Effect of Active Agent SiO₂ on Microstructure and Wear Resistance of Cobalt-Based Boride TIG Weld Overlay Layers
1. Definition and Fundamental Principles
1.1 Active Agent SiO₂ in TIG Weld Overlay
Silicon dioxide (SiO₂) is a thermodynamically stable oxide with a melting point of approximately 1,713 °C. When introduced as an active agent into the molten pool of a cobalt-based boride overlay deposited via Gas Tungsten Arc Welding (GTAW/TIG), SiO₂ participates in a series of metallurgical reactions that fundamentally alter the composition, microstructure, and mechanical properties of the resulting cladding layer. The active agent is typically introduced in powder form, applied as a pre-placed layer on the substrate surface prior to arc ignition, or fed into the weld pool as a consumable powder during the overlay process.
1.2 Metallurgical Reaction Mechanism
The primary mechanism by which SiO₂ influences cobalt-based boride overlay microstructure involves the following reactions within the high-temperature weld pool (typically 1,400–1,600 °C):
- Oxidation-reduction reactions: SiO₂ reacts with dissolved boron and carbon in the cobalt matrix, promoting the formation of fine boride and carbide precipitates. The reaction Co₃B + SiO₂ → Co₃B₂O₇ + Si (simplified) leads to redistribution of boron species and creation of oxide-boride composite phases.
- Nucleation enhancement: SiO₂ particles act as heterogeneous nucleation sites, reducing the grain size of the cobalt solid solution matrix and promoting a finer, more uniform microstructure. This grain refinement directly contributes to improved hardness and wear resistance.
- Modification of phase distribution: The presence of SiO₂ alters the relative proportions of Co₃B, Co₂B, and metallic boride phases, shifting the phase balance toward harder, more wear-resistant configurations.
- Modification of cooling kinetics: SiO₂ particles increase the thermal resistance of the molten pool, locally slowing solidification rates and influencing dendrite arm spacing and secondary phase morphology.
1.3 Typical Cobalt-Based Boride Overlay Compositions
| Component | Typical Range (wt%) | Role in Overlay |
|---|---|---|
| Co (Cobalt) | Balance (60–75%) | Base matrix; provides corrosion resistance and high-temperature strength |
| B (Boron) | 5–12% | Forms Co₃B and Co₂B hard phases; primary wear-resistance contributor |
| C (Carbon) | 2–5% | Forms Co₃C and Co₇W₆C₆ carbides; enhances hardness |
| W (Tungsten) | 5–15% | Forms hard carbides; improves hot hardness |
| Cr (Chromium) | 10–20% | Improves oxidation and corrosion resistance |
| SiO₂ (Active Agent) | 1–5% (added) | Modifies microstructure; refines grains; adjusts phase balance |
2. Category and Business Positioning
2.1 Positioning Within Cladding Technology Portfolio
This technical entry belongs to the research and development (R&D) optimization category within the company's TIG/MIG weld overlay technology route. It represents a fundamental materials science investigation aimed at enhancing the performance of existing cobalt-based boride overlay products. The study of SiO₂ as an active agent falls under the broader umbrella of microstructure engineering—a systematic approach to tailoring overlay properties through controlled modification of the solidification environment.
2.2 Strategic Importance
In the competitive landscape of surface engineering and cladding services, the ability to fine-tune overlay properties through active agent addition provides significant differentiation. This research directly supports:
- Product performance enhancement: Achieving higher hardness (HV30–HV1000), improved wear life, and better microstructural uniformity in cobalt-based boride overlays.
- WPS qualification expansion: Developing and qualifying new Welding Procedure Specifications that incorporate active agent technology, expanding the company's certified capability envelope.
- Customer value proposition: Offering customers overlays with demonstrably superior wear resistance and service life compared to conventional cobalt boride deposits.
- Intellectual property development: Generating patentable process innovations around active agent addition techniques and optimized parameter combinations.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The investigation of SiO₂ as an active agent in cobalt-based boride TIG weld overlay serves the following specific technical objectives:
- Microstructure refinement: Reduce grain size and dendrite arm spacing in the cobalt solid solution matrix through heterogeneous nucleation, thereby improving toughness and reducing crack susceptibility.
