Zhuojin Pipe Fitting Co., LtdWear Mechanism of Induced Draft Fan Impeller and Anti-Wear Overlay Welding
Literature Overview
This paper by Peng Zhenwei, Liu Yongsheng, Wang Li, and Wang Zongyu, published in 1999 in the Journal of Jiamusi University (Natural Science Edition), investigates the wear mechanism of induced draft fan impellers in power plant applications and proposes an anti-wear overlay welding solution. The research was conducted by personnel from Jiamusi Municipal Company and Jiamusi Power Plant, reflecting a direct industry-academia collaboration. The classification codes TM621 (power plant equipment) and TG455 (welding processes) indicate the interdisciplinary nature of the work.
Core Technical Content
The authors analyze the wear mechanism of induced draft fan impellers and identify the root cause of the observed wear patterns. The wear surfaces exhibit a plough-like morphology, which the authors attribute to the presence of hard quartz and feldspar particles in low-quality coal. These abrasive particles are entrained in the flue gas and impact the impeller blades at high velocity, causing progressive material removal.
Wear Mechanism Analysis
The wear mechanism can be classified as abrasive wear with a cutting mechanism, where hard particles plough through the softer impeller material, removing material in the form of chips or grooves. The severity of wear is influenced by several factors:
| Factor | Effect on Wear Rate | Typical Value in Power Plant Application |
|---|---|---|
| Particle hardness | Higher hardness increases wear | Quartz: 7 Mohs, Feldspar: 6 Mohs |
| Particle size | Larger particles cause deeper grooves | 10-200 micrometers |
| Particle concentration | Higher concentration increases wear | 10-50 g/m3 |
| Gas velocity | Higher velocity increases impact energy | 15-25 m/s |
| Impeller material hardness | Higher hardness reduces wear | Base material: 150-200 HB |
| Impact angle | Normal impact maximizes wear | 30-60 degrees |
The plough-like wear pattern is characteristic of the cutting mechanism, where the hard particle acts as a cutting tool, removing material from the impeller surface in a manner analogous to machining. This mechanism is particularly severe at the leading edges of the impeller blades and at the areas where the gas flow direction changes.
Overlay Welding Solution
The authors propose using a special welding electrode designated as "Te Dui 707" (a tungsten carbide-containing electrode) to overlay weld the impeller blades. The tungsten carbide particles in the overlay layer provide exceptional hardness and wear resistance, significantly extending the service life of the impeller.
Overlay Welding Process Parameters
| Parameter | Specification |
|---|---|
| Electrode type | Te Dui 707 (WC-containing) |
| Electrode diameter | 3.2-4.0 mm |
| Welding current | 120-180 A (DC) |
| Arc voltage | 22-28 V |
| Travel speed | 150-250 mm/min |
| Preheat temperature | 150-250 degrees C |
| Interpass temperature | Maximum 250 degrees C |
| Overlay layer thickness | 3-5 mm per pass |
| Number of passes | 2-3 passes |
| Post-weld treatment | Stress relief at 550-600 degrees C |
The overlay welding process requires careful control to ensure adequate bonding between the overlay layer and the base metal while maintaining the hardness of the tungsten carbide particles. The preheat temperature is selected to reduce thermal gradients and minimize the risk of cracking in the base metal HAZ. The interpass temperature limit prevents excessive softening of the previously deposited overlay layer.
Microstructural Considerations
The overlay layer microstructure consists of tungsten carbide particles dispersed in a martensitic or austenitic matrix, depending on the electrode composition and welding parameters. The hardness of the overlay layer is typically in the range of 60-70 HRC, compared to 150-200 HB for the base impeller material. This hardness differential provides excellent wear resistance while maintaining adequate toughness in the base metal.
The dilution between the base metal and the overlay layer is a critical factor in determining the final overlay hardness. Excessive dilution can reduce the overlay hardness below the required threshold, while insufficient dilution can lead to poor bonding and cracking. The welding parameters must be optimized to achieve a dilution ratio of 20-30 percent, which provides an optimal balance between hardness and bonding strength.
Engineering Practice Implications
For power plant engineers, the overlay welding solution presented in this paper offers a cost-effective method to extend the service life of induced draft fan impellers. The replacement cost of a new impeller can be substantial, and the downtime associated with replacement can result in significant production losses. Overlay welding repairs can extend the impeller service life by 3-5 times compared to the original material, providing a compelling economic and operational benefit.
Maintenance Schedule and Inspection
The overlay welded impeller should be inspected at regular intervals to assess the wear condition of the overlay layer. The inspection frequency depends on the severity of the service conditions and the expected wear rate. A typical inspection schedule might include:
- Visual inspection every 3 months for surface condition and cracking
- Ultrasonic thickness measurement every 6 months to assess overlay layer remaining thickness
- Full replacement or re-overlay when the overlay layer thickness falls below 2 mm
- Metallographic examination of worn areas to assess wear mechanism and overlay integrity
Study Insights and Reflections
This paper represents a practical, field-driven approach to solving a specific industrial problem. The identification of the wear mechanism through observation of the wear pattern morphology is a straightforward but effective diagnostic method that can be applied to other wear problems in industrial equipment. The selection of a tungsten carbide-containing electrode is a well-established approach for abrasive wear protection, and the process parameters described are consistent with industry practice.
However, the paper is limited in its discussion of the long-term performance of the overlay layer under cyclic thermal loading, which is common in power plant applications. The thermal fatigue behavior of the overlay layer, particularly at the overlay-base metal interface, can significantly affect the service life of the repaired impeller. Future research should investigate the thermal fatigue resistance of tungsten carbide overlay layers under conditions representative of induced draft fan service.
In conclusion, this study provides a valuable case study in the application of overlay welding technology to solve a specific industrial wear problem, and its methodology can be adapted to similar wear protection challenges in other industrial equipment including pipe fittings, valves, and pump components.