Zhuojin Pipe Fitting Co., LtdStrip Electrode Submerged Arc Overlay Welding on Nuclear Pressurizer Hemispherical Heads
Literature Overview
This paper by Liao Guoping, published in the journal "Pressure Vessel" (2008, Vol. 25, No. 5, pp. 21-25), addresses a critical engineering challenge in nuclear power plant construction: the large-area overlay welding of ultra-low-carbon austenitic stainless steel corrosion-resistant layers on hemispherical heads of reactor pressurizers. The work was conducted at Dongfang Boiler (Group) Co., Ltd., one of China's leading nuclear equipment manufacturers, and represents a significant milestone in domestic nuclear-grade overlay welding technology.
The pressurizer is a vital component in pressurized water reactor (PWR) systems, responsible for maintaining primary circuit pressure stability. Its hemispherical head, typically fabricated from low-alloy steel (such as 15CrMoR or SA-387 Gr.11 Cl.2), requires an internal corrosion-resistant overlay to withstand the aggressive boric acid–lithium hydroxide coolant environment. The conventional approach of using stainless steel cladding plates introduces additional weld joints and potential leakage paths, whereas overlay welding provides a metallurgically bonded, leak-free corrosion barrier.
Core Technical Approach and Equipment Configuration
The authors designed and manufactured a dedicated assembly fixture and auxiliary curved backing plates to accommodate the complex geometry of the hemispherical head. The entire overlay welding operation was performed on a 100-ton welding positioner using an ESAB strip electrode submerged arc welding (SAW) machine. This equipment configuration was necessary because the hemispherical head diameter and wall thickness demand both the rigidity of a heavy-duty positioner and the high deposition rate of strip electrode SAW.
Key Equipment Parameters
| Equipment/Parameter | Specification |
|---|---|
| Welding Positioner | 100 t capacity, capable of rotating and tilting the hemispherical head |
| Welding Machine | ESAB strip electrode submerged arc welding system |
| Strip Electrode | Ultra-low-carbon austenitic stainless steel (likely ESAB SFA-15.11 or equivalent) |
| Flux | Submerged arc welding flux, low-hydrogen type |
| Workpiece | Nuclear pressurizer hemispherical head, low-alloy steel substrate |
| Quantity Completed | 2 pressurizers, 4 hemispherical heads total |
The use of a 100-ton positioner is noteworthy because it ensures that the hemispherical head can be rotated into any welding position, maintaining consistent gravity conditions and arc stability throughout the entire overlay process. The auxiliary curved backing plates serve a dual purpose: they provide a stable reference surface for the strip electrode torch and they act as thermal mass to reduce the cooling rate at the weld root, thereby minimizing the risk of hot cracking in the dilution zone.
Process Parameter Analysis
The paper systematically investigates five key process parameters and their influence on overlay welding quality. This systematic approach reflects a rigorous engineering methodology, consistent with the PDCA (Plan-Do-Check-Act) cycle.
Parameter Influence Summary
| Parameter | Effect on Quality | Recommended Range |
|---|---|---|
| Welding Current | Higher current increases dilution and penetration; excessive current causes undercuts and spatter | Optimized to balance deposition rate and dilution control |
| Arc Voltage | Affects bead width and profile; higher voltage increases bead width but may reduce penetration | Adjusted in conjunction with current to maintain stable arc |
| Welding Speed | Directly affects heat input per unit length; slower speed increases dilution | Controlled to limit dilution to acceptable levels (typically <30%) |
| Bead Overlap (Lap Amount) | Insufficient overlap causes lack of fusion between adjacent beads; excessive overlap increases heat input and dilution | Typically 1/3 to 1/2 of bead width overlap |
| Workpiece Levelness | Non-level positioning causes flux accumulation, arc instability, and uneven bead profile | Maintained within ±0.5° tolerance |
Dilution Rate Control
Dilution rate is the most critical quality metric in overlay welding of stainless steel onto low-alloy steel. The dilution rate is defined as the ratio of base metal mixed into the overlay layer to the total overlay layer composition. For nuclear pressurizer applications, the dilution rate must be controlled to ensure that the overlay layer maintains its corrosion resistance, typically requiring the carbon content to remain below 0.03% and chromium content above 18%.
