Zhuojin Pipe Fitting Co., LtdStudy Note on Wear-Resistant Surfacing Electrode Development for Centrifugal Induced Draft Fans
Literature Overview
Liu Pengshan's paper, published in "Fan Technology" (风机技术) in 1991 (Volume 33, Issue 4, pages 30-32), addresses the development of specialized surfacing electrodes for centrifugal induced draft fans in power generation and industrial process applications. These fans operate in extremely harsh environments, handling flue gas containing fine particulate matter (fly ash, dust) at elevated temperatures. The blade erosion in such fans can reduce efficiency by 15-25% within months of operation, making wear-resistant surfacing a critical maintenance strategy.
Technical Background and Failure Analysis
Centrifugal induced draft fans in coal-fired power plants handle flue gas at temperatures of 120-200°C, containing fly ash particles with hardness ranging from Mohs 5-7 (quartz, feldspar). The erosion mechanism is primarily two-body abrasion, where hard particles impact the blade surface at angles of 15-45 degrees. This oblique impact angle produces a cutting-type wear mechanism that is particularly damaging to ductile materials.
Failure Mode Analysis Using FMEA Approach
| Failure Mode | Severity | Occurrence | Detection | RPN |
|---|---|---|---|---|
| Leading edge erosion | 9 | 8 | 5 | 360 |
| Trailing edge thinning | 7 | 7 | 6 | 294 |
| Blade root fatigue cracking | 8 | 5 | 7 | 280 |
| Coating spalling | 6 | 6 | 4 | 144 |
| Base metal corrosion under coating | 5 | 4 | 8 | 160 |
The leading edge of the fan blade is the most critical area, as it first encounters the erosive flow and experiences the highest impact velocity of particles. The surfacing strategy must therefore provide maximum protection at this location.
Electrode Development and Material Design
The paper focuses on the development of specialized SMAW electrodes with coating compositions optimized for fan blade service. The key design considerations include:
Electrode Coating Composition Optimization
| Component | Content Range | Function |
|---|---|---|
| Carbon (C) | 1.8-3.5% | Carbide formation for hardness |
| Chromium (Cr) | 15-25% | Oxidation resistance, carbide stabilization |
| Molybdenum (Mo) | 2-5% | Solid solution strengthening, wear resistance |
| Vanadium (V) | 1-3% | Fine carbide precipitation (VC, V4C3) |
| Titanium (Ti) | 0.5-2.0% | Carbide formation, microstructure refinement |
| Silicon (Si) | 0.5-1.5% | Deoxidizer, strength enhancement |
| Manganese (Mn) | 1.0-2.5% | Desulfurizer, hardenability improvement |
Microstructural Requirements
The optimal deposit microstructure for fan blade surfacing consists of:
- A martensitic matrix providing base hardness of HRC 45-55
- Fine dispersion of Cr7C3, Cr23C6, and VC carbides providing resistance to abrasive cutting
- Absence of coarse carbide agglomerates that could act as crack initiation sites
- Uniform grain size throughout the deposit thickness
Process Parameters and Application
The surfacing application on fan blades requires careful management of heat input to prevent distortion of the thin-walled blade structure (typically 3-8 mm thick). The following parameters are critical:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Preheating temperature | 100-150°C | Reduce thermal stress, prevent cold cracking |
| Interpass temperature | ≤200°C | Control cooling rate for fine microstructure |
| Arc voltage | 24-28V | Moderate penetration for good fusion |
| Travel speed | 8-12 cm/min | Balance deposit thickness and heat input |
| Electrode angle | 70-80° from horizontal | Optimal penetration and bead shape |
| Number of passes | 2-3 | Achieve required thickness (2-4 mm) |
Post-Weld Heat Treatment
For fan blades requiring both wear resistance and fatigue resistance, a tempering treatment at 550-600°C for 2-4 hours is recommended after surfacing. This treatment:
- Relieves welding residual stresses that could initiate fatigue cracks
- Transforms brittle martensite to tempered martensite, improving toughness
- Maintains adequate hardness (HRC 45-52 after tempering) for wear resistance
Engineering Practice and Maintenance Strategy
In power plant practice, fan blade surfacing is typically performed during scheduled outages (every 6-12 months of operation). The inspection protocol involves:
- Thickness measurement: Ultrasonic thickness gauging at designated measurement points to determine remaining blade thickness.
- Surface profiling: Three-dimensional surface scanning to map erosion patterns and identify critical areas.
- Crack inspection: Magnetic particle or dye penetrant testing of the blade root and weld joints.
- Hardness mapping: Portable hardness testing to verify coating integrity and identify areas requiring re-surfacing.
The economic analysis shows that proper surfacing can extend blade life by 3-5 times compared to uncoated blades, with the total cost of ownership being significantly lower than blade replacement at each outage.
Study Insights and Reflections
This paper represents an important contribution to the development of specialized welding consumables for power generation equipment in China. The systematic approach to electrode development—starting from failure analysis, through material design, to process optimization—provides a model for similar applications in other industries. The emphasis on the specific erosion mechanism (oblique particle impact) and its implications for material microstructure is particularly valuable.
The work also highlights the importance of considering the entire component lifecycle in surfacing design. The coating must not only resist the primary wear mechanism but also maintain adequate toughness to withstand cyclic thermal and mechanical loading. This holistic approach, which considers both wear resistance and structural integrity, is essential for reliable long-term performance in power generation service.