CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Centrifugal Fan Overlay Welding Electrodes
Literature Overview and Background
Centrifugal fans operating in abrasive service environments such as cement plants, power stations, and mineral processing facilities suffer severe erosion of impeller blades and shrouds. The literature under review investigates the microstructural evolution and tribological performance of overlay welding electrodes specifically designed for centrifugal fan applications. The study examines how electrode composition, welding parameters, and heat input influence the resulting overlay microstructure and its resistance to abrasive wear.
The motivation for this research stems from the practical observation that conventional carbon steel or low-alloy steel impellers fail within weeks under high-velocity particulate-laden gas streams. Overlay welding with hardfacing electrodes offers a cost-effective alternative to full replacement with exotic materials. However, the selection of appropriate electrode type and welding procedure remains a critical engineering challenge, as improper selection can lead to cracking, spalling, or insufficient hardness retention after multiple thermal cycles.
Core Technical Content
Electrode Classification and Composition
The study categorizes overlay welding electrodes for centrifugal fan applications into several compositional families, each offering distinct wear mechanisms:
| Electrode Type | Typical Composition | Hardness (HV) | Primary Wear Mechanism |
|---|---|---|---|
| Martensitic (Cr-C) | 5-10% Cr, 0.3-0.6% C | 450-600 | Hardness-based abrasion resistance |
| High-Cr Martensitic | 18-26% Cr, 0.6-1.2% C | 550-700 | Combined hardness and corrosion resistance |
| Austenitic (Ni-Cr-C) | 20-30% Ni, 5-10% Cr | 200-350 | Work-hardening under impact |
| Hardfacing (Cr-Fe-B-C) | 15-25% Cr, 2-5% B, 2-4% C | 800-1200 | Ceramic carbide phase reinforcement |
| Stellite-based (Co-Cr) | 20-30% Cr, 0.5-1.5% C, balance Co | 350-500 | High-temperature oxidation and abrasion |
Microstructural Analysis
The study employs metallographic examination to identify phase transformations within the overlay weld. For martensitic electrodes, the rapid solidification during welding produces a lenticular martensite structure with retained austenite content typically between 5% and 15%. The retained austenite fraction is strongly influenced by carbon and alloy content, and its stability under thermal cycling determines long-term wear behavior.
For high-chromium martensitic electrodes, the formation of M7C3 and M23C6 carbides along grain boundaries contributes significantly to hardness. The study notes that excessive carbide network formation can compromise toughness and lead to intergranular cracking during service, particularly when the overlay is subjected to thermal shock from cyclic gas temperature variations.
Wear Test Results
Abrasive wear testing was conducted using the ASTM G65 dry sand-rubber wheel method and the ASTM G99 sliding wear test. Key findings include:
- Martensitic overlays with HV 550-600 achieved specific wear rates of 8-12 mg/N·m against silica sand at 28 μm grain size.
- Hardfacing Cr-Fe-B-C overlays demonstrated specific wear rates of 2-4 mg/N·m, representing a 3-4× improvement over martensitic types.
- Austenitic overlays showed initial low hardness but exhibited progressive work-hardening, reaching HV 400-500 after extended sliding distance, indicating their suitability for impact-abrasion environments.
Process Parameters and Their Influence
Heat Input Effects
The study systematically varied welding heat input from 1.2 kJ/mm to 4.5 kJ/mm and observed distinct microstructural transitions:
| Heat Input (kJ/mm) | Overlay Microstructure | Hardness (HV) | Wear Rate (mg/N·m) |
|---|---|---|---|
| 1.2-1.8 | Fine martensite, 5-8% retained austenite | 580-620 | 9-11 |
| 2.0-3.0 | Coarse martensite, 10-15% retained austenite | 520-560 | 11-14 |
| 3.5-4.5 | Coarse martensite + carbide network, 15-20% retained austenite | 480-520 | 14-18 |
The study concludes that optimal heat input for centrifugal fan overlay applications lies in the range of 1.5-2.5 kJ/mm, balancing hardness retention with crack resistance. Excessive heat input promotes carbide coarsening and retained austenite instability, both detrimental to long-term wear performance.
