CLADDING TECHNOLOGY SHANXI CO., LTDHardfacing Cladding Repair of Electric Shovel Drive Wheels and Bucket Teeth
Literature Overview and Engineering Context
This 1992 publication by Xu Guoling and Shi Meiyu from Ningbo Port Authority Automobile Repair Factory and Ningbo Equipment Installation Company addresses the repair and extension of service life for electric shovel drive wheels and bucket teeth through hardfacing cladding. Electric shovels are critical pieces of mining and port equipment that operate under extremely severe wear conditions, with bucket teeth experiencing direct contact with abrasive rock and ore, and drive wheels subjected to high contact stresses and sliding friction.
The technical significance of this work lies in its practical approach to extending the service life of heavy-duty mining components through weld overlay. Rather than replacing entire components, which is costly and time-consuming, hardfacing cladding allows the restoration of worn surfaces to acceptable dimensions while simultaneously improving wear resistance. This approach aligns with modern lean manufacturing principles that emphasize repair and reuse over replacement.
Hardfacing Materials and Process Selection
The selection of hardfacing materials for drive wheels and bucket teeth is governed by the type of wear mechanism encountered in service. Bucket teeth typically experience abrasive wear from contact with rock and ore, while drive wheels experience a combination of abrasive and adhesive wear from contact with the ground or conveyor surfaces.
| Component | Wear Mechanism | Recommended Hardfacing Material | Hardness (HRC) | Typical Cladding Thickness |
|---|---|---|---|---|
| Bucket teeth | Abrasive (rock/ore) | Stellite 6 or high-carbon martensitic | 45-50 | 6-10 mm |
| Drive wheel tread | Abrasive + adhesive | High-silicon cast iron or martensitic | 40-48 | 4-8 mm |
| Drive wheel flange | Moderate wear | Low-alloy martensitic | 38-42 | 3-5 mm |
Welding Process Considerations
The primary welding processes used for hardfacing in this application are SMAW (shielded metal arc welding) and SAW (submerged arc welding). SMAW offers excellent portability and flexibility, making it suitable for field repair of large components. SAW provides higher deposition rates and better quality consistency, making it preferred for workshop repair of drive wheels.
The key process parameters for hardfacing include:
- Preheat temperature: 200-300°C for high-carbon materials to reduce cracking susceptibility
- Interpass temperature: Maintained between 200-300°C to promote repeated tempering of the hardfacing alloy
- Travel speed: Optimized to achieve the desired dilution rate (typically 15-25% for high-alloy hardfacing)
- Weld pass configuration: Multi-pass with back gouging between passes to improve fusion and reduce cracking
The dilution rate is a critical parameter that must be carefully controlled. Excessive dilution from the base metal reduces the hardness and wear resistance of the hardfacing layer, while insufficient dilution can lead to poor bond strength and cracking. The dilution rate is influenced by the welding process, electrode diameter, travel speed, and the number of passes.
Microstructural Analysis and Performance Evaluation
The effectiveness of hardfacing cladding is ultimately determined by the microstructure and mechanical properties of the overlay layer. For martensitic hardfacing alloys, the microstructure consists of a matrix of martensite with dispersed carbides. The hardness and wear resistance are primarily governed by the carbide phase, which provides resistance to abrasive wear.
The typical microstructural features of Stellite 6 hardfacing include:
- A fully austenitic matrix with high nickel and chromium content
- Dispersed M6C and MC carbides of chromium and molybdenum
- High solid solution strengthening from the austenitic matrix
- Excellent hot hardness retention up to 500°C
For high-carbon martensitic hardfacing, the microstructure consists of:
- A tempered martensite matrix with high carbon content
- Dispersed cementite (Fe3C) and alloy carbides
- Hardness primarily from martensite and carbide phases
- Susceptible to cracking if not properly tempered
Performance Testing and Validation
The performance of the hardfacing cladding should be validated through the following tests:
| Test Method | Standard | Acceptance Criteria | Purpose |
|---|---|---|---|
| Hardness test | ASTM B187 | ≥45 HRC for Stellite 6 | Verify hardness uniformity |
| Wear test | ASTM G99 | Specific wear rate < 0.5 mm³/N·m | Quantify abrasive wear resistance |
| Bond strength | ASTM B755 | ≥250 MPa | Verify interface integrity |
| Impact test | Internal | No cracking at 25°C | Assess ductility and toughness |
| Metallographic | ASTM E3 | No unmelted flux or cracks | Verify microstructure quality |
Engineering Practice and Field Application
In practical application, the hardfacing repair of electric shovel components follows a systematic procedure:
- Surface preparation: Remove existing worn material by grinding to expose sound base metal. Ensure the surface is free of scale, rust, and contamination.
- Weld preparation: Create a suitable weld groove geometry to ensure adequate fusion between the hardfacing and the base metal. A U-groove or J-groove is preferred for thick cladding layers.
- Preheating: Apply uniform preheat to the entire component to reduce thermal gradients and minimize residual stress.
- Welding: Apply the hardfacing in multiple passes, maintaining the interpass temperature within the specified range. Use a balanced welding sequence to minimize distortion.
- Post-weld treatment: Apply tempering treatment to reduce residual stress and improve toughness. For martensitic hardfacing, temper at 550-650°C for 2 hours.
- Inspection: Perform visual inspection, magnetic particle testing, and dimensional verification.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in hardfacing | High carbon equivalent; excessive cooling rate | Increase preheat; use low-hydrogen electrode; reduce travel speed |
| Poor fusion | Insufficient heat input; contamination | Increase current; clean surface; use proper technique |
| Excessive porosity | Moisture in flux or electrode coating | Dry electrodes; use dry flux; control environment |
| Inclusion | Flux contamination or incomplete fusion | Improve flux quality; optimize travel speed and angle |
| Distortion | Excessive heat input; asymmetric welding | Use balanced sequence; apply back-up ring; reduce heat input |
Study Insights and Practical Implications
The work by Xu and Shi demonstrates the practical value of hardfacing cladding in extending the service life of heavy-duty mining equipment. The key insight is that hardfacing is not merely a surface treatment but a comprehensive repair technology that must be carefully designed, executed, and validated to achieve reliable performance.
The economic benefit of hardfacing repair is substantial. For a typical electric shovel bucket tooth, the cost of hardfacing repair is approximately 20-30% of the cost of a new tooth, while the service life extension is 2-3 times the original. This makes hardfacing an attractive option for fleet maintenance programs.
From a metallurgical perspective, the challenge is to balance hardness and toughness. Excessive hardness leads to brittle failure, while insufficient hardness results in poor wear resistance. The optimal design requires a careful selection of hardfacing material, process parameters, and post-weld treatment to achieve the desired combination of properties.
This literature study reinforces the importance of systematic approach to hardfacing repair, emphasizing the need for proper material selection, process optimization, and quality assurance to achieve reliable and cost-effective results in heavy equipment maintenance.