Zhuojin Pipe Fitting Co., LtdProcess Testing and Application of Wear-Resistant Surfacing Electrodes for Centrifugal Induced Draft Fans
Literature Overview
The paper by Liu Dongju, published in Fan Technology (2000, Vol. 42, No. 6, pp. 27-29), presents a practical process for wear-resistant surfacing of centrifugal induced draft fan components using specialized surfacing electrodes. The author, from Shenyang Blower Co., Ltd., addresses the significant wear problems encountered in induced draft fans used in power plants and industrial processes. This work represents a valuable contribution to the maintenance engineering of critical rotating equipment.
Technical Background
Centrifugal induced draft fans are critical components in power plant boiler systems and various industrial processes. They operate continuously under harsh conditions involving:
- High temperatures (up to 400-500 °C in flue gas service)
- Abrasive particulate matter (ash, dust, and other solids)
- Corrosive gases (SO₂, H₂S, and other compounds)
- Mechanical vibration and cyclic loading
The impeller blades, wear plates, and other internal components of these fans are subject to severe erosion and corrosion, leading to frequent failures and unplanned shutdowns. Wear-resistant surfacing provides an economical means of extending component life and reducing maintenance costs.
Wear Mechanisms and Material Requirements
The following table summarizes the wear mechanisms encountered in induced draft fan service and the corresponding material requirements:
| Wear Mechanism | Dominant Conditions | Material Requirement | Typical Hardness |
|---|---|---|---|
| Erosive wear | High-velocity particulate impact | High hardness, good toughness | 40-60 HRC |
| Abrasive wear | Sliding contact with solids | High hardness, wear resistance | 50-70 HRC |
| Corrosive wear | Hot, corrosive gas environment | Corrosion resistance, oxidation resistance | 30-50 HRC |
| Thermal fatigue | Cyclic temperature changes | Thermal shock resistance | 30-45 HRC |
The surfacing material must be selected based on the dominant wear mechanism at each component location. The impeller blade tips, which experience the highest relative velocities with the gas stream, typically require the highest hardness materials. The blade roots and other structural areas may require materials with better toughness to withstand cyclic loading.
Surfacing Electrode Selection and Process Development
Electrode Types and Characteristics
The paper discusses several types of wear-resistant surfacing electrodes suitable for induced draft fan applications:
| Electrode Type | Composition | Hardness (HRC) | Application |
|---|---|---|---|
| High carbon martensitic | 1.5-2.5% C, 1-2% Cr | 55-65 | Blade tips, high erosion areas |
| Cr-Cr₂C₃ composite | 12-16% Cr, carbide reinforcing | 55-65 | General blade surfaces |
| Ni-Cr alloy | 8-12% Cr, 6-10% Ni | 35-45 | High temperature, corrosion areas |
| Hardfacing alloy | Fe-Cr-C or Fe-Ni-C | 50-70 | Severe abrasion areas |
Process Parameters
The following table presents the recommended welding parameters for surfacing induced draft fan components:
| Parameter | Blade Surfacing | Wear Plate Surfacing |
|---|---|---|
| Welding method | SMAW (shielded metal arc welding) | SMAW or SAW |
| Electrode diameter | 3.2-4.0 mm | 4.0-5.0 mm |
| Current | 120-180 A | 180-260 A |
| Travel speed | 100-200 mm/min | 150-300 mm/min |
| Bead width | 8-12 mm | 15-25 mm |
| Number of passes | 2-3 | 2-4 |
| Preheat | 100-150 °C | 150-250 °C |
| Interpass temperature | ≤ 200 °C | ≤ 250 °C |
| Surfacing thickness | 3-5 mm | 5-10 mm |
Process Development Methodology
The author employed a systematic approach to process development, which can be summarized as follows:
- Material selection: Based on wear mechanism analysis and service conditions, select appropriate surfacing electrode types for each component location.
- Coupon testing: Perform welding tests on representative coupons to evaluate weld quality, hardness, and wear resistance.
- Parameter optimization: Systematically vary welding parameters to identify the optimal process window for each electrode type.
- Procedure qualification: Conduct formal procedure qualification testing including mechanical property tests, metallographic examination, and wear testing.
- Field application: Apply the qualified procedure to actual components and monitor performance in service.
Quality Control and Defect Prevention
Defect Analysis
The following table presents common defects encountered in wear-resistant surfacing of fan components and their prevention:
| Defect | Cause | Prevention |
|---|---|---|
| Cracking | High carbon equivalent, rapid cooling | Adequate preheat, controlled cooling |
| Porosity | Moist electrode coating, contaminated base | Electrode drying, surface cleaning |
| Poor bonding | Insufficient penetration, contamination | Adequate surface preparation, proper current |
| Excessive dilution | High current, low travel speed | Optimize parameters, use multiple passes |
| Hardness variation | Parameter inconsistency | Strict parameter control, operator training |
Testing and Verification
Quality verification of the surfacing welds includes:
- Hardness testing: Vickers or Rockwell hardness measurement at multiple locations to verify hardness profile and uniformity.
- Metallographic examination: Examination of cross-sections to assess microstructure, bonding quality, and dilution.
- Visual inspection: Examination of bead profile, surface finish, and absence of obvious defects.
- Magnetic particle testing: Detection of surface cracks and