High-Chromium Hardfacing Overlay for Scraper Conveyor Middle Troughs: Microstructural Characteristics and Sliding Wear Performance
1. Definition and Technical Principles
High-chromium hardfacing overlay is a surface engineering technique in which a wear-resistant alloy deposit, typically containing 12–30 wt% Cr, is applied to a structural substrate (such as Q235, Q345, or 16Mn steel) via TIG or MIG welding processes. The resulting composite structure leverages the toughness of the base metal while providing exceptional resistance to abrasive, sliding, and impact wear on the surface. In the context of scraper conveyor middle troughs (刮板输送机中部槽), this technology is critical because the trough is subjected to continuous sliding abrasion from chain scrapers, loaded with abrasive coal, rock, and ore particles under high contact pressure.
The fundamental wear mechanisms in scraper conveyor service include:
- Abrasive sliding wear — dominant mechanism caused by hard particles (quartz, feldspar) embedded in transported material scraping across the trough bottom surface
- Adhesive wear — micro-welding and tearing at asperity contacts between the scraper chain and trough surface
- Impact-abrasion synergy — combined effect of falling material impact and subsequent sliding abrasion
- Oxidative wear — secondary mechanism under elevated temperatures generated by friction
High-chromium alloys resist these mechanisms through a combination of hard, thermodynamically stable chromium carbides (primarily M7C3, M2C, and M6C types) dispersed within a martensitic or austenitic matrix. The carbide hardness typically ranges from HV 1200–1800, compared to HV 200–350 for the base steel substrate.
2. Microstructural Characteristics of High-Chromium Hardfacing Deposits
2.1 Carbide Morphology and Distribution
The wear performance of high-chromium hardfacing is directly governed by the type, size, shape, and distribution of chromium carbides formed during solidification. The primary carbide types and their characteristics are summarized below:
| Carbide Type | Composition | Hardness (HV) | Morphology | Wear Resistance Contribution |
|---|---|---|---|---|
| M7C3 | Cr7C3 | 1200–1500 | Cellular/rosette, network at grain boundaries | Excellent abrasive wear resistance; moderate fracture toughness |
| M2C | Cr2C | 1600–1800 | Rosette/cross-shaped, coarse | Very high hardness; prone to chipping under impact |
| M6C | Cr6C | 1400–1600 | Blocky, at grain boundaries | Good wear resistance; acts as crack initiation sites if excessive |
| MC | Cr3C, Cr4C | 1800–2000 | Large, irregular | Highest hardness; very brittle; detrimental in excess |
2.2 Matrix Structure
The matrix phase surrounding the carbides significantly influences the overall toughness and fatigue resistance of the overlay:
- Martensitic matrix (Cr ≥ 12%, with Mo, V additions): Provides HV 600–900 base hardness; good combination of wear resistance and impact toughness. Preferred for high-impact scraper conveyor applications.
- Austenitic matrix (Cr ≥ 18%, with Ni additions): Provides strain-hardening capability during sliding; excellent resistance to high-temperature oxidation; used where thermal stability is required.
- Duplex matrix (martensite + retained austenite): Balances hardness and toughness; commonly achieved with Cr 18–22%, Mo 1–3%, V 0.5–1.5% compositions.
2.3 Microstructural Zones in the Overlay
A typical multi-pass high-chromium hardfacing deposit exhibits three distinct microstructural zones from the surface downward:
- Surface zone (0–0.5 mm): Fine-grained structure with refined carbide distribution due to rapid cooling; highest hardness (HV 1200–1600); primary wear-resisting layer.
- Mid zone (0.5–2.0 mm): Coarser carbide morphology with M7C3 networks; hardness HV 900–1200; provides bulk wear resistance.
- Heat-affected zone (HAZ, 2.0–5.0 mm): Partial melting and grain growth; possible softening to HV 200–350; critical for evaluating overlay-substrate bonding strength and potential crack initiation.
