Peel Mechanism Analysis of Stainless Steel Strip Electroslag Welding Overlay on Nuclear Island Main Equipment - A Technical Study Note

Literature Overview and Background

The nuclear island main equipment — including reactor pressure vessels (RPVs), steam generators, pressurizers, and main coolant piping — demands the highest level of material integrity and weld quality in the nuclear industry. Stainless steel strip electroslag welding (ESW) overlay is a widely used technique for creating corrosion-resistant cladding layers on carbon steel or low-alloy steel substrates that form the pressure boundary of these critical components. The peel mechanism — referring to the separation or delamination of the overlay layer from the substrate — represents a fundamental failure mode that can compromise the integrity of the entire pressure boundary.

This literature review focuses on the metallurgical and mechanical mechanisms responsible for overlay layer peeling during ESW cladding, including thermally induced cracking, interfacial bonding failures, and residual stress-driven delamination. Understanding these mechanisms is essential for optimizing the ESW overlay process parameters and ensuring long-term reliability of nuclear-grade bimetallic components.

Core Failure Mechanisms

Interfacial Bonding and Dilution Control

The interface between the stainless steel overlay and the carbon steel substrate is the weakest link in the bimetallic structure. During ESW overlay, the electroslag pool achieves temperatures of 1600-1900°C, creating a deep, uniform melt pool that ensures metallurgical bonding. However, the dilution rate — the percentage of base metal melted into the weld metal — critically affects the interface quality.

For nuclear-grade ESW overlay, the dilution rate must be tightly controlled to ensure the overlay meets the specified chemical composition. Typical requirements include:

Excessive dilution in the first layer can lead to the formation of a hard, brittle intermetallic layer (such as Fe-Cr sigma phase) at the interface, which becomes a preferential crack initiation site under thermal cycling or mechanical loading. Insufficient dilution, on the other hand, may result in incomplete fusion or a weak mechanical bond.

Residual Stress-Driven Peeling

The ESW process generates significant residual stresses due to the large heat input and directional solidification pattern. The overlay layer experiences compressive stresses in the transverse direction and tensile stresses in the longitudinal direction, creating a stress state that can drive interfacial delamination.

The residual stress distribution in ESW overlay is characterized by:

• Longitudinal stress: Tensile, reaching 100-200 MPa in the overlay layer
• Transverse stress: Compressive, reaching -50 to -150 MPa
• Through-thickness stress: Compressive at the surface, tensile at the interface

The tensile through-thickness stress at the interface is the primary driver of peel-type failures. When this stress exceeds the interfacial fracture toughness of the bond, crack initiation occurs at the interface and propagates along the overlay-substrate boundary.

Thermally Induced Cracking

Nuclear island equipment is subjected to significant thermal cycling during operation, including:

• Normal operating cycles (hot start to cold shutdown)
• Transient events (loss of coolant accidents, reactivity insertions)
• Steam generator tube failures (for SG tubes with ESW overlay)

The thermal cycling creates cyclic thermal stresses at the overlay interface due to the mismatch in thermal expansion coefficients between the stainless steel overlay (α ≈ 17.5 × 10⁻⁶ /K for 304L) and the carbon steel substrate (α ≈ 12 × 10⁻⁶ /K). This mismatch generates a cyclic tensile stress at the interface that can drive fatigue crack growth and eventual peel separation.

Process Optimization for Peel Prevention

ESW Parameter Optimization

The ESW overlay process parameters must be carefully optimized to minimize the risk of peel failure:

Multi-Pass Strategy

A multi-pass overlay strategy is essential for achieving both metallurgical bonding and composition control:

1. First pass (bonding pass): Higher current, lower travel speed to achieve adequate penetration and metallurgical bonding. Dilution rate target: 20-35%.
2. Second pass (transition pass): Moderate parameters to transition from high-dilution to low-dilution composition. Dilution rate target: 10-20%.
3. Subsequent passes (build-up passes): Lower current, higher travel speed to minimize dilution and maintain overlay composition. Dilution rate target: 5-10%.
4. Final pass (surface pass): Optimized for surface quality and dimensional accuracy. May use GTAW for final surface finishing.

