Wear Mechanism Analysis of Exhaust Fan Impellers and Anti-Wear Weld Overlay Technology
1. Introduction and Technical Context
The exhaust fan impeller (排粉机叶轮) is a critical rotating component in coal-fired power plant forced draft and induced draft fan systems, as well as in cement kiln exhaust systems and industrial dust collection networks. These impellers operate under extremely harsh conditions characterized by high-temperature flue gas, abrasive particulate matter (fly ash, silica particles, and coal fines), high rotational speeds, and continuous cyclic loading. The wear mechanism of exhaust fan impellers is complex and multifactorial, involving a combination of abrasive wear, erosive wear, corrosion-wear synergy, and fatigue spalling. Understanding these mechanisms is essential for selecting appropriate overlay metallurgy and welding processes to extend component service life and reduce unplanned shutdowns.
This technical entry represents a knowledge synthesis document produced by Cladding Technology Shanxi Co., Ltd., reflecting the company's deep engagement with industrial wear protection applications. It demonstrates the company's capability to not only execute weld overlay operations but also to diagnose failure modes, select appropriate metallurgical solutions, and deliver value-engineered repair and protection strategies for heavy-duty rotating equipment.
2. Wear Mechanism Analysis
2.1 Abrasive and Erosive Wear
The dominant wear mechanism in exhaust fan impellers is abrasive erosion caused by high-velocity impact of solid particulates (typically 10–200 μm in size) carried in flue gas streams. The erosive wear rate is governed by the following factors:
- Particle velocity: Erosive wear rate increases approximately with the square to cube of particle impact velocity. At impeller blade surfaces operating at peripheral speeds of 25–50 m/s, combined with gas flow velocities of 15–30 m/s, effective impact velocities can exceed 40 m/s.
- Particle hardness: Fly ash particles containing free SiO₂ (Mohs hardness 7) and Al₂O₃ (Mohs hardness 9) cause severe micro-cutting and ploughing damage on softer base materials.
- Impact angle: For ductile materials, maximum erosion occurs at shallow angles (15°–30°); for brittle materials, maximum erosion occurs at near-normal incidence (70°–90°).
- Particle size distribution: Larger particles cause deeper gouging; fine particles cause cumulative micro-abrasion leading to surface roughening.
2.2 Corrosion-Wear Synergy
In coal-fired boiler exhaust systems, flue gas contains SO₂, SO₃, HCl, and trace HF, which create acidic condensation on cooler blade surfaces. The corrosion-wear synergy mechanism operates as follows:
- Acidic condensate corrodes the base metal, forming soft oxide/hydroxide layers (FeOOH, FeSO₄).
- These corrosion products are mechanically removed by particle impact, exposing fresh metal.
- The exposed metal undergoes rapid re-corrosion, accelerating material loss by 2–5× compared to either mechanism alone.
2.3 Fatigue Spalling and Thermal Fatigue
Cyclic thermal loading (flue gas temperature fluctuations of 150°C–350°C) combined with centrifugal stress at the blade root induces micro-cracking. These micro-cracks serve as initiation sites for spalling, where sections of the base material detach as flakes, exposing fresh surface to accelerated erosion. This mechanism is particularly critical at the leading edge of blades where aerodynamic loading is highest.
3. Anti-Wear Weld Overlay Technology
3.1 Design Philosophy
The anti-wear weld overlay strategy for exhaust fan impellers is designed to create a functionally graded surface layer that combines:
- High hardness core: Carbide-forming alloys (Cr-C-Mo, Co-Cr, or Ni-Cr-C) providing hardness of HRC 50–65 to resist micro-cutting.
- Thermal stability: Alloy systems maintaining hardness above 350°C without significant softening.
- Toughness reserve: Adequate fracture toughness (KIC ≥ 20 MPa·m^½) to prevent brittle spalling under cyclic loading.
- Corrosion resistance: Chromium-rich microstructure (≥ 25% Cr) providing resistance to acid condensation corrosion.
- Metallurgical compatibility: Smooth transition from overlay to base material without brittle intermetallic formation.
