Dinesh W. Rathod

Diffusion Control and Metallurgical Behavior of Successive Buttering on SA508 Steel Using Ni–Fe Alloy and Inconel 182

The metallurgical deterioration and carbon diffusion caused during welding in the vicinity of the fusion interfaces between dissimilar metals have been investigated in case of gas tungsten arc and shielded metal arc welded SA508Gr.3Cl.1 steel substrate with Ni–Fe alloy and Inconel 182. The study was conducted to investigate the effect of Ni–Fe matrix as buffer layer and SMAW process for buttering deposition on the carbon diffusion and metallurgical changes. Quantitative measurement and validation of carbon/alloying elements distribution in as-welded, thermally aged, and postweld heat-treated conditions were performed in buttering deposits by using optical emission spectrometry and electron probe microanalysis. The extent of carbon diffusion has been estimated using Groube’s diffusion couple and confirmed with microstructure microhardness and X-ray diffraction. Martensite formation has been estimated for its thickness and validated with metallurgical properties. The effect of buffer layer is significant for carbon diffusion and tempering of martensite with thermal aging, PWHT, and multipass deposition. The concentration and activity gradient of carbon has been established due to Ni–Fe matrix as buffer layer and higher dilution for buttering. The obtained results are confirming the control of carbon diffusion and lesser metallurgical deterioration in suggested buttering procedure.

Metallurgical Behaviour and Carbon Diffusion in Buttering Deposits Prepared With and Without Buffer Layers

Use of a buttering deposit on ferritic steel in dissimilar metal weld (DMW) joint is a common practice in nuclear plants to connect pressure vessel components (ferritic steel) to pipelines (austenitic stainless steel). Carbon migration and metallurgical changes near fusion interface (ferritic steel–austenitic stainless steel) lead to a steeper gradient in material properties, and minimizing this gradient is the major challenge in the manufacturing of DMW joints. Inconel 82 is often deposited on ferritic steel material as buttering to reducing the gradient of physical and attaining material compatibility. Inconel 82/182 fillers are used to minimize the carbon migration, but the results are not truly adequate. In this paper, Ni–Fe alloy (chromium-free) has been used as the intermediate buffer layer in the weld buttering deposit to address the issue of carbon migration and subsequent metallurgical deterioration. The weld pads with and without buffer layers of Ni–Fe alloy have been investigated and compared in detail for metallurgical properties and carbon diffusivities. Ni–Fe buffer layer can significantly control the carbon migration which resists the metallurgical deterioration. It showed the better results in post-weld heat treatment and thermally aged conditions. The buttering deposit with Ni–Fe buffer layer could be the better choice for DMW joints requirements.

Friction stir welding of nuclear grade SA508Gr.3Cl.1 and SS304LN dissimilar steels

The present work has been carried out to justify the feasibility of dissimilar metal welds between nuclear grade SA508Gr.3Cl.1 ferritic steel and SS304LN stainless steel using friction stir welding. The evaluation has been made by analysing the metallurgical and mechanical properties of the friction stir welded dissimilar joint. Martensite formation and chemical variations in the weld nugget have been confirmed with optical microscopy, micro-hardness measurement and X-ray diffraction. The resultant chemistry variations owing to solid-state mixing on metallurgical and associated mechanical properties are significant. Transverse tensile tests and Charpy impact tests suggest the required strength of the joint. The tensile specimens were fractured from the SS304LN side, and similar sections of stainless steel have been torn with severe plastic deformation. The significant formation of martensite has not shown any adverse effect on the joint properties. The tool wear rate of tungsten carbide was high for this dissimilar weld.

Design and Manufacture of Industrially Representative Weld Mock-ups for the Quantification of Residual Stresses in a Nuclear Pressure Vessel Steel

