Experimental Study on Carbon Arc Surfacing of Scraper Conveyor Middle Troughs

Literature Overview

This 1983 article from the Zhangjiakou Coal Mine Machinery Factory (Ministry of Coal) and the Carbon Arc Surfacing Trial Group of the Harbin Welding Research Institute (Ministry of Machinery), published in Coal Science and Technology (煤炭科学技术), Vol. 11, Issue 7, pp. 31-32, reports on the experimental development of a carbon arc surfacing process for repairing wear on scraper conveyor middle troughs in underground coal mines. The study, classified under TG455, describes a practical, field-applicable process for applying high-carbon, high-chromium, boron-containing wear-resistant alloy overlays to the easily worn corners of the middle trough center plate made of 16Mn steel.

The significance of this work lies in its practical orientation: it addresses a real-world maintenance challenge in coal mining operations where scraper conveyor middle troughs suffer severe abrasive wear from coal and rock particles, requiring frequent repair or replacement. The carbon arc surfacing process described offers a cost-effective, field-deployable solution.

Process Description and Technical Features

The carbon arc surfacing process (also known as carbon arc gouging or carbon arc welding) uses a carbon electrode to create an arc that melts both the base metal and a pre-placed alloy powder or block. The molten pool solidifies to form a wear-resistant overlay with the desired chemical composition and microstructure.

Process Steps

1. Surface preparation: The worn area on the middle trough center plate is cleaned by grinding or chipping to remove loose material, rust, and scale. The area is prepared to a sound base metal surface with a slight undercut to promote mechanical interlocking of the overlay.
2. Alloy placement: Pre-fabricated wear-resistant alloy powder blocks are placed on the prepared surface at the four corners of the center plate, which are the most severely worn locations during conveyor operation.
3. Carbon arc melting: A carbon electrode is used to create an arc that melts the alloy powder block and a portion of the base metal simultaneously. The molten pool is directed to flow across the prepared surface, forming a uniform overlay.
4. Solidification and cooling: The molten pool solidifies to form the wear-resistant overlay. The cooling rate is relatively slow in field conditions, which promotes the formation of a favorable microstructure.

Process Characteristics Compared to Plasma Surfacing

The authors provide a comparison between carbon arc surfacing and plasma surfacing (plasma arc welding) for this application:

The high dilution rate of carbon arc surfacing is both an advantage and a disadvantage. On one hand, the deep penetration and high dilution ensure a strong metallurgical bond between the overlay and the base metal, reducing the risk of overlay spalling during service. On the other hand, the high dilution means that the final overlay composition is a mixture of the alloy powder and the base metal, which may not achieve the full hardness and wear resistance of a pure alloy overlay.

Overlay Composition and Microstructure

The wear-resistant alloy used in this study contains high carbon (2-4%), high chromium (15-25%), and boron (0.5-1.5%). The resulting overlay microstructure typically consists of:

• A matrix of martensite or austenite, depending on the cooling rate and alloy composition
• Dispersed carbide particles of Cr7C3, Cr23C6, and possibly boron carbides (B4C)
• The high carbon and chromium content promotes the formation of hard, wear-resistant carbides that provide the primary wear resistance

The hardness of the overlay is typically in the range of 50-60 HRC, significantly higher than the base 16Mn steel (which is typically 150-200 HB). The increased hardness provides the wear resistance needed to extend the service life of the middle trough in abrasive coal mining environments.

Field Application and Performance Evaluation

The carbon arc surfacing process was developed specifically for underground coal mine applications, where the following constraints must be considered:

1. Limited power availability: The process must work with portable power sources that can operate in underground environments, typically diesel generators or battery-powered inverters.
2. Space constraints: The equipment must be compact enough to be transported to the repair location in the mine and to operate in the confined spaces around the conveyor.
3. Safety requirements: The process must not produce excessive fumes, sparks, or heat that could ignite methane or coal dust in the mine atmosphere.
4. Speed of repair: The process must allow rapid repair to minimize conveyor downtime, which directly affects mine production.

The authors report that the carbon arc surfacing process meets these requirements effectively. The simple equipment, ease of operation, and low cost make it an attractive option for mine maintenance operations. The deep penetration ensures a strong bond between the overlay and the base metal, reducing the risk of overlay failure during the harsh service conditions of a scraper conveyor.

Performance Comparison with Alternative Processes

The study also compares the carbon arc surfacing process with plasma surfacing in terms of the resulting overlay properties:

The slightly lower hardness of the carbon arc overlay is compensated by the excellent bond strength and the practical advantages of the process for field use. In the harsh environment of a coal mine, the ability to achieve a reliable repair with simple equipment is often more important than achieving the maximum possible overlay hardness.

Study Insights and Practical Implications

This 1983 study represents an important contribution to the practical application of surfacing technology in the mining industry. The carbon arc surfacing process described here is a classic example of process development driven by practical constraints rather than theoretical optimization. The process was designed to meet the specific needs of underground coal mine maintenance operations, where reliability, simplicity, and cost-effectiveness are paramount.

The high dilution rate of carbon arc surfacing, which is often viewed as a disadvantage in laboratory settings, becomes an advantage in this application. The deep penetration ensures a metallurgical bond that is resistant to the mechanical stresses and thermal cycling experienced by the middle trough during conveyor operation. The overlay is less likely to spall or delaminate under impact loading from coal and rock particles.

The study also highlights the importance of field testing and practical validation in process development. The carbon arc surfacing process was not merely developed in a laboratory but was tested and refined in actual mine conditions, ensuring that the process works reliably in the real-world environment where it will be applied.

For modern mining operations, the carbon arc surfacing process remains relevant, particularly in remote or underground locations where advanced welding equipment may not be available. The process can be adapted to use modern power sources and improved alloy compositions, but the fundamental principles remain the same. Engineers working on maintenance and repair programs in the mining industry should consider this process as a viable option for extending component life in abrasive service environments.

The study demonstrates the value of practical, field-oriented research in advancing welding technology. While laboratory studies provide fundamental understanding, it is the application of this knowledge to real-world problems that drives the practical adoption of new processes and materials.