Microstructure and Wear Resistance Analysis of Weld Overlay Electrodes for Centrifugal Fan Applications
1. Definition and Fundamental Principles
1.1 Technical Definition
The research on microstructure and wear resistance of weld overlay electrodes for centrifugal fans constitutes a metallurgical engineering discipline focused on characterizing the phase composition, grain morphology, carbide distribution, hardness profile, and tribological performance of deposited overlay layers produced by arc welding processes. Centrifugal fans operating in abrasive, erosive, or corrosive environments—such as coal handling, cement grinding, mineral processing, and flue gas transport—suffer from accelerated blade and housing degradation due to particulate erosion, impingement wear, and chemical attack. Weld overlay electrodes are purpose-formulated consumables designed to deposit hardfacing or corrosion-resistant alloys onto base fan components to extend service life.
1.2 Metallurgical Principles
The wear resistance of a weld overlay deposit is governed by the following metallurgical mechanisms:
- Carbide hardening phase: Chromium carbides (Cr₇C₃, Cr₃C), titanium carbides (TiC), and molybdenum carbides (Mo₂C) form during solidification and subsequent heat treatment. Their volume fraction, size, morphology, and distribution density directly correlate with abrasion resistance.
- Matrix microstructure: The binder matrix—typically martensitic, austenitic, or ferritic—provides the load-bearing framework that supports the dispersed hard phases. Martensitic matrices offer high hardness but may crack under thermal cycling; austenitic matrices provide toughness at the expense of peak hardness.
- Grain refinement: Fine grain structures increase grain boundary area, impeding crack propagation and improving both wear and fatigue resistance.
- Dilution control: The interaction between the deposited alloy and the base metal (typically carbon steel or low-alloy steel) introduces dilution that modifies the final chemistry and microstructure of the overlay. Managing dilution through preheating, electrode composition design, and multi-pass techniques is critical.
2. Category and Business Positioning
2.1 Positioning Within Company Capability Framework
This research entry falls under the company's Weld Overlay Technology division, specifically within the TIG/MIG weld overlay route. It represents a fundamental materials science and process engineering capability that underpins the company's ability to select, qualify, and deploy appropriate overlay consumables for industrial fan repair and protection programs.
2.2 Strategic Value in the Company Portfolio
Understanding electrode microstructure and wear resistance enables the company to:
- Recommend optimal electrode selections tailored to specific fan operating conditions (temperature, particle size, velocity, corrosivity)
- Qualify WPS (Welding Procedure Specifications) with documented performance data
- Provide customers with evidence-based technical justification for overlay specifications
- Reduce warranty claims by matching overlay chemistry to service environment
3. Technical Purpose and Value
3.1 Primary Objectives
The research addresses the following technical objectives:
- Microstructure characterization: Identification of phases present (martensite, austenite, carbides, retained phases) through optical microscopy, SEM/EDS, and X-ray diffraction analysis.
- Hardness mapping: Determination of hardness distribution from the fusion line through the overlay surface, identifying dilution zones and ensuring uniform hardness across the deposit.
- Wear testing: Quantification of abrasion resistance using standardized test methods (pin-on-disk, dry sand abrasion, or industry-relevant tribological simulators).
- Correlation development: Establishing relationships between electrode composition, welding parameters, microstructure, and measured wear life to enable predictive overlay selection.
3.2 Deliverable Value
The output of this research directly contributes to:
- Development of qualified overlay material databases for centrifugal fan applications
- Technical proposals with quantified service life extensions (typically 3–10× compared to unprotected base material)
- WPS qualification packages supported by metallurgical evidence
- Customer training materials and technical consultation capability
4. Key Process and Implementation Points
4.1 Electrode Selection Criteria for Centrifugal Fan Overlay
| Overlay Type | Typical Composition | Hardness (HV) | Wear Mechanism Addressed | Temperature Limit |
|---|---|---|---|---|
| High-Chromium Carbide (Type A) | Cr 25–35%, C 2.5–4.0%, Mo 3–6% | 800–1100 | Abrasive wear (mineral, coal) | ≤350°C |
| High-Chromium Carbide (Type B) | Cr 28–36%, C 3.0–5.0%, W 5–10% | 900–1200 | Severe abrasion, erosion | ≤300°C |
| Stellite-type (Co-Cr) | Co 55–65%, Cr 20–28%, W 5–10% | 400–550 (as-cast) | Erosion-corrosion, high temp | ≤650°C |
| Austenitic Ni-Cr | Cr 20–25%, Ni 18–25% | 200–300 | Corrosive environments | ≤500°C |
| Transition layer (309L-type) | Cr 22–25%, Ni 12–15% | 180–220 | Dilution control, crack prevention | ≤600°C |
4.2 Welding Process Parameters
For centrifugal fan blade and housing overlay applications using SMAW (Shielded Metal Arc Welding) or GTAW (Tungsten Inert Gas Welding), the following parameters are typically employed:
| Parameter | SMAW Hardfacing | GTAW Hardfacing | Notes |
|---|---|---|---|
