CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Cladding on Spiral Conveyor Flight Blades
Application Context and Problem Definition
Spiral conveyor flight blades (helical screw blades) in bulk material handling systems—particularly in cement, mining, power generation, and agricultural processing—experience severe abrasive and adhesive wear from continuous contact with abrasive particulates such as limestone, coal, fly ash, and ore. The typical service life of unclad carbon steel flight blades in such applications ranges from 3 to 6 months, with some severe duty applications requiring replacement every 6 to 8 weeks. Weld overlay cladding represents a proven technology for extending service life by 3 to 10 times through the application of hardfacing alloys with controlled hardness, toughness, and wear mechanisms.
Overlay Process Selection and Parameters
The study evaluates three primary overlay processes for spiral conveyor applications:
| Process | Consumable Type | Typical Hardness (HV30) | Application Suitability | Cost per Unit Area |
|---|---|---|---|---|
| SAW (SAW-overlay) | Wire + flux cored | 450–550 | Large flat areas, high deposition rate | Low |
| GMAW (flux-cored) | FCAW wire | 400–500 | Medium areas, good flexibility | Medium |
| GTAW (TIG) | Wire electrode | 500–650 | Small areas, repair work | High |
| Submerged arc (multi-pass) | SAW wire | 550–650 | Heavy-duty, thick overlay | Medium |
For spiral conveyor flights, the geometry presents unique challenges. The helical shape requires either:
- Machining then cladding: The blade is machined to final geometry and then clad, requiring precision post-weld grinding to restore dimensional tolerances.
- Cladding then machining: A thicker overlay is applied and then machined to final dimensions, preserving the hardened microstructure at the wear surface.
The study recommends the "cladding then machining" approach for new fabrication and the "machining then cladding" approach for repair applications where dimensional accuracy of the conveyor shaft coupling must be maintained.
Microstructure and Wear Mechanism Analysis
The overlay alloys evaluated include:
- Cr-C-Mo system (Type 1): High-carbon martensitic with M7C3 and M23C6 carbides, hardness 500–550 HV30. Suitable for moderate abrasive wear with low impact loading.
- Cr-C-Mo-Ni system (Type 2): Higher carbon content (3.0–4.5% C) with retained austenite, hardness 550–600 HV30. Superior for severe abrasive wear with good impact resistance.
- Ni-Cr-B-Si system (Type 3): Amorphous/metallic glass-like structure, hardness 650–700 HV30. Excellent for high-temperature abrasive wear but limited to specific applications.
The dominant wear mechanism in spiral conveyor service is primarily abrasive (two-body and three-body) with secondary contributions from adhesive wear at high contact pressures. The Cr-C-Mo-Ni system with 3.5–4.0% carbon content demonstrated optimal performance, achieving hardness retention above 550 HV30 after 500 hours of service in limestone conveying applications. The retained austenite in this system provides transformation toughening—when the material is plastically deformed under abrasive contact, austenite transforms to martensite, creating compressive residual stresses that resist crack initiation and propagation.
Common Defects and Quality Control
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking in overlay | High carbon + low preheat | MT / PT | Preheat to 150–200 °C, control interpass |
| Porosity | Contaminated flux / wire | RT / UT | Proper flux storage, wire cleaning |
| Undercut | Excessive current / speed | Visual / PT | Optimize welding parameters |
| Delamination | Poor base preparation | UT / tap test | Thorough surface cleaning, grinding |
| Hardness inconsistency | Parameter drift | Hardness survey | In-process parameter monitoring |
Engineering Practice Recommendations
Based on the study's findings and engineering experience, the following specifications are recommended for spiral conveyor flight blade overlay:
- Base material preparation: Grind the wear surface to bare metal with a 30° V-groove preparation, width 6–8 mm, depth 3–4 mm. Remove all rust, paint, and scale.
- Preheat: Apply 150–200 °C preheat to the entire blade section using induction heating or torch preheating.
- Overlay build-up: Apply 2–3 passes of SAW overlay with a total thickness of 8–12 mm on the wear surface.
- Post-weld treatment: Allow natural cooling in still air; do not quench. Perform stress relief at 500–550 °C for 1 hour.
- Post-weld machining: Machine the overlay surface to final dimensions with a carbide tool, feed rate 0.1–0.15 mm/rev, cutting speed 80–120 m/min.
- Inspection: Perform hardness survey (minimum 5 points per 100 mm²), magnetic particle testing for surface cracks, and ultrasonic testing for subsurface defects.
Study Insights and Reflections
The most significant insight from this study is that overlay hardness alone does not determine service life in spiral conveyor applications. The interaction between overlay hardness, toughness, and the specific wear mechanism is critical. Overly hard overlays (above 650 HV30) may exhibit chipping and spalling under impact-abrasive conditions, while overly soft overlays (below 450 HV30) suffer rapid abrasive wear. The optimal window of 500–600 HV30, achieved with properly designed Cr-C-Mo-Ni alloys, provides the best balance. Furthermore, the study highlights that overlay geometry—specifically the transition from clad surface to unclad surface—must be carefully designed with a gradual taper (30°–45° fillet) to prevent stress concentration and premature delamination at the overlay edge.