Large-Area Pattern Weld Overlay on Scraper Conveyor Middle Trough Center Plates
1. Definition and Technical Principles
Large-area pattern weld overlay (大面积花样堆焊) on scraper conveyor middle trough center plates refers to the systematic application of multi-pass, geometrically arranged weld beads onto the wear-critical inner surface of the center plate (中板) within the middle trough assembly of a scraper conveyor system. The "pattern" designation (花样) denotes a deliberate, engineered arrangement of weld bead geometry, spacing, direction, and overlap that maximizes wear resistance, minimizes residual stress accumulation, and ensures uniform coverage across large flat or contoured plate surfaces—typically ranging from 1,200 mm × 600 mm to 3,000 mm × 1,500 mm per center plate.
The fundamental metallurgical principle is the creation of a dilution-controlled, hardfacing alloy layer on the base structural steel (typically Q345B, Q355B, or 45# carbon steel) that exhibits superior abrasion and impact resistance against coal, rock, and slurry media encountered during underground mining operations. The pattern design ensures that each successive weld bead partially dilutes into the previously deposited layer, achieving a metallurgical gradient from the base steel to the full-composition hardfacing alloy at the surface. This gradient eliminates brittle intermetallic phases at the base/weld interface while maintaining a high-hardness (typically 50–65 HRC) wear surface.
The overlay process leverages the self-fluxing and self-shielding characteristics of specialized hardfacing consumables, combined with precise thermal management through bead sequencing, interpass temperature control, and optional preheating/interpass heating protocols. The pattern geometry—whether chevron, zigzag, transverse, or multi-directional cross-hatch—is selected based on the primary wear vector, plate geometry constraints, and distortion sensitivity.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG weld overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents a high-volume, high-value-added service segment targeting the coal mining equipment manufacturing and aftermarket maintenance market. Unlike hydraulic explosive bonding or explosion welding—which are primarily used for corrosion-resistant cladding on pressure vessels and heat exchangers—the pattern weld overlay technology serves a distinct market: bulk material handling equipment requiring extreme abrasion resistance under severe impact loading.
The business positioning is as a specialized surface engineering and wear protection service provider for mining equipment OEMs and aftermarket repair shops. Key business characteristics include:
- Volume manufacturing capability: Large-area, repetitive overlay operations on standardized center plate geometries require high-throughput MIG-based processes with automated or semi-automated torch manipulation.
- Customization capability: Pattern design must be tailored to specific conveyor models (e.g., SGZ-800/40, SGZ-1000/630, SGZ-1250/800 series), coal type, mining method, and expected service life.
- Qualification-driven market entry: OEM customers require WPS/PQR qualification packages, material traceability, and NDT documentation for overlay operations on safety-critical conveyor components.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The overlay operation serves three simultaneous engineering objectives:
- Wear life extension: Increase the service life of the center plate by 3–8 times compared to bare carbon steel, depending on coal abrasiveness and operating conditions. Typical un-overlayed center plates require replacement every 400–800 operating hours; overlayed plates achieve 2,000–6,000 hours.
- Impact resistance preservation: Unlike surface hardening treatments (induction hardening, flame hardening), weld overlay maintains the toughness of the base material beneath the hardfacing layer, preventing catastrophic brittle fracture under scraper chain impact loads.
- Repairability: The overlay can be re-applied in the field or at a repair facility, extending component life through multiple overlay cycles.
3.2 Economic Value
For a single scraper conveyor middle trough center plate, the overlay operation typically adds 800–2,500 RMB in material and labor cost, while the replacement cost of a new center plate ranges from 5,000–15,000 RMB. When amortized over the extended service life, the overlay cost per operating hour is reduced by 60–80%, delivering significant total cost of ownership savings for mine operators.
