Zhuojin Pipe Fitting Co., LtdWear-Resistant Surfacing Repair of Centrifugal Exhaust Fan Impellers in Cement Equipment
Literature Overview
This 1997 paper by Huang Zhiquan and colleagues from the Zhengzhou Research Institute of Mechanical Engineering addresses a persistent operational challenge in cement grinding circuits: the progressive erosion of centrifugal exhaust fan impellers due to abrasive particulate loading. The study documents a practical surfacing repair methodology using dedicated wear-resistant welding electrodes, offering a cost-effective alternative to full impeller replacement. The work appeared in the journal "Cement" (水泥), reflecting its direct industrial relevance to the cement manufacturing sector.
Core Technical Approach
The fundamental problem lies in the interaction between high-velocity cement dust-laden gas streams and the impeller blade surfaces. In cement mill ventilation systems, gas velocities typically range from 25 to 40 m/s at the impeller tip, carrying fine particles (median size 10-60 μm) that induce severe erosive wear over time. The authors evaluated several surfacing electrode compositions and identified the optimal combination for this specific service environment.
The repair methodology follows a systematic sequence:
- Surface preparation – Removal of the worn layer and any loose material through grinding or wire brushing to expose sound base metal
- Preheating – Application of controlled preheat (typically 150-250°C) to mitigate hydrogen-induced cracking in the HAZ and reduce thermal stress gradients
- Surfacing deposition – Multi-pass SMAW welding using wear-resistant electrode compositions, with careful control of travel speed and overlap
- Post-weld treatment – Stress relief annealing where required, followed by dimensional verification and balancing
Surfacing Electrode Selection Criteria
| Parameter | Typical Specification | Rationale |
|---|---|---|
| Electrode type | Nickel-based or high-carbon steel with Cr-Cr₂O₃ | Provides composite structure resistant to both abrasion and thermal fatigue |
| Hardness (as-welded) | 45-55 HRC | Balances wear resistance against brittleness and spalling risk |
| Dilution rate | 15-25% (multi-pass) | Ensures sufficient hard phase content in the final surface layer |
| Maximum interpass temperature | 150°C | Prevents over-tempering and maintains microstructural integrity |
| Preheat temperature | 150-250°C | Reduces cooling rate to avoid martensitic transformation in HAZ |
Metallurgical Considerations
The wear mechanism in cement service is predominantly three-body abrasion, where hard particles trapped between the impeller surface and the gas stream cause progressive material removal. The surfacing layer must therefore incorporate hard phases (Cr₇C₃, Cr₃C₂ carbides, or Ni₃B) dispersed in a ductile matrix to accommodate thermal cycling without catastrophic spalling. The authors emphasized that single-pass surfacing often results in excessive dilution, leading to hardness values below 35 HRC and premature failure. A minimum of two to three passes is recommended to achieve the target composite microstructure.
Engineering Practice Integration
From a production planning perspective, this repair approach offers significant economic advantages. A typical cement kiln exhaust fan impeller weighs 2-8 tonnes, and replacement requires not only material cost but also extended downtime for removal, machining, and reinstallation. Surfacing repair can be completed in 2-3 days versus 4-6 weeks for a new fabrication. However, the repair must be followed by precise dynamic balancing to prevent vibration issues at operating speeds (typically 1500-2980 rpm). The residual stress introduced by surfacing can cause dimensional distortion of 0.5-1.5 mm over a blade length of 600-1200 mm, necessitating post-weld machining to restore aerodynamic profiles.
Study Insights and Reflections
This paper, while dated, addresses a fundamentally relevant engineering challenge that persists in modern cement plants. The systematic approach to electrode selection, process parameter control, and post-weld verification remains valid. The key insight is that wear-resistant surfacing is not merely a coating application but a metallurgical design exercise requiring understanding of the base metal's thermal response, the deposit's phase evolution, and the operational loading conditions. The economic case for repair over replacement is compelling when lifecycle costs are properly assessed, including environmental considerations of avoiding large-scale steel fabrication and disposal.