3.2 Overlay Alloy Selection Matrix
| Application Zone | Wear Mechanism | Recommended Overlay Alloy | Hardness (HRC) | Key Standards |
|---|---|---|---|---|
| Blade Leading Edge | High-velocity erosion + fatigue | ASTM A213 T91 / Stellite 6 / D2 (high-carbon Cr) | 55–65 | ASTM A511, AWS A5.15 |
| Blade Suction Surface | Abrasive erosion (shallow angle) | Cr-C-Mo (e.g., D168 equivalent) / Ni-Cr-C | 45–55 | GB/T 12470, AWS A5.21 |
| Blade Pressure Surface | Erosion + corrosion synergy | Co-Cr (Stellite 6/21) / Ni-Cr-C-B | 50–60 | ASTM A511, NACE MR0175 |
| Hub and Shroud | Low-velocity wear + fatigue | Cr-Mo (D154 equivalent) / 309L transition | 35–45 | ASME IX, AWS D10.6 |
3.3 Multi-Layer Overlay Strategy
For critical impeller repairs, a multi-layer approach is recommended:
- Transition layer (1–2 passes): 309L or 310L stainless steel deposited via TIG to buffer dilution and prevent chromium carbide precipitation at the fusion boundary. Minimum thickness: 1.5 mm.
- Intermediate layer (2–3 passes): Cr-Mo alloy (e.g., D154/D168) deposited via MIG to build bulk hardness and thickness. Target thickness: 3–5 mm.
- Surface layer (2–4 passes): High-carbon Cr or Co-Cr alloy deposited via TIG with controlled interpass temperature to maximize carbide dispersion. Final thickness: 2–4 mm total overlay.
4. Key Process Parameters and Implementation Points
4.1 TIG Weld Overlay Parameters
| Parameter | Transition Layer (309L) | Intermediate Layer (Cr-Mo) | Surface Layer (High-Cr) |
|---|---|---|---|
| Shielding Gas | Ar (99.99%) | Ar (99.99%) | Ar + 2% H₂ or Ar + 5% He |
| Current (DCEN) | 80–120 A | 100–160 A | 70–110 A |
| Travel Speed | 80–120 mm/min | 60–100 mm/min | 50–80 mm/min |
| Wire Feed Rate | 250–350 mm/min | 300–450 mm/min | 200–300 mm/min |
| Interpass Temperature | ≤ 150°C | ≤ 120°C | ≤ 80°C |
| Bead Width | 12–18 mm | 15–22 mm | 10–15 mm |
| Pass Thickness | 1.0–1.5 mm | 1.5–2.5 mm | 0.8–1.5 mm |
4.2 Critical Implementation Controls
- Pre-heating: Base material pre-heated to 150–250°C for high-carbon steel impellers (e.g., 20G, 15CrMo) to reduce residual stress and prevent cold cracking. For stainless impellers, pre-heat to 100–150°C maximum.
- Dilution management: Penetration into base metal controlled at 20–30% maximum to preserve overlay alloy composition. Achieved through low current, high travel speed, and minimal arc dwell time.
- Weld direction: Overlay passes applied in a direction perpendicular to the primary wear trajectory to maximize surface hardness uniformity and minimize directional anisotropy.
- Post-weld treatment: Stress relief at 550–620°C for 2 hours for Cr-Mo substrates; 700–750°C for 1 hour for high-carbon overlay layers to convert retained austenite to fine carbides.
- Surface finishing: Post-overlay machining to specified aerodynamic profile (±0.5 mm tolerance) followed by shot peening (Al₂O₃ media, 100–150 μm, 0.3–0.5 m/s) to introduce compressive residual stress and enhance fatigue life.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- ASME Section IX: Qualification of welding procedures for overlay welding (Part QW-400 series for overlay requirements).
- AWS D10.6: Welding Procedure and Performance Qualification for Weld Overlay.
- GB/T 12470-2008: Welding consumables for weld overlay — General technical conditions.
- NB/T 20306-2015: Technical code for weld overlay in pressure equipment (if impeller is part of pressure system).
