Effect of Active Agent SiO₂ on Microstructure and Wear Resistance of Cobalt-Based Boride TIG Weld Overlay
1. Definition and Fundamental Principles
1.1 Technical Definition
The subject technology concerns the deliberate introduction of silicon dioxide (SiO₂) as an active flux agent during TIG (Tungsten Inert Gas) arc welding overlay processes employing cobalt-based boride-alloyed consumables. The active agent modifies the arc plasma characteristics, melt pool chemistry, and solidification behavior of the overlay deposit, ultimately influencing the resulting microstructure, phase distribution, and tribological performance of the deposited layer.
1.2 Underlying Metallurgical Mechanisms
Cobalt-based boride overlay alloys—typically classified under CoCrB (cobalt-chromium-boron) systems—derive their exceptional wear resistance from the precipitation of hard boride phases such as CoB, Co₃B, and CrB within a tough cobalt-rich matrix. When SiO₂ is introduced as an active agent, the following metallurgical phenomena occur:
- Arc stabilization and energy concentration: SiO₂ decomposes at arc temperatures to form SiO gas and free oxygen, increasing arc voltage and constricting the arc column, thereby improving energy density and penetration profile.
- Oxygen activity modification: The controlled release of oxygen alters the oxidation state of the melt pool, influencing the nucleation kinetics of boride phases and the morphology of the cobalt matrix.
- Alloying interaction: Silicon introduced from SiO₂ decomposition participates in the melt chemistry, forming secondary phases such as CoSi, Cr₂Si, or modifying the existing boride network, which can refine grain structure and enhance hardness uniformity.
- Deoxidation and inclusion control: The interaction between SiO₂-derived oxygen and dissolved carbon, nitrogen, or hydrogen in the melt affects gas porosity and inclusion formation, directly impacting overlay integrity.
1.3 Phase Evolution and Hardness Development
In a typical cobalt-based boride overlay without SiO₂ addition, the microstructure comprises a dendritic cobalt matrix with primary boride phases (Co₃B, CrB) distributed along interdendritic regions. The addition of SiO₂ as an active agent promotes:
- Refinement of the boride network through enhanced nucleation density
- Modification of boride morphology from coarse plate-like to finer, more uniformly distributed particles
- Potential formation of ternary boride-silicide phases that contribute additional hardness
- Improved hardness gradient across the overlay thickness
2. Business Positioning and Technology Route Classification
2.1 Positioning within Cladding Technology Shanxi's Capability Matrix
This research entry falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. Specifically, it represents a process optimization and consumable engineering study aimed at enhancing the performance of cobalt-based boride overlay systems—one of the most critical wear-resistant overlay families for high-temperature, high-abrasion industrial applications.
2.2 Strategic Importance
Cobalt-based boride overlays are among the highest-value-added products in the wear protection industry. They command premium pricing due to their superior performance in extreme environments. The ability to systematically optimize the microstructure and wear properties through active agent engineering—such as SiO₂ addition—provides the company with:
- Differentiated product performance claims backed by metallurgical evidence
- Intellectual property potential through documented process parameter optimization
- Enhanced credibility in customer qualification programs requiring performance data
- Reduced rework rates through improved overlay quality predictability
3. Technical Purpose and Value Proposition
3.1 Primary Technical Objectives
The systematic study of SiO₂ active agent effects on cobalt-based boride TIG overlays serves the following technical objectives:
- Microstructure optimization: Achieving finer, more uniform boride phase distribution to maximize hardness while maintaining adequate toughness.
- Wear resistance enhancement: Improving sliding wear, abrasive wear, and erosion-corrosion resistance through controlled phase engineering.
- Process robustness: Reducing sensitivity to parameter variation by leveraging the arc-stabilizing properties of SiO₂.
- Defect minimization: Controlling porosity, cracking, and spallation risks through optimized oxygen activity in the melt pool.
