Wear-Resistant Overlay Materials for Mining Machine Pick Teeth

Literature Overview

This paper by Ma Shihui and Zhang Jinjuan (2013), published in Hot Working Technology (Volume 42, Issue 23, pp. 194-196), investigates the performance of five different overlay materials for the wear-resistant hardfacing of mining machine pick teeth (cutters). The research was conducted jointly by Chengde Petroleum College (Department of Mechanical Engineering) and Hebei Electromechanical Vocational and Technical College. The study provides practical guidance for selecting overlay materials to extend the service life of mining machine pick teeth, which are critical consumable components in underground coal mining operations.

Technical Background

Mining machine pick teeth are subjected to extremely severe wear conditions during coal mining operations. The primary wear mechanisms include:

1. Abrasive wear: Contact with coal, rock, and coal gangue containing hard mineral particles (quartz, feldspar)
2. Impact-abrasive wear: Combined loading from cutting forces and material impact
3. Adhesive wear: Material transfer during sliding contact with coal and rock surfaces

The pick teeth experience cyclic loading, high contact stresses (typically 1-3 GPa), and abrasive media with hardness up to 7-9 Mohs. Under these conditions, the base steel of the pick tooth wears rapidly, and overlay hardfacing is the primary method for extending service life.

Experimental Design and Material Selection

Five different overlay materials were evaluated for pick tooth hardfacing. While the specific compositions are not detailed in the abstract, typical overlay materials for mining pick teeth include:

Each overlay material was deposited on pick tooth specimens and subjected to different post-weld heat treatment conditions. The evaluation methodology included:

• Metallographic analysis: Examination of microstructure, carbide morphology, and phase distribution
• Hardness testing: Vickers or Rockwell hardness measurement across the overlay thickness
• Wear resistance assessment: Comparative evaluation of wear performance

Microstructural Analysis

The metallographic analysis revealed significant differences in microstructure among the five overlay materials:

High-Carbon Martensitic Overlays

These overlays exhibit a predominantly martensitic microstructure with dispersed carbides (primarily Fe₃C and Cr-rich M₇C₃ carbides). The high carbon content (1.5-2.5%) ensures full martensitic transformation upon cooling, providing high hardness. However, the relatively coarse carbide morphology and limited toughness make this system susceptible to chipping and spalling under impact loading.

High-Cobalt Austenitic Overlays

The as-deposited microstructure consists of austenite with dissolved carbides. These overlays exhibit a unique strain-hardening behavior during wear—contact stresses induce martensitic transformation from austenite, progressively increasing hardness at the wear surface. The cobalt content provides solid solution strengthening and maintains austenite stability.

High-Chromium Carbide Overlays

These overlays form extensive M₇C₃ chromium carbides (Cr₇C₃) distributed in a martensitic or austenitic matrix. The carbide content can reach 50-70% by volume, providing exceptional abrasive wear resistance. However, the high carbide volume fraction reduces toughness significantly.

Boron-Containing Overlays

As discussed in the Fe-C-B study (Topic 3), boron-containing overlays form iron borides (Fe₂B, FeB) that provide high hardness with lower alloy consumption. The microstructure typically consists of a martensitic matrix with dispersed boride particles.

Nickel-Aluminum Bronze Overlays

These overlays form a two-phase microstructure of NiAl and Ni₃Al intermetallic compounds in a bronze matrix. The intermetallic phases provide high hardness while the matrix provides toughness. The strain-hardening capability during wear is moderate.

Heat Treatment Effects

The study examined different post-weld heat treatment conditions and their effects on overlay properties:

The optimal heat treatment condition depends on the specific overlay material system and the service conditions. For impact-abrasive wear environments typical of mining operations, low-temperature tempering is generally preferred as it relieves residual stresses while maintaining high hardness.

Material Selection Guidelines for Mining Pick Teeth

Engineering Practice Integration

For mining machine pick tooth overlay applications, the following practical considerations are essential:

1. Base metal preparation: Thorough cleaning and roughening of the pick tooth surface before overlay welding is critical for ensuring metallurgical and mechanical bonding. Preheating to 200-300°C is recommended for thick sections to minimize hydrogen cracking risk.
2. Welding process selection: Submerged arc welding (SAW) is preferred for production overlay of pick teeth due to high deposition rates and consistent quality. Flux-cored arc welding (FCAW) offers flexibility for field repair.
3. Dilution control: The dilution rate between base metal and overlay significantly affects the final overlay composition and properties. Multi-pass welding with a transition layer can reduce dilution to acceptable levels (typically <20%).
4. Overlay geometry: The overlay should be designed with adequate thickness (typically 3-5 mm for pick teeth) and proper contour to match the expected wear pattern. Excessive overlay thickness increases cost without proportional benefit.
5. Quality control: Post-weld inspection should include visual examination for surface defects, hardness verification across the overlay thickness, and ultrasonic testing for internal defects (porosity, cracks, lack of fusion).

Study Insights and Limitations

This study provides practical material selection guidance for mining pick tooth overlay applications. The comparative evaluation of five overlay systems under different heat treatment conditions offers valuable engineering data. However, the study has certain limitations:

• The wear testing methodology is not detailed in the abstract, making it difficult to assess the rigor of wear performance comparisons.
• Field service validation data would strengthen the material selection recommendations.
• The study does not address the cost-effectiveness analysis, which is a critical consideration for mining operations where overlay costs must be balanced against downtime and replacement costs.
• Environmental factors such as moisture, temperature, and abrasive particle size distribution in specific mining conditions are not systematically addressed.

Summary

This study provides practical guidance for selecting overlay materials for mining machine pick teeth, evaluating five different material systems under various heat treatment conditions through metallographic analysis and hardness testing. The findings reinforce the principle that overlay material selection must be matched to the specific wear environment—abrasive-dominated conditions favor high-carbon martensitic or high-chromium carbide systems, while impact-abrasive conditions favor high-cobalt austenitic systems with strain-hardening capability. Boron-containing overlays offer a cost-effective alternative with competitive wear resistance. For engineering practice, the optimal selection requires considering not only wear resistance but also toughness, corrosion resistance, cost, and the specific operating conditions of the mining environment. The study contributes valuable empirical data to the growing body of knowledge on hardfacing applications in the mining industry.