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A research project has recently been launched in the UK investigating residual stress (RS) in nuclear power plant [1]. At the outset there is a need to review techniques available for modifying/relieving residual stress levels in weldments, since it is well known that large tensile RS levels generated in welds can be detrimental in terms of fatigue, fracture resistance and environmentally assisted cracking (EAC). Therefore current RS mitigation methods have been reviewed. Mitigation methods can be categorised into three main groups as follows: a) Surface treatment to induce compressive skin stress; b) Stress relief through thickness; c) Weld design optimisation to produce low/favourable RS levels and minimize distortion. A brief description is provided of how each method works, together with the capability and potential benefit in terms of RS reduction, as well as references for further information. Metallurgical effects of treatment are also an important consideration. The practicality of application to nuclear plant is considered, both in manufacture and in-service, together with any limitations and risks. Several techniques are identified that are likely to be beneficial and warrant funding for further development. RS mitigation should be targeted at key/critical weld locations in the plant, where loadings and degradation mechanisms (such as corrosion, fatigue, EAC or fracture) are most significant. Treatment would be carried out in order to improve plant integrity and reliability (eg safety margins). There are potentially substantial cost savings since through-life inspection/maintenance work could be reduced and expensive repairs and shutdowns avoided. Note that it is important to understand whether the benefits in terms of RS improvement are likely to be long term. In certain systems large thermal transients are applied that might generate additional surface plastic strains, thereby modifying RS magnitudes and distributions.

Finite element methods are used increasingly to predict weld residual stresses. This is a relatively complex use of the finite element method, and it is important that its practitioners are able to demonstrate their ability to produce accurate predictions. Extensively characterised benchmark problems are a vital tool in achieving this. However, existing benchmarks are relatively complex and not suitable for analysis by novice weld modellers. This paper describes two benchmarks based upon a simple beam specimen with a single autogenous weld bead laid along its top edge. This geometry may be analysed using either 3D or 2D FE models and employing either block-dumped or moving heat source techniques. The first, simpler, benchmark is manufactured from AISI 316 steel, which does not undergo solid state phase transformation, while the second, more complex, benchmark is manufactured from SA508 Cl 3 steel, which undergoes solid state phase transformation during welding. A number of such beams were manufactured using an automated TIG process, and instrumented with thermocouples and strain gauges to record the transient temperature and strain response during welding. The resulting residual stresses were measured using diverse techniques, and showed markedly different distributions in the austenitic and ferritic beams. The paper presents the information necessary to perform and validate finite element weld residual stress simulations in both the simple austenitic beam and the more complex ferritic beam, and provides performance measures for the austenitic beam problem.

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This paper provides an overview over the work of the European Network on Neutron Techniques Standardization for Structural Integrity (NeT). The network involves some 35 organisations from industry and academia and these partners undertake the application of modern experimental and numerical techniques to problems related to the structural integrity of components, mainly relevant to nuclear applications. While being built around neutron scattering techniques, which are predominantly applied for analyses of welding residual stresses, one of the major strengths of the consortium is the diversity in available experimental and numerical techniques. In the residual stress area, for example, many types of materials characterizations testing, several methods for residual stress analysis, including neutron and X-ray diffraction, deep hole drilling, the contour method and others, and many different ways of numerical analysis employing several commercially available FEM codes can be covered by the partners. Currently the network has embarked on five different Task Groups. Four of these are dealing with welding residual stress assessment, and one applies Small Angle Neutron Scattering for studying thermal ageing processes in duplex stainless steels used for reactor core internals. The work already performed in the context of NeT and the envisaged investigations for the ongoing Task Groups are briefly outlined in this paper. The aim is to give the reader a comprehensive overview of the work of NeT and to shed some light on the potential present in this kind of collaborative effort.