Preheat and Interpass Temperature
Preheating the base metal to 150-250°C is recommended to reduce hydrogen-induced cracking susceptibility, particularly for high-carbon martensitic electrodes. However, the study warns that preheat temperatures exceeding 300°C can promote excessive retained austenite formation and subsequent softening during cooling. The interpass temperature should be maintained below 250°C to ensure adequate martensitic transformation in subsequent passes.
Common Defects and Countermeasures
The study identifies several prevalent defects in centrifugal fan overlay welding:
- Cracking: Longitudinal cracks in the overlay weld metal, typically associated with high carbon content and low ductility of martensitic microstructure. Countermeasures include selecting lower-carbon electrodes, applying controlled preheat, and implementing post-weld stress relief at 550-600°C.
- Spalling: Delamination of the overlay from the base metal, caused by poor metallurgical bonding or differential thermal expansion. The study recommends ensuring a clean, oxide-free interface and using transition layers where necessary.
- Insufficient penetration: Resulting in incomplete fusion at the overlay-base metal interface. This is mitigated by optimizing electrode diameter, current density, and travel speed.
Engineering Practice Integration
From a practical standpoint, the study's findings align with industry experience in cement plant fan maintenance. A typical centrifugal fan impeller operating at 1450 rpm with a gas flow of 50,000 m³/h carrying 15 g/m³ of silica dust can achieve 6-12 months of service life with a 6-8 mm martensitic overlay, compared to 2-3 months with bare carbon steel. The study's recommendation of controlled heat input and preheat temperature is directly applicable to field repair procedures where portable welding equipment is used.
The PDCA cycle can be applied to overlay welding quality management: Plan by selecting appropriate electrode composition based on wear mechanism analysis; Do by implementing the optimized welding procedure with controlled parameters; Check by conducting hardness surveys, metallographic examination, and wear testing on coupon samples; Act by adjusting parameters based on inspection results and field performance feedback.
Key Questions and Reflections
The study raises several important questions for further investigation. First, the long-term stability of retained austenite under cyclic thermal loading remains uncertain, as laboratory wear tests typically involve isothermal conditions that do not replicate the thermal cycling experienced in service. Second, the interaction between overlay microstructure and gas-side erosion angle is not adequately addressed; oblique impingement of abrasive particles can produce different wear mechanisms than normal incidence.
The study's emphasis on microstructural control through heat input management is particularly valuable. In practice, many field operators focus solely on achieving target hardness without considering the underlying microstructural drivers of wear resistance. The study demonstrates that hardness alone is an insufficient predictor of wear life; the type, distribution, and stability of carbide phases are equally important.
Study Insights and Implications
This literature provides a solid foundation for understanding the structure-property relationships in centrifugal fan overlay welding. The systematic correlation between electrode composition, welding parameters, microstructure, and wear performance enables more rational material and procedure selection. The emphasis on microstructural analysis rather than empirical hardness measurement represents a significant methodological advancement in overlay welding practice.
For engineers involved in fan maintenance and repair, the practical implications are clear: selecting the appropriate overlay electrode requires consideration of the specific wear mechanism (sliding, impinging, or impact-abrasion), the operating temperature regime, and the available welding equipment. The study's parameter recommendations serve as a reliable starting point for procedure development, subject to qualification testing on representative coupon configurations.
Conclusion
The study of microstructure and wear resistance in centrifugal fan overlay welding electrodes offers valuable insights into the complex interplay between composition, processing, and performance. By systematically examining how welding heat input governs microstructural evolution and subsequently influences abrasive wear behavior, the research provides actionable guidance for engineers tasked with extending fan service life through overlay welding. The key takeaway is that overlay welding success depends not merely on achieving a target hardness but on engineering a stable, wear-resistant microstructure through careful control of composition and thermal parameters.