submerged arc (SA) welds were produced in grooved plates made from a low-alloy ferritic (SA508) steel. A thermal-metallurgical-mechanical model was developed to simulate multi-pass welding while accounting for tempering kinetics. Significant tempering of martensite, in the heat affected zone that was produced by the first pass, occurred during the second and third passes, resulting in a coarsened lath structure, increased carbide precipitation and reduced hardness. The tempering effect was more extensive in the SA weldments than in the GTA weldments, since the tempering mainly occurred in a martensite-dominant region without re-austenitisation for the former while in a partially re-austenitised region for the latter. The predictions for tempered microstructures were consistent with microscopic observations, and the predicted micro-hardness agreed well with measurements when tempering was considered in modelling. The peaks in predicted tensile and compressive residual stresses were reduced by considering tempering effects, since the local yield strength reduced as a consequence of the tempering, thereby limiting the stresses that could be sustained. [ABSTRACT FROM AUTHOR]

In this study we aim to determine how the choice of welding process might impact on the through-life performance of critical nuclear components such as the reactor pressure vessel, steam generators and pressuriser in a pressurised water reactor. Attention is devoted to technologies that are currently employed in the fabrication of such components, i.e. narrow-gap variants of gas-tungsten arc welding (GTAW) and submerged arc welding (SAW), as well as a technology that might be applied in the future (electron beam welding). The residual stresses that are introduced by welding operations will have an influence on the integrity of critical components over a design lifetime that exceeds 60 years. With a view to making an assessment based on residual stress as pertinent as possible, weld test pieces were manufactured with each process at a thickness that is representative for such components, i.e. 130 mm. In Part 1, the manufacture of the welds is documented, together with the underpinning rationale, so that the value of the resulting measurements (which are presented in Part 2) will be maximised. • 130 mm thick welds were manufactured in a nuclear steel using gas-tungsten arc, submerged-arc and electron beam welding. • Residual stresses were measured using the contour method and incremental deep-hole drilling, before and after PWHT. • Part 1 of this article documents the full history of the weld test pieces. • Part 2 deals with the residual stress measurements, and presents the major conclusions that arise from the work. • Results show that the effectiveness of PWHT is best assessed on large weld mock-ups. [ABSTRACT FROM AUTHOR]

Safety-critical pressure vessels in pressurised water reactors (PWRs), such as the reactor pressure vessel and the steam generators, have been fabricated using arc welding processes for several decades. However, recent interest in nuclear new-build programmes has stimulated interest in reassessing the technologies that are available for the welding of these components. For example, considerable effort has been directed at the development of reduced-pressure electron beam (RPEB) welding, owing to the substantial productivity gains it would offer, while developmental work has also been carried out on multipass narrow-gap laser welding (NGLW). In this work we assess the effects that two current technologies, and two candidate technologies for future build programmes, are likely to have on the generation of residual stresses within critical nuclear components. Single-sided welds were manufactured in 30 mm thick plates of SA508 steel, using four welding processes: gas-tungsten arc welding (GTAW), submerged-arc welding (SAW), multipass NGLW and RPEB welding. The residual stress distributions for each type of weld have been measured both in the as-welded condition and after post-weld heat treatment, using neutron diffraction and the contour method. The results are compared and discussed. [ABSTRACT FROM AUTHOR]

The prediction of weld-induced residual stress assists the structural integrity assessment of welded structures. In this study, residual stress measurements of girth welded austenitic stainless-steel pipes were used to develop two Artificial Neural Network (ANN) ensemble models to predict through-thickness residual stress profiles in Weld Centre Line (WCL). One model was developed for axial and the other for hoop residual stress prediction. The inputs of the models were the pipe radius to thickness ratio, the thickness, the heat input (weld arc electrical energy per unit run length [kJ/mm]) and the normalised through-thickness position. The hyperparameters were tuned, and the models were trained with various initial weight vectors, creating an ensemble of ANNs. The models' performance was assessed by a test set and by sensitivity studies which revealed the models' output trends. • Ensemble machine learning for small and noisy residual stress dataset • Model predictions below R6 Level 2 profiles and close to experimental measurements • Sensitivity studies revealed the influence of model inputs on residual stress [ABSTRACT FROM AUTHOR]

Abstract: A well-defined residual stress field was introduced into modified single edge notched bend, SEN(B), specimens by the ‘in-plane compression’ procedure in order to investigate the interaction between residual stress and applied mechanical loading. Numerical predictions of the residual stress field arising from the in-plane compression procedure are given along with details of the numerical fracture modelling and experimental fracture test results made on A533B ferritic steel specimens in the lower transition region at −150°C. Use was made of a recently developed finite element post-processor capable of determining path-independent J-integral values in the presence of residual stress fields. The paper compares the experimental results to predictions made using a probabilistic ‘global approach’ based on the conventional crack-tip parameters K and J and predictions made using a well-known structural integrity assessment code, R6 (Revision 4). It is shown that obtaining more accurate estimates of the crack driving force created by residual stresses leads to better correlation between experiments and predictions, and less conservatism in the assessment code. [Copyright &y& Elsevier]

Abstract: Full 3-dimensional (3-D) simulation of multi-pass weld repairs is now feasible and practical given the development of improved analysis tools and significantly greater computer power. This paper presents residual stress results from 3-D finite element (FE) analyses simulating a long (arc length of 62°) and a short (arc length of 20°) repair to a girth weld in a 19.6mm thick, 432mm outer diameter cylindrical test component. Sensitivity studies are used to illustrate the importance of weld bead inter-pass temperature assumptions and to show where model symmetry can be used to reduce the analysis size. The predicted residual stress results are compared with measured axial, hoop and radial through-wall profiles in the heat affected zone of the test component repairs. A good overall agreement is achieved between neutron diffraction and deep hole drilling measurements and the prediction at the mid-length position of the short repair. These results demonstrate that a coarse 3-D FE model, using a ‘block-dumped’ weld bead deposition approach (rather than progressively depositing weld metal), can accurately capture the important components of a short repair weld residual stress field. However, comparisons of measured with predicted residual stress at mid-length and stop-end positions in the long repair are less satisfactory implying some shortcomings in the FE modelling approach that warrant further investigation. [Copyright &y& Elsevier]

A thermal-metallurgical-mechanical model was developed to study the effects of dilution in each weld pass for multipass gas tungsten arc and submerged arc welding in low alloy steel (i.e. SA508) plates. Hardness distributions and residual stresses were measured on the transverse sections perpendicular to the welding direction of the manufactured weldments. The predicted hardness and residual stresses were compared with the measurement data and shown to be reasonably accurate. The results showed that dilution can significantly affect both the hardness and the residual stress field in the weld metal. It was found that, for the base and filler materials used, increased dilution led to greater weld-metal hardness and reduced the magnitude of tensile stress or promoted compressive stress in the as-deposited and reheated weld metals. This mechanical behaviour is associated with the tendency for diluted weld metal to experience delayed austenite decomposition, owing to the high hardenability of SA508 steel relative to the filler materials used. Although dilution is irrelevant for the hardness of the base material and its transformation products adjacent to the weld metal, it affected the full-field residual stresses via the equilibrium interaction between the stresses in the base and weld metals. • Dilution effect is investigated for multipass arc welds with base material having higher hardenability than filler material. • Predictions of hardness and residual stresses are in reasonable agreement with measurements. • Higher dilution leads to higher hardness of the studied weld metal in both as-deposited and reheated states. • Higher dilution reduces tensile stress or promotes compressive stress in the as-deposited and reheated weld metals. [ABSTRACT FROM AUTHOR]