Surfacing of High-Speed Steel Coating on Coal Mining Machine Picks

Literature Overview

This paper by Yao Shuyu and Li Huiqi, published in Heat Treatment of Metals in 2004 (Vol. 29, No. 8, pp. 51–54), investigates the surfacing of high-speed steel coatings onto coal mining machine picks using surfacing welding technology. The study demonstrates that the coating achieves metallurgical bonding with the substrate and that the coating composition resembles that of high-speed steel. The hardening mechanisms include solid solution strengthening, dislocation strengthening, dispersion strengthening, and grain refinement strengthening. The final coating microstructure consists of network ledeburite, martensite, tempered martensite, and lower bainite.

Core Technical Findings

Coal mining machine picks are subjected to extreme abrasive wear from coal and rock, making them a critical consumable component in mining operations. The application of high-speed steel surfacing coatings to these picks is a cost-effective approach to extending their service life. The study demonstrates that the surfacing process achieves true metallurgical bonding between the coating and the substrate, which is essential for withstanding the high impact and abrasive loading conditions encountered in mining.

The coating composition, which mimics that of conventional high-speed steel, contains high levels of carbon, chromium, tungsten, molybdenum, and vanadium. These elements contribute to the hardness and wear resistance through multiple mechanisms. Solid solution strengthening arises from the dissolution of alloying elements in the austenite matrix, while dislocation strengthening results from the high dislocation density in the martensitic structure. Dispersion strengthening is provided by the fine carbide particles (WC, Mo₂C, VC) that form during solidification and subsequent tempering. Grain refinement strengthening operates through the Hall-Petch relationship, with the fine grain structure of the surfacing layer contributing to increased yield strength.

The final microstructure of the coating is a complex mixture of network ledeburite, martensite, tempered martensite, and lower bainite. The network ledeburite forms at the grain boundaries and provides a hard, wear-resistant phase. The martensite and tempered martensite constitute the primary matrix phases, providing a balance of hardness and toughness. The lower bainite phase, which forms in regions with intermediate cooling rates, provides additional toughness. This multi-phase microstructure is characteristic of high-speed steels and is responsible for the superior wear resistance of the coating.

Microstructure Analysis and Wear Mechanism Discussion

The network ledeburite in the coating microstructure is a critical feature. Ledeburite consists of cementite (Fe₃C) and austenite, and in the presence of alloying elements, it transforms into a complex mixture of carbides and retained austenite. The network morphology at grain boundaries provides a hard, continuous phase that resists abrasive wear. However, the brittleness of the network ledeburite can be a concern for impact resistance, as cracks may initiate at the ledeburite network and propagate through the coating.

The presence of both martensite and tempered martensite indicates that the coating experienced a range of cooling rates during solidification. The martensite forms in regions with rapid cooling, while the tempered martensite forms in regions where the cooling rate was slower or where the interpass reheating acted as a tempering treatment. The lower bainite phase forms in the intermediate cooling rate regions and provides a transition between the hard martensite and the softer austenite.

The wear mechanism in this coating is primarily abrasive, with the hard carbide particles and network ledeburite providing resistance to material removal. The matrix phases (martensite, tempered martensite, and lower bainite) provide the ductility necessary to accommodate the stresses generated during abrasion. The metallurgical bond between the coating and substrate ensures that the coating does not delaminate under the high impact loads encountered in mining operations.

Engineering Practice Implications and Reflections

For mining equipment manufacturers and maintenance engineers, this study demonstrates that high-speed steel surfacing coatings can be an effective solution for extending the life of mining picks. The metallurgical bonding achieved through surfacing welding is superior to mechanical attachment methods such as welding-on or press-fit inserts, as it provides a continuous transition from substrate to coating without a distinct interface that could serve as a crack initiation site.

The multi-mechanism hardening approach in the coating design is worth emulating in other surface engineering applications. By combining solid solution, dislocation, dispersion, and grain refinement strengthening, the coating achieves a balance of hardness and toughness that would be difficult to attain through any single mechanism alone. Engineers designing coatings for other abrasive applications should consider this multi-mechanism approach.

A practical consideration is the deposition of the coating on the pick geometry. Mining picks typically have a complex shape with a cutting edge, a body, and a shank. The surfacing process must be adapted to deposit the coating primarily on the cutting edge and body while avoiding the shank, which must retain its original properties for insertion into the pick holder. This requires careful shielding and process control to limit spatter and avoid unwanted deposition.

This study provides a practical and cost-effective solution for a critical mining application. The use of high-speed steel surfacing coatings on mining picks represents a straightforward application of well-established surfacing technology to a demanding industrial problem. The multi-mechanism hardening approach and the metallurgical bonding achieved through surfacing welding make this a robust solution for extending pick life in coal mining operations.