Zhuojin Pipe Fitting Co., LtdWide-Strip Submerged Arc and Electroslag Overlay Welding with Magnetic Control for Nuclear Equipment Corrosion-Resistant Cladding
Literature Overview
Published in 2010 in the journal Welder (电焊机), this paper by Li Pengfei and Wang Jiantao from Xi'an Nuclear Equipment Co., Ltd. investigates wide-strip electroslag and submerged arc overlay welding for corrosion-resistant cladding of nuclear equipment components. The study employs domestic strip electrode and flux materials with Swedish ESAB welding equipment, and evaluates the overlay deposits through intergranular corrosion testing, lateral bending tests, metallographic examination, and hardness measurements. A notable innovation is the use of a magnetic control device to improve weld bead quality in strip electrode submerged arc overlay welding.
Core Technical Content
Nuclear equipment components often require corrosion-resistant cladding to withstand aggressive coolant environments, including pressurized water reactor (PWR) coolant conditions. The wide-strip overlay welding processes investigated—S-ESOW and S-SAOW—are well-suited for producing thick, uniform cladding layers on large-diameter components such as reactor vessel internals, heat exchanger tubesheets, and containment vessel linings.
Process Parameters and Equipment Configuration
| Parameter | Electroslag Overlay (S-ESOW) | Submerged Arc Overlay (S-SAOW) |
|---|---|---|
| Equipment | ESAB mechanized system | ESAB mechanized system |
| Strip electrode width | 30–60 mm | 30–60 mm |
| Flux type | Domestic granular flux | Domestic granular flux |
| Current range | 800–1500 A | 800–1500 A |
| Travel speed | 150–300 mm/min | 150–400 mm/min |
| Magnetic control | Not applicable | Applied to S-SAOW |
| Target overlay composition | 304L-equivalent austenitic stainless steel | 304L-equivalent austenitic stainless steel |
The use of domestic materials with imported equipment represents a practical approach to balancing cost considerations with process reliability. The ESAB equipment provides precise control of current, voltage, travel speed, and electrode feed, which is essential for achieving consistent overlay quality.
Magnetic Control Device for S-SAOW
The magnetic control device is a key innovation in this work. In strip electrode submerged arc welding, the weld pool is susceptible to magnetic arc blow, which can cause asymmetric bead profiles, uneven penetration, and increased dilution. The magnetic control device generates a controlled magnetic field that counteracts arc blow effects, resulting in:
- Symmetric and uniform weld bead profiles
- Reduced dilution rate variation across the bead width
- Improved surface quality with fewer surface defects
- More consistent overlay composition across the weld width
This is particularly important for nuclear applications where overlay uniformity is critical for ensuring consistent corrosion resistance across the cladding surface.
Corrosion Resistance and Mechanical Performance Evaluation
The intergranular corrosion testing is of paramount importance for nuclear applications. Austenitic stainless steel cladding is susceptible to intergranular corrosion if carbon content is elevated or if sensitization occurs during welding. The 304L composition (low carbon, ≤0.03% C) is specifically selected to minimize sensitization susceptibility. The intergranular corrosion test results confirm that the overlay deposits achieve corrosion resistance comparable to wrought 304L stainless steel.
The lateral bending test evaluates the ductility and crack resistance of the overlay layer. For nuclear components, the cladding must withstand thermal cycling and mechanical loading without cracking or delamination. The lateral bending results demonstrate adequate ductility in the overlay deposits, indicating that the microstructure provides sufficient toughness despite the high alloy content required for corrosion resistance.
Metallographic Analysis and Hardness
The metallographic examination reveals the microstructural characteristics of the overlay deposits. Wide-strip overlay welding produces deposits with columnar grain structures growing perpendicular to the fusion line, with grain size increasing with distance from the fusion boundary. The Z-direction (through-thickness) grain size is typically smaller than the X and Y-direction grain sizes, reflecting the directional solidification pattern inherent to overlay welding.
Hardness measurements across the overlay cross-section show a gradient from the fusion boundary (lower hardness due to dilution) to the top surface (higher hardness reflecting the full alloy composition). The hardness profile is important for evaluating the overlay's resistance to mechanical wear and erosion in nuclear service.
Engineering Practice and Nuclear Application Considerations
Nuclear equipment cladding imposes additional requirements beyond those of conventional industrial applications:
- Traceability: All materials and consumables must be fully traceable to meet nuclear regulatory requirements
- Qualification: Welding procedures must be qualified according to applicable codes (ASME BPV Section VIII, RCC-M, or applicable Chinese nuclear codes)
- Inspection: Non-destructive examination requirements are more stringent, typically requiring 100% UT or RT coverage of the cladding
- Documentation: Complete welding records must be maintained for the lifetime of the component
The use of domestic consumables with imported equipment addresses the supply chain concern while maintaining process reliability. However, qualification of domestic consumables for nuclear service requires additional testing and documentation beyond what is described in this paper.
Key Questions and Reflections
The paper raises an important question about the long-term performance of the overlay under nuclear service conditions. Intergranular corrosion resistance is evaluated through standard testing, but the actual service environment may include radiation damage, impurity ingress from coolant chemistry, and cyclic thermal loading that can accelerate degradation. Long-term exposure testing or simulation studies would provide additional confidence in the overlay's durability.
The magnetic control device is an interesting process improvement, but its effect on residual stress distribution in the overlay layer was not evaluated. Residual stresses in nuclear cladding can influence stress corrosion cracking susceptibility, and the magnetic field application may alter the stress state in ways that require further investigation.
Summary
This paper demonstrates the viability of wide-strip electroslag and submerged arc overlay welding for producing corrosion-resistant cladding on nuclear equipment components. The use of domestic consumables with ESAB equipment provides a cost-effective solution that meets the stringent requirements of nuclear applications. The magnetic control device represents a practical process improvement for S-SAOW that enhances weld quality and consistency. The comprehensive evaluation through intergranular corrosion testing, mechanical testing, and metallographic analysis provides confidence in the overlay's performance. The work contributes to the body of knowledge on advanced overlay welding technologies for nuclear applications and highlights the potential for domestic consumable development in critical infrastructure sectors.