CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Cladding Processes for Sintering Blower Impeller Blades
Background and Operational Challenges
Sintering blower impeller blades in iron and steel manufacturing plants are critical components that operate under harsh conditions involving high-temperature gas flow, abrasive dust particles (iron ore fines, scale, and slag), and continuous cyclic loading. The blades experience erosion-corrosion wear, where the combination of high-temperature oxidation and particle impact accelerates material loss. Without protective cladding, blade life is typically limited to 6–12 months, requiring frequent shutdowns for replacement. The cladding process must therefore provide a durable, erosion-resistant surface while maintaining the structural integrity of the blade.
Material Selection and Metallurgical Considerations
The base material of sintering blower blades is typically low-carbon steel or low-alloy steel (e.g., Q345R or 16Mn). The cladding material must resist both abrasive wear and high-temperature oxidation. The following materials were evaluated in the study:
| Cladding Material | Hardness (HRC) | Oxidation Resistance | Abrasion Resistance | Suitability |
|---|---|---|---|---|
| 1Cr13 (Martensitic SS) | 40–48 | Moderate | Moderate | Good general-purpose option |
| 1Cr18Ni9Ti (Austenitic SS) | 25–35 | Excellent | Low | Suitable for transition layer |
| Ni-Cr-Si-B (Stellite-type) | 40–50 | Excellent | High | Premium option, high cost |
| Cr-Mo-V (High-speed steel type) | 55–62 | Low | Very high | Not suitable for high-temperature zones |
| Ni-based alloy (Inconel 625 type) | 35–42 | Excellent | Moderate | Excellent for hot gas zones |
The study recommended a two-layer approach: an austenitic stainless steel transition layer (e.g., 310 or 309-type) followed by a Ni-based or Cr-based hardfacing layer. The transition layer serves dual purposes: it reduces thermal stress during welding due to its lower carbon content and better ductility, and it provides a metallurgically compatible base for the hardfacing layer.
Welding Process Design
The geometry of impeller blades presents unique challenges for cladding. The thin blade section (typically 8–15 mm) requires low heat input to prevent distortion, while the leading edge and pressure side require sufficient overlay thickness (2–3 mm) for adequate protection.
| Process Parameter | Specification | Justification |
|---|---|---|
| Welding Process | GTAW (TIG) for root, GMAW or FCAW for fill | GTAW provides precise heat control on thin sections |
| Preheating | 150–200 °C | Reduce cooling rate, prevent cracking in base material |
| Heat Input | 8–15 kJ/cm (GTAW), 15–25 kJ/cm (GMAW) | Limit distortion while ensuring adequate fusion |
| Shielding Gas | Pure Ar (GTAW), Ar + 2% O2 (GMAW) | Minimize contamination, stabilize arc |
| Wire Diameter | 1.6 mm (GTAW), 1.2 mm (GMAW) | Match to heat input requirements |
| Layer Thickness | 1.5–2.5 mm total overlay | Balance protection with weight and cost |
A critical finding was that welding sequence significantly affects blade distortion. The study found that welding from the root outward, in a symmetrical pattern, minimized angular distortion. Additionally, using a backing plate with a copper backing strip helped prevent burn-through on thin blade sections.
Quality Control and Inspection
The quality of the cladding directly affects blade performance and safety. The following inspection methods were implemented:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual Inspection (VT) | Surface defects, porosity, undercut | No visible cracks, porosity < 1 mm |
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | No linear indications > 1 mm |
| Ultrasonic Testing (UT) | Bond strength, lack of fusion | No lack of fusion at overlay/base interface |
| Hardness Testing | Verify overlay hardness | Within specified range ± 5 HRC |
| Bond Strength Test | Verify metallurgical bond | Minimum 150 MPa (per relevant standards) |
| Chemical Analysis | Verify overlay composition | Within specified ranges |
The bond strength test was performed according to the wedge test method specified in relevant standards. The average bond strength achieved was 185 MPa, well above the minimum requirement. This confirmed that the transition layer approach effectively prevented brittle intermetallic formation at the interface.
Performance Results and Lessons Learned
Field trials on sintering blower blades with the optimized cladding process showed significant improvements:
- Blade life extended from 8 months to 24 months (3-fold improvement)
- Blade leading edge erosion reduced by 85% compared to uncoated blades
- No cracking or spalling observed after 12 months of continuous operation
- Distortion after welding was within 0.5 mm/m, acceptable for blade balance requirements
The study also highlighted the importance of surface preparation. Blades with residual scale or oxidation from previous service showed significantly lower bond strength. The recommendation was to machine the cladding area to bare metal with a roughness of Ra 3.2–6.3 μm before welding.
Summary and Outlook
This study demonstrated that a carefully designed two-layer cladding process can dramatically extend the life of sintering blower impeller blades. The key success factors were the selection of an appropriate transition layer, precise control of heat input to minimize distortion, and rigorous quality inspection protocols. Future improvements could include the application of thermal spray coatings as a secondary protection layer on top of the weld overlay, or the use of laser cladding for more precise dilution control on thin blade sections. The economic analysis clearly supports the investment in cladding, with a payback period of less than 6 months based on reduced replacement frequency and downtime.