4. Key Process and Implementation Points
4.1 Consumable Selection
The selection of hardfacing consumables is the single most critical factor determining overlay performance. The following table summarizes commonly used consumable types for scraper conveyor center plate applications:
| Consumable Type | Typical Composition | Hardness (HRC) | Primary Application | Key Advantage |
|---|---|---|---|---|
| Cr-Cr2C (Chrome-Carbide) | Fe-25Cr-5C with Cr7C3 particles | 55–62 | General coal and rock abrasion | Excellent dry abrasion resistance |
| Cr-Ni-CrB (Stellite-type) | Co-28Cr-5W-7Fe with CrB | 45–55 | Wet/slurry abrasion, high temperature | Corrosion-resistant, good impact toughness |
| Cr2C-Cr (High Chrome) | Fe-30Cr-7C with Cr23C6 | 58–65 | Severe abrasive conditions | Highest hardness, fine carbide distribution |
| Fe-Cr2C (Iron-Carbide) | Fe-12Cr-3C with Cr7C3 | 45–52 | Transition/base layer, high impact | Good weldability, low dilution sensitivity |
4.2 Pattern Design and Bead Sequencing
The "pattern" (花样) is the engineered arrangement of weld beads that determines overlay quality. Key design parameters include:
- Bead width: Typically 25–40 mm for MIG process, 15–25 mm for TIG process
- Bead height: 2–4 mm per pass, with total overlay thickness of 8–25 mm depending on service severity
- Overlap ratio: 30–50% lateral overlap between adjacent beads to ensure uniform coverage and prevent gaps
- Travel speed: 150–350 mm/min for MIG, 80–200 mm/min for TIG
- Bead direction: Alternating 90° between passes to minimize residual stress and distortion
Common pattern configurations include:
- Chevron pattern: V-shaped bead arrangement with the apex pointing in the direction of primary wear. Provides directional wear resistance and stress relief along the chevron axis.
- Transverse zigzag pattern: Beads deposited in alternating left-to-right and right-to-left directions with a zigzag lateral movement. Most common for large flat center plates.
- Cross-hatch pattern: Two perpendicular sets of beads. Used for severe multi-directional wear applications.
- Longitudinal staggered pattern: Beads run parallel to the conveyor travel direction with staggered start/stop points. Minimizes stress concentration at bead terminations.
4.3 Process Parameters
The following table presents typical process parameters for MIG (GMAW) pattern overlay on scraper conveyor center plates:
| Parameter | Base Layer (Pass 1) | Build-Up Layer (Passes 2–3) | Final Surface Layer (Passes 4+) |
|---|---|---|---|
| Consumable | Fe-Cr2C low-dilution wire | Cr-Cr2C hardfacing wire | Cr2C-Cr high-hardness wire |
| Wire diameter | 1.6 mm | 1.6 mm or 2.0 mm | 1.6 mm or 2.0 mm |
| Voltage (V) | 22–24 | 24–26 | 24–27 |
| Current (A) | 180–220 | 200–260 | 200–280 |
| Travel speed (mm/min) | 200–250 | 180–250 | 150–220 |
| Shielding gas | Ar + 5% CO2 | Ar + 5% CO2 | Ar + 5% CO2 or Ar + 2% O2 |
| Gas flow rate (L/min) | 15–20 | 15–20 | 15–20 |
| Interpass temperature | < 200°C | < 250°C | < 250°C |
| Preheat temperature | 100–150°C (for plates > 20 mm) | Maintain at 100–150°C | |
| Target dilution | 30–40% | 15–25% | 10–20% |
4.4 Surface Preparation
Proper surface preparation is essential for achieving metallurgical bond between the base plate and the overlay:
- Weld preparation: Machining or grinding of the center plate surface to a consistent flatness within ±1 mm/m. Removal of scale, rust, oil, and paint to bare metal. Surface roughness of Ra 3.2–6.3 μm is optimal for weld adhesion.
- Edge preparation: A 3–5 mm chamfer or U-groove along the perimeter of the overlay area to ensure full penetration at the overlay boundary and prevent undercut at the transition zone.
- Preheat: Application of 100–150°C preheat using induction heating or propane torches, distributed uniformly across the entire plate area to be overlaid. Preheat is mandatory for plates exceeding 20 mm thickness or for ambient temperatures below 10°C.
4.5 Distortion Control
Large-area overlay operations generate significant thermal input, leading to plate warping and dimensional deviation. Control measures include:
- Back-up plate: Welding on a rigid, preheated backing plate (typically 20–30 mm thick steel plate) to constrain out-of-plane distortion.
- Clamping: Mechanical clamping of the center plate to the backing plate using evenly spaced clamps (spacing ≤ 300 mm) with controlled clamp force.
