CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair Technology for Vertical Mill Liner Plates
Literature Overview
This technical note addresses the weld overlay repair technology applied to vertical mill liner plates, which are critical wear components in mineral processing and cement grinding operations. Vertical mills are extensively used for grinding ores, coal, and cement clinker, and their liner plates are subjected to severe abrasive and impact wear. When liner plates reach their service limit, they can be economically restored through weld overlay repair rather than complete replacement. This study examines the technical aspects of overlay repair, including material selection, process parameters, and quality assurance.
Core Technical Points
Vertical mill liner plates are typically made of high-manganese steel (such as Mn13) or high-chromium cast iron, materials selected for their excellent wear resistance under impact and abrasion. However, these materials present significant welding challenges due to their high carbon and alloy content, which makes them susceptible to cracking. The weld overlay repair process must therefore balance the need for a wear-resistant overlay with the weldability requirements of the base material.
The typical overlay repair process involves the following steps:
| Step | Description | Key Considerations |
|---|---|---|
| Assessment | Inspection of wear depth and damage | UT or thickness measurement |
| Preparation | Removal of damaged material | Gouging, grinding, or machining |
| Preheating | Temperature control before welding | 200–400°C depending on base material |
| Overlay welding | Multi-pass deposit application | Process selection and parameters |
| Post-treatment | Stress relief or quenching | Temperature and time control |
| Inspection | NDT and dimensional verification | UT, MT, dimensional checks |
The choice of overlay material depends on the specific wear mechanism. For abrasive wear from ore particles, high-chromium white cast iron or tungsten carbide composite overlays are suitable. For impact-dominated wear, high-manganese austenitic overlays such as Mn13 are preferred. For mixed wear conditions, a graded overlay with a hard surface layer and a tough intermediate layer may be optimal.
Process Analysis and Material Selection
Several welding processes are applicable for vertical mill liner plate repair:
| Process | Deposition Rate | Dilution Control | Cost | Suitability |
|---|---|---|---|---|
| Submerged arc welding (SAW) | High | Moderate | Low | Thick overlays |
| Flux-cored arc welding (FCAW) | Medium | Good | Medium | Medium thickness |
| Gas metal arc welding (GMAW) | Medium | Good | Medium | General repair |
| Plasma transferred arc (PTA) | Medium | Excellent | High | Thin, precise overlays |
| Laser cladding | Medium | Excellent | High | High-quality overlays |
For vertical mill liner plates, SAW and FCAW are the most commonly used processes due to their high deposition rates and cost-effectiveness. The SAW process with a high-alloy flux can achieve dilution ratios as low as 20–30 percent, which is critical for maintaining the wear resistance of the overlay. The FCAW process offers better dilution control and is suitable for repairs in difficult access positions.
The welding parameters for overlay repair of high-manganese steel liner plates typically include:
- Current: 300–500 A
- Voltage: 25–35 V
- Travel speed: 150–350 mm/min
- Wire diameter: 1.6–2.4 mm
- Preheat temperature: 250–350°C
- Interpass temperature: below 350°C
The preheat temperature is critical for preventing cold cracking in high-carbon base materials. For high-manganese steels, preheating to 250–350°C reduces the cooling rate and minimizes the formation of hard, brittle martensite in the heat-affected zone. Post-weld heat treatment, such as austenitizing at 1000–1100°C followed by water quenching, may be applied to restore the austenitic structure of the base material and improve its toughness.
Defect Analysis and Quality Control
Common defects in vertical mill liner plate overlay repair include:
- Cracking in the heat-affected zone: Caused by the formation of hard martensite in high-carbon base materials. Mitigated by adequate preheating and post-weld heat treatment.
- Lack of fusion: Resulting from insufficient current, excessive travel speed, or poor surface preparation. Ensuring proper cleaning and adequate heat input addresses this issue.
- Porosity: Associated with contaminated base metal or flux. Maintaining dry flux and clean surfaces is essential.
- Spatter: Common in GMAW and FCAW processes, which can reduce deposition efficiency and create surface irregularities. Using proper gas shielding and flux coverage minimizes spatter.
Quality control involves ultrasonic testing to detect lack of fusion and internal defects, magnetic particle testing for surface cracks, and hardness testing to verify the overlay composition and dilution level. For critical applications, metallographic examination of cross-sections may be required to assess the microstructure and bond quality.
Engineering Practice and Study Insights
A practical case involved the repair of vertical mill liner plates in a cement grinding operation. The original liner plates experienced severe wear after 8,000 operating hours, with a maximum wear depth of 25 mm. The repair process involved removing the worn material by mechanical gouging, preheating to 300°C, and applying a four-pass overlay using a high-chromium white cast iron flux with a carbon steel band electrode. The final overlay achieved a hardness of 600 HV and demonstrated excellent wear resistance during subsequent service.
The study highlights that the success of liner plate repair depends not only on the welding process and parameters but also on the thoroughness of surface preparation and the appropriateness of the overlay material selection. Engineers should conduct a detailed assessment of the wear mechanism before selecting the overlay material, as using an inappropriate material can lead to premature failure and costly re-repairs. The economic analysis of repair versus replacement should also consider the downtime costs, as vertical mill shutdowns can be expensive and should be minimized through efficient repair planning.