Application of Surfacing Technology in Coal Mine Fully Mechanized Equipment Repair

Literature Overview

This 2007 paper by Zhang Xinhua and Li Jing from Shandong Jining Mining Group, published in Mining Machinery (矿山机械), Volume 35, Issue 6, addresses the application of surfacing welding technology for repairing scraper conveyor troughs in fully mechanized coal mining equipment. The paper is classified under TD407 (coal mining methods and equipment) and reflects the practical challenges of maintaining heavy-duty mining equipment in high-production-rate operations.

Core Technical Content

The Jining Mining Group operates a large modern mine with a design capacity of 1.5 million tonnes per year, equipped with two sets of fully mechanized longwall mining systems. The scraper conveyors (刮板输送机) that transport coal from the working face are subjected to extreme wear conditions:

• Abrasive wear: Coal particles, including embedded rock fragments, abrade the trough surfaces continuously.
• Impact loading: Large coal chunks and rock pieces impact the trough bottom and side plates.
• Fatigue: Cyclic loading from the reciprocating scraper chain causes fatigue cracks in high-stress regions.

After one production cycle, significant localized wear occurs on the troughs, particularly at the bottom plate, side plate edges, and chain groove areas. The paper notes that multiple materials and processes were attempted for trough repair, but results were unsatisfactory—suggesting that the selection of surfacing materials and process parameters was not optimized for the specific service conditions.

Surfacing Material Selection for Trough Repair

The selection of surfacing material for scraper conveyor troughs must balance wear resistance, toughness, and weldability:

The key insight from this paper is that a single surfacing material may not be sufficient for the entire trough. Different zones experience different wear mechanisms, and a zoned surfacing strategy—applying different materials to different areas—would likely improve repair longevity.

Process Optimization

The paper implicitly addresses the challenge of achieving satisfactory repair results. Based on engineering practice, the following process parameters are critical:

1. Surface preparation: The worn surface must be ground to remove all damaged material, including microcracks and delaminated layers. A minimum removal of 2–3 mm is recommended to ensure sound base material.
2. Preheating: For medium-carbon and high-carbon steel troughs, preheating to 200–300 °C is essential to prevent cold cracking in the HAZ. The preheat temperature should be maintained throughout the welding sequence.
3. Welding sequence: A symmetric welding sequence should be used to minimize distortion. For troughs, welding should proceed from the center outward, alternating sides.
4. Layer control: Each surfacing layer should be limited to 2–3 mm in thickness. Multiple thin layers reduce residual stress and improve metallurgical quality.
5. Post-weld treatment: Stress relief at 550–600 °C is recommended to eliminate residual stresses that could cause delayed cracking or premature wear.

Common Defects and Root Cause Analysis

Engineering Practice and FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework to trough repair operations reveals the following critical failure modes:

1. Premature spalling: The most common failure mode, occurring when the surfacing layer detaches from the base material. Root causes include insufficient fusion, high residual stress, and thermal cycling during service. The recommended detection method is tap testing with a soft hammer, and the recommended prevention is multi-pass surfacing with controlled interpass temperature.
2. Abrasive wear breakthrough: The surfacing layer wears through faster than expected. This occurs when the surfacing material hardness is insufficient for the coal-rock abrasive mixture. The countermeasure is to use harder surfacing materials (HV > 550) in high-abrasion zones and to increase layer thickness to 6–8 mm.
3. Fatigue cracking at surfacing interface: Cyclic loading initiates cracks at the weld interface. Prevention requires smooth transitions from surfacing to base material (no sharp edges) and post-weld stress relief.

Study Insights

This paper, while brief, highlights a recurring theme in mining equipment repair: the gap between material capability and practical application. The authors note that multiple materials and processes were tried with unsatisfactory results, suggesting that the fundamental issue may not be the surfacing material itself but the process control and quality assurance surrounding the repair operation. In my experience, the most common cause of surfacing repair failure in mining applications is inadequate surface preparation and process discipline, rather than material selection.

The paper also underscores the economic rationale for repair over replacement. For a mine producing 1.5 million tonnes per year, the cost of replacing worn troughs is substantial, and the downtime associated with replacement is often more costly than the repair itself. Surfacing repair, when properly executed, can extend trough life by 50–100% at a fraction of the replacement cost.

From a broader perspective, this case illustrates the need for standardized repair procedures in mining operations. The development of welding procedure specifications (WPS) and qualified welding procedure records (WQR) for trough surfacing repair would significantly improve consistency and reliability of repair outcomes.