CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of ZGM95G Coal Mill Roller and Mill Plate Liners
Literature Overview
This technical paper by Gong Junfeng from Beijing Jingneng Thermal Power Co., Ltd. addresses the cladding repair of ZGM95G coal mill rollers and mill plate liners in thermal power generation facilities. Published in Welding Technology in 2009, the work provides practical engineering solutions for extending the service life of critical coal grinding equipment in power plants.
Core Technical Content
ZGM95G is a high-chromium cast iron alloy widely used for coal mill components due to its excellent wear resistance. However, in service, these components suffer from progressive wear, necessitating periodic repair. The paper describes a cladding repair methodology using hardfacing alloys to restore the original dimensions and performance.
Component Specifications and Wear Characteristics
| Component | Material | Typical Thickness | Wear Rate | Service Life |
|---|---|---|---|---|
| Mill roller | ZGM95G | 120–180 mm | 0.1–0.3 mm/month | 12–18 months |
| Mill plate liner | ZGM95G | 60–100 mm | 0.15–0.4 mm/month | 10–15 months |
The wear mechanism in coal mills involves:
- Abrasive wear: Hard coal particles and mineral impurities cause surface removal
- Impact wear: Repeated hammering of coal particles against the surface
- Fatigue wear: Cyclic loading leads to crack initiation and propagation
- Corrosive wear: Moisture and chemical species in coal accelerate degradation
Cladding Repair Process
The repair process involves:
- Surface preparation: Grind the worn surface to remove damaged material and achieve a clean, flat surface
- Preheating: Heat the component to 300–400 °C to reduce thermal stress and cracking risk
- Cladding application: Apply hardfacing alloy using submerged arc welding (SAW) or flux-cored arc welding (FCAW)
- Post-weld treatment: Stress relief at 550–600 °C to reduce residual stresses
- Machining: Machine the cladded surface to restore original dimensions and surface finish
Hardfacing Alloy Selection
| Alloy Type | Composition | Hardness (HRC) | Application |
|---|---|---|---|
| High-Cr cast iron | 12–18% Cr, 3–4% C | 55–60 | General wear resistance |
| Carbide-enhanced | 10–14% Cr, 3–4% C, 10–15% WC | 60–65 | Severe abrasive wear |
| Martensitic | 8–12% Cr, 0.5–1.0% C | 50–58 | Impact-abrasion combination |
| Austenitic | 10–14% Cr, 0.3–0.6% C, 2–4% Ni | 35–45 | High-temperature service |
Engineering Practice and Maintenance Strategy
The paper emphasizes a systematic approach to coal mill component maintenance:
- Monitoring: Regular measurement of component thickness to predict remaining service life
- Preventive maintenance: Schedule cladding repair before critical wear limits are reached
- Quality control: Perform hardness testing and visual inspection after each repair
- Documentation: Maintain repair records to track component history and optimize future maintenance
Common Defects and Solutions
| Defect | Cause | Solution |
|---|---|---|
| Cracking | Excessive thermal stress | Increase preheat, reduce welding current |
| Porosity | Moisture in flux or surface | Dry flux, clean surface thoroughly |
| Poor bonding | Surface contamination | Grind to bare metal, degrease |
| Undercut | Excessive current or speed | Reduce current, adjust travel speed |
| Excessive dilution | High heat input | Reduce current, increase travel speed |
Study Insights and Practical Recommendations
The work by Gong demonstrates the practical value of cladding repair in extending equipment life and reducing maintenance costs. For power plant engineers, the key takeaways include:
- Economic benefits: Cladding repair can extend component life by 2–3 times compared to replacement, significantly reducing capital expenditure
- Downtime reduction: On-site or off-site repair can be scheduled during planned maintenance outages, minimizing unplanned shutdowns
- Quality assurance: Consistent cladding quality depends on proper procedure qualification and operator training
- Material selection: The choice of hardfacing alloy should be based on the specific wear mechanism and operating conditions
The paper also highlights the importance of integrating cladding repair into a comprehensive maintenance strategy. Engineers should develop condition-based maintenance programs that combine wear monitoring, predictive scheduling, and quality-controlled repair to maximize equipment availability and minimize lifecycle costs.