This paper describes work carried out under the NNUMAN research programme. This work focuses on the measurement and modelling of residual stresses in weld test pieces that have a thickness that is representative of primary components in a pressurised water reactor, such as the steam generators and the pressuriser. Weld test pieces at thicknesses of 30 mm and 130 mm have been and are being manufactured in SA508 Grade 3 Class 1 steel. Attention has been given to welding processes that are currently applied in nuclear manufacturing, such as narrow-groove arcbased welding processes, as well as to candidate processes for future build programmes, such as electron beam welding. The manufacture, characterisation and modelling of large test pieces each present challenges over and above those that arise when dealing with the smaller test pieces that are more typically manufactured in research laboratories. Some of those challenges, and the approaches that have been used to overcome them, are described. Plans for future work are briefly mentioned. Introduction Residual stresses can play a significant role in affecting the long-term structural performance of safety-critical components in nuclear power plants. Residual stresses can contribute to the driving force for crack growth [1] but, in nuclear environments, they can also activate degradation mechanisms such as creep [2] and stress-corrosion cracking [3] even in the absence of operating stresses. This is significant because many safety-critical components in a nuclear plant undergo welding during manufacture, and welding is known introduce substantial levels of residual stress [4]. The primary options for quantifying residual stresses in welds are to measure them, or to make predictions based on numerical models. However, both approaches are not straightforward since, on the one hand, many measurement techniques are either destructive or not suited to applications on large components and, on the other hand, numerical models must undergo rigorous validation before they can be used with confidence, through comparisons with measurements made with multiple and dissimilar techniques on test cases for which the manufacturing history has been documented in detail. This highlights the need for carefully designed weld mock-ups that can serve as weld modelling benchmarks. Such benchmarks enable weld models to be validated on well-defined test cases before they are applied to make predictions for real components. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 581-586 doi: http://dx.doi.org/10.21741/9781945291173-98 582 As part of the NNUMAN research programme, we have designed and are manufacturing weld mock-ups that are larger than those that are typically manufactured in a laboratory environment. In doing so, our aim is to gain a better appreciation of the manufacturing challenges that are encountered when large nuclear components are manufactured in practice. Our hope is that, by manufacturing weld mock-ups that are more representative of welds that are made on an industrial scale, we will use welding procedures that are closer to those applied in practice, we will capture a greater range of relevant phenomena, and we will thereby reduce the uncertainties associated with inferring trends for industrial scale welded joints based on extrapolation from laboratory scale test pieces. While the motivation is straightforward, the manufacture of these weld mock-ups has presented different challenges to those that we encounter when making smaller weld test pieces. Thus the purpose of this paper is to highlight some of these challenges and the approaches we are employing to meet them. Research Objectives Welding research under the NNUMAN programme has the overarching aim of developing an understanding of how the choice of manufacturing process impacts on the performance of reactor components over the design life of the component, which for new build applications is typically 60+ years. This aim is quite distinct from the basic manufacturing requirement to produce a weld that will meet start-of-life weld quality requirements and pass inspection so that it will go into service. In this work, we have focused on welds made in SA508 Grade 3 Class 1 steel, which is a low-alloy steel that is typically used in the manufacture of primary components in a pressurised water reactor (PWR) such as the reactor pressure vessel or the steam generators. For this steel, and for such components, the parameters that will have the greatest influence of long-term structural integrity include the fracture toughness of the weld region and the weld residual stress distributions. Accordingly, the NNUMAN welding research programme has the following objectives: • To manufacture industrially-representative weld mock-ups with welding processes that are currently applied to nuclear components, as well as with welding processes that may be used in future build programmes. The welding processes that have been chosen for investigation are narrow-groove gas-tungsten-arc welding (NG-GTAW), narrow-groove submerged arc welding (NG-SAW), electron beam (EB) welding and laser welding; • To carefully record and document all aspects of the manufacturing processes and steps that are employed in each case so that the weld mock-ups can be used as the basis for validating numerical models for welding; • To characterise the weld residual stress distributions in each of the weld mock-ups using multiple measurement techniques; • To develop a methodology for the modelling of each weld and welding process so that residual stresses can be predicted and the interaction between the welding process and material can be understood. Design Requirements for Weld Mock-Ups The design requirements included (but were not limited to) the following: • The thickness of the weld mock-ups should be representative of the thicknesses that are applicable to primary components in a PWR; • The length and width of the weld mock-ups should be sufficient to enable steady-state welding conditions to be achieved along the length of the weld and for the residual stress distributions to be substantially unaffected by the width of the test pieces; • The boundary conditions, particularly with respect to weld restraint, should be either clearly defined or well characterised. In terms of the first requirement, a thickness of 130 mm was identified as being representative. The last requirement that is listed above is of great importance since welds usually need to be restrained in some way during manufacture in order to prevent excessive distortion. When this restraint is released, there will inevitably be some spring back and associated relaxation of residual stress. While Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 581-586 doi: http://dx.doi.org/10.21741/9781945291173-98 583 this can (in principle) be accounted for, unfortunately, common methods of restraint (e.g. clamping and tack welding of test pieces to backing plates) tend to provide levels of restraint that are not easily quantified. This means that such approaches can be difficult to represent accurately in numerical models. In addition to manufacturing welds at a thickness of 130 mm, it was decided that test pieces should also be manufactured at a thickness of 30 mm, as such an exercise would serve as a stepping stone in terms of the development of welding procedures and associated weld models, and it would also enable residual stresses to be characterised with neutron diffraction (not feasible at a thickness of 130 mm). Basic Geometry The basic geometries of the 30 mm and 130 mm thick specimens are shown in Figure 1. The 30 mm thick arcand laser-welds had a geometry that was similar to that shown in Figure 1 (left). It can be seen that these 30 mm thick welds are not full-length groove welds, and that a ligament of parent material remained intact at either end. These ligaments served to provide a degree of self-restraint, so that the specimens did not need to be clamped or restrained with any significant force. Instead, light clamping could be employed simply to prevent the specimens from moving during welding. Furthermore, the specimen geometry could be captured in finite-element models so that the extent of self-restraint could be represented accurately. For the electron beam weld, it was possible to leave the weld free of restraint, due to the process requiring only a single weld pass. For the 130 mm thick specimens, a self-restraining approach was not feasible, and another method of restraint was developed. Fig. 1: Photograph of a 30 mm thick submerged arc welded specimen (left) showing ligaments of parent material that remain intact at each end of the specimen, and schematic representation of basic specimen geometry for the 130 mm thick weld mock-ups (right). The weld seam runs along the centre of the specimen in both cases. Challenges Associated with Distortion One of the major challenges in developing welding procedures for the 130 mm thick specimens was controlling the distortion of the welds, and designing weld grooves that would enable welds to be completed successfully in spite of this distortion. There are two components to this distortion, namely butterfly distortion, and transverse contraction, and both are represented schematically in Figure 2. Multipass welds made from one side experience both components of distortion. For these welds, butterfly distortion in particular can create problems with respect to the weld groove progressively closing as successive weld passes are deposited. If a narrow groove weld torch is used, then butterfly distortion can raise the possibility of the torch becoming trapped between the two Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 581-586 doi: http://dx.doi.org/10.21741/9781945291173-98 584 plates be