Plasma Surfacing Coatings for Mining Picks Wear Resistance Enhancement

Literature Overview

This paper, published in the Journal of North University of China (Natural Science Edition) in 2017 by Cheng Bo and colleagues from North University of China and Capital Aerospace Machinery Company, addresses a critical problem in coal mining equipment: the rapid wear of pick heads on mining picks under harsh underground operating conditions. The authors employed transfer-type plasma arc surfacing to deposit wear-resistant alloy coatings onto pick surfaces, investigating four different alloy powder compositions with varying TiC content. The study was supported by the National Natural Science Foundation of China (Grant No. 21604074).

Core Technical Approach

The plasma transfer arc (PTA) surfacing method uses a compressed plasma arc as the heat source to melt metal alloy powders and achieve metallurgical bonding with the base material. The key advantage of this technique lies in the high energy density of the compressed plasma arc, which can reach temperatures exceeding 10,000°C, enabling the melting of refractory carbide-containing powders that would be difficult to process with conventional arc welding methods.

The experimental matrix involved four alloy powder compositions differing primarily in TiC content. The researchers systematically evaluated the wear resistance of the resulting overlay layers through standardized abrasion testing. The central finding was that increasing the TiC content in the alloy powder led to a significant improvement in the wear resistance of the surfacing layer on mining picks.

Technical Parameters and Process Considerations

The process window for plasma surfacing is relatively narrow compared to conventional welding, requiring careful control of arc current, travel speed, and powder feed rate to achieve a uniform overlay without excessive dilution from the base metal. The dilution rate is particularly critical for carbide-containing alloys because excessive base metal dilution reduces the volume fraction of hard carbide phases in the overlay, thereby degrading wear resistance.

Microstructural Analysis and Wear Mechanism

The wear resistance improvement with increasing TiC content can be attributed to several metallurgical mechanisms. TiC is a very hard refractory carbide with a hardness exceeding 2000 HV, and its presence in the overlay matrix creates a composite structure where the hard carbide particles resist abrasive wear through ploughing and cutting resistance. The TiC particles act as obstacles to dislocation motion and also resist the micro-ploughing mechanism that dominates abrasive wear.

However, engineers should be aware that excessively high TiC content can introduce brittleness and may lead to cracking during the cooling stage due to thermal expansion mismatch between the TiC particles and the metallic binder matrix. The optimal TiC content represents a balance between wear resistance and mechanical integrity, a classic trade-off in tribology engineering.

Engineering Practice Implications

For mining equipment manufacturers, this research provides a practical pathway to extend the service life of mining picks through surface engineering. The plasma surfacing method offers several advantages over traditional hardfacing with submerged arc welding: better compositional control, lower dilution, and the ability to apply multiple layers with different compositions. In practice, a multi-layer approach could be considered where the first layer serves as a transition layer to reduce residual stress, and subsequent layers contain progressively higher TiC content to maximize surface wear resistance.

The cost-benefit analysis for mining operations must consider the increased production cost of plasma surfacing against the extended replacement interval. Given that mining picks are replaced frequently in high-production coal seams, even a modest improvement in wear life translates into substantial savings in downtime and material costs.

Key Questions and Reflections

One important question that this study does not fully address is the long-term performance of the coating under actual mining conditions, where the wear mechanism is not purely abrasive but also involves impact loading, thermal cycling from frictional heating, and potentially corrosive attack from moisture and coal dust. The laboratory abrasion test, while providing relative comparisons, may not capture the full complexity of field conditions.

Another consideration is the re-weldability and repairability of the plasma-surfaced picks. In the field, when a pick is damaged, operators need to either replace the entire pick or repair it. If the plasma-surfaced coating is difficult to grind off or re-weld, this could limit its practical adoption.

Study Insights and Implications

This research reinforces the principle that surface engineering through advanced welding techniques is a highly effective strategy for extending the life of mining components. The systematic investigation of TiC content as the key variable demonstrates good experimental design methodology. For engineers working in the mining equipment sector, the key takeaway is that plasma surfacing with TiC-reinforced alloys represents a viable technology for pick hardfacing, and the optimal composition should be determined through both laboratory testing and field trials under actual operating conditions.