Large-Area Wear-Resistant Pattern Weld Overlay on Scraper Conveyor Middle Sleds
1. Definition and Technical Principles
The weld overlay of large-area wear-resistant patterns on scraper conveyor middle sleds (中部槽) is a surface engineering technique applied to critical wear surfaces of underground coal mining scraper conveyors (刮板输送机). The middle sled serves as the load-bearing trough through which the scraper chain and scraper plates transport coal and rock. These components endure severe abrasive wear from coal, rock fragments, and water-sand mixtures, leading to progressive material loss, structural degradation, and eventual failure of the conveyor system.
The core principle involves depositing a wear-resistant alloy layer onto the base steel surface of the middle sled using arc welding processes (typically MIG or TIG), forming a hardfacing layer with superior abrasion resistance, impact toughness, and corrosion resistance compared to the base material. The "pattern" (花纹) refers to the geometric configuration of the overlay—typically a ribbed, chevron, or cross-hatch pattern—that serves a dual function: (1) providing a mechanically interlocked wear surface that resists delamination under high impact loads, and (2) creating a textured surface that improves material retention and reduces coal/rock sliding, thereby enhancing conveyor transport efficiency.
The metallurgical mechanism relies on the dilution-controlled deposition of carbide-forming alloy systems (Cr-C, Cr-Mo-C, Co-Cr-W, or Ni-Cr-C) that produce fine, uniformly distributed hard carbides (Cr₇C₃, Cr₃C, WC, Mo₂C) within a tough matrix. The pattern geometry further enhances mechanical bonding through geometric interlocking, reducing the likelihood of overlay spalling under the dynamic loading conditions encountered in underground mining environments.
2. Category and Business Positioning
Within the company's technical capability portfolio, this entry falls under the TIG/MIG Weld Overlay technology route, specifically in the category of hardfacing and wear-resistant overlay for heavy-duty mining equipment. It represents a high-value-added service that bridges the gap between conventional welding repair and advanced surface engineering solutions.
Business positioning considerations include:
- Target Market: Underground coal mining operations (particularly longwall mining systems) where scraper conveyor middle sleds experience 3,000–8,000 hours of continuous abrasive service before requiring replacement or refurbishment.
- Value Proposition: Extending service life of middle sleds by 3–5 times compared to unprotected carbon steel, reducing equipment downtime, spare parts inventory costs, and total cost of ownership (TCO) for mining operators.
- Competitive Differentiation: Large-area pattern overlay (as opposed to simple flat hardfacing) requires specialized fixture design, multi-pass deposition strategies, and pattern-specific WPS qualification—creating technical barriers to entry.
- Revenue Streams: New-build overlay on OEM middle sleds, refurbishment of in-service worn sleds, and technical consulting for mine-specific wear pattern design.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Achieve overlay hardness of ≥ HV 500 (or specified per customer requirement) with controlled dilution from base material (typically < 20%)
- Ensure overlay adhesion strength ≥ 100 N/mm² (per ASTM G139 or equivalent) to prevent spalling under impact
- Maintain overlay thickness of 4–8 mm per pass series, with total build-up of 12–25 mm on critical wear zones
- Produce a uniform pattern geometry with consistent rib height (typically 6–12 mm) and spacing (30–80 mm center-to-center)
- Minimize residual stress and distortion to maintain sled structural integrity and dimensional accuracy for conveyor alignment
3.2 Quantifiable Value Metrics
| Performance Metric | Unprotected Base Steel (Q345B/Q235) | Wear-Resistant Pattern Overlay | Improvement Factor |
|---|---|---|---|
| Abrasion Resistance (ASTM G65) | Baseline (1.0×) | 4.0–6.0× baseline | 400–600% |
| Service Life (Typical) | 6–12 months | 24–60 months | 3–5× |
| Overlay Hardness | HV 200–250 | HV 500–700 | 2–3× |
| Impact Toughness (CVN at -20°C) | 27 J (base) | ≥ 20 J (overlay) | Maintained |
| Annual Cost per Tonne (Mining Op.) | High (frequent replacement) | Low (extended service) | 40–60% reduction |
4. Key Process and Implementation Points
4.1 Base Material Preparation
Proper surface preparation is critical to ensuring overlay adhesion and minimizing defect formation. The following steps are mandatory:
- Visual Inspection: Identify existing weld seams, repairs, inclusions, and surface damage on the middle sled wear surface.
