Welding Performance of Middle Trough Based on Plasma Hardfacing

Literature Overview

The paper by Wang Cunfei, Wang Xudong, Wei Ziliang, Li Keming, and Yang Zengfu from Shenhua Shendong Coal Group Co., Ltd. and the State Key Laboratory of Metallurgical Materials, Xi'an Jiaotong University, published in Coal Mine Machinery (2019, Vol. 40, No. 9, pp. 46-49), investigates the welding performance of the middle trough (a critical conveyor component in underground coal mining) when repaired using plasma hardfacing technology. The study examines macro and micro structures, measures hardness profiles, and conducts wear resistance tests to evaluate the effectiveness of plasma hardfacing for middle trough repair. This work addresses a significant industrial need in the coal mining sector, where conveyor middle troughs are subjected to severe abrasive wear from coal and rock particles.

Core Technical Findings

The study demonstrates that plasma hardfacing can achieve a weld overlay thickness of up to 8 mm on middle trough components. The key findings include:

Solid solution strengthening from C and Cr elements in the alloy powder significantly contributes to overlay hardness.
Dispersion strengthening from carbides further enhances wear resistance.
The wear resistance of the hardfacing overlay is improved by a factor of 1.77 compared to the base material.
The overlay wear resistance slightly exceeds that of the base material, meeting the welding standard requirements for middle trough repair.

Microstructural Analysis

The plasma hardfacing process produces a microstructure characterized by:

Feature	Description	Function
Matrix	Ferrite + martensite + retained austenite	Base strength and toughness
Primary carbides	Cr₇C₃, Cr₂₃C₆ type	Dispersion strengthening
Secondary carbides	M₇C₃, M₂₃C₆	Additional hardness contribution
Grain structure	Fine dendritic with interdendritic phases	Strength and toughness balance

The combination of solid solution strengthening (from dissolved C and Cr in the matrix) and dispersion strengthening (from carbide precipitates) provides the enhanced wear resistance observed in the study.

Plasma Hardfacing Process Parameters

Plasma hardfacing (PTA - Plasma Transfer Arc) is particularly well-suited for middle trough repair due to several advantageous characteristics:

Low dilution rate: Typically 5-15%, compared to 30-50% for conventional arc hardfacing. This preserves the alloy composition of the overlay.
Controlled heat input: The plasma arc provides concentrated, stable heat input that minimizes HAZ effects.
High deposition rate: Enables efficient repair of large worn areas.
Good metallurgical bonding: Produces strong fusion between overlay and substrate.
Flexible process: Can be performed on-site or in workshop conditions.

Application Context in Coal Mining

The middle trough is a critical component of scraper conveyor systems used in underground coal mining. It is subjected to:

Continuous abrasive wear from coal and embedded rock particles
Impact loading from large rock fragments
Fatigue loading from cyclic scraper chain engagement
Corrosive environment from mine water and coal dust

The service life of middle troughs is primarily limited by wear of the trough surface. When the trough depth exceeds the allowable limit (typically 20-30% of original depth), the component must be repaired or replaced. Plasma hardfacing offers an economical repair solution that restores the trough surface geometry while providing enhanced wear resistance.

Wear Mechanism Analysis

The wear mechanisms in middle trough service are predominantly:

Wear Type	Mechanism	Dominant Phase
Abrasive wear	Hard particles plough through surface	Carbides resist ploughing
Impact wear	Repeated loading causes material removal	Tough matrix absorbs impact
Fatigue wear	Cyclic loading causes crack initiation and growth	Fine grain structure resists crack growth
Adhesive wear	Material transfer between surfaces	Hard surface resists adhesion

Quality Control and Standards Compliance

The welding performance evaluation must address several quality aspects:

Overlay thickness: Must be sufficient to restore geometry and provide wear life (typically 5-10 mm for middle troughs).
Hardness profile: Must show adequate hardness gradient from overlay to base material.
Bonding strength: Interface must withstand impact and fatigue loading.
Crack resistance: Overlay must not contain cracks that could propagate under service loading.
Dimensional accuracy: Repaired surface must maintain proper geometry for scraper engagement.

The relevant standards for this application include GB/T standards for coal mining equipment, and welding procedure qualification requirements per GB/T 19866 or ISO 15614.

Engineering Practice Considerations

Several practical considerations arise from this study for engineers implementing plasma hardfacing repair of middle troughs:

Surface preparation: Thorough cleaning of the worn surface is essential for good metallurgical bonding. Rust, oil, and loose material must be removed.
Preheating: Moderate preheating (100-200°C) may be beneficial for thick sections to reduce thermal stresses and cracking tendency.
Multi-pass deposition: For 8 mm overlay thickness, multiple passes are required. Interpass temperature control is important to maintain overlay properties.
Post-weld treatment: Stress relief treatment may be beneficial to reduce residual stresses, particularly for components subjected to cyclic loading.
Inspection: Visual inspection, magnetic particle testing (MT), and hardness testing should be performed on all repairs.

Key Questions and Reflections

The 1.77 times improvement in wear resistance reported in this study is significant but should be interpreted in the context of the specific test conditions. Laboratory wear tests (typically pin-on-disk or block-on-ring) may not fully replicate the complex loading conditions encountered in actual mining service. The actual service life improvement may be different from the laboratory wear resistance improvement ratio.

From my experience in mining equipment maintenance, the economic viability of hardfacing repair depends on the ratio of repair cost to replacement cost, multiplied by the life extension factor. For middle troughs, which are relatively inexpensive components compared to other mining equipment, the economics of hardfacing repair must be carefully evaluated. However, when considering the cumulative cost of multiple replacements over the life of a mining operation, hardfacing repair can be significantly more economical.

The study also raises the question of whether the overlay properties remain stable under prolonged service. Carbide coarsening and phase transformations at elevated temperatures (if present in the mining environment) could degrade overlay properties over time. Long-term service monitoring would provide valuable data on this aspect.

Study Insights and Implications

This work demonstrates that plasma hardfacing is a technically viable and economically attractive solution for middle trough repair in coal mining applications. The combination of low dilution, controllable process parameters, and excellent metallurgical bonding makes plasma hardfacing superior to conventional arc hardfacing for this application. The 1.77 times wear resistance improvement, while based on laboratory testing, provides strong evidence for the technology's effectiveness.

For mining companies considering implementation, I recommend a phased approach: begin with a limited pilot program on non-critical equipment, collect service performance data, and then scale up to full implementation. The key success factors are proper surface preparation, controlled process parameters, and thorough post-weld inspection. The long-term value of this technology lies not only in the immediate wear resistance improvement but also in the reduction of unplanned downtime and the extension of equipment service intervals, which directly contribute to mining operation productivity and safety.