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Residual Stresses in Thick Bi-metallic Fusion Welds: A Neutron Diffraction Study
No full textWelding is applied in many industrial sectors to join components, and has become an important manufacturing process because it enables the fabrication of structures that could not otherwise be constructed. Weld regions have inhomogeneous microstructures and are more susceptible to crack initiation and crack propagation than the surrounding base material regions. Residual stresses are also formed, which superimpose with applied loads, resulting in a reduction of the maximum applied load a component can sustain. In particular for nuclear installations, the limited failure tolerance and the relative abundance of rather large welds require a reliable assessment of component integrity for lifetime management. Residual stresses need to be considered in such assessments because they can contribute to initiation and propagation of defects. Commonly, residual stresses are more difficult to determine experimentally or to predict by numerical methods than stresses resulting from applied loads; hence residual stress assessment in welded nuclear components is an important area of research. The present work concerns the experimental determination of residual stresses by neutron diffraction in three full-scale mock-ups of components found in nuclear power installations. Two of these mock-ups represent dissimilar metal girth welds joining ferritic steel pressure vessel nozzles to austenitic stainless steel primary piping sections. The third represents a welded clad layer on a section of a reactor pressure vessel wall. In this work neutron diffraction has been used as the technique for residual stress determination. This technique is based on the principle of Bragg diffraction and measures changes in lattice spacing; i.e., strain. Residual stresses can be determined in three directions in the bulk of a component at a spatial resolution of typically 1-5 mm. Such a resolution is appropriate in view of the distances over which the residual stresses normally vary in welded metallic components. There are a number of challenges associated with neutron diffraction measurements addressed in the current study. One of these is that component dimensions and geometries necessitate machining to facilitate access of the neutron beams to the measurement locations. Neutron diffraction measurements are also known to be sensitive to the inhomogeneity in the microstructure and to local variations in chemical composition in the weld region; therefore dedicated reference specimens are needed in order to calibrate the strain determinations. Prior to the description of the experimental work undertaken, background information is provided on the main aspects of the work. Definitions are given for residual stresses and strains and their main characteristics are explained (chapter 2). A short overview of the most important techniques for strain and stress measurement being used today is given in chapter 3. These are the relaxation techniques, where the relaxation of strains due to material removal is measured; and the diffraction techniques, where lattice deformations caused by stresses are observed through Bragg diffraction. A detailed account of residual stress determination based on neutron diffraction is followed by a description of the facility at the Joint Research Centre that was used for the present investigations. An overview of the industrially relevant welding techniques is given in chapter 4, where additional detail is provided on the arc welding methods used for the manufacture of the components investigated in this study. The mechanism responsible for residual stress formation in welds, namely local plastic deformation caused by steep temperature gradients, and the dendritic microstructure of stainless steel fusion welds similar to those investigated here are discussed based on a few examples. The experimental work and the results obtained are described in chapters 5, 6 and 7 with each chapter covering one of the three components. The first component is a “thin" walled (25 mm wall thickness) bi-metallic girth welded pipe. The second component is a ferritic steel block with a 10 mm thick welded stainless steel clad layer applied to one of its surfaces. This component represents a nuclear reactor pressure vessel wall with a thickness of 146 mm. The third component is a thick walled (51 mm wall thickness) bi-metallic girth welded pipe. Both, the thin and the thick walled bi-metallic pipes, represent joints between ferritic steel pressure vessel nozzles and austenitic stainless steel pipes. For each component, details are provided concerning the manufacturing techniques employed, including the welding processes, the associated heat treatments and the final machining applied to the thick walled pipe. For the access of the neutron beams in three measurement orientations windows and access slots were cut into each of the bi-metallic piping weld specimens. The clad layer specimen thickness was locally reduced from 146 mm to 25 mm. This significant reduction was necessary to achieve a high spatial resolution in the measurements. The alterations described lead to stress relaxation and stress redistribution within the components. For the thick bi-metallic pipe the relaxation of strains was monitored by strain gauges during cutting showing negligible impact at the location of the neutron diffraction measurements. All