Zhuojin Pipe Fitting Co., LtdDevelopment of Wear-Resistant Surfacing Electrodes for Fan Blades Study Note
Literature Overview
The 1991 paper by Liu Pengshan, published in "Fan Technology" (Vol. 33, Issue 4, pp. 30-32), addresses the development of specialized welding electrodes for wear-resistant surfacing of centrifugal induced draft fan components. The classification under TG422.1 (welding materials) indicates the paper's primary focus on electrode composition and manufacturing rather than on welding process parameters alone. This work is particularly relevant to power plant engineers, where induced draft fans operate under extreme conditions of high temperature, corrosive flue gas, and erosive particulate loading.
Service Conditions and Wear Mechanisms
Centrifugal induced draft fans in coal-fired power plants are exposed to a combination of wear mechanisms that accelerate material degradation:
| Wear Mechanism | Contributing Factor | Typical Operating Condition |
|---|---|---|
| Abrasive wear | Fly ash particles (SiO2, Al2O3) | 120-200°C flue gas with 50-300 mg/m3 particulate |
| Erosion | High-velocity particle impact | Gas velocity 15-25 m/s at blade leading edge |
| Corrosive wear | SO2, SO3, HCl in flue gas | Acid dew point corrosion at blade surface |
| Thermal fatigue | Cyclic temperature variation | Start-up and shutdown thermal cycling |
The combination of these mechanisms creates a synergistic degradation effect where corrosion weakens the surface, making it more susceptible to abrasive removal, and abrasion exposes fresh material to corrosive attack. This is why simple hardness enhancement alone is insufficient — the overlay must also demonstrate corrosion resistance and thermal stability.
Electrode Composition Development
The paper describes the development of a specialized SMAW electrode with a coating composition engineered to deposit a wear-resistant overlay with the following target properties:
- Hardness: 50-58 HRC after air cooling
- Microstructure: Austenite-martensite dual-phase with dispersed carbides
- Carbon content: 1.5-2.5% in deposited metal
- Chromium content: 18-22% for corrosion resistance
- Molybdenum content: 2-4% for thermal strength and carbide stability
- Welding position: All positions (F, H, V, OH)
- Electrode type: Cellulose or rutile coating for good arc stability
The electrode design philosophy follows a dual-phase approach where the austenitic phase provides ductility and corrosion resistance, while the martensitic phase provides hardness and wear resistance. The balance between these phases is controlled by the cooling rate after welding, which can be influenced by the number of layers, interpass temperature, and base material thickness.
Process Parameters and Quality Control
| Process Parameter | Recommended Value | Effect on Overlay Quality |
|---|---|---|
| Welding current | 100-160 A (for 3.2 mm electrode) | Controls dilution and bead geometry |
| Arc voltage | 22-28 V | Affects penetration and fusion ratio |
| Travel speed | 200-300 mm/min | Influences cooling rate and microstructure |
| Preheat temperature | 100-150°C | Prevents hydrogen cracking in thick sections |
| Interpass temperature | Below 200°C | Controls martensite transformation and residual stress |
| Number of layers | 2-4 passes | Reduces dilution, builds adequate thickness |
| Post-weld stress relief | 550°C × 1 hour | Reduces residual stress, improves toughness |
Quality control of the surfacing operation involves hardness testing of the deposited metal, visual inspection for porosity and undercut, and periodic metallographic examination of the fusion line for cracks or lack of fusion. The dilution rate should be verified by hardness traverse testing, with the target being less than 20% dilution to maintain the overlay hardness above 50 HRC.
Engineering Application and Performance
Field application of these specialized electrodes on fan blade and impeller components has demonstrated significant life extension. In power plant service, the overlay life typically ranges from 8-16 months compared to 2-4 months for uncoated blades, depending on the coal quality and fly ash abrasiveness. The overlay thickness is typically maintained at 4-6 mm, with periodic rebuilding during scheduled maintenance outages.
The paper's contribution extends beyond the specific electrode composition to the broader principle that welding material development must be integrated with service condition analysis. The electrode was developed through iterative testing, where each composition modification was evaluated against the specific wear and corrosion conditions encountered in the target application. This systematic development approach — analyzing failure, hypothesizing material improvement, testing, and refining — is a model for welding consumable development in any wear-protection application.
Study Insights and Reflections
This paper highlights the importance of matching the overlay microstructure to the specific wear mechanism. In fan blade service, where abrasive and corrosive mechanisms act synergistically, a single-phase hard overlay would fail prematurely due to corrosion-induced degradation. The dual-phase austenite-martensite design provides the necessary combination of properties, demonstrating that wear-resistant surfacing requires a holistic approach to material design.
The paper also underscores the practical importance of electrode qualification for field use. Unlike laboratory-based surfacing processes that require specialized equipment, SMAW with qualified electrodes can be performed by trained welders in maintenance shops and even in the field. This practical accessibility is a significant advantage for power plant maintenance operations where equipment availability is critical and specialized surfacing equipment may not be available.