- Surface Cleaning: Remove rust, scale, oil, and contaminants using G9.5-grade grinding (per ISO 8501-1 Sa 2.5 equivalent) or shot blasting to achieve a clean, matte finish with minimum surface roughness Ra of 12.5 μm.
- Edge Beveling: Prepare a 45° V-groove or U-groove along pattern rib boundaries to facilitate proper weld metal flow and ensure adequate root penetration.
- Preheating: Apply localized preheat of 150–250°C (for low-carbon steel base) or 250–350°C (for medium-carbon or high-strength steel) to reduce thermal gradient and prevent cold cracking. Monitor with infrared pyrometer.
4.2 Welding Process Parameters
The following table summarizes typical process parameters for large-area pattern overlay using MIG (GMAW) process, which is preferred for productivity on large sled surfaces:
| Parameter | Value / Range | Notes |
|---|---|---|
| Process | GMAW (MIG) – Solid Wire / Flux-Cored Wire | TIG (GTAW) for root passes and transition zones |
| Wire Type | Cr-C system (e.g., AWS A5.15 E71T-8 equivalent) or Ni-Cr-C | Select based on required hardness/toughness balance |
| Wire Diameter | 1.2 mm or 1.6 mm | 1.6 mm for build-up passes; 1.2 mm for transition |
| Shielding Gas | Argon + 5–10% CO₂ or 100% Argon | Argon for Ni-base; Ar+CO₂ for Cr-C |
| Gas Flow Rate | 15–25 L/min | Adjust for ambient wind conditions |
| Welding Current | 180–280 A | Dependent on wire diameter and pass type |
| Welding Voltage | 22–28 V | Maintain short-arc transfer mode |
| Travel Speed | 150–300 mm/min | Slower for build-up; faster for cap pass |
| Preheat Temperature | 150–300°C | Maintain between passes (interpass ≤ 350°C) |
| Interpass Temperature | ≤ 350°C (Cr-C); ≤ 250°C (Ni-base) | Critical for controlling dilution and microstructure |
| Number of Passes | 3–5 (transition + build-up + cap) | Typical for 12–20 mm total build-up |
4.3 Pattern Geometry and Deposition Strategy
The wear-resistant pattern is typically one of the following configurations, selected based on conveyor application and wear mechanism:
| Pattern Type | Geometry | Typical Application | Advantage |
|---|---|---|---|
| Chevron (人字形) | V-shaped ribs at 45–60° to conveyor axis; rib height 8–12 mm; spacing 50–80 mm | High-throughput longwall conveyors with heavy rock content | Guides material flow; resists lateral wear |
| Transverse Ribs (横筋) | Perpendicular ribs to conveyor axis; height 6–10 mm; spacing 40–60 mm | Standard coal transport with moderate abrasion | Simple fabrication; good material retention |
| Cross-Hatch (网格) | Intersecting ribs at 90°; height 6–8 mm; spacing 50–70 mm | High-impact zones (bends, transfer points) | Maximum mechanical interlock; resists multi-directional wear |
| Pyramid/Bump (凸台) | Discrete raised pyramids; base 20–30 mm; height 5–8 mm | Localized high-wear spots (scraper plate contact zones) | Concentrated hardfacing; minimal distortion |
4.4 Multi-Pass Deposition Sequence
- Transition Layer (Pass 1): Deposit a single pass of 309L or 312L stainless steel wire (per AWS A5.4/A5.9) to establish a dilution buffer between the low-alloy base steel and the high-alloy hardfacing. This reduces carbon pickup and prevents cracking at the base-metal/overlay interface.