component modifications are described in detail as well as the design and manufacture of the reference specimens needed for the calibration of the strain measurements. The measurement procedures and the data analyses are explained, and for each specimen the neutron diffraction results are presented in terms of residual strains and the derived stresses. In the thin walled bi-metallic pipe, tensile residual stresses have been found in the welding direction within the fusion zone. These tensile stresses reach values not far below the yield level of the material near the outer surface; and they decrease to almost 0 MPa toward the inner surface of the pipe. In the ferritic part of the pipe compressive residual stresses have been found near the austenitic-ferritic material interface. Here, the highest compression is observed close to the inner surface, decreasing toward the outer surface. In the welding transverse direction, tensile stresses have been obtained near the outer surface changing gradually to compressive stresses near the inner surface. The maximum stress levels attained in tension and compression are slightly lower than those in the welding direction. The piping radial direction exhibits overall the lowest stress levels in this component with values varying between +100 MPa and -100 MPa. The stainless steel clad layer of the clad component exhibits high tensile stresses in the welding longitudinal direction as well as in the welding transverse direction. The stress level ranges between 250 and 500 MPa with a considerable scatter of the data. The ferritic steel substrate is found to be in compression in both directions with minimum stress levels between -150 and -200 MPa near the austenitic-ferritic material interface. The stresses in the interface normal direction observed in this component scatter about 0 MPa. In the fusion zone of the thick walled bi-metallic piping component, tensile residual stresses have been found in the welding direction. In the ferritic part compressive residual stresses have been found in this direction near the austenitic-ferritic interface. Both of these observations are similar to those made for the thin walled component. The maximum stress levels observed are slightly higher than in the thinner component; the tensile stresses in the fusion zone reach the nominal yield level. No clear trend is observed for the welding longitudinal stresses between the outer and the inner surface. For the other two measurement directions the scatter of the data is so high that the magnitude of the stresses cannot be determined with sufficient accuracy. For all three components the data recorded from the fusion zones exhibit higher scatter than those from the base materials. The neutron diffraction results have been compared to third party numerical predictions of the residual stresses and to stress measurements by strain relaxation techniques. The numerical predictions have been performed by finite element analyses. Simplified and more detailed approaches have been applied. In the simplified approaches the welding process itself has been neglected unlike in the detailed models. In all cases the detailed numerical assessments produced a better agreement with the neutron diffraction results than the simplified ones. In particular, in the fusion zones the simplified numerical approaches under predict the residual stresses found by neutron diffraction. The applied third party strain relaxation techniques were the ring core method and deep hole drilling for the clad layer component, and surface hole drilling and the crack compliance method for the thick walled bi-metallic pipe. Most comparisons with the neutron diffraction data show a qualitative, but not a close quantitative agreement. The discrepancies are attributed to the use of differently extracted test pieces from the original components and to the differences in the measurement geometries that apply to the different methods used. Subsequent to the presentation of the measurements and their results, an assessment of the experimental methods and the analysis of the results are performed. In particular attention is given to the method of obtaining the measurement uncertainties. In stress determination by neutron diffraction it is common practice to calculate the uncertainties solely from the fitting uncertainties of the neutron data; a practice that has been applied in this work as well. The detailed assessment of the results indicates that this approach can provide appropriate uncertainty values only for materials that are sufficiently homogenous. This is the case, for example, for the ferritic steel substrate of the clad layer component. For the less homogeneous regions, like the welds in these components, it is observed that the experimental data exhibit larger scatter than one would expect on the basis of the counting statistics. The analysis of the effect suggested that the uncertainty is underestimated by a factor of up to 10 in the worst case presented. The second uncertainty contributor analysed in more detail is the uncertainty in the detector position. . The analysis shows that, for the highest strains measured, an uncertainty in detector position as small as 1° or 2° results in an additional strain uncertainty comparable to that stemming from the fitting uncertainties. Other sources of uncertainty, such as possible errors in specimen positioning or variations of the specimen temperature, are also briefly analysed. It is found that