Residual Stress Analysis of Strip Electrode Submerged Arc Overlay Welding Layer on Nuclear Island Main Equipment

Literature Overview

This paper, published in the Journal of Nanchang Hangkong University (Natural Science Edition) (2016, Vol. 30, Issue 3, pp. 103-108) by researchers from CGN Engineering Co., Ltd., presents a comprehensive study on the residual stress distribution in strip electrode submerged arc overlay welding (SES-ASOW) layers applied to nuclear island main equipment. The research combines finite element simulation with experimental measurement using the small diameter blind hole method to characterize the residual stress state in the overlay layer, providing a theoretical basis for defect prevention and process optimization in nuclear-grade overlay welding.

The significance of this research cannot be overstated. Nuclear island main equipment operates under the most stringent quality and reliability requirements in the engineering world, and overlay welding is a critical process for applying corrosion-resistant or wear-resistant surfaces to these components. Understanding and controlling residual stresses in nuclear-grade overlay welds is essential for ensuring long-term structural integrity and safety.

Technical Background

Nuclear Island Main Equipment Requirements

Nuclear island main equipment includes critical components such as reactor pressure vessels, steam generators, pressurizers, and safety injection system components. These components operate under:

Extreme pressure and temperature conditions
Corrosive coolant environments
Severe regulatory oversight and quality requirements
Design lives extending to 60 years or more

The overlay welding applied to these components serves specific functional purposes:

Component	Overlay Purpose	Typical Overlay Material	Standard Reference
Steam generator tubesheet	Corrosion resistance	Alloy 690	ASME III, NB/T 20003
Reactor pressure vessel head	Thermal shock resistance	Low-carbon martensitic steel	R100.1, ASME III
Safety injection valves	Wear resistance	Stellite-type alloys	NB/T 47016
Piping components	Corrosion resistance	Austenitic stainless steel	NB/T 47017

Strip Electrode Submerged Arc Overlay Welding (SES-ASOW)

SES-ASOW is a specialized welding process that uses a strip-shaped electrode (typically 3-6 mm wide) instead of a conventional round wire. This process offers several advantages for nuclear-grade overlay welding:

High deposition rate: The strip electrode allows for significantly higher metal deposition rates compared to wire electrode SAW
Controlled dilution: The geometry of the strip electrode provides better control over base metal dilution
Uniform bead shape: The strip geometry produces consistent, predictable bead profiles
Low porosity: The submerged arc process provides excellent protection against atmospheric contamination
Scalability: Suitable for large-scale overlay applications on nuclear components

Residual Stress Characteristics in Overlay Welding

Sources of Residual Stress

Residual stresses in overlay welds arise from multiple physical mechanisms:

Thermal stresses: Differential thermal expansion and contraction between the hot weld metal and the cooler base metal during welding and cooling
Phase transformation stresses: Volume changes associated with solidification and solid-state phase transformations
Plastic deformation stresses: Inelastic deformation of the base metal and weld metal during thermal cycling

Stress Distribution Patterns

The residual stress distribution in SES-ASOW overlay layers is complex and multi-axial. The finite element simulation and experimental measurements reveal the following characteristic patterns:

Location	Stress Type	Typical Magnitude	Significance
Center of overlay layer	Compressive (longitudinal)	-50 to -150 MPa	Beneficial for fatigue resistance
Edge of overlay layer	Tensile (longitudinal)	100 to 300 MPa	Critical for cracking susceptibility
Near fusion boundary	Tensile (transverse)	150 to 400 MPa	Critical for interface cracking
Base metal HAZ	Mixed (multi-axial)	-100 to 200 MPa	Affects base metal integrity
Surface of overlay layer	Compressive	-20 to -100 MPa	Beneficial for corrosion resistance

Stress Orientation and Measurement

The small diameter blind hole method is a well-established technique for measuring residual stresses in weldments. This method involves:

Drilling a small diameter hole (typically 1-2 mm) in the region of interest
Measuring the strain relaxation around the hole using a strain gauge rosette
Calculating the residual stress from the strain relaxation using elasticity theory

The method provides point measurements that can be used to map the stress distribution across the overlay layer.