- Phase composition optimization: Shift the Co₃B/Co₂B ratio toward Co₂B (harder, more wear-resistant) through controlled boron redistribution induced by SiO₂ reactions.
- Wear resistance improvement: Achieve measurable increases in sliding wear resistance, abrasion resistance, and erosion resistance through the combined effects of grain refinement and phase modification.
- Residual stress management: Evaluate whether SiO₂ addition affects thermal gradients and solidification stresses, potentially reducing residual stress levels in the overlay.
- Crack resistance enhancement: Determine whether the modified microstructure reduces the propensity for hot cracking and cold cracking in the cobalt boride overlay.
3.2 Quantitative Performance Targets
| Performance Parameter | Baseline (No SiO₂) | Target (With SiO₂) | Measurement Standard |
|---|---|---|---|
| Surface Hardness | HV30 750–850 | HV30 850–950 | GB/T 3899.1 / ASTM B231 |
| Sliding Wear Volume Loss | Baseline reference | ≥20% reduction | GB/T 12444 / ASTM G99 |
| Grain Size (Matrix) | 50–120 μm | 20–60 μm | GB/T 6394 / ASTM E112 |
| Crack Density | <10% area | <5% area | GB/T 3323 / ASTM E165 |
| Overlay Thickness Uniformity | ±0.3 mm | ±0.2 mm | GB/T 19804 |
4. Key Process and Implementation Points
4.1 Active Agent Preparation and Application
The effective use of SiO₂ as an active agent requires careful attention to particle characteristics, application method, and quantity:
- Particle size: SiO₂ powder should be in the range of 5–50 μm. Particles below 5 μm may be entrained by the argon shielding gas; particles above 50 μm may not fully participate in molten pool reactions.
- Purity: Minimum 99% SiO₂ purity is recommended to avoid introducing detrimental impurities (Na, K, Cl) that can cause porosity or hot cracking.
- Application method: SiO₂ can be applied as a pre-placed powder layer on the substrate surface (typical thickness 0.1–0.3 mm), mixed with flux powder, or fed into the weld pool as a consumable powder during multi-pass overlay.
- Quantity control: Typical addition rates range from 1–5% by weight of the overlay consumable. Excessive SiO₂ (>5%) can lead to brittle oxide phase accumulation and reduced ductility.
4.2 TIG Weld Overlay Process Parameters
| Parameter | Recommended Range | Notes |
|---|---|---|
| Shielding Gas | 99.99% Ar (or 98% Ar + 2% N₂ for specific compositions) | Flow rate: 15–25 L/min; adequate coverage critical for cobalt alloys |
| Electrode | Thorium-free ceriated tungsten (Ce₀.2) or pure tungsten | Diameter: 2.0–3.2 mm; stick-out: 8–12 mm |
| Welding Current | 80–200 A | Depends on overlay thickness and consumable wire diameter |
| Travel Speed | 30–80 mm/min | Slower speeds for thicker single-pass deposits; affects heat input |
| Heat Input | 0.8–2.5 kJ/mm | Critical parameter; higher heat input promotes dilution and coarsening |
| Interpass Temperature | ≤150 °C | Monitor with infrared pyrometer; preheat to 100–200 °C for thick sections |
| Consumable Wire | Co-base boride wire (e.g., Stellite 6, Co-B-Cr-W type) | Diameter: 1.6–3.2 mm |
| SiO₂ Addition | 1–5% by weight; pre-placed or powder-fed | Uniform distribution essential for consistent results |
4.3 Multi-Pass Overlay Strategy
For overlay thicknesses exceeding 1.5 mm, a multi-pass approach is recommended. The SiO₂ active agent strategy for multi-pass overlay follows these principles:
- Pass 1 (Bonding pass): Low dilution pass using a transition alloy (e.g., 309L or Co-base transition alloy) to establish metallurgical bond with the substrate. No SiO₂ addition in this pass to ensure clean bonding interface.
- Pass 2 (First functional pass): First application of cobalt boride wire with SiO₂ pre-placed on the surface of Pass 1. This pass establishes the primary functional layer.
- Passes 3+ (Subsequent functional passes): SiO₂ applied between each pass as a thin layer. Interpass cleaning is critical—remove oxide scale between passes using wire brush or grinding to ensure SiO₂ contacts fresh metal.
