CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Centrifugal Fan Overlay Electrode
Literature Overview
The study by Xu Xiangyang (North China Electric Power University, 2001), published in the journal China Electric Power, investigates the microstructure and wear resistance of overlay electrodes used for centrifugal fan blade repair. Centrifugal fans are critical components in power plant boiler systems, where they are used to supply combustion air and flue gas. The fan blades are subjected to severe abrasive wear from the continuous impact of air and particulate matter, and the overlay electrode must be designed to provide maximum wear resistance under these conditions. The study focuses on the relationship between the microstructure of the overlay deposit and its wear resistance, which is essential for optimizing the overlay material and process for centrifugal fan applications.
Core Technical Content
The microstructure of the overlay deposit is the primary determinant of its wear resistance. For centrifugal fan blade applications, the overlay deposit must have a microstructure that provides a balance of hardness, toughness, and abrasion resistance. The following microstructural features are critical for wear resistance:
- Hard phases: Carbides, borides, and intermetallic compounds provide the primary abrasion resistance.
- Matrix hardness: The hardness of the matrix phase affects the overall wear resistance of the deposit.
- Grain size: Fine grains provide better toughness and abrasion resistance than coarse grains.
- Phase distribution: A uniform distribution of hard phases provides better wear resistance than a segregated distribution.
The overlay electrode composition is typically a high-carbon martensitic alloy with 1.5-2.5% C and 8-12% Cr. The high carbon content promotes the formation of hard carbides, while the chromium content improves the hardenability and oxidation resistance of the alloy. The microstructure of the overlay deposit is characterized by the following phases:
| Phase | Composition | Hardness (HV) | Function |
|---|---|---|---|
| Martensite | Fe, C, Cr | 600-800 | Matrix, provides toughness |
| Chromium carbides | Cr7C3, Cr23C6 | 1200-1500 | Abrasion resistance |
| Cementite | Fe3C | 800-1000 | Secondary hard phase |
| Ferrite | Fe | 200-300 | Soft phase, reduces toughness |
The hardness of the overlay deposit is typically in the range of HRC 50-60, which provides good abrasion resistance while maintaining adequate toughness for impact loading. The wear resistance of the overlay deposit is measured by the standard pin-on-disk test, and the results show that the wear resistance increases with increasing hardness and decreasing grain size.
Process Parameters and Microstructure Control
The welding process parameters have a significant effect on the microstructure and wear resistance of the overlay deposit. The following parameters are critical for controlling the microstructure:
- Welding current: Controls the heat input and the deposition rate
- Welding voltage: Controls the arc length and the penetration depth
- Welding speed: Controls the heat input and the cooling rate
- Shielding gas: Affects the oxidation and nitrogen pickup in the deposit
- Interpass temperature: Controls the thermal cycle and the phase transformation
The cooling rate is the most critical parameter for controlling the microstructure of the overlay deposit. A high cooling rate promotes the formation of fine, needle-like martensite and carbides that provide high hardness but low toughness. A low cooling rate allows for the formation of coarser martensite and carbides that provide lower hardness but better toughness. The optimal cooling rate depends on the specific application requirements.
The heat input is also critical because it affects the dilution ratio between the base metal and the overlay material. A high heat input increases the dilution ratio, which can alter the composition of the overlay deposit and reduce the hardness. A low heat input reduces the dilution ratio, which can lead to a more compositionally pure overlay deposit with higher hardness. The optimal heat input depends on the base material and the desired overlay properties.
Defect Analysis and Countermeasures
The following table summarizes the common defects and their countermeasures in centrifugal fan blade overlay welding:
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Cracking | Excessive brittleness, high residual stress | Use proper material, control heat input, preheat adequately, PWHT |
| Porosity | Gas porosity from hydrogen or nitrogen | Use dry flux, clean surface, ensure adequate shielding |
| Excessive hardness | High cooling rate, low heat input | Control cooling rate, increase heat input, use proper welding parameters |
| Insufficient hardness | High dilution, improper composition | Control dilution, use proper material, ensure adequate composition |
| Spalling | Poor metallurgical bond, thermal cycling | Use proper bond layer, control interpass temperature, PWHT |
Engineering Practice Integration
In power plant applications, the overlay repair of centrifugal fan blades is a routine maintenance task that requires careful attention to the metallurgical and mechanical properties of the overlay deposit. The overlay material must be selected based on the specific service conditions, including the temperature, the particulate loading, and the impact loading. The welding process must be optimized to ensure that the overlay deposit has the required microstructure and properties.
The study by Xu Xiangyang provides valuable insights into the relationship between the microstructure and wear resistance of overlay electrodes for centrifugal fan applications. The key finding is that the wear resistance of the overlay deposit is primarily determined by the hardness and grain size of the microstructure. The optimal welding process parameters must be selected to ensure that the overlay deposit has the required microstructure and properties. For engineers working on centrifugal fan blade repair, the key lessons are: always select the overlay material based on the specific service conditions, always optimize the welding process parameters to control the microstructure, and always perform post-weld heat treatment to improve the mechanical properties.
Summary and Reflections
The research by Xu Xiangyang provides a comprehensive understanding of the microstructure and wear resistance of overlay electrodes for centrifugal fan blade repair. The study demonstrates that the wear resistance of the overlay deposit is primarily determined by the hardness and grain size of the microstructure, and that the welding process parameters must be carefully controlled to achieve the required microstructure. The optimal overlay material and process parameters depend on the specific service conditions, and the selection must be based on a thorough understanding of the metallurgical and mechanical requirements. For engineers working on centrifugal fan blade repair, the key takeaway is that the overlay deposit must be designed and processed to provide a balance of hardness, toughness, and abrasion resistance, and that this balance is achieved through careful control of the material composition and welding process parameters. This work exemplifies how a systematic approach to material selection and process optimization can lead to significant improvements in the performance and service life of critical power plant components.