Residual stresses in arc and electron-beam welds in 130 mm thick SA508 steel: Part 2 –Measurements

A N Vasileioua, M C Smitha,c, J A Francisa, D W Rathoda, J. Balakrishnanb, and N M Irvinea

aThe University of Manchester, Sackville Street, Manchester, M13 9PLbNuclear Advanced Manufacturing Centre, Advanced Manufacturing Park, Brunel Way, Rotherham, S60 5WGcCorresponding Author

Abstract

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.

Introduction

After a hiatus spanning more than two decades, the prospect of a new nuclear build programme in the United Kingdom has stimulated interest in reviewing the manufacturing technologies that are available [1]. Over that time, the most significant changes in the landscape, in the context of fabricating components in the primary circuit of a light water reactor, have related to advancements in narrow-gap (NG) arc welding technologies, and the potential for applying electron beam (EB) welding to large components outside of a vacuum chamber [2]. In other countries, narrow-gap variants of arc welding technologies such as gas-tungsten arc welding (GTAW) and submerged arc welding (SAW) have already been applied in the manufacture of nuclear pressure vessels [3], but EB welding does not yet appear to have been utilised in a civil reactor, despite attracting the interest of some fabricators [4]. Given that incremental changes in manufacturing practice have already been adopted in some quarters, and that further changes are likely to be considered in the near future, it would seem appropriate to assess how the choice of welding process might influence the performance of critical nuclear components over a design lifetime that is typically 60 years or more.

Weld residual stresses are a major contributor to premature material degradation and in-service structural performance problems [5]. Structural integrity assessment procedures such as R6 and API 579 [6, 7] give detailed advice on their estimation and impact, and there has been a large body of work aimed at improving the quality of those estimates, ranging from extensive parametric studies based upon modelling [8-11], through formulations based upon a combination of measurements on weldment mock-ups and modelling [12], to artificial neural network techniques based upon weldment mock-up measurements [13, 14]. All these studies examine similar metal welds in plain pipes, and the measurements used both for profile generation and modelling validation are derived almost entirely from conventional J-groove and V-groove arc welds. The electron beam welding process is relatively novel, and narrow-gap arc welds are not used in existing UK civil nuclear plants, so no specific advice on residual stress levels and distributions for these weld types is currently available in procedures such as R6.

Residual stress measurements in thick section welds can be challenging. The thickness of the components limits the residual measurement techniques that can be used, with the deep-hole-drilling (DHD) technique being the most common [15-18]. The contour method may also be used, but the size of the components to be cut is at the upper limit of current practice. Residual stress distributions in electron beam welds are non-uniform, with short characteristic lengths[footnoteRef:1], and limited to the narrow fusion zone and small adjacent regions [19-21]. In ferritic steels that undergo low temperature bainitic or martensitic phase transformations an “M-shaped” distribution can develop, with high tensile stresses just outside the HAZ and low tensile or compressive stresses in the weld fusion zone [20, 22]. This further reduces the characteristic length of the distribution. Narrow gap arc welds are expected to develop more conventional residual stress distributions, but the problems of thickness remain. [1: The “characteristic length” of a residual stress field may be thought of as the length scale over which it self-equilibrates.]

Thick section ferritic steel welds normally undergo post-weld heat treatment (PWHT) prior to entry into service. This reduces, but does not eliminate the weld residual stresses [4].

This paper describes the residual stress measurements that have been carried out on narrow gap welds made in SA508 Grade 3 Class 1 steel at the plant-representative thickness of 130 mm. Results are presented for welds made with the following techniques, in the as-welded condition and after post-weld heat treatment:

· Narrow-gap gas-tungsten arc welding (NG-GTAW);

· Narrow-gap submerged arc welding (NG-SAW); and

· Reduced Pressure Electron beam welding (RPEB).

Residual stress measurements were made using two independent strain-relief techniques, DHD and the contour method, to increase their reliability. The results give insights both into the effect of welding process on the development of residual stress in thick-section welds made from low alloy steels, and into the impact on those stresses of conventional post-weld heat treatment.

A companion paper[23] describes manufacture and characterization of the weldments in more detail, sufficient both to put the residual stress measurements reported here into context, and to allow finite element modeling of the welding process to predict the as-welded stresses, using procedures such as those laid out in R6 [6, 24]

Manufacture of WeldmentsDesign Requirements

The design requirements included the following:

· The thickness of the weldments should be representative of the thicknesses of primary circuit components in a pressurised water reactor (PWR).

· All weldments should be fabricated from a single well-characterized cast (or heat in US parlance) of material.

· The length and width of the completed weldments 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 simple (unrestrained) or well characterized.

The last requirement is important, since welds usually need to be restrained in some way during manufacture, to prevent excessive distortion. When this restraint is released, there will inevitably be some spring back and associated relaxation of residual stress. While this can be accounted for, unfortunately common methods of restraint such as 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.

A plate thickness of 130 mm was chosen for the thick-section welds. Although thinner than a typical reactor pressure vessel in a PWR, 130 mm is representative both of other PWR primary circuit components and small modular reactor designs. A simple rectangular butt-welded plate geometry was chosen, with completed dimensions of about 575-580 mm 330 mm 130 mm (length x width x thickness). The precise length and width varied slightly with the welding process, with the EB weld being 5 mm longer than the two arc welds.

