CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Repair Method for Roller Mill Roller Surfaces
Literature Overview
The study by Wang Xin, Zhang Yongsheng, Huang Zhiquan, and Xu Jian from the Zhengzhou Machinery Research Institute, published in Cement Engineering in 2004, presents a systematic approach to overlay repair of roller mill roller surfaces. Roller mills are critical grinding equipment in the cement industry, and their roller surfaces undergo severe abrasion from the grinding of raw materials and clinker. The research addresses the economic and technical challenges of restoring worn roller surfaces to serviceable condition through overlay welding techniques.
Core Technical Content
Roller mill rollers in cement grinding applications are typically made of medium-carbon steel or low-alloy steel with a hardened surface. The operating conditions involve continuous abrasive contact with hard cement raw materials, resulting in material loss rates of 0.1-0.5 mm per month depending on the specific application. When wear exceeds the allowable limit, the roller must be either refurbished or replaced. Full replacement is economically prohibitive, making overlay repair the preferred approach.
Wear Mechanisms and Material Requirements
| Wear Mechanism | Dominant Condition | Material Requirement |
|---|---|---|
| Abrasive wear | Hard particles in grinding media | High hardness, carbide-rich overlay |
| Impact wear | Hammering of feed material | Tough binder phase, crack resistance |
| Adhesive wear | Metal-to-metal contact | Hardness differential, lubricity |
| Corrosive wear | Moisture in raw meal | Oxidation resistance, alloying elements |
Overlay Material Selection
The choice of overlay material depends on the specific grinding application. For raw mill rollers grinding limestone and clay, a medium-hardness overlay with good impact resistance is appropriate. For coal mill rollers, a softer, more ductile overlay may be preferred to minimize dust explosion risks. For clinker grinding, the hardest available overlay materials are typically required due to the extreme abrasiveness of the material.
Common overlay compositions include:
- High-chromium white iron: Cr 12-25%, C 2.5-3.5%, hardness 58-65 HRC
- Manganese steel: Mn 12-18%, C 1.0-1.5%, hardness 25-35 HRC (work-hardening type)
- Stainless steel overlay: Cr 18-22%, Ni 8-10%, hardness 35-45 HRC
- Composite overlays: Hard carbide layer over tough base layer
Process Methodology
Surface Preparation
The worn roller surface must be prepared to ensure proper bond between the base metal and the overlay. The preparation sequence includes:
- Grinding: Remove worn material and any loose scale to expose sound base metal
- Cleaning: Shot blast or grind to Sa 2.5 surface cleanliness
- Preheating: Heat to 150-250 degrees C to reduce cracking risk and remove moisture
- Defect repair: Grind out any existing cracks or defects to a smooth, rounded profile
Welding Process Selection
The choice of welding process depends on roller size, geometry, and available equipment. Common processes for roller surface overlay include:
| Process | Deposition Rate | Dilution | Surface Quality | Cost |
|---|---|---|---|---|
| Submerged arc welding (SAW) | High (5-10 kg/h) | Moderate | Good | Low |
| Gas metal arc welding (GMAW) | Medium (2-4 kg/h) | Low | Good | Medium |
| Electroslag welding (ESW) | High (8-15 kg/h) | Low | Excellent | Medium |
| Manual metal arc welding (SMAW) | Low (0.5-1.5 kg/h) | Variable | Fair | Low |
| Plasma transferred arc (PTA) | Medium (2-5 kg/h) | Very low | Excellent | High |
For large cement mill rollers, submerged arc welding or electroslag welding is typically preferred due to the high deposition rates and excellent surface quality. For smaller rollers or field repairs, GMAW or SMAW may be more practical.
Multi-Pass Overlay Strategy
Building up the overlay in multiple passes is essential for achieving adequate thickness and controlling dilution. A typical strategy involves:
- Pass 1: Bond layer using a low-dilution filler to ensure metallurgical compatibility with the base metal
- Passes 2-3: Transition layers gradually increasing the alloy content
- Final pass: Surface layer with the full alloy composition for optimal wear resistance
The total overlay thickness is typically 6-12 mm, depending on the expected service life and wear rate.
Quality Assurance and Post-Weld Treatment
After overlay welding, the roller surface must undergo machining to achieve the required dimensional accuracy and surface finish. The overlay material must be machinable, which places constraints on the hardness and carbide morphology. Post-weld heat treatment may be required to relieve residual stresses and improve the mechanical properties of the overlay.
Inspection requirements include:
- Visual examination: Check for surface defects, undercuts, and irregularities
- Magnetic particle testing: Detect surface cracks in the overlay and heat-affected zone
- Ultrasonic testing: Verify bond strength and detect internal defects
- Hardness testing: Profile hardness across the overlay thickness
- Dimensional verification: Confirm roller diameter and runout are within tolerance
Engineering Practice and Economic Analysis
The economic case for overlay repair versus roller replacement is compelling. A typical cement mill roller can cost several hundred thousand dollars to replace, while overlay repair costs a fraction of that amount. However, the repair must be performed correctly to ensure a service life comparable to a new roller. Poor overlay quality, characterized by cracking, spalling, or premature wear, can result in unplanned downtime that far exceeds the cost savings.
The Zhengzhou Machinery Research Institute's approach emphasizes systematic process control and quality assurance. Their methodology includes detailed welding procedure specifications, welder qualification requirements, and comprehensive inspection protocols. This systematic approach has been validated through extensive field experience in cement plant maintenance operations.
Study Insights and Practical Recommendations
This research provides valuable guidance for engineers responsible for roller mill maintenance. The key insight is that overlay repair is not simply a matter of welding hard material onto a worn surface; it requires careful attention to material selection, process parameters, surface preparation, and quality control. The selection of overlay material must be matched to the specific grinding application, considering factors such as feed material characteristics, operating conditions, and required service life.
Engineers should also consider the long-term maintenance strategy. A well-planned overlay repair program can extend roller life significantly, reducing overall maintenance costs and improving plant availability. However, this requires investment in proper equipment, trained personnel, and quality assurance systems. The return on investment is typically realized within the first repair cycle, making it economically justified for most cement grinding applications.