CLADDING TECHNOLOGY SHANXI CO., LTDStripe Method Cladding Repair Process Parameter Optimization for Scraper Conveyor Middle Trough
Literature Overview
This study focuses on the repair of worn scraper conveyor middle troughs using a stripe (striped) welding cladding method, with systematic optimization of process parameters to maximize repair quality and service life. Scraper conveyors are critical equipment in coal mining, power plants, and bulk material handling operations, where the middle trough is subjected to severe sliding abrasion from the scraper chain and transported material. The literature presents a methodology for optimizing welding parameters to achieve uniform stripe patterns with adequate bond strength, minimal cracking, and improved wear resistance.
Background and Problem Statement
The middle trough of a scraper conveyor experiences:
- Continuous sliding abrasion from the scraper chain links
- Impact loading from falling material
- Corrosive attack from moisture and coal dust
- Thermal cycling from ambient conditions and material temperature
Typical service life of an unclad middle trough is 6–12 months in severe service. Conventional repair methods include full replacement, which is expensive and causes extended downtime, or simple overlay welding, which often suffers from cracking and poor wear resistance. The stripe method offers a compromise between cost and performance by creating alternating hard-facing and base metal stripes that provide wear resistance while maintaining structural integrity.
Process Description
The stripe cladding method involves depositing parallel stripes of hard-facing alloy along the wear surface of the middle trough, with the stripe width and spacing optimized to provide:
- Sufficient hard material coverage for wear protection
- Adequate base metal exposure for thermal stress accommodation
- Uniform stress distribution to prevent cracking
- Minimum welding time for cost-effective repair
| Stripe Parameter | Typical Value | Rationale |
|---|---|---|
| Stripe width | 15–30 mm | Balances coverage with thermal stress relief |
| Stripe spacing | 10–20 mm | Allows base metal to absorb thermal expansion |
| Stripe height | 3–6 mm | Provides adequate wear life without excessive buildup |
| Number of passes per stripe | 2–3 | Achieves required thickness with controlled heat input |
| Overlap between stripes | 10–20% | Ensures continuous coverage without excessive dilution |
Process Parameter Optimization
The optimization study employed orthogonal experimental design (L9 orthogonal array) to evaluate the effects of key parameters on repair quality:
| Factor | Level 1 | Level 2 | Level 3 | Level 4 |
|---|---|---|---|---|
| Welding current (A) | 220 | 250 | 280 | 310 |
| Travel speed (mm/min) | 300 | 400 | 500 | 600 |
| Preheat temperature (°C) | 100 | 200 | 300 | 400 |
| Interpass temperature (°C) | 150 | 250 | 350 | 450 |
Key findings from the optimization:
- Current: 250–280 A provides optimal penetration without excessive dilution. Currents above 310 A cause excessive base metal melting and reduce overlay hardness by 10–15 HRC.
- Travel speed: 400–500 mm/min achieves the best balance between deposition rate and cooling rate. Higher speeds produce thinner, more brittle layers.
- Preheat: 200–300°C is optimal for preventing cold cracking in the base metal HAZ while avoiding excessive grain growth.
- Interpass temperature: Maintaining below 350°C prevents austenite coarsening in the overlay layer and maintains martensitic transformation on cooling.
Defect Analysis Using FMEA Approach
| Failure Mode | Severity | Occurrence | Detection | RPN | Mitigation |
|---|---|---|---|---|---|
| Cracking in overlay stripe | 9 | 5 | 4 | 180 | Preheat to 250°C; use low-hydrogen filler |
| Poor bond strength | 8 | 4 | 5 | 160 | Thorough surface preparation; control first pass |
| Excessive dilution | 7 | 6 | 3 | 126 | Reduce current; increase travel speed |
| Uneven stripe height | 5 | 7 | 3 | 105 | Use backing plate; maintain consistent speed |
| Porosity in overlay | 6 | 4 | 4 | 96 | Ensure dry filler; maintain shielding gas coverage |
Performance Testing Results
The optimized stripe cladding process achieved the following performance metrics:
| Test Parameter | Result | Specification Requirement |
|---|---|---|
| Overlay hardness | 58–65 HRC | ≥ 55 HRC |
| Bond strength (shear) | 32–38 MPa | ≥ 25 MPa |
| Crack length (visual) | None detected | No cracks > 0.5 mm |
| Wear rate (ASTM G99) | 0.8–1.2 mg/1000 cycles | ≤ 2.0 mg/1000 cycles |
| Impact toughness (Charpy V) | 25–35 J at 20°C | ≥ 20 J |
Engineering Practice Recommendations
For field application of the stripe cladding repair method, the following practices are recommended:
- Surface preparation: Remove all oxide, rust, and existing worn material to a clean, sound base surface using grinding or shot blasting. The surface should be within 0.5 mm of flatness for uniform stripe deposition.
- Welding sequence: Deposit stripes in a staggered pattern, starting from the center and working outward, to minimize overall distortion. Each stripe should be completed in one continuous pass to avoid restart cracks.
- Post-weld treatment: Apply post-weld heat treatment at 550–600°C for 1 hour to relieve residual stresses and improve toughness without significantly reducing hardness.
- Inspection protocol: Perform visual inspection of all stripes, followed by magnetic particle testing (MT) for surface cracks. Hardness testing should be performed on at least 10% of stripes, and bond strength testing on a representative sample.
- Service monitoring: Implement a periodic inspection schedule to monitor stripe wear depth. When 50% of the original stripe height is consumed, re-cladding should be performed before complete loss of protective coverage.
Study Insights
The stripe method represents a practical engineering solution that balances cost, quality, and service life for scraper conveyor repair. The optimization study demonstrates that careful control of welding parameters, particularly current and travel speed, is essential for achieving consistent overlay quality. The staggered deposition pattern and controlled interpass temperature are critical for preventing distortion and cracking in the thin-walled trough geometry.
A notable insight is that the stripe pattern itself provides inherent thermal stress relief compared to continuous overlay, as the exposed base metal between stripes accommodates differential thermal expansion. This self-reinforcing characteristic makes the stripe method particularly suitable for thin-walled structures where continuous overlay would induce unacceptable distortion.
The study reinforces the importance of systematic parameter optimization over empirical trial-and-error approaches. The orthogonal array methodology identified parameter interactions that would not have been discovered through single-factor testing, particularly the interaction between preheat temperature and travel speed on crack susceptibility.