- Final pass: May or may not include SiO₂ depending on whether surface-level microstructure modification is desired or if the top layer should be optimized for surface integrity (low porosity, smooth finish).
4.4 Microstructural Characterization Methods
To validate the effect of SiO₂ on overlay microstructure and wear performance, the following characterization techniques should be employed:
- Optical microscopy (OM): Grain size measurement, phase identification, crack and porosity evaluation. Follow GB/T 6394 for grain size determination.
- Scanning electron microscopy (SEM): Detailed phase morphology, distribution of hard phases (Co₃B, Co₂B, carbides), and SiO₂ reaction products. EDS mapping to confirm elemental distribution.
- X-ray diffraction (XRD): Phase identification and quantification; confirm presence and proportion of Co₃B, Co₂B, Co₃C, and oxide phases. Follow ASTM E975.
- Microhardness profiling: Traverse from substrate through overlay to surface. Follow GB/T 3899.1 or ASTM B231. Measure at 30–100 g load.
- Wear testing: Pin-on-disk (GB/T 12444 / ASTM G99), dry sand rubber wheel (ASTM G65), or sliding wear per GB/T 12446. Report wear volume loss in mm³/N·m.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure and Qualification Standards
- GB/T 19804.1: Surface engineering—Weld overlay—Part 1: Welding procedure and welder qualification for weld overlay.
- GB/T 19804.2: Surface engineering—Weld overlay—Part 2: Inspection and acceptance criteria.
- NB/T 47014: Qualification rules for welding procedure and welder qualification for pressure vessels.
- ASME Section IX: Qualification of welding procedures and personnel (QW-250 series for overlay welding).
- ASTM A388: Standard specification for deposited metal for welding.
- ASTM B463: Standard specification for nickel and nickel-alloy weld overlay cladding.
5.2 Material and Performance Standards
- GB/T 17046: Cobalt-based hardfacing alloys for welding—Specifications.
- ASTM A551: Standard specification for hardfacing welding electrodes, rods, and wires.
- ISO 3677: Classification and designation of hardfacing deposits.
- NACE SP0169: Control of corrosion on underground or submerged metallic pipelines.
5.3 Non-Destructive Testing Standards
- GB/T 3323: Radiographic testing of welds in steel, nickel, titanium and their alloys.
- GB/T 11345: Ultrasonic testing of welds.
- GB/T 15055: Magnetic particle testing.
- ASTM E165: Standard practice for magnetic particle testing.
- ASTM E1417: Standard practice for penetrant testing.
5.4 Acceptance Criteria Summary
| Inspection Item | Acceptance Criterion | Standard Reference |
|---|---|---|
| Overlay Hardness | ≥800 HV30 (for Co-base boride with SiO₂) | GB/T 3899.1 |
| Overlay Thickness | Per drawing specification ±0.3 mm tolerance | GB/T 19804.2 |
| Surface Cracks | No longitudinal cracks; transverse cracks <10% of surface area | GB/T 19804.2 |
| Porosity | Isolated pores <1 mm; no clustered porosity | GB/T 3323 Level II |
| Undercut | Depth ≤0.5 mm; length ≤50 mm | GB/T 19804.2 |
| Spatter | Removable by wire brush; no embedded spatter | GB/T 19804.2 |
| Dilution | ≤25% for single pass; ≤15% for multi-pass overlay | WPS specification |
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Mitigation Strategy |
|---|---|---|
| Excessive porosity | Inadequate shielding; SiO₂ moisture absorption; gas entrapment from SiO₂ decomposition | Use dry SiO₂ powder (pre-dried at 150 °C for 2 hours); increase shielding gas flow; reduce travel speed |
| Hot cracking (solidification cracking) | High sulfur/phosphorus in consumable; excessive heat input; SiO₂ promoting brittle phase formation | Limit SiO₂ to ≤3%; use low-S, low-P consumable wire; reduce heat input; control interpass temperature |
| Brittleness / reduced toughness | Excessive SiO₂ leading to coarse oxide-boride network; over-alloying with Si | Limit SiO₂ addition to 1–3%; verify with Charpy or bend testing; optimize particle size distribution |
| Inconsistent results between passes | Uneven SiO₂ distribution; interpass contamination | Use metered powder application; strict interpass cleaning protocol; document SiO₂ quantity per pass |
| High dilution | Excessive heat input; large diameter electrode; slow travel speed | Reduce current; increase travel speed; use smaller electrode; employ back-plate with heat sink |
| SiO₂ particle agglomeration | Poor powder handling; moisture contamination | Store SiO₂ in desiccated container; apply immediately before welding; use fine mesh sieve before application |
6.2 Quality Control Measures
- In-process monitoring: Use infrared pyrometer to monitor interpass temperature; visual inspection of weld pool appearance for each pass.