- Sequencing: Depositing beads in a balanced sequence that maintains thermal symmetry—starting from the center and working outward, or using a "back-and-forth" sequence that alternates between opposite ends of the plate.
- Post-weld stress relief: For critical applications, applying a controlled stress relief cycle (550–650°C for 1 hour per 25 mm of plate thickness, furnace or induction) after overlay completion.
- Post-weld straightening: Induction heating-based straightening for residual warping exceeding ±2 mm/m, applied in a controlled sequence to avoid re-introducing residual stress.
4.6 Multi-Layer Overlay Strategy
The overlay is typically executed in three metallurgical layers:
- Base/Transition layer: A low-dilution, high-toughness consumable (e.g., Fe-Cr2C or 309L stainless steel wire) is deposited in 1–2 passes. This layer provides a metallurgical bridge between the carbon steel base and the high-alloy hardfacing, reducing dilution in subsequent layers and preventing cracking at the interface.
- Build-up layer: 1–2 passes of medium-hardness hardfacing wire to achieve the required overlay thickness. This layer provides the bulk of the wear-resistant material with controlled dilution (15–25%).
- Surface layer: 1–3 passes of high-hardness hardfacing wire deposited at the final pattern geometry. This layer determines the wear resistance and surface quality of the finished overlay. Dilution is minimized to 10–20% through careful parameter control and adequate overlap with the build-up layer.
5. Applicable Standards and Acceptance Criteria
5.1 Process Qualification Standards
| Standard | Scope | Relevance to Overlay Operation |
|---|---|---|
| GB/T 985.1-2008 | Welding procedure qualification | WPS/PQR qualification framework for weld overlay processes |
| GB/T 985.2-2008 | Welding procedure qualification (non-ferrous) | Reference for overlay qualification methodology |
| GB/T 19866-2005 | Weld overlay—General requirements | Primary standard for weld overlay qualification and acceptance |
| ISO 12535:2017 | Welding—Qualification of welding procedures | International qualification framework for overlay WPS |
| ASME Section IX, Part QW-400 | Welding procedure qualification | Qualification requirements for weld overlay procedures |
| ASTM A404 | Standard specification for weld overlay | Material and performance requirements for overlay deposits |
| NB/T 47014-2011 | Welding procedure qualification for pressure equipment | Qualification requirements when overlay is applied to pressure-containing components |
5.2 Acceptance Criteria
The following acceptance criteria apply to the completed overlay:
- Visual inspection (VT): 100% visual inspection per GB/T 3323 or equivalent. No undercut exceeding 0.5 mm depth, no porosity exceeding 1 mm diameter or 2% surface area, no cracks, no incomplete overlap between adjacent beads. Surface smoothness within ±1 mm over 100 mm length.
- Hardness testing: Minimum 3 hardness readings per 100 mm × 100 mm area, using Rockwell C (HRC) method per GB/T 230.1. Target hardness: 50–65 HRC for the surface layer, with a hardness gradient transition to < 35 HRC at the base/overlay interface. No individual reading below 45 HRC for the surface layer.
- Dilution testing: Metallographic examination of a cross-section at a minimum of 2 locations per overlay area. Dilution at the base/overlay interface measured per ASTM E10 or equivalent. Maximum allowable dilution: 25% for the surface layer, 40% for the base layer.
- Penetrant testing (PT): 100% PT inspection of the overlay surface per GB/T 18851 or ASTM E709. No linear indications (cracks, lack of fusion) permitted. Round indications (porosity) limited to 2 per 100 mm length, maximum 1 mm diameter.
- Dimensional verification: Overlay thickness measured at a minimum of 5 points per plate using ultrasonic thickness gauge or micrometer. Thickness within ±0.5 mm of specified value. Plate flatness within ±2 mm/m after overlay.
- Mechanical testing (for qualification): Transverse tensile test, bend test (face bend and side bend), and impact test (Charpy V-notch, 20°C) per GB/T 19866. Minimum tensile strength: 550 MPa. No cracking in bend tests at 5T bend diameter. Impact energy: ≥ 27 J at 20°C for the overlay/base interface zone.
