Zhuojin Pipe Fitting Co., LtdPlasma Surfacing of Mining Picks for Wear Resistance
Literature Overview
The paper by Cheng Bo and colleagues (2017, Journal of North University of China, Vol. 38, No. 4, pp. 446-451) addresses a critical practical problem in coal mining: the rapid wear of pick heads used in continuous miners operating under harsh underground conditions. The authors employed plasma surfacing technology to deposit wear-resistant alloy coatings on mining picks and systematically evaluated four different alloy powder compositions. The research was funded by the National Natural Science Foundation of China (Grant No. 21604074), reflecting its significance in the field of surface engineering for mining equipment.
Core Technical Approach
The study utilizes transferred plasma arc as the heat source. The compressed plasma arc generates instantaneous high temperatures capable of melting metal powder mixtures containing wear-resistant phases. The melted alloy forms a metallurgical bond with the base material of the pick, creating a coating with significantly enhanced wear resistance compared to the uncoated substrate.
Key Process Parameters
| Parameter | Description |
|---|---|
| Heat Source | Transferred plasma arc (compressed) |
| Powder Delivery | Metal alloy powder mixture |
| Bonding Mechanism | Metallurgical bonding between coating and substrate |
| Test Variable | TiC content in alloy powder (4 compositions) |
| Test Method | Wear resistance evaluation of deposited layer |
Technical Points and Analysis
The central finding is that increasing TiC content in the alloy powder leads to a clear improvement in the wear resistance of the deposited coating. This is consistent with the well-established role of TiC as a hard ceramic phase that resists abrasive and adhesive wear through its high hardness (approximately 2,800 HV) and thermal stability.
Wear Mechanism Considerations
Mining picks experience complex combined wear mechanisms including:
- Abrasive wear: Contact with coal, rock, and embedded mineral particles during cutting operations
- Adhesive wear: Material transfer between the pick surface and the coal seam
- Impact wear: Repeated impact loading during the cutting cycle
- Oxidative wear: Elevated temperature conditions during high-speed cutting
The plasma surfacing process offers several advantages over alternative surface engineering methods for this application:
- The high temperature and rapid cooling rate of the plasma arc produce a refined microstructure with fine grain size
- Dilution between the coating alloy and base material can be controlled through process parameter optimization
- The process is amenable to automation, enabling consistent coating quality across production batches
- Coating thickness can be built up layer by layer to achieve desired dimensions
Connection to Engineering Practice
In coal mining operations, the economic cost of pick replacement is substantial. A typical continuous miner may require pick replacement every 50-200 hours of operation depending on the coal-rock ratio and geological conditions. The plasma surfacing approach provides a cost-effective alternative to full pick replacement, extending service life significantly.
Practical Considerations for Implementation
- Pre-treatment of the pick surface is essential to ensure proper metallurgical bonding; oxide layers and scale must be removed through grinding or shot blasting
- The base material of mining picks is typically low-carbon steel or medium-carbon steel, which must be compatible with the surfacing alloy system
- Post-deposition heat treatment may be necessary to relieve residual stresses and prevent cracking in the coating
- Coating thickness typically ranges from 0.5 to 3 mm depending on the expected wear rate and operating conditions
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracking | High residual stress, incompatible CTE | Preheating, controlled cooling, multiple thin passes |
| Poor bonding | Surface contamination, excessive dilution | Surface preparation, process parameter optimization |
| Porosity | Gas entrapment, powder moisture | Powder drying, shielding gas flow control |
| Incomplete melting | Insufficient arc energy | Increase arc current, reduce travel speed |
Study Insights and Reflections
The study demonstrates a clear materials engineering principle: the wear resistance of a composite coating is dominated by the hard phase content and its distribution within the matrix. The finding that TiC content positively correlates with wear resistance aligns with the rule of mixtures and the threshold effect of hard phase volume fraction in tribological systems.
However, several aspects warrant further investigation from an engineering standpoint. The study does not appear to address the trade-off between wear resistance and toughness. In mining applications, picks are subjected to significant impact loading, and an overly hard, brittle coating may spall under impact. A balanced approach considering both hardness and fracture toughness is essential for practical application.
Additionally, the long-term performance under actual mining conditions—which involve cyclic thermal loading, chemical attack from coal moisture, and varying mechanical loads—remains an open question. Laboratory wear tests, while useful for comparative evaluation, may not fully capture the complexity of in-service degradation.
The work represents a solid contribution to the surface engineering of mining tools, and the plasma surfacing technique is well-suited for industrial scale-up. Future research should incorporate field trials and develop comprehensive qualification protocols that account for the multi-mechanism wear environment of underground mining operations.