Post-Weld Heat Treatment

Post-weld stress relief (PWSR) is a critical step in preventing peel failures. The recommended PWSR parameters for ESW overlay on nuclear island components are:

The PWSR treatment reduces residual stresses by 70-90%, significantly lowering the driving force for interfacial delamination. However, the PWSR temperature must be carefully selected to avoid sensitization of the austenitic stainless steel overlay (which would reduce corrosion resistance) or precipitation hardening of the low-alloy substrate (which would reduce toughness).

Quality Verification and Acceptance Criteria

Non-Destructive Testing

Comprehensive NDT is mandatory for ESW overlay on nuclear island components:

• Ultrasonic testing (UT): TOFD or phased array UT (PAUT) for interfacial bond verification. The overlay-substrate interface must be free of lack of fusion, cracks, and delamination.
• Eddy current testing (ET): For surface and near-surface defect detection in the overlay layer.
• Visual testing (VT): For surface quality, porosity, and dimensional verification.
• Radiographic testing (RT): For volumetric flaw detection in thicker overlay sections.

Mechanical Property Testing

Metallographic Analysis

Metallographic examination of the overlay interface provides critical information about bonding quality and microstructural integrity. The examination should include:

• Interface morphology: Verification of complete fusion, absence of unmelted substrate, and absence of intermetallic compounds
• Microstructure mapping: Identification of grain structure, phase distribution, and any anomalous phases (such as sigma phase or martensite)
• Hardness traverse: Hardness measurements across the overlay-HAZ-substrate interface to identify any localized hardening
• Crack examination: Identification of any microcracks at or near the interface

Common Defects and Root Cause Analysis

Engineering Practice Insights

The study of ESW overlay peel mechanisms has several important implications for nuclear engineering practice:

1. Design margin for thermal cycling: The design of nuclear island components with ESW overlay must account for the thermal fatigue life of the overlay-substrate interface. Finite element analysis (FEA) of the thermal stress distribution during operating cycles should be performed to verify that the interface stress remains below the fatigue threshold.
2. Procedure qualification rigor: The ESW overlay procedure qualification must include specific tests for interfacial bond strength and thermal fatigue resistance, not just mechanical properties of the overlay metal itself. The qualification should simulate the expected thermal cycling conditions of the component.
3. In-service monitoring: Nuclear island components with ESW overlay should be included in the in-service inspection (ISI) program with specific attention to the overlay-substrate interface. UT/PAUT examinations at regular intervals can detect early-stage delamination before it progresses to a critical failure.
4. Material compatibility: The selection of the stainless steel overlay grade must be carefully matched to the substrate and the expected service environment. For example, 304L is suitable for general nuclear service, but 316L or 321 may be required for specific corrosion environments. The thermal expansion mismatch should be minimized by selecting grades with similar expansion coefficients.

Study Insights and Outlook

The analysis of ESW overlay peel mechanisms reveals that the interface between the overlay and substrate is a complex, multi-scale phenomenon governed by the interplay of metallurgical, mechanical, and thermal factors. No single parameter or treatment can guarantee peel resistance — it requires a holistic approach that integrates process optimization, material selection, post-weld treatment, and quality verification.

Future research directions should include:

• Advanced simulation of residual stress evolution during multi-pass ESW overlay
• Development of in-situ monitoring techniques for real-time interfacial bond quality assessment
• Investigation of novel overlay materials (such as nickel-based alloys or duplex stainless steels) with improved thermal expansion matching
• Long-term thermal fatigue testing of ESW overlay interfaces under simulated nuclear service conditions

The ultimate goal is to achieve ESW overlay systems that are not only resistant to initial peel failure but also capable of withstanding the full service life of nuclear island components — potentially 60 years or more — without degradation of the overlay-substrate bond.