- Build-Up Passes (Passes 2–4): Deposit 2–3 passes of the selected hardfacing wire to achieve required thickness. Maintain interpass temperature and ensure each pass has ≥ 70% overlap with the previous pass to prevent cold cracking and ensure uniform dilution.
- Cap Pass (Final Pass): Apply a final capping pass to achieve the desired pattern profile geometry, smooth the surface, and ensure consistent rib height. Use slightly reduced current and voltage for better profile control.
- Pattern Forming: For raised patterns, use a backing plate or temporary fixture to define the rib shape during deposition. Alternatively, use a multi-wire or multi-electrode technique to build the pattern in a single continuous pass.
4.5 Distortion Control
Large-area overlay on thin-walled middle sleds (typical wall thickness 8–14 mm) introduces significant thermal distortion. Control measures include:
- Weld Sequence Planning: Use a symmetric, balanced welding sequence (e.g., starting from center and proceeding outward in alternating directions) to minimize directional distortion.
- Intermittent Welding: For very large areas, use a "skip-weld" technique where weld beads are deposited in a staggered pattern, allowing thermal equilibrium between sections.
- Rigidity Fixtures: Clamp the sled to a rigid backing plate or welding fixture to resist bending and warping during deposition.
- Post-Weld Stress Relief: Apply local or global stress relief annealing at 550–650°C (depending on base material) for 1–2 hours per 25 mm thickness to reduce residual stresses below 50 MPa.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| GB/T 11365-2009 | Welding consumables – Classification | Classification of hardfacing electrodes and wires |
| GB/T 12467-2009 | Welding consumables – Hardfacing electrodes | Chemical composition, hardness, mechanical properties of hardfacing electrodes |
| GB/T 13143-2009 | Welding consumables – Wire for gas shielded arc welding | Specifications for hardfacing welding wires |
| GB/T 19864-2005 | Welding procedure qualification | WPS/PQR qualification requirements for overlay welding |
| GB 50236-2011 | Code for construction and acceptance of steel structures welding | General welding quality requirements |
| ASTM A5.15 | Specification for Carbon and Alloy Steel Welding Electrodes | Classification of hardfacing electrodes (E71T-8, E81T-1, etc.) |
| ASTM A5.9 | Specification for Stainless Steel Welding Electrodes | 309L/312L transition layer wire specifications |
| ASTM G65 | Standard Test Method for Wear Testing by Dry Abrasion | Abrasion resistance testing of overlay |
| ASTM G139 | Standard Test Method for Adhesion of Weld Overlay Hardfacing | Adhesion strength verification (≥ 100 N/mm² typical) |
| ASTM A262 | Standard Practices for Detecting Chloride Stress Corrosion | Applicable for Ni-base overlays in corrosive environments |
| NACE MR0175/ISO 15156 | Materials for Use in H₂S-Containing Environments | Hardness limits for H₂S service (≤ HV 220 for Ni-base; overlay design consideration) |
| EN ISO 14271 | Welding – Procedure qualification for hardfacing | European procedure qualification standard |
| EN ISO 14272 | Welding – Examination of welders for hardfacing | Welder certification requirements |
| JB/T 7693 | Technical conditions for scraper conveyor middle sled | Product-specific requirements for middle sled geometry and material |
5.2 Acceptance Criteria
- Visual Inspection (VT): No cracks, porosity > 1 mm, undercut > 0.5 mm, or spatter on the overlay surface. Pattern geometry within ±1 mm tolerance of design.
- Hardness Testing: Minimum 5 test points per 100 mm² of overlay area. Average hardness must meet specification (typically HV 500–700 for Cr-C; HV 450–600 for Ni-Cr-C). Maximum individual reading ≤ HV 800 to maintain toughness.
- Dilution Analysis: Chemical analysis at the base-metal/overlay interface (transverse section, 5 mm depth). Dilution from base material must be ≤ 20% (for Cr-C) or ≤ 15% (for Ni-base). Carbon content at interface ≤ 0.3%.
