Weld Overlay (Hardfacing) Technology for Underground Scraper Chain Conveyor Maintenance
1. Definition and Technical Principles
Weld overlay, commonly referred to as hardfacing or surfacing in mining applications, is the process of depositing one or more layers of wear-resistant, impact-resistant, or corrosion-resistant alloy onto the surface of a base component through arc welding, flame spraying, or thermal spraying. In the context of underground scraper chain conveyor (刮板输送机) maintenance, the technology is applied to restore dimensional integrity and extend service life of critical wear components—including chain guides (溜槽/链轨), scraper chains (刮板链), chain plates (链板), transition troughs (过渡槽), and chain sprockets (链轮)—that suffer from severe abrasive wear caused by continuous contact with coal, rock, and slurry in underground coal mine environments.
The fundamental metallurgical principle involves the dilution control between the base metal (typically Q235A, Q345B, or Q345C low-carbon structural steel) and the overlay alloy. The overlay material, usually a high-carbon martensitic alloy (e.g., Cr-C-Mo high-speed steel type, Cr-Ni-Cobalt alloy, or Ni-Cr-Ce alloy), forms a hardened microstructure upon cooling. The hardness of the overlay layer typically ranges from HRC 45–65 depending on the alloy system, providing significantly superior abrasion resistance compared to the base material (HB 120–180).
The bonding mechanism between the overlay and substrate relies on metallurgical fusion at the interface, ensuring cohesive strength. In multi-pass applications, transition layers (e.g., 309L or 312 stainless steel consumables) may be deposited between the base metal and the final hardfacing layer to minimize dilution, reduce cracking susceptibility, and improve adhesion.
2. Category and Business Positioning
This technical capability falls squarely within the company's TIG/MIG Weld Overlay technology route, specifically in the subcategory of field-service hardfacing for heavy mining equipment. It represents a critical service offering that differentiates the company from conventional repair shops by combining:
- Portable welding capability — Equipment and consumables designed for deployment in confined underground environments with limited power supply and ventilation constraints.
- Process qualification — WPS/PQR documentation for specific component geometries and operating conditions.
- Performance-based specification — Overlay material selection tied to measured wear rates and service life requirements rather than generic material recommendations.
Within the company's business portfolio, this capability serves as a high-frequency, high-value-add service line that generates recurring revenue through maintenance contracts with coal mining enterprises. It also serves as a qualification-building platform, generating PQR records and field performance data that support expansion into more complex overlay applications (e.g., boiler tube cladding, pressure vessel repair).
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Wear life extension — Increase component service intervals from typical 30–60 days to 180–365 days or more through appropriate overlay selection and process control.
- Dimensional restoration — Rebuild worn surfaces (chain guide grooves, scraper chain contact surfaces) to original dimensional specifications, eliminating the need for complete component replacement.
- Impact resistance — Provide a surface that resists both abrasive wear and impact loading from rocks and debris entrained in coal flow.
- Cost reduction — Reduce annual spare parts expenditure by 40–70% through in-place repair versus replacement cycles.
3.2 Quantifiable Customer Value
| Value Metric | Without Overlay Repair | With Qualified Overlay Repair | Benefit |
|---|---|---|---|
| Chain guide replacement cycle | 45–60 days | 200–365 days | 3–6× life extension |
| Annual spare parts cost (per conveyor line) | ¥800,000–1,200,000 | ¥200,000–350,000 | 60–70% reduction |
| Equipment downtime per year | 120–180 hours | 30–50 hours | 70–80% reduction |
| Scrap chain disposal volume | 15–25 tons/year | 3–5 tons/year | Environmental benefit |
4. Key Process and Implementation Points
4.1 Overlay Material Selection Matrix
| Component | Wear Mechanism | Recommended Overlay Alloy | Consumable Type | Target Hardness | Typical Thickness |
|---|---|---|---|---|---|
| Chain guide (溜槽) groove surface | Abrasive (coal-rock) | Cr-Mo high-carbon martensitic (e.g., D2/D3 type) | CB-113, CB-114 (MIG/TIG) | HRC 52–58 | 3–5 mm |
