Residual stress and strain measurements were performed on a narrow gap weld mock-up consisting of a 130 mm thick lowalloy ferritic steel plate of type SA508 Grade 3 Class 1. A submerged arc welding (SAW) process with two beads per layer [1] was used. The thermal response during welding was recorded to provide information for numerical modelling of heat transfer. Detailed through- thickness measurements of residual stresses 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 [2]. In this paper, a finite element model of the mockup is described as well as some recent modelling improvements achieved thanks to internal EDF R&D activities and within the framework of NeT Task Group 8 [3]. Comparison between measurements and simulations are in very good agreement. The results presented in this paper are useful for the validation of numerical models for the prediction of residual stress states, including the description of the material behavior which depends on phase fraction evolution and tempering effects. Post-weld heat treatment significantly relieves the welding residual stresses and a focus on its modelling is presented in this paper as it is seen as a key step of the manufacturing operation, reducing the level of residual stresses in reactor pressure vessel's welded joints.

Residual stresses are a crucial factor in assessing the integrity of welded joints. These stresses are known to influence the joint's strength under additional loading, with the altered grain structure at and near the joint a complicating factor. Consequently, a mesoscale model is essential to understand the accumulation of damage in components subjected to external loading, as well as the impact of prior loads on failure. This study addresses the interplay between loading direction and grain morphology, explicitly investigating damage accumulation. The mesoscale model includes a coupled crystal plasticity and a phase field fracture model to estimate the deformation induced during a laser beam weld of 316H stainless steel. The displacement boundary condition was derived from a mechanical model of the weld, with the application of a Chaboche model. The temperature field required for the grain growth and mechanical models were obtained through a thermal fluid dynamics framework. Investigation of crack initiation and propagation was carried using a phase-field fracture model, which allowed the consideration of prior loading. This study indicated that the direction of loading plays an important role in damage susceptibility. The modified grain structure based on the welding simulation showed a different strain at failure compared to the 316H stainless steel parent material. The achieved strain at failure was found to be lower in normal loading compared to the transverse direction. Presently, the crystal plasticity model fails to estimate the macroscopic residual stresses, illustrated by damage propagation resulting in earlier than expected ductile failure upon reloading. The potential causes are addressed and discussed in detail.

Whilst Electron Beam Welding (EBW) has been in existence for several decades, it typically requires the welded component to be housed within a conventional vacuum chamber in order to minimize diffusion of the electron beam, thus limiting the maximum overall size of the component. Recent advances in EBW by TWI (UK) have led to the development of a local vacuum system, whereby a local vacuum is implemented around only the welded section rather than the whole component, thereby avoiding the component size limitations associated with more conventional systems. A direct comparison between the two EBW systems does not appear to have been published hitherto and thus forms one aspect of the work reported here. For the EBW low alloy steel SA508 materials employed in this study, the post weld heat treatment (PWHT) regime has also been investigated, since there has been some concern over the appropriateness of employing conventional PWHTs typically used for traditional arc welds. Charpy impact testing was performed to assess the initial toughness of the welds, with the notch located on the weld centre-line, within the HAZ, and within the parent materials. For SA508 Gr.3, Charpy testing suggested that only the full austenitise and temper heat treatment gave weld centre-line properties similar to the parent material. However, for SA508 Gr. 4N, a modified single step PWHT restored a high level of toughness performance to the weld metal, in comparison with the parent. Finally, fracture toughness testing on the SA508 Gr. 3 and SA508 Gr. 4N materials has been undertaken to determine the upper shelf fracture toughness ( J1C) at 250 degrees C, using multi-specimen compact tension (C( T)) test specimens. These data showed that there were little observable differences in the performance of welds produced by either the conventional high vacuum welding system, or the local vacuum system.

The European Network on Neutron Techniques Standardization for Structural Integrity (NeT) aims to develop experimental and numerical techniques and standards for the reliable characterization of residual stresses in structural welds. NeT has developed over 20 years and involves over 30 organizations including industrial, academic, and research facilities around the world. The network tackled each specific problem by creating a dedicated Task Group (TG), which undertakes both measurement and modelling studies. NeT TG8 follows on from the successful TG1 and TG4 benchmarks, which studied the dissimilar metal welds (DMW) between 18MnD5 and Alloy 52. The material properties characterization, residual stress measurements and modelling have been conducted for NeT TG8. This paper only presents the NeT TG8 benchmark residual stress measurement in the as welded condition and preliminary simulation results.