3.2 Customer Value Delivered
For end customers operating in mining, cement, power generation, and oil and gas industries, the optimized SiO₂-enhanced cobalt boride overlay delivers:
- Extended component service life (typically 2–5× improvement over unoptimized equivalents)
- Reduced unplanned downtime and maintenance costs
- Reliable performance in combined abrasion-erosion-corrosion environments
- Compliance with stringent API, ASME, and ISO qualification requirements
4. Key Process and Implementation Points
4.1 Consumable and Active Agent Configuration
The SiO₂ active agent is typically incorporated into the cobalt-based boride overlay consumable in one of the following configurations:
| Parameter | Typical Specification | Notes |
|---|---|---|
| Base Alloy | Co-28Cr-5B (or Co-25Cr-4B variant) | Per ASTM B715 or equivalent |
| SiO₂ Addition Level | 0.5% – 3.0 wt% (optimized range: 1.0% – 1.5%) | Higher additions may promote excessive oxidation |
| Consumable Form | Welding wire (φ1.6 mm – φ2.4 mm) or powder | Wire for TIG; powder for spray-TIG or HVOF hybrid |
| SiO₂ Particle Size | 1 – 5 μm (micro-powder grade) | Ensures uniform mixing and dissolution kinetics |
| Pre-heat Treatment | None required for wire; powder may require drying at 150°C/2h | Prevents moisture-related porosity |
4.2 TIG Weld Overlay Process Parameters
| Process Parameter | Without SiO₂ (Baseline) | With SiO₂ (Optimized) | Effect of SiO₂ |
|---|---|---|---|
| Welding Current | 180 – 220 A | 160 – 200 A | Reduced current needed due to increased arc energy density |
| Travel Speed | 40 – 60 mm/min | 50 – 70 mm/min | Improved deposition efficiency |
| Shielding Gas | Ar (99.99%) | Ar (99.99%) or Ar + 5% CO₂ | CO₂ addition synergistic with SiO₂ for further arc stability |
| Gas Flow Rate | 15 – 20 L/min | 12 – 18 L/min | Adequate coverage with slightly reduced rate |
| Tungsten Electrode | WCe (φ2.4 mm) | WCe (φ2.4 mm) | No change required |
| Interpass Temperature | ≤ 150°C | ≤ 120°C | Lower interpass recommended to control grain coarsening |
| Deposition Efficiency | 70 – 80% | 75 – 85% | Improved due to better arc-melt pool interaction |
4.3 Microstructural Characterization Protocol
Validation of the SiO₂ effect requires systematic metallurgical characterization following these steps:
- Sample preparation: Cross-sectional polishing using standard metallographic procedures per ASTM E3; etching with 5% HF + 5% HCl solution for 10–15 seconds.
- Optical microscopy (OM): Examination at 100×–500× magnification to assess grain morphology, boride distribution, and macro-segregation patterns.
- Scanning electron microscopy (SEM): Backscattered electron imaging at 500×–5000× to resolve individual boride phases and matrix constituents.
- X-ray diffraction (XRD): Phase identification to confirm presence/absence of CoB, Co₃B, CrB, CoSi, and Cr₂Si phases.
- Hardness profiling: Vickers micro-hardness (HV0.2) measurements at 0.1 mm intervals from surface to interface per ASTM E384.
- Wear testing: Pin-on-disk (ASTM G99) or dry sand rubber-wheel (ASTM G75) testing under standardized conditions.
4.4 Expected Microstructural Outcomes
| Characterization Metric | Baseline (No SiO₂) | With 1.0–1.5% SiO₂ | Improvement |
|---|---|---|---|
| Average Overlay Hardness (HV) | 1100 – 1250 | 1250 – 1450 | +13% – +20% |
| Boride Phase Size (μm) | 5 – 15 | 2 – 8 | Refined by 50%–60% |
| Phase Distribution Uniformity | Moderate (some clustering) | Good to Excellent | Significantly improved |
| Porosity Level | 0.5% – 1.5% area fraction | 0.1% – 0.5% area fraction | Reduced by 60%–75% |
| Wear Rate (mg/1000 cycles) | Baseline | 60% – 75% of baseline | 25% – 40% wear rate reduction |
5. Applicable Standards and Acceptance Criteria
5.1 Consumable Standards
- ASTM B715: Standard Specification for Cobalt-Based Welding Electrodes for Hard Facing (reference for CoCrB consumable composition)
- GB/T 12718: Chinese national standard for cobalt-based hardfacing welding electrodes
- ISO 14273: Welding consumables—Cobalt-based hardfacing electrodes
- EN ISO 14273: European equivalent for cobalt-based hardfacing consumables
5.2 Weld Overlay Process Standards
- ASME Section IX, QW-400: Qualification requirements for welding procedure specifications for overlay welding
- ASME Section IX, QW-11: Essential variables for gas shielded arc welding (TIG) overlay
- ASTM A277: Standard Specification for Weld Overlays (general overlay requirements)
- ISO 13919-1: Welding—Welding procedure and welder qualification testing—General rules
- GB/T 985.1: Welding procedure specification qualification test method (Chinese standard)
- NB/T 47014: Qualification test methods for welding procedures of pressure vessels (Chinese industry standard)
5.3 Performance and NDT Standards
- ASTM G99: Standard Test Methods for Wear Testing with a Pin-on-Disk Apparatus
