High-Speed Steel Coating on Shearer Picks by Surfacing Welding

Literature Overview

This study, published in Metal Heat Treatment (2004, Vol. 29, No. 8, pp. 51-54) by Yao Shuyu and Li Huiqi from Shandong University of Science and Technology, investigates the application of high-speed steel surfacing welding on shearer picks used in coal mining equipment. The research focuses on achieving metallurgical bonding between the high-speed steel coating and the pick substrate, and characterizes the resulting coating composition, microstructure, hardness, and wear resistance.

Core Technical Findings

The study demonstrates that high-speed steel wear-resistant welding electrodes can be successfully applied to shearer pick substrates to produce coatings that are metallurgically bonded to the base material. The coating composition is similar to conventional high-speed steel, containing high levels of tungsten, chromium, vanadium, and carbon. These alloying elements contribute to the coating's exceptional hardness and wear resistance through multiple strengthening mechanisms.

The strengthening mechanisms identified include solid solution strengthening from dissolved alloying elements, dislocation strengthening from the high dislocation density in the rapidly solidified martensitic structure, dispersion strengthening from fine carbide precipitates, and grain refinement strengthening from the fine grain structure produced during rapid solidification.

The final coating microstructure consists of a network of ledeburite, martensite, tempered martensite, and lower bainite. This complex microstructure is characteristic of high-speed steel and is responsible for the combination of high hardness, good toughness, and excellent wear resistance.

Microstructural Analysis

The presence of ledeburite network in the coating microstructure is significant. Ledeburite is a eutectic mixture of austenite and cementite that forms during the solidification of high-carbon iron-carbon alloys. In the context of high-speed steel cladding, the ledeburite network forms at the interdendritic regions during rapid solidification and provides a continuous hard phase framework that enhances wear resistance.

The martensite and tempered martensite phases provide the bulk of the coating's hardness. The rapid solidification rates in surfacing welding produce a fine martensitic structure with high carbon supersaturation, which is subsequently tempered during the cooling cycle or post-weld heat treatment. The lower bainite phase, which forms at intermediate cooling rates, provides a complementary toughness contribution.

The metallurgical bonding between the coating and substrate is achieved through the melting and mixing of the base metal at the fusion boundary. The resulting dilution zone contains a mixture of substrate and coating compositions, which may exhibit intermediate properties. For shearer picks, where the substrate is typically a medium-carbon steel or low-alloy steel, the dilution zone provides a gradual transition in properties from the tough substrate to the hard coating.

Engineering Practice Implications

Shearer picks are critical consumable components in underground coal mining operations, subjected to severe abrasion from coal, rock, and debris. The application of high-speed steel coatings via surfacing welding provides a cost-effective method of extending pick life without requiring complete replacement of the pick body. This approach is particularly advantageous for large-diameter picks where the cost of complete replacement is prohibitive.

From a production standpoint, the surfacing welding process must be carefully controlled to ensure uniform coating thickness and consistent metallurgical bonding. The welding parameters, including current, voltage, travel speed, and interpass temperature, must be optimized to achieve the target coating composition and microstructure. Multi-pass welding may be required to achieve sufficient coating thickness, with careful attention to interpass temperature to avoid excessive grain coarsening or cracking.

Quality control of high-speed steel coatings on shearer picks should include hardness testing across the coating cross-section, metallographic examination of the fusion zone for cracks or incomplete bonding, and wear testing under simulated mining conditions. The coating thickness should be verified to ensure adequate material remains after wear to the substrate.

Study Insights and Reflections

The successful application of high-speed steel coatings to shearer picks demonstrates the versatility of surfacing welding as a surface engineering technique. The multi-mechanism strengthening approach inherent in high-speed steel microstructures provides a robust combination of hardness and toughness that is difficult to achieve with simpler alloy systems.

One important consideration for field engineers is the residual stress state in the coating. Surfacing welding deposits typically contain tensile residual stresses in the as-welded condition, which can contribute to spalling or cracking under cyclic loading. Post-weld stress relief treatment or the use of multi-directional welding sequences may be necessary to manage residual stresses in critical applications.

In summary, this study provides a clear demonstration that high-speed steel surfacing welding is a viable and effective method for enhancing the wear resistance of shearer picks. The complex microstructure produced by the rapid solidification process, combining ledeburite, martensite, tempered martensite, and lower bainite, delivers the multi-mechanism strengthening necessary for severe mining abrasion conditions. The technique offers a practical and economical solution for extending the service life of mining equipment components.