| Scraper chain (刮板) contact face | Abrasive + impact | Ni-Cr-Ce cast iron alloy | CB-303, CB-307 | HRC 55–62 | 4–6 mm |
| Chain plate (链板) bearing surface | Abrasive + fatigue | Cr-C-Ni-Ce alloy | CB-217, CB-219 | HRC 50–56 | 3–5 mm |
| Chain sprocket teeth (链轮) | Impact + abrasion | Cr-Mo-V alloy steel | CB-108, CB-112 | HRC 48–55 | 4–8 mm |
| Transition trough (过渡槽) bottom | Abrasive (wet) | Cr-Ni-Cobalt alloy | CB-316, CB-318 | HRC 45–52 | 3–4 mm |
| Base-to-overlay transition | Dilution/cracking control | 309L/312 austenitic stainless | ER309L (MIG), E309L-16 (TIG) | HRC 20–25 | 1.5–2.5 mm |
4.2 Welding Process Parameters
| Parameter | TIG Overlay (GTAW) | MIG Overlay (GMAW) | Submerged Arc Overlay (SAW) |
|---|---|---|---|
| Shielding gas | Argon 99.99% or Ar+2%O₂ | Ar+15–20%CO₂ or pure CO₂ | Flux (SJ-201, SJ-301) |
| Welding current | 120–220 A | 200–380 A | 400–650 A |
| Welding voltage | 14–22 V | 22–30 V | 28–36 V |
| Travel speed | 60–150 mm/min | 200–450 mm/min | 400–800 mm/min |
| Wire diameter | 2.4–3.2 mm | 1.2–1.6 mm | 3.2–5.0 mm |
| Preheat temperature | 150–250°C | 150–250°C | 200–350°C |
| Interpass temperature | ≤250°C | ≤250°C | ≤300°C |
| Post-weld cooling | Controlled (≤50°C/hr) | Controlled (≤50°C/hr) | Controlled (≤40°C/hr) |
| Typical application | Precision small-area repair; tight tolerances | Large-area production repair; high deposition rate | Heavy overlay buildup; high efficiency |
4.3 Critical Implementation Steps
- Pre-weld assessment — Measure remaining base material thickness using ultrasonic thickness gauge (UTG). Minimum remaining thickness must exceed 1.5× the planned overlay thickness plus 5 mm for structural integrity. Inspect for subsurface cracks using magnetic particle testing (MT) per GB/T 26952.
- Surface preparation — Remove all oxidation, scale, oil, and previous weld deposits to a minimum of Sa 2½ cleanliness (ISO 8501-1). Grind worn surfaces to expose sound base metal. Establish a chamfer or V-groove at the edge of the wear surface to ensure proper fusion and avoid undercutting.
- Preheating — Apply induction heating or propane torch preheating to achieve uniform 150–250°C across the entire weld zone plus 100 mm beyond. Verify with infrared thermometer at minimum 3 measurement points. Maintain preheat during welding to prevent thermal cracking.
- Transition layer deposition — For high-carbon overlay alloys on low-carbon steel substrates, deposit 1–2 passes of austenitic stainless steel (ER309L/312) as a transition layer. This reduces carbon dilution in the final overlay and minimizes cold cracking susceptibility.
- Overlay layer deposition — Apply hardfacing passes using a weave pattern or circular pattern depending on component geometry. Maintain overlap of ≥50% between adjacent passes. For thick overlays (>4 mm), deposit in 2–3 layers with interpass grinding to remove oxide between passes.
- Post-weld heat treatment — For overlay alloys susceptible to hydrogen cracking (high-carbon martensitic types), apply post-weld stress relief at 550–620°C for 1 hour per 25 mm thickness, followed by controlled cooling in an insulated container or furnace.
- Final machining and inspection — Machine overlay surface to specified dimensional tolerance (typically ±0.5 mm for chain guide grooves). Perform visual inspection (VT), hardness testing (≥5 points per 100 mm²), and if required, magnetic particle inspection of the overlay and heat-affected zone.
4.4 Underground Environment-Specific Considerations
- Power supply limitations — Underground mining areas typically provide 380V/50Hz three-phase power with limited capacity. Welding equipment must be rated for continuous operation within available amperage (typically 250–400 A per welding station). Portable diesel generators may supplement but require strict ventilation compliance.
- Ventilation and gas detection — Welding operations in underground coal mines require continuous monitoring of methane (CH₄), carbon monoxide (CO), and oxygen (O₂) levels per GB 6222 and AQ 1029. Welding must be suspended if CH₄ exceeds 1.0% or O₂ falls below 19.5%.
- Equipment portability — All welding equipment must be designed for transport via mine roadway (width ≤3.0 m, height ≤2.2 m). Typical portable TIG units weigh 15–25 kg; portable MIG units weigh 35–55 kg. Consumable storage must comply with flammable material regulations.
- Humidity and moisture — Underground environments have relative humidity of 85–100%. Consumables (especially flux-cored wires and coated electrodes) must be stored in heated ovens at 150–250°C and transported in insulated containers. Moisture contamination leads to porosity and hydrogen cracking.
