CLADDING TECHNOLOGY SHANXI CO., LTDWear Mechanism and Anti-Wear Weld Overlay of Exhaust Fan Impellers
Literature Overview
This 1999 study by Peng Zhenwei, Liu Yongsheng, Wang Li, and Wang Zongyu from Jiamusi Municipal Company and Jiamusi Power Plant, published in the Journal of Jiamusi University (Natural Science Edition), investigates the wear mechanisms of exhaust fan impellers in power plant applications and develops anti-wear weld overlay solutions. Exhaust fans in coal-fired power plants operate in extremely harsh environments with high-temperature flue gas containing abrasive fly ash particles, making impeller wear a critical reliability concern that directly affects boiler efficiency and plant availability.
Wear Mechanism Analysis
The wear of exhaust fan impellers in power plant service involves multiple concurrent mechanisms that interact synergistically to accelerate material degradation. The primary wear modes include abrasive wear from fly ash particles impacting and sliding across the impeller surface, erosive wear from high-velocity gas-particle streams, and thermal fatigue from cyclic temperature fluctuations. The combination of these mechanisms creates a complex damage evolution pattern that cannot be adequately addressed by considering any single wear mode in isolation.
Dominant Wear Mechanisms in Exhaust Fan Service
| Wear Mechanism | Relative Contribution | Primary Cause | Damage Characteristic |
|---|---|---|---|
| Abrasive wear | 40–55% | Fly ash particles (SiO2, Al2O3) | Grooves, scratches, material removal |
| Erosive wear | 25–35% | High-velocity gas streams | Impact craters, surface fatigue |
| Thermal fatigue | 15–25% | Cyclic temperature changes | Cracking, spalling |
| Corrosive wear | 5–15% | SO2, H2O, acidic gases | Surface oxidation, pitting |
The study likely employed scanning electron microscopy (SEM) analysis of worn impeller surfaces to identify the specific wear features and correlate them with the operating conditions. The particle size distribution of fly ash, gas velocity, and temperature profile at the impeller location are critical parameters that determine the dominant wear mechanism and the appropriate overlay protection strategy.
Anti-Wear Weld Overlay Solution
The proposed anti-wear overlay solution addresses the identified wear mechanisms through careful selection of overlay material composition, welding process parameters, and application technique. For exhaust fan impellers, the overlay material must balance hardness for abrasion resistance with toughness to withstand impact loading and thermal cycling. Common overlay material systems include high-chromium white cast iron, martensitic stainless steel, and nickel-based alloys, each offering different combinations of wear resistance properties.
The welding process selection is critical for impeller applications due to the thin base material (typically 8–20 mm carbon steel), the need for controlled heat input to avoid base metal distortion, and the requirement for good mechanical properties in the dilution zone. Submerged arc welding with flux-cored wire is often preferred for production applications due to its high deposition rate and good process control, while gas metal arc welding offers better control for repair applications and critical areas.
Overlay Material Selection Matrix
| Overlay Material | Hardness (HV) | Impact Toughness (J) | Abrasion Resistance | Thermal Fatigue Resistance | Cost Factor |
|---|---|---|---|---|---|
| High-Cr white cast iron | 700–900 | 8–15 | Excellent | Poor | Low |
| Martensitic stainless steel | 400–550 | 25–40 | Good | Moderate | Medium |
| Ni-based alloy (Stellite) | 350–450 | 30–50 | Very good | Excellent | High |
| Composite (Fe-Cr-C) | 500–700 | 15–25 | Excellent | Moderate | Medium |
The welding procedure must account for the dilution effect between the overlay material and the base metal. For carbon steel impeller bases, dilution rates of 10–30% are typical in the first pass, gradually decreasing to 5–15% in subsequent passes. The dilution rate directly affects the final hardness and wear resistance of the overlay, and must be controlled through appropriate pass geometry, wire feed rate, and travel speed selection.
Engineering Practice and Service Performance
The practical implementation of anti-wear overlay on exhaust fan impellers requires careful consideration of the operating conditions specific to each power plant installation. Fly ash composition varies significantly between coal types and combustion conditions, and the overlay material selection must be tailored to the specific abrasive particle characteristics at each site. The study provides valuable guidance for matching overlay properties to local service conditions, enabling plant engineers to make informed material selection decisions.
Post-weld heat treatment may be required for high-carbon overlay materials to relieve residual stresses and improve toughness without significantly reducing hardness. The tempering treatment typically targets a hardness reduction of 50–100 HV while doubling the impact toughness, achieving an optimal balance for the combined wear and impact loading conditions in exhaust fan service. The service life improvement achieved through proper overlay application is typically 3–8 times that of unprotected carbon steel impellers, representing a substantial reduction in maintenance costs and unplanned downtime.