- Adhesion Testing: Per ASTM G139, minimum adhesion strength of 100 N/mm². Test coupon must show failure in the base metal, not at the interface.
- Microstructural Examination: Transverse cross-section examination at 100× and 500× magnification. No macro-segregation, no continuous grain boundary carbide network, no HAZ cracking. Carbide morphology should be fine and uniformly distributed.
- Dimensional Tolerance: Overlay thickness within ±0.5 mm of specified value. Pattern rib height within ±1 mm. Total sled distortion ≤ 2 mm/m after overlay and stress relief.
- Impact Testing: Charpy V-notch impact test on weld-metal coupon at service temperature (-20°C for cold-region mining). Minimum absorbed energy per customer specification (typically ≥ 20 J).
6. Common Risks and Controls
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Hot Cracking | High sulfur/phosphorus in base metal; excessive dilution; high carbon equivalent | Overlay cracking during cooling; loss of service life | Preheat to 250°C; use low-sulfur consumables; control interpass temperature; add transition layer |
| Cold Cracking (Hydrogen-Induced) | Hydrogen pickup from moisture; high carbon equivalent base metal; rapid cooling | Delayed cracking in HAZ or overlay; catastrophic failure | Preheat to 150–250°C; use low-hydrogen consumables; control cooling rate; post-weld bake at 200°C for 2 hours |
| Overlay Spalling/Delamination | Insufficient adhesion; thermal fatigue cycling; high residual stress | Loss of wear protection; sudden failure in service | Verify adhesion per ASTM G139; perform stress relief; ensure proper surface prep; use geometric interlock (pattern) |
| Excessive Dilution | Large heat input; deep penetration; high base-metal carbon pickup | Softening of overlay; reduced hardness; loss of wear resistance | Use transition layer; reduce heat input; control travel speed; monitor interpass temperature |
| Thermal Distortion | Large heat input on thin-walled structure; asymmetric welding sequence | Dimensional out-of-tolerance; conveyor alignment issues; mechanical interference | Use balanced welding sequence; apply rigidity fixtures; perform post-weld straightening; stress relief |
| Porosity | Contaminated surface; inadequate shielding; wire moisture | Reduced overlay density; weakened mechanical properties | Ensure Sa 2.5 surface prep; use proper gas flow; bake consumables; use trailing gas nozzle |
| Hardness Non-Uniformity | Inconsistent wire feed; varying travel speed; dilution variation | Predictable wear pattern; premature failure at soft spots | Use automated welding; monitor parameters; perform hardness mapping; rework non-conforming areas |
7. Application Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for This Application)
This entry directly falls under the MIG/GMAW hardfacing route. The MIG process is preferred for large-area middle sled overlay due to its high deposition rate (5–10 kg/h vs. 1–2 kg/h for TIG), which reduces production time and cost for large sled surfaces (typically 2,000–4,000 mm² per sled). TIG is reserved for:
- Transition layer deposition (Pass 1) where precise control of dilution is critical
- Repair of localized defects in the overlay
- Overlay on thin sections (< 6 mm) where heat input must be minimized
- Welder qualification and WPS development
The TIG/MIG route offers the greatest flexibility in pattern geometry, as manual or semi-automated welding allows real-time adjustment of bead profile to achieve the desired rib shape. Fully automated MIG systems (with wire feed control and torch manipulation) are increasingly used for production-scale overlay, offering superior consistency and reduced labor costs.
7.2 Hydraulic Explosive Bonding (Complementary Application)
While hydraulic explosive bonding is not directly applicable to the overlay of wear-resistant patterns (which requires surface deposition rather than bulk bonding), it serves a complementary role in the broader scraper conveyor manufacturing ecosystem:
- Clad Middle Sled Fabrication: Hydraulic explosive bonding can be used to produce bilayer middle sled plates where a wear-resistant alloy (e.g., 13Cr, 27Cr, or Ni-base) is bonded to a structural steel backing (Q345B/Q460). This provides a base clad plate that can then receive a pattern overlay via MIG welding, combining the advantages of both technologies.