- Confined space welding — Scraper conveyor repair often occurs in confined spaces between the conveyor trough and the mine roof. Welder positioning, cable routing, and fume extraction must be planned to ensure operator safety and weld quality.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- GB/T 985.1-2008 — Butt weld preparation, position, and gap dimensions for steel (applied to groove preparation for overlay)
- GB/T 19231-2003 — Welding procedure qualification for ferrous metals
- GB/T 19866-2005 — Welding procedure qualification and WPS for ferrous metals
- NB/T 47014-2011 — Qualification rules for welding procedures of pressure vessels
- ASME Section IX — Qualification of welding procedures, welders, and welding operators (for components under ASME jurisdiction)
- ISO 15614-1:2017 — Qualification testing of welding procedures for metallic materials
5.2 Inspection and Acceptance Standards
- GB/T 3323-2005 — Radiographic testing of welds (RT, for subsurface defect detection)
- GB/T 26952-2011 — Magnetic particle testing of welds (MT, for surface and near-surface defects)
- GB/T 11345-2013 — Ultrasonic testing of welds (UT, for internal defects in thick sections)
- GB/T 26494-2011 — Visual testing of welds (VT, general surface quality)
- GB/T 11354-2013 — Penetrant testing of welds (PT, for surface-breaking defects in non-ferrous or non-magnetic materials)
- GB/T 11353-2010 — Hardness testing of welds and heat-affected zones
- GB 6222-2005 — Safety rules for underground metal mine gas detection (applied to coal mine welding safety)
5.3 Acceptance Criteria Summary
| Inspection Method | Acceptance Level | Reference Standard | Application |
|---|---|---|---|
| Visual Testing (VT) | No cracks, undercut ≤1 mm depth, porosity ≤3 per 100 mm | GB/T 26494 | 100% inspection of all overlay welds |
| Magnetic Particle Testing (MT) | No linear indications; rounded indications ≤3 mm | GB/T 26952 | 100% for critical components; 20% for standard components |
| Ultrasonic Testing (UT) | Level II acceptance; no indications above Q1 level | GB/T 11345 | 10% random sampling for overlay thickness >5 mm |
| Hardness Testing | Overlay: ≥90% of specified hardness; HAZ: ≤HB 300 | GB/T 11353 | ≥5 points per 100 mm² of overlay area |
| Dilution Testing | Carbon dilution in first overlay layer ≤2.0% C | ASTM A396 / GB/T 11261 | Verification coupon for each WPS qualification |
6. Common Risks and Controls
| Risk Category | Specific Risk | Cause | Control Measure |
|---|---|---|---|
| Cold cracking | Hydrogen-induced cracking in HAZ | High carbon dilution, inadequate preheat, high travel speed | Preheat to 200–250°C; use transition layer; control travel speed; post-weld bake at 200°C for 2 hours |
| Hot cracking | Weld cracking in overlay passes | High sulfur/phosphorus in base metal, improper alloy composition | Limit S≤0.030%, P≤0.035% in base; use appropriate filler alloy; control interpass temperature |
| Delamination/spalling | Overlay layer separates from base | Inadequate fusion, contamination, excessive dilution | Thorough surface preparation; adequate current settings; verify fusion on test coupon; use transition layer |
| Excessive hardness (brittleness) | Overlay too hard, prone to chipping | Wrong alloy selection for impact loading; no tempering | Select alloy with appropriate toughness; apply post-weld tempering at 550–620°C for martensitic alloys |
| Undercut | Edge groove in overlay reducing effective thickness | Excessive current, fast travel speed, improper torch angle | Reduce current by 10%; slow travel speed; maintain torch angle at 10–15°; use proper weave technique |
| Porosity | Gas pores in overlay weld | Moisture contamination, inadequate shielding, surface oxide | Dry consumables; ensure gas flow rate (TIG: 12–18 L/min; MIG: 15–20 L/min); grind surface to bare metal |
| Dimensional inaccuracy | Overlay surface not meeting tolerance | Inadequate post-weld machining; thermal distortion | Allow 2–3 mm machining allowance in overlay buildup; machine after full stress relief; use fixture clamping |
| Safety (underground) | Methane explosion, CO poisoning, electric shock | Inadequate ventilation, equipment failure, procedural non-compliance | Continuous gas monitoring; explosion-proof equipment; lockout-tagout; trained operators only |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application Route)
The scraper chain conveyor repair application is the core use case for the company's TIG/MIG weld overlay technology. The underground mining environment specifically favors portable TIG and MIG equipment due to:
- TIG (GTAW) — Preferred for precision repair of scraper chain contact surfaces, chain sprocket teeth, and transition sections where tight dimensional control is required. TIG provides superior arc stability, low spatter, and excellent visual monitoring of the weld pool—critical in confined underground spaces.
- MIG (GMAW) — Preferred for large-area overlay of chain guide grooves and conveyor trough bottoms where deposition rate is the priority. MIG achieves 3–5× the deposition rate of TIG, making it suitable for rebuilding heavily worn surfaces in minimum downtime.
