Residual Stress Distribution in Strip Electrode Submerged Arc Surfacing of Nuclear Island Main Equipment

Literature Overview

The paper by Wu Yidang and colleagues from CGN Engineering Co., Ltd. presents a comprehensive investigation of residual stress distribution in the surfacing layers of nuclear island main equipment, specifically using strip electrode submerged arc surfacing (SESAS). Nuclear island main equipment—such as reactor pressure vessels, steam generators, and main piping—requires extensive surfacing of corrosion-resistant alloy layers to ensure long-term integrity under nuclear service conditions. The surfacing process introduces significant residual stresses that can compromise the integrity of the deposited layer and the base material. This study combines finite element simulation with experimental measurement to characterize the residual stress field and provide a theoretical basis for process optimization and defect prevention.

Methodology and Technical Approach

The research employed a dual methodology combining computational modeling and experimental validation:

The blind hole method is a well-established residual stress measurement technique that involves drilling a small diameter hole (typically 1–3 mm) in the surface of interest and measuring the elastic strain relief around the hole using strain gauges. The measured strain is then converted to residual stress using calibration factors that account for material properties, hole geometry, and stress depth. This method is particularly suitable for measuring surface residual stresses in thick sections, which is the typical configuration of nuclear island main equipment.

Residual Stress Characteristics

The strip electrode submerged arc surfacing process generates residual stresses through two primary mechanisms: thermal contraction during cooling and phase transformation strains in the deposited metal. The following table summarizes the typical residual stress values observed:

The residual stress distribution exhibits several important features. First, the maximum tensile stress occurs at the fusion line interface, where the combination of thermal contraction and phase transformation strains is most severe. Second, the longitudinal stress (in the direction of welding travel) is consistently higher than the transverse stress, reflecting the anisotropic nature of the welding thermal cycle. Third, the stress magnitude decreases with increasing distance from the fusion line in both the deposited layer and the base metal HAZ.

Engineering Implications for Nuclear Equipment

The residual stress field in nuclear island main equipment surfacing has direct implications for:

• Stress corrosion cracking (SCC) susceptibility: High tensile residual stresses at the fusion line interface increase the driving force for SCC initiation in sensitized austenitic stainless steel cladding layers
• Fatigue crack initiation: Cyclic loading superimposed on high residual stresses reduces the fatigue life of the surfacing layer, particularly in regions subject to thermal cycling during reactor operation
• Dimensional stability: Residual stresses cause distortion and warpage of the component, which can affect assembly tolerances and fit-up quality
• Post-weld heat treatment (PWHT) effectiveness: The residual stress distribution determines the effectiveness of stress relief annealing and the risk of re-introducing stresses during cooling

The study provides a theoretical basis for optimizing the surfacing sequence and layer arrangement to minimize peak residual stresses. For example, alternating the welding direction between layers, reducing the number of layers in a single pass, and controlling the inter-pass temperature are all strategies that can be informed by the residual stress simulation results.

Process Optimization Recommendations

Based on the residual stress analysis, the following process optimization strategies are recommended:

1. Layer sequence optimization: Implement a symmetric layer sequence to balance the thermal input and reduce net distortion
2. Inter-pass temperature control: Maintain inter-pass temperature between 150–250°C to reduce thermal gradients and stress accumulation
3. Welding direction alternation: Alternate the welding direction between consecutive layers to partially compensate for longitudinal stress buildup
4. Post-weld stress relief: Apply a comprehensive PWHT cycle (typically 600–650°C for austenitic stainless steel cladding) to reduce residual stresses to below 100 MPa
5. Layer thickness control: Limit individual layer thickness to 3–5 mm to reduce the thermal gradient within each pass

Connection to Pipe and Fitting Standards

The residual stress management principles discussed in this paper are directly applicable to the surfacing of nuclear-grade piping components governed by ASME B31.12 (Nuclear Piping and Components) and NB/T 20001 series standards (Chinese nuclear industry standards). The control of residual stresses in nuclear piping surfacing is critical for ensuring resistance to stress corrosion cracking in chloride-containing environments and for maintaining the integrity of the corrosion-resistant overlay during the 40–60 year design life of nuclear facilities.

Study Insights and Outlook

The integration of finite element simulation with experimental validation provides a robust framework for residual stress analysis that can be extended to other nuclear equipment surfacing applications. The study demonstrates that the fusion line interface is the most critical location for residual stress concentration, which aligns with the observation that most surfacing-related failures in nuclear service initiate at or near the fusion line. Future work should focus on developing predictive models that can be integrated into process planning software to enable real-time residual stress monitoring and control during the surfacing operation. The results of this study provide a valuable reference for establishing residual stress acceptance criteria in nuclear equipment surfacing quality assurance programs.