| Deposition rate | 0.5–1.5 kg/h | 0.15–0.40 kg/h | Higher rate for housing, lower for blades |
| Travel speed | 30–80 mm/min | 10–40 mm/min | Slower for uniform carbide distribution |
| Weld current (SMAW) | 150–300 A | — | Depends on electrode diameter (3.2–5.0 mm) |
| Weld current (GTAW) | — | 80–200 A | DCEN polarity for hardfacing |
| Argon flow (GTAW) | — | 15–25 L/min | Shielding + trailing gas for post-arc protection |
| Preheat temperature | 100–250°C | 100–200°C | Reduces cracking tendency in high-carbon deposits |
| Interpass temperature | ≤250°C | ≤200°C | Controls grain growth and carbide coarsening |
| Overlay thickness per pass | 2–4 mm | 1–3 mm | Multi-pass for total thickness 3–10 mm |
| Minimum total overlay thickness | ≥3 mm (abrasive); ≥2 mm (corrosive) | Ensures dilution zone is fully covered | |
4.3 Microstructural Characterization Methods
- Optical Microscopy (OM): Macrostructure evaluation, grain size measurement, crack detection at 50×–500× magnification
- Scanning Electron Microscopy (SEM) with EDS: Phase identification, carbide morphology, elemental mapping across fusion line
- X-ray Diffraction (XRD): Quantitative phase analysis (martensite, austenite, carbide percentages)
- Vickers Hardness Testing: HV0.3 or HV1 indentations along a traverse from fusion line to surface
- Rockwell Hardness (HRC): Surface hardness verification for as-deposited condition
- SEM fractography: Wear surface analysis after tribological testing to identify dominant wear mechanism
4.4 Wear Testing Protocols
Wear resistance is typically evaluated using the following standardized approaches:
- Dry sand abrasion test (GB/T 16497): Standardized sand abrasion using 220# SiC abrasive paper or ASTM G65 equivalent
- Pin-on-disk test: Al₂O₃ or SiC counterface against rotating overlay specimen under controlled load
- Impingement erosion test: Particle impact at specified velocity (30–60 m/s) and angle (30°–90°) simulating fan blade erosion
- Service life correlation: Comparison of lab wear data with field performance data from actual fan components
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Relevance to Fan Overlay |
|---|---|---|
| GB/T 13814 | Welding consumables—Classification of welding electrodes for hardfacing | Electrode classification and specification requirements |
| GB/T 12469 | Welding consumables—Cast hardfacing alloys for arc welding | Cast overlay material specifications |
| ASTM A526 | Standard Specification for Cast High-Chromium-Iron Hardfacing Alloys | Chemistry and performance requirements for Cr-based overlays |
| ASTM A540 | Standard Specification for High-Chromium-Iron Hardfacing Alloys for Welding Electrodes | Electrode qualification and chemical composition |
| ASTM A220 | Standard Specification for Cast Hardfacing Alloys for Welding Electrodes | General hardfacing electrode requirements |
| ASME Section IX | Welding, Brazing, and Fusing Qualifications | WPS/PQR qualification framework |
| NB/T 47014 | Qualification of Welding Procedures and Welders for Pressure Vessel Welding | WPS qualification for pressure-containing fan housings |
| GB/T 16497 | Wear testing—Determination of wear resistance of materials | Standardized wear test methodology |
| GB/T 6393 | Welding consumables—Chemical analysis methods | Electrode composition verification |
| API 16C | Standard for Hardfacing and Overlay for Industrial Equipment | Industrial overlay qualification and acceptance |
| NACE SP0388 | Recommended Practice for Corrosion Control in Refractory Heat-Resisting Steel Systems | High-temperature overlay applications |
5.2 Acceptance Criteria
- Hardness: Overlay surface hardness must meet or exceed specified minimum (e.g., ≥800 HV for high-chromium carbide Type A deposits), measured at ≥1 mm from surface
- Dilution: Dilution at the fusion line shall not exceed 25–30% for high-carbon hardfacing deposits; transition layer must be applied where base metal is austenitic or high-alloy
- Crack-free: No transverse or longitudinal cracks exceeding 1.5 mm length in the overlay deposit (per GB/T 11345 visual inspection criteria)
- Porosity: Porosity level not exceeding Level 2 per GB/T 3323 (radiographic) or Level 1 per GB/T 11345 (visual)
- Overlay thickness: Minimum specified thickness achieved across all weld passes with tolerance ±0.5 mm
- Wear rate: Lab-measured wear rate must demonstrate ≥3× improvement over unprotected base material under relevant test conditions
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in high-carbon deposits | Last-dripping eutectic phases, high sulfur/phosphorus | Control interpass temperature ≤250°C; use low-S/P electrodes; apply appropriate preheat |
| Excessive dilution reducing hardness | High heat input, thin first pass, ferrous base metal | Reduce heat input; apply transition layer (309L); use multiple thin passes; control travel speed |
| Carbide coarsening | High interpass temperature, slow cooling, excessive post-weld heat treatment | Maintain interpass ≤200°C; avoid unnecessary post-weld heating; consider controlled cooling |
| Residual stress cracking | Mismatch between overlay and base metal thermal expansion; high carbon content | Apply post-weld stress relief at 450–550°C for 1–2 hours per 25 mm thickness; use multi-pass technique |
| Graphitization in as-cast condition | High carbon + slow cooling rates | Use high-current, fast-travel parameters; apply rapid cooling between passes |
6.2 Process Risks
