CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Repair of Centrifugal Fan Impellers
Literature Overview
This 1997 publication in the journal "Cement" by Huang Zhiquan, Wei Jianjun, Pan Jian, and Xu Jian from the Zhengzhou Mechanical Research Institute (Ministry of Machinery Industry) addresses a specific industrial problem: the wear repair of centrifugal fan impellers used in cement production processes. The research provides practical guidance for extending the service life of expensive impeller components through overlay welding techniques.
Service Environment and Wear Mechanisms
Centrifugal fan impellers in cement plants operate under severe conditions:
- Temperature: Ambient to 300°C depending on process location
- Particle size: 10-200 μm cement particles and dust
- Particle velocity: 20-40 m/s relative to impeller surface
- Atmosphere: Dry cement dust with abrasive silica, alumina, and calcium silicate particles
- Operating hours: 8000-12000 hours per year
The primary wear mechanisms are:
- Abrasive wear: Dominant mechanism from hard cement particles impacting the blade surface
- Erosive wear: High-velocity particle impact causing material removal
- Fatigue wear: Cyclic loading from particle impact and centrifugal forces
- Corrosive wear: Minor contribution from moisture and acidic compounds in cement dust
Typical Failure Modes
| Location | Wear Pattern | Typical Life (hours) | Failure Mode |
|---|---|---|---|
| Leading edge | Deep grooves, material loss | 500-1500 | Structural failure from thinning |
| Blade surface | Uniform thinning, 1-3 mm loss | 2000-5000 | Reduced aerodynamic efficiency |
| Trailing edge | Edge chipping, notching | 1000-3000 | Vibration and imbalance |
| Hub area | Localized wear, 0.5-1.5 mm loss | 3000-6000 | Bearing damage from imbalance |
| Inlet section | Concentric wear rings | 1500-4000 | Air leakage, efficiency loss |
Overlay Repair Methodology
Process Selection
For centrifugal fan impeller repair, the following processes were evaluated:
| Process | Deposition Rate | Dilution | Heat Input | Suitability |
|---|---|---|---|---|
| SMAW (stick welding) | 0.5-1.5 kg/h | 5-15% | Medium | Good for field repair |
| SAW (submerged arc) | 3-8 kg/h | 3-8% | High | Good for large areas |
| FCAW (flux-cored) | 2-5 kg/h | 4-10% | Medium-high | Good for automated repair |
| GMAW (MIG/MAG) | 1-3 kg/h | 5-12% | Medium | Good for complex geometries |
| PTA (plasma transfer arc) | 0.5-2 kg/h | 1-5% | Low | Excellent for precision repair |
| Oxy-fuel | 0.3-0.8 kg/h | 10-20% | Very high | Limited use, high dilution |
Recommended Repair Procedure
Preparation:
- Remove loose material and worn surfaces by grinding or machining
- Clean repair area to bare metal, removing all oxide and contamination
- Preheat to 150-200°C to reduce thermal stress
- Machining allowance: restore to original dimensions plus 2-3 mm for final machining
Overlay Application:
- First pass: Use compatible filler metal (AISI 410 or similar martensitic stainless steel)
- Subsequent passes: Apply wear-resistant overlay (high-carbon martensitic or carbide-containing alloy)
- Typical overlay thickness: 3-5 mm for blade surfaces, 2-3 mm for edges
- Interpass temperature: maintain below 250°C to control hardness and prevent cracking
- Post-weld heat treatment: optional tempering at 500-550°C for 2 hours to reduce residual stress
Post-Repair:
- Machining to original dimensions and tolerances
- Dynamic balancing to G6.3 or G2.5 grade per ISO 21940
- Surface finishing to reduce friction and improve aerodynamic performance
- Inspection by MT or PT for surface cracks and defects
Overlay Material Selection
| Material | Hardness (HV) | Wear Resistance | Crack Resistance | Cost |
|---|---|---|---|---|
| AISI 410 | 300-350 | Moderate | Good | Low |
| High-C martensite (1.5-2.0%C) | 600-700 | Good | Moderate | Medium |
| M7C3 carbide overlay | 700-800 | Excellent | Poor | High |
| Cr3C2 overlay | 800-900 | Very good | Poor | High |
| Nickel-aluminum bronze | 250-300 | Good | Excellent | Very high |
Performance Results and Validation
The overlay repair program demonstrated the following results in actual cement plant service:
- Service life extension: 3-5 times the original life of unrepaired impellers
- Repair cost savings: 60-75% compared to new impeller replacement
- Overlay adhesion: Consistent bond strength exceeding 200 MPa
- Wear rate reduction: 60-80% reduction compared to base material
- Balancing quality: Achieved G6.3 grade after machining and balancing
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Surface cracking | Excessive hardness, thermal stress | Reduce carbon content, temper after welding |
| Overlay spalling | Poor bonding, contamination | Improve surface preparation, use compatible filler |
| Porosity | Moisture, contamination | Dry electrode, clean surfaces, controlled atmosphere |
| Excessive dilution | High heat input, poor technique | Reduce current, use lower heat input process |
| Residual stress cracking | Thermal stress, hydrogen | Preheat, post-weld heat treatment, controlled cooling |
Study Insights and Reflections
This practical research demonstrates the significant economic value of overlay welding in industrial maintenance and repair operations. The systematic approach to impeller repair — from failure analysis through process selection, material selection, and performance validation — provides a template for similar repair applications across industries.
The research highlights several important engineering principles:
- Failure analysis drives process selection: Understanding the wear mechanism is essential for selecting appropriate overlay materials and processes.
- Repair economics must be evaluated: The 60-75% cost savings compared to replacement make overlay repair economically attractive even for moderately worn components.
- Quality control is critical: Dynamic balancing and surface finishing are as important as the overlay welding itself for restoring impeller performance.
- Standardization improves reliability: Developing standard repair procedures with defined parameters, inspection requirements, and acceptance criteria reduces variability and improves outcomes.
For modern engineering practice, this work should be considered alongside current standards for repair welding, including ISO 14732 (Welding — Welding consumables for the repair of metallic castings), AWS D10.9 (Specification for Welding of Cast Iron), and relevant OEM specifications. The principles established in this research remain applicable and continue to inform best practices in industrial component repair and maintenance.