3. Sliding Wear Performance Evaluation
3.1 Wear Test Methodology
Sliding wear performance of high-chromium hardfacing plates for scraper conveyor applications is typically evaluated using the following standardized methods:
- Pin-on-disk test (ASTM G99): Simulates sliding contact between scraper chain (pin, often GCr15 bearing steel) and overlay surface (disk). Normal loads of 5–20 N applied; sliding distance 1000–5000 m; wear volume measured by profilometry or weight loss.
- Block-on-ring test (ASTM G113): More representative of scraper conveyor contact geometry; block represents trough surface, rotating ring represents scraper chain link.
- Roller abrasion test: Simulates continuous sliding abrasion with abrasive slurry (SiC particles in oil/water) at controlled normal pressure.
- Field service testing: Direct installation in operating scraper conveyor; wear depth measured at defined intervals (typically 500–2000 operating hours) using ultrasonic thickness gauging or profilometric mapping.
3.2 Key Performance Metrics
| Performance Metric | Typical Requirement (Scraper Conveyor) | Measurement Method | Acceptance Criterion |
|---|---|---|---|
| Surface hardness | ≥ HV 1000 (average) | Vickers microhardness (HV0.3) | ASTM E92 / GB/T 6398 |
| Wear rate (sliding abrasion) | ≤ 0.5 mg/N·m | Pin-on-disk, ASTM G99 | Relative to unhardened Q235 baseline |
| Wear life improvement | ≥ 5× base material | Field comparison testing | Service life comparison |
| Overlay thickness (wear-resistant layer) | ≥ 3.0 mm (minimum 2.0 mm) | Ultrasonic / cross-section | GB/T 12469 / ASTM A270 |
| Impact toughness (overlay + substrate) | ≥ 27 J @ -20°C | Charpy V-notch, full-thickness | GB/T 229 / ASTM E23 |
| Peel/bend adhesion | No delamination | Longitudinal bend test | GB/T 12469 / ASTM A270 |
4. Application to Scraper Conveyor Middle Troughs
4.1 Component Description and Service Conditions
Scraper conveyor middle troughs (中部槽) are structural components in longwall mining and bulk material handling systems. They form the continuous channel along which scraper chains transport mined material. The bottom surface and side walls of the trough experience:
- Continuous sliding contact with scraper chain links (typically 20–40 kg per link) at speeds of 0.3–1.0 m/s
- Abrasive particles (coal, rock, quartz, feldspar) embedded in transported material
- Impact loading from material falling into the trough
- Normal operating temperatures of 40–80°C, occasionally exceeding 100°C in deep mines
- High cyclic loading with 10⁶–10⁷ cycles over service life
4.2 Overlay Application Strategy
For scraper conveyor middle troughs, high-chromium hardfacing overlay is applied to the following areas:
- Trough bottom surface: Primary wear zone; full-width overlay, typically 3–5 mm thick in 2–3 passes
- Side wall contact surfaces: Where scraper chain links contact the trough walls; 2–4 mm overlay
- End wall/transition zones: Where material enters/exits the trough; may require thicker overlay (5–8 mm)
5. Key Process Parameters and Implementation Points
5.1 Recommended Welding Consumables
| Application | Welding Wire/Rod | Key Chemistry (wt%) | Deposited Hardness (HV) | Standards Reference |
|---|---|---|---|---|
| General scraper trough | CrMoV high-carbon hardfacing wire | C 2.5–3.5, Cr 20–25, Mo 1.5–3.0, V 0.5–1.5 | 1000–1400 | GB/T 12469 / ASTM A270 |
| High-impact service | Austenitic high-Cr hardfacing | C 1.5–2.5, Cr 18–22, Ni 4–8, Mo 1–2 | 500–800 (strain-hardens to 800–1000) | ASTM A270 Type C-3 |
| Severe abrasive wear | High-Cr high-C hardfacing | C 3.0–4.0, Cr 22–28, Mo 2–4, W 2–5 | 1200–1600 | GB/T 12469 |
| Transition layer (if needed) | 309L stainless steel | C ≤ 0.03, Cr 22–25, Ni 12–15 | 200–300 | GB/T 983 / AWS A5.4 |
5.2 Process Parameters for TIG/MIG Weld Overlay
| Parameter | TIG (GTAW) Overlay | MIG (GMAW) Overlay |
|---|---|---|
| Base material | Q235 / Q345 / 16Mn trough plate | Q235 / Q345 / 16Mn trough plate |
