Zhuojin Pipe Fitting Co., LtdWear-Resistant Overlay Welding Strategy for Screw Conveyor Blades
Literature Overview
This technical paper by Ma Tiantian and Wang Bingjun from Tangshan Sanyou Chemical Engineering Design Co., Ltd., published in Soda Industry (2018, No. 1, pp. 29-31), presents a practical engineering approach to extending the service life of screw conveyor blades through wear-resistant overlay welding. The study addresses a common industrial problem in chemical processing plants where screw conveyors handle abrasive materials such as soda ash, salts, and granular chemicals.
The research adopts a systematic methodology—analyzing wear patterns, selecting appropriate materials, and implementing overlay welding strategies—to provide actionable guidance for manufacturers and maintenance technicians.
Core Technical Analysis
Wear Pattern Analysis
Screw conveyor blades experience complex wear mechanisms that vary by location and operating conditions:
| Wear Zone | Wear Mechanism | Severity | Typical Failure Mode |
|---|---|---|---|
| Blade leading edge | Abrasive wear (sliding) | High | Progressive thinning |
| Blade trailing edge | Abrasive wear (impact) | Medium-High | Edge chipping |
| Blade root (hub connection) | Fatigue + abrasive | Medium | Cracking and delamination |
| Blade surface (upper) | Abrasive wear (sliding) | Medium | Surface roughening |
| Blade surface (lower) | Abrasive + adhesive | Low-Medium | Material transfer |
The wear rate is influenced by material hardness, conveyor speed, material moisture content, and blade geometry. In soda ash handling service, the angular particles of soda ash create severe abrasive wear, with wear rates of 0.5-2.0 mm/year on unprotected carbon steel blades.
Material Selection Strategy
Based on wear analysis, the following material selection approach is recommended:
| Application Condition | Recommended Overlay Material | Hardness (HRC) | Expected Life Extension |
|---|---|---|---|
| Mild abrasion (dry powder) | Cr-Mo alloy (e.g., D2 equivalent) | 55-60 | 3-5× |
| Moderate abrasion (granular) | High-carbon alloy (e.g., Stellite 6) | 45-50 | 4-6× |
| Severe abrasion (wet/slurry) | Ni-Cr-Mo alloy | 40-45 | 5-8× |
| Impact + abrasion | Martensitic stainless (e.g., 410) | 45-52 | 3-4× |
Technical Implementation
Pre-Welding Preparation
The surface preparation procedure is critical for achieving sound overlay welds on existing blades:
- Removal of existing wear surface: Machine or grind the worn surface to a uniform thickness, removing all oxide and contaminated material
- Surface roughening: Create a mechanical anchor pattern (grooves or dimples) to enhance metallurgical and mechanical bonding
- Cleaning: Remove all oil, grease, and particulate contamination using solvent cleaning followed by wire brushing
- Fit-up: If blade thickness reduction from wear is significant, consider adding a backing plate before overlay welding
Welding Process Selection
| Process | Applicability | Advantages | Limitations |
|---|---|---|---|
| SMAW | Field repair; small areas | Portable; low equipment cost | Lower deposition rate; higher dilution |
| FCAW | Workshop; medium areas | High deposition rate; good penetration | Requires gas supply |
| SAW | Large flat surfaces | Very high deposition rate; consistent quality | Requires flat surface; low flexibility |
| GTA (TIG) | Precision areas; root passes | Excellent control; low dilution | Low deposition rate |
For screw conveyor blades, FCAW with flux-cored wire is generally the most practical choice, offering a good balance between deposition rate, flexibility, and equipment requirements.
Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Wire diameter | 1.2-1.6 mm | Depends on overlay thickness required |
| Current | 200-300 A | Adjust for wire diameter |
| Voltage | 28-34 V | Maintain stable arc |
| Travel speed | 100-150 mm/min | For 3-5 mm overlay thickness |
| Shielding gas | CO₂ or Ar/CO₂ (80/20) | For FCAW; pure CO₂ for self-shielded |
| Preheat | 100-150°C | For low-alloy steel blades |
| Interpass temperature | ≤ 200°C | Prevent overheating and grain coarsening |
Quality Control and Acceptance Criteria
Inspection Requirements
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | Surface defects; undercut | No cracks, porosity > 2 mm, undercut > 1 mm |
| Magnetic particle testing (MT) | Surface/subsurface cracks | No linear indications > 3 mm |
| Hardness testing | Overlay composition verification | Within specified range ± 5 HRC |
| Bend test (qualification) | Ductility verification | 180° bend on 10 mm coupon without cracking |
| Wear testing | Performance verification | Wear rate < 0.5 mm/year in service |
Common Defects and Root Causes
| Defect | Root Cause | Corrective Action |
|---|---|---|
| Cracking at root | Excessive restraint; high carbon equivalent | Increase preheat; reduce travel speed; use low-H electrode |
| Delamination | Poor surface preparation; insufficient penetration | Improve cleaning; increase current; add transition layer |
| Uneven thickness | Inconsistent travel speed; operator error | Use mechanized welding; implement parameter monitoring |
| Excessive dilution | Low current; high travel speed | Increase current; reduce travel speed; use transition pass |
Integration with Engineering Practice
Case Study: Soda Ash Conveyor Blade Repair
A typical implementation scenario involves:
- Equipment: Horizontal screw conveyor, 300 mm diameter, 6 m length
- Blade material: Q235 carbon steel, original thickness 8 mm
- Service: Soda ash handling, 24/7 operation, 8,000 hours/year
- Original life: 12-18 months before replacement
- Wear rate: 0.8-1.2 mm/year on blade surface
Overlay welding solution:
- Process: FCAW with high-carbon martensitic alloy wire
- Overlay thickness: 4 mm on blade working surface
- Number of passes: 3 (1 transition + 2 build-up)
- Resulting hardness: 52-58 HRC
- Expected life: 4-5 years (3-4× improvement)
- Cost: Overlay material and labor cost is 15-20% of new blade cost
Maintenance Strategy
Implement a preventive maintenance schedule:
- Monthly inspection: Visual check of blade edges and surfaces
- Quarterly measurement: Ultrasonic thickness measurement at 5-10 critical locations
- Annual overlay renewal: Add 1-2 mm of fresh overlay when thickness reaches minimum specification
- Complete replacement: When total blade thickness (base + overlay) falls below 6 mm
Study Insights and Implications
This research provides a practical, field-oriented approach to extending screw conveyor blade life through overlay welding. The systematic analysis of wear patterns followed by targeted material selection and process optimization represents a sound engineering methodology that can be applied to similar components across the chemical processing industry.
The key insight is that overlay welding is not merely a surface treatment but a comprehensive engineering solution that requires integration of materials science, welding technology, and operational knowledge. The selection of overlay material must consider not only hardness but also toughness, weldability, and compatibility with the base metal. A hard overlay that cracks or delaminates provides no benefit and may accelerate failure through stress concentration at crack tips.
For engineering practice, the recommended approach is to establish a qualification procedure for each specific application, testing the selected overlay material under actual service conditions before full-scale implementation. This qualification should include wear testing, bend testing, and field trial periods to validate the expected life extension. The economic benefits of overlay welding—reduced replacement frequency, lower downtime, and reduced material consumption—typically justify the initial investment within the first maintenance cycle.