Large-Area Wear-Resistant Pattern Surfacing Welding Application on Mine Conveyor Middle Trough

Literature Overview

This 1995 paper by Zheng Yanting, Ge Changlu, Liu Yugao, and Li Yong, published in Coal Science and Technology, reports on the application of large-area wear-resistant pattern surfacing welding on the middle trough of scraper conveyors in underground coal mines. The collaboration between China University of Mining and Technology and Yungang Mine of Datong Mining Bureau represents an early example of university-industry cooperation in addressing practical mining engineering challenges. The paper documents the development and field application of specialized wear-resistant welding electrodes (KD-1 and KD-2 types, equivalent to EDCrWB) for extending the service life of conveyor middle troughs.

Technical Background

The middle trough (also called the trough section or center trough) is a critical component of underground scraper conveyors used in coal mining operations. It serves as the channel through which the scraper chain and coal are transported. The trough is subjected to:

• Abrasive wear from coal particles, rock fragments, and dust
• Impact loading from falling coal and rock
• Corrosive attack from mine water and chemical agents
• Fatigue stress from cyclic loading during continuous operation

Typical SGB-764/400 series scraper conveyors used in Chinese coal mines have middle troughs that originally last only 3-6 months before requiring replacement. Given the large number of conveyors in operation and the difficulty of replacement in underground environments, extending trough life is a significant economic priority.

Wear-Resistant Electrode Development

The China University of Mining and Technology developed two specialized electrodes for this application:

KD-1 Type Electrode

KD-2 Type Electrode (EDCrWB)

The addition of boron in the KD-2 electrode produces hard chromium boride (CrB) particles that provide superior wear resistance compared to chromium carbide alone. However, the increased hardness comes at the expense of some toughness, making KD-2 more suitable for applications where pure abrasion dominates and impact loading is moderate.

Pattern Surfacing Design

The paper describes a "pattern" (花样) surfacing approach that differs from uniform full-surface welding. This pattern approach involves:

Design Principles

1. Selective coverage: Weld material is applied only to the areas experiencing the most severe wear, typically the bottom surface and side walls of the trough.
2. Geometric optimization: The weld pattern is designed to provide wear resistance while maintaining the structural integrity of the base material.
3. Heat input management: By welding only specific areas, the total heat input is reduced, minimizing thermal distortion of the trough.
4. Material economy: Selective welding reduces the volume of expensive alloy material required.

Typical Pattern Layout

The surfacing pattern for the SGB-764/400 trough includes:

• Bottom surface: Full-width overlap-welded pattern with 2-3 layers of 8-12 mm wide beads
• Side walls (upper portion): Partial coverage on the wear-prone upper edge, 1-2 layers
• Scraper chain contact zone: Heavier build-up (3-4 layers) in the area where the scraper chain runs
• End connections: Reinforced welds at trough joints where stress concentration occurs

Weld Bead Geometry

Welding Process Parameters

The surfacing welding is performed using SMAW (shielded metal arc welding) with the following parameters:

The use of DCEP (direct current electrode positive) polarity is critical for cast iron electrodes, as it provides deeper penetration and better fusion with the base metal while maintaining the hard microstructure of the overlay.

Performance Results

The paper reports significant improvements in trough service life:

These results represent a 3-6 fold improvement in service life, which translates directly into significant cost savings for mining operations.

Technical Challenges and Solutions

Cracking Prevention

The high carbon content of cast iron weld metal creates a significant risk of cracking:

• Hot cracking: Controlled by maintaining adequate arc length and travel speed to avoid excessive heat concentration
• Cold cracking: Prevented by preheating and controlled cooling to reduce hydrogen-induced cracking susceptibility
• Microcracking: Minimized by using multiple thin layers rather than thick single passes

Dilution Management

When welding cast iron overlay onto carbon steel base metal, dilution is inevitable:

• First pass dilution: 40-60% base metal in the weld metal
• Second pass dilution: 20-30% base metal
• Final surface layer dilution: Below 15%

The multi-layer approach ensures that the final surface achieves the target hardness despite dilution from the base metal.

Dimensional Control

The trough must maintain precise geometry to ensure proper conveyor operation:

• Total thickness variation: Within ±1.0 mm
• Flatness: Within 1.0 mm per meter
• Alignment of adjacent trough sections: Within ±0.5 mm

Post-weld machining or grinding is typically required to restore dimensional accuracy.

Economic Analysis

The economic benefits of the surfacing welding approach are substantial:

The payback period for the initial investment in welding equipment and training is typically less than 6 months.

Engineering Practice Considerations

For engineers implementing similar surfacing welding programs in mining operations, the following considerations are important:

1. Welder training: Cast iron welding requires specialized skills; welders should be certified and regularly evaluated.
2. Quality control: Hardness testing of weld deposits should be performed periodically to verify that specifications are being met.
3. Surface preparation: Thorough cleaning and grinding of the worn surface is essential for achieving proper fusion.
4. Environmental factors: Underground mining environments may present challenges with ventilation, humidity, and confined spaces.
5. Material compatibility: The base material composition should be verified to ensure compatibility with the selected electrode.

Critical Reflection

The paper represents a successful application of surfacing welding technology in a demanding industrial environment. However, several aspects could be improved with modern techniques:

• Welding process automation: Manual SMAW is labor-intensive; FCAW or GMAW with automatic or semi-automatic torches would improve productivity and consistency.
• Alternative materials: Modern high-alloy surfacing materials (e.g., CoCr alloys, ceramic-filled wires) could potentially provide even better wear resistance.
• Process monitoring: Real-time monitoring of weld parameters and in-situ hardness measurement could improve quality assurance.
• Design optimization: Finite element analysis of the trough stress distribution could optimize the surfacing pattern for maximum benefit.

Summary

The application of large-area pattern surfacing welding on mine conveyor middle troughs demonstrates the significant potential of overlay welding technology to extend component life in abrasive wear environments. The development of specialized high-chromium cast iron electrodes with boron additions, combined with a rational pattern design that targets the most wear-prone areas, achieved a 3-6 fold improvement in trough service life. This represents a straightforward yet highly effective engineering solution that delivers substantial economic benefits to mining operations. For engineers in similar applications, the paper provides a proven methodology for selecting appropriate surfacing materials, designing effective weld patterns, and implementing quality control procedures.