Finite element analyses of the effect of weld overlay sizing on residual stresses of the dissimilar metal weld in PWRs
Introduction
Many welding processes are used in nuclear power facilities. Close attention must be paid to the safety of welded structures because nuclear safety is critical. Since the mid-1980 s, nuclear power plants have begun to exhibit stress corrosion cracking (SCC), which is an important mechanism of degradation of metal components (IAEA, 2011). In pressurized water reactors (PWRs), primary water stress corrosion cracking (PWSCC) has been observed in an increasing number of Alloy 82/182/600 components, such as pressurizer nozzles, control rod drive mechanism (CRDM) penetrations, reactor pressure vessel (RPV) outlet/inlet nozzles, and steam generator tubes. In boiling water reactors (BWRs), the intergranular stress corrosion cracking (IGSCC) of piping and other components that are made of austenitic stainless steel or nickel-based alloys has become a major safety concern. This mechanism may cause material failure and serious accidents when a susceptible material undergoes tensile stresses in a corrosive environment. In response to incidents of IGSCC, the United States Nuclear Regulatory Commission (USNRC) published NUREG-0313, Rev. 2 as a technical basis for controlling the susceptibility of BWR piping to IGSCC. One suggested method is weld overlay, which can successfully mitigate further crack growth and subsequent leakage in SCC materials.
Cracked welds have been commonly repaired using the weld overlay process, which is a primary technique for repairing SCC welds and has been utilized in the nuclear industry for years. A weld overlay ensures the structural integrity of the piping system because it provides a redundant pressure boundary as a structural reinforcement to meet the structural margins that are required by the American Society of Mechanical Engineers (ASME) Code, Section XI when cracks are present; it also increases SCC resistance by generating low tensile or compressive residual stresses in the SCC-susceptible inner portion of the original pipe using overlay materials, such as Alloy 52 M. As well as generating compressive stresses in metal components, a weld overlay provides various benefits, such as reinforcing the cross-section of interest, preventing possible leakage, and facilitating inspection by moving the examination volume to the outside surface of the nozzle structure. The design of Full Structural Weld Overlay (FSWOL) at stainless steel piping welds is based on the ASME Code Case N-504-4, which was approved by the USNRC, as documented in Revision 18 of Regulatory Guide 1.147, and has proved to be reliable in long-term plant operations. The ASME Code Case N-740-2 has been modified to specify FSWOL design at dissimilar metal welds and similar metal welds. Although Code Case N-740-2 has not been approved by the USNRC, it has been applied in the planning of weld overlays for large-diameter RPV nozzles.
Over the last two decades, the weld overlay process has been used preemptively at locations that have not yet exhibited any cracking but are regarded as susceptible to PWSCC. Providing additional guidance concerning areas that are not addressed by the ASME Code Cases, in the form of residual stress analyses, fatigue usage factor evaluation and fatigue crack growth analyses, MRP-169-1A defines the methodology and criteria for the use of preemptive weld overlay (PWOL) as a mitigation measure for PWSCC in PWR primary coolant piping and nozzles. An optimized weld overlay (OWOL) is also specified as an acceptable alternative to the FSWOL when a weld has no flaws or flaws of limited size. Approved by the USNRC, MRP-169-1A has provided an adequate technical basis for demonstrating that a dissimilar metal weld (DMW) that is overlaid with either FSWOL or OWOL provides reasonable assurance of the maintenance of the structural integrity of the piping system. Therefore, in MRP-169-1A, a crucial aspect of weld overlay design is the demonstration that favorable residual stress reversal mitigates PWSCC initiation and growth. If a nozzle-specific weld overlay design is demonstrated to generate favorable residual stresses in the severe case, then effective mitigation against future PWSCC in the DMW is ensured.
Validated methods for predicting welding residual stresses are sought because of the complexity of the welding process, which involves non-uniform heating, temperature-dependent material properties, multipass welding, and a moving heat source. Welding residual stress analysis must be demonstrated to yield results that agree with the results of an experiment. For instance, Woo et al. (2011) determined the through-thickness distribution of residual stresses in a dissimilar welded pipe using neutron diffraction and compared the changes in residual stresses from tension to compression induced by a weld overlay. Enabled by the development of computer hardware and software, the finite element method (FEM) has been extensively used to predict welding residual stresses. Using FEM simulation, Kim et al. (2009) developed a two-dimensional plane-strain model, consisting of the SUS316 plate and the SA 508 plate, to estimate the residual stresses in the DMW. Their FEM results showed a trend that was consistent with experimental measurements (X-ray). The above methodology has also been used to predict residual stresses that were generated by overlay welding on the plate. Song et al. (2010) carried out a FEM simulation of a PWR pressurizer safety/relief nozzle using a two-dimensional axisymmetric model to elucidate the effect of weld overlay on residual stresses. They used the Code-recommended minimum weld overlay thickness of one-third of the pipe thickness. Their results demonstrated that the applied overlay thickness sufficed to mitigate residual stresses at the inside surface of the DMW.
The use of multipass welding simulation to elucidate complications in the welding on large components has attracted a lot of attention in recent years. Welding simulation using the lumped-pass technology, in which several weld passes are assumed to be one lumped pass, can save substantial computational time and cost. Tan et al. (2014) investigated the effect of the lumped-pass simulation on the distribution of residual stresses before and after heat treatment in a thick-walled nuclear power rotor pipe with an 89-pass narrow gap welding process. Zhang et al. (2012) carried out FEM analyses to simulate subject butt welding at pressurizer surge, spray, and safety/relief nozzles to predict the magnitude of residual stresses in every DMW of the nozzles. The analysis involved not only the pass-by-pass welding steps but also other essential steps in the fabrication of the nozzles. Their results revealed the residual stresses in DMW region were reduced by overlay welding, providing a region of more uniform distribution and rather large compressive stresses through the DMW thickness. Liu and Huang (2013) applied the lumped-pass technology to predict residual stresses in a BWR feedwater nozzle following overlay welding. Jiang et al. (2014) proposed a new method in which overlay welding on the inner surface of a nozzle reduced residual stresses in the penetration joint, which has been validated by FEM simulation. Deng and Kiyoshima (2010) developed a computational approach that was based on thermal elastic–plastic FEM analyses to clarify the effect of the initial stresses that were generated by heat treatment on the welding-induced residual stresses in an austenitic stainless steel pipe.
Although welding is a three-dimensional procedure, it has been widely proved that cylindrical structure welding can be simulated using a two-dimensional axisymmetric model to determine residual stresses therein. Thus, two-dimensional FEM simulation, which is a considerably faster and easier-to-use method for investigating residual stresses, has been used extensively. The Electric Power Research Institute (EPRI) has issued a series of technical reports that establish guidelines for the use of two-dimensional welding residual stress analyses in the nuclear industry. MRP-33 outlines elastic–plastic FEM analyses that determine the welding residual and operating condition stresses in idealized single-V and double-V weld joints that are similar to those at the V.C. Summer and Ringhals plants. MRP-106 presents elastic–plastic FEM analyses of Alloy 182 butt welds for a range of nozzle geometries, sizes, weld repairs, and external pipe loading conditions. The stress distributions thus obtained provide insights regarding the potential for PWSCC and possible PWSCC crack growth rates in Alloy 182 butt welds. To demonstrate the favorable residual stress effect on the PWOL, MRP-169-1A specifies a fully circumferential, 50% of wall ID repair to simulate in the example nozzle welds, and the resulting stress fields are used as initial stress states for welding residual stress analyses.
With regard to the application of the Gas Tungsten Arc Weld (GTAW) temper bead technique on low alloy steel (LAS) components, the ASME Code Case N-638-1 requires that the maximum area of an individual weld based on the finished surface shall be 100 in2. As plants age and inspection techniques continue to improve, increasing the area limit becomes increasingly important. The limitation of 100 in2 that was imposed by Code Case N-638-1 for ambient temperature temper bead welding was too conservative, and has significantly influenced the design of weld overlays at large-diameter RPVs. EPRI report 1,011,898 presents the results of residual stress analyses that technically support increasing the allowed amount of temper bead welding on LAS components. Their study examined effects of increasing the area on which weld material was deposited using ambient temperature temper bead procedures on a BWR feedwater nozzle. Of key interest is the effect of the amount of applied material – possibly beyond, the 100 in2 limit and of geometrical constraints on residual stresses. The results in the report constitute a significant part of the justification for the eliminating the 100 in2 temper bead welding limit that has been imposed on ASME Section XI repair activities. EPRI report 1,011,898 concludes that the limitation on the surface area of repairs should be increased to 500 in2, based on the residual stress results. Furthermore, EPRI report 1,021,073 (EPRI 1021073) presents residual stress analyses that justify the extension of the temper bead limit to 1000 in2 for a weld overlay of carbon and LAS materials of large-diameter, thicker PWR primary coolant piping and nozzles. Since the revision of Code Case N-638-3, the ASME Code limitation has been increased to 500 in2.
To permit the ultrasonic testing (UT) examination of the adjacent base metal and to minimize the stress concentration on the LAS nozzle, the FSWOL design of BWR nozzle welds has often been extended and blended into the nozzle taper surface. The Westinghouse-designed PWR pressure vessel is supported using support pads under the RPV nozzles. Integrated into the nozzles that support the vessel, steel pads rest on steel base plates on top of a support structure that is attached to the concrete foundation. Therefore, the weld overlay metal could not be extended around the full circumference of the nozzle taper surface so the design of the weld overlay at PWR nozzle welds may not satisfy the FSWOL requirements of Code Case N-504-4 and N-740-2. Considering the obstruction of space due to PWR design characteristic, the purpose of this investigation is to evaluate the effect of weld overlay sizing on residual stresses of the DMW in a typical PWR.
Section snippets
Weld overlay sizing
In this section, both FSWOL and OWOL sizing calculations were performed for the DMW in a typical PWR based on plant-specific nozzle designs and loads. In accordance with ASME Section XI IWB-3640, the maximum allowable depth of axial and circumferential flaws is 75% of the wall thickness for wrought base metals, cast stainless steel, GTAW, and gas-metal arc welds (GMAW). Therefore, the maximum allowable depth of 75% of the wall thickness was used in the weld overlay design. With this maximum
Welding residual stress analyses
This section elucidates the development of the finite element models for investigating the effect of weld overlay sizing on residual stresses of the DMW in a typical PWR. The methodology is essentially the same as that used to develop the models in MRP-33, MRP-106, MRP-169-1A and EPRI report 1011898.
Analysis results
This section presents residual stress results that were obtained by the FEM simulation using ANSYS software. The stress distributions following overlay welding along two paths are reported in each of the five sets of cases. The first path is the center line within the DMW in a typical PWR, and the second is along the inside surface from the nozzle to the safe end. Both axial and hoop stress components are presented.
Conclusions
From the present results obtained, one draw the effect of weld overlay sizing on residual stresses of the DMW in a typical PWR using elastic–plastic FEM simulation. Various weld overlay sizes were simulated to determine residual stresses. Although overlay welding reduced residual stresses at the inside surface of the original weld, the results herein demonstrate that the mitigation of residual stresses is significant over the applied overlay length. The applied length that is shorter than that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the financial support of Taiwan Power Company (under grant no. 05601000501).
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