Plant-representative weldments can only be manufactured in limited numbers, are not easily portable, and present challenges for both residual stress measurement and computational modelling. Two further weldment types were therefore designed and manufactured, at a reduced thickness of 30 mm, but from the same heat of steel:

· "Residual stress measurement" plates

· "Modelling validation" plates

The residual stress measurement plates were designed to allow neutron diffraction residual stress measurements, to be readily portable, and to act as stepping stones for both weld procedure development and computational modelling. The modelling validation plates were designed to allow the development and validation of both welding heat source models and solid state phase transformation models in welds with very limited numbers of passes, where key modelling variables are separable. Both these weldment types are discussed in more detail elsewhere [1, 25-27].

Materials

A single forging of SA508 Gr 3 Cl 1 steel was procured and used for the manufacture of every welded specimen. This was purchased from Sheffield Forgemasters International Ltd. in the form of a prolongation ring forging together with thinner segments of material extracted from the dome that was contiguous with the prolongation ring. The forged material had undergone a standard quench and temper heat treatment. Its chemical composition, and those of the welding consumables, are presented in Part 1 of this study.

Weldment manufacture

All the thick-section welded specimens were manufactured and inspected to quality standards consistent with ASME requirements.

The reduced pressure electron beam weld (RPEB) was manufactured by TWI Ltd in the 2G position, with the two rectangular plates butted together and secured by tack welds at the plate ends. The weld was completed in a single autogenous pass, see Figure 1 [20].

The narrow-gap gas tungsten arc (NG-GTAW) and submerged arc (NG-SAW) welds were made in the Manufacturing Technology Research Laboratory (MTRL) at the University of Manchester. It is not possible to successfully complete a single-sided narrow gap GTAW or SAW plate weld without some form of restraint, because the combination of a narrow groove weld preparation and angular "butterfly" distortion will lead to torch trapping. A bespoke restraint rig was thus designed and manufactured to mitigate butterfly distortion. Full details are given in Part 1 of this study [23]. Both the arc-welds made use of run-on and run-off plates to ensure that the weld remained uniform over the entire specimen, see Figure 1. These were not removed after welding was complete. Arc welds were made in the 1G position. Full details of weldment manufacture are also given in given in Part 1.

The complete process flowchart for the arc-welded plates is given in Figure 2. That for the RPEB weld was similar, but the lack of any restraint system simplified the manufacturing process. Only a single weldment was manufactured from SA508 Gr 3 Cl 1 material for each process. Residual stress measurement and characterization therefore required progressive subdivision of the completed welds, as seen in Figure 2.

The manufacture of the 130 mm arc-welded plates was preceded by manufacture of both the 30 mm residual stress measurement plates and arc-welded plates at a thickness of 80 mm. The 80 mm welds [28] were manufactured from SA533 material and were made both for procedure development and to test the restraint rig. These trials allowed final adjustment of the welding parameters and the groove preparation as the thickness increased from 30 mm to 130 mm.

Weld geometry

The weld groove geometry and the weld bead stacking pattern are shown in Figure 3 for the three welding processes. The NG-SAW plate was made with two stringer beads per layer, with a total number of 104 passes. The NG-GTAW plate was made with a single weaved bead per layer, with a total number of 73 passes.

The macrographs of Figure 4 reveal details of the weld fusion zone and heat-affected zone profiles for all three processes. The scale in the figure allows direct comparison of the three welds. It is noticeable that both the widest HAZ and the narrowest weld fusion zone occur in the RPEB weld. The fusion zones of the arc welds are clearly wider than on the RPEB weld, due to their multi-pass nature and the use of filler metal. The HAZ in the NG-SAW plate is visibly narrower than that in the NG-GTAW plate.

Sequencing of residual stress measurements and post-weld heat treatment

After welding, the characterization strategy described in Figure 2 required residual stress measurements in both the as-welded and the post-weld heat treated conditions. This was achieved by first making DHD measurements in the as-welded condition, and then performing a mid-length transverse contour method cut, again in the as-welded condition. One half of each cut plate was then sent for post-weld heat treatment, followed by further DHD measurements. The other half was preserved in the as-welded condition for further characterization.

The heat treatment procedure was based upon standard ASME practice. It placed no restrictions on the heating and cooling rates below 300°C, but both heating and cooling rates were restricted to a maximum of 20°C per hour between 300°C and 607 °C, and the soak took place at a temperature of 607 +/- 13 °C for a duration of 6 hours.

Residual stress measurement programme

Residual stresses were measured in all three types of weld. The originally planned programme is shown in Figure 2. All measurements were made at or close to mid-length where possible, in a transverse-normal plane.

The 130 mm plates were too thick for neutron diffraction, so residual stress measurements were made using both hole drilling and the contour method. Measurements using both conventional and incremental deep hole drilling (iDHD), were carried out by Veqter Limited in Bristol, while contour method cuts and surface profile measurements were performed by StressMap at the Open University, and the data analyzed at the University of Manchester.

Two iDHD measurements were made first in the as-welded condition, close to weld mid-length. One hole was vertical, on the weld centerline, and recovered longitudinal and transverse stresses on that vertical line. The second hole was horizontal, drilled in the transverse direction at plate mid-thickness, and recovered longitudinal and normal stresses on that horizontal line. A contour method cut was then made at weld mid-length, to measure the longitudinal stress distribution on a transverse-normal plane.

