CLADDING TECHNOLOGY SHANXI CO., LTDInterface Delamination in Nozzle Inner Wall Cladding for Nuclear Power Equipment
Literature Overview
The 2019 publication by Zhang Wenyang, Yang Juwen, and Gu Jialei from Shanghai Electric Nuclear Power Equipment Co., Ltd., published in Pressure Vessels (压力容器), addresses the critical issue of interface delamination in nozzle inner wall overlay welding for nuclear power pressure vessels. This is a safety-critical topic because nozzle connections are integral to the pressure boundary of nuclear reactor vessels, and any delamination at the overlay interface could compromise the structural integrity and leak tightness of the vessel. The paper represents a significant contribution to nuclear pressure vessel manufacturing technology.
Core Technical Content
Nuclear power pressure vessel nozzles require overlay welding of corrosion-resistant materials (typically stainless steel or nickel-based alloys) on the inner wall to resist the corrosive effects of reactor coolant water (RCW). The overlay must be applied to the internal surface of the nozzle neck, which presents unique geometric and thermal challenges compared to flat or external surface overlay. The interface delamination phenomenon occurs when the metallurgical bond between the overlay layer and the base metal is insufficient to resist the residual stresses and thermal stresses generated during welding and subsequent service.
Root Cause Analysis of Interface Delamination
| Cause Category | Specific Mechanism | Contributing Factor |
|---|---|---|
| Metallurgical | Brittle intermetallic compounds at interface | Fe-Ni, Fe-Cr phases from base metal dilution |
| Metallurgical | Delta ferrite formation in overlay | High Cr/Ni ratio in weld metal |
| Metallurgical | Hydrogen-induced cracking | Hydrogen absorption from shielding gas or flux |
| Thermal | Thermal mismatch residual stress | CTE difference between overlay and base metal |
| Thermal | Rapid cooling rate | High thermal mass of thick nozzle wall |
| Geometric | Poor wetting due to nozzle curvature | Inadequate base metal penetration at inner wall |
| Procedural | Insufficient preheat | Excessive thermal gradient at interface |
| Procedural | Excessive interpass temperature | Grain coarsening, reduced ductility |
Process Parameters for Nozzle Inner Wall Overlay
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Base metal | SA-516 Gr.70 or SA-508 Gr.3 | Typical nuclear vessel material |
| Overlay material | 308L or 309L wire (for 304L/316L overlay) | Low carbon to prevent sensitization |
| Process | GTAW (single or multi-pass) | Low dilution, good control |
| Preheat | 150–250°C | Reduce thermal gradient, prevent HIC |
| Interpass temperature | ≤250°C | Control grain growth |
| Current | 120–180 A | Adequate penetration without excessive dilution |
| Travel speed | 80–150 mm/min | Control heat input per pass |
| Heat input per pass | 1.0–2.5 kJ/mm | Balance penetration and dilution |
| Layers | 3–5 layers minimum | Achieve required overlay thickness |
| First layer penetration | 0.5–1.0 mm into base metal | Ensure metallurgical bond |
Inspection and Acceptance Criteria
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| UT (contact method) | Detect interface delamination | No indications >0.5 mm equivalent |
| UT (tandem probe) | Detect sub-surface defects | No indications per ASME V Art.4 |
| MT (surface) | Detect surface cracks | No linear indications |
| PT (surface) | Detect surface-breaking defects | No linear indications |
| Metallographic examination | Verify dilution, microstructure, bond quality | Dilution <15%; no intermetallics; full bond |
| Hardness survey | Verify microstructural uniformity | 200–300 HV within overlay |
| Hydrostatic test | Verify leak tightness | No leakage at 1.3× design pressure |
Prevention Measures
The following systematic approach is recommended to prevent interface delamination:
- Base metal preparation: Machine the inner wall to a smooth finish (Ra < 6.3 μm) to ensure uniform wetting by the overlay weld metal. Remove all scale, rust, and contamination.
- Preheat control: Apply uniform preheat to the entire nozzle assembly (not just the welding area) to minimize thermal gradients. Use thermocouples to monitor preheat temperature at multiple locations.
- First layer control: The first overlay layer is critical for establishing the metallurgical bond. Use slightly higher current and slower travel speed to ensure adequate penetration into the base metal (0.5–1.0 mm) without excessive dilution.
- Welding sequence: Use a spiral or circumferential welding sequence that progresses uniformly around the nozzle circumference to minimize localized thermal distortion.
- Post-weld heat treatment: Apply PWHT at 590–620°C for a minimum of 2 hours per 25 mm of maximum section thickness to relieve residual stresses.
- Interface inspection: Perform UT of the overlay interface using tandem probes or phased array techniques before proceeding to the next layer. If delamination is detected, remove the affected area and re-weld.
Engineering Practice Insights
The interface delamination problem in nozzle inner wall overlay is particularly challenging because of the confined geometry and the difficulty of visual inspection of the internal surface. Unlike flat plate overlay, where the interface can be readily inspected from both sides, nozzle inner wall overlay requires indirect inspection methods and careful process control to ensure bond integrity.
From a nuclear safety perspective, the integrity of the overlay bond is paramount because it defines the corrosion protection boundary of the pressure vessel. Any delamination creates a potential pathway for reactor coolant water to reach the carbon steel base metal, leading to corrosion and eventual loss of containment. The regulatory framework governing nuclear pressure vessel fabrication (such as ASME BPV Code Section III, Division 1, Appendix G) imposes stringent requirements on overlay welding qualification, inspection, and documentation.
The following engineering recommendations emerge from this analysis:
- Qualification welding procedures must include nozzle geometry simulation to verify that the procedure is applicable to curved internal surfaces.
- Welder qualification must include demonstration of capability to weld on internal nozzle surfaces, not merely flat or external surfaces.
- In-process inspection (UT of each layer before proceeding to the next) is strongly recommended for critical nuclear applications, even though it increases production cost.
- Documentation of all welding parameters (current, voltage, travel speed, gas flow rate) for each pass should be maintained for traceability.
- Post-fabrication UT of the completed overlay should be performed using phased array ultrasonic testing (PAUT) for superior defect detection capability at curved interfaces.
Key Reflections
The research by Zhang et al. addresses a problem that has been recognized in the nuclear industry for decades but continues to challenge manufacturers. The confined geometry of nozzle inner walls creates thermal and mechanical conditions that are fundamentally different from flat or external surface overlay, and processes qualified on flat specimens may not be directly applicable. The key insight from this research is that interface delamination is rarely caused by a single factor but rather by the synergistic interaction of metallurgical incompatibility, thermal stress, and procedural deviations. In my professional assessment, the most effective prevention strategy combines careful process qualification with in-process inspection, rather than relying solely on post-fabrication testing. Engineers working on nuclear pressure vessel fabrication should treat nozzle inner wall overlay as a critical process requiring enhanced quality controls beyond those applied to standard overlay welding. The lessons from this research are directly transferable to other critical applications such as hydrogenation reactor internals and high-pressure chemical equipment where overlay integrity is safety-critical.