5.3 Equipment and Component Standards
| Standard | Description | Application |
|---|---|---|
| MT/T 105-2006 | Scraper conveyor—General technical requirements | Design and performance requirements for scraper conveyor assemblies |
| MT/T 113-2010 | Scraper conveyor—Middle trough | Specific requirements for middle trough components including center plates |
| GB/T 1591-2018 | High-strength low-alloy structural steel | Base material specification for center plates (Q345, Q355 grades) |
| GB/T 3077-2015 | Hot rolled steel bars for mechanical and structural purposes | Material specification for 45# carbon steel center plates |
6. Common Risks and Controls
| Risk | Cause | Detection Method | Control Measure |
|---|---|---|---|
| Cracking at base/overlay interface | Excessive dilution, high carbon equivalent of base steel, insufficient preheat, rapid cooling | PT, MT, metallographic cross-section | Use transition layer with low-carbon consumable; preheat to 150–250°C; control interpass temperature < 250°C; post-weld stress relief |
| Porosity in overlay | Inadequate shielding gas coverage, surface contamination, excessive travel speed | VT, PT, radiographic testing | Ensure gas flow ≥ 15 L/min; use gas shielding cup in windy conditions; clean surface to bare metal; maintain travel speed within qualified range |
| Plate warping/distortion | Excessive thermal input, unbalanced bead sequencing, lack of back-up constraint | Visual inspection, flatness measurement with straightedge | Use back-up plate with clamping; alternate bead direction; control interpass temperature; apply post-weld straightening |
| Inadequate hardness | Excessive dilution, incorrect consumable selection, insufficient number of passes | Hardness testing (HRC) | Verify consumable chemistry; increase number of surface passes; reduce travel speed to increase deposition rate per pass |
| Undercut at overlay boundary | Inadequate edge preparation, excessive current, incorrect torch angle | VT, dimensional measurement | Prepare 3–5 mm chamfer at overlay boundary; reduce current by 10–15% at edges; use correct torch angle (10–15° from vertical) |
| Incomplete overlap between beads | Excessive travel speed, incorrect torch manipulation, consumable feeding irregularities | VT, PT | Maintain 30–50% overlap; use automatic wire feed with consistent speed; train operators on torch control |
| Spalling/delamination in service | Poor metallurgical bond, hydrogen-induced cracking, residual stress | In-service inspection, acoustic emission testing | Ensure proper surface preparation; use low-hydrogen consumables; apply post-weld stress relief; verify dilution and bond quality during qualification |
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This technology is the core application of the TIG/MIG weld overlay route. MIG (GMAW) is the preferred process for large-area overlay due to its higher deposition rate (3–5 kg/h vs. 0.5–1.0 kg/h for TIG), while TIG is reserved for:
- Repair of localized wear areas: Small patches on heavily worn sections of center plates in the field.
- Overlay of high-alloy consumables: Materials such as cobalt-based Stellite that are difficult to weld with MIG due to sensitivity to gas composition and heat input.
- Transition layers: Critical base/overlay interface layers where precise dilution control is required.
- Overlay on thin plates: Center plates below 12 mm thickness where TIG's lower heat input minimizes distortion risk.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding is not directly applicable to scraper conveyor center plates (which are non-pressure, non-clad components), the technology route provides complementary capabilities in the following scenarios:
- Wear-resistant pipe overlays for slurry transport systems: Hydraulic explosive bonding of stainless steel or nickel alloy cladding on carbon steel pipes used in mine slurry transport, where corrosion and abrasion are simultaneous concerns.
- Corrosion-resistant cladding on conveyor support structures: In wet mining environments, hydraulic explosive bonding of 304L/316L stainless steel on structural steel supports provides corrosion protection that complements the wear overlay on center plates.
- Material processing capability: The same metallurgical expertise and NDT infrastructure used for hydraulic explosive bonding qualification supports the overlay qualification program.
7.3 Explosion Welding Route (Complementary Application)
Explosion welding provides the following complementary capabilities:
- Large-area clad plate production: Production of pre-clad center plates with a corrosion-resistant stainless steel backing layer (to protect the non-wear side from environmental corrosion) and a wear-resistant overlay on the wear side. This hybrid approach combines explosion welding (for corrosion cladding) with weld overlay (for wear protection).
- High-integrity bond production: For applications requiring both corrosion and wear resistance (e.g., wet coal handling systems), explosion-welded clad plates provide a metallurgically sound base for subsequent overlay operations.
- Technology synergy: The metallurgical understanding of dilution, bond quality, and interface characterization developed through explosion welding qualification directly informs overlay process optimization.
