Zhuojin Pipe Fitting Co., LtdTubesheet Overlay Welding Technology for 1000MW PWR Steam Generator
Literature Overview
The paper by Li Shuangyan (2011), published in Welding (No. 10, pp. 36–41), documents the tubesheet overlay welding technology employed in the fabrication of steam generators (SG) for 1000 MW-class pressurized water reactor (PWR) nuclear power plants. The work was conducted at Shanghai Electric Nuclear Power Equipment Co., Ltd., a leading Chinese nuclear equipment manufacturer. This paper is of considerable significance given the critical role of the SG in PWR safety systems and the stringent quality requirements of nuclear-grade manufacturing.
Core Technical Content
Steam Generator Tubesheet Function and Requirements
The SG tubesheet is a large-diameter, thick-walled component (typically 300–500 mm thick, 2000–3000 mm diameter for 1000 MW class) that serves multiple critical functions:
- Primary/secondary boundary: Provides pressure containment between the primary coolant circuit and the secondary steam-generating side.
- Tube support: Houses thousands of U-tubes (typically 10,000–14,000 tubes) that form the heat exchange surface.
- Corrosion barrier: The overlay layer provides corrosion resistance against the secondary side (feedwater/steam).
- Welding substrate: Serves as the base material for tube-to-tubesheet welds, which are critical safety-related joints.
The tubesheet base material is typically a carbon steel or low-alloy steel (such as SA-516 Gr. 70, 15CrMoR, or equivalent Chinese grades), while the overlay material is a stainless steel (typically 304L or 316L) to resist corrosion from the secondary side.
Overlay Welding Process Configuration
The paper describes a dual-process approach for tubesheet overlay:
| Region | Process | Parameters | Rationale |
|---|---|---|---|
| Tubesheet plane (flat surface) | Electrode strip electroslag welding (ESW) | Current: 3000–5000 A; Voltage: 30–40 V; Travel speed: 50–100 mm/min | High deposition rate, uniform thickness, low dilution |
| Tubesheet edge (curved surface) | SMAW (shielded metal arc welding) | E309L electrodes; Preheat: 150–200 °C | Flexibility for curved geometry, accessibility |
The electrode strip ESW process is particularly advantageous for large flat surfaces because:
- It provides extremely high deposition rates (5–10 kg/h per electrode)
- The slag pool provides excellent shielding and atmospheric protection
- Dilution is inherently low due to the self-shielding slag layer
- The process is highly repeatable and amenable to automated control
Quality Assurance and Testing Requirements
Nuclear-grade overlay welding requires extensive qualification and testing:
| Test Category | Standard/Method | Acceptance Criteria |
|---|---|---|
| Weld procedure qualification (WPQ) | ASME Section IX, AWS D10.9 | Full qualification with mechanical, metallurgical, and NDE testing |
| Radiographic testing (RT) | ASME Section V, T-274 | No lack of fusion, cracks, or excessive porosity |
| Ultrasonic testing (UT) | ASME Section V, T-428 | No internal defects exceeding acceptance criteria |
| Magnetic particle testing (MT) | ASME Section V, T-771 | No surface cracks, lack of fusion |
| Hardness testing | ASTM E18 (Rockwell) | Within ±50 HV of base metal, no localized hardening |
| Intergranular corrosion (IGC) | ASTM A262 Practice E | No intergranular attack after 24h exposure |
| Tensile testing | ASTM E8 | Overlay weld metal meets minimum strength requirements |
| Impact testing | ASTM E23 | Minimum 20 J at -29 °C (or service temperature) |
Tubesheet-to-Tube Weld Interface Considerations
The overlay layer serves as the substrate for the critical tube-to-tubesheet welds. The quality of the overlay directly affects:
- Weldability of subsequent tube welds: Poor overlay quality (porosity, lack of fusion) propagates into tube welds.
- Corrosion resistance at the weld toe: Any discontinuity in the overlay creates a corrosion initiation site.
- Residual stress distribution: Overlay welding residual stresses affect the stress state of subsequent tube welds.
The paper recommends automatic overlay welding methods (such as ESW or automated GTAW) for the overlay to ensure uniformity, and specifies that overlay repairs should use GTAW (argon arc welding) to minimize thermal input and dilution.
Engineering Practice and Nuclear Safety Context
The SG tubesheet overlay is classified as a Class 1 weld in nuclear safety terms, requiring:
- Complete welder qualification: Each welder must be individually qualified for the specific process, position, and material combination.
- Weld procedure specification (WPS): Must be qualified through full-scale test plates with comprehensive testing.
- In-process monitoring: Real-time monitoring of welding parameters with automatic shutoff for parameter excursions.
- 100% NDE coverage: Both volumetric (RT/UT) and surface (MT/PT) testing on all overlay welds.
- Documentation and traceability: Complete records of all materials, procedures, welders, and test results maintained for the life of the plant (60+ years).
The manufacturing timeline for a single SG tubesheet overlay can extend over several weeks due to the sequential nature of multi-pass welding, interpass NDE, and repair cycles. Any repair requires documented justification and potentially requalification of the welding procedure.
Key Technical Insights and Reflections
The dual-process approach (ESW for flat areas, SMAW for edges) represents a practical optimization that balances production efficiency with geometric flexibility. The ESW process, while less flexible in terms of geometry, provides superior productivity and quality consistency for large flat surfaces—a critical advantage when manufacturing hundreds of tubesheets for a fleet of nuclear power plants.
The emphasis on automatic welding methods reflects the nuclear industry's philosophy of minimizing human variability in safety-critical operations. While manual welding can achieve excellent results in skilled hands, the consistency required for nuclear applications favors automated or semi-automated processes with parameter monitoring and recording capabilities.
The overlay-to-tube-weld interface is perhaps the most technically challenging aspect of this application. The overlay layer must provide a clean, defect-free surface for tube welding, while also maintaining its own structural integrity and corrosion resistance. Any compromise in overlay quality directly impacts the safety case for the entire steam generator.
Study Insights and Implications
This paper provides valuable documentation of nuclear-grade overlay welding technology as practiced in Chinese nuclear power plant fabrication. The systematic approach to quality assurance, the careful selection of welding processes for different geometries, and the emphasis on the overlay-to-tube-weld interface all reflect the mature engineering practices of the nuclear industry. For engineers entering the nuclear fabrication field, this work demonstrates that overlay welding in nuclear applications is not merely a manufacturing operation but a safety-critical process requiring rigorous qualification, monitoring, and documentation throughout the entire manufacturing lifecycle.