Vertical Mill Liner Plate Weld Overlay Repair Technology for Cement Industry

Literature Overview

This technical paper, published in China Cement in 2006 by Zhang Xingsheng, Gong Jinhua, Li Huansheng, and Chang Yongli from Tangshan Jidong Cement Co., Ltd. and Tangshan Metallurgical Mining Machinery Factory, presents the application of weld overlay repair technology on vertical mill liner plates in cement grinding operations. The study documents the technical approach, process parameters, and performance results of overlay repair for vertical roller mill liners, which are critical wear components in modern cement grinding circuits.

Core Technical Content

Application Context

Vertical roller mills (VRMs) have become increasingly popular in cement grinding due to their energy efficiency, compact design, and ability to handle a wide range of feed materials. The liner plates (also called grinding segments or mill segments) are the primary wear components that come into direct contact with both the grinding table and the grinding rollers. These components are subjected to:

• High contact stresses from grinding rollers (typically 20-30 MPa)
• Abrasive wear from cement clinker and raw materials
• Impact loading from falling materials
• Thermal cycling during operation (ambient to 150-200 °C)
• Continuous sliding contact with grinding rollers

The typical service life of unhardened liner plates ranges from 2-6 months, depending on the material being ground and operating conditions. Weld overlay repair offers a cost-effective alternative to complete replacement.

Overlay Repair Process

The repair process for vertical mill liner plates involves the following steps:

1. Inspection and assessment: Determine the remaining thickness and condition of the liner plate
2. Surface preparation: Remove worn material to expose sound substrate; clean and degrease
3. Preheating: Apply preheat to reduce thermal stresses and prevent cracking
4. Overlay welding: Apply wear-resistant hardfacing material in multiple passes
5. Post-weld treatment: Stress relief or controlled cooling
6. Surface finishing: Machine to achieve required surface profile
7. Quality verification: Inspect for defects and verify hardness and bond strength

Welding Parameters and Materials

Technical Analysis of Vertical Mill Liner Overlay

Wear Mechanisms in Vertical Mills

The wear mechanisms affecting vertical mill liner plates are distinct from those in ball mills:

1. Abrasive wear: Primary mechanism; caused by sliding contact between liner and grinding rollers
2. Adhesive wear: Secondary mechanism; material transfer between contacting surfaces
3. Impact wear: Occurs when materials fall onto the liner surface
4. Fatigue wear: Cyclic loading can cause surface fatigue and spalling

The overlay material must be selected to resist the dominant wear mechanism while maintaining adequate toughness to resist impact and fatigue.

Material Selection for Vertical Mill Liners

Overlay Design Considerations

The design of the overlay layer must consider:

1. Overlay thickness: Sufficient thickness to provide wear life, but not so thick as to cause excessive distortion or cracking
2. Overlay profile: The surface profile must match the original liner geometry for proper grinding action
3. Edge treatment: Edges of the liner are more susceptible to wear and require special attention
4. Transition zones: Smooth transition between overlay and base material to prevent stress concentration
5. Thermal expansion compatibility: The overlay material must have compatible thermal expansion with the base to prevent delamination

Performance Results and Economic Analysis

Performance Data

The study reports the following performance results:

Economic Comparison

The return on investment is typically achieved within the first repair cycle, making the technology economically attractive.

Quality Control and Defect Prevention

Common Defects in Liner Overlay Repair

Quality Verification Procedures

1. Visual inspection: Verify overlay coverage, uniformity, and absence of visible defects
2. Hardness testing: Rockwell hardness testing at multiple locations (minimum 5 points per liner)
3. Bond strength testing: Peel test or bend test to verify metallurgical bond
4. Surface roughness: Verify surface finish meets grinding requirements (Ra 12.5-25 μm typical)
5. Dimensional inspection: Verify overlay thickness and surface profile
6. Non-destructive testing: Magnetic particle testing for surface cracks; ultrasonic testing for subsurface defects

Integration with Engineering Practice

Operational Considerations

Several operational factors must be considered for successful liner overlay repair:

1. Surface preparation quality: Inadequate surface preparation is the primary cause of overlay failure; ensure complete removal of worn material and contamination
2. Heat input control: Excessive heat input can cause base material softening, cracking, or distortion; maintain parameters within specified ranges
3. Cooling rate management: Rapid cooling can cause hardening and cracking in high-carbon overlay materials; use controlled cooling or stress relief
4. Stress relief: Post-weld stress relief is critical for preventing delayed cracking and improving long-term reliability
5. Repeatability: Multiple repair cycles must maintain overlay quality; monitor cumulative effects of repeated thermal cycling

Best Practices for Liner Overlay Repair

Based on the study findings and practical experience, the following best practices are recommended:

1. Pre-repair inspection: Thoroughly inspect the liner for cracks, excessive wear, or other damage that may affect repair quality
2. Surface preparation: Grind or machine the worn surface to expose sound substrate; clean and degrease thoroughly
3. Preheating: Apply preheat of 200-350 °C uniformly across the repair area to reduce thermal stresses
4. Welding sequence: Use a symmetric welding sequence to minimize distortion; weld from the center outward
5. Parameter control: Monitor welding parameters continuously; adjust as needed to maintain consistent bead quality
6. Interpass cleaning: Clean each pass before applying the next; remove slag and spatter
7. Post-weld treatment: Apply stress relief at 550-650 °C for 2 hours to reduce residual stresses
8. Surface finishing: Machine the overlay surface to achieve the required profile and finish
9. Quality verification: Perform all required quality checks before returning the liner to service
10. Documentation: Record all repair parameters, inspection results, and performance data for future reference

Key Questions and Reflections

The application of weld overlay repair to vertical mill liner plates raises several important engineering considerations:

1. Long-term reliability: How does the overlay performance degrade over multiple repair cycles? Is there a practical limit to the number of repair cycles?
2. Material compatibility: What are the long-term metallurgical compatibility issues between the overlay material and the base casting? Can repeated thermal cycling cause interface degradation or cracking?
3. Standardization: Are there established standards or specifications for weld overlay repair of cement grinding equipment? The lack of standardization can lead to inconsistent quality and reliability.
4. Inspection protocols: What inspection methods are most appropriate for verifying overlay quality in the field? Should magnetic particle testing, ultrasonic testing, or hardness mapping be mandatory?
5. Operator training: What level of training is required for welders performing overlay repair? The skill requirements for wear-resistant overlay welding are higher than for general structural welding.
6. Cost-benefit analysis: Under what conditions is overlay repair more economical than replacement? The answer depends on liner thickness, wear rate, and repair costs.

Study Insights and Implications

The most significant finding of this study is the dramatic improvement in wear life achieved through weld overlay repair of vertical mill liner plates, with wear life increasing by 3-5 times compared to the unhardened condition. This translates directly into reduced maintenance costs, improved equipment availability, and lower production costs for cement manufacturers.

The technology also demonstrates the versatility of weld overlay methods in addressing abrasive wear in cement grinding equipment. The ability to repair worn components in situ or in a workshop setting, rather than replacing entire components, represents a significant cost and resource saving. The environmental benefits are also notable, as reduced material consumption and energy usage contribute to sustainability goals.

For engineers involved in equipment maintenance and repair, this study highlights the importance of:

• Proper material selection based on wear mechanisms
• Rigorous surface preparation procedures
• Controlled welding parameters to prevent defects
• Post-weld treatment to ensure long-term reliability
• Systematic quality control to verify repair quality
• Documentation and performance tracking to optimize future repairs

The economic benefits are substantial, with annual savings of ¥49,000-80,000 per liner plate, making the technology highly attractive for industrial applications. The technology should be considered as a standard maintenance practice for cement grinding equipment, rather than a reactive repair measure.

Conclusion

The weld overlay repair of vertical mill liner plates demonstrates the practical effectiveness of hardfacing technology in cement industry applications. The dramatic improvement in wear life (3-5× extension), combined with significant cost savings and reduced maintenance downtime, makes this technology highly attractive for cement manufacturers. Engineers should adopt systematic approaches to overlay repair, including proper material selection, rigorous quality control, and ongoing performance monitoring, to maximize the benefits of this technology. The study provides a valuable reference for implementing weld overlay repair programs in cement grinding equipment maintenance, and the principles can be applied to other wear components in cement and mineral processing industries.