Safety sensitive industries are increasingly facing challenges such as reducing their environmental impact and bringing down their cost. Part of the solution to such challenges is economical use of assets while maintaining the safety level if not increasing it. Thus, more informed and reliable decision making on repairing or replacing key components is becoming even more important where a simple binary safe/unsafe choice is no longer desirable. Instead, a realistic assessment which inevitably would be probabilistic is needed. However, obtaining the required level of data to suitably underpin a probabilistic assessment can be prohibitively expensive as carrying out hundreds if not thousands of full-scale tests is no longer economically possible. In this work, we explore an alternative approach in which micromechanical characterisations, which due to their small scale, are more affordable, are carried out and informed a meso-scale model of the material behaviour. The meso-scale simulation, that is a crystal plasticity finite element model, is informed by the variations within the material microstructure thus returning a representative material response. The model variation can be estimated by machine learning algorithm such as polynomial chaos expansion thus returning material response variability in a sensible time-scale. The material variability, in turn, is input into a surrogate model of a process modelling, in our case welding simulation, to produce variability in a parameter important for assessment such as weld residual stress.

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.

Within the NeT project [1], Task Group 8 examines a steel plate (French grade 18MND5 close to ASTM A508 Gr.3) containing a five pass “slot” weld made with Ni base alloy (Alloy 52) consumables. NeT TG8 has been organized to address welding repair issues. The TG8 round robin specimen is closely based upon the TG4 design [2], except for the plate thickness that is increased up to 30 mm for more thermal inertia, self-clamping conditions and to reduce distortions that may affect the accurate definition of residual measurement position. It thus presents all the advantages and challenges of the TG4 specimens, namely, the generation of a complex 3D residual stress distribution in a compact, portable specimen that is amenable to rapid measurement of residual stresses by diverse techniques, with a significant volume of weld metal that undergoes multiple high temperature thermo-mechanical load cycles. The use of a nickel-based alloy as filler metal adds considerable residual stress measurement challenges, while this configuration undergoes a complex mismatch behaviour with the base metal where phase transformations and tempering effects occur in the Heat Affected Zone (HAZ). The residual stress measurement and simulation round robins for TG8 are ongoing. What is presented here is therefore the first phase results of the behaviour of the TG8 specimens. Finite element simulation results are compared with residual stresses determined experimentally thanks to different techniques. The agreement between stresses measured by the Contour Method and by neutron diffraction is generally very good, simulation exhibits complex behaviour that help to interpreted some of the measurements and the lessons learned will allow to conduct a second phase benchmark with possible improvement in residual stresses assessments and in-depth analyses for a better understanding of such a dissimilar weldment.

Determining residual stresses in thin-walled pipes is challenging. They are potentially difficult targets for simulation, because they may not behave as simple axisymmetric structures during welding. Thin-walled pipes are more sensitive to changes in the welding heat input than thick-walled pipes. They are also under-represented in the existing population of residual stress measurements used to generate upper bound residual stress profiles for structural integrity assessments. In this paper, residual stress characterisation of two thin-walled austenitic girth welded pipes is presented. The overall geometries of the two mock-up designs were the same; they differed in the linear heat input per pass and the total number of weld passes. The residual stress characterisation was carried out using two independent measurement techniques; the contour method and neutron diffraction. The multiple-cut contour strategy was implemented to measure the cross-sectional maps of hoop and axial stresses on axial-radial and hoop-radial planes respectively. The contour method results are compared with stresses measured using neutron diffraction and specific residual stress distribution signatures observed are discussed.

The mission of the European Network on Neutron Techniques Standardization for Structural Integrity (NeT) is to develop experimental and numerical techniques and standards for the reliable characterisation of residual stresses in structural welds. NeT was first established in 2002, and involves over 30 organisations from Europe and beyond. It operates on a “contribution in kind” basis from industrial, academic, and research facility partners. Each problem examined by the network is tackled by creating a dedicated Task Group (TG), which undertakes measurement and modelling studies and the interpretation of the results. NeT achieves this by conducting parallel measurement and prediction round robins on closely controlled and well characterized benchmark weldments. NeT TG8 follows on from the successful NeT TG1 and NeT TG4 benchmarks, which both examined welds in AISI 316L material and also NeT TG6 which examines an Alloy 600 plate containing a three pass “slot” weld made with Alloy 82 consumables. A new benchmark, NeT TG8, which examines an 18MnD5 (French nuclear code grade also named 18 MnNiMo 05) plate containing a five pass “slot” weld made with Alloy 52 consumables, has been organized to address welding repair issues. This paper describes the NeT TG8 benchmark as a whole, and presents preliminary materials characterization, residual stress measurement, and residual stress modelling results.

