Delamination Mechanism Analysis in Stainless Steel Strip Electroslag Welding Overlay for Nuclear Island Main Equipment

Overview and Context

The nuclear island main equipment, including steam generators, reactor pressure vessel internals, and main coolant pumps, demands extremely high-integrity cladding interfaces. The strip electroslag welding (ESW) overlay process is widely adopted for producing austenitic stainless steel clad surfaces on carbon and low-alloy steel substrates due to its high deposition rate and excellent metallurgical bonding quality. However, delamination or spalling defects at the cladding-substrate interface remain a critical concern in nuclear applications, where even minor interface degradation can compromise containment integrity and corrosion resistance over decades of service. This study note summarizes the core findings regarding the metallurgical mechanisms, process factors, and engineering countermeasures associated with interface delamination in nuclear-grade ESW overlay production.

Core Technical Findings

The fundamental mechanism of interface delamination in ESW overlay involves a combination of metallurgical incompatibility, residual stress accumulation, and microstructural degradation at the transition zone. The key contributing factors identified include:

1. Dilution and composition gradient control: The ESW process inherently produces a wider dilution zone compared to GMAW or PTA methods. The high heat input (typically 400–800 kJ/cm) results in a broader transition region where the carbon content drops sharply from the substrate to the cladding layer. This creates a zone of reduced hardness and potentially embrittled microstructure susceptible to intergranular cracking.
2. Residual stress and thermal cycling effects: The large temperature gradients during multi-pass ESW overlay generate significant longitudinal and transverse residual stresses. Repeated thermal cycling during subsequent hot forming or heat treatment operations can exacerbate stress concentrations at the interface, leading to fatigue-driven delamination initiation.
3. Interfacial microstructural evolution: The transition zone typically exhibits a gradient of ferrite-austenite microstructure in the dilution region, transitioning to fully austenitic in the upper cladding layers. Inadequate control of the ferrite content (below the minimum required per ASME or NB/T standards) can lead to hot cracking and subsequent interface separation during service.
4. Slag inclusion and oxide film effects: Residual slag or oxide films at the interface, particularly if the preheating and flux management are inadequate, act as crack initiation sites. The nuclear-grade requirements for cleanliness make this a particularly critical concern.

Process Parameters and Control Strategy

Defect Analysis and Countermeasures

The study identifies three primary delamination morphologies:

• Type A – Interfacial separation: Occurs at the original cladding-substrate boundary, typically associated with inadequate preheating, excessive cooling rates, or poor flux coverage. Countermeasures include strict preheat control, optimized flux composition with higher Mn and Si content to promote wetting, and post-weld stress relief at 620–650°C for 2–4 hours.
• Type B – Sub-interface cracking: Cracks initiate in the dilution zone and propagate toward the interface, often associated with excessive carbon enrichment or insufficient ferrite content. The recommended approach is to employ a multi-layer strategy with a dedicated transition layer of 309L or 310L composition before the final 304/321 cladding layers.
• Type C – Service-induced spalling: Occurs during long-term thermal cycling or under corrosive environments, particularly in high-temperature water or steam conditions. This is associated with chromium carbide precipitation at grain boundaries in the dilution zone. The countermeasure is to use stabilized grades (321 with Ti, 347 with Nb) and to ensure the post-weld solution treatment effectively dissolves any carbide precipitates.

Standards and Quality Assurance

For nuclear island applications, the overlay process must comply with multiple standards simultaneously:

The non-destructive examination (NDE) protocol for nuclear-grade ESW overlay includes ultrasonic testing (UT) with phased array (PAUT) for interface bonding verification, magnetic particle testing (MT) for surface and near-surface defects in the cladding, and radiographic testing (RT) for volumetric defect detection. The acceptance criteria are significantly more stringent than those for conventional industrial applications.

Engineering Practice Insights

From a practical standpoint, the delamination issue in nuclear ESW overlay cannot be addressed through any single parameter adjustment. A systems-level approach is required, integrating material selection, process design, equipment capability, and quality assurance into a unified quality plan. The key insight from this literature is that the interface integrity is determined during the first few passes of the overlay, where the metallurgical bonding is established. Subsequent passes primarily build thickness and achieve the desired composition but cannot repair a compromised initial bond. This underscores the importance of rigorous procedure qualification with detailed microstructural characterization of the transition zone, including transmission electron microscopy (TEM) analysis of the interface morphology and energy dispersive spectroscopy (EDS) mapping of the compositional gradient.

The study reinforces the principle that in nuclear applications, the margin between acceptable and unacceptable performance is narrow, and the cost of failure is unacceptable. A conservative approach to process parameter selection, combined with comprehensive metallurgical characterization and conservative acceptance criteria, is the only defensible engineering practice for ensuring long-term interface integrity in nuclear island main equipment.