One plate half then underwent PWHT, after which a transverse DHD measurement was made at its mid-length. This repeated the horizontal measurement previously made in the as-welded state. A second vertical DHD measurement on the weld centerline was made solely on the NG-GTAW plate after post-weld heat treatment. This required the previously archived as-welded plate half to be heat treated, to provide sufficient weld length for a representative measurement to be made.

The measurement locations chosen for the most extensively characterized plate, the NG-GTAW weld, are shown in Figure 5.

Deep-Hole Drilling measurements

The DHD technique is a strain relaxation method for residual stress measurement [15-17]. An initial reference hole is drilled through the component along the line where residual stresses are to be measured, and its diametric profile measured with high accuracy. A core surrounding the reference hole is then trepanned out using electric discharge machining (EDM). This process relaxes the residual stresses in the core. The diametric profile of the reference hole is then re-measured, allowing stresses in a plane normal to the drilling direction to be recovered as a function of through-wall position.

In the as-welded condition, conventional DHD was used for regions remote from the welds and the more complex iDHD technique was applied for regions close to and within the weld. Higher stresses are expected in the weld fusion zone and the heat affected zone, the relaxation of which might cause plasticity during the drilling and trepanning operations. The iDHD method is less sensitive to plasticity-induced errors [18]. The conventional DHD technique was used for measurements after post-weld heat treatment, as the stresses everywhere were expected to be less than 30% of the yield stress.

Two measurements were made on each of the plates in the as-welded state. The first was made in the transverse direction at mid-thickness, and at 55 mm from mid-length, see Figure 1 and Figure 5. This location was chosen to ensure the hole drilling measurement did not perturb the residual stress field at the location of the subsequent contour method measurement. The transverse measurement was made with a 3 mm diameter reference hole, and a 10 mm diameter trepanned core, and extended 225 mm into the plate, whose width was 330 mm. This measurement recovered residual stresses in the longitudinal and through-wall directions.

The second measurement was made in the through-thickness direction on the weld centre line close to mid-length (as shown in Figure 6). The intention was to position the hole at about 110 mm from mid-length, to avoid any interaction with the transverse measurement, and this was achieved for the two arc welds. However, a failed first attempt on the RPEB weld meant that the vertical measurement in this weld was made 160 mm from mid-length. The vertical measurement was made with a 1.5 mm diameter reference hole, and a 5 mm diameter core, and over the whole depth of the weld. This measurement recovered residual stresses in the longitudinal and transverse directions.

A transverse conventional DHD measurement at mid-thickness was made on all three welds after post-weld heat treatment. This was made close to mid-length of the heat-treated half-plate, at 160 mm from the contour cut location (recall that the half-plate had a total length of 290 mm). It used a 3 mm diameter reference hole and a 10 mm diameter trepanned core, extended to a depth of 225 mm, and recovered longitudinal and normal stresses. The heat-treated half-plates contained no holes from as-welded iDHD measurements.

A single vertical DHD measurement was also made on the NG-GTAW test piece after post-weld heat treatment. This was an addition to the planned programme. It was made at mid-length of the half plate that was originally retained in the as-welded condition.

Contour Method Measurements

Contour method residual stress measurements were made on all three weld types in the as-welded condition. The EDM cuts were made in a transverse plane at weld mid-length, thereby recovering a map of longitudinal stresses on this plane.

The clamping and cut sequences varied with the welding process. The RPEB welds were cut without any clamping and without pilot holes, with the wire moving from one side of the plate to the other. The arc welds were expected to be more susceptible to cutting induced plasticity [29-32], so a different cutting strategy was adopted. The cuts were made between pre-drilled pilot holes 2 mm in diameter, positioned ~ 35 mm millimeters from the edge of the plate. The remaining ligaments provided a self-restraining feature that helped to minimise errors due to plasticity during cutting. The cut was also intentionally interrupted at two “planned” stop positions, either side of the weld region. The cutting strategy for the arc welds is illustrated on Figure 7.

In broad terms, contour method measurements involve four steps. These are:

· EDM cutting.

· Measuring out-of-plane deformations (the contour) on the EDM-cut surface.

· Processing the raw point cloud data from the surface profile measurements to produce an out-of-plane displacement profile suitable for application to a finite element model.

· Elastic finite element analysis to calculate stresses normal to the cut plane, by imposing the measured out-of-plane displacement profile as a boundary condition.

Once the cut surface profile was characterized, some data manipulation was required in order to import it into a finite-element (FE) model. The FE analysis involved forcing the cut surface to be flat. The contour method analysis procedure assumes that the cut surface was perfectly flat prior to making the cut, and that the measured deformations resulted from the elastic relaxation of residual stresses that were present prior to cutting. The elastic FE analysis can then reveal the stress distribution that would have been required to restore the cut surface to a flat surface, and this stress distribution will be the negative of the calculated residual stress distribution. One of the inherent assumptions in making contour method measurements is that all out-of-plane deformations result from the elastic relaxation of stress. While the method can be employed with excellent results, one must always be alert to the potential for plasticity-induced errors when interpreting the results.

