CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Cutting Tooth Bodies in Coal Mining Machinery
Literature Overview
This 2008 study by a researcher from Tangshan Vocational and Technical College of Science and Technology addresses the weld overlay repair of cutting tooth bodies used in coal mining machinery. Cutting teeth (also called pick teeth or auger bits) are critical components in continuous mining machines and roadheaders, where they fracture coal and rock through high-impact, high-abrasion conditions. The cutting tooth body is typically manufactured from high-strength steel, and when worn or damaged, replacement of the entire assembly is costly and time-consuming. Weld overlay repair offers an economical alternative by restoring the functional geometry and surface properties.
Core Technical Challenges
The cutting tooth body presents several distinctive challenges for overlay repair:
- Severe impact loading: Cutting teeth experience repeated impact loads during mining operations, requiring the overlay to have excellent toughness in addition to hardness.
- Abrasive wear environment: Coal and rock abrasives require overlay materials with high hardness (HRC 55–65) and wear resistance.
- Complex geometry: The tooth body has an irregular shape with a tapered cutting edge, making automated cladding difficult and often requiring manual or semi-automated techniques.
- Space constraints: Repair must be performed in confined workshop conditions, limiting the available equipment and technique selection.
- Fatigue resistance: The overlay and its HAZ must resist fatigue cracking under cyclic impact loading.
Overlay Material Selection
The selection of overlay material for cutting tooth repair is governed by the specific mining conditions:
| Material Type | Hardness (HRC) | Key Properties | Application Condition |
|---|---|---|---|
| High-carbon high-chromium cast iron | 55–65 | Excellent abrasion resistance | Coal with moderate abrasives |
| Hardfacing alloy (Cr-C-Fe type) | 58–66 | High hardness, good toughness | General coal cutting |
| Tungsten carbide composite | 70–80 | Extreme wear resistance | Hard rock interbedded with coal |
| Cobalt-based alloy (Stellite) | 45–50 | Excellent hot hardness, toughness | High-temperature, high-impact service |
| Manganese steel (Austenitic) | 20–30 (work-hardens to 50+) | Impact resistance, strain hardening | Pure coal, low abrasion |
Process Parameters and Technique Selection
Given the geometry and repair nature of the application, the following welding techniques and parameters are typically employed:
| Parameter | Manual Arc Welding (SMAW) | Submerged Arc Welding (SAW) |
|---|---|---|
| Filler metal | Hardfacing electrode (e.g., CH-501, CH-505) | Hardfacing flux + wire |
| Current type | AC or DCEP | DC |
| Current range | 150–300 A | 400–600 A |
| Voltage | 22–32 V | 28–35 V |
| Travel speed | 50–100 mm/min | 100–200 mm/min |
| Layer thickness per pass | 3–5 mm | 4–8 mm |
| Number of layers | 2–4 | 2–3 |
| Preheat | 150–250 °C | 200–300 °C |
The manual arc welding technique is preferred for cutting tooth repair due to the irregular geometry and the need for precise deposition control at the cutting edge. Multi-layer welding is essential to achieve the desired overlay chemistry with controlled dilution from the base material.
Defect Analysis and Quality Control
| Defect | Cause | Detection | Countermeasure |
|---|---|---|---|
| Overlay spalling | Excessive dilution; thermal shock | Visual; UT | Reduce heat input; increase layers |
| Cracking | High carbon equivalent; restraint | MT/PT | Preheat; low-hydrogen consumables |
| Incomplete fusion | Poor surface preparation; low current | UT | Clean surface; increase current |
| Uneven hardness | Inconsistent travel speed; dilution variation | Hardness survey | Uniform technique; post-weld heat treatment |
| Excessive dilution | Single thick pass; high heat input | Chemical analysis; hardness | Multiple thin passes; lower heat input |
Engineering Practice Considerations
- Surface preparation: The worn surface must be ground back to sound metal, removing all worn material, cracks, and oxide contamination. The preparation depth typically exceeds the damaged area by 2–3 mm.
- Post-weld machining: After cladding, the cutting edge must be ground to restore the original geometry within tolerance (typically ±0.5 mm), which also serves to relieve surface residual stresses.
- Heat treatment: A low-temperature tempering treatment (200–300 °C) may be applied to relieve residual stresses without reducing overlay hardness.
- Service life evaluation: Properly repaired cutting teeth typically achieve 60–80% of the service life of new teeth at 30–40% of the replacement cost.
- Repeatability: Multiple repair cycles may be performed on the same tooth body, but each repair requires verification of the remaining base material thickness to ensure structural integrity.
Study Insights and Implications
This study demonstrates the practical application of weld overlay technology in a demanding industrial repair scenario. The key engineering insight is that successful cutting tooth repair requires balancing hardness (for wear resistance) with toughness (for impact resistance)—a classic materials science trade-off. The selection between high-hardness, low-toughness materials (such as high-chromium white iron) and moderate-hardness, high-toughness materials (such as cobalt-based alloys) depends on the specific mining conditions. For engineers managing mining equipment maintenance, the economic case for overlay repair is compelling: a properly executed repair cycle can reduce spare parts inventory requirements and minimize equipment downtime. However, the quality of repair is highly dependent on welder skill, consumable quality, and process discipline—factors that must be addressed through proper training, qualification (per NB/T 47014 or ASME IX), and quality assurance procedures. The vocational college origin of this research also highlights the importance of skills development in specialized welding operations that remain labor-intensive despite advances in automation.