Nickel-Based Alloy Strip Electrode Electroslag Overlay for Nuclear Power Equipment

Literature Overview and Nuclear Industry Context

The paper by Li Shuangyan from Shanghai Electric Nuclear Power Equipment Co., published in Pressure Vessel in 2011, addresses the application of strip electrode electroslag welding (ESW) overlay for nickel-based alloy deposits on steam generator tube sheets in pressurized water reactor (PWR) nuclear power plants. The specific alloy studied is Inconel 690, which is the standard cladding material for PWR steam generator tubes due to its exceptional resistance to stress corrosion cracking (SCC) in high-temperature, high-pressure water environments.

The steam generator tube sheet is a critical component that interfaces the primary coolant side with the secondary steam side. The tube sheet holes must be overlaid with Inconel 690 to ensure metallurgical compatibility with the Inconel 690 tubes and to provide a barrier against primary coolant corrosion. The large surface area of the tube sheet and the demanding quality requirements of nuclear applications make this a particularly challenging overlay task.

Electroslag Overlay Process Principles

Electroslag welding is a high-deposition-rate process in which the heat for melting is generated by the electrical resistance of a molten slag pool rather than by the arc itself. In strip electrode ESW, a continuous strip of metal serves as both the electrode and the filler material. The process is inherently well-suited for building up thick, uniform deposits on large flat or gently curved surfaces because:

• The slag pool provides excellent heat insulation, resulting in a deep, stable molten pool with minimal spatter
• The continuous strip electrode ensures consistent composition and geometry of the deposited metal
• The high deposition rate (typically 3–5 kg/h) significantly reduces production time compared to arc welding processes
• The slag covering protects the molten pool from atmospheric contamination, reducing inclusion formation

Process Qualification and Simulation Testing

The paper describes a rigorous qualification program consisting of three stages: process qualification testing, simulation piece trials, and product application verification. This staged approach is consistent with nuclear industry quality assurance practices, which require extensive demonstration of process capability before production application.

The process qualification stage involved systematic variation of welding parameters to establish the acceptable process window. Key quality criteria included:

• Deposition geometry: Uniform thickness and smooth surface finish without undercut, excessive reinforcement, or lack of fusion
• Chemical composition: Inconel 690 composition within ASTM B637 specification limits, with particular attention to Ni (50–55%), Cr (22–28%), and Fe (balance)
• Microstructure: Predominantly austenitic structure with controlled delta ferrite content (typically below 5%) to minimize the risk of solidification cracking
• Mechanical properties: Tensile strength and hardness consistent with Inconel 690 specifications
• Non-destructive testing: Full radiographic or ultrasonic inspection to detect internal defects

Nuclear-Specific Quality Requirements

Nuclear applications impose quality requirements that far exceed those of conventional industrial welding. For the steam generator tube sheet overlay, the following nuclear-specific considerations apply:

• Code compliance: The welding procedure must qualify under applicable nuclear codes such as ASME Section III, Appendix X or GB/T 19146 for Chinese nuclear plants
• Operator qualification: Welders must hold current nuclear welder qualifications with documented performance records
• Traceability: Complete traceability of all consumables, equipment, and operator records is mandatory
• Defect acceptance criteria: Nuclear codes typically impose stricter defect acceptance criteria than industrial codes, requiring more extensive NDT coverage
• Post-weld heat treatment: PWHT may be required to relieve residual stresses and stabilize the microstructure

Engineering Practice Reflections

The successful application of strip electrode ESW for Inconel 690 overlay on nuclear steam generator tube sheets represents a significant engineering achievement. The process combines the high productivity of electroslag welding with the metallurgical quality required for nuclear service. From a practical standpoint, the key success factors are:

1. Maintaining consistent slag pool stability through precise current and travel speed control
2. Ensuring adequate preheating to minimize thermal stresses and prevent cold cracking in the nickel-based alloy
3. Implementing rigorous interpass cleaning to prevent slag inclusion entrapment between layers
4. Conducting full volumetric NDT (radiographic or phased array ultrasonic testing) to detect any internal defects

The staged qualification approach described in the paper—starting from parameter optimization, moving to simulation trials, and culminating in product application—is a model that should be adopted for all critical nuclear overlay projects. It provides a structured path to confidence in the process while managing the inherent risks of working with nickel-based alloys, which are notoriously susceptible to cracking.

In conclusion, this paper demonstrates that strip electrode electroslag welding is a viable and productive method for applying Inconel 690 overlays to nuclear steam generator tube sheets, provided that the qualification program is thorough and the process parameters are tightly controlled. The work is a valuable reference for engineers involved in nuclear equipment fabrication and repair, particularly those tasked with developing or qualifying overlay processes for critical nuclear components.