- ASTM A511: Specification for castings, iron cast, for general engineering purposes (Stellite alloys).
- AWS A5.15/A5.21: Standard specifications for consumable electrodes and wire for weld overlay.
5.2 Acceptance Criteria
| Inspection Item | Method | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Weld soundness (defects) | PT (Penetrant Testing) | No linear indications > 2 mm; no indications at stress concentration points | ASME V Article 7 / GB/T 18851 |
| Sub-surface defects | MT (Magnetic Particle Testing) | No Type 1 or 2 indications | ASME V Article 8 / NB/T 47013 |
| Overlay thickness | Ultrasonic thickness measurement | ≥ 90% of specified minimum thickness at all points | GB/T 19624 |
| Hardness | Micro-Vickers (HV 0.5) traverse | Uniform within ±5 HV across overlay; no soft spots < 90% of specification | ASTM E92 / GB/T 4340 |
| Dilution | Optical emission spectroscopy (OES) at weld root | Base metal dilution ≤ 30% (measured by Cr, Mo, Ni content deviation) | ASTM E1251 |
| Geometry (aerodynamic profile) | 3D laser scanning / CMM | Deviation ≤ ±0.5 mm from CAD model; surface roughness Ra ≤ 3.2 μm | ISO 1101 / Customer specification |
| Stress relief effectiveness | Residual stress measurement (XRD or hole-drilling) | Longitudinal residual stress ≤ 50 MPa in overlay | ASTM E653 / GB/T 17421 |
6. Common Risks and Controls
6.1 Metallurgical Risks
- Cracking at fusion boundary: High-carbon overlay alloys (D2, D168) are susceptible to hot cracking due to low melting point eutectics at grain boundaries. Control: Maintain interpass temperature below 80°C, use narrow bead width, and ensure proper pre-heat. Avoid sulfur and phosphorus exceeding 0.02% and 0.03% respectively.
- Intermetallic formation: Long dwell times or excessive heat input can form brittle Fe-Cr intermetallics at the transition layer boundary. Control: Limit single-pass heat input to 0.5–1.2 kJ/mm, use 309L (not 316L) for transition to avoid molybdenum enrichment.
- Softening of base material: Excessive heat input can temper the base material (particularly for quenched-and-tempered steels like 25Cr2MoV), reducing strength. Control: Limit total heat input per pass, use back-gas cooling, and monitor base metal hardness post-weld.
6.2 Process Risks
- Porosity: Hydrogen-induced porosity in high-carbon overlay layers. Control: Bake filler wire at 150°C for 2 hours prior to use; ensure shielding gas purity ≥ 99.99%; minimize arc length.
- Unbonded overlay: Insufficient fusion to base or previous layer creates delamination risk. Control: Maintain consistent travel speed; ensure proper edge preparation (V-groove or chamfer to expose clean base metal); verify fusion by MT inspection.
- Aerodynamic imbalance: Uneven overlay thickness distribution creates vibration at operating speed. Control: Apply overlay in symmetric pattern; machine to final profile post-weld; perform dynamic balance verification (ISO 21940, Grade G2.5 or better for impellers > 150 mm diameter).
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
This is the primary technology route for exhaust fan impeller anti-wear protection. The company's TIG/MIG weld overlay capability directly addresses this application through:
- Large-scale surface protection: MIG overlay (GMAW) for rapid deposition of intermediate layers on large blade surfaces (productivity: 0.5–1.0 kg/h).
- Precision surface finishing: TIG overlay (GTAW) for critical leading edges and thin-walled sections requiring tight geometry control.
- WPS qualification: Development and qualification of welding procedures per ASME IX and AWS D10.6 for specific base/overlay combinations (e.g., 20G base + 309L transition + D168 intermediate + D2 surface).
- Field repair capability: Portable TIG/MIG equipment for on-site repair of impellers without disassembly from fan housing.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding is primarily used for clad plate/pipe manufacturing, its relevance to impeller applications includes:
- Pre-fabricated impeller blanks: Manufacturing of duplex impeller blanks where a wear-resistant face layer (e.g., high-chromium cast iron) is bonded to a tough backing material (e.g., low-carbon steel or ductile iron) for the hub and shroud regions.
