Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Pick Teeth Bodies
Overview and Application Context
The referenced paper by Liu Cheng (2008, Coal Mine Machinery, Vol. 29, No. 12, pp. 186-187) addresses the practical engineering challenge of repairing worn pick teeth used in coal cutting machines. Pick teeth are critical components in underground coal mining equipment, subjected to severe abrasive wear from continuous contact with coal and rock. The paper presents a method for extending pick tooth service life through overlay welding repair, combining material selection, welding process optimization, and post-weld heat treatment to achieve balanced hardness, toughness, and wear resistance.
Core Technical Content
The paper describes a systematic approach to pick tooth repair that addresses the fundamental challenge of balancing competing material properties. Pick teeth require high hardness for wear resistance but also sufficient toughness to resist chipping and fracture under impact loading. The overlay welding approach allows for the creation of a wear-resistant surface layer while maintaining a tough base material.
Material Selection and Weldability Analysis
The paper begins with a weldability analysis of the worn pick tooth material. The base material is typically a high-carbon alloy steel or tool steel, which presents welding challenges due to:
- High carbon equivalent, increasing cold cracking susceptibility
- Hard and brittle microstructure in the heat-affected zone
- Potential for excessive hardness in the weld and HAZ
- Residual stress development leading to distortion or cracking
Welding Process Parameters
The repair process employs flux-cored arc welding (FCAW/MAG) with specific parameter selection:
| Parameter | Typical Value | Purpose |
|---|---|---|
| Welding current | 180-250 A | Sufficient penetration without excessive heat input |
| Arc voltage | 25-32 V | Stable arc and good deposition rate |
| Travel speed | 200-300 mm/min | Control heat input and weld geometry |
| Shielding gas | CO2 or CO2/Ar mixture | Cost-effective shielding with good penetration |
| Wire diameter | 1.2-1.6 mm | Balance between deposition rate and control |
| Preheating temperature | 150-250°C | Reduce cold cracking risk |
| Interpass temperature | 200-300°C | Control cooling rate and residual stress |
Post-Weld Heat Treatment
The post-weld heat treatment is critical for achieving the desired mechanical properties. The paper describes a tempering process that reduces hardness while improving toughness:
- Tempering temperature: 550-650°C
- Tempering time: 1-2 hours per 25 mm thickness
- Cooling method: Air cooling or furnace cooling
- Resulting hardness: 45-55 HRC (balance of hardness and toughness)
Engineering Practice Applications
The overlay welding repair technique has several practical applications in mining equipment maintenance:
- Cost reduction: Repairing worn pick teeth through overlay welding is significantly less expensive than replacing them with new teeth, particularly for large-scale mining operations.
- Downtime reduction: On-site repair capabilities reduce equipment downtime and improve mining productivity.
- Performance optimization: The overlay welding process allows for the selection of specific wear-resistant materials, potentially improving performance over the original design.
- Sustainability: Repair and reuse of components reduces waste and environmental impact.
Key Technical Considerations
Several technical challenges must be addressed in the overlay welding repair of pick teeth:
- Weld geometry: The overlay weld must be shaped to match the original pick tooth geometry, ensuring proper cutting performance.
- Layer thickness: The overlay layer must be thick enough to provide adequate wear resistance but not so thick as to distort the tooth shape or consume excessive material.
- Multi-pass welding: Multiple passes may be required to build up the overlay layer, requiring careful control of interpass temperature and welding sequence.
- Welding position: Pick teeth may require welding in various positions, which affects heat input, weld geometry, and residual stress development.
- Surface preparation: The worn surface must be properly prepared (grinding, cleaning) to ensure good weld fusion and bonding.
Study Insights and Reflections
This paper exemplifies the practical application of welding technology to solve real-world engineering problems. The overlay welding repair of pick teeth represents a straightforward but effective solution to a common maintenance challenge in the mining industry. The approach demonstrates the importance of understanding material properties, welding processes, and heat treatment in achieving balanced mechanical properties.
From a broader perspective, the pick tooth repair problem illustrates the fundamental engineering challenge of balancing competing requirements. In this case, the competing requirements are wear resistance (requiring high hardness) and impact resistance (requiring sufficient toughness). The overlay welding approach resolves this conflict by creating a composite structure with a hard, wear-resistant surface and a tougher, more ductile substrate. This philosophy of composite design through welding is applicable to many other engineering problems where single materials cannot simultaneously satisfy all performance requirements.
The paper's emphasis on process optimization and heat treatment highlights the importance of post-weld processing in achieving desired mechanical properties. In many welding applications, the as-welded condition is not the final condition, and post-weld heat treatment is essential for optimizing properties. Understanding the relationship between welding parameters, microstructure, and mechanical properties is fundamental to successful welding engineering practice.