Residual Stress Distribution in Submerged Arc Overlay Welding on Nuclear Island Main Equipment

Literature Overview

This paper by Wu Yidang and colleagues from CGN Engineering Co., Ltd., published in the Journal of Nanchang Hangkong University in 2016, investigates the residual stress distribution in submerged arc overlay welding 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 at different locations within the overlay weld layer. The study provides a theoretical basis for preventing defects in nuclear-grade overlay welds and offers guidance for process optimization.

Nuclear Island Equipment and Overlay Welding Context

Nuclear island main equipment, including reactor pressure vessels, steam generators, and various piping components, must meet exceptionally stringent quality and safety requirements. Overlay welding is employed on these components to provide corrosion resistance, erosion resistance, or to repair surface damage. The choice of submerged arc welding with strip electrode (SAW-SE) is driven by the need for high deposition rates, consistent quality, and the ability to produce thick overlay layers in a single pass or few passes.

The residual stress state in overlay welds is of particular concern in nuclear applications because it can influence the initiation and propagation of cracks, particularly under thermal cycling and mechanical loading conditions. Unlike conventional welds where residual stress is a secondary consideration, in nuclear applications it must be explicitly characterized and managed as part of the quality assurance framework.

Finite Element Simulation Methodology

The finite element analysis was conducted to predict the residual stress distribution throughout the overlay weld layer. The simulation incorporated the thermal-mechanical coupling inherent to the welding process, accounting for the temperature-dependent material properties of both the base metal and the overlay metal. The SAW-SE process parameters, including heat input, travel speed, and electrode configuration, were modeled to reproduce the actual thermal cycle experienced by the weld metal.

The finite element model captures the progressive nature of residual stress development during multi-pass overlay welding. Each successive pass modifies the stress state established by previous passes, and the final residual stress distribution reflects the cumulative effect of all passes. This progressive accumulation is critical for understanding why multi-pass overlay welds often exhibit higher residual stresses than single-pass welds.

Experimental Measurement and Validation

The small diameter blind hole method was employed for experimental measurement of residual stress. This method involves drilling a small diameter hole into the surface of the weld and measuring the strain relief around the hole using strain gauges. The measured strain is then converted to residual stress using appropriate calibration factors.

The blind hole method was selected because it provides point measurements that can be taken at specific locations within the weld layer, allowing for detailed spatial characterization of the stress field. Multiple measurement points were established at different locations and depths within the overlay layer to capture the three-dimensional stress distribution.

Comparison of Simulation and Experimental Results

The comparison between finite element predictions and experimental measurements revealed good agreement in terms of the overall stress distribution pattern. Both methods identified the weld centerline as the region of maximum tensile residual stress, with compressive stresses developing at the outer edges of the weld to maintain equilibrium. The magnitude of the residual stresses, as determined by both methods, provides a quantitative basis for assessing the risk of crack initiation and propagation.

Discrepancies between simulation and experimental results were observed at certain locations, particularly near the fusion line where the metallurgical complexity is greatest. These discrepancies are attributed to the simplified material property models used in the finite element analysis, which cannot fully capture the complex phase transformations and microstructural variations that occur in the heat-affected zone.

Engineering Implications for Nuclear Applications

The residual stress data obtained from this study has direct implications for the qualification and licensing of nuclear island equipment. Regulatory frameworks for nuclear components require explicit consideration of residual stresses in fitness-for-service assessments, and the quantitative data provided by this research supports these assessments with reliable measurements.

The identification of high-stress regions within the overlay weld layer provides guidance for non-destructive examination planning. Regions of high tensile residual stress are more susceptible to stress corrosion cracking and fatigue crack initiation, and these regions should be prioritized for detailed NDE inspection. The process optimization recommendations derived from this study can help reduce peak residual stresses through parameter adjustment, post-weld heat treatment, or mechanical stress relief techniques.

Key Questions and Reflective Analysis

A significant question arising from this work is the long-term stability of the residual stress field under service conditions. Nuclear island equipment experiences thermal cycling, mechanical loading, and irradiation, all of which can cause stress relaxation and redistribution. The residual stress measurements reported in this study represent the as-welded condition, and understanding the evolution of this stress state under service conditions is essential for lifetime predictions.

Another consideration is the interaction between residual stresses and the metallurgical properties of the overlay weld. High residual stresses in a hard, brittle overlay metal are particularly detrimental, as the combination of high stress and low toughness promotes brittle fracture. The process optimization strategies suggested by this study should be evaluated in conjunction with the metallurgical properties of the specific overlay metal used.

Summary

This paper provides valuable quantitative data on the residual stress distribution in submerged arc overlay welding applied to nuclear island main equipment. The combination of finite element simulation and experimental measurement using the blind hole method offers a comprehensive characterization of the stress field that supports both quality assurance and fitness-for-service assessments. The findings have direct implications for nuclear regulatory compliance and provide a foundation for process optimization aimed at reducing residual stress-related risks. The work underscores the importance of residual stress management in nuclear applications, where the consequences of failure are unacceptable and the margins for error are minimal.