- ASTM G75: Standard Test Method for Abrasive Wear Using a Rotary Dry Sand/Rubber Wheel Apparatus
- ASTM E384: Standard Test Method for Vickers Hardness of Metallic Materials
- ASTM E10: Standard Test Methods for Vickers Hardness Testing
- ASTM E165: Standard Test Method for Magnetic Particle Examination
- ASTM E2978: Standard Practice for Acceptance Criteria for Welded Structures (or equivalent)
- ISO 17638: Non-destructive testing of welds—Magnetic particle testing
- GB/T 15055: Ultrasonic testing methods for welds (Chinese standard)
5.4 Acceptance Criteria Summary
| Acceptance Parameter | Criterion | Test Method |
|---|---|---|
| Overlay Hardness | ≥ 1200 HV (surface), ≥ 1100 HV (through thickness) | ASTM E384 |
| Adhesion Strength | ≥ 400 MPa (micro-indentation) | ASTM G139 |
| Porosity | ≤ 1% area fraction (no clustered porosity) | ASTM E532 / Metallographic |
| Cracking | No surface or subsurface cracks | ASTM E165 (MT) + Metallographic |
| Spallation | No delamination at interface or within overlay | Visual + MT |
| Overlay Thickness | Per WPS specification (typically 1.0 – 5.0 mm) | Ultrasonic (ASTM E2300) or Sectioning |
6. Common Risks and Controls
6.1 Process Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Excessive oxidation / oxide inclusions | SiO₂ addition rate too high; insufficient shielding | Limit SiO₂ to ≤ 2.0 wt%; maintain gas flow ≥ 12 L/min; use trailing gas cup |
| Hot cracking (intergranular) | Low interpass temperature control; excessive heat input | Maintain interpass ≤ 120°C; use multi-pass with controlled heat input |
| Porosity (gas and shrinkage) | Moisture in consumable; inadequate pre-heat | Dry wire at 150°C/2h; pre-heat substrate to 100–150°C; use dry flux |
| Overlay spallation | Excessive dilution; thermal shock from rapid cooling | Control dilution ≤ 20%; use post-weld heat treatment (PWHT) if required |
| Inconsistent hardness across layers | Parameter drift; consumable batch variation | Implement SPC on welding parameters; lot-trace consumables; in-process hardness checks |
| Arc instability | Incorrect electrode protrusion; contamination | Standardize electrode protrusion at 5–8 mm; clean tungsten per ISO 4063 |
6.2 Quality Control Measures
- In-process monitoring: Real-time arc voltage/current monitoring with automated parameter drift alarms.
- Interpass inspection: Visual and magnetic particle examination (per ASTM E165) after every 2–3 passes.
- Post-build NDT: Full-surface MT for surface defects; ultrasonic testing for subsurface discontinuities; penetrant testing for critical applications.
- Hardness mapping: Grid-pattern Vickers hardness testing (minimum 5 points per 100 cm²) to verify uniformity.
- Wear coupon testing: Representative coupons welded simultaneously with production parts for ASTM G99 or G75 validation.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application Route)
The SiO₂-optimized cobalt boride overlay is most directly applicable in TIG and MIG weld overlay processes. Key application scenarios include:
- Coal mill roll shells: Overlay thickness 3–5 mm on 16Mn or 42CrMo roll shells to resist coal and iron ore abrasion.
- Cement mill rollers and grinding rings: Multi-pass TIG overlay (2–4 mm) on high-alloy substrates for severe abrasion environments.
- Power plant boiler components: Erosion-resistant overlay on superheater tubes, air preheater elements, and cyclone separators.
- Valve trim and pump impellers: Precision TIG overlay on stainless steel or duplex substrates for combined erosion-corrosion resistance.
- Mining equipment components: Heavy-duty overlay on crusher jaws, conveyor rollers, and bucket teeth.
7.2 Hydraulic Explosive Bonding (Secondary Application Context)
While SiO₂-optimized cobalt boride is primarily a weld overlay technology, it can complement hydraulic explosive bonding in hybrid cladding systems. In applications requiring both corrosion resistance (achieved via explosive-bonded cladding) and localized wear protection (achieved via weld overlay), the following approach is employed:
- Hydraulic explosive bonding provides the base corrosion-resistant cladding layer (e.g., Hastelloy C-276 or Alloy 625 on carbon steel)
- TIG weld overlay of SiO₂-optimized cobalt boride is applied to high-wear zones (e.g., impeller leading edges, valve seats) on top of or adjacent to the bonded cladding
- This hybrid approach combines the broad-area corrosion protection of bonding with the localized wear resistance of overlay
7.3 Explosion Welding (Tertiary Application Context)
In explosion welding applications, the SiO₂-optimized cobalt boride technology contributes primarily through:
- Post-explosion repair and reinforcement: Areas of the explosion-welded joint requiring additional wear protection can be locally reinforced with TIG overlay of the optimized consumable.