- Incomplete fusion at fusion line: Ensure adequate root preparation (V-groove or bevel); verify weld current and travel speed; perform visual and UT inspection
- Contamination of deposit: Clean base metal of rust, paint, oil, and scale prior to welding; use fresh electrode coating; maintain proper shielding gas purity (>99.95% Ar for GTAW)
- Weld spatter and slag inclusion: Remove slag between passes; control arc length; use appropriate electrode angle (5–15° from vertical)
- Geometric distortion of thin fan blades: Use balanced welding sequence; apply clamping fixtures; limit total heat input; consider back-up plates for thermal mass
6.3 Quality Assurance Controls
- Pre-weld: Verify electrode lot qualification certificates; confirm base metal chemistry; inspect surface preparation
- In-process: Monitor welding parameters; maintain interpass temperature records; perform periodic hardness spot checks
- Post-weld: Conduct full visual inspection; perform MT or PT for surface crack detection; UT or RT for subsurface defects; hardness survey at defined intervals
- Documentation: Maintain complete WPS/PQR records; compile microstructure and wear test reports; archive in company qualification database
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This research directly supports the company's TIG/MIG weld overlay capability for centrifugal fan protection. Key applications include:
- Fan blade leading edges: Overlay of high-chromium carbide deposits using GTAW with consumable insert technique for erosion protection at impingement zones
- Fan housing wear plates: MIG overlay of thick deposits (6–12 mm) on housing interiors exposed to circulating abrasive media
- Diffuser vanes: Precision TIG overlay of transition layer + hardfacing for combined corrosion and wear protection
- Inlet cones and scroll sections: Multi-pass overlay with controlled dilution management
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While the research focuses on weld overlay, the understanding of microstructure and wear resistance informs material selection for hydraulic explosive bonding applications where clad fan components require:
- Corrosion-resistant cladding (316L, duplex 2205) on carbon steel fan housings where overlay would be too thick or impractical
- Selection of base/clad material pairs based on wear and corrosion performance data developed through overlay research
- Post-bonding machining considerations informed by knowledge of hardened surface layers
7.3 Explosion Welding Route (Material Selection Input)
The metallurgical knowledge gained from overlay research contributes to explosion welding material selection by:
- Providing comparative data on wear performance of different alloy systems (Cr-based, Co-based, Ni-based) that may be considered as clad materials
- Informing qualification of post-explosion-welding surface treatments where localized overlay repair may be required
- Establishing baseline wear resistance benchmarks against which bonded clad performance is evaluated
8. Qualification Building and Customer Value
8.1 Qualification Framework Contribution
This research entry strengthens the company's qualification portfolio by:
- Generating documented PQR (Procedure Qualification Records) for multiple overlay systems on centrifugal fan base materials (Q235, 20#, 16Mn, 15CrMo)
- Establishing qualified electrode databases with verified microstructure and wear performance data
- Supporting ASME Section IX and NB/T 47014 WPS qualification packages with metallurgical evidence
- Building IP and technical differentiation through proprietary wear performance databases
8.2 Customer Value Proposition
- Evidence-based recommendations: Customers receive overlay specifications backed by laboratory-verified wear performance data rather than generic product catalog selections
- Reduced total cost of ownership: Optimized overlay selection extends fan component life by 3–10×, reducing unplanned shutdowns and replacement costs
- Technical partnership: The company positions itself as a metallurgical engineering partner rather than a simple welding contractor
- Warranty confidence: Documented qualification data reduces warranty risk and supports performance guarantees
8.3 Product Delivery Impact
The research findings translate directly into operational improvements:
- Standardized overlay selection matrices reducing engineering time per project
- Pre-qualified WPS packages accelerating project mobilization
- Reduced rework rates through improved understanding of failure mechanisms
- Enhanced capability to handle complex fan overlay projects requiring multi-material, multi-zone specifications
9. Conclusion
The systematic research on microstructure and wear resistance of weld overlay electrodes for centrifugal fans represents a foundational metallurgical engineering capability that underpins the company's entire weld overlay service line. By establishing rigorous correlations between electrode composition, welding parameters, microstructural features, and measured wear performance, the company can deliver technically superior overlay solutions, accelerate qualification timelines, and provide customers with quantifiable service life improvements. This capability directly feeds into WPS qualification packages, supports all three technology routes through material selection intelligence, and establishes the company as a technically differentiated provider in the industrial fan protection market.