- Transition Layer Alternative: For applications requiring a metallurgically bonded transition between dissimilar materials (e.g., austenitic steel to martensitic hardfacing), explosive bonding provides a defect-free interface without dilution, which can then be used as the base for subsequent weld overlay.
- Large-Format Cladding: For very large middle sled assemblies (e.g., > 3,000 mm length), explosive bonding of full-size clad plates is more economical than full-surface weld overlay, with MIG overlay reserved for localized high-wear zones.
7.3 Explosion Welding (Specialized Application)
Explosion welding (using controlled detonation of high explosives) is applicable in specialized scenarios:
- High-Performance Clad Slides: For premium scraper conveyor applications requiring maximum wear resistance (e.g., ultra-hard Ni-Cr-Mo or Co-base overlays), explosion welding can produce a clad plate with a metallurgically bonded, high-alloy surface layer that serves as the base for pattern overlay.
- Thick Overlay Requirements: When overlay thickness exceeds 20 mm, explosion welding of a thick clad layer followed by MIG pattern overlay is more economical than full weld buildup.
- Multi-Layer Cladding: For extreme service conditions (e.g., high-temperature + high-abrasion + corrosive environments), explosion welding can produce multi-layer clad plates (e.g., structural steel / austenitic steel / martensitic hardfacing / Ni-base) that are then patterned via MIG welding.
7.4 Technology Route Integration Matrix
| Application Scenario | Primary Route | Secondary Route | Rationale |
|---|---|---|---|
| Standard coal conveyor middle sled (new build) | MIG Weld Overlay | — | High productivity; flexible pattern design; cost-effective for moderate thickness |
| Refurbishment of worn in-service sleds | MIG/TIG Weld Overlay | — | Field-applicable; no need for bulk material removal; rapid turnaround |
| Premium high-wear conveyor (rock-heavy) | Explosion Welding (clad base) + MIG Pattern | Hydraulic Explosive Bonding | Maximum wear resistance; thick overlay; metallurgical bonding without dilution |
| Large-format clad sled (OEM supply) | Hydraulic Explosive Bonding | MIG Weld Overlay (pattern) | Full-surface cladding; pattern overlay on clad surface for localized protection |
| Corrosive + abrasive environment (wet mining) | Explosion Welding (Ni-base clad) + MIG Pattern | — | Corrosion resistance from Ni-base; abrasion resistance from pattern; no dilution |
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR Development: Each pattern type and overlay specification requires a qualified Welding Procedure Specification (WPS) and Performance Qualification Record (PQR) per GB/T 19864 or EN ISO 14271. Developing these qualifications demonstrates technical capability and enables market entry into specific mining OEM supply chains.
- Welder Certification: Welders must be certified per EN ISO 14272 or equivalent national standard for hardfacing overlay work. Maintaining a certified welder pool ensures consistent quality and enables bidding on large contracts that require certified personnel.
- NDT Qualification: Level II/III NDT personnel (per EN ISO 9712 or GB/T 9445) qualified in VT, MT, and hardness testing are required for overlay acceptance inspection. Building this NDT capability supports full-service qualification.
- Customer-Specific Qualifications: Major mining OEMs (e.g., North American Coal Corp., DBT, Jindal, KET) require supplier-specific WPS qualifications and factory audits. Completing these builds a portfolio of customer-approved procedures that accelerates future order fulfillment.
8.2 Product Delivery Capability
- Scalable Production: The MIG-based overlay process is inherently scalable—from single-sled repair to batch production of 100+ sleds per month. Investing in automated MIG welding cells (with CNC torch manipulation) enables high-volume, consistent production.
- Pattern Customization: The ability to produce custom pattern geometries (chevron, cross-hatch, pyramid, etc.) based on customer-specific wear analysis differentiates the company from generic hardfacing suppliers and enables value-added engineering services.
- Turnkey Solutions: Combining overlay welding with surface preparation (shot blasting), stress relief (furnace or induction), and NDT inspection creates a complete turnkey service that reduces customer coordination burden and increases contract value.
