Zhuojin Pipe Fitting Co., LtdHardfacing Process for Sintering Blower Blades
Literature Overview
This 1989 paper published in Fan Technology (Vol. 31, No. 1, pp. 51–53) by Zhao Jiancang addresses the hardfacing repair of blower blades in sintering furnaces. Sintering blowers are critical components in iron ore sintering plants, where they supply combustion air to the sintering bed. The blades of these centrifugal blowers operate under severe conditions of high temperature, abrasive gas flow, and cyclic mechanical loading, making them prone to wear and requiring periodic repair or replacement.
Service Environment Analysis
Operating Conditions
Sintering blower blades experience a complex combination of degradation mechanisms:
| Degradation Mechanism | Description | Severity |
|---|---|---|
| Abrasive wear | Erosion by silica dust and iron oxide particles in the gas stream | High |
| Thermal fatigue | Cyclic temperature variation (150–400°C) causing thermal stress | Moderate |
| Oxidation | High-temperature oxidation in the presence of oxygen | Moderate |
| Mechanical fatigue | Cyclic loading from gas pressure fluctuations | Moderate |
| Corrosion | Attack by acidic gases (SO2, CO2) in the sintering atmosphere | Low to Moderate |
The combination of abrasive wear and thermal fatigue is the primary failure mechanism. The leading edge of the blade, which faces the incoming gas flow, experiences the most severe wear. The trailing edge and suction side also experience significant wear due to the high-velocity gas flow.
Base Material Considerations
Blower blades are typically fabricated from carbon steel (Q235 or Q345) or low-alloy steel (16Mn). These materials provide adequate structural strength for the cyclic loading but have insufficient surface hardness to resist abrasive wear. The typical hardness of the base material is 150–200 HB, which is far too soft for the service conditions.
Hardfacing Process Design
Material Selection
The hardfacing material selection for sintering blower blades must balance several competing requirements:
- Hardness: Sufficient to resist abrasive wear (target: 45–55 HRC)
- Toughness: Adequate to withstand impact loading from gas pressure fluctuations
- Thermal stability: Hardness retention at elevated temperatures (up to 400°C)
- Crack resistance: Ability to accommodate thermal cycling without cracking
- Weldability: Compatibility with the base material (carbon or low-alloy steel)
Based on these requirements, the following hardfacing materials were considered:
| Material Type | Hardness (HRC) | Toughness | Thermal Stability | Suitability |
|---|---|---|---|---|
| High-carbon martensitic (Cr-Mo) | 50–58 | Low | Good | Good for leading edge |
| Austenitic (Ni-Cr) | 35–45 | High | Excellent | Good for trailing edge |
| Stellite (Co-Cr-W) | 40–48 | High | Excellent | Premium option |
| Hardfacing alloy (Cr-C-Ni) | 45–55 | Moderate | Good | Good general purpose |
Process Selection
Given the geometry of blower blades (curved surfaces, thin sections, complex contours), the following welding processes were evaluated:
| Process | Advantages | Disadvantages | Suitability |
|---|---|---|---|
| SMAW (Shielded Metal Arc) | Simple, portable, good for complex geometries | Lower deposition rate, higher operator dependence | Good for field repair |
| GTAW (Tungsten Inert Gas) | Precise control, low dilution, good for thin sections | Lower deposition rate, requires skilled operator | Good for critical areas |
| SAW (Submerged Arc) | High deposition rate, consistent quality | Requires flat or slightly curved surfaces | Limited applicability |
| FCAW (Flux-Cored Arc) | Good deposition rate, good for thick deposits | Higher cost, requires gas shielding | Good for thick deposits |
For sintering blower blades, SMAW was selected as the primary process due to its flexibility and ability to handle complex geometries. GTAW was used for the first pass on thin sections to ensure good fusion and minimize dilution.
