Zhuojin Pipe Fitting Co., LtdNickel-Based Alloy Tape Electrode Electroslag Overlay Welding for Nuclear Power Equipment
Literature Overview
This paper by Li Shuangyan from Shanghai Electric Nuclear Power Equipment Co., Ltd., published in Pressure Vessel in 2011 (Volume 28, Issue 3, pages 33-37), describes the application of tape electrode electroslag overlay welding for Inconel 690 nickel-based alloy on the tube sheets of steam generators in pressurized water reactor (PWR) nuclear island main equipment. The study covers welding procedure qualification, mock-up testing, and product application, demonstrating the feasibility of this process for large-area nickel-based alloy overlay in nuclear applications.
Process Description and Technical Rationale
Electroslag welding (ESW) with tape electrodes is a high-deposition-rate process that offers unique advantages for overlay welding large, flat areas such as steam generator tube sheets. The electroslag process operates by passing current through a molten slag pool, which provides the heat for melting the base metal and filler material. This fundamentally different heat source mechanism compared to arc welding processes results in lower cooling rates, which is beneficial for reducing residual stress and cracking susceptibility in thick sections.
Inconel 690 is a nickel-chromium-iron alloy specifically designed for nuclear steam generator tubing applications due to its excellent resistance to stress corrosion cracking (SCC) in primary coolant environments. The overlay of Inconel 690 on carbon steel tube sheets creates a corrosion-resistant barrier that protects the base metal from the aggressive reactor coolant environment.
| Parameter | Typical Value | Rationale |
|---|---|---|
| Tape width | 60-90 mm | High deposition rate for large areas |
| Tape thickness | 0.5-1.0 mm | Adequate feeding characteristics |
| Current | 800-1200 A | Sufficient heat for slag pool maintenance |
| Voltage | 30-40 V | Arc stability and penetration control |
| Travel speed | 100-200 mm/min | Balance between deposition and quality |
| Preheat temperature | 150-250°C | Reduce cracking susceptibility |
Nuclear Quality Requirements and Qualification
Nuclear equipment fabrication demands the highest level of welding quality assurance, governed by stringent codes such as ASME Section III, RCC-M, and applicable national standards. The qualification process described in this paper follows the typical three-stage approach: welding procedure qualification (WPQ), mock-up testing, and product application verification.
The WPQ stage establishes the essential variables and performance requirements for the welding procedure. For nuclear applications, these variables are more restrictive than for conventional industrial welding, with tighter control on parameters such as heat input, interpass temperature, and welding sequence. The mock-up testing stage verifies that the qualified procedure produces acceptable weld quality on a representative component, including full non-destructive examination (NDE) and destructive testing.
The product application stage confirms that the procedure can be successfully executed on actual production components, accounting for factors such as component geometry, access constraints, and production schedule pressures. The successful transition from qualification to production demonstrates that the process is robust enough to withstand the variability inherent in manufacturing operations.
Engineering Practice Considerations
The application of electroslag overlay welding to nuclear steam generator tube sheets presents several unique challenges. The tube sheet is a thick carbon steel component with thousands of tube holes, and the overlay must be applied uniformly across the entire surface without damaging the tube holes or creating distortion that could compromise tube-to-tube-sheet joints.
Residual stress management is critical in this application. The large thermal gradient between the nickel-based overlay and the carbon steel substrate creates significant residual stresses that could contribute to stress corrosion cracking of the overlay or distortion of the tube sheet. Post-weld stress relief heat treatment is typically required, but must be carefully controlled to avoid sensitization or other adverse microstructural changes.
The NDE requirements for nuclear welds are extensive, typically including 100% ultrasonic testing (UT) for volumetric defects, magnetic particle testing (MT) or penetrant testing (PT) for surface defects, and potentially radiographic testing (RT) for critical areas. The tape electrode electroslag process must be capable of producing welds that consistently pass these stringent inspection requirements.
Key Reflections
The use of electroslag welding for nuclear overlay applications represents a significant engineering achievement, as this process is not commonly associated with nuclear fabrication. The high deposition rate and low dilution characteristics of the process are well-suited to the requirements of large-area overlay on thick sections. However, engineers should be aware that electroslag welding produces a different weld metal microstructure compared to arc welding processes, with coarser grain structures that may affect mechanical properties and corrosion resistance. Additionally, the process requires specialized equipment and operator training, which may limit its applicability in some fabrication environments.
Summary
This paper demonstrates the successful application of tape electrode electroslag overlay welding for Inconel 690 on PWR steam generator tube sheets, providing a viable process for large-area nickel-based alloy overlay in nuclear applications. The comprehensive qualification approach and the successful product application confirm that this process can meet the stringent quality requirements of nuclear equipment fabrication, offering a productive alternative to conventional arc welding methods for this specific application.