| Preheating temperature | 150–250°C | 200–350°C |
| Interpass temperature | ≤ 300°C | ≤ 400°C |
| Current (A) | 120–200 | 180–320 |
| Voltage (V) | 12–18 | 20–28 |
| Travel speed (mm/s) | 3–8 | 8–20 |
| Shielding gas | Ar 99.99% | Ar 80% + CO₂ 20% or Ar 95% + CO₂ 5% |
| Wire diameter (mm) | 2.0–3.2 (filler rod) | 1.2–1.6 |
| Pass thickness (mm) | 1.5–3.0 | 2.0–4.0 |
| Number of passes | 2–3 | 1–2 (higher deposition rate) |
| Post-weld treatment | Peening (optional); controlled cooling | Peening (recommended); controlled cooling |
5.3 Critical Implementation Points
- Surface preparation: Grind the trough surface to a uniform, oxide-free finish (minimum 3 mm depth of sound material exposed). Remove all rust, scale, oil, and prior coatings. Surface roughness Ra ≤ 10 μm.
- Preheating control: Maintain preheat temperature of 150–250°C for carbon steel substrates to reduce hydrogen-induced cracking susceptibility. Use infrared pyrometry for temperature verification.
- Weld pass sequencing: For multi-pass overlays, apply a "stringer bead" technique with 20–40% overlap between adjacent passes. Avoid excessive overlap which causes dilution and softening.
- Dilution management: Maintain dilution rate below 30% for optimal hardness. Verify by microhardness traverse across the overlay-substrate interface. Higher dilution reduces surface hardness below required minimum.
- Peening: Apply shot peening or mechanical peening after each pass to compress residual tensile stresses, refine grain structure, and improve fatigue resistance. Peening intensity (Almen spot) of 0.2–0.4 mm A-10.
- Cooling rate control: For high-carbon hardfacing alloys, control cooling rate to 20–50°C/min to promote favorable carbide precipitation without excessive brittleness. Use insulating blankets for slow cool if necessary.
- Weld geometry: Maintain consistent bead width-to-height ratio of 1.5–2.5:1 for uniform hardfacing thickness and reduced stress concentration.
6. Applicable Standards and Acceptance Criteria
6.1 Product and Material Standards
- GB/T 12469 — Clad steel plate and strip for pressure vessels and other equipment (hardfacing overlay specifications)
- ASTM A270 — Standard Specification for Clad Steel Plate and Strip for Pressure Vessels and Other Equipment
- GB/T 983 — Stainless steel welding electrodes (for transition layers)
- ASTM A5.4 (AWS A5.4) — Specification for stainless steel welding electrodes and rods
- GB/T 14957 — Welding consumables for hardfacing
- ISO 9518 — Welding consumables — Classification of filling metals for hardfacing
6.2 Process and Qualification Standards
- GB/T 19866 — Welding procedure qualification and approval
- ASME Section IX — Qualification rules for welding, brazing, and fusing
- ISO 15614 — Qualification procedures for welding of metallic materials
- GB/T 19418 — Welding procedure specification (WPS) requirements
6.3 Testing and Acceptance Standards
- GB/T 6398 (ASTM E92) — Vickers microhardness testing
- GB/T 229 (ASTM E23) — Charpy impact testing
- GB/T 2650 — Bend testing of welds in plates
- GB/T 2649 — Peel testing for overlay/clad welds
- ASTM G99 — Pin-on-disk wear testing
- GB/T 16493 — Non-destructive testing of welds (PT/MT)
- GB/T 11345 — Ultrasonic testing of welds
- ASTM E165 — Magnetic particle testing
6.4 Acceptance Criteria Summary
| Test Item | Acceptance Criterion | Standard |
|---|---|---|
| Visual inspection (VT) | No cracks, pores > 2 mm, undercut > 1 mm, overlap | GB/T 3323 / ISO 17637 |
| Magnetic particle testing (MT) | No linear indications > 5 mm; no indications in overlay | GB/T 15825 / ASTM E709 |
| Surface hardness | ≥ HV 1000 (minimum), average ≥ HV 1200 | GB/T 6398 |
| Hardness traverse | No sharp gradient; gradual transition over ≥ 2 mm | Engineering specification |
| Bend test (longitudinal) | 180° bend, no cracking at overlay/substrate interface | GB/T 2650 |
