Special Welding Electrode Overlay for Anti-Wear Protection on Pulverized Coal Fan Impellers
1. Definition and Technical Principles
Special welding electrode overlay for anti-wear protection on pulverized coal fan (ID fan / FD fan) impellers is a surface engineering technique that deposits a hardfacing alloy layer onto the wear-critical surfaces of fan blades and shrouds using consumable welding electrodes designed with specific metallurgical compositions. The fundamental principle relies on the dilution-controlled deposition of carbide-forming or oxide-forming alloys—typically high-carbon chromium, nickel-based, or cobalt-based compositions—onto a carbon steel or low-alloy steel substrate (commonly Q235, Q345, or 16Mn). The resulting overlay layer exhibits microhardness values in the range of HRC 45–65, significantly exceeding the base material hardness of HRC 20–25, thereby dramatically extending the service life of impellers operating in abrasive coal-pulverized-air environments.
The metallurgical mechanism involves three critical zones: the dilution zone at the interface where base metal alloys with the deposited metal, the transition zone where microstructural transformation occurs (martensite, carbides, or intermetallic phases), and the surface zone where the primary wear-resisting microstructure (e.g., M₇C₃, M₆C, or Ni₃B particles) is fully established. The dilution ratio, which typically ranges from 25% to 60% depending on the electrode composition and welding parameters, is the single most important variable governing the final hardness and wear resistance of the overlay.
2. Category and Business Positioning
This technology falls under the consumable electrode weld overlay category within Cladding Technology Shanxi Co., Ltd.'s broader portfolio of surface engineering solutions. It represents a cost-effective, field-applicable, and rapidly deployable anti-wear technology that complements the company's higher-end TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding capabilities.
Within the company's business architecture, this technology serves as:
- Entry-level qualification capability: Establishing the company's credentials in the power generation and coal-handling maintenance market
- Cross-selling platform: Identifying customers whose initial needs are simple hardfacing, who may later require advanced clad plate or pipe solutions
- Field service differentiation: Providing on-site repair and refurbishment services that reduce customer downtime versus complete impeller replacement
- Technical training foundation: Serving as the learning platform for welding personnel before they advance to TIG/MIG overlay and more complex bonding technologies
3. Technical Purpose and Value
The primary technical purpose is to extend the operational life of pulverized coal fan impellers by 3 to 10 times compared to unprotected carbon steel blades, reducing unplanned shutdowns and replacement costs in coal-fired power plants, cement kilns, and coal handling facilities.
Key value propositions include:
- Availability improvement: Reducing impeller replacement frequency from 6–12 months to 24–60+ months depending on coal abrasiveness
- Cost reduction: Overlay repair costs typically represent 15–30% of new impeller fabrication costs
- Weight management: Unlike bolted or welded-on wear plates, overlay deposits add minimal mass (typically 2–5 mm), preserving impeller balance and dynamic characteristics
- Geometric fidelity: The overlay conforms to the aerodynamic profile of the blade, maintaining fan efficiency
- Maintenance simplicity: Electrode-based overlay can be performed with portable SMAW equipment, enabling rapid field repairs