For the surface profilometry, a co-ordinate measuring machine (CMM) was used with a sampling interval of 250 microns. The data processing included the cleaning of data, capturing the outline of the cross-sections, averaging and fitting. In the case of the arc welds, data cleaning was performed manually at the locations of the “planned” stop positions. The averaged surface was fitted using a bi-variate cubic spline, based on a least-squares fitting procedure. The knot-spacing (with ‘knots’ being the points where the 3rd order polynomials of the spline are joined) was selected to be 1.5 mm. The finite element analysis (FEA) model for the calculation of stresses was created in ABAQUS, using quadratic (C3D20) elements. The negative surface deformations were applied as displacement boundary conditions. Additional boundary conditions were applied to prevent rigid body motion.

Residual Stress Measurement ResultsMeasurements in the As-Welded Condition

Contour maps of the longitudinal residual stress distributions measured using the contour method for all three weld processes are presented in Figure 8. Recall that all measurements were made on a transverse plane at weld mid-length. The RPEB welded plate appears to be wider than the two arc-welded plates. This is due to the absence of pilot holes in this weld. Figure 8 thus presents stresses across the entire plate width for the RPEB weld, while data beyond the pilot holes in the two arc-welded plates are omitted, so the maps stop at the pilot hole locations. Some measurement artefacts are visible close to the pilot holes in the two arc welds, especially in the NG-SAW plate. Although care was taken to minimise measurement artifacts at the wire stopping positions, these remain visible in both arc-welded plates. In particular, the NG-GTAW plate exhibits an elongated stripe of clearly erroneous data, associated with the left hand stop position (Figure 7).

The EB weld contains regions of high tensile stresses just outside the HAZ, with reduced stresses in the fusion zone. This M-shaped distribution is caused by low temperature solid-state phase transformation in the HAZ and weld fusion zone. The highest stresses are observed close to mid-thickness of the plate.

The arc welds of Figure 8 appear to develop a more conventional residual stress distribution, with regions of high tensile stresses concentrated within both weld and HAZ. In both GTAW and SAW plates the highest tensile stresses are observed near top and bottom surfaces of the plates, with significantly reduced stresses at about 2/3 depth.

Figure 9 presents comparisons of the longitudinal and normal direction residual stresses measured on a transverse line at mid-thickness using deep hole drilling in all three welds. The data presented are composites: where plasticity effects were not expected the conventional DHD technique was used, and where stresses were high, the more involved iDHD method was used. The DHD method is continuous, so its results appear on the plots as a continuous line with frequent error bars. The iDHD method recovers data at discrete points, so its results appear as individual points. The error bars on the plots are the quoted measurement uncertainties. They do not reflect any possible systematic errors due to plasticity effects, although these should be minimized by use of the iDHD technique in regions of high stress.

The longitudinal stresses in the RPEB weld show the same features already observed in the contour method measurement, namely peak tensile stresses, of about 550 MPa, developed in parent material on either side of the weld, with reduced stresses, of about 350 MPa, in the weld fusion zone and HAZ. This distribution is similar to that already observed in 30 mm thickness RPEB welds made from the same steel forging [22, 27], albeit with a much less marked depression in the stresses in the weld fusion zone and HAZ, despite the two welds being manufactured from the same material. The difference in response is caused by the weld characteristics: the beam power is higher and the advance speed lower for the thick section weld, and the thermal mass of the plate is much larger. These factors result in lower cooling rates in weld and HAZ for the thick-section weld. This leads to phase transformations from austenite to low temperature micro-constituents at higher temperatures in the thick section weld, forming a predominantly bainitic microstructure rather than a mixture of martensite and bainite, and introducing the transformation strains at a higher temperature where they have less impact on the final residual stress field [33].

Normal direction stresses in the RPEB weld show a similar pattern, with tensile stresses in parent material either side of the weld and HAZ, and virtually no stress on the weld centerline.

The residual stresses in the arc welds show more conventional distributions, with peak tensile stresses in the weld fusion zone, of approximately equal magnitude for the two processes. It must be remembered however that the transverse mid-thickness line presented in Figure 9 does not pass through the regions of peak tensile stress in the two arc welds.

Interestingly, the widest extent of tensile residual stresses is in the RPEB weld, because the peak stresses occur in untransformed parent material either side of the weld and HAZ. The transverse extent of the tensile stress region in the GTAW plate is less than in the EB weld, and that in the SAW plate is lower still, a reflection of its narrower HAZ and lower heat input per pass.

There is some level of asymmetry either side of the weld in the measured stresses for all three processes. This is probably a reflection of the residual effects of plastic deformation during the trepanning process [34].

Through wall distributions of longitudinal and transverse stress measured on the weld centerline using iDHD are plotted in Figure 10a and Figure 10b respectively. The top surface (i.e. the weld cap) is at zero depth. Examination of the residual stress maps in Figure 8 shows that a through wall line on the weld centerline passes through the regions of peak longitudinal tension in the two arc welds, but samples only the reduced stresses developed in weld metal in the RPEB weld.