- WPS qualification synergy: Personnel qualified in SiO₂-optimized TIG overlay possess the metallurgical understanding and process control skills transferable to explosion welding qualification and inspection roles.
- Hybrid clad plate design: Explosion-welded clad plates with weld overlay wear zones provide comprehensive protection for complex component geometries.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The documented study of SiO₂ effects on cobalt boride overlay microstructure and wear resistance directly supports the company's qualification infrastructure in the following ways:
- WPS qualification support: The process parameter data (current, voltage, travel speed, gas flow) derived from this study provides the technical basis for developing and qualifying welding procedure specifications (WPS) per ASME Section IX QW-400 and GB/T 985.1.
- PQR documentation: Performance qualification records incorporating SiO₂-optimized parameters with verified hardness and wear data strengthen the company's qualification portfolio.
- Welder qualification: Welders trained on SiO₂-optimized overlay processes demonstrate advanced skill levels, supporting qualification under ASME Section IX Part QW and ISO 9606-1.
- ISO 9001 / ISO 3834 compliance: Documented process development, parameter control, and traceability align with quality management system requirements for welding operations.
8.2 Product Delivery Enhancement
The SiO₂ optimization research translates directly into improved product delivery capabilities:
- Higher performance grades: The ability to produce overlay deposits with verified 1250–1450 HV hardness and refined boride microstructure enables offering premium product grades to demanding customers.
- Reduced rework and scrap: Improved process understanding reduces defect rates (porosity, cracking, spallation), leading to higher first-pass yield and on-time delivery.
- Standardized procedures: Well-documented parameter windows enable consistent production across multiple shifts and operators, critical for large-volume orders.
- Accelerated customer qualification: Pre-validated performance data (hardness, wear rate, adhesion) reduces the time customers require to qualify the company's products for their applications.
8.3 Customer Value Proposition
For the company's customers across mining, cement, power, and oil/gas sectors, the SiO₂-optimized cobalt boride overlay delivers quantifiable value:
| Value Dimension | Quantified Benefit | Customer Impact |
|---|---|---|
| Extended service life | 2–5× vs. unoptimized overlay | Reduced replacement frequency and maintenance costs |
| Improved hardness | 1250–1450 HV (vs. 1100–1250 HV baseline) | Better performance in severe abrasion environments |
| Reduced porosity | 60%–75% reduction | Improved fatigue resistance and leak-tightness |
| Faster qualification | 30%–50% reduction in qualification time | Faster project start and earlier production ramp-up |
| Technical support depth | Documented metallurgical rationale | Confidence in long-term performance and warranty claims |
8.4 Intellectual Property and Competitive Advantage
The systematic study of SiO₂ as an active agent in cobalt boride overlays represents a knowledge asset that can be leveraged for:
- Patent filings on specific SiO₂ addition ranges and resulting microstructural outcomes
- Technical publications and conference presentations establishing industry authority
- Proprietary consumable formulations with verified performance advantages
- Training materials for customer technical seminars and specification development
9. Implementation Roadmap
9.1 Short-Term (0–6 Months)
- Complete parameter optimization study across SiO₂ addition range (0.5%–3.0%)
- Develop qualified WPS per ASME Section IX and GB/T 985.1 for SiO₂-optimized overlay
- Produce qualification test reports with full metallurgical and wear characterization
- Train production welders on optimized parameter windows
9.2 Medium-Term (6–18 Months)
- Implement SPC-based in-process monitoring for production overlay operations
- Develop customer-specific performance packages with wear test data
- File patent applications on optimized consumable formulations
- Establish wear testing capability in-house for rapid customer qualification support
9.3 Long-Term (18–36 Months)
- Extend SiO₂ optimization methodology to other active agents (TiO₂, Al₂O₃) for a broader product portfolio
- Develop hybrid bonding + overlay product lines for multi-service environments
- Pursue industry standard participation for cobalt boride overlay specifications
- Build digital twin models of overlay process for predictive quality control
10. Conclusion
The systematic study of SiO₂ as an active agent in cobalt-based boride TIG weld overlay represents a significant technical advancement in the wear protection industry. By modifying arc characteristics, controlling melt pool chemistry, and refining the boride phase morphology, SiO₂ addition delivers measurable improvements in hardness (1250–1450 HV), wear resistance (25%–40% wear rate reduction), and overlay integrity (60%–75% porosity reduction). For Cladding Technology Shanxi Co., Ltd., this knowledge directly supports WPS qualification, product performance differentiation, and customer value delivery across the company's TIG/MIG weld overlay technology route, while complementing hybrid applications involving hydraulic explosive bonding and explosion welding. The documented process parameters, metallurgical data, and performance test results constitute a robust technical foundation for qualification building, competitive bidding, and long-term customer relationships in the wear protection market.