Alloy Weld Overlay Technology for Dry Quenching Coke Blast Fan Components
1. Definition and Technical Principles
Alloy weld overlay technology, as applied to blast fan (induced draft and forced draft) components in dry quenching coke (DQCK) systems, involves the controlled deposition of wear-resistant and corrosion-resistant alloy layers onto the working surfaces of fan impellers, blades, inlet vanes, and housing sections. The fundamental principle relies on the fusion of a specially selected alloy filler metal onto a carbon steel or low-alloy steel base substrate through a thermal process, creating a metallurgically bonded overlay layer that significantly enhances the service life of the component under severe abrasive and corrosive conditions.
In the context of DQCK systems, the blast fans operate in environments characterized by continuous exposure to fine coke dust particles (typically 10–100 μm), high-temperature flue gases (150–400°C), and corrosive species including CO, CO₂, H₂S, and moisture. The combination of high-velocity particle impingement, thermal cycling, and chemical attack creates a synergistic degradation mechanism that rapidly erodes unprotected carbon steel surfaces. Weld overlay technology addresses this challenge by introducing a surface layer with superior hardness, erosion resistance, and thermal stability without requiring complete component replacement.
The metallurgical bond between the overlay and the base material is achieved through full fusion at the interface, with the dilution rate between base and filler metal carefully controlled to maintain the desired microstructure and mechanical properties of the overlay. Common overlay microstructures include martensitic, austenitic, and high-chromium cast iron types, each selected based on the specific wear mechanism and service environment encountered in the DQCK fan application.
2. Category and Business Positioning
This technology falls squarely within the company's TIG/MIG weld overlay capability route, representing a high-value industrial application that demonstrates the company's expertise in extending the service life of critical rotating equipment in the metallurgical and coking sectors. The application to Jigang's DQCK blast fan system positions the company as a specialist provider of surface engineering solutions for the steel and iron industry's most demanding thermal and abrasive service conditions.
From a business perspective, this entry serves multiple strategic functions:
- Qualification Building: Successful deployment on a major integrated steel plant such as Jigang establishes a verified track record that can be leveraged for similar applications at other steel producers, including Baosteel, Shougang, Ansteel, and Handan Iron and Steel.
- Product Delivery: The technology enables the company to deliver field-repair and re-overlay services directly at the customer's facility, minimizing downtime and eliminating the need for expensive component replacement.
- Customer Value: By extending fan component service life by 3–8 times compared to bare carbon steel, the company delivers substantial capital and operational savings to the customer, creating a compelling total cost of ownership argument.
3. Technical Purpose and Value
3.1 Problem Statement
DQCK blast fans are critical process equipment that circulate nitrogen inert gas through the quenching chamber, absorbing heat from the red hot coke and maintaining the inert atmosphere necessary for safe quenching. The fan impellers and inlet guide vanes experience continuous erosion from entrained coke fines, while the housing and volute sections suffer from thermal degradation and corrosion. Typical failure modes include:
- Impeller blade leading-edge erosion reducing aerodynamic efficiency and causing vibration
- Blade thickness loss leading to structural failure and catastrophic fan breakdown
- Inlet vane erosion disrupting flow uniformity and increasing power consumption
- Housing corrosion and thinning compromising structural integrity
3.2 Value Delivered
The application of alloy weld overlay technology to DQCK blast fan components delivers quantifiable value through:
- Extended Service Life: Overlay layers with hardness of 45–60 HRC can extend component life by 3–8 times compared to unprotected carbon steel surfaces.
- Reduced Downtime: Field-applied overlay repairs eliminate the need to remove and ship components to a foundry for replacement, reducing shutdown duration from days to hours.
- Energy Savings: Maintaining original blade geometry through overlay repair preserves aerodynamic efficiency, reducing specific power consumption by 5–12% compared to eroded blades.
- Safety Improvement: Preventing blade failure eliminates the risk of catastrophic fan breakdown, which in a DQCK system could lead to inert gas loss and potential coke ignition events.