Process Parameters
SMAW Parameters
| Parameter | Value | Notes |
|---|---|---|
| Electrode type | E70NiCrMo (or equivalent) | Low-hydrogen, high-alloy |
| Electrode diameter | 3.2 mm | For general areas; 2.5 mm for thin sections |
| Current | 120–160 A | DCEN (Direct Current Electrode Negative) |
| Arc length | 3–5 mm | Maintain consistent arc length |
| Travel speed | 200–300 mm/min | Adjust based on section thickness |
| Preheat temperature | 150–200°C | Reduce cracking tendency |
| Interpass temperature | < 250°C | Control cooling rate |
| Number of passes | 2–3 | Build up to required thickness |
GTAW Parameters (First Pass)
| Parameter | Value | Notes |
|---|---|---|
| Tungsten electrode | 2.4 mm, ceriated | DCEN |
| Filler wire | ERNiCrMo (or equivalent) | Match hardfacing composition |
| Current | 80–120 A | DCEN |
| Shielding gas | Argon (99.99%) | 15–20 L/min |
| Travel speed | 150–200 mm/min | Ensure good fusion |
| Preheat temperature | 150–200°C | Same as SMAW |
Microstructural and Mechanical Analysis
Deposit Microstructure
The hardfacing deposit microstructure consisted of:
- Primary phase: Tempered martensite (approximately 70–80%)
- Secondary phase: Carbide precipitates (Cr7C3, Mo2C) distributed throughout the matrix
- Carbide size: 1–3 μm, providing adequate wear resistance without excessive brittleness
The tempered martensite structure was achieved through the combination of:
- Post-weld tempering at 250–300°C for 2 hours
- The inherent tempering response of the Cr-Mo alloy system
Mechanical Properties
| Property | Base Material | Hardfacing Deposit | Improvement Factor |
|---|---|---|---|
| Hardness (HB) | 180–200 | 480–520 | 2.5–2.8× |
| Hardness (HRC) | 20–22 | 48–52 | 2.2–2.4× |
| Wear resistance (pin-on-disc) | Baseline | 3–4× baseline | 3–4× |
| Impact energy (Charpy V, 20°C) | 40–50 J | 15–25 J | Reduced (expected) |
The hardfacing deposit demonstrated significantly improved wear resistance (3–4 times that of the base material) at the cost of reduced impact energy. This trade-off is acceptable given the primary failure mechanism is abrasive wear rather than impact loading.
Field Application Results
The hardfacing repair was applied to blower blades in a sintering plant. The service life comparison was:
| Component | Service Life (New Blade) | Service Life (Hardfaced Blade) |
|---|---|---|
| Leading edge | 3–4 months | 8–10 months |
| Trailing edge | 4–6 months | 10–14 months |
| Overall blade | 3–4 months | 8–12 months |
The hardfacing extended the service life by approximately 2–3 times, representing a significant economic benefit. The hardfaced blades maintained acceptable surface hardness throughout the service life, with hardness remaining above 45 HRC after 12 months of operation.
Engineering Practice Insights
Key Process Considerations
- Preheating is essential: The high carbon content of the hardfacing material and the thermal cycling service conditions require adequate preheating to prevent cracking. A preheat temperature of 150–200°C is recommended.
- Interpass temperature control: Maintaining interpass temperature below 250°C is critical to avoid excessive grain growth and reduced hardening response.
- Post-weld heat treatment: Tempering at 250–300°C for 2 hours relieves residual stresses without significantly reducing hardness. This is particularly important for blades subjected to cyclic loading.
- Surface preparation: The base material surface must be thoroughly cleaned of rust, scale, and paint. Surface roughness should be controlled to Ra 12.5–25 μm for adequate mechanical anchoring.
- Welding sequence: Start from the leading edge (most critical area) and work towards the trailing edge. This ensures that the most critical area receives the best weld quality.
Defect Analysis and Countermeasures
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Cracking at fusion boundary | High cooling rate, hydrogen | Increase preheat, use low-hydrogen electrode, control interpass temp |
| Porosity | Moisture in electrode, inadequate shielding | Dry electrode storage, verify gas flow |
| Incomplete fusion | Surface contamination, improper travel speed | Thorough surface preparation, maintain consistent parameters |
| Excessive dilution | High travel speed, low current | Optimize parameters, use backing layer if needed |
| Hardness variation | Inconsistent thermal input | Maintain consistent parameters, use automated welding if possible |
Study Reflections
This work demonstrates the effectiveness of hardfacing repair for blower blades in sintering applications. The key insight is that the hardfacing material must be selected based on the specific failure mechanism — in this case, abrasive wear is the dominant mechanism, so a high-hardness martensitic deposit is appropriate.
The economic analysis should consider that hardfacing repair extends blade life by 2–3 times, significantly reducing replacement frequency and downtime. The additional processing cost (welding, heat treatment, inspection) is typically recovered within the first month of extended service life.
Engineers should also consider that the hardfacing build-up thickness should be designed to allow for regrinding operations during the service life. A typical build-up thickness of 3–5 mm provides adequate material for 2–3 regrinding operations, each removing approximately 1–1.5 mm.
The methodology employed — selecting appropriate hardfacing materials, optimizing process parameters, and validating through field trials — is a rigorous engineering approach that should be replicated for other hardfacing applications in fan and blower maintenance.