4. Key Process and Implementation Points
4.1 Substrate Preparation
Proper substrate preparation is the foundation of overlay quality. The following steps are mandatory:
- Removal of existing coating: Strip all rust, paint, scale, and prior weld deposits down to bare metal using grinding or sandblasting to Sa 2.5 (ISO 8501-1)
- Edge preparation: If the impeller has machined surfaces, create a slight bevel or undercut at the overlay boundary to prevent cracking at the transition
- Surface roughening: Lightly roughen the deposition surface (grit blast or grit wheel) to improve mechanical bonding
- Preheating: Apply preheat to the substrate to control cooling rates and minimize cracking risk
4.2 Welding Parameter Selection
| Parameter | Typical Value | Notes |
|---|---|---|
| Welding Current (SMAW) | 120–220 A | Depends on electrode diameter (2.5–4.0 mm) |
| Deposition Thickness per Pass | 2.0–4.0 mm | Single pass for thin deposits; multi-pass for thicker layers |
| Total Overlay Thickness | 3.0–8.0 mm | Minimum 3 mm for reliable abrasion resistance |
| Preheat Temperature | 150–300°C (carbon steel) | 300–400°C for thicker sections or low-ductility substrates |
| Interpass Temperature | ≤ 250°C | Critical for maintaining overlay hardness; overheating causes softening |
| Welding Speed | 150–300 mm/min | Slower for better dilution control; faster for higher productivity |
| Post-Weld Heat Treatment | Generally not required | Exception: stress relief at 500–550°C for high-stress applications |
4.3 Electrode Selection Matrix
| Electrode Type | Typical Composition | Achieved Hardness | Application |
|---|---|---|---|
| High-Carbon Chromium (Type A) | C 2.0–4.0%, Cr 20–30%, Mn 1.0–2.0% | HRC 50–60 | General coal abrasion; moderate impact loading |
| High-Carbon Chromium (Type B) | C 3.0–5.0%, Cr 12–20%, Ni 3–6% | HRC 55–65 | Severe abrasion; dry coal handling |
| Nickel-Based (Type C) | Ni 60–70%, Cr 5–10%, B 0.5–1.0% | HRC 50–58 | Wet/abrasive conditions; thermal shock resistance |
| Transition Electrode | Cr 23–30%, Ni 10–15% (309L-type) | HRC 30–38 | First pass on dissimilar substrates; crack prevention |
4.4 Multi-Pass Deposition Strategy
For overlay thicknesses exceeding 4 mm or where dilution control is critical, a multi-pass strategy is employed:
- Pass 1 (Transition/Binding): Apply a 1–2 mm layer using a transition electrode (e.g., E309L equivalent) to ensure metallurgical compatibility and reduce cracking susceptibility
- Pass 2 (Build-up): Apply 2–3 mm using the selected hardfacing electrode at controlled current
- Pass 3 (Surface finish): Apply 1–2 mm final pass to achieve surface hardness and smooth the topography
- Post-weld machining: Grind or machine the overlay surface to the required aerodynamic profile and dimensional tolerance (±0.5 mm)
4.5 Impeller-Specific Considerations
- Dynamic balance: After overlay, the impeller must be dynamically balanced to G6.3 or G2.5 (ISO 21940-11) depending on rotational speed and service criticality
- Thermal distortion: Weld sequentially from the hub outward, alternating sides, to minimize blade warpage; dimensional deviation should not exceed 1.0 mm
- Stress relief: For large impellers (diameter > 800 mm) or thick sections (> 40 mm), post-weld stress relief at 550–580°C for 2 hours per 25 mm of section thickness is recommended
- Crack inspection: 100% visual examination plus spot PT (penetrant testing) on each blade; full PT or MT (magnetic particle testing) for critical service impellers
5. Applicable Standards and Acceptance Criteria
5.1 Design and Material Standards
- GB/T 33975-2017 — Welding consumables — Hardfacing electrodes — Classification and specifications
- GB/T 5117-2012 — Classification of welding consumables (transition electrodes)