- Lead Time Advantage: MIG overlay is significantly faster than explosion welding or hydraulic explosive bonding for patterned surfaces, enabling shorter lead times (2–4 weeks vs. 6–12 weeks) for overlay-only orders.
8.3 Customer Value Creation
- Reduced Total Cost of Ownership (TCO): A well-executed wear-resistant pattern overlay can extend middle sled life by 3–5 times, reducing replacement frequency, spare parts inventory, and conveyor downtime. For a mine operating 100+ middle sleds, this translates to annual savings of $50,000–$200,000 per mine.
- Improved Conveyor Efficiency: The pattern geometry not only resists wear but also improves material retention on the conveyor trough, reducing material spillage and improving transport efficiency by 5–15%.
- Enhanced Safety: Reduced conveyor downtime means fewer emergency stoppages, fewer manual interventions, and improved overall mine safety. The pattern overlay also reduces the risk of structural failure due to progressive wear.
- Technical Partnership: By providing wear analysis, pattern design optimization, and periodic inspection services, the company transitions from a commodity supplier to a strategic technical partner, increasing customer loyalty and contract duration.
- Environmental Benefit: Extended service life of middle sleds reduces steel consumption, manufacturing emissions, and waste disposal associated with frequent replacement—supporting mine operators' ESG (Environmental, Social, and Governance) objectives.
9. Implementation Recommendations
- Establish a Wear Analysis Protocol: Develop a standardized procedure for analyzing customer-provided worn sleds to determine wear pattern, severity, and optimal overlay specification. This data-driven approach ensures the correct hardfacing alloy and pattern geometry are selected.
- Develop a Library of Qualified WPS: Qualify WPS for at least 3–4 common hardfacing alloy systems (Cr-C, Ni-Cr-C, Co-Cr-W, Cr-Mo-C) across the range of pattern types. This library enables rapid WPS selection for new orders.
- Invest in Automation: For production volumes exceeding 50 sleds/month, invest in semi-automated or fully automated MIG welding systems with CNC torch manipulation to ensure consistency, reduce labor costs, and improve throughput.
- Implement Statistical Process Control (SPC): Monitor key process parameters (current, voltage, travel speed, interpass temperature) and output parameters (hardness, dilution, adhesion) using SPC charts to maintain quality consistency and identify drift early.
- Develop Field Service Capability: Equip field service teams with portable MIG/TIG welding equipment, preheat ovens, hardness testers, and NDT instruments to provide on-site refurbishment services for mines with limited logistics access.
- Pursue OEM Partnerships: Approach major scraper conveyor OEMs (e.g., DBT, Jindal, KET, North American Coal Corp.) with qualified WPS packages and test data to become an approved supplier for new-build overlay. OEM partnerships provide stable, high-volume order flow.
- Conduct Customer Training: Offer technical training to mine maintenance personnel on overlay inspection, wear monitoring, and re-overlay criteria. This builds customer trust and creates recurring service opportunities.
10. Conclusion
The large-area wear-resistant pattern weld overlay of scraper conveyor middle sleds represents a high-value, technically demanding application within the TIG/MIG weld overlay technology route. It requires mastery of hardfacing metallurgy, process parameter optimization, pattern geometry design, distortion control, and rigorous quality assurance. By developing qualified WPS, certified welder pools, automated production capability, and field service infrastructure, the company can position itself as a leading provider of wear-resistant overlay solutions for the mining industry. The integration of this capability with the hydraulic explosive bonding and explosion welding routes—through clad base plate fabrication and multi-layer cladding—creates a comprehensive surface engineering portfolio that addresses the full spectrum of wear, corrosion, and impact protection requirements in heavy-duty mining equipment.
This entry, while originating as a learning experience (学习心得), represents a critical knowledge asset that should be formalized into qualified procedures, documented in the company's technical library, and leveraged for customer-facing technical proposals and qualification submissions. The systematic conversion of operational experience into certified, repeatable, and auditable technical capability is the foundation of sustainable growth in the surface engineering and cladding industry.