Zhuojin Pipe Fitting Co., LtdOverlay Welding Repair of Pick Bodies for Mining Shearer Cutter Heads
Literature Overview
The article by Liu Cheng, published in Coal Mine Machinery (Vol. 29, No. 12, 2008, pp. 186–187), addresses the overlay welding repair of worn pick bodies used in mining shearer cutter heads. The author, affiliated with Tangshan Science and Technology Vocational and Technical College, presents a practical approach to extending the service life of these critical mining components through surface overlay welding using flux-cored wire MAG (Magnetic Arc Gas) welding. The study is significant because pick bodies are subjected to severe abrasive wear in coal mining operations, and their timely repair directly impacts mining productivity and equipment availability.
Core Technical Content
Pick bodies, also known as pick holders or cutter bodies, are integral components of shearer machines used in longwall coal mining. They are subjected to extreme impact loading, abrasive wear from coal and rock, and sometimes corrosive conditions from mine water. The wear patterns are typically concentrated on the cutting edges and contact surfaces, leading to dimensional loss and reduced cutting efficiency. When the wear exceeds acceptable limits, the pick body must be repaired or replaced.
Weldability Analysis of Pick Body Materials
The pick body material is typically a low-alloy steel or a medium-carbon steel with a hardness in the range of 250–350 HB. The weldability of such materials is generally good, but the high carbon equivalent can lead to cold cracking in the heat-affected zone (HAZ) if proper preheating and post-weld heat treatment are not applied. The author conducted a weldability analysis considering the carbon equivalent (CE) value, the hardness of the base metal, and the cooling rate expected during welding.
Overlay Welding Process Selection
The author selected flux-cored wire MAG welding (FCAW-G) as the primary overlay welding process. This process was chosen for several reasons:
- High deposition rate, which is essential for repairing large worn areas
- Good penetration control, allowing for precise overlay thickness management
- Compatibility with flux-cored wires that can be formulated for specific wear resistance requirements
- Suitability for field conditions, as the process is less sensitive to wind and drafts compared to GMAW
Welding Procedure and Parameters
The overlay welding procedure involves multiple passes to build up the required thickness. The first pass is a transition layer that ensures good metallurgical compatibility between the base metal and the overlay material. Subsequent passes use the wear-resistant overlay material to restore the original dimensions and provide the desired surface hardness.
| Process Parameter | Typical Value | Notes |
|---|---|---|
| Welding process | FCAW-G (MAG) | Flux-cored wire |
| Wire diameter | 1.2–1.6 mm | Depending on repair size |
| Shielding gas | CO2 or Ar/CO2 mix | Per wire manufacturer |
| Preheat temperature | 150–250°C | Based on CE value |
| Interpass temperature | ≤ 250°C | To prevent cold cracking |
| Heat input | 0.5–1.5 kJ/mm | Controlled to manage HAZ |
| Overlay material | High-carbon martensitic | Hardness 50–60 HRC |
| Post-weld heat treatment | 550–650°C | Stress relief |
| Final hardness | 45–55 HRC | Surface wear resistance |
Post-Weld Heat Treatment
The post-weld heat treatment is critical for achieving the desired balance of hardness and toughness in the repaired pick body. The treatment is performed at 550–650°C for a duration sufficient to relieve welding residual stresses without causing excessive softening of the overlay layer. The cooling rate from the heat treatment temperature is controlled to prevent the formation of brittle microstructures. The final microstructure of the overlay layer should consist of tempered martensite with fine carbide precipitation, providing excellent wear resistance while maintaining adequate toughness.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cold cracking in HAZ | High CE, rapid cooling | Increase preheat, reduce heat input |
| Porosity in overlay | Flux contamination, moisture | Dry flux, proper gas shielding |
| Lack of fusion | Insufficient penetration | Increase current, optimize travel speed |
| Excessive dilution | High heat input | Reduce heat input, use multiple thin passes |
| Cracking in overlay | High hardness, low toughness | Optimize overlay composition, proper PWHT |
| Surface irregularity | Poor bead control | Use proper welding technique, maintain consistent parameters |
Engineering Practice Integration
In mining operations, the repair of pick bodies is a routine maintenance activity that directly affects the availability and productivity of the shearer. The overlay welding repair method described in this paper offers several advantages over complete replacement:
- Significant cost savings, as the base material is retained and only the worn surface is replaced
- Reduced downtime, as the repair can be performed in the workshop without waiting for new parts
- Improved performance, as the overlay material can be selected to provide superior wear resistance compared to the original material
The repair process should be integrated into a preventive maintenance program, with regular inspection of pick bodies to identify wear before it becomes critical. The welding procedure should be qualified according to applicable standards, and the repaired components should undergo non-destructive examination (UT or MT) to verify the quality of the overlay weld.
Key Questions and Reflections
The study raises several important considerations for engineering practice. First, the long-term durability of the overlay weld under cyclic impact loading is a concern. The overlay layer, while hard, may be susceptible to fatigue cracking if the bond with the base metal is not robust. Second, the repair process must be repeatable and consistent, which requires careful control of welding parameters and operator skill. Third, the economic viability of repair versus replacement should be evaluated on a case-by-case basis, considering the cost of the repair, the expected remaining service life, and the cost of downtime.
The paper also highlights the importance of materials selection in overlay welding. The choice of overlay material must balance wear resistance with toughness, and the metallurgical compatibility with the base metal must be ensured to prevent cracking and delamination during service.
Study Insights and Implications
This literature provides a practical and economically viable solution for extending the service life of mining pick bodies through overlay welding repair. The approach combines sound metallurgical understanding with practical welding technology, offering a method that can be readily implemented in mining workshops. The emphasis on weldability analysis, proper process parameter selection, and post-weld heat treatment demonstrates the importance of a systematic approach to welding repair. For engineers in the mining industry, this study underscores the value of welding repair as a cost-effective maintenance strategy, provided that the welding procedure is properly qualified and the quality of the repair is rigorously controlled.