Weld Overlay Reinforcement of Blast Furnace Equalizing Valves
1. Definition and Technical Principles
The blast furnace equalizing valve (均压阀) is a critical pressure-control component in ironmaking blast furnace operation, responsible for managing the pressure differential between the furnace interior and the tuyere blast system during tuyere cleaning and maintenance cycles. These valves operate under extreme conditions: temperatures ranging from 200°C to 800°C, high-velocity blast gas flow containing abrasive coke dust and iron oxide particles, cyclic pressure loading between atmospheric and 0.2–0.4 MPa gauge pressure, and frequent thermal shock events. The valve seat, sealing surfaces, and flow channels are subjected to severe erosion-corrosion, which leads to dimensional degradation, loss of sealing integrity, and eventual valve failure.
Weld overlay reinforcement (堆焊强化) for blast furnace equalizing valves involves the application of a metallurgically compatible, wear- and erosion-resistant alloy layer onto critical surfaces of the valve body, valve seat, and flow passages using TIG (GTAW) or MIG (GMAW) welding processes. The overlay layer is engineered to provide a surface hardness significantly exceeding the base material (typically 35–45 HRC for the base cast steel versus 45–58 HRC for the overlay), thereby extending service life by 3–8 times compared to the unprotected base material.
The fundamental metallurgical principles underlying this technology include:
- Dilution control: Limiting the base metal dilution in the overlay to below 5–8% to maintain the required microstructural characteristics of the overlay alloy.
- Thermal management: Controlling interpass temperature and heat input to prevent base material microstructural degradation (e.g., temper softening, grain coarsening) while ensuring proper fusion and bonding of the overlay.
- Residual stress management: Implementing preheating, interpass heating, and post-weld heat treatment (PWHT) to mitigate weld-induced residual stresses that could lead to cracking or distortion.
- Microstructural optimization: Selecting overlay alloys that form hard carbides (Cr₇C₃, Cr₃C, VC, TiC) and/or martensitic/bainitic matrices to resist abrasive and erosive wear mechanisms.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay business route of Cladding Technology Shanxi Co., Ltd. It represents a specialized application of the company's weld overlay capabilities targeting the ironmaking and steelmaking sector, specifically blast furnace auxiliary equipment maintenance and rehabilitation.
Business Positioning:
- Market segment: Iron and steel production — blast furnace maintenance and reliability engineering.
- Service model: On-site repair and overlay reinforcement services, as well as workshop-based component refurbishment for returned valve assemblies.
- Customer value proposition: Extending equalizing valve service intervals from typical 2–4 months (unprotected) to 12–24 months (overlaid), reducing unplanned shutdown costs, spare part inventory, and furnace campaign disruption risk.
- Competitive differentiation: Proprietary WPS (Welding Procedure Specification) qualification for specific overlay alloys and thicknesses tailored to blast furnace operating conditions, supported by NDT verification and performance tracking data.
3. Technical Purpose and Value
The primary technical objectives of weld overlay reinforcement on blast furnace equalizing valves are:
- Erosion resistance enhancement: Providing a hard, erosion-resistant surface layer capable of withstanding high-velocity blast gas flow (typically 30–80 m/s) laden with abrasive particulates.
- Sealing surface restoration: Rebuilding worn valve seat geometry to restore precise sealing clearance (typically 0.05–0.15 mm) between the valve disc and seat.
- Thermal shock resistance: Selecting overlay alloys with appropriate thermal expansion coefficients and toughness to resist cracking during rapid temperature cycling.
- Dimensional accuracy: Achieving overlay thickness tolerances of ±0.2 mm on critical sealing surfaces and ±0.5 mm on flow channel surfaces.
- Service life extension: Targeting a minimum 3× improvement in valve service life, with documented cases achieving 5–8× extension.
Economic Value Assessment:
| Parameter | Unprotected Valve | Overlay-Reinforced Valve | Improvement |
|---|---|---|---|
| Service Life | 2–4 months | 12–24 months | 3–8× |
| Annual Replacement Cost | High (frequent spare parts) | Low (extended intervals) | 50–70% reduction |
| Unplanned Shutdown Risk | High | Low | Significantly reduced |
| Surface Hardness | 200–250 HB | 500–650 HB | 2–3× |
4. Key Process and Implementation Points
4.1 Base Material Assessment and Preparation
Before overlay application, thorough assessment of the valve base material is mandatory:
- Material identification: Confirm base material grade (commonly ZG230-450, ZG270-500 cast steel, or 16Mn/20# carbon steel per GB/T 11352). Conduct chemical analysis if markings are illegible.
- Wear damage evaluation: Measure remaining material thickness, assess pitting depth, and determine the extent of erosion on sealing surfaces and flow channels.
- Surface preparation: Grind worn surfaces to sound metal, removing all oxide, scale, and contaminated material to a minimum of 2 mm depth. Final surface should be clean, dry, and free of oil and moisture.
- Crack inspection: Perform MPI (Magnetic Particle Inspection) per GB/T 26952 or ASME V Article 7 to detect existing cracks in the base material. Any cracks must be ground out, re-inspected, and confirmed crack-free before overlay.
4.2 Overlay Alloy Selection
| Overlay Alloy Type | Typical Composition | Hardness (HRC) | Application Zone | Key Properties |
|---|---|---|---|---|
| Cr-based martensitic | 5–6% Cr, 0.3–0.6% C | 48–55 | Valve seat sealing surface | Good toughness, moderate erosion resistance |
| Cr-Mo-C high-carbon | 4–5% Cr, 2–3% Mo, 1.2–1.8% C | 55–62 | Flow channel, high-erosion zones | High hardness, excellent abrasion resistance |
| Stellite-type (Co-Cr) | 50% Co, 28% Cr, 5% W, 6% Ni | 45–52 | Severe erosion + temperature zones | Outstanding hot erosion resistance, oxidation resistance |
| Transition layer (309L/310L) | 22–25% Cr, 12–14% Ni | 22–28 | Interface between base and hard overlay | Low dilution, strain compatibility, crack resistance |
4.3 Multi-Layer Overlay Strategy
For blast furnace equalizing valves, a multi-layer overlay approach is strongly recommended:
- Layer 1 — Transition layer (1–2 passes): Apply a low-dilution, ductile alloy (e.g., ER309L or ER310L per AWS A5.4) to ensure metallurgical compatibility between the carbon steel base and the subsequent hard overlay. This layer accommodates differential thermal expansion and prevents cracking at the base/overlay interface.
- Layer 2 — Intermediate layer (2–3 passes): Apply a medium-hardness alloy (e.g., Cr-Mo-C alloy at 50–55 HRC) to build bulk thickness and provide the primary erosion resistance. This layer also serves as a buffer against dilution from the base metal.
- Layer 3 — Surface hardening layer (2–4 passes): Apply the final high-hardness overlay (e.g., high-carbon Cr-Mo-C at 58–62 HRC or Stellite-type alloy) to the critical erosion and sealing surfaces. This layer provides the ultimate wear resistance.
4.4 Welding Process Parameters
| Parameter | TIG (GTAW) — Transition Layer | TIG (GTAW) — Hard Overlay | MIG (GMAW) — Intermediate Layer |
|---|---|---|---|
| Welding Current | 80–140 A | 100–180 A | 120–200 A |
| Travel Speed | 3–6 cm/min | 4–8 cm/min | 5–10 cm/min |
| Shielding Gas | Ar (99.99%) | Ar (99.99%) | Ar + 5% CO₂ or Ar + 2% O₂ |
| Gas Flow Rate | 12–18 L/min | 15–20 L/min | 15–22 L/min |
| Preheat Temperature | 150–250°C | 150–250°C | 150–250°C |
| Interpass Temperature | ≤300°C | ≤250°C | ≤300°C |
| Weld Bead Width | 8–12 mm | 10–15 mm | 12–20 mm |
| Weld Bead Height | 1.5–2.5 mm | 2.0–3.0 mm | 2.5–4.0 mm |
| Overlap Between Passes | 50–70% | 50–70% | 50–70% |
4.5 Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is critical for blast furnace equalizing valve overlay reinforcement:
- Purpose: Relieve residual welding stresses, prevent delayed cracking, and stabilize the overlay microstructure.
- Treatment: Tempering at 550–600°C for 2–4 hours for Cr-based martensitic overlays, or stress-relief annealing at 650–700°C for 2–3 hours for the entire valve assembly.
- Cooling: Furnace cool to below 300°C before air cooling. Avoid quenching or rapid cooling which could induce cracking.
- Acceptance: Post-PWHT hardness should remain within 5 HRC of as-welded values for the overlay layer. Base material hardness should not exceed 30 HRC.
4.6 Dimensional Control and Machining
After overlay and PWHT, the valve assembly must be machined to final dimensions:
- Sealing surface flatness: ≤0.02 mm/m (per GB/T 1184 Grade 1 or tighter).
- Valve seat angle accuracy: ±0.5° from nominal (typically 90° or 115° depending on valve design).
- Flow channel roughness: Ra ≤ 6.3 μm on flow surfaces; Ra ≤ 1.6 μm on sealing surfaces.
- Overlay thickness verification: Minimum 3 mm on high-erosion zones; minimum 2 mm on sealing surfaces (measured after machining).
- Valve clearance: Final assembly clearance between disc and seat should be 0.05–0.15 mm, verified by feeler gauge or optical measurement.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application |
|---|---|
| GB/T 26952 | Magnetic particle testing of welds — general requirements |
| GB/T 3323 | Non-destructive testing — radiographic testing of welds |
| GB/T 1184 | Geometrical tolerances — general tolerances for linear and angular dimensions |
| GB/T 131 | Hardness of metals — Brinell hardness test |
| GB/T 230.1 | Rockwell hardness test — Part 1: Test method |
| GB/T 11352 | Gray and malleable iron castings for general engineering purposes |
| ASME V | Non-destructive examination (Articles 1, 2, 7, 8, 16) |
| ASME IX | Welding, brazing, and fusing qualifications |
| AWS D10.9 | Standard for qualification and certification of weld overlay procedures and welders |
| AWS A5.4 | Stainless steel electrode and rod specifications (ER309L, ER310L) |
| AWS A5.21 | Stainless steel welding wire specifications |
| NACE MR0175/ISO 15156 | Materials for use in H₂S-containing environments (if applicable to blast gas) |
| JB/T 10953 | Welding consumables for wear-resistant overlay welding |
| ISO 16312 | Welding — requirements for the qualification of welding procedures |
5.2 Acceptance Criteria
- Visual inspection (VT): No undercut exceeding 0.5 mm depth, no porosity clusters exceeding 3 mm in any 100 mm length, no surface cracks, no excessive reinforcement (≤1.5 mm above adjacent surface).
- Magnetic particle inspection (MPI): 100% coverage of all overlay welds. No linear indications exceeding 3 mm in length. No cluster indications exceeding 10 mm in any 100 mm area. Per GB/T 26952 or ASME V Article 7.
- Radiographic testing (RT): If required for thick overlay sections (>6 mm), RT per GB/T 3323 or ASME V Article 2. Acceptance: no cracks, no slag inclusions exceeding 2 mm, porosity area ≤1% of weld area.
- Hardness verification: Overlay layer hardness within specified range (±5 HRC of target). Hardness gradient from overlay to base material should be gradual with no abrupt transition exceeding 10 HRC within 1 mm depth.
- Macrograph examination: On a sacrificial coupon welded under identical conditions, macrograph should show sound fusion, no lack of fusion, no cracks, and dilution in the first overlay pass ≤10% (ideally ≤5%).
- Dimensional verification: All critical dimensions (sealing surface geometry, valve clearance, flow channel profile) within specified tolerances after machining.
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Overlay cracking | High carbon dilution, excessive restraint, rapid cooling | Use transition layer; control interpass temperature ≤250°C; preheat to 150–250°C; apply PWHT |
| Lack of fusion | Inadequate heat input, poor joint preparation, excessive travel speed | Grind base surface to sound metal; verify current and travel speed; use weave technique for wider penetration |
| Excessive dilution | High heat input, wide bead, insufficient preheating control | Reduce current; use short arc length; increase travel speed; apply narrow bead technique; use transition layer |
| Distortion of valve body | Asymmetric welding sequence, high heat input | Use balanced welding sequence (symmetric, alternating); control heat input per pass; use backing plate or fixture |
| Hardness degradation after PWHT | Over-tempering, excessive PWHT temperature or time | Optimize PWHT parameters based on alloy type; verify post-PWHT hardness; adjust overlay composition if needed |
| Spalling/delamination in service | Poor metallurgical bond, residual stresses, thermal cycling | Ensure proper transition layer; apply adequate PWHT; select alloy with compatible thermal expansion; verify bond strength via macrograph |
| Porosity in overlay | Moisture-contaminated consumables, inadequate shielding | Dry electrodes per manufacturer specs; maintain shielding gas purity ≥99.99%; use trailing cup for back-side protection |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
The blast furnace equalizing valve overlay reinforcement is primarily executed through the TIG/MIG weld overlay route. This technology route is the company's core competency for this application due to:
- Precision control: TIG welding provides excellent control over heat input, dilution, and bead geometry — essential for the thin, precise overlay layers required on valve sealing surfaces.
- Multi-layer capability: The TIG/MIG route enables the multi-layer overlay strategy (transition + intermediate + hard surface) with controlled dilution at each layer interface.
- On-site applicability: TIG/MIG equipment is portable and can be deployed for on-site repair of large valve assemblies that cannot be removed from the furnace area.
- WPS qualification: The company maintains qualified WPS per AWS D10.9 and ASME IX for multiple overlay alloy combinations, base material types, and thickness ranges relevant to blast furnace components.
7.2 Hydraulic Explosive Bonding
While hydraulic explosive bonding is not typically applied directly to valve components, it contributes to the supply chain for blast furnace equalizing valve reinforcement:
- Clad plate fabrication: Hydraulic explosive bonding produces bulk clad steel plates (e.g., 16Mn + Cr-Mo-C wear-resistant overlay, or carbon steel + stainless steel) used as replacement valve body materials with integrated wear-resistant cladding.
- Material supply: The company's hydraulic explosive bonding capability provides custom clad plate stock for valve manufacturers who require pre-clad materials for new valve fabrication, reducing the need for extensive field welding.
- Complementary role: For valves requiring very thick overlay layers (>10 mm), pre-clad material from hydraulic explosive bonding can be machined into valve components, with only minimal TIG/MIG finishing welds required.
7.3 Explosion Welding
Explosion welding (air explosion welding) plays a supporting role in the blast furnace equalizing valve reinforcement ecosystem:
- Large component cladding: For large valve housings or valve body assemblies where extensive surface coverage is needed, explosion welding provides a metallurgically sound bond between a wear-resistant cladding sheet and the base valve material.
- Surface preparation for overlay: Explosion-welded cladding can serve as a pre-treated surface onto which additional TIG/MIG overlay is applied for final sealing surface finishing.
- Specialty applications: For valves operating in hydrogen sulfide (H₂S) environments (per NACE MR0175/ISO 15156), explosion welding enables the application of resistant alloys (e.g., duplex stainless steel, Alloy 6) that cannot be reliably achieved through weld overlay alone due to dilution and microstructural concerns.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS qualification portfolio: Each successful blast furnace equalizing valve overlay project contributes to the company's WPS qualification database, expanding the range of qualified base materials, overlay alloys, thicknesses, and process parameters.
- Welder certification: Welders performing valve overlay work maintain and build their AWS D10.9 and ASME IX certifications, ensuring a qualified workforce capable of meeting customer and regulatory requirements.
- NDT capability: Regular MPI and RT inspection of overlay welds maintains and develops the company's NDT Level II/III personnel qualifications per ASME V and GB/T 26952.
- Performance data accumulation: Systematic tracking of overlay performance in service (hardness retention, erosion rate, service life) builds a proprietary database that supports future WPS optimization and customer proposals.
8.2 Product Delivery
- Standardized delivery packages: The company delivers overlay-reinforced equalizing valves as complete, inspected, and certified assemblies with full documentation including WPS/PQR, welder certifications, NDT reports, hardness maps, dimensional inspection reports, and material traceability records.
- On-site service capability: For critical furnace campaigns where valve removal is impractical, the company deploys qualified technicians with portable TIG/MIG equipment for on-site overlay and repair, minimizing downtime.
- Turnkey refurbishment: The company offers complete valve refurbishment services including disassembly, NDT assessment, overlay welding, PWHT, machining, assembly, and functional testing — delivered as a turnkey product.
8.3 Customer Value
- Reduced total cost of ownership: By extending valve service life 3–8×, customers reduce spare part inventory, replacement frequency, and associated logistics costs.
- Improved furnace campaign reliability: Reliable equalizing valves prevent unplanned furnace shutdowns, which can cost $50,000–$200,000 per hour of downtime for a large blast furnace.
- Technical partnership: The company's deep understanding of blast furnace operating conditions enables proactive recommendations for overlay alloy selection, maintenance scheduling, and preventive replacement strategies.
- Quality assurance: Full traceability from WPS qualification through NDT verification provides customers with documented confidence in overlay integrity, meeting audit requirements for ISO 9001 and industry-specific quality systems.
9. Lessons Learned and Best Practices
The "learning summary" (学习心得) aspect of this technical entry emphasizes the following hard-won lessons that form the foundation of the company's blast furnace equalizing valve overlay expertise:
- Transition layer is non-negotiable: Direct application of high-carbon hard alloys to carbon steel base material consistently results in cracking. A 309L or 310L transition layer is mandatory.
- Interpass temperature control is critical: Exceeding 250°C interpass temperature on hard overlay layers significantly degrades final hardness and can cause base material temper softening.
- Welding sequence determines distortion: Symmetric, alternating weld sequences minimize angular and bow distortion. Starting at the center of long surfaces and working outward is preferred.
- PWHT cannot be skipped: Even for thin overlay layers, stress relief is essential to prevent delayed cracking during thermal cycling in service.
- Post-overlay machining is essential: As-welded overlay surfaces are rough and irregular. Precision machining to final geometry is required for proper valve sealing and flow characteristics.
- Field conditions demand flexibility: On-site repairs often encounter non-ideal conditions (vibration, limited access, ambient temperature variations). Procedures must be flexible enough to accommodate these while maintaining quality.
10. Conclusion
Weld overlay reinforcement of blast furnace equalizing valves represents a high-value, technically demanding application within the company's TIG/MIG weld overlay portfolio. It requires mastery of multi-layer overlay metallurgy, precise process parameter control, rigorous NDT verification, and deep understanding of blast furnace operating conditions. The technology delivers substantial economic value to iron and steel customers through extended component service life, reduced downtime risk, and lower total maintenance costs. As the company continues to build its qualification database, accumulate field performance data, and refine its WPS library for this application, it positions itself as a preferred technical partner for blast furnace reliability engineering in the ironmaking sector.