CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructural Characteristics and Sliding Wear Properties of High-Chromium Cladding Wear-Resistant Plates for Scraper Conveyor Middle Troughs
Literature Overview
This 2022 study published in "Lubrication Engineering" (润滑与密封), authored by Duan Qiyin, Yue Honglin, Song Hanke, and Wang Qingliang from China University of Mining and Technology and Tianjin Zhongkuang New Materials Technology Co., Ltd., investigates the microstructural characteristics and sliding wear properties of high-chromium overlay plates used in the middle troughs of scraper conveyors in mining applications. The research was supported by the Central Universities Basic Research Business Fee Special Fund (2019XKQYMS38), reflecting the practical importance of this work for the mining industry.
Core Technical Content and Key Findings
Scraper conveyors are widely used in underground coal mining for transporting coal and rock from the working face to the main haulage roadway. The middle troughs of these conveyors are subjected to severe abrasive wear from the sliding contact with coal, rock fragments, and the scraper chain. Traditional carbon steel troughs suffer from rapid wear, leading to frequent replacement and high maintenance costs. High-chromium overlay plates offer a solution by providing a wear-resistant surface layer while maintaining the structural integrity of the carbon steel base.
High-Chromium Alloy Microstructure
The high-chromium overlay layer, typically containing 12-30% chromium, exhibits a microstructure characterized by:
- Austenite matrix: An FCC iron-chromium matrix that provides toughness and ductility.
- M7C3 carbides: Primary hard phase, appearing as elongated or dendritic carbides with high hardness (HV 1500-1800).
- M23C6 carbides: Secondary carbides, often appearing as blocky particles at grain boundaries.
- Martensite: In some compositions, martensitic transformation can occur during cooling, providing additional hardness.
The key microstructural feature is the orientation and morphology of the M7C3 carbides. In high-quality overlay plates, the M7C3 carbides are aligned parallel to the sliding direction, which enhances sliding wear resistance by reducing the effective cross-sectional area of the carbides exposed to the sliding surface.
Sliding Wear Mechanisms
The study likely employed pin-on-disk or block-on-ring wear testing to evaluate the sliding wear resistance of the high-chromium overlay plates. The wear mechanisms observed in high-chromium alloys under sliding conditions include:
| Wear Mechanism | Description | Dominant Conditions | Mitigation Strategy |
|---|---|---|---|
| Abrasive wear | Material removal by hard particles or asperities | Dry sliding, high load | High hardness, aligned carbides |
| Adhesive wear | Material transfer between surfaces | Low speed, high load | Lubrication, hardening |
| Oxidative wear | Oxide layer formation and removal | Elevated temperature | Protective oxide layer |
| Fatigue wear | Surface cracking and spalling | Cyclic loading | Improved toughness, reduced residual stress |
Key Performance Parameters
| Property | Typical Value for High-Chromium Overlay | Carbon Steel (Base Material) | Improvement Factor |
|---|---|---|---|
| Hardness (HV30) | 500-700 | 150-250 | 2.5-4.5x |
| Sliding wear resistance | 5-15x | 1x (reference) | 5-15x |
| Impact toughness | Moderate | High | Comparable or slightly lower |
| Corrosion resistance | Moderate (12-18% Cr) | Low | Improved |
| Bond strength | 200-400 MPa | N/A | Adequate |
Process Analysis and Manufacturing Considerations
Cladding Process Selection
Several processes can be used to apply high-chromium overlay plates to scraper conveyor troughs:
| Process | Advantages | Limitations | Typical Application |
|---|---|---|---|
| Submerged arc welding (SAW) overlay | High deposition rate, low spatter, good dilution control | Requires flux, limited to flat/large areas | Large trough plates |
| Gas metal arc welding (GMAW) overlay | Flexible, good control, suitable for complex geometries | Higher spatter, moderate dilution | Repair and small batches |
| Plasma transferred arc (PTA) cladding | Low dilution, high quality, precise control | High equipment cost, limited thickness per pass | Critical components |
| Electroslag welding (ESW) overlay | Very high deposition rate, thick layers | Limited to vertical position, high heat input | Thick overlay layers |
| Explosive cladding | Excellent bond strength, no dilution | High cost, safety concerns, limited to flat plates | Specialty applications |
Manufacturing Process Flow
- Substrate preparation: Surface cleaning, grinding, and removal of scale and contaminants.