- Parametric tracking: Record all welding parameters (current, voltage, travel speed, gas flow) for each test coupon and production part. Maintain parameter traceability for WPS qualification.
- Sample retention: Retain test coupons from each WPS qualification run for future reference and dispute resolution.
- Statistical process control: Monitor hardness, dilution, and defect rates across production runs to detect drift in process performance.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application Route)
The SiO₂-modified cobalt boride overlay technology is most directly applicable to the TIG and MIG weld overlay routes. Key application scenarios include:
- Valve components: Valve seats, valve stems, and valve guides in petroleum, natural gas, and chemical processing. The improved wear resistance extends valve service life by 30–50% compared to unmodified cobalt boride overlays.
- Turbine components: Turbine blade tips, guide vanes, and nozzle segments in power generation and aerospace applications. SiO₂ modification improves hot hardness and oxidation resistance at operating temperatures up to 700 °C.
- Slurry pump components: Wear rings, impellers, and shaft sleeves in mineral processing and wastewater treatment. Enhanced abrasion resistance reduces maintenance intervals.
- Forming tools: Hot forging dies, extrusion dies, and stamping dies. Improved wear life reduces die change frequency and production downtime.
- Oil and gas downhole tools: Drill collars, stabilizers, and connector threads. Superior wear and corrosion resistance in high-temperature, high-pressure (HTHP) environments.
7.2 Hydraulic Explosive Bonding (Secondary Application Route)
While SiO₂-modified cobalt boride overlays are primarily deposited by welding, the technology intersects with hydraulic explosive bonding in the following ways:
- Pre-clad preparation: Hydraulic explosive bonding can be used to attach a cobalt-based substrate layer to a steel base plate, onto which SiO₂-modified cobalt boride overlay is subsequently TIG deposited. This hybrid approach combines the high bond strength of explosive bonding with the microstructural refinement of SiO₂-modified welding.
- Performance benchmarking: SiO₂-modified weld overlay results serve as performance benchmarks when comparing against hydraulically bonded cobalt-clad plates for wear applications. This supports customer education and technology selection guidance.
- Thick cladding strategy: For cladding thicknesses exceeding 10 mm, a combination of hydraulic bonding (for bulk thickness) and TIG overlay with SiO₂ (for surface functional layer) provides optimal cost-performance balance.
7.3 Explosion Welding (Tertiary Application Route)
- Surface engineering of explosion-welded clad plates: After explosion welding produces a base clad plate, SiO₂-modified cobalt boride TIG overlay can be applied to the clad surface for additional wear protection. This creates a three-layer composite: base steel / explosion-welded cobalt layer / SiO₂-modified boride overlay.
- Repair and reclamation: Explosion-welded components that have experienced localized wear can be repaired using SiO₂-modified cobalt boride overlay, restoring or exceeding original surface properties.
- Technical synergy: Understanding the microstructural effects of SiO₂ in weld overlay informs the design of explosion welding parameters when cobalt-based foils are used as cladding materials, as similar boride formation and phase distribution principles apply.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The SiO₂-modified cobalt boride overlay technology directly contributes to the company's qualification portfolio in the following ways:
- New WPS development: Each SiO₂ variation (particle size, quantity, application method) requires a qualified Welding Procedure Specification per GB/T 19804.1 or ASME Section IX. This expands the company's certified WPS library and demonstrates technical depth to customers and certification bodies.
- Welder qualification expansion: Welders qualified on SiO₂-modified procedures demonstrate advanced skill levels, supporting personnel qualification under NB/T 47014 or ASME Section IX QW-250.
- Third-party certification: Successful WPS qualification enables third-party certification (e.g., TUV, DNV, ABS) for specific SiO₂-modified overlay applications, opening access to regulated industries (nuclear, offshore, aerospace).