Both the NG-SAW and the NG-GTAW plates show a broadly co-sinusoidal through-wall distribution of longitudinal stress, with peaks of about 600 MPa near the top surface, reduced stresses below mid-thickness, and higher stresses near the lower surface. The fall in stresses in the lower part of the plate is more marked for the NG-GTAW weld and the peak near the lower surface is lower, at ~ 450 MPa rather than ~ 550 MPa in the NG-SAW plate. The RPEB weld shows an inverse parabolic distribution of longitudinal stress, with the peak stress of ~ 350 MPa developed close to mid-depth.

Comparison of Figure 9a and Figure 10a shows that the peak longitudinal stresses developed in the arc welds and the RPEB weld are broadly similar in magnitude at ~ 550 MPa in the RPEB weld and ~ 600 MPa in the arc welds, but, as noted above, very different in distribution – see Figure 8.

The longitudinal stresses measured at the intersection of the two iDHD measurements are consistent, at a mid-width, mid-depth location where all three welds have very similar stress levels. This observation gives confidence in the repeatability of the iDHD measurements.

The distributions of transverse stress plotted in Figure 10b are similar in shape to the longitudinal stresses, namely co-sinusoidal for the arc welds and inverse parabolic for the RPEB weld. Stresses in the NG-GTAW plate are somewhat higher than in the NG-SAW plate, with peak tensile stresses of ~ 500 MPa near the top surface, falling to about -250 MPa in the lower part of the plate, rising to ~ 600 MPa at the lower surface. This compares with ~ 400 MPa near the top surface, falling to about -250 MPa in the lower part of the plate, rising to ~ 550 MPa at the lower surface in the NG-SAW plate. Transverse stresses in the EB weld are compressive at the upper and lower surfaces, at about -200 MPa, with a tensile peak of ~ 300 MPa near mid-thickness.

Measurements in the Post-weld Heat Treated Condition

A transverse DHD measurement was made on each of the welds after post-weld heat treatment, repeating the transverse iDHD measurement made in the as-welded state. This recovered longitudinal and normal stresses. Figure 11 compares longitudinal residual stresses before and after post-weld heat treatment. It is evident that heat treatment has generally been effective in relieving residual stresses, as the stresses are greatly reduced when compared to the corresponding measurements for the plates in the as-welded condition. However the residual stresses were not removed altogether: in the EB weld residual stresses of ~ 100 MPa remain even after an ASME-compliant post-weld heat treatment. From a structural integrity standpoint, this may not be of concern at the start of life when ferritic steel components exhibit high upper shelf toughness. However, it may become significant as components age within a reactor, particularly if neutron irradiation leads to shifts in the ductile-to-brittle transition temperature, thereby creating a situation in which components have toughness values that reside on the shoulder of the transition curve.

A vertical DHD measurement was made on the weld centerline of the NG-GTAW test piece after post-weld heat treatment, repeating the measurement made in the as-welded state. This recovered longitudinal and transverse stresses, and it sampled the regions in the weld where the highest stresses were measured in the as-welded condition. Figure 12 compares the stresses measured before and after post-weld heat treatment. It is again evident that heat treatment has generally been effective in relieving residual stresses, as the stresses are greatly reduced. However, stresses approaching 100 MPa remain.

Comparison of Stresses Measured with Different Techniques

Longitudinal residual stresses were measured using both iDHD/DHD and the contour method in the as-welded condition, so may be compared on the two lines chosen for iDHD measurements. The two techniques are compared in Figure 13 for the three welding processes. In general, good agreement is achieved, which provides increased confidence in the measurements. Peak tensile residual stresses of about 600 MPa were measured for each welding process. This is consistent with previously published measurements on this type of steel [19, 20, 22, 25-27] and corresponds approximately to the ultimate tensile strength (UTS) for the steel. It is worth noting that residual stresses in the order of the UTS are feasible when multi-axial residual stresses are present.

The characteristic “M-shaped” residual stress measurement profile was obtained with both measurement techniques in the RPEB weld, which is both reassuring and a reflection of the care taken in optimising the data analysis pipeline for this weld.

The contour method measurements made on arc welds suffered from cutting artefacts, and might be expected to be more seriously impacted by cutting plasticity effects. The good agreement achieved within the welds themselves, remote from the cut start-stop positions, is thus very encouraging.

Discussion

Residual stress measurements on thick-section ferritic steel welds tend to be few and far between. The weldments themselves are expensive to manufacture, the measurements themselves are difficult and expensive, and only a limited number of measurement techniques are available. The measurement programme reported here is probably unique. It examines three different welding processes, applied to very tightly characterised weldments made from a single forging, applies diverse, state-of-the-art measurement techniques and reports measurements made in both as-welded and heat-treated conditions. The chosen welding processes also look forward, to potential new-build plant, rather than back, to weld processes and geometries in existing nuclear plant. The level of process characterisation is also sufficient to allow reliable finite element simulation of residual stress development.

Making measurements on representative thick-section welds is important, as results from thinner, cheaper and more portable weldments may not be fully representative. Thick section welds develop higher levels of triaxiality, which can have two effects: raising the peak stress levels in the as-welded state, and potentially limiting stress relaxation during post-weld heat treatment, since creep relaxation is normally assumed only to affect the deviatoric stresses. The measured stresses after post-weld heat treatment in the thick section welds examined here do indeed appear to be higher than those measured in the accompanying 30 mm thick welds made at the same time from the same forging material [27].

