CLADDING TECHNOLOGY SHANXI CO., LTDTungsten Carbide Alloy Wear-Resistant Overlay Welding on Single-Tooth Rolls
Overview of the Technical Challenge
Single-tooth rolls are critical components in mining, crushing, and grinding operations where they are subjected to severe abrasive wear from hard rock and mineral particles. The conventional approach of manufacturing the entire roll body from tungsten carbide is prohibitively expensive. The overlay welding approach offers a cost-effective alternative by depositing a hardfacing layer on a steel substrate, combining the toughness of the base material with the extreme hardness of the overlay. This literature review focuses on the practical aspects of applying tungsten carbide-containing hardfacing alloys to single-tooth rolls using various welding processes, and the study reveals significant insights into process selection, dilution control, and service life optimization.
Key Technical Parameters and Process Selection
The selection of welding process is the first critical decision when overlay welding tungsten carbide alloys onto single-tooth rolls. The following table summarizes the most commonly used processes and their characteristics:
| Process | Typical Heat Input | Dilution Control | Suitable Geometry | Hardness Achieved (HV) |
|---|---|---|---|---|
| SAW (Submerged Arc) | High (15-30 kJ/mm) | Poor (30-50%) | Flat/slightly curved surfaces | 1200-1800 |
| GTAW (TIG) | Low (1-5 kJ/mm) | Good (10-20%) | Complex geometries, thin sections | 1500-2200 |
| GMAW (MIG) | Medium (5-15 kJ/mm) | Moderate (20-35%) | Large areas, production runs | 1300-1900 |
| FCAW (Flux-Cored) | Medium-High (8-20 kJ/mm) | Moderate (20-35%) | Thick sections, outdoor work | 1400-2000 |
| PTA (Plasma Transfer Arc) | Controlled (3-8 kJ/mm) | Excellent (5-15%) | Precision surfaces | 1600-2400 |
| Laser Cladding | Very Low (0.5-3 kJ/mm) | Excellent (5-10%) | Precision, thin coatings | 1800-2500 |
The study emphasizes that for single-tooth rolls, the geometry of the tooth profile creates significant challenges. The convex surface of the tooth and the concave valleys between teeth require different approaches. GTAW with tungsten carbide powder feeding or cored wire is recommended for the tooth tips where precision is critical, while SAW or FCAW can be used for the flat sections of the roll body. The interpass temperature must be carefully controlled between 150°C and 300°C to prevent excessive thermal cycling and cracking in the hardfacing layer.
Dilution Control and Microstructural Considerations
Dilution is the single most important factor affecting the final hardness and wear resistance of tungsten carbide overlay layers. When the base metal dissolves into the weld pool, it reduces the effective concentration of tungsten carbide particles and can lead to the formation of brittle intermetallic compounds. The study recommends several strategies to minimize dilution:
- Preheating the base metal to 200-250°C to reduce thermal gradient and cracking tendency
- Using a sacrificial transition layer (typically a Ni-Cr-C alloy or Co-Cr-C alloy) between the steel substrate and the tungsten carbide overlay
- Applying multiple thin passes rather than a few thick passes to reduce the volume of base metal melted per pass
- Using low-current, high-speed techniques such as PTA or laser cladding for the final overlay layers
- Maintaining a single-layer dilution rate below 15% for optimal hardness retention
The microstructure of a properly deposited tungsten carbide overlay should show well-dispersed WC particles (typically 5-50 μm in size) in a Ni-based or Co-based binder matrix. The binder matrix provides toughness while the WC particles provide wear resistance. If the dilution is excessive, the WC particles tend to dissolve and reprecipitate in a coarser form, or the matrix becomes too hard and brittle, leading to spalling during service.
Common Defects and Countermeasures
The following defects are frequently encountered in tungsten carbide overlay welding on single-tooth rolls:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking (hot/cold) | Excessive carbon content, thermal stress | Preheat 200-250°C, post-weld stress relief at 450°C |
| Spalling/delamination | Poor bond strength, high dilution | Reduce dilution, use transition layer, ensure clean substrate |
| Porosity | Gas entrapment, flux contamination | Clean wire/powder, use dry flux, proper shielding |
| Excessive hardness (brittleness) | Overheating, high cooling rate | Control interpass temperature, use slower cooling |
| Uneven thickness on tooth profile | Poor torch positioning | Use CNC-guided torch, multiple passes with proper overlap |
The study particularly highlights that spalling is the most common failure mode in service. This occurs when the bond strength between the overlay and the substrate is insufficient to withstand the impact loading during crushing operations. A minimum bond strength of 200 MPa (as measured by the bond strength test per ASTM A780) is recommended for heavy-duty applications. The substrate surface preparation is critical — it must be ground to a smooth, clean finish free of rust, scale, and oil contamination.
Engineering Practice Insights
From an engineering practice perspective, the study offers several valuable insights. First, the economics of overlay welding versus full tungsten carbide manufacturing should be evaluated on a life-cycle basis. For a typical single-tooth roll used in a mining operation, the overlay approach can extend service life by 3-5 times compared to uncoated steel rolls, at a cost that is only 20-30% of a full tungsten carbide roll. Second, the repairability of overlay-welded rolls is a significant advantage — worn sections can be re-grinded and re-overlay without scrapping the entire component. Third, the study recommends implementing a documented welding procedure qualification (WPQ) per NB/T 47014 or ASME IX, including a specific qualification test for the bond strength and hardness of the overlay on the particular substrate material used. The overall conclusion is that tungsten carbide overlay welding on single-tooth rolls is a mature and reliable technology, provided that dilution is carefully controlled, the process is properly qualified, and the substrate preparation receives adequate attention.