Zhuojin Pipe Fitting Co., LtdWear Mechanism of Exhaust Fan Impeller and Anti-Wear Overlay Welding
Literature Overview
The 1999 paper by Peng Zhenwei, Liu Yongsheng, Wang Li, and Wang Zongyu published in the Journal of Jiamusi University (Natural Science Edition) investigates the wear mechanism of exhaust fan (pulverized coal exhaust fan) impellers in power plant applications and proposes an anti-wear overlay welding solution using tungsten carbide (WC) electrode. The study combines tribological analysis with practical welding process development, providing a comprehensive approach to extending the service life of critical fan components.
Wear Mechanism Analysis
The authors identified the wear mechanism through detailed examination of the worn impeller surfaces:
Microscopic Wear Analysis
The worn surfaces exhibited characteristic ploughing grooves (犁沟状) caused by hard abrasive particles entrained in the pulverized coal-laden gas stream. The wear mechanism was classified as three-body abrasion, where hard particles (primarily quartz SiO₂ and feldspar) trapped between the impeller surface and the flowing medium cause progressive material removal.
| Wear Characteristic | Observation | Mechanism |
|---|---|---|
| Surface morphology | Deep ploughing grooves, directional scratches | Three-body abrasion |
| Groove depth | 0.5–2.0 mm | Particle hardness and impact energy |
| Wear direction | Aligned with gas flow direction | Material removal by sliding |
| Surface hardness loss | Base material 180–220 HV reduced to exposed substrate | Progressive material removal |
| Particle identification | Quartz (SiO₂), feldspar (KAlSi₃O₈) | Low-quality coal combustion products |
Wear Rate Estimation
Based on the service conditions and observed wear depth:
- Fan operating speed: 1480–2980 rpm
- Gas velocity at impeller tip: 40–60 m/s
- Particle size: 20–200 μm
- Particle concentration: 50–300 g/m³
- Annual wear depth: 1.5–3.0 mm on leading edges
- Service interval before replacement: 6–12 months
Overlay Welding Solution: Tungsten Carbide Electrode Application
The authors selected the Special Welding 707 (特堆707) tungsten carbide electrode for overlay welding the fan impeller. This electrode deposits a weld metal containing 60–70% WC particles in a cobalt or nickel-based binder matrix, providing exceptional abrasion resistance.
Welding Process Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Electrode type | Special Welding 707 (WC-Co) | High abrasion resistance |
| Welding current | 60–90 A | Low heat input to preserve WC particles |
| Arc voltage | 18–22 V | Stable arc with low dilution |
| Travel speed | 15–25 mm/min | Controlled deposition rate |
| Layer thickness | 2–4 mm per pass | Adequate coverage of wear zones |
| Total overlay thickness | 6–12 mm | 2–3 years additional service life |
| Preheat temperature | 150–200°C | Prevent cold cracking in base material |
| Post-weld cooling | Controlled (wrapped in insulation) | Prevent thermal shock cracking |
Process Sequence
- Surface preparation: Remove existing wear damage by machining or grinding; roughen the surface to 40–60 μm Ra for mechanical bonding
- Base layer: Deposit a transition layer using a compatible alloy (e.g., nickel-based) to reduce dilution of the hardfacing layer and improve interface bonding
- Hardfacing layers: Apply 2–3 passes of WC-Co overlay electrode with careful control of heat input
- Interpass cleaning: Remove oxide and slag between passes to ensure layer bonding
- Post-weld treatment: Allow controlled cooling to prevent thermal cracking; stress relief if required
Technical Challenges and Countermeasures
| Challenge | Cause | Countermeasure |
|---|---|---|
| WC particle degradation | Excessive heat input melts and decomposes WC | Low heat input, short arc length |
| Cracking in overlay | High thermal expansion mismatch | Base layer, controlled cooling |
| Poor adhesion | Contamination, inadequate roughness | Thorough cleaning, proper surface prep |
| Uneven coverage | Inconsistent technique | Jig fixtures, systematic welding pattern |
| Pore formation | Hydrogen absorption | Electrode baking, dry conditions |
Engineering Practice Integration
The study provides practical guidance for power plant maintenance engineers:
Application to Other Wear Components
The same overlay welding approach can be applied to:
- ID fan blades: Similar wear conditions from fly ash-laden gas
- Boiler burners: Abrasion from pulverized coal flow
- Dust collector hoppers: Wear from particle impact and sliding
- Pneumatic conveying pipes: Internal abrasion from material flow
Economic Analysis
| Cost Item | Without Overlay | With Overlay | Saving |
|---|---|---|---|
| Impeller replacement (annual) | ¥85,000 | ¥0 (replaced every 3 years) | ¥56,667 |
| Overlay welding (initial) | ¥0 | ¥12,000 | - |
| Downtime cost (per replacement) | ¥30,000 | ¥10,000 (less frequent) | ¥20,000 |
| Annual total | ¥115,000 | ¥21,667 | ¥93,333 |
Key Questions and Reflections
Several technical considerations deserve further examination:
- The long-term stability of WC-Co overlay welds under thermal cycling conditions (common in power plant fans operating near hot gas) requires investigation. Thermal fatigue may cause microcracking in the overlay layer over extended service periods.
- The dilution control strategy is critical. If the base material dilution exceeds 35–40%, the hardness and abrasion resistance of the overlay will be significantly reduced.
- The transition layer (base layer) approach mentioned by the authors is essential for preventing cracking but adds complexity to the welding procedure. The selection of transition layer material must be carefully matched to both the base material and the hardfacing alloy.
Study Insights and Implications
This paper represents a classic example of integrating tribological analysis with welding technology to solve a practical engineering problem. The systematic approach of first understanding the wear mechanism and then selecting an appropriate overlay welding solution is the correct engineering methodology.
The key insight for practicing engineers is that overlay welding is not a universal solution—it must be matched to the specific wear mechanism. For three-body abrasion (as in this case), a WC-containing overlay is appropriate. For adhesive wear, a different alloy system would be required. For erosion-corrosion, a chromium-based overlay might be more suitable. Understanding the wear mechanism is therefore the essential first step in any overlay welding application.
The economic analysis demonstrates that even relatively expensive overlay welding operations can provide substantial cost savings when compared to component replacement, particularly when downtime costs are included. This economic argument is often the most compelling justification for implementing overlay welding solutions in industrial maintenance programs.