these do not produce significant additional uncertainty contributions in these investigations. Based on the observations and subsequent analyses of the findings and comparisons several conclusions are derived. The most significant conclusions can be summarized as follows: \u95 The applied cutting schemes for the specimens and the use of the dedicated reference specimens have made the neutron diffraction measurements possible. The necessary alterations to the specimens have an impact on the stresses under investigation. It is demonstrated that the impact should be quantified through experiments or modelling, in order to relate the stresses measured to the original stresses present in the test piece. \u95 It is shown that for a material like the welds studied in this work, the impact of the material inhomogeneity on the neutron diffraction measurements must be considered in a complete uncertainty analysis. It is found that this uncertainty contribution can be larger than the contribution from the fitting uncertainty of the neutron data. \u95 The neutron diffraction stress measurements can be used for the validation of numerical stress prediction methods. It is demonstrated that the simplified numerical approaches for these specimens are not sufficient. Following from the above, a number of recommendations are formulated for further improvements in the application of neutron diffraction for future residual stress measurements in large welded components. In similar cases it is recommended to apply experimental techniques capable of mitigating the problem of the high scatter in the results obtained for the fusion zones. The time-of-flight technique is presented as one option, or where possible, rocking of the specimen during measurements could be applied to increase the number of grains sampled during a diffraction measurement. Furthermore, the importance of quantifying the stress relaxation due to modifications of the specimens is pointed out. This quantification could be achieved by numerical simulation, but preferably by measurement, in particular when the recommendations just mentioned are followed.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stress distribution in a 316L(N) steel narrow gap TIG model weld for Gen IV nuclear applications
Get PDFThe stress distribution in a model weld developed for nuclear application has been determined non-destructively by means of neutron diffraction, in the frame of the Horizon 2020 Project Generation IV Materials Maturity (GEMMA). The investigated sample is a narrow gap Tungsten Inert Gas (TIG) austenitic 316L(N) steel weld, prepared following RCC-MRx Code specifications. Two lines perpendicular to the welding direction, at the middle of the sample, were scanned at 6 mm and 14 mm depth; additional measurements were carried out in the middle of the weld, down to 16 mm depth. At 6 mm depth and within ± 5 mm distance from the weld centre, marked tensile stress gradients are found, with the residual stresses reaching maximum values up to 400 MPa in the longitudinal direction. At 14 mm depth, the stresses decrease to around 200 MPa for the longitudinal component and get compressive for the transverse and normal components, down to -200 MPa for the transverse one, with smoother stress gradients around the weld. The in-depth measurements inside the weld confirm that the main integrity concern for the investigated sample may arise from the tensile longitudinal stresses. Additional micro-structural information has been obtained by qualitative comparison of diffraction line profiles in the weld and in the base metal. These experimental results are discussed with reference to the expected service conditions of such welds and to their capability to fulfill Gen IV safety goals and requirements.JRC.G.I.4 - Nuclear Reactor Safety and Emergency Preparednes
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Stress Distributions in P91 Martensitic Steel and in AISI 316LN Steel Welds for Gen IV Nuclear Applications
No full textAbstract: Neutron diffraction has been used to investigate the stress field in two different welds developed for nuclear applications, namely those obtained by tungsten inert gas welding of P91 martensitic steel (Cr 9, Mn 6, Mo 1, C 0.1, and Fe bal wt %) and hybrid (laser beam and gas metal arc) welding of AISI316LN austenitic steel (17.8 Cr, 12.3 Ni, 1.7 Mn, 2.4 Mo, 0.3 Si, and Fe bal wt %). The sizes of the investigated samples were 100 × 50 × 12 mm for the P91 weld and 220 × 160 × 15 mm for the 316LN weld; unstrained references were prepared for both welds. The neutron diffraction measurements have been carried out utilizing the E3 diffractometer at the BER II reactor in Berlin, with a gauge volume of 2 × 2 × 2 mm3. Lines perpendicular to the weld direction were scanned at different depths inside the material and at different distances from the weld centerline, including the heat affected zone and the weld centerline. Strain and stress values were determined in the three principal directions. In the TIG P91 weld, the stresses are almost completely relieved after a post weld heat treatment for 2 h at 760°C. In the laser beam and gas metal arc 316LN weld not submitted to post weld heat treatment, nearly balancing longitudinal and transverse stress components as high as 300–350 MPa are found within a range of approximately 3 mm around the centerline of weld
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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