Electron beam (EB) welding can efficiently join large-scale components using one single autogenous pass, but it still faces challenges associated with weld-induced distortion and stress. This study investigates EB welding in a low-alloy steel thick-section shell-flange structure for a small modular reactor. A 3D thermal-metallurgicalmechanical model is developed to assess the weld-induced distortion and stress, as well as the strategy to mitigate them. When no restraint is imposed on the circumferential weld plane, an opening and sliding gap develops during the EB welding, which can cause weld defects and even process failure. Restraint through tack welds can effectively mitigate the gapping distortion, but it generates high transient tensile stress in the tack weld. Circumferentially continuous tack weld is preferential over circumferentially discrete tack welds to minimise the tensile stress. The final residual stress is insensitive to the tack-weld restraint, and the stress distribution in the steady-state welding region is broadly similar to that found in plate butt welds. However, concentration of residual stresses with high triaxiality occurs in the weld stop region, with high tensile stresses generated just behind the beam stop location, which cannot be diminished by overlap welding or change of weld stop position. The mechanisms responsible for the distortion and the transient and residual stresses are analysed. This study could provide rational basis for designing weld restraint to control distortion and guiding stress mitigation strategy for crack-susceptible region in EB weldments.

The activities within a European network to develop accurate experimental and numerical methods to assess residual stresses in structural weldments are reported. The NeT Task Group 6 or NeT-TG6 project examined an Alloy 600 plate containing a three-pass slot weld made with Alloy 82 consumables. A number of identical specimens were fabricated and detailed records of the manufacturing history were kept. Parallel measurement and simulation round robins were performed. Residual stresses were measured using neutron diffraction via five different instruments. The acquired database is large enough to generate reliable mean profiles, to identify clear outliers, and to establish the systematic uncertainty associated with this non-destructive technique. NeT-TG6 gives a valuable insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods. The mean measured profiles were used to validate the accuracy achieved by the network in the prediction of residual stresses.

In this work a novel mathematical framework, that fully describes the fusion and vapourisation state transitions in multi-component systems, has been applied to assist in understanding the fundamental mechanisms of defect formation and chemical homogenisation in the laser powder bed fusion process (L-PBF). Specifically, the role of vapourisation and condensation of the multi-component metallic substrate is investigated to determine the importance of properly capturing the state transitions when understanding the substrate evolution. The framework is applied to a ternary metallic system; it is revealed that entrained vapour bubbles in chemically dissimilar flows promote greater homogenisation during the condensation and collapse of these bubbles when compared to non-condensing phases. It is further shown that as the laser power density is increased, there is a greater tendency for preferential element evaporation of the lighter elements; this preferential element evaporation is quantified numerically for the first time, and shown to be a non-linear function of power density.

The complex flow within the molten metallic pool, during advanced manufacturing and joining processes such as welding and additive manufacturing, directly affects the performance of the final components; through the generation of various microstructures and strain gradients. Understanding of the flow within such melt pools can enhance the predictability of the final microstructures and therefore component performance. In situ synchrotron X-ray imaging is employed to observe and map the evolving flow patterns within gas tungsten arc weld pools using tracking particles. The experimentally observed flow patterns are compared with numerical simulations to gain additional insights on accurate predictability in relevance to the physical driving forces within the flow. The spatio-temporal distribution of the flow using the mapped data is analyzed by considering the evolution of driving forces acting throughout the weld pool. The results demonstrate quantitative benchmark flow mapping with confirmation of the overall mechanisms of weld pool flow.

The complex interplay between thermal, hydrodynamic, and electromagnetic, forces governs the evolution of multi-phase systems in high technology applications, such as advanced manufacturing and fusion power plant operation. In this work, a new formulation of the time dependent magnetic induction equation is fully coupled to a set of conservation laws for multi-phase fluid flow, energy transport and chemical species transport that describes melting and solidification state transitions. A finite-volume discretisation of the resulting system of equations is performed, where a novel projection method is formulated to ensure that the magnetic field remains divergence free. The proposed framework is validated by accurately replicating a Hartmann flow profile. Further validation is performed through correctly predicting the experimentally observed trajectory of Argon bubbles rising in a liquid metal under varying applied magnetic fields. Finally, the applicability of the framework to technologically relevant processes is illustrated through the simulation of an electrical arc welding process between dissimilar metals. The proposed framework addresses an urgent need for numerical methods to understand the evolution of multi-phase systems with large electromagnetic property contrast.