The observations made on EB welds also emphasise the value of examining representative weldments. Stresses in these single pass autogenous welds are strongly affected by solid-state phase transformations during cooling. Cooling rates in the thick-section welds examined here were slower than in the 30 mm thick welds examined in [22, 27], resulting in transformation to a bainitic structure at higher temperatures than the mixed bainite/martensite transformation observed in 30 mm welds. This led to tensile longitudinal stresses in the weld fusion zone, rather than the compressive stresses observed in thinner section welds. Electron beam welding is a relatively novel process for thick-section welds. The results reported here are consistent with measurements reported for a similar low alloy ferritic steel in [19] for thicknesses of 80 mm and 160 mm.

Conclusions

Residual stresses have been measured in thick-section plate butt welds made with three different processes: narrow-gap submerged arc (NG-SAW), narrow-gap gas tungsten arc (NG-GTAW), and reduced pressure electron beam, using two independent methods, deep hole drilling and the contour method. Measurements in the as-welded condition were made using both techniques, while measurements after post-weld heat treatment were made only using deep hole drilling.

1. Good overall agreement was achieved between the longitudinal residual stresses measured using the contour method and incremental deep hole drilling in the as-welded condition.

2. All three welding processes produced peak tensile residual stresses in the order of the ultimate tensile strength (UTS) for SA508 Grade 3 Class 1 steel, but the distributions differed markedly.

3. The single-pass, autogenous RPEB welds developed a longitudinal residual stress distribution that displayed a characteristic M-shape, with lower magnitude stresses on the weld centerline, and regions of higher tensile residual stress immediately outside the heat affected zone. This distribution is a result of the solid-state phase transformations that take place in SA508 Grade 3 Class 1 steel during the single welding thermal cycle that was imposed. The highest stresses were measured at mid-thickness of the plate.

4. The longitudinal stress distributions for the arc welds were more conventional, with tensile residual stresses concentrated in the weld fusion zone and adjacent HAZ. Peak tensile residual stresses were measured on the weld centerline in weld metal close to the top and bottom surfaces of the welds, with somewhat lower stresses at mid-thickness.

5. The RPEB weld developed the widest region of tensile longitudinal residual stress, because peak stresses occurred in parent material either side of the weld and HAZ.

6. In the multi-pass arc-welded plates, the region of peak longitudinal tensile residual stresses was slightly wider in the NG-GTAW plate, compared with the NG-SAW plate. This observation is consistent with the heat inputs for these two processes.

7. Transverse stresses on the weld centerline were highest near top and bottom surfaces for the two arc-welded plates, with reduced tensile stresses at mid-thickness.

8. In contrast, transverse stresses in the RPEB weld were compressive near the top and bottom surfaces, and tensile at mid-thickness.

9. Post-weld heat treatment (PWHT) was effective in reducing the levels of residual stress that were present in all the welds. However, residual stresses of about 100 MPa were measured after an ASME-compliant PWHT procedure. This value is noticeably higher than what has historically been assumed when carrying out structural integrity assessments on nuclear pressure vessels.

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4K Ayres, P R Hurrell, C M Gill, K Bridger, L D Burling, C S Punshon, L Wei and N Bagshaw, Development of Reduced Pressure Electron Beam Welding Process for Thick Section Pressure Vessel Welds, ASME Pressure Vessels and Piping Conference, Belleview, Washington, USA, July 18-22, 2010, PVP2010-25957, 39-48 (2010).

5P J Withers, Residual stress and its role in failure, Rep Prog Phys 70(12), 2211-2264 (2007).

6R6, Assessment of the integrity of structures containing defects, EDF Energy (2015).

7Fitness for Service, API 579-1/ASME FFS-1, Second Edition (2007).

8S Song, P Dong and X Pei, A full-field residual stress estimation scheme for fitness-for-service assessment of pipe girth welds: Part I – Identification of key parameters, Int Jnl Press Vess and Piping 126-127, 58-70 (2015).

9S Song, P Dong and X Pei, A full-field residual stress estimation scheme for fitness-for-service assessment of pipe girth welds: Part II – A shell theory based implementation, Int Jnl Press Vess and Piping 128, 8-17 (2015).

10S Song and P Dong, A framework for estimating residual stress profile in seam-welded pipe and vessel components part I: Weld region, Int Jnl Press Vess and Piping 146, 74-86 (2016).

11S Song and P Dong, A framework for estimating residual stress profile in seam welded pipe and vessel components Part II: Outside of weld region, Int Jnl Press Vess and Piping 146, 65-73 (2016).

12P J Bouchard, Validated residual stress profiles for fracture assessments of stainless steel pipe girth welds, Int Jnl Press Vess and Piping 84(4), 195-222 (2007).

13J Mathew, R J Moat, S Paddea, M E Fitzpatrick and P J Bouchard, Prediction of residual stresses in girth welded pipes using an artificial neural network approach, Int Jnl Press Vess and Piping 150, 89-95 (2017).

14J Mathew, R J Moat, S Paddea, J A Francis, M E Fitzpatrick and P J Bouchard, Through-Thickness Residual Stress Profiles in Austenitic Stainless Steel Welds: A Combined Experimental and Prediction Study, Metallurgical and Materials Transactions A 48(12), 6178-6191 (2017).

15D B F George, Determination of Residual Stresses in Large Section Stainless Steel Welds, PhD Thesis, University of Bristol (2000).