- ASTM A441/A441M — Standard Specification for Wear-Resistant Steel Plates
- GB/T 11352-2009 — General technical conditions for castings of carbon steel and alloy steel (for impeller substrate)
- GB/T 1236-2000 — Performance testing of fans (for post-repair performance verification)
5.2 Welding Procedure and Qualification Standards
- GB/T 9452-2015 — Welding procedure qualification and welder qualification requirements
- GB/T 19866-2005 — Welding procedure specification qualification for carbon steel and low alloy steel
- ASME Section IX — Qualification rules for welding, brazing, and fusion bonding
- ISO 15614-1:2017 — Qualification procedures for welding of metallic materials — Part 1: Qualification rules for fusion welding
- ISO 9606-1:2012 — Qualification testing of welders — Fusion welding — Part 1: Steel
5.3 NDT and Acceptance Standards
- GB/T 18750-2008 — Welding work quality requirements for carbon steel and low alloy steel
- GB/T 3323-2005 — Radiographic testing of welds
- GB/T 26955-2011 — Penetrant testing
- JB/T 4730.3-2005 — Non-destructive testing of pressure equipment — Magnetic particle testing
- ASTM E709 — Standard practice for magnetic particle testing
5.4 Acceptance Criteria Summary
| Acceptance Item | Criterion | Method |
|---|---|---|
| Surface hardness | ≥ HRC 45 (minimum); target HRC 50–62 | Shore D or Rockwell C (ASTM E18) |
| Dilution rate | ≤ 50% (metallographic cross-section) | Spectrochemical analysis (GB/T 223) |
| Crack-free | No cracks ≥ 1 mm length in overlay or HAZ | PT per GB/T 26955 |
| Adhesion strength | ≥ 200 MPa (tensile bond test) | GB/T 11354-2013 |
| Dimensional accuracy | ±0.5 mm from drawing; blade profile within 1.0 mm | CMM or template measurement |
| Dynamic balance | ≤ 2.5 mm/s (G2.5 per ISO 21940-11) | Dynamic balancing machine |
6. Common Risks and Controls
6.1 Cracking
Cracking is the most prevalent defect in hardfacing overlay, occurring in the weld metal, HAZ, or at the fusion boundary. Root causes include high carbon equivalent of the substrate, rapid cooling, hydrogen embrittlement, and thermal stresses.
- Control measures: Use a transition layer (E309L-type) on high-carbon-equivalent substrates; maintain preheat at 200–300°C; limit interpass temperature to 250°C; select electrodes with low hydrogen content (H ≤ 5 mL/100 g); apply post-weld stress relief where cracking susceptibility is high
6.2 Excessive Dilution
High dilution (> 60%) reduces overlay hardness below the required threshold and compromises wear resistance.
- Control measures: Use larger electrode diameter for first pass to increase deposited volume; apply multiple thin passes rather than one thick pass; use a backing bar or root reinforcement to limit base metal melting; train welders to maintain consistent travel speed and arc length
6.3 Thermal Distortion
Localized welding heat input causes blade warpage, impeller imbalance, and dimensional deviation.
- Control measures: Weld in a planned sequence (hub → tip, alternating blades); use intermittent welding (skip-weld pattern); apply mechanical clamping fixtures; perform post-weld stress relief; verify dimensional accuracy after cooling
6.4 Poor Adhesion / Spalling
Inadequate fusion between the overlay and substrate, or between overlay passes, leads to spalling under service loads.
- Control measures: Ensure thorough substrate cleaning (Sa 2.5 minimum); avoid excessive arc length (maintain arc length ≤ electrode diameter); ensure proper overlap between passes (≥ 50% overlap); conduct 100% PT inspection of all overlay surfaces
6.5 Hydrogen-Induced Delayed Cracking
Especially relevant when welding on low-alloy steels with higher carbon equivalent.
