CLADDING TECHNOLOGY SHANXI CO., LTDImproving the Service Life of Middle Troughs by Weld Overlay Method
Literature Overview
This 2006 paper by Li Chuangji from Weinan High-Tech Zone Kaituo Special Welding Materials Factory addresses the application of weld overlay technology to extend the service life of middle troughs, which are critical components in mining conveyor systems. Middle troughs are subjected to severe abrasive wear from coal and rock particles, as well as impact loading from falling material. The paper describes the use of hardfacing weld overlay to protect the wear surfaces of middle troughs, thereby extending their service interval and reducing maintenance costs.
Wear Mechanisms and Overlay Strategy
The wear experienced by middle troughs in mining conveyor systems is primarily a combination of abrasive wear and impact wear. Abrasive wear occurs when hard particles in the conveyed material scrape across the trough surface, gradually removing material. Impact wear results from the kinetic energy of falling material striking the trough surface. The severity of wear depends on factors such as material hardness, particle size, conveyor speed, and the angle of impact.
The weld overlay strategy for middle troughs involves depositing a hardfacing alloy on the wear surfaces to provide a sacrificial layer that resists wear better than the base structural steel. Common hardfacing alloys used for this application include:
| Alloy Type | Typical Composition | Hardness (HRC) | Wear Resistance | Impact Toughness |
|---|---|---|---|---|
| Carbide-forming | Fe-Cr-C with WC, Cr3C2 | 45–60 | Excellent | Moderate |
| High-chromium cast iron | Fe-12-15% Cr-2-4% C | 50–60 | Very good | Low |
| Nickel-hardiron | Fe-Ni-C with Cr, Mo | 40–55 | Good | Moderate |
| Stellite-type | Co-Cr-W | 40–50 | Excellent | Good |
The selection of the overlay alloy must balance wear resistance with impact toughness, as excessive hardness can lead to spalling under impact loading. For middle troughs in coal mining applications, carbide-forming iron-based alloys with WC or Cr3C2 are commonly used, as they provide an excellent combination of hardness and toughness.
Overlay Process Parameters
The weld overlay process for middle troughs typically employs either submerged arc welding (SAW) or flux-cored arc welding (FCAW), depending on the production volume and the geometry of the trough. The key process parameters include:
| Parameter | SAW Overlay | FCAW Overlay |
|---|---|---|
| Welding current | 400–600 A | 250–400 A |
| Welding voltage | 25–35 V | 28–38 V |
| Travel speed | 200–400 mm/min | 300–600 mm/min |
| Wire/feed diameter | 2.4–3.2 mm | 1.2–1.6 mm |
| Number of passes | 1–3 | 1–2 |
| Overlay thickness | 3–8 mm | 2–5 mm |
For SAW overlay, a submerged flux is used to protect the molten pool and provide alloying elements to the overlay deposit. The flux composition must be carefully controlled to achieve the desired hardness and microstructure in the overlay layer. The number of passes depends on the required overlay thickness and the dilution ratio of the base metal.
Quality Control and Performance Verification
The quality of the weld overlay on middle troughs is verified through several methods:
- Hardness testing: The overlay hardness is measured using Vickers or Rockwell hardness testers at multiple locations across the overlay surface. The hardness should be uniform and within the specified range for the selected alloy.
- Metallographic examination: Cross-sectional samples are examined to verify the microstructure of the overlay layer, the dilution zone, and the bond interface. The presence of cracks, porosity, or incomplete bonding at the interface is a critical defect.
- Bond strength testing: The bond strength between the overlay layer and the base metal is tested using a tensile or shear test. The bond strength should be at least equal to the tensile strength of the base metal.
- Wear testing: In some cases, a bench-scale wear test is conducted to compare the wear resistance of the overlay layer with the base metal. This provides quantitative data on the expected life extension.
Engineering Practice and Life Extension
The practical application of weld overlay to middle troughs has demonstrated significant life extension in mining operations. Typical results show that the service life of overlay-protected middle troughs can be extended by 3 to 5 times compared to unprotected troughs, depending on the operating conditions and the quality of the overlay application. The economic benefit is substantial, as the cost of overlay application is typically a fraction of the cost of replacing the trough.
However, the long-term performance of the overlay depends on proper application. Common failure modes include overlay spalling due to impact, cracking of the overlay layer due to thermal stresses, and premature wear through the overlay layer. These failures are often attributed to improper process parameters, inadequate surface preparation, or selection of an overlay alloy that is too hard and brittle for the application.
Study Insights
This paper provides a practical and straightforward demonstration of how weld overlay technology can be applied to extend the service life of heavily worn components in mining equipment. The key lesson is that the selection of the overlay alloy must be driven by a thorough understanding of the wear mechanism and the loading conditions. A hardfacing alloy that is too hard may fail prematurely under impact, while one that is too soft may wear through quickly. The optimal solution is often a medium-hardness alloy with good carbide dispersion and adequate toughness. This paper is a valuable reference for engineers working on wear protection solutions in mining and bulk material handling applications.