CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment and Application of High-Speed Electroslag Welding Overlay
Literature Overview
This paper by Zhang Liangcheng and Ding Bixue from Shanghai Boiler Works Co., Ltd., published in 2003 in the journal "Boiler Technology" (锅炉技术), documents the development and industrial application of high-speed electroslag welding (HS-ESW) for overlay cladding on boiler and pressure vessel components. The study represents a significant advancement in electroslag welding technology, achieving deposition rates 2–3 times higher than conventional ESW while maintaining acceptable metallurgical quality.
Technical Background
Conventional electroslag welding overlay has long been recognized for its high deposition rate and low dilution characteristics, making it ideal for depositing thick overlay layers on large components such as boiler drum heads, reactor shells, and heat exchanger tubesheets. However, the traditional ESW process operates at travel speeds of 80–150 mm/min with deposition rates of 15–30 kg/h, which limits its application to large, thick-walled components. The high-speed variant addresses this limitation by optimizing the electromagnetic stirring, flux composition, and electrode configuration.
High-Speed ESW Process Configuration
The high-speed ESW overlay process employs several key modifications to the conventional ESW setup:
| Feature | Conventional ESW | High-Speed ESW |
|---|---|---|
| Travel speed (mm/min) | 80–150 | 200–400 |
| Deposition rate (kg/h) | 15–30 | 40–80 |
| Electrode diameter (mm) | 8–10 | 6–8 |
| Flux layer thickness (mm) | 15–25 | 10–15 |
| Electromagnetic stirring | Single-phase AC | Multi-phase AC, optimized frequency |
| Electrode oscillation | None or mechanical | Electromagnetic oscillation |
| Current density (A/mm²) | 15–25 | 25–40 |
| Slag pool depth (mm) | 8–12 | 5–8 |
Process Optimization Parameters
The study systematically investigated the influence of key parameters on overlay quality:
Electromagnetic Stirring Optimization
The electromagnetic stirring field plays a critical role in high-speed ESW by:
- Homogenizing the composition of the molten pool
- Refining the grain structure through controlled solidification
- Reducing segregation and banding in the overlay
- Enabling higher travel speeds without compromising quality
The optimal stirring frequency was determined to be 50–100 Hz for single-phase operation and 50/100/150 Hz for three-phase operation, with the three-phase configuration providing superior compositional uniformity.
Flux Composition and Properties
The flux serves multiple functions in ESW overlay: shielding, alloying, slag viscosity control, and heat input regulation. For high-speed operation, the flux must maintain adequate viscosity at the reduced slag pool depth while providing sufficient alloying to compensate for the reduced contact time between slag and metal.
| Flux Property | Conventional ESW | High-Speed ESW |
|---|---|---|
| Viscosity at 1300 °C (Pa·s) | 0.8–1.5 | 0.6–1.0 |
| Alloying capacity (Cr pickup) | 2–4% | 3–5% |
| Granule size (mm) | 3–8 | 2–5 |
| Moisture content (%) | < 0.5 | < 0.3 |
Metallurgical Quality Assessment
The overlay quality was evaluated through comprehensive testing:
| Test Method | Acceptance Criteria | Measured Results |
|---|---|---|
| Bond strength (shear) | ≥ 250 MPa (per ASTM A264) | 280–350 MPa |
| Dilution rate | ≤ 15% (per API 934) | 8–12% |
| Hardness (overlay) | Within specified range | Within ±10% of nominal |
| Grain size (overlay) | ≤ 0.5 mm (coarse grain boundary) | 0.2–0.4 mm |
| Macrostructure uniformity | No banding or segregation | Uniform throughout |
| NDE (MT) | No indications per ASME V | Passed 100% |
Industrial Application Cases
Boiler Drum Head Overlay
The primary application documented was the overlay of 304L stainless steel on carbon steel boiler drum heads for a 600 MW power plant. The overlay thickness was 6 mm, achieved in a single ESW pass at 350 mm/min travel speed, completing the full circumference in approximately 45 minutes compared to 2 hours with conventional ESW.
Heat Exchanger Tubesheet Overlay
For a petrochemical heat exchanger, 1.5 mm of Hastelloy C276 was overlaid on a carbon steel tubesheet using high-speed ESW with a 6 mm electrode. The process achieved a deposition rate of 45 kg/h with 10% dilution, and the overlay passed intergranular corrosion testing per ASTM G48.
Nuclear Component Overlay
The technology was also applied to overlaying nickel-based alloy on a reactor vessel head, requiring strict control of dilution (< 8%) and comprehensive NDE including UT per ASME V Section 4.
Defect Analysis and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cold cracks | High residual stress, hydrogen | Preheat 200 °C, post-weld stress relief at 600 °C |
| Incomplete bonding | Insufficient penetration, oxide | Increase current, proper surface preparation |
| Slag inclusion | Inadequate slag removal between passes | Mechanical removal, visual inspection |
| Porosity | Moisture in flux, gas absorption | Flux drying, proper shielding |
| Composition segregation | Insufficient stirring | Increase stirring frequency, optimize flux alloying |
Key Reflections and Engineering Implications
The high-speed ESW overlay technology represents a significant productivity improvement for large-scale cladding applications. The key insight from this research is that the electromagnetic stirring mechanism is not merely an auxiliary feature but a fundamental enabler of high-speed operation—it compensates for the reduced solidification time by actively homogenizing the molten pool composition and promoting uniform grain growth. However, the technology also introduces challenges: the reduced slag pool depth increases susceptibility to cold cracks, and the higher current density requires more robust power supply equipment. For modern applications in hydrogen energy storage vessels and advanced nuclear components, high-speed ESW overlay offers a viable pathway to achieving thick, uniform overlay layers with minimal dilution and maximum productivity.