- Large-format cladding: Production of wear-resistant panels that can be mechanically attached or weld-bonded to impeller blade surfaces for modular protection systems.
- Material development: Optimization of bonding interface quality (shear strength ≥ 150 MPa) for novel wear alloy combinations.
7.3 Explosion Welding Route
Explosion welding contributes to this application through:
- Functionally graded impeller substrates: Production of explosion-welded laminates combining erosion-resistant surface layers with fatigue-resistant core materials for next-generation impeller designs.
- Co-Cr / Steel bonded plates: Manufacturing of Stellite 6 / carbon steel explosion-welded plates used as repair patches or reinforcement plates for heavily worn impeller sections.
- Research-grade material development: Exploration of novel high-entropy alloy / steel bonded systems for extreme wear environments (e.g., cement kiln exhaust at > 400°C).
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technical knowledge base directly supports the company's qualification development in the following ways:
- WPS/PQR documentation: The wear mechanism understanding enables rational selection of overlay alloys and process parameters, resulting in qualified welding procedures with documented performance data (hardness traverse, dilution analysis, erosion test results).
- Industry-specific certification: Demonstrated understanding of power plant component failure modes positions the company for qualification as a certified repair vendor for major equipment manufacturers (e.g., Dongfang Electric, Harbin Electric, Shanghai Electric) and their service networks.
- NDT capability validation: Knowledge of expected defect modes (hot cracks, lack of fusion, porosity) drives appropriate NDT method selection and acceptance criteria definition, supporting ASME "N" and "NT" stamp qualification.
8.2 Product Delivery Enhancement
- Extended service life: Properly executed anti-wear overlay extends impeller blade life from 6–12 months to 24–36 months in typical coal-fired boiler applications, reducing customer MRO costs by 40–60%.
- Reduced downtime: On-site TIG/MIG overlay capability enables repair during planned maintenance windows rather than requiring impeller replacement and extended shutdown.
- Customized solutions: Understanding of wear mechanism variation across blade sections enables tailored overlay thickness and alloy selection for each zone, optimizing cost-effectiveness.
8.3 Customer Value Proposition
The integration of wear mechanism analysis with precision weld overlay execution provides customers with a scientifically grounded, data-driven approach to impeller protection. Rather than applying generic overlay specifications, the company delivers solutions calibrated to the specific wear regime (erosion rate, particle characteristics, temperature profile, corrosion severity) of each customer's operating environment. This approach is validated through post-service retrieval and microstructural analysis, creating a continuous improvement loop that strengthens customer trust and long-term service relationships.
9. Performance Validation and Erosion Testing
Post-overlay performance is validated through standardized erosion testing:
- ASTM G73 (dry sand erosion): Overlay specimens subjected to 100 μm alumina particles at 30 m/s impact velocity. Target: erosion rate < 0.01 mg/g·m (≥ 5× improvement over bare base material).
- ASTM G76 (liquid solid particle erosion): For applications with wet flue gas conditions. Target: erosion rate < 0.5 mg/g·m.
- Field performance monitoring: In-service thickness measurement at 3-month intervals using ultrasonic gauges; erosion rate calculated from thickness loss vs. operating hours. Data feeds back into WPS optimization.
10. Conclusion
The technical understanding of exhaust fan impeller wear mechanisms, as documented in this entry, forms the intellectual foundation for Cladding Technology Shanxi Co., Ltd.'s anti-wear weld overlay service delivery. By combining rigorous metallurgical analysis with qualified TIG/MIG welding execution, supported by explosion welding and hydraulic explosive bonding for advanced substrate development, the company delivers comprehensive wear protection solutions that extend asset life, reduce operational costs, and minimize unplanned downtime for power generation and industrial process customers. This knowledge-to-execution pipeline represents a core competitive advantage in the industrial wear protection market and directly supports the company's strategic positioning as a technically differentiated cladding and overlay manufacturer.