8. Qualification Building and Customer Value
8.1 Qualification Package Development
Building a comprehensive qualification package for large-area pattern weld overlay requires the following deliverables:
- WPS (Welding Procedure Specification): Documented per GB/T 19866 or ISO 12535, specifying all essential variables including consumable type, process parameters, preheat/interpass temperatures, pattern geometry, and post-weld treatment.
- PQR (Procedure Qualification Record): Witness coupon fabrication and testing demonstrating that the qualified WPS produces a weld overlay meeting all acceptance criteria. Includes hardness, dilution, tensile, bend, and impact test results.
- Welder qualification records: Operator certification per GB/T 985.1 or ISO 9606-1, demonstrating proficiency in the specific overlay pattern, consumable, and plate geometry.
- Material traceability documentation: Mill certificates for base plates, consumable certificates for hardfacing wires, and lot-level traceability from raw material to finished overlay.
- NDT documentation: Full VT and PT records for 100% of overlay surfaces, with hardness test maps for each plate.
8.2 Customer Value Proposition
The qualification and execution of large-area pattern weld overlay on scraper conveyor center plates delivers the following customer value:
- Extended equipment availability: 3–8× service life extension reduces conveyor downtime for center plate replacement, directly improving mine production output.
- Reduced total cost of ownership: Overlay cost is 15–30% of new plate replacement cost, with net savings of 60–80% per operating hour over the service life.
- Customized wear protection: Pattern design and consumable selection tailored to specific mining conditions (coal type, moisture content, rock fraction, conveyor speed) ensures optimal performance for each application.
- Quality assurance: Full NDT documentation, hardness mapping, and dilution verification provide traceable quality records that satisfy OEM and mine operator quality management systems.
- Field repair capability: The ability to re-overlay worn center plates in the field extends the service life of existing conveyor installations without full component replacement.
8.3 Strategic Positioning for Cladding Technology Shanxi Co., Ltd.
This technology entry represents a strategic capability that bridges the company's core cladding expertise with the high-volume mining equipment aftermarket. Key strategic advantages include:
- Geographic advantage: Shanxi Province is China's largest coal-producing region, providing direct access to the primary market for scraper conveyor overlay services.
- Technical differentiation: The combination of metallurgical expertise (from cladding technology), NDT capability, and qualification documentation provides a competitive advantage over local repair shops that lack systematic quality management.
- Cross-sell opportunity: Mining customers requiring overlay services may also need corrosion-resistant cladding for slurry systems, hydraulic equipment, and pressure vessels—creating a natural cross-sell pathway to the company's other technology routes.
- Recurring revenue: The consumable nature of overlay (wear plates require periodic re-overlay) creates a recurring revenue model with predictable demand cycles tied to mine production schedules.
9. Implementation Checklist
The following checklist ensures consistent quality and compliance for each overlay operation:
- Verify base plate material grade and thickness against WPS requirements
- Confirm consumable type, chemistry, and lot number against WPS
- Inspect and document surface preparation (cleanliness, flatness, edge preparation)
- Apply and verify preheat temperature (thermocouple or infrared pyrometer)
- Set and verify MIG/TIG machine parameters against WPS
- Execute overlay in the specified pattern sequence with documented interpass temperature checks
- Perform 100% VT inspection during and after overlay
- Perform 100% PT inspection after cooling to ambient temperature
- Perform hardness testing and dilution verification per acceptance criteria
- Measure and document overlay thickness and plate flatness
- Compile and submit full NDT and quality documentation package
- Apply post-weld stress relief if specified in WPS
- Perform final dimensional and visual inspection after stress relief
10. Conclusion
Large-area pattern weld overlay on scraper conveyor middle trough center plates is a technically demanding, quality-critical process that combines metallurgical science, welding engineering, and quality management. The success of this technology depends on precise consumable selection, rigorous pattern design, controlled process parameters, comprehensive NDT, and systematic qualification documentation. As a core capability within the TIG/MIG weld overlay technology route, this technology provides significant economic value to mining equipment operators while leveraging the company's metallurgical expertise and quality management infrastructure developed through cladding technology. The qualification package built through this technology not only enables direct product delivery but also strengthens the company's position as a comprehensive surface engineering solutions provider across all three technology routes.