- SAW (Submerged Arc Welding) — Deployed for heavy overlay buildup (>6 mm) on chain plates and structural components where maximum efficiency is needed and surface quality can be restored by subsequent machining.
7.2 Hydraulic Explosive Bonding (Complementary Application)
While hydraulic explosive bonding is not directly applied to conveyor component repair, the technology contributes to the supply chain by manufacturing high-performance clad steel plates used in conveyor structural components:
- Clad steel plate supply — The company's hydraulic explosive bonding capability produces Ni-Cr-Ce/low-carbon steel clad plates and Cr-Mo/low-carbon steel clad plates used as raw material for fabricating new chain guides, transition troughs, and conveyor structural members with built-in wear resistance.
- Value-added integration — Combining factory-applied clad plate manufacturing with field weld overlay repair creates a complete lifecycle solution: new components with explosive-bonded wear layers, maintained through periodic weld overlay refreshment in the field.
7.3 Explosion Welding (Strategic Material Development)
Explosion welding technology supports the scraper conveyor application through:
- Custom clad material development — The company can develop proprietary clad plate compositions optimized for specific mine conditions (e.g., high-silica coal seams requiring silicon-resistant overlay alloys) that are not available from commercial suppliers.
- Large-format component fabrication — Explosion welding produces large-format clad plates (up to 2000 mm × 6000 mm) suitable for direct fabrication of full conveyor trough sections, eliminating the need for field welding of overlay layers on individual components.
- R&D and qualification — Explosion welding serves as a research platform for developing new overlay alloy systems, with qualified materials subsequently adapted for TIG/MIG field repair applications.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
- WPS/PQR accumulation — Each scraper conveyor repair project generates qualified welding procedures for specific combinations of base metal, overlay material, joint geometry, and process parameters. These accumulate into a comprehensive WPS library that demonstrates technical capability across multiple material systems.
- Field performance data — Systematic tracking of overlay service life in actual mining conditions provides empirical data for material selection recommendations, strengthening technical authority and customer confidence.
- Operator certification — Welder qualification records (per GB/T 15059 or ASME Section IX) accumulated through conveyor repair projects demonstrate a trained workforce capable of deploying to customer sites.
- System certification support — Weld overlay capability on mining equipment supports the company's pursuit of ISO 9001 quality management certification, ISO 3834 welding quality management certification, and industry-specific certifications for mining equipment repair.
8.2 Product Delivery and Customer Value
- Reduced capital expenditure — Customers avoid purchasing new chain guides, scraper chains, and conveyor structural components. A single overlay repair at ¥3,000–8,000 per component versus replacement cost of ¥25,000–80,000 per component.
- Reduced operational downtime — In-place repair eliminates the need to remove and ship components to surface workshops. Typical repair time: 4–8 hours per component versus 5–10 days for replacement cycle.
- Extended equipment life — Comprehensive overlay maintenance programs can extend conveyor component life by 3–5× over original design life, deferring capital replacement of entire conveyor systems.
- Technical partnership positioning — The learning and documentation of this application (as reflected in the "学习心得" technical paper) demonstrates the company's commitment to continuous improvement and knowledge transfer, positioning it as a technical partner rather than a commodity service provider.
9. Implementation Recommendations
- Establish a standardized repair protocol — Develop a company-level technical standard (enterprise standard) for scraper conveyor overlay repair covering material selection, process parameters, inspection requirements, and documentation formats.
- Create a consumable management system — Maintain inventory of qualified overlay consumables (CB-113, CB-114, CB-303, CB-316, ER309L, etc.) with proper storage, traceability, and expiration tracking.
- Develop a field service package — Package portable TIG/MIG equipment, consumables, inspection instruments (portable hardness tester, MT kit, UT thickness gauge), and trained personnel into a deployable underground repair team.
- Implement a performance tracking database — Record each repair with component identification, overlay specification, process parameters, inspection results, and subsequent service life. Use this data to refine material selection and process parameters.
- Pursue customer qualification — Submit WPS/PQR packages and field performance data to major coal mining enterprises (Shaanxi Coal, Shaanxi Coal Group, Yanzhou Mining, etc.) for inclusion in their approved vendor lists for welding repair services.
10. Conclusion
The application of weld overlay technology to underground scraper chain conveyor maintenance represents a high-value, technically demanding service that leverages the company's core TIG/MIG weld overlay capability in a demanding real-world environment. The technical paper documenting this learning experience serves as both an internal knowledge asset and an external demonstration of technical depth. By systematically developing qualified procedures, accumulating field performance data, and building a trained field service capability, the company can establish a sustainable revenue stream from mining equipment repair while simultaneously strengthening its overall qualification portfolio and technical reputation in the clad and weld overlay industry.