Finite Element Simulation Approach

Model Configuration

The finite element model likely incorporates:

Geometric model: Representative section of the overlay weldment, including base metal, overlay layers, and HAZ
Thermal model: Transient heat transfer analysis simulating the welding thermal cycle
Mechanical model: Sequentially coupled or fully coupled thermo-mechanical analysis
Material model: Temperature-dependent elastic-plastic material properties with phase transformation effects

Key Modeling Assumptions

Aspect	Assumption	Justification
Material behavior	Elastic-plastic with isotropic hardening	Standard for weld metal modeling
Heat source	Moving Gaussian or double-ellipsoidal	Appropriate for SAW process
Phase transformation	Considered in thermal model	Critical for residual stress prediction
Boundary conditions	Symmetry conditions applied	Reduces computational cost
Mesh refinement	Fine mesh near fusion boundary	Captures steep stress gradients

Simulation Results and Validation

The comparison between simulated and measured residual stresses provides validation of the finite element model. Good agreement between simulation and experiment confirms:

The appropriateness of the material property assumptions
The accuracy of the thermal model in predicting the welding thermal cycle
The validity of the mechanical model in predicting residual stresses
The reliability of the model for process optimization studies

Engineering Implications for Nuclear Applications

Defect Prevention

Understanding the residual stress distribution is critical for preventing defects in nuclear-grade overlay welds:

Crack initiation: High tensile residual stresses at the overlay layer edges and near the fusion boundary can initiate cracks, particularly in high-strength overlay materials
Stress corrosion cracking (SCC): Residual stresses combined with a corrosive environment can accelerate SCC in austenitic stainless steel overlay layers
Fatigue cracking: Tensile residual stresses reduce the fatigue life of the overlay layer under cyclic loading conditions
Hydrogen-induced cracking: Residual stresses can promote hydrogen-induced cracking in high-strength overlay materials

Process Optimization Strategies

Based on the residual stress analysis, several process optimization strategies can be identified:

Strategy	Effect on Residual Stress	Implementation
Post-weld heat treatment (PWHT)	Reduces peak stresses by 30-50%	Stress relief annealing at 550-650 °C
Interpass temperature control	Reduces thermal gradient	Maintain interpass at 150-250 °C
Weld sequence optimization	Reduces peak stresses	Weld from center to edges or vice versa
Peening	Introduces surface compressive stress	Shot peening or hammer peening after welding
Multi-pass welding	Reduces peak stresses per pass	Use multiple thin passes instead of single thick pass

Quality Assurance Considerations

For nuclear island main equipment, the residual stress analysis must be integrated into the quality assurance program:

Welding Procedure Qualification (WPQ): Residual stress data should be included in the WPQ documentation
Non-Destructive Examination (NDE): Residual stress maps guide the selection of NDE methods and inspection areas
Fitness-for-Service (FFS) Assessment: Residual stress data is essential for FFS assessments of in-service nuclear components
Lifetime Assessment: Residual stresses affect the predicted lifetime of nuclear components under creep, fatigue, and SCC conditions

Key Questions and Reflections

Questions for Further Investigation

How do the residual stresses evolve during long-term service under creep conditions? The initial residual stress state may relax significantly over the design life of nuclear components.
What is the effect of residual stresses on the initiation and propagation of stress corrosion cracks in the overlay layer? This is a critical concern for austenitic stainless steel overlays in corrosive environments.
How effective are post-weld heat treatment procedures in reducing residual stresses without adversely affecting the overlay layer microstructure and properties?
Can the residual stress analysis be extended to predict the effects of different welding sequences on multi-pass overlay welds?

Independent Thinking

This research demonstrates the critical importance of residual stress analysis in nuclear-grade overlay welding. In conventional industrial applications, residual stresses are often treated as a secondary concern, but in nuclear applications, they must be carefully characterized and controlled as part of the safety case.

The combination of finite element simulation and experimental measurement provides a powerful approach to understanding residual stress distributions. The simulation provides a continuous stress field that can be used for design optimization, while the experimental measurements provide validation and ground truth data.

From a practical standpoint, the residual stress data presented in this paper can be directly applied to:

Optimizing welding procedures to minimize cracking susceptibility
Selecting appropriate post-weld heat treatment parameters
Guiding NDE inspection planning and acceptance criteria
Supporting fitness-for-service assessments of in-service nuclear components

The methodology developed in this research is also applicable to other high-integrity overlay welding applications, such as pressure vessel heads, heat exchanger tubesheets, and piping components in the petrochemical and power generation industries.

Summary

This research provides a comprehensive characterization of residual stress distributions in strip electrode submerged arc overlay welding layers applied to nuclear island main equipment. Through the combined use of finite element simulation and small diameter blind hole experimental measurement, the study reveals complex multi-axial stress states with tensile stresses reaching 300-400 MPa near the fusion boundary and compressive stresses in the center of the overlay layer. These residual stress patterns have significant implications for defect prevention, including cracking, stress corrosion cracking, and fatigue cracking. The research provides a theoretical foundation for process optimization strategies, including post-weld heat treatment, interpass temperature control, and weld sequence optimization. For nuclear applications, where the consequences of failure are unacceptable, this level of residual stress characterization is essential for ensuring long-term structural integrity and regulatory compliance. The methodology and findings of this research are directly applicable to other high-integrity overlay welding applications in the nuclear, petrochemical, and power generation industries.