CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Specialized Hardfacing Electrodes for Anti-Wear Cladding on Pulverized Coal Fan Impellers
Technical Context and Challenge
Pulverized coal fan impellers in coal-fired power plants endure extreme abrasive wear from high-velocity coal-laden air streams, with erosion rates reaching 0.5–2.0 mm per month in severe service. The literature describes the application of specialized hardfacing welding electrodes designed specifically for this demanding environment, achieving overlay layers that resist the combined effects of abrasion, impact, and thermal cycling.
Electrode and Process Selection
The specialized electrodes utilize a high-carbon, high-chromium composition with carbide-forming elements that produce a hard, wear-resistant microstructure. The selection between different hardfacing types depends on the specific wear mechanism:
| Hardfacing Type | Hardness (HRC) | Primary Wear Resistance | Typical Application |
|---|---|---|---|
| High-Cr martensitic (Cr20–Cr30) | 50–58 | Abrasion + moderate impact | Fan blades, impeller vanes |
| High-C austenitic (Cr25–Ni20) | 35–45 | Impact + corrosion | Severe impact zones |
| High-Cr high-Mo | 55–62 | High-temperature abrasion | Hot gas zones |
| Stellite-based (Co-Cr-W) | 45–55 | Abrasion + corrosion | Critical wear areas |
The welding process typically employs SMAW for field repairs and GMAW or SAW for shop fabrication, with the following critical parameters:
- Current range: 250–350 A for SMAW with 4.0 mm electrode diameter.
- Travel speed: 200–300 mm/min, slower for thicker deposits.
- Preheat: 150–250°C to prevent cold cracking in the base steel.
- Interpass temperature: Maintained below 250°C to preserve martensitic hardness.
- Overlay thickness: 2–4 mm, with a 1.5–2.0 mm machining allowance.
- Post-weld treatment: No heat treatment for martensitic types (maintains hardness); controlled air cooling for austenitic types.
Microstructural Considerations
The wear resistance of the overlay layer is fundamentally governed by its microstructure. High-chromium martensitic hardfacing produces primary chromium carbides (Cr₇C₃) dispersed in a martensitic matrix, providing excellent abrasive wear resistance. However, excessive hardness introduces brittleness, making the overlay susceptible to spalling under impact loading. The literature emphasizes the importance of matching hardfacing type to the specific wear mechanism: pure abrasive environments favor harder martensitic compositions, while environments involving impact or thermal shock require tougher austenitic or duplex structures.
Dilution control is critical. Excessive dilution from the base carbon steel reduces the effective chromium and carbon content in the overlay, degrading hardness and wear resistance. Practical dilution rates should be maintained below 30% for martensitic hardfacing and below 40% for austenitic types. This is achieved through proper groove preparation, controlled first-pass parameters, and appropriate electrode selection.
Engineering Practice and Maintenance Strategy
In power plant maintenance, the hardfacing repair of pulverized coal fan impellers follows a planned maintenance schedule rather than reactive repair. The typical service life of a properly applied hardfacing overlay is 12–24 months before reclamation is required. A structured approach includes:
- Scheduled inspection: Monthly visual and ultrasonic thickness measurements of the overlay layer.
- Preventive reclamation: Removing worn material and re-applying hardfacing before complete overlay consumption.
- Surface preparation: Grinding to bare metal, removing all oxide and contamination.
- Multi-pass deposition: Build-up pass followed by 2–3 hardfacing passes with cross-hatch pattern.
- Post-repair testing: Hardness verification at multiple locations, magnetic particle inspection for cracks.
Reflections and Recommendations
The literature demonstrates that specialized hardfacing electrode selection is not merely a material choice but a systems engineering decision that must account for the specific tribological conditions, thermal environment, and maintenance philosophy of the application. A common pitfall observed in practice is the use of generic hardfacing electrodes without considering the specific wear mechanism—this leads to premature failure and repeated repair cycles. Engineers should conduct a thorough wear analysis before specifying hardfacing composition, considering factors such as particle hardness, particle velocity, temperature, and the presence of corrosive media. The economic case for specialized hardfacing is compelling when calculated on a cost-per-year-of-service basis, as the initial premium for specialized electrodes is offset by dramatically extended service intervals.