CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Technology for Vertical Mill Grinding Rollers and Discs
Literature Overview
This 2005 publication by researchers from Kunshan Huifeng Wear-Resistant Industry Co., Ltd. provides a focused examination of wear-resistant overlay welding technology applied to vertical mill grinding rollers and grinding discs in cement and mineral processing industries. Vertical mills are widely used for fine grinding of cement clinker, raw meal, and coal, where the grinding rollers and discs are subjected to intense abrasive and impact loading. The overlay layer on these components is critical for maintaining grinding efficiency and extending component service life.
The paper takes a practical, application-oriented approach, emphasizing the selection of appropriate overlay materials, optimization of welding parameters, and implementation of quality control measures. The authors draw upon their industrial experience to present practical guidelines that can be directly applied in manufacturing and maintenance settings.
Core Technical Content
Vertical Mill Operating Environment
Vertical mill grinding components operate under demanding conditions that challenge the performance of overlay layers:
| Parameter | Typical Value | Effect on Overlay |
|---|---|---|
| Grinding pressure | 5–15 MPa | Plastic deformation, fatigue |
| Sliding velocity | 1–3 m/s | Frictional heating, smearing |
| Material hardness | 3–7 MPa (Mohs) | Abrasive wear |
| Particle size | 0.05–3 mm | Three-body abrasion |
| Temperature | 80–180°C | Thermal softening, oxidation |
| Moisture content | 1–15% | Corrosive wear, embrittlement |
| Operating hours | 6000–8000 h/year | Cumulative wear |
The combination of high pressure, sliding contact, and abrasive particles creates a wear environment that is predominantly three-body abrasion, supplemented by adhesive wear and fatigue wear. The overlay layer must therefore possess high hardness for abrasive resistance, adequate toughness for impact resistance, and good thermal stability for temperature resistance.
Overlay Material Systems
The study evaluates several overlay material systems for vertical mill applications:
| Material Type | Composition | Hardness (HV) | Wear Resistance | Cost | Application |
|---|---|---|---|---|---|
| High-Cr carbide | Cr25Ni4Mo, C 3.0% | 700–850 | Excellent | Medium | Primary overlay |
| Medium-Cr alloy | Cr18Ni10, C 1.5% | 500–600 | Good | Low-Medium | Transition layer |
| Leaded alloy | Pb5, Cr20Ni10 | 600–700 | Good | Medium | Self-lubricating |
| Ceramic composite | WC-Co, Cr20Ni10 | 900–1200 | Excellent | High | High-wear zones |
| Medium-alloy | 42CrMo, C 0.4% | 350–450 | Fair | Low | Substrate repair |
For vertical mill grinding rollers and discs, the high-chromium carbide system (Cr25Ni4Mo) is the most commonly used overlay material. The high carbon content (2.5–3.5%) promotes the formation of primary chromium carbides (Cr₇C₃ and Cr₃C₂) that provide excellent abrasive wear resistance. The nickel addition stabilizes retained austenite in the matrix, providing inherent toughness and work-hardening capacity.
Welding Process Comparison
Several welding processes are applicable for overlaying vertical mill components, each with distinct advantages and limitations:
| Process | Deposit Rate (kg/h) | Dilution (%) | Heat Input | Equipment Cost | Best Application |
|---|---|---|---|---|---|
| SAW | 5–10 | 20–35 | High | Low | Large flat areas |
| GMAW | 2–5 | 10–20 | Medium | Medium | Complex geometries |
| FCAW | 3–6 | 15–25 | Medium-High | Low | Field repairs |
| PTA | 0.5–2 | <10 | Low-Medium | High | Precision overlay |
| Laser Cladding | 0.3–1.5 | <5 | Very Low | Very High | Thin precision layers |
For full overlay of grinding rollers and discs, submerged arc welding (SAW) is typically the most economical choice due to its high deposit rate and consistent quality. However, the high heat input of SAW can lead to excessive dilution and grain coarsening in the overlay. Gas metal arc welding (GMAW) provides better control over dilution and heat input, making it suitable for precision overlay work and localized repairs.
Overlay Layer Microstructure
The microstructure of the overlay layer is characterized by a matrix of austenite and martensite with dispersed carbide particles. The key features include:
- Primary carbides: Cr₇C₃ and Cr₃C₂ particles, 2–10 μm in size, providing high hardness (2000–2300 HV) and excellent abrasive wear resistance
- Matrix phase: A mixture of retained austenite (γ') and tempered martensite, with hardness of 500–700 HV
- Secondary carbides: Mo₂C and Ni₃C, 0.5–2 μm in size, contributing to thermal stability
- Transition zone: A region of 50–200 μm thickness where the overlay microstructure gradually transitions to the substrate microstructure
The microhardness distribution across the overlay layer typically shows a gradient from 750–850 HV at the surface to 300–400 HV at the interface. This gradient provides a smooth transition in mechanical properties that reduces the risk of delamination under cyclic loading.
Welding Parameter Optimization
Optimal welding parameters are essential for achieving the desired overlay properties. The following parameter ranges have been found to be effective for high-chromium overlay welding:
| Parameter | SAW | GMAW | FCAW |
|---|---|---|---|
| Current (A) | 400–600 | 250–350 | 280–380 |
| Voltage (V) | 32–38 | 28–34 | 32–38 |
| Travel speed (mm/min) | 200–350 | 150–250 | 180–300 |
| Wire feed (m/min) | — | 5–8 | 5–8 |
| Shielding gas | Flux | Ar + 5% CO₂ | Flux |
| Preheat (°C) | 150–250 | 100–200 | 150–250 |
| Interpass temp (°C) | <250 | <200 | <250 |
The dilution rate is the most critical parameter affecting overlay performance. A dilution rate of 15–25% is generally acceptable for vertical mill applications, balancing hardness and toughness. Dilution rates above 30% result in excessive softening, while rates below 10% may lead to cracking due to high carbon and alloy concentrations.