| Impact test (full-thickness) | ≥ 27 J @ -20°C (mining applications) | GB/T 229 |
| Wear rate (sliding) | ≤ 0.5 mg/N·m (pin-on-disk) | ASTM G99 |
| Overlay thickness | ≥ 3.0 mm nominal (±0.5 mm tolerance) | Product specification |
7. Common Risks and Control Measures
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Cracking in overlay | High carbon equivalent; rapid cooling; hydrogen | Reduced service life; premature failure | Preheat 200–250°C; interpass ≤ 300°C; use low-hydrogen consumables; post-weld bake 200°C/2h |
| Excessive dilution | Deep penetration; excessive travel speed; high current | Reduced hardness; soft overlay | Optimize parameters; use shallow stringer beads; verify dilution by hardness traverse |
| Porosity | Moisture in consumables; inadequate shielding; surface contamination | Reduced mechanical properties; surface defects | Dry consumables at 150–200°C/2h; maintain gas flow ≥ 15 L/min; clean surface thoroughly |
| Delamination at interface | Insufficient heat input; poor surface preparation; high residual stress | Overlay spalling during service | Adequate preheat; proper surface preparation; peening; controlled cooling |
| Excessive brittleness | Over-rapid cooling; coarse carbide network; high C content | Chipping and spalling under impact | Control cooling rate; optimize alloy composition; consider lower-carbon formulation |
| Uneven overlay thickness | Inconsistent welding technique; poor fit-up | Non-uniform wear life; stress concentrations | Welding positioner/automation; consistent travel speed; fixture jigs |
8. Integration with Company Technology Routes
8.1 TIG/MIG Weld Overlay Route (Primary Application)
This entry directly supports the company's TIG/MIG weld overlay technology route. The technical knowledge gained from studying the microstructural characteristics and sliding wear performance of high-chromium hardfacing plates provides the following value:
- WPS development: The understanding of carbide morphology and its relationship to wear performance enables the development of optimized welding procedure specifications for scraper conveyor applications. Each WPS can be tailored to achieve specific carbide distributions based on service conditions.
- Material selection: Knowledge of how Cr, C, Mo, V, and Ni content affects carbide type and matrix structure enables precise consumable selection for different wear regimes (abrasive vs. impact-abrasive vs. high-temperature).
- Quality control: Microstructural knowledge enables development of rapid quality assessment methods — correlating surface hardness, metallographic examination, and wear test results to predict field performance.
- Process optimization: Understanding the relationship between welding parameters (heat input, cooling rate) and resulting microstructure enables optimization of deposition rate, dilution control, and final hardness uniformity.
8.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding is primarily used for metallurgical cladding of dissimilar metals (e.g., copper to steel, stainless to carbon steel), the knowledge from this entry contributes in the following ways:
- Hybrid cladding strategy: For scraper conveyor troughs requiring both corrosion resistance and wear resistance, a hybrid approach can be employed: hydraulic explosive bonding for a stainless steel corrosion-resistant layer, followed by TIG hardfacing overlay for wear resistance on the working surface.
- Substrate preparation: Understanding the microstructural requirements of the overlay substrate (from hardfacing studies) informs the specification of base material properties for explosively bonded clad plates used as hardfacing substrates.
- Interface integrity: The knowledge of overlay-substrate bonding mechanisms (from hardfacing microstructural studies) complements the understanding of explosive bonding interfaces, enabling development of multi-layer composite structures with optimized property gradients.