- Control measures: Use low-hydrogen electrodes (E7018-type or equivalent); bake electrodes at 300–350°C for 1 hour before use; apply post-weld bake at 350–400°C for 2–4 hours to allow hydrogen diffusion; store electrodes in heated ovens between uses
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
The SMAW electrode overlay technology on fan impellers establishes the foundational process knowledge for the company's more advanced TIG and MIG weld overlay capabilities. Key integrations include:
- Process transfer: Welders trained on SMAW hardfacing can transition to MIG overlay (using wire feed systems with similar alloy compositions) for higher deposition rates and better dilution control on large impeller surfaces
- TIG precision overlay: For thin-walled impellers or areas requiring tight dimensional control (e.g., blade tips, trailing edges), TIG overlay with filler wire provides superior control over heat input and dilution
- Hybrid approach: Use TIG for the transition layer (first pass) followed by MIG for build-up passes, combining TIG's low-dilution advantage with MIG's productivity
7.2 Hydraulic Explosive Bonding Relevance
While hydraulic explosive bonding is not directly applied to impeller overlay, the metallurgical knowledge gained from understanding dilution, interface bonding, and metallurgical compatibility directly informs the company's hydraulic bonding qualification work:
- Interface characterization: Understanding the dilution zone in weld overlay informs the interpretation of bonded interface microstructure in hydraulic bonding
- Material selection: The alloy compatibility knowledge (e.g., carbon steel to high-carbon chromium) transfers to selecting clad material pairings for hydraulic bonding of wear plates and liners
- Customer education: The company can position hydraulic bonding as a superior alternative for applications where weld dilution is unacceptable (e.g., overlaying corrosion-resistant alloys on thick carbon steel plates)
7.3 Explosion Welding Relevance
Explosion welding, like hydraulic bonding, provides a non-dilutive bonding method. The relationship to this technology entry is primarily in the following areas:
- Market intelligence: Understanding the limitations of weld overlay (dilution, cracking, distortion) enables the company to position explosion welding for applications where these limitations are unacceptable
- Process qualification: The NDT procedures developed for weld overlay (PT, MT, hardness profiling) are directly applicable to explosion weld bond quality assessment
- Comparative proposals: For large impeller refurbishment projects, the company can offer a comparative analysis of weld overlay vs. explosion-welded wear plates, quantifying lifecycle cost differences
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technology entry represents a critical qualification milestone for Cladding Technology Shanxi Co., Ltd. in the following respects:
- WPS qualification: Development and qualification of welding procedure specifications (WPS) for hardfacing overlay on carbon steel substrates per GB/T 19866-2005 and ISO 15614-1 establishes the company's formal procedural credentials
- WPQ qualification: Welder performance qualification records (WPQ) per ISO 9606-1 demonstrate the company's workforce competency in hardfacing applications
- Industry entry: Successful delivery of impeller overlay projects provides reference cases and customer testimonials for the power generation and coal handling market
- Equipment capability: Investment in hardfacing welding equipment, preheat systems, NDT equipment, and hardness testing capabilities creates infrastructure reusable across all three technology routes
8.2 Product Delivery Excellence
The systematic approach to impeller overlay—encompassing substrate preparation, parameter control, multi-pass deposition, NDT, and dimensional verification—establishes a repeatable quality framework that ensures consistent product delivery:
- Standardized work instructions: Documented procedures for each step ensure that every impeller overlay project is executed to the same quality standard regardless of the specific welder or site conditions
- Traceability: Electrode lot numbers, welding parameters, NDT results, and hardness measurements are recorded for each project, enabling full traceability from raw material to delivered product
- Performance warranty: Based on documented hardness values and dilution control, the company can offer performance warranties (e.g., minimum 24 months of service life under specified operating conditions)
8.3 Customer Value Creation
The technical value delivered to customers through this capability is quantifiable:
- Availability improvement: Reducing impeller replacement cycles from 6 months to 24+ months translates to 75%+ reduction in unplanned maintenance downtime
- Cost avoidance: A single large ID fan impeller replacement can cost ¥150,000–500,000; overlay repair costs ¥30,000–80,000 while achieving equivalent or superior wear life
- Rapid turnaround: On-site overlay repair can be completed in 3–5 days versus 4–8 weeks for new impeller fabrication and installation
- Emissions reduction: Extending impeller life reduces the frequency of manufacturing, transportation, and disposal of replacement components, contributing to the customer's carbon footprint reduction goals
- Technical partnership: Establishing a relationship through impeller overlay opens pathways for the company to provide higher-value clad plate, pipe, and bonding solutions for the customer's broader plant infrastructure
9. Conclusion
The application of special welding electrode overlay for anti-wear protection on pulverized coal fan impellers represents a technically mature, economically compelling, and strategically valuable capability for Cladding Technology Shanxi Co., Ltd. It provides a foundation of metallurgical understanding, process discipline, and quality management that underpins the company's more advanced TIG/MIG overlay, hydraulic explosive bonding, and explosion welding technologies. By mastering this technology to the highest qualification standard, the company positions itself as a comprehensive surface engineering solutions provider capable of addressing the full spectrum of wear protection challenges in the power generation, cement, and mining industries.