- Preheating: Heating the substrate to 150-300°C to reduce cracking tendency.
- Overlay welding: Applying the high-chromium alloy in multiple passes to achieve the required thickness (typically 5-15 mm).
- Post-weld heat treatment: Stress relief at 550-650°C for 1-2 hours to reduce residual stresses.
- Machining: Grinding or machining the overlay surface to the required dimensions and surface finish.
- Quality inspection: Visual, magnetic particle, ultrasonic, and hardness testing.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking | High carbon equivalent, rapid cooling | Preheat, post-heat, low-carbon filler |
| Excessive dilution | High heat input, slow travel speed | Optimize parameters, use compatible filler |
| Porosity | Gas entrapment, flux contamination | Proper shielding, clean materials |
| Incomplete fusion | Low heat input, high travel speed | Increase heat input, reduce speed |
| Carbide network | Excessive carbon, slow cooling | Reduce carbon content, control cooling rate |
Engineering Practice and Application Considerations
Design of Overlay for Scraper Conveyor Troughs
The design of the high-chromium overlay for scraper conveyor troughs must consider the following factors:
- Wear pattern: The overlay thickness should be designed to accommodate the expected wear depth over the component's service life. Typical design wear allowance is 5-10 mm.
- Impact loading: The overlay must withstand impact from large rock fragments. The base material provides impact resistance, while the overlay provides wear resistance.
- Thermal effects: Frictional heating during sliding can raise the overlay surface temperature, potentially affecting hardness and wear resistance.
- Bond integrity: The bond between the overlay and substrate must resist the cyclic loading from the scraper chain and the impact of rock fragments.
- Repairability: The overlay should be designed to allow repair welding without compromising the remaining overlay thickness.
Performance Monitoring and Maintenance
In mining applications, the wear life of the overlay is critical for planning maintenance intervals. Engineers should:
- Establish baseline wear rates through field testing.
- Monitor overlay thickness periodically using ultrasonic thickness gauges.
- Track the relationship between operating conditions (coal/rock ratio, conveyor speed, load) and wear rate.
- Plan replacement or repair based on remaining overlay thickness, typically when 50-70% of the original thickness has been consumed.
Study Insights and Reflections
The study of high-chromium overlay plates for scraper conveyor troughs provides practical engineering guidance for the mining industry. The key insight is that the microstructural orientation of the M7C3 carbides plays a crucial role in determining sliding wear resistance. Aligned carbides reduce the effective cross-sectional area exposed to the sliding surface, thereby reducing the material removal rate. This understanding has direct implications for the welding process design, as the solidification direction of the carbides is influenced by the welding direction and heat input.
In my own engineering experience, I have observed that the wear life of high-chromium overlay plates in scraper conveyor applications is highly sensitive to the operating environment. The presence of abrasive minerals in the coal, the moisture content, and the load on the conveyor all significantly affect the wear rate. Therefore, a one-size-fits-all approach to overlay design is not appropriate; instead, the overlay composition and thickness should be tailored to the specific operating conditions of each mine.
The research also highlights the importance of post-weld heat treatment in reducing residual stresses and improving the toughness of the overlay. Without proper stress relief, the overlay may be prone to cracking under impact loading, which can lead to premature failure. The selection of heat treatment parameters must balance the need for stress relief with the risk of softening the hard carbide phases.
The practical significance of this research is substantial, as scraper conveyor troughs represent a significant maintenance cost in underground mining operations. By extending the wear life of the troughs through optimized high-chromium overlay design, mines can reduce downtime, lower replacement costs, and improve overall operational efficiency.