Quality Control and Inspection
Pre-Weld Inspection
Before overlay welding begins, the following inspections must be completed:
- Surface condition: Verify that the substrate surface is free of rust, scale, oil, and contamination
- Substrate integrity: Inspect for cracks, grooves, or material loss using magnetic particle inspection (MT)
- Dimensional verification: Confirm that the component dimensions are within acceptable tolerances
- Material verification: Verify the substrate material composition through spectrometric analysis
- Welding consumables: Check wire composition, flux dryness, and shielding gas purity
In-Process Monitoring
During overlay welding, the following parameters should be monitored:
- Welding current and voltage (recorded at regular intervals)
- Travel speed (maintained within specified range)
- Interpass temperature (measured with infrared thermometer or thermocouple)
- Shielding gas flow rate (verified with flow meter)
- Weld bead appearance (visual inspection for porosity, undercut, and excessive spatter)
Post-Weld Inspection
After overlay welding is completed, the following inspections must be performed:
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Surface cracks | MT (magnetic particle) | No linear indications >2 mm |
| Subsurface defects | UT (ultrasonic) | No indications >3 mm |
| Hardness | Vickers (HV10) | 700–850 HV, uniformity ±50 HV |
| Thickness | UT or caliper | Within ±0.5 mm of specified |
| Bond strength | Peel test or UT | No delamination |
| Microstructure | Metallographic | No excessive grain growth or cracking |
Common Defects and Countermeasures
| Defect | Cause | Detection | Countermeasure |
|---|---|---|---|
| Surface cracking | High residual stress, high cooling rate | MT | Stress relief, preheat, controlled cooling |
| Porosity | Inadequate shielding, contaminated wire | RT, UT | Improve gas coverage, use dry flux |
| Excessive dilution | High heat input, low travel speed | Metallographic | Optimize parameters, use transition layer |
| Delamination | Poor fusion, hydrogen embrittlement | UT, peel test | Surface preparation, low-hydrogen consumables |
| Uneven hardness | Inconsistent parameters, poor mixing | Hardness mapping | Process control, parameter monitoring |
Engineering Practice and Case Studies
Case Study: Vertical Mill Roller Overlay in a Cement Plant
A cement plant with a vertical mill of 1200 mm grinding roller diameter and 400 mm width experienced frequent roller failures with overlay life of only 40–60 days. The root cause analysis identified the following issues:
- High dilution: The original overlay procedure resulted in a dilution rate of 35–45%, producing an overlay hardness of only 450–550 HV
- Inadequate thickness: The overlay thickness was only 2–3 mm, providing insufficient material for the expected wear rate of 0.05–0.1 mm/day
- Poor surface preparation: Rust and contamination on the substrate led to incomplete fusion and early delamination
- No stress relief: High residual stresses contributed to fatigue cracking and spalling
- Inconsistent parameters: Manual welding without parameter monitoring led to variable quality
After implementing the improved procedures recommended in this study, the overlay life increased to 180–250 days, representing a 3–4 times improvement. The key changes included:
- Reducing dilution to 15–20% by using optimized welding parameters and a low-dilution wire
- Increasing overlay thickness to 5–6 mm
- Implementing proper surface preparation with grinding and cleaning
- Adding a stress relief treatment at 580°C for 2 hours
- Establishing parameter monitoring and recording procedures
- Implementing a regular inspection and maintenance schedule
Maintenance Strategy
An effective maintenance strategy for vertical mill overlay layers should include:
- Regular inspection: Inspect roller and disc surfaces every 200–500 operating hours
- Wear measurement: Measure overlay thickness at multiple locations to track wear rate
- Surface repair: Grind and re-weld localized wear areas before they become critical
- Alignment verification: Check roller and disc alignment to prevent uneven wear
- Predictive maintenance: Use wear rate data to predict overlay life and schedule maintenance proactively
Key Questions and Reflections
The study raises several important questions for further consideration. First, the interaction between the overlay microstructure and the grinding process is complex. The effect of particle size distribution, material hardness, and moisture content on wear rate is not fully understood. Further research is needed to develop predictive models for overlay wear that can be used for maintenance planning.
Second, the economic optimization of the overlay process deserves attention. While thicker overlay layers provide longer service life, they also increase manufacturing cost and downtime. A life-cycle cost analysis should be conducted to determine the optimal overlay thickness and material selection for specific operating conditions.
Third, the potential of advanced welding technologies such as laser cladding and cold spray for vertical mill applications merits investigation. These technologies offer excellent dilution control and minimal heat input, but their higher equipment costs and lower deposit rates may limit their practical application for large-scale overlay work.
Study Insights and Implications
This study provides practical and comprehensive guidance for the wear-resistant overlay of vertical mill grinding rollers and discs. The systematic approach of addressing material selection, process optimization, quality control, and maintenance procedures offers a complete framework that can be adapted to other heavy-duty grinding applications.
The key insight is that overlay performance is determined by the integrated effect of material composition, process parameters, surface preparation, post-weld treatment, and maintenance practices. A holistic approach to overlay management is essential for achieving optimal performance and economic viability.
For engineering practice, the recommendation is to establish a documented overlay procedure with specific parameters for each step, from surface preparation through to final inspection. Regular training of welding personnel and maintenance staff is essential to ensure consistent quality and enable early detection of potential problems. The implementation of parameter monitoring and recording systems can significantly improve overlay quality and consistency.