CLADDING TECHNOLOGY SHANXI CO., LTDImproving Service Life of Middle Trough Using Weld Overlay Method
Literature Overview
This study addresses the wear problem encountered in middle troughs (also known as intermediate chutes or transfer troughs) used in bulk material handling systems, particularly in mining, cement, and aggregate processing industries. The research evaluates the effectiveness of weld overlay techniques as a cost-effective alternative to full component replacement, focusing on extending service life through the application of abrasion-resistant overlay coatings on the wear-critical surfaces of the trough interior.
Core Technical Findings
The middle trough operates under severe abrasive conditions where bulk material slides across the trough surface at velocities of 3–8 m/s, generating impact-abrasion wear that progressively thins the trough wall. The study compared three overlay approaches: single-layer hard-facing with high-carbon martensitic alloy, multi-layer composite overlay with alternating hard and tough layers, and electroslag welding (ESW) overlay with low-dilution consumables.
| Overlay Method | Consumable Type | Achieved Hardness (HRC) | Dilution Ratio (%) | Service Life Improvement |
|---|---|---|---|---|
| Single-layer GMAW | Cr-C-Mo martensitic | 52–55 | 18–22 | 2.5× |
| Multi-layer GMAW | Alternating Cr-C and Ni-based | 58–62 | 12–15 | 4.0× |
| ESW overlay | Low-dilution alloy wire | 55–58 | 8–10 | 3.5× |
The multi-layer composite approach demonstrated the most significant service life improvement, extending trough life from approximately 3 months to over 12 months under equivalent operating conditions. The alternating layer strategy provides a tough substrate layer that resists cracking while the hard surface layer provides primary wear resistance.
Interpretation of Technical Points
The wear mechanism in middle troughs is predominantly three-body abrasion, where hard particles trapped between the material stream and the trough surface cause material removal through micro-ploughing and micro-cutting. The overlay layer must therefore possess both high hardness to resist penetration by abrasive particles and adequate toughness to absorb impact energy from falling material.
The multi-layer approach achieves this balance through strategic layer composition. The first layer (closest to base metal) employs a nickel-chromium-cobalt alloy with 45–48 HRC hardness, providing excellent bonding strength to the carbon steel substrate (typically Q345 or Q235 grade) while maintaining ductility. Subsequent layers alternate between high-carbon martensitic alloy (58–62 HRC) and the nickel-based alloy, creating a gradient hardness profile that distributes wear across multiple layers rather than concentrating it at the surface.
The ESW overlay method, while slower in application, produces overlays with the lowest dilution ratio (8–10%) due to the protective slag layer that shields the molten pool from atmospheric contamination and limits base metal entrainment. This low dilution is particularly important when the base metal is high-carbon steel, where excessive carbon dilution can lead to brittleness and cracking in the overlay.
Process and Standards Analysis
The overlay qualification procedure must comply with relevant standards including NB/T 47014 for welding procedure qualification and JB/T 4730 for non-destructive testing requirements. The study recommends magnetic particle testing (MT) of the overlay surface after each pass to detect surface cracks, with acceptance criteria based on zero tolerance for longitudinal cracks longer than 5 mm and no more than two transverse cracks per 100 mm of overlay length.
The base metal preparation requirements include grinding to a uniform surface with no undercut or geometric discontinuities exceeding 0.5 mm depth. Surface roughness should be controlled to Ra 3.2–6.3 μm to ensure proper bond strength between the base metal and the first overlay layer. Preheating to 150–200°C is recommended for trough walls thicker than 12 mm to reduce the risk of hydrogen-induced cracking.
The study also references the importance of interpass temperature control, maintaining a range of 150–250°C between passes. Exceeding 300°C interpass temperature leads to grain coarsening in previously deposited layers and a subsequent reduction in hardness by 3–5 HRC.
Integration with Engineering Practice
In practical implementation, the middle trough overlay repair can be performed in-situ or in a workshop setting. In-situ repair offers the advantage of minimal downtime but requires careful shielding against ambient contamination, particularly in outdoor environments where wind and moisture can compromise weld quality. Workshop repair allows for better process control but requires component removal and reinstallation, which may be impractical for large troughs.
The economic analysis presented in the study demonstrates that overlay repair costs approximately 15–20% of the cost of full trough replacement, with a payback period of less than 4 months based on extended service life. This makes the overlay approach highly attractive for planned maintenance programs where trough replacement would require extended production shutdowns.
For troughs operating in wet abrasive environments (such as wet sand or slurry handling), the overlay composition should be modified to include higher chromium content (8–12%) to provide additional corrosion resistance alongside wear protection. The study recommends a duplex overlay approach where the surface layer contains chromium carbide-forming elements for corrosion resistance while maintaining adequate hardness through carbon and alloy additions.
Key Questions and Reflections
A critical question raised by this study is the long-term durability of overlay coatings under impact-abrasion conditions where the wear mechanism may transition from abrasion-dominated to fatigue-dominated over extended service periods. The overlay layer, being thinner than the base material, may eventually reach a critical thickness below which substrate wear becomes the governing failure mode.
The study does not adequately address the effect of thermal cycling on overlay integrity during repeated heating and cooling cycles inherent to hot material handling. In cement plant applications, troughs may experience temperature fluctuations from ambient to 200°C, which can induce thermal fatigue cracking at the overlay-substrate interface if the coefficient of thermal expansion mismatch is not properly managed.
From a practical standpoint, the study's recommendation of multi-layer overlay, while technically superior, increases application time and cost. Engineers must evaluate whether the additional 3–4× life extension justifies the additional 2–3× overlay cost, particularly for troughs in remote locations where repair access is difficult and overlay equipment availability is limited.
Study Insights and Implications
This study provides valuable engineering guidance for extending the service life of bulk material handling components through targeted weld overlay applications. The multi-layer composite approach represents the most technically advanced solution, offering the best combination of wear resistance, toughness, and service life extension.
The key practical insight is that overlay repair is not merely a surface treatment but a metallurgical engineering solution that requires careful consideration of the base material properties, operating conditions, and expected service environment. Engineers should develop overlay specifications that are tailored to specific applications rather than applying generic hard-facing solutions, as the wear mechanism and environmental conditions vary significantly between different industrial applications.