Application of Weld Overlay Technology in Mining Pick Teeth

Literature Overview

This study, published in Mining Machinery in 2003, was conducted by researchers from Shandong University of Science and Technology and Taian Keda Jina Plasma Technology Co., Ltd. The research focuses on the application of weld overlay technology to enhance the wear resistance and service life of mining pick teeth, which are critical consumable components in continuous miner machines used in underground coal mining operations. Pick teeth are subjected to extremely severe abrasive and impact wear conditions, making material selection and surface engineering solutions critical for operational efficiency.

Core Technical Content

Mining pick teeth are subjected to a complex combination of wear mechanisms including abrasion by coal and rock particles, impact from hard rock fragments, and adhesive wear from coal dust. The conventional pick teeth manufactured from quenched and tempered medium carbon steel (typically 45 steel or 50Cr) exhibit limited service life, often requiring replacement after 50-200 meters of mining advance, depending on the geology. This frequent replacement results in significant downtime, increased maintenance costs, and reduced mining productivity.

The study investigates the application of weld overlay technology to deposit hardfacing alloys on the working surfaces of pick teeth, creating a wear-resistant overlay layer that extends service life while maintaining the toughness of the base material. The overlay alloys examined include iron-based alloys (Fe-Cr-C, Fe-Cr-W), cobalt-based alloys (Co-Cr-W, such as Stellite 6), and nickel-based alloys (Ni-Cr-Mo, such as Inconel 625). The welding processes evaluated include submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) welding, each offering different advantages in terms of deposition rate, dilution control, and microstructural quality.

Overlay Alloy Selection and Performance

The selection of overlay alloy is governed by the specific mining conditions, including coal hardness, rock content, moisture level, and particle size distribution:

The iron-based martensitic alloys (Fe-Cr-C) offer the highest hardness and are suitable for moderate impact conditions with predominantly abrasive wear. However, they are susceptible to cracking during welding due to their high carbon equivalent and the formation of brittle martensitic microstructure. The cobalt-based alloys (Stellite 6) provide an excellent balance of hardness, toughness, and hot hardness, making them ideal for severe impact-abrasion conditions. The nickel-based alloys (Inconel 625) offer superior toughness and corrosion resistance but lower hardness, making them suitable for high-impact, low-abrasion conditions.

Process Parameters and Welding Procedure

The welding procedure for pick teeth overlay requires careful optimization to ensure proper fusion with the base metal, adequate overlay thickness, and controlled dilution. The following table presents typical process parameters for different welding methods:

The dilution rate is a critical parameter that directly affects the final overlay composition and properties. Excessive dilution from the base metal can reduce the hardness and wear resistance of the overlay layer, particularly for high-alloy overlays where the beneficial alloying elements are diluted below critical thresholds. PTA welding offers the lowest dilution rates and is preferred for high-alloy overlays where composition control is essential.

Welding Sequence and Defect Prevention

The welding sequence for pick teeth overlay is designed to minimize distortion and ensure uniform coverage of the wear surface. A typical sequence involves:

1. Preparation: Grinding of the wear surface to remove oxide scale and provide a clean, slightly roughened surface for improved fusion.
2. Base pass: A low-dilution pass using a compatible filler material to establish a metallurgical bond between the base metal and the overlay alloy.
3. Fill passes: Multiple passes to build up the required overlay thickness, typically 3-5 mm for pick teeth applications.
4. Final pass: A carefully controlled pass to achieve the desired surface profile and finish.

Common defects in pick teeth overlay welding include:

Engineering Practice and Performance Evaluation

The performance of weld-overlay pick teeth is evaluated through field testing in actual mining conditions, comparing service life against conventional uncladded pick teeth. The study reports that properly applied weld overlay coatings can extend pick tooth service life by 3-6 times, depending on the overlay alloy selection and welding quality. The economic benefit is significant, as reduced pick replacement frequency translates to lower downtime, reduced consumable costs, and improved mining productivity.

The FMEA (Failure Mode and Effects Analysis) approach is applied to identify potential failure modes in the overlay process:

Study Insights and Practical Recommendations

This research demonstrates the significant potential of weld overlay technology for extending the service life of mining pick teeth, with practical implications for improving mining efficiency and reducing operational costs. The key insight is that the selection of overlay alloy must be matched to the specific wear conditions encountered in the mining environment, as no single alloy provides optimal performance across all conditions. Iron-based martensitic alloys offer the best cost-performance ratio for moderate wear conditions, while cobalt-based alloys provide superior performance for severe impact-abrasion conditions at a higher material cost.

The study also highlights the importance of welding procedure qualification and welder skill in achieving consistent overlay quality. The dilution rate, which directly affects overlay composition and properties, is highly sensitive to welding parameters and operator technique. Standardized procedures, combined with visual and dimensional inspection of each weld, are essential for maintaining consistent quality in production environments.

For engineering practice, the following recommendations emerge: (1) conduct a detailed wear analysis of the mining conditions to select the appropriate overlay alloy; (2) qualify the welding procedure according to NB/T 47014 or equivalent standards, with specific attention to dilution control; (3) implement a preheat and post-weld treatment regime to prevent cracking; (4) perform regular field performance monitoring to track overlay wear rate and identify optimization opportunities; and (5) consider the total cost of ownership, including overlay material cost, welding labor, and service life extension, when evaluating the economic viability of overlay protection. These measures collectively enable the effective application of weld overlay technology in mining pick tooth protection, delivering substantial operational benefits in underground coal mining operations.