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

Literature Overview

This study by Li Shuangyan, published in Pressure Vessel Technology (Volume 28, Issue 3, 2011, pages 33-37), investigates the application of strip electrode electroslag surfacing (ESS) for nickel-based alloy overlay on steam generator tubesheets in pressurized water reactor (PWR) nuclear power plants. Using Inconel 690 as the model alloy, the research demonstrates the feasibility of ESS for large-area nickel-based alloy surfacing through process qualification, simulation trials, and product application. This work is of critical importance to nuclear power engineering, as steam generator tubesheet overlay is a key component requiring high-quality nickel-based alloy protection against corrosion and stress corrosion cracking.

Technical Background

Nuclear Power Equipment Requirements

Steam generators in PWR nuclear power plants are critical safety-related components that transfer heat from the primary (reactor) coolant to the secondary (steam) side. The tubesheet, which holds thousands of heat transfer tubes, is exposed to:

• High-temperature water (280-330°C)
• High pressure (15-17 MPa)
• Boric acid and lithium hydroxide chemical treatment
• Potential chloride contamination
• Thermal cycling during reactor operation

The tubesheet overlay must provide:

• Resistance to stress corrosion cracking (SCC)
• Resistance to general and localized corrosion
• Adequate mechanical properties at operating temperature
• Long service life (30-60 years)

Inconel 690 Properties

Inconel 690 (UNS N06690) is the standard alloy for PWR steam generator tubes and tubesheet overlays. Its key properties include:

Electroslag Surfacing Process Analysis

Process Principle

Electroslag surfacing is a specialized welding process that uses a strip electrode and flux to deposit thick overlays with high productivity and excellent metallurgical quality. The process involves:

1. Slag pool formation: A pool of molten slag is maintained on the substrate surface.
2. Electrode feeding: The strip electrode is fed into the slag pool, where it melts and deposits onto the substrate.
3. Heat transfer: Heat is transferred primarily through the slag pool, resulting in uniform heating and reduced thermal gradients.
4. Solidification: The deposited metal solidifies beneath the slag pool, producing a fine-grained microstructure.

Process Advantages for Nuclear Applications

Process Parameters

The study establishes the following process parameters for Inconel 690 ESS:

Quality Verification and Acceptance Criteria

Nuclear power applications require rigorous quality verification. The study employs the following testing methods:

Nuclear-Specific Requirements

Nuclear power applications impose additional requirements beyond conventional industrial standards:

1. Traceability: All consumables, equipment, and operators must be fully traceable throughout the manufacturing process.
2. Procedure qualification: Welding procedure qualification must follow nuclear-specific standards (ASME Section III, RCC-M, or equivalent).
3. Operator qualification: Welders and operators must be qualified through nuclear-specific certification programs.
4. Quality assurance: Manufacturing must comply with nuclear quality assurance standards (ASME NQA-1, RCC-M, or equivalent).
5. Documentation: Complete documentation of all process parameters, test results, and quality records is required.

Integration with Engineering Practice

Steam Generator Tubesheet Overlay Application

The steam generator tubesheet is typically a thick carbon steel or low-alloy steel plate (100-200mm thick) with thousands of tube holes. The overlay covers the secondary-side surface (exposed to secondary coolant) and provides corrosion and SCC protection.

Typical overlay specifications:

Process Implementation Sequence

The study demonstrates the following implementation sequence for steam generator tubesheet overlay:

1. Substrate preparation: Grind and clean the tubesheet surface, removing oxide, paint, and contamination.
2. Process qualification: Qualify the welding procedure using simulation specimens that replicate the actual tubesheet geometry and material.
3. Preheat: Apply uniform preheat to the tubesheet surface using induction heating or radiant heaters.
4. Overlay application: Apply the Inconel 690 overlay using strip electrode ESS, following the qualified procedure.
5. Post-weld inspection: Perform NDT, dimensional measurement, and mechanical testing to verify overlay quality.
6. Post-weld heat treatment: If required, perform PWHT to reduce residual stress and optimize microstructure.
7. Final inspection: Complete final quality verification and documentation.

Productivity and Economics

Strip electrode ESS offers significant productivity advantages for large-area overlay applications:

The productivity advantage of ESS makes it economically attractive for nuclear power applications, where the cost of labor and quality assurance documentation is substantial.

Key Questions and Reflections

The study raises several important questions for nuclear power engineering:

• How does the ESS process perform on tubesheets with tube holes? The presence of tube holes creates geometric discontinuities that may affect slag pool stability and deposit quality.
• What is the long-term performance of the overlay under actual nuclear service conditions? Laboratory testing may not fully capture the complex degradation mechanisms experienced in reactor service.
• How does the overlay quality vary with substrate thickness? Thicker substrates may require different parameter settings to achieve the same overlay quality.
• What are the implications of the overlay for the subsequent tube installation and welding? The overlay must provide a suitable surface for tube-to-tubesheet welding.

The study's demonstration of ESS feasibility for nuclear power applications is significant, as it provides an alternative to the traditional wire electrode methods that have been used for decades. The productivity and quality advantages of ESS make it a compelling option for future nuclear power projects.

Study Insights and Implications

This research demonstrates that strip electrode electroslag surfacing is a viable and effective technology for nickel-based alloy overlay on nuclear power equipment. The key insight is that ESS combines the metallurgical quality of electroslag welding with the productivity of strip electrode technology, making it suitable for large-area overlay applications in the nuclear power industry.

The study's systematic approach to process qualification, simulation testing, and product application provides a model for nuclear power manufacturing. The emphasis on quality verification and traceability aligns with nuclear quality assurance requirements, ensuring that the overlay meets the rigorous standards required for safety-related components.

The demonstrated ability to produce high-quality Inconel 690 overlays using ESS confirms that this method is suitable for steam generator tubesheet applications. The productivity advantages of ESS over conventional methods are substantial, making this technology economically attractive for nuclear power projects where manufacturing efficiency is critical.

The study contributes to the advancement of nuclear power manufacturing technology by demonstrating an innovative approach to a critical component. The successful application of ESS for Inconel 690 overlay represents a significant step forward in nuclear power equipment manufacturing, offering improved productivity, quality, and cost-effectiveness for future nuclear power projects.