Electron beam (EB) welding has a low tolerance to inter-part gapping distortion and can generate complicated stresses, which pose challenges to weld quality and integrity. This study investigates welding distortion and stresses in an EB welded plate made from SA508 Grade 4N low-alloy steel. A thermal-metallurgical-mechanical model was developed to predict the temperature, micro-constituents, hardness, distortion and stresses in the EB weldment; the predictions are in good agreement with experimental results. Different restraint conditions on the weld plane were modelled to examine their effects on distortion and stresses. If welding is performed with no restraint, inter-part gapping develops ahead of the beam position that could exceed the tolerance for a sound weld. In contrast, tack welds at the plate ends significantly reduce this gapping, but induce additional tensile stress at the stop-end tack weld. This stress is particularly high as the beam approaches the tack weld. Increasing the extent of the tack weld reduces the tensile stress, while increasing number of distantly distributed narrow tack welds does not help. A full through-length restraint eliminates the opening gap and minimises the development of tensile stresses ahead of the beam that could potentially break the restraint. The applied restraint on the weld plane has little effect on the final residual stress field, since this field mostly develops during cooling after the EB weld is complete. The weld-induced martensitic transformation suppressed tension or promoted compression in the EB weld and heat affected zone (HAZ). A steep gradient of residual stress exists, with high tensile stress concentrated in a narrow region immediately outside the HAZ.

High energy density advanced manufacturing processes, such as power beam welding and additive manufacturing, are notoriously difficult to simulate. Such processes initiate fusion, and vapourisation, state transitions in their respective (normally metallic) substrates generating complex metallic flows over incredibly short time scales. To mathematically model such processes, equations describing the conservation of momentum, conservation of energy, and an equation that describes the evolution of the metallic substrate interface must be considered. In this work, we present beamWeldFoam, an OpenFOAM solver capable of simulating these high energy density advanced manufacturing processes. In beamWeldFoam, the metallic substrate, and shielding gas phase, are treated as incompressible. The volumetric dilation due to the vapourisation state transition is neglected, instead, a phenomenological recoil pressure term is used to capture the contribution to the momentum and energy fields due to vaporisation events. beamWeldFoam is released under the GNU general public license, and its source code is available on Github.

Reduced-pressure electron beam (EB) plate butt welds were manufactured in two low-alloy pressure-vessel steels, SA508 Gr 3 Cl 1 and SA508 Gr 2, at two thicknesses in both steels, 30 mm and 130 mm. Transient temperatures during welding were recorded using thermocouple arrays. Residual stresses in the as-welded condition and after post-weld heat treatment were measured using diverse methods: neutron diffraction and the contour method at 30 mm thickness; and deep hole drilling and the contour method at 130 mm. Incremental centre hole drilling measurements were performed at 130 mm thickness to better understand near-surface stresses. Weld and heat-affected zone microstructures and microconstituents were evaluated using a combination of hardness mapping, optical microscopy and electron microscopy. The as-welded residual stresses exhibit the characteristic M-shaped distribution for hardenable steels, reaching 500–600 MPa in tension in both steels at both thicknesses. However, the modest changes to the chemical composition and the change in plate thickness both significantly influenced microstructures, mechanical properties and residual stress distributions. These sensitivities underline the need for physically faithful models. This extensive characterisation study enables the development and validation of models that predict the development of microstructures and residual stresses in EB welds in low alloy pressure vessel steels.

The activities within a European network to develop accurate experimental and numerical methods to assess residual stresses (RS) in structural weldments are reported. The NeT Task Group 6 or TG6 project examined an Alloy 600 plate containing a three-pass slot weld made with Alloy 82 consumables. A number of identical specimens were fabricated and detailed records of the manufacturing history were kept. Parallel RS measurement and simulation round robins were performed. RS were measured using neutron diffraction at five different instruments. The acquired database is large enough to generate reliable mean profiles, to identify clear outliers, and to establish the systematic uncertainty associated with this non-destructive technique. TG6 gives a valuable insight into the real-world variability of diffraction-based RS measurements, and forms a reliable foundation against which to benchmark other measurement methods. The mean profile of measured RS was used to validate the accuracy achieved by the network on the prediction of RS.