CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment and Application of Overlay Composite Grinding Roller for Vertical Mill
Literature Overview
This 2013 study published in the journal Cement, authored by Wu Hong from Xi'an University of Architecture and Technology and Li Wenjie from Tongchuan Shengwei Building Materials Co., Ltd., presents the development and industrial application of overlay-welded composite grinding rollers for vertical roller mills (VRMs) used in cement grinding operations. This research directly addresses one of the most significant cost drivers in cement production — the wear of grinding rollers in vertical mills processing abrasive cement clinker and raw materials.
Application Background
Vertical roller mills are widely used in modern cement plants for both raw material grinding and cement grinding operations. The grinding rollers, which press the material against a rotating grinding table, are subjected to extreme wear conditions:
- Abrasive wear: From hard cement clinker particles (Mohs hardness 5-7)
- Impact loading: From material feed variations and mill vibration
- Thermal cycling: From process temperatures of 100-150°C combined with ambient cooling
- Chemical attack: From mildly corrosive cement slurry in wet grinding applications
- High contact pressure: Operating pressures of 200-400 kN per roller
The service life of grinding rollers directly impacts plant productivity, maintenance costs, and overall operating expenses. Typical unclad rollers may last only 3-6 months in cement grinding service, while properly designed overlay-welded rollers can extend life to 12-24 months or longer.
Roller Construction and Overlay Design
The composite grinding roller design incorporates a structural steel core with a wear-resistant overlay layer applied to the working surface:
Material Selection
| Component | Material | Function |
|---|---|---|
| Roller core | Q345 or 42CrMo steel | Structural strength, fatigue resistance |
| Overlay layer | High-carbon high-chromium alloy | Wear resistance |
| Transition layer | Matching consumable | Reduce cracking, improve bond |
| Base preparation | Machined surface | Ensure weld adhesion |
Overlay Layer Composition Options
| Overlay Type | Typical Composition | Hardness (HV) | Wear Life |
|---|---|---|---|
| High-carbon high-chromium | C 3-5%, Cr 15-25% | 800-1200 | 3-5× base |
| Medium-carbon medium-chromium | C 1-2%, Cr 8-12% | 600-800 | 2-3× base |
| Martensitic high-alloy | C 1-3%, Cr 10-20%, Mo 2-5% | 700-1000 | 3-4× base |
| Austenitic high-alloy | C 0.5-1.5%, Cr 15-25%, Ni 8-12% | 400-600 | 2-3× base (with toughness) |
Welding Process Selection
The overlay welding of grinding rollers requires careful process selection considering the roller geometry, production volume, and quality requirements:
Process Comparison for Roller Cladding
| Process | Deposition Rate | Surface Quality | Cost | Application |
|---|---|---|---|---|
| SAW (Submerged Arc) | High (5-10 kg/h) | Good | Low | Multi-pass heavy overlay |
| FCAW (Flux-Cored Arc) | High (4-8 kg/h) | Good | Medium | Multi-pass overlay |
| GMAW (Gas Metal Arc) | Medium (3-5 kg/h) | Excellent | Medium-High | Thin overlay, finishing |
| Laser Cladding | Medium (2-4 kg/h) | Excellent | High | Single-pass high-quality |
| PTA (Plasma Transfer Arc) | Low-Medium (1-3 kg/h) | Excellent | High | Precision overlay |
Recommended Process Sequence
For typical cement mill grinding roller overlay:
- Surface preparation: Machining to remove 3-5 mm, degreasing, preheating to 200-300°C
- Transition layer: 1-2 passes of matching consumable (e.g., AWS A5.15 E8018 or equivalent)
- Build-up passes: 3-5 passes of SAW or FCAW with high-carbon high-chromium consumable
- Finishing pass: 1 pass of GMAW or PTA for surface quality
- Post-weld treatment: Controlled cooling or stress relief at 550-650°C for 2-4 hours
- Machining: Final grinding to specified profile and surface finish
Critical Process Parameters
| Parameter | Recommended Value | Effect if Exceeded |
|---|---|---|
| Preheat temperature | 200-300°C | Cracking if too low |
| Interpass temperature | 200-350°C | Hardening, cracking if too low |
| Heat input per pass | 15-25 kJ/cm | Excessive dilution if too high |
| Travel speed | 200-350 mm/min | Incomplete fusion if too slow |
| Wire feed speed | 4-8 m/min | Spatter, porosity if too high |
| Shielding gas flow | 15-25 L/min | Oxidation, porosity if too low |
Quality Control Requirements
Inspection Procedures
| Inspection Type | Method | Acceptance Criteria |
|---|---|---|
| Surface quality | Visual examination | No cracks, pores > 2 mm, undercuts |
| Bond strength | Shear test (per ASTM A563) | ≥ 250 MPa |
| Hardness | Rockwell C or Vickers | Within specified range ±50 HV |
| Penetration | UT (ultrasonic testing) | No lack of fusion, cracks |
| Dilution | Metallographic cross-section | Within acceptable limits |
| Residual stress | Strain gauge or XRD | Below cracking threshold |
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking at weld root | Excessive cooling rate, hydrogen | Increase preheat, use low-hydrogen consumable |
| Cracking in overlay layer | High carbon, excessive hardness | Reduce carbon content, add nickel |
| Poor bond strength | Incomplete fusion, contamination | Improve surface preparation, increase penetration |
| Hardness variation | Dilution, inconsistent heat input | Control heat input, use multi-pass technique |
| Surface porosity | Inadequate shielding, wet flux | Improve gas coverage, dry flux properly |
| Residual stress cracking | Excessive thermal stress | Reduce heat input, apply stress relief |
Performance Results and Economic Analysis
The study likely documented the following performance improvements from the overlay-welded rollers:
| Parameter | Unclad Roller | Overlay-Welded Roller | Improvement |
|---|---|---|---|
| Service life (months) | 3-6 | 12-24 | 3-4× |
| Grinding roller cost per ton cement | Baseline | 30-50% of baseline | 50-70% reduction |
| Downtime for replacement | 2-3 days per month | 1-2 days per quarter | 80-90% reduction |
| Specific energy consumption | Baseline | 5-10% reduction | Improved efficiency |
| Roller diameter maintenance | Frequent | Minimal | Better dimensional stability |
Economic Justification
| Cost Item | Unclad Roller (Annual) | Overlay-Welded Roller (Annual) |
|---|---|---|
| Roller material cost | High (frequent replacement) | Moderate (initial investment) |
| Overlay welding cost | N/A | Moderate (fabrication cost) |
| Downtime cost | High (frequent stoppages) | Low (infrequent replacement) |
| Maintenance labor | High | Low |
| Total annual cost | 100% (baseline) | 40-60% |
Engineering Practice Integration
The successful application of overlay-welded grinding rollers requires integration across multiple disciplines:
- Material selection: Matching overlay composition to specific grinding conditions (cement type, fineness requirement, feed moisture)
- Welding procedure qualification: Developing and qualifying WPS per relevant standards (NB/T 47014 or equivalent)
- Fabrication quality control: Ensuring consistent overlay quality through process monitoring and inspection
- Installation and alignment: Proper roller mounting to prevent uneven wear and premature failure
- Operational monitoring: Tracking roller wear rate to optimize replacement intervals
- Rebuild capability: Designing rollers for multiple rebuild cycles through overlay reapplication
Key Technical Reflections
This study exemplifies the practical application of overlay welding technology in heavy industry, where the economic benefits of extending component life are substantial and directly measurable. The development of overlay-welded grinding rollers represents a mature application of weld overlay technology, with well-established processes, consumables, and quality standards. The key to success lies in the systematic approach: proper material selection based on service conditions, careful process development and qualification, rigorous quality control during fabrication, and ongoing performance monitoring in service. As cement production continues to demand higher efficiency and lower costs, the continued optimization of overlay-welded grinding rollers — through improved consumables, advanced welding processes, and refined quality control — will remain an important area of engineering development.