4. Key Process and Implementation Points
4.1 Overlay Material Selection
The selection of overlay material is the most critical technical decision and must be matched to the specific wear mechanism, temperature exposure, and corrosive environment at each component location within the DQCK fan system.
| Component Location | Primary Wear Mechanism | Recommended Overlay Material | Typical Hardness (HRC) | Key Alloying Elements |
|---|---|---|---|---|
| Impeller Blade Leading Edge | High-velocity particle erosion | High-Cr Martensitic (e.g., Stellite-type or D2-type) | 50–58 | Cr 20–30%, Mo 5–10% |
| Impeller Blade Trailing Edge | Combined erosion and corrosion | High-Cr High-Mo Martensitic | 48–55 | Cr 25–35%, Mo 8–12% |
| Inlet Guide Vanes | Sliding abrasion from dust-laden gas | Austenitic or Duplex with Carbide Reinforcement | 35–45 | Cr 20–25%, Ni 8–12% |
| Housing / Volute Interior | Corrosion and thermal cycling | Cast Iron-type (e.g., 40Cr25Ni20 or equivalent) | 40–50 | Cr 20–25%, Ni 18–22% |
| Hub and Root Regions | Mixed wear with structural requirement | Transition layer (309L) + Hard facing (Stellite) | 35 (transition) / 50–55 (hard facing) | Cr 22–25%, Ni 12–14% (transition) |
4.2 Surface Preparation
Proper surface preparation is essential for achieving a sound metallurgical bond and preventing defects such as porosity, cracking, and delamination. The recommended surface preparation sequence is:
- Mechanical Cleaning: Remove all rust, scale, coatings, and contaminants using grinding or shot blasting to a minimum Sa 2.5 surface cleanliness per ISO 8501-1.
- Beveling: Machine a V-groove or J-groove bevel on the component surface to ensure adequate fusion and overlay thickness control. Typical groove dimensions are 45° included angle with a 6–8 mm root radius.
- Preheating: Apply localized or global preheating to reduce thermal stress and prevent cold cracking. Preheat temperature is typically 200–350°C depending on base material carbon equivalent and section thickness.
- Surface Degreasing: Apply a high-temperature flux or cleaning agent to the beveled surface to prevent oxide inclusion during welding.
4.3 Weld Overlay Process Parameters
The TIG (GTAW) process is preferred for the transition layer and thin overlay applications due to its superior control over heat input and dilution rate. The MIG (GMAW) process is employed for thicker overlay builds where higher deposition rates are required.
| Parameter | Transition Layer (TIG) | Hard Facing Layer (TIG) | Build-up Layer (MIG) |
|---|---|---|---|
| Shielding Gas | Ar 100% | Ar 100% | Ar 95% + CO₂ 5% or Ar 100% |
| Current Type | DCEN | DCEN | DCEP or AC |
| Current Range (A) | 80–150 | 100–180 | 150–250 |
| Voltage Range (V) | 18–22 | 20–25 | 22–28 |
| Travel Speed (mm/min) | 150–300 | 120–250 | 250–500 |
| Welding Wire Diameter (mm) | 1.6–2.4 | 2.4–3.2 | 1.2–1.6 |
| Interpass Temperature (°C) | ≤ 200 | ≤ 250 | ≤ 300 |
| Post-Weld Heat Treatment | Stress relief 550–600°C / 2h | Stress relief 550–600°C / 2h | Stress relief 550–600°C / 2h |
4.4 Layer Design and Build Strategy
The overlay layer is typically designed in a multi-pass configuration to ensure uniform composition, adequate thickness, and freedom from defects:
- Wet-on Layer (Pass 1): A 309L or 309 stainless steel transition layer is deposited directly onto the base carbon steel to prevent chromium carbide precipitation at the interface and to provide a compatible dilution buffer.
- Build-up Layers (Passes 2–n-1): Additional passes of the hard-facing alloy are deposited to achieve the required overlay thickness (typically 3–6 mm total). Each pass is deposited with a slight overlap of the previous pass to ensure full fusion.
- Final Surface Layer (Pass n): A final pass with slightly reduced heat input is applied to optimize surface hardness and minimize dilution from the underlying layers.
For large fan impellers, the overlay is applied in a systematic circumferential pattern, with the welder rotating the impeller on a fixture to ensure uniform coverage. The deposition rate for TIG overlay is typically 0.5–1.5 kg/h, while MIG overlay achieves 1.5–4.0 kg/h.
4.5 Post-Weld Processing
After overlay deposition, the following post-processing steps are required:
- Stress Relief: Apply a post-weld stress relief heat treatment at 550–600°C for 2 hours to relieve residual stresses and prevent delayed cracking.
- Surface Finishing: Grind or machine the overlay surface to the required aerodynamic profile. The grinding operation should remove only the decarburized and oxidized surface layer (0.2–0.5 mm) while preserving the bulk hardness of the overlay.
- Dynamic Balancing: After overlay and finishing, the impeller must be dynamically balanced to a tolerance of G6.3 or better per ISO 21940-11 to prevent vibration issues at operating speed.
- Final Inspection: Perform dimensional verification, hardness testing, and non-destructive examination before returning the component to service.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Relevance to Application |
|---|---|---|
| GB/T 12469 | Welding consumables — Classification and specification of welding wires for hard facing | Defines composition and mechanical properties of overlay filler metals |
| GB/T 22751 | Welding consumables — Classification and specification of welding electrodes for hard facing | Governs electrode selection and qualification |
| GB/T 985.1 | Welding — Bevels, grooves and weld preparations for steel | Defines groove geometry for overlay preparation |
| GB/T 3323 | Non-destructive testing — Radiographic testing of welds | Acceptance criteria for radiographic examination of overlay welds |
| GB/T 11345 | Non-destructive testing — Ultrasonic testing of welds | Acceptance criteria for ultrasonic examination of overlay welds |
| GB/T 13298 | Non-destructive testing — Magnetic particle testing of welds | Acceptance criteria for surface defect detection |
| ASTM A396 | Standard specification for austenitic stainless steel welding electrodes | Governs 309L transition layer electrode specification |
| ASTM A552 | Standard specification for austenitic cast iron welding electrodes | Governs hard-facing electrode specification |
| ASTM E10 | Standard test methods for Vickers hardness of metallic materials | Hardness verification of overlay layers |
| ISO 21940-11 | Balance quality requirements for rigid rotors | Dynamic balancing tolerance for repaired impellers |
| ASME BPV Section IX | Welding and Brazing Qualifications | WPS/PQR qualification framework for weld overlay procedures |
| NACE MR0175 | Sour Service Materials Requirements | Applicable when H₂S exposure is a concern in DQCK gas streams |
5.2 Acceptance Criteria
The acceptance criteria for weld overlay on DQCK blast fan components are defined as follows:
- Visual Inspection: The overlay surface shall be free from cracks, excessive porosity, undercuts greater than 0.5 mm, and spatter. The weld profile shall conform to the required aerodynamic shape within ±0.5 mm tolerance.
- Radiographic Testing (RT): Performed on a minimum of 10% of overlay welds (or 100% for critical blade root areas) per GB/T 3323. Acceptance level: Grade II or better per GB/T 3323.1.
- Ultrasonic Testing (UT): Performed on 100% of overlay welds at blade root and hub transition areas per GB/T 11345. No indications exceeding 20% of reference reflector size shall be present.
- Magnetic Particle Testing (MT): Performed on 100% of accessible overlay surfaces per GB/T 13298. No linear indications (cracks, laps) shall be present.
- Hardness Testing: Vickers hardness per ASTM E10 shall be within the specified range for the selected overlay material, with a minimum of 3 test points per component. The hardness gradient from overlay to base shall be verified to ensure adequate dilution control.
- Dimensional Verification: Blade profile, thickness, and overall impeller geometry shall conform to the original design drawings within ±0.5 mm tolerance.
- Dynamic Balance: The repaired impeller shall achieve a residual unbalance of ≤ G6.3 per ISO 21940-11 at the operating speed.
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in overlay welds | Low melting point eutectics in high-Cr alloys; excessive heat input | Limit heat input per pass; use low-S, low-P filler metals; apply proper interpass temperature control |
| Cold cracking at overlay-base interface | High carbon equivalent of base material; hydrogen embrittlement | Preheat to 200–350°C; use low-hydrogen consumables; apply post-weld stress relief |
| Excessive dilution reducing overlay hardness | Large groove dimensions; high travel speed; improper welder technique | Optimize groove geometry; use multi-pass build strategy; monitor dilution by hardness profile testing |
| Porosity in overlay welds | Inadequate shielding; surface contamination; excessive arc length | Ensure clean surface preparation; use proper gas flow rates (8–12 L/min for TIG); maintain consistent arc length |
| Delamination during service | Thermal mismatch; residual stress; insufficient bond strength | Apply proper preheat and post-weld heat treatment; verify bond strength by microtensile testing on qualification coupons |
| Impeller imbalance after overlay | Non-uniform overlay thickness; asymmetric deposition | Apply overlay in balanced pairs; perform final dynamic balancing to G6.3; use CNC-controlled deposition where feasible |
| Thermal distortion of thin-walled components | Excessive heat input; inadequate fixture support | Use low heat input parameters; apply back-of-weld backing; use rigid fixtures; apply sequential welding sequence to minimize distortion |
6.2 Quality Assurance Controls
The following quality assurance measures are implemented to mitigate technical risks:
- WPS/PQR Qualification: Each overlay procedure shall be qualified per ASME BPV Section IX or equivalent, with qualification coupons subjected to hardness testing, microstructural examination, and bend testing to verify mechanical properties and metallurgical bond.
- Welder Qualification: All welders performing overlay work shall be qualified per the relevant WPS, with periodic requalification to maintain proficiency.
- In-Process Monitoring: Real-time monitoring of welding parameters (current, voltage, travel speed, gas flow) is performed using portable data loggers. Interpass temperature is recorded at each pass junction.
- Material Traceability: All filler metals are traceable to the heat of manufacture, with certificates of conformity retained for each batch. Consumables are stored in a controlled environment to prevent contamination.
- Statistical Process Control: Hardness measurements, dilution ratios, and weld geometry are tracked across multiple components to identify process drift and implement corrective actions.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for This Application)
The DQCK blast fan application is delivered exclusively through the company's TIG/MIG weld overlay route. This route is the most appropriate technology for fan component repair because:
- Field Applicability: TIG and MIG equipment can be deployed directly at the customer's facility, enabling on-site repair without component removal and shipping.
- Material Flexibility: A wide range of overlay compositions can be applied using the same equipment, allowing customization for different wear mechanisms at different component locations.
- Thickness Control: Multi-pass overlay allows precise control of total layer thickness, from thin (1 mm) protective coatings to thick (6+ mm) heavy-duty wear surfaces.
- Cost Efficiency: Weld overlay repair costs 30–60% less than new component replacement, providing significant economic advantage to the customer.
The company's TIG/MIG overlay capability includes qualified procedures for carbon steel to stainless steel transitions, high-chromium martensitic hard facing, austenitic stainless steel overlay, and multi-layer composite overlay systems. The Jigang DQCK application has contributed directly to the company's WPS library by qualifying specific procedures for carbon steel impeller repair with high-chromium overlay.
7.2 Hydraulic Explosive Bonding (Secondary Route — Limited Applicability)
Hydraulic explosive bonding is not typically applicable to DQCK blast fan component repair due to the complex geometry of fan impellers and the relatively thin wall sections involved. However, this technology route is relevant to the broader DQCK system, where hydraulic explosive bonding is used for:
- Heat Exchanger Tubes: Producing clad tubes with wear-resistant inner liners for DQCK heat recovery systems.
- Piping Systems: Manufacturing clad pipes for conveying abrasive coke dust slurries in the DQCK gas handling system.
- Pressure Vessel Linings: Applying corrosion-resistant linings to DQCK quenching chambers and gas collection headers.
The company's hydraulic explosive bonding capability complements the weld overlay route by providing permanent, full-bond clad components for the DQCK system's static equipment, while weld overlay addresses the rotating component repair needs.
7.3 Explosion Welding (Tertiary Route — Component Manufacturing)
Explosion welding (explosive cladding) is applicable to the manufacturing of new DQCK fan components with built-in wear protection. This technology route can produce:
- Clad Impeller Blanks: Explosion-welded plates of wear-resistant alloy on carbon steel substrates, which are then machined into impeller blades with the wear surface formed by the clad layer.
- Clad Housing Sections: Explosion-welded panels for fan housings and volute sections, providing a permanent wear-resistant lining throughout the component's life.
- Clad Inlet Vanes: Pre-clad sheet material for manufacturing inlet guide vanes with integrated erosion protection.
The explosion welding route offers advantages over weld overlay for new component manufacturing, including uniform clad layer thickness, freedom from residual stresses, and elimination of the need for post-manufacture overlay application. However, the capital investment and lead time for explosion-welded components are higher than for field-applied weld overlay, making the latter more suitable for repair applications.
8. Qualification Building and Strategic Contribution
8.1 Technical Qualification Development
The Jigang DQCK blast fan application has contributed to the company's technical qualification portfolio in several significant ways:
- WPS Qualification: New welding procedure specifications have been qualified for overlay of high-chromium martensitic alloys on carbon steel fan impellers, covering the specific base material grades, groove geometries, and process parameters used in the DQCK application.
- Material Qualification: Specific overlay consumable combinations have been validated for the DQCK service environment, including hardness retention at elevated temperatures (up to 400°C) and resistance to coke dust erosion.
- Welder Qualification: A pool of qualified welders with demonstrated proficiency in overlay welding on rotating equipment has been developed, ensuring consistent quality delivery.
- NDT Protocol Development: Inspection procedures specific to overlay welds on thin-walled rotating components have been developed and validated, including optimized UT techniques for detecting lack of fusion at the overlay-base interface.
8.2 Customer Value Demonstration
The successful application at Jigang provides a quantifiable case study that demonstrates the company's value proposition to prospective customers:
- Service Life Extension: Documented extension of impeller blade life from approximately 6 months to 3–5 years, representing a 5–8× improvement in service life.
- Downtime Reduction: Field-applied overlay repairs completed within 48–72 hours versus 4–6 weeks for new component procurement and installation.
- Cost Savings: Overlay repair costs approximately 35–55% less than new impeller replacement, with additional savings from reduced downtime and avoided production losses.
- Energy Efficiency: Maintaining original blade profile through overlay repair preserves fan aerodynamic efficiency, reducing specific power consumption by 5–10% compared to eroded blades.
8.3 Market Positioning and Business Development
The Jigang application positions the company as a recognized specialist in DQCK system component repair and protection. This qualification enables:
- Framework Agreements: The demonstrated capability supports the negotiation of multi-year framework agreements with major steel producers for ongoing DQCK fan component repair services.
- Geographic Expansion: The technical qualification obtained at Jigang is transferable to similar DQCK installations across China and Southeast Asia, enabling market expansion into new regions.
- Technology Platform: The overlay technology developed for DQCK fans is directly transferable to similar applications in power generation (flue gas fans), cement (kiln ID fans), and mining (ventilation fans), broadening the company's addressable market.
- Integrated Solutions: The company can now offer integrated surface protection solutions for the entire DQCK system, combining weld overlay for rotating components with hydraulic explosive bonding and explosion welding for static components, providing a single-source solution for comprehensive asset protection.
9. Lessons Learned and Continuous Improvement
9.1 Key Technical Lessons
The Jigang DQCK blast fan application has yielded several important technical lessons that have been incorporated into the company's standard operating procedures:
- Preheat is Critical: Insufficient preheat on thin-walled impeller sections led to micro-cracking at the overlay-base interface in early trials. The company now mandates a minimum preheat of 250°C for carbon steel impellers with wall thickness below 15 mm.
- Sequential Welding Sequence: A systematic welding sequence starting from the impeller hub and progressing outward to the blade tips minimizes thermal distortion. This sequence has been standardized and documented in the company's WPS library.
- Dynamic Balancing Integration: Overlay deposition must be planned in conjunction with the final dynamic balancing operation. The company now performs overlay in balanced pairs and reserves a small amount of material for final balancing correction.
- Temperature Monitoring: Real-time monitoring of interpass temperature using infrared pyrometers has proven essential for maintaining consistent overlay quality. The company has implemented automated temperature logging as a standard practice.
9.2 Future Development Directions
Based on the lessons learned from the Jigang application, the company is pursuing the following development directions:
- Robotized Overlay: Development of automated TIG/MIG overlay systems for fan impellers to improve consistency, reduce welder fatigue, and increase deposition rate.
- Advanced Consumables: Evaluation of new overlay alloys with improved high-temperature erosion resistance, including nanostructured and functionally graded compositions.
- Predictive Maintenance Integration: Development of overlay thickness monitoring systems that can predict remaining service life based on measured erosion rates, enabling proactive maintenance scheduling.
- Digital Documentation: Implementation of digital record-keeping for all overlay repairs, including parameter logging, NDT results, and hardness data, to enable traceability and continuous improvement.
10. Conclusion
The application of alloy weld overlay technology to DQCK blast fan components at Jigang represents a significant technical achievement that demonstrates the company's capability to deliver high-value surface engineering solutions for the most demanding industrial applications. The technology extends component service life by 3–8 times, reduces downtime by eliminating the need for component replacement, and delivers substantial cost savings to the customer.
From a strategic perspective, this application has contributed to the company's qualification portfolio, established a verified track record with a major steel producer, and positioned the company as a specialist provider of surface protection solutions for the metallurgical and coking sectors. The lessons learned have been incorporated into standardized procedures, and the technology platform is directly transferable to similar applications across multiple industries.
The company's commitment to continuous improvement, supported by robust quality management systems and qualified personnel, ensures that the DQCK blast fan overlay technology will continue to evolve and deliver increasing value to customers. By maintaining a comprehensive capability set spanning TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding, the company is uniquely positioned to provide integrated surface protection solutions for the full range of industrial equipment requirements.