The paper discusses how welding current, speed, and bead overlap collectively determine the dilution rate. Higher currents increase base metal melting and thus increase dilution. Slower welding speeds deposit more heat per unit length, also increasing dilution. The bead overlap strategy is particularly important: overlapping beads excessively in a single pass can cause localized overheating and increased dilution, while insufficient overlap risks lack of fusion.
Bead Overlap Strategy
The "lap amount" (搭接量) parameter discussed in the paper refers to the horizontal overlap between adjacent longitudinal beads. In strip electrode SAW, the bead width can be substantial (often 30-50 mm depending on strip width and parameters), so the overlap strategy directly determines the number of passes required to cover the entire inner surface of the hemispherical head.
A practical overlap ratio of approximately 30-40% of the bead width is generally recommended. This provides adequate fusion between adjacent beads without excessive heat input. The authors emphasize that maintaining consistent overlap throughout the curved surface requires careful positioner control, as the geometry changes continuously during rotation.
Engineering Practice Insights
Fixture Design Considerations
The dedicated assembly fixture described in the paper is a critical enabler of the entire process. For hemispherical heads with diameters potentially exceeding 2 meters, the fixture must:
- Provide stable support without deforming the thin-walled hemispherical shell
- Allow the positioner to rotate the head smoothly without vibration
- Accommodate the strip electrode torch at various angular positions
- Permit access for flux loading and post-weld inspection
The auxiliary curved backing plates are particularly important because they provide a flat reference surface for the torch carriage to traverse, compensating for the curvature of the hemispherical head. Without these plates, the torch would need to follow the curved surface, which is mechanically complex and prone to error.
Quality Assurance Approach
The completion of 4 hemispherical heads for 2 pressurizers indicates a successful scale-up from prototype to production. Key quality assurance measures would include:
| NDT Method | Purpose |
|---|---|
| Penetrant Testing (PT) | Surface crack detection on each bead |
| Magnetic Particle Testing (MT) | Surface and near-surface defect detection |
| Ultrasonic Testing (UT) | Subsurface defect detection, especially lack of fusion |
| Hardness Testing | Verification of microstructure and dilution control |
| Spectroscopic Analysis | Confirmation of overlay composition (C, Cr, Ni, Mo contents) |
Connection to Standards
This work aligns with the requirements of Chinese nuclear standards such as RBA (Reactors, Components, and Associated Hardware) and GB/T 150, as well as international standards including ASME Section III (for nuclear components) and ASME Section IX (for welding qualifications). The overlay welding procedure qualification would need to demonstrate compliance with specified composition, hardness, and NDT acceptance criteria.
Key Reflections and Study Insights
The most significant contribution of this paper is the demonstration that large-area overlay welding on complex curved geometries is technically feasible with proper equipment and process control. The systematic investigation of five process parameters, culminating in a qualified production procedure, represents a mature engineering approach.
One area that could benefit from further discussion is the residual stress management. Large-area overlay welding on a hemispherical head introduces significant thermal stresses that can affect the dimensional accuracy of the pressurizer head. Post-weld stress relief heat treatment may be required, but this must be carefully controlled to avoid sensitization of the austenitic overlay layer.
Another practical consideration is the repair strategy. If defects are detected during NDT, the repair welding must be performed without compromising the integrity of the surrounding overlay. This requires careful parameter selection and possibly the use of a narrower electrode or different process (such as GTAW) for localized repairs.
The experience documented in this paper is directly transferable to other nuclear components requiring corrosion-resistant overlay, such as steam generator tubesheet, reactor coolant pump casings, and control rod drive mechanisms. The key lessons—rigorous parameter optimization, dedicated fixture design, and systematic NDT—are universally applicable.
Reference Value and Outlook
This paper provides a valuable engineering reference for nuclear equipment manufacturers seeking to develop domestic overlay welding capabilities. The successful completion of 4 hemispherical heads demonstrates that the technology has matured to a production-ready level. Future work could explore the application of multi-wire or twin-strip electrode configurations to further increase deposition rates, as well as the integration of online monitoring systems for real-time dilution rate control. The experience also highlights the importance of equipment investment (100-ton positioner, ESAB welding system) as an enabler of advanced welding technology.