16D J Smith, P J Bouchard and D George, Measurement and prediction of residual stresses in thick-section steel welds, The Journal of Strain Analysis for Engineering Design 35(4), 287-305 (2000).

17R H Leggatt, D J Smith, S D Smith and F Faure, Development and experimental validation of the deep hole method for residual stress measurement., J Strain Analysis Eng Des 31(3), 177-186 (1996).

18A H Mahmoudi, S Hossain, C E Truman, D J Smith and M J Pavier, A new procedure to measure near yield residual stresses using the deep hole drilling technique, Experimental Mechanics 49(4), 595-604 (2008).

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Figure 1: Showing completed 130mm thick welds: NG-GTAW (top), NG-SAW (middle) and RPEB (bottom). The mid-length contour measurement plane is highlighted in red, and the close-to-mid-length, mid-thickness, transverse line chosen for DHD/iDHD measurements is highlighted in yellow.

Figure 2 130mm arc-welded plate process and characterization flowchart.

(a)

(b)

(c)

Figure 3: Details of weld groove geometry and bead stacking pattern for 130mm NG-SAW (a), NG-GTAW (b), and RPEB (c) welds.

Figure 4: Transverse weld macrographs at mid-length of 130mm NG-SAW, NG-GTAW, and RPEB welds (sections were polished to a 1 m finish before etching using a solution of 5 ml nitric acid and 95 ml ethanol).

Figure 5: Residual stress measurement locations in the NG-GTAW plate butt weld.

Figure 6. Transverse iDHD measurement underway on a RPEB 130mm weld in the as-welded state.

Figure 7: Through-thickness iDHD measurement underway for NG-SAW specimen in the as-welded state.

Figure 8: A schematic drawing of the cutting strategy followed for the arc welds.

As-welded

NG-SAW

NG-GTAW

RPEB

Figure 9: RS Contour maps in the as-welded condition for 130mm thick NG-SAW, NG-GTAW and RPEB welds.

a) Longitudinal direction stresses

b) Normal direction stresses

Figure 10: Comparative plots of the residual stresses measured using deep hole drilling on a transverse line at mid-thickness, in the as-welded condition.

a) Longitudinal stresses

b) Transverse stresses

Figure 11: Comparative plots of the residual stresses measured using deep hole drilling on a vertical line at the centre of the weld, in the as-welded condition.

Figure 12: Showing the impact of post-weld heat treatment on the longitudinal stresses measured on a transverse line at mid-thickness: (a) NG-SAW, (b) NG-GTAW, (c) RPEB, (d) comparative plot of the PWHT stress profiles for all processes.

Figure 13: Showing the impact of post-weld heat treatment on the longitudinal and transverse stresses measured on a vertical line at the weld centerline in the NG-GTAW weld

Through thickness line on weld centre-line

Transverse line at mid-thickness

NG-SAW

NG-GTAW

RPEB

Figure 14: Comparisons of longitudinal stresses measured using iDHD and the contour method in the as-welded condition (left: through-thickness line at the centre of the weld; right: transverse line at mid-thickness).

Machine plate halves for a single weld from SA508 Gr 3 Cl 1 forging ring

Instrument with thermo-couples, and strain gauges if required

Measure transient temperatures during welding

Maintain full welding process parameter log

Record distortion development with laser scanner and conventional reference markers

Perform fusion-zone metallography

Perform hardness mapping

Perform advanced microscopy and characterisation

Retain for archive

Install arc-welded plates in welding restraint system

Make weld

After welding is complete, release restraints and measure elastic springback

Perform two iDHD measurements in as-welded state near to weld mid-length

Perform mid-length transverse contour cut

Send one plate half for PWHT

Perform single DHD measurement at mid-length of plate half after PWHT

Extract slice(s) for metallography/microscopy/hardness mapping

Extract cross weld tensile DIC specimens and Charpy specimens

Retain one half in the as-welded state

Perform fusion-zone metallography

Perform hardness mapping

Perform advanced microscopy and characterisation

Extract slice(s) for metallography/microscopy/hardness mapping

Perform Charpy testing

Perform cross-weld DIC tensile testing

-300-200-10001002003004005006007000255075100125150175200225

Longitudinal Stress [MPa]Distance through specimen from one side [mm]

NG-GTAW, Longitudinal - DHDNG-GTAW, Longitudinal - iDHDNG-SAW, Longitudinal - DHDNG-SAW, Longitudinal - iDHDRPEB, Longitudinal - DHDRPEB, Longitudinal - iDHD

-300

-200

-100

0

100

200

300

400

500

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0 25 50 75 100 125 150 175 200 225

Stre

ss [

MPa

]

Distance through specimen from one side [mm]

NG-GTAW, Normal - DHD

NG-GTAW, Normal - iDHD

NG-SAW, Normal - DHD

NG-SAW, Normal - iDHD

RPEB, Normal - DHD

RPEB, Normal - iDHD

-300

-200

-100

0

100

200

300

400

500

600

0255075100125150175200225

S

t

r

e

s

s

[

M

P

a

]

Distance through specimen from one side [mm]

NG-GTAW, Normal - DHD

NG-GTAW, Normal - iDHD

NG-SAW, Normal - DHD

NG-SAW, Normal - iDHD

RPEB, Normal - DHD

RPEB, Normal - iDHD

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Stre

ss [

MP

a]