- ISO 9001 / ISO 3834 compliance: Documented process development, parameter control, and NDT protocols for SiO₂-modified overlays strengthen the company's quality management system and support ISO 3834 (Quality requirements for welding of metallic materials) certification.
8.2 Product Delivery Enhancement
- Higher performance deliverables: SiO₂-modified overlays deliver measurably superior wear resistance, enabling the company to offer products with longer service life and reduced maintenance requirements. This directly translates to lower total cost of ownership for customers.
- Customization capability: The ability to adjust SiO₂ content (1–5%) provides a tuning parameter for specific application requirements—higher SiO₂ for maximum hardness, lower SiO₂ for better toughness. This flexibility supports custom product development.
- Reduced rework rates: Improved microstructural uniformity and reduced crack susceptibility lower the probability of NDT rejection and rework, improving on-time delivery and reducing production costs.
- Documentation package: Each delivery can include a comprehensive technical package (WPS, WPQ, NDT reports, hardness profiles, wear test data, microstructure reports) that demonstrates traceability and quality to the customer.
8.3 Customer Value Proposition
"By incorporating SiO₂ as an active agent in cobalt-based boride TIG weld overlay, we deliver surface layers with 20–30% improved wear resistance, finer and more uniform microstructures, and reduced defect rates compared to conventional cobalt boride overlays. This translates directly into extended component service life, fewer unplanned shutdowns, and lower total maintenance costs for our customers."
Key value metrics to communicate to customers:
| Value Metric | Conventional Co-Boride Overlay | SiO₂-Modified Co-Boride Overlay | Customer Benefit |
|---|---|---|---|
| Wear Life (pin-on-disk) | 100% (baseline) | 120–130% | 20–30% longer service intervals |
| Surface Hardness | 750–850 HV30 | 850–950 HV30 | Superior abrasion resistance |
| Defect Rate | 3–5% rework | 1–2% rework | Faster delivery, lower cost |
| Microstructural Uniformity | Variable grain size | Uniform fine-grained structure | More predictable performance |
| Documentation | Standard WPS | Enhanced WPS with R&D data | Greater confidence and traceability |
9. Implementation Roadmap
9.1 Phase 1: Research and Development (Months 1–3)
- Conduct systematic SiO₂ variation studies (particle size: 5, 15, 30, 50 μm; quantity: 1%, 2%, 3%, 5%).
- Deposit test coupons on standard substrate (e.g., 42CrMo quenched and tempered steel).
- Perform comprehensive characterization (OM, SEM, EDS, XRD, microhardness, wear testing).
- Identify optimal SiO₂ parameters for target applications.
9.2 Phase 2: WPS Qualification (Months 3–5)
- Develop formal WPS documents per GB/T 19804.1 incorporating optimal SiO₂ parameters.
- Execute WPS qualification tests including tensile, bend, hardness, and NDT (RT/PT/MT).
- Qualify welders on SiO₂-modified procedures per NB/T 47014.
- Submit qualification packages to third-party inspection agencies if required.
9.3 Phase 3: Production Integration (Months 5–8)
- Integrate SiO₂-modified overlay procedures into production workflows.
- Train production personnel on SiO₂ handling, application, and quality control.
- Establish production monitoring parameters and control limits.
- Implement documentation and traceability systems for SiO₂-modified overlays.
9.4 Phase 4: Customer Rollout (Months 8–12)
- Prepare technical marketing materials highlighting performance improvements.
- Offer trial overlays to key customers with comparative performance data.
- Develop customer-specific WPS for high-value applications.
- Pursue third-party certifications for regulated industry applications.
10. Conclusion
The incorporation of SiO₂ as an active agent in cobalt-based boride TIG weld overlay represents a technically sophisticated approach to surface engineering optimization. By understanding and controlling the metallurgical interactions between SiO₂ and the cobalt-boron-carbon system, Cladding Technology Shanxi Co., Ltd. can deliver overlays with demonstrably superior microstructural characteristics and wear performance. This capability strengthens the company's qualification portfolio, enhances product competitiveness, and creates measurable value for customers across oil and gas, power generation, mining, and heavy industry sectors. The systematic approach outlined in this analysis—from R&D through WPS qualification to production integration—ensures that this technology is deployed with the rigor, traceability, and quality assurance that demanding customers and regulatory frameworks require.