8.3 Explosion Welding Route (Substrate Fabrication)
Explosion welding (explosive cladding) is applicable for fabricating clad plates where the base material is the scraper conveyor trough steel and the cladding layer provides corrosion or moderate wear resistance:
- Explosively clad substrate: Explosion welding can produce a uniform, full-bonded cladding layer (e.g., Cr-Ni stainless steel or medium-Cr alloy) on the trough plate, which then serves as a substrate for subsequent hardfacing overlay of the high-chromium wear layer.
- Microstructural compatibility: Understanding the microstructural characteristics of high-chromium hardfacing (from this study) ensures that the explosion-welded interface microstructure is compatible with subsequent hardfacing welding — avoiding brittle intermetallic formation at the triple junction (base metal / explosive clad / hardfacing).
- Through-thickness property gradient: Combining explosion welding (for corrosion/structural integrity) with hardfacing overlay (for wear resistance) creates an optimal property gradient through the trough plate thickness, maximizing both durability and wear life.
9. Contribution to Qualification Building, Product Delivery, and Customer Value
9.1 Qualification Building
- WPS qualification support: The microstructural and wear performance data directly support welding procedure qualification under GB/T 19866 and ASME Section IX. Performance-qualified WPS (as opposed to procedure-qualified only) provide stronger customer confidence.
- Material qualification: Hardfacing consumable qualification data (hardness, wear rate, microstructure) supports material certifications for procurement and supply chain management.
- Personnel qualification: Understanding of microstructural characteristics and their relationship to process parameters supports welder qualification programs and technique training.
9.2 Product Delivery
- Performance prediction: The ability to predict sliding wear life from microstructural analysis enables accurate product life-cycle projections for customer procurement planning.
- Customization capability: Knowledge of how alloy composition and process parameters affect wear performance enables customization of hardfacing solutions for specific mining conditions (coal type, moisture content, particle size, conveyor speed).
- Quality assurance: Non-destructive and destructive testing protocols derived from microstructural understanding ensure consistent product quality across production batches.
9.3 Customer Value
- Reduced maintenance costs: High-chromium hardfacing overlay typically extends scraper conveyor trough life by 5–10× compared to unhardened steel, reducing replacement frequency and associated downtime.
- Improved operational continuity: Longer trough life means fewer conveyor shutdowns for trough replacement, directly improving mine production output.
- Technical consulting value: The company's deep understanding of microstructure-property relationships enables value-added technical consulting for customers — optimizing overlay specifications for their specific operating conditions.
- Total cost of ownership reduction: While hardfacing overlay increases initial trough cost by 15–25%, the extended service life typically results in 40–60% reduction in total cost of ownership over the trough's operational life.
10. Summary and Recommendations
The study of microstructural characteristics and sliding wear performance of high-chromium hardfacing plates for scraper conveyor middle troughs represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. The key takeaways and actionable recommendations are:
- Establish a microstructure-property database: Systematically correlate welding parameters, consumable chemistry, cooling conditions, and resulting microstructure with measured wear performance. This database becomes the foundation for all WPS development and product engineering.
- Develop standardized overlay specifications: Create tiered product specifications (Standard, Enhanced, Premium) corresponding to different service severity levels, each backed by qualified WPS and validated wear performance data.
- Invest in characterization capabilities: Equip the laboratory with optical microscopy, SEM/EDS, XRD, and microhardness testing capabilities to support ongoing microstructural analysis and quality control.
- Implement field monitoring programs: Establish systematic wear monitoring of installed troughs to validate predicted performance and continuously refine the microstructure-property correlations.
- Develop multi-layer composite solutions: Leverage knowledge across all three technology routes (TIG/MIG overlay, hydraulic explosive bonding, explosion welding) to develop innovative multi-layer solutions combining wear resistance, corrosion resistance, and structural integrity.
Key Insight: The transition from empirical hardfacing practice to microstructure-guided process engineering represents the fundamental differentiator in competitive positioning. Companies that can predict and control overlay microstructure — and thereby predict and guarantee wear performance — command premium positioning in the mining and bulk materials handling markets.