Depth through specimen from front surface [mm]

NG-GTAW 130mm, Longitudinal - DHDNG-GTAW 130mm, Longitudinal - iDHDNG-SAW 130mm, Longitudinal - DHDNG-SAW 130mm, Longitudinal - iDHDRPEB 130mm, Longitudinal - DHDRPEB 130mm, Longitudinal - iDHD

0

100

200

300

400

500

600

700

0102030405060708090100110120130

S

t

r

e

s

s

[

M

P

a

]

Depth through specimen from front surface [mm]

NG-GTAW 130mm, Longitudinal - DHD

NG-GTAW 130mm, Longitudinal - iDHD

NG-SAW 130mm, Longitudinal - DHD

NG-SAW 130mm, Longitudinal - iDHD

RPEB 130mm, Longitudinal - DHD

RPEB 130mm, Longitudinal - iDHD

-400

-300

-200

-100

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Stre

ss [

MP

a]

Depth through specimen from front surface [mm]

NG-GTAW 130mm, Transverse- DHDNG-GTAW 130mm, Transverse - iDHDNG-SAW 130mm, Transverse - DHDNG-SAW 130mm, Transverse - iDHDRPEB 130mm, Transverse - DHDRPEB 130mm, Transverse - iDHD

-400

-300

-200

-100

0

100

200

300

400

500

600

700

0102030405060708090100110120130

S

t

r

e

s

s

[

M

P

a

]

Depth through specimen from front surface [mm]

NG-GTAW 130mm, Transverse- DHD

NG-GTAW 130mm, Transverse - iDHD

NG-SAW 130mm, Transverse - DHD

NG-SAW 130mm, Transverse - iDHD

RPEB 130mm, Transverse - DHD

RPEB 130mm, Transverse - iDHD

-200-10001002003004005006000255075100125150175200225 Stress [MPa]Distance through specimen from one side [mm]AW, NG-SAW, Longitudinal - DHDAW, NG-SAW, Longitudinal - iDHDPWHT, NG-SAW, Longitudinal - DHD

(a)

-200-10001002003004005006000255075100125150175200225Stress [MPa]Distance through specimen from one side [mm]AW, NG-GTAW 130mm, Longitudinal - DHDAW, NG-GTAW 130mm, Longitudinal - iDHDPWHT, NG-GTAW 130mm, Longitudinal - DHD(b)

-200-10001002003004005006000255075100125150175200225 Stress [MPa]Distance through specimen from one side [mm]AW, RPEB, Longitudinal - iDHDAW, RPEB, Longitudinal - DHDPWHT, RPEB, Longitudinal - DHD

(c)

-100-500501001502002500255075100125150175200225 Stress [MPa]Distance through specimen from one side [mm]PWHT, Longitudinal - DHD, 130mm NG-SAWPWHT, Longitudinal - DHD, 130mm NG-GTAWPWHT, Longitudinal - DHD, 130mm RPEB

(d)

-300

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600

700

0 20 40 60 80 100 120 140

Resid

ual S

tres

s (M

Pa)

Depth through thickness of specimen from weld cap surface (mm)

Longitudinal, AW, DHD

Transverse, AW, DHD

Longitudinal, AW, iDHD

Transverse, AW, iDHD

Longitudinal, after PWHT

Transverse, after PWHT

-300

-200

-100

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140

R

e

s

i

d

u

a

l

S

t

r

e

s

s

(

M

P

a

)

Depth through thickness of specimen from weld cap surface (mm)

Longitudinal, AW, DHD

Transverse, AW, DHD

Longitudinal, AW, iDHD

Transverse, AW, iDHD

Longitudinal, after PWHT

Transverse, after PWHT

01002003004005006007000102030405060708090100110120130Stress [MPa]Depth through specimen from front surface [mm]NG-SAW 130mm, Longitudinal - DHDNG-SAW 130mm, Longitudinal - iDHDNG-SAW 130mm, Longitudinal - CM

-300-200-10001002003004005000255075100125150175200225

Stress [MPa]

Distance through specimen from one side [mm]NG-SAW, Longitudinal - DHDNG-SAW, Longitudinal - iDHDNG-SAW, Longitudinal - CM

01002003004005006007000102030405060708090100110120130Stress [MPa]Depth through specimen from front surface [mm]NG-GTAW 130mm, Longitudinal - DHDNG-GTAW 130mm, Longitudinal - iDHDNG-GTAW 130mm, Longitudinal - CM

-300-200-10001002003004005000255075100125150175200225Stress [MPa]Distance through specimen from one side [mm]NG-GTAW, Longitudinal - DHDNG-GTAW, Longitudinal - iDHDNG-GTAW, Longitudinal - CM

-100-500501001502002503003504000102030405060708090100110120130Stress [MPa]Depth through specimen from front surface [mm]RPEB 130mm, Longitudinal - DHDRPEB 130mm, Longitudinal - iDHDRPEB 130mm, Longitudinal - CM

-200-10001002003004005006007000255075100125150175200225Stress [MPa]Distance through specimen from one side [mm]RPEB, Longitudinal - DHDRPEB, Longitudinal - iDHDRPEB, Longitudinal - CM