CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Overlay Welding Repair Method for Conveyor Scraper Plates
Literature Overview
This study investigates the application of plasma transferred arc (PTA) welding for the repair and enhancement of conveyor scraper plates in industrial material handling systems. Conveyor scrapers are critical components that remove accumulated material from belt surfaces, and they are subject to severe abrasive wear from continuous contact with conveyed materials. The research documents the PTA repair process, evaluates the resulting overlay properties, and compares performance against conventional repair methods.
Service Environment and Failure Analysis
Operating Conditions
| Parameter | Typical Value |
|---|---|
| Conveyor speed | 1.0-3.0 m/s |
| Material temperature | 20-200°C (ambient to hot material) |
| Abrasive material type | Coal, ore, sand, slag |
| Contact pressure | 0.5-3.0 MPa |
| Service life (unrepaired) | 3-8 months |
| Original material | Q235 / Q345 carbon steel |
| Original surface hardness | 120-180 HV |
Failure Modes
The scraper plates fail through progressive material removal at the leading edge and contact surface. The wear pattern typically follows:
- Initial stage — Surface polishing and shallow grooving (0-2 months)
- Progressive stage — Deep grooving and localized material loss (2-5 months)
- Critical stage — Through-thickness wear approaching structural failure (5-8 months)
PTA Process Parameters
Optimized Process Parameters
| Parameter | Value | Justification |
|---|---|---|
| Arc current | 200-280 A | Adequate melting with controlled heat input |
| Arc voltage | 30-35 V | Stable plasma jet; proper transfer |
| Travel speed | 100-200 mm/min | Control dilution and bead profile |
| Powder feed rate | 80-120 g/min | Match to arc parameters |
| Gas flow rate (Ar) | 15-20 L/min | Adequate shielding |
| Powder diameter | 75-150 μm | Fine powder for smooth surface |
| Powder composition | Ni-Cr-Mo-C (Stellite-type) | High hardness; good wear resistance |
| Number of passes | 2-3 | Achieve target thickness (2-4 mm) |
Powder Composition Options
| Powder Type | Composition (wt%) | Hardness (HV) | Application |
|---|---|---|---|
| Ni-Cr-C | Ni-20Cr-5C | 400-500 | General abrasion |
| Ni-Cr-Mo-C | Ni-18Cr-5Mo-4C | 500-600 | Severe abrasion |
| Co-Cr-W-C | Co-30Cr-5W-3C | 600-700 | Extreme abrasion |
| Fe-Cr-C | Fe-20Cr-6C | 550-650 | Cost-effective option |
Repair Process Procedure
The PTA repair process follows a systematic procedure:
- Surface preparation — Remove existing worn material to sound metal using grinding; clean to SA 2.5 minimum; degrease thoroughly
- Edge preparation — Create a 45° chamfer of 2-3 mm at the leading edge for optimal overlay adhesion
- Preheating — Apply 150-200°C preheat using induction heater or oxy-fuel torch to reduce thermal shock
- First pass (transition layer) — Apply a transition powder (e.g., Ni-Fe type) at reduced current (180 A) to minimize dilution
- Second pass (functional layer) — Apply the primary wear-resistant powder at optimized parameters
- Third pass (if required) — Apply additional pass for thickness or to achieve target surface finish
- Post-weld cooling — Allow controlled cooling to ambient; avoid quenching
- Surface finishing — Light grinding to achieve smooth surface and remove spatter
Performance Evaluation
Hardness Profile Through Overlay Thickness
| Depth from Surface (mm) | Hardness (HV) | Microstructure |
|---|---|---|
| 0.0-0.5 | 580-620 | Fine M7C3 + M23C6 in austenitic matrix |
| 0.5-1.0 | 520-560 | Coarser carbides; some ferrite |
| 1.0-2.0 | 450-490 | Mixed structure; transition zone |
| 2.0-3.0 | 350-400 | Diluted zone; increased base metal influence |
| >3.0 | 200-250 | Base metal |
Wear Resistance Comparison
| Test Method | Unrepaired Plate | PTA Overlay | Improvement Factor |
|---|---|---|---|
| Dry sliding wear (Al2O3 disc) | 2.8×10⁻⁴ mm³/N·m | 0.3×10⁻⁴ mm³/N·m | 9.3× |
| Abrasive wear (sand rubber) | 4.5×10⁻⁴ mm³/N·m | 0.6×10⁻⁴ mm³/N·m | 7.5× |
| Erosion wear (particle impact) | 1.2×10⁻³ mm³/g | 0.2×10⁻³ mm³/g | 6.0× |
Service Life Extension
| Repair Method | Original Life | Repaired Life | Cost per Month |
|---|---|---|---|
| No repair (replacement) | 6 months | 6 months | 100% (baseline) |
| Arc welding (SMAW) | 6 months | 10-12 months | 65-70% |
| PTA overlay | 6 months | 18-24 months | 35-45% |
Quality Control Requirements
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Surface quality | Visual | No cracks, no excessive spatter, smooth finish |
| Overlay thickness | UT / caliper | Minimum 2.0 mm at leading edge |
| Bond strength | Peel test | ≥50 MPa shear strength |
| Hardness | Rockwell / Vickers | ≥450 HV across top 2 mm |
| Dilution | Spectroscopic analysis | Top layer dilution <15% |
| Cracks | MT / PT | No linear indications >1 mm |
Engineering Practice Recommendations
Based on the study findings and practical experience, the following recommendations are provided:
- Process selection — PTA is preferred over conventional arc welding for scraper plate repair due to its lower dilution (typically 5-15% vs. 20-40% for FCAW), superior surface finish, and reduced heat input that minimizes distortion.
- Powder selection — For coal handling applications, Ni-Cr-Mo-C powders provide the best balance of wear resistance and cost. For hot slag service, Co-Cr-W-C powders are recommended despite higher cost.
- Repair timing — Implement a preventive repair schedule at 4-5 months of service life rather than waiting for critical wear. This extends total service life and avoids unplanned downtime.
- Equipment considerations — Ensure the PTA equipment has adequate powder feed control and arc stability features. Manual PTA requires skilled operators; automated PTA with CNC positioning provides superior consistency for production environments.
- Environmental factors — PTA produces minimal spatter and fume compared to conventional arc processes, making it suitable for indoor maintenance operations. However, adequate ventilation is still required during the process.
Study Insights and Conclusions
The PTA repair technique for conveyor scraper plates represents a significant advancement over conventional repair methods, offering 3-4 times the service life extension with reduced total cost of ownership. The key advantages lie in the process's ability to produce high-quality overlay deposits with minimal dilution, excellent metallurgical bonding, and controlled heat input.
From a metallurgical perspective, the PTA process creates a unique microstructure characterized by fine carbide dispersion within a tough matrix, providing both hardness and fracture resistance. The rapid cooling rates achievable in PTA (typically 10-50 K/s) promote fine grain structures and dense carbide networks that contribute to superior wear performance.
For engineers managing material handling equipment, the PTA repair approach should be incorporated into the maintenance strategy as a planned preventive measure rather than a reactive repair. The economic analysis clearly demonstrates that the upfront cost of PTA repair (approximately 60-70% of component replacement cost) is more than offset by the 3-4 fold extension in service life, resulting in significant reduction in total maintenance expenditure and equipment downtime. This technology represents a practical application of advanced welding metallurgy to solve real-world industrial problems with measurable economic benefits.