CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Toothed Rollers and Grates for Sintering Machines
Literature Overview
This study focuses on the repair of toothed rollers and grate plates in sintering machines using weld overlay technology. Sintering machines are critical equipment in the iron and steel industry, where toothed rollers and grate plates operate under extreme conditions of high temperature, severe abrasion, impact loading, and chemical corrosion from molten iron ore and flux materials. The base materials are typically medium-carbon structural steels such as Q235 or Q345, which suffer rapid wear under these harsh operating environments. The research evaluates various weld overlay consumables, process parameters, and repair strategies to extend component service life and reduce maintenance downtime.
Core Technical Points
The fundamental challenge in repairing toothed rollers and grate plates lies in the combination of high-temperature oxidation resistance, abrasion resistance, and impact toughness required simultaneously. The operating temperature at the working surface of a sintering machine grate can reach 800–1000 °C, while the toothed rollers experience temperatures of 600–900 °C. The materials must resist thermal fatigue cracking, spalling, and abrasive wear from the sinter cake material.
The study examines several overlay approaches including:
- Hardfacing alloys based on Cr-C system (such as Cr15-Cr20 type) for high-temperature abrasion resistance
- Nickel-based hardfacing alloys for improved thermal shock resistance
- Multi-layer overlay strategies combining different alloy systems
- Preheating and interpass temperature control to prevent cracking
Typical Process Parameters
| Parameter | Recommended Range | Purpose |
|---|---|---|
| Preheat temperature | 200–300 °C | Reduce thermal gradient and prevent cracking |
| Interpass temperature | ≤ 250 °C | Control cooling rate and HAZ hardness |
| Welding current (SAW) | 350–500 A | Ensure adequate penetration and fusion |
| Welding speed | 250–400 mm/min | Control dilution and overlay composition |
| Number of layers | 2–4 layers | Achieve required thickness and reduce residual stress |
| Post-weld stress relief | 550–650 °C, 2 h | Eliminate residual stresses |
Process Analysis and Standards
The welding process selection depends on the geometry and accessibility of the component. For toothed rollers, submerged arc welding (SAW) and flux-cored arc welding (FCAW) are preferred due to their high deposition rates and ability to build thick overlay layers. For grate plates with complex geometries, gas metal arc welding (GMAW) with flux-cored wire offers better positional flexibility.
Key considerations include:
- Dilution control: The base metal dilution must be controlled below 15–20% to maintain the desired overlay composition and properties. Multi-pass welding with decreasing dilution from root to cap helps achieve this.
- Thermal cycling: The high-temperature operating environment subjects the overlay to repeated thermal cycling. The coefficient of thermal expansion mismatch between the overlay and base metal creates cyclic stresses that can lead to spalling.
- Bond strength: The overlay-to-base bond strength must exceed 150 MPa (in shear) to prevent delamination during service.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Overlay spalling | Thermal expansion mismatch, high residual stress | Increase preheat, add stress relief, use intermediate layer |
| Cracking in HAZ | High carbon equivalent, rapid cooling | Increase preheat, use low-carbon consumables |
| Insufficient bond strength | Excessive dilution, poor surface preparation | Improve cleaning, reduce dilution, add bonding layer |
| Hardness variation | Uneven cooling rate, composition segregation | Control interpass temperature, use multi-wire SAW |
Integration with Engineering Practice
In actual plant operations, the repair strategy must balance performance requirements with economic considerations. A typical sintering machine toothed roller repair cycle involves:
- Removal of the worn overlay and surface preparation by grinding to bare metal
- Visual and magnetic particle inspection of the base metal for existing cracks
- Application of a bonding layer (typically low-carbon steel weld metal)
- Application of 2–3 layers of hardfacing overlay
- Post-weld heat treatment for stress relief
- Surface finishing and dimensional verification
The field experience indicates that proper surface preparation is the single most critical factor in achieving reliable overlay adhesion. Any residual oxide, rust, or contamination from previous service must be completely removed. The overlay should be applied in thin layers (2–3 mm per pass) to minimize thermal input per layer and reduce the risk of cracking.
Key Questions and Reflections
One important question raised by this study is the optimal combination of overlay materials for different zones of the sintering machine. The toothed roller teeth experience primarily abrasive wear with moderate thermal loading, while the grate plate surface faces both abrasion and significant thermal cycling. This suggests that a single overlay material may not be optimal for all locations, and a zoned approach using different alloy systems may provide better overall performance.
Another reflection concerns the long-term behavior of the overlay under thermal cycling. Laboratory testing typically evaluates static properties (hardness, impact toughness, wear rate), but the actual service involves millions of thermal cycles. The fatigue life of the overlay-base bond under cyclic thermal loading remains an area requiring further investigation.
Study Insights and Implications
This study reinforces the principle that weld overlay repair is not merely a matter of depositing hard material on a worn surface but requires a systematic approach considering the full service environment. The success of overlay repair depends on the synergy between material selection, process parameter optimization, and proper execution. For sintering machine components, the use of Cr-C type hardfacing alloys with appropriate dilution control provides a practical solution that balances cost, performance, and repairability. The study also highlights the importance of post-weld stress relief in preventing early failure due to thermal stress-induced cracking. Future improvements may include the development of functionally graded overlays that transition gradually from the base metal to the wear-resistant surface layer, thereby reducing the thermal mismatch stresses at the interface.