Zhuojin Pipe Fitting Co., LtdResidual Stress Analysis of Dissimilar Metal Weld Overlay Repair on Pressurizer Nozzle at Qinshan Nuclear Power Plant
Literature Overview
The paper by Zhang Shiwei, Chen Xuede, Zhang Yong, Sun Lei, and Li Xihua from the China Nuclear Power Research and Design Institute (published in Nuclear Power Engineering, 2016, Vol. 37, S2, pp. 4-6) presents a finite element analysis of residual stress distribution during overlay repair welding of a dissimilar metal weld (DMW) at the Qinshan Phase I nuclear power plant. The specific component is the pressurizer spray nozzle, which is a critical pressure boundary component in the reactor coolant system. This study is of significant importance to nuclear engineering because it addresses the well-documented vulnerability of Alloy 81/182 DMWs to primary water stress corrosion cracking (PWSCC).
Core Technical Points
The Alloy 81/182 DMW configuration is ubiquitous in nuclear power plant reactor coolant systems, where Alloy 600 or Alloy 800 nozzles are welded to carbon steel piping using Alloy 81 filler metal and Alloy 182 cladding. The PWSCC susceptibility of these welds arises from the combination of:
- Sensitization - Chromium depletion in the heat-affected zone (HAZ) during welding creates a narrow band of susceptible microstructure
- Residual tensile stress - Welding-induced residual stresses provide the driving force for crack initiation and propagation
- Primary water chemistry - The high-purity, high-temperature water in the reactor coolant system provides the corrosive environment
The overlay repair approach involves depositing a corrosion-resistant cladding layer (typically Alloy 690 or Alloy 182) over the existing DMW to cover the sensitized zone and provide a barrier against PWSCC initiation.
Finite Element Methodology
The authors established an axisymmetric finite element model based on the actual dimensions of the pressurizer spray nozzle. The welding process was simulated using the birth-and-death element technique, which activates and deactivates elements to represent the sequential deposition of weld passes. This approach captures:
- Transient temperature field evolution during multi-pass welding
- Thermal stress development and relaxation
- Final residual stress state after cooling to ambient temperature
| Modeling Parameter | Value/Method |
|---|---|
| Model type | Axisymmetric |
| Element type | Thermal-mechanical coupled |
| Birth-death technique | Sequential pass activation |
| Material model | Elastic-plastic with temperature-dependent properties |
| Thermal load | Calculated temperature history |
| Boundary conditions | Symmetry and geometric constraints |
| Cooling condition | Air cooling (no post-weld stress relief) |
The analysis revealed that after overlay repair completion, the residual stress state at the inner wall of the DMW weld region was predominantly compressive. This is a critical finding because compressive residual stresses are known to inhibit PWSCC initiation and propagation by counteracting the tensile stresses that drive crack growth.
Residual Stress Distribution Characteristics
The residual stress distribution in the repaired DMW follows a predictable pattern based on welding mechanics:
- Near-surface (inner wall): Compressive residual stress due to the last deposited layer contracting upon cooling, pulling the surface into compression
- Mid-thickness: Transition from compressive to tensile stress
- Outer surface (outer wall): Tensile residual stress due to restraint by the cooler interior material
The magnitude of compressive stress at the inner wall surface is sufficient to provide PWSCC resistance, as the threshold stress for PWSCC initiation in sensitized Alloy 81 weld metal is typically in the range of 100-200 MPa tensile stress. If the overlay repair produces compressive stresses exceeding this threshold at the susceptible zone, the risk of PWSCC is significantly reduced.
Engineering Practice Implications
This study provides quantitative support for the overlay repair methodology that has been widely adopted in the nuclear industry since the early 2000s, following the discovery of PWSCC in Alloy 81/182 DMWs. Key practical considerations include:
- Welding sequence design - The order of pass deposition directly affects the final residual stress state. The last pass should be deposited at the inner wall surface to ensure compressive stress at the most critical location.
- Post-weld heat treatment (PWHT) - While the study focuses on the as-welded condition, PWHT can further reduce residual stresses but must be carefully controlled to avoid sensitization or sigma phase formation in the overlay material.
- Inspection requirements - Despite the beneficial residual stress state, thorough inspection (typically using eddy current testing or ultrasonic testing) is required to verify the integrity of the overlay repair and ensure no cracks exist.
- Regulatory acceptance - Nuclear regulatory bodies require demonstration that repair procedures maintain or improve the safety margins of the original component. Residual stress analysis provides this quantitative basis.
Study Insights and Reflections
This paper exemplifies the increasing sophistication of nuclear welding repair methodologies. The transition from empirical repair procedures to quantitatively supported approaches based on finite element analysis represents a maturation of the field. The authors' decision to model the specific geometry of the Qinshan pressurizer nozzle, rather than using a generic configuration, demonstrates the engineering rigor required for nuclear applications.
From my experience in welding engineering, I note that the birth-death element technique, while widely used, has known limitations in accurately representing the thermal-mechanical coupling during multi-pass welding. The technique assumes instantaneous cooling of previously deposited passes, which may not accurately capture the thermal history experienced by earlier passes. More advanced methods, such as moving heat source models with adaptive meshing, can provide more accurate results but at significantly higher computational cost.
The finding that compressive residual stresses at the inner wall are achievable through overlay repair is encouraging and validates the industry-wide approach. However, engineers must recognize that the residual stress state is only one factor in PWSCC resistance. The quality of the overlay deposit (absence of lack of fusion, porosity, and cracking) and the integrity of the bond between the overlay and the base DMW are equally critical.
This work contributes to the growing body of knowledge that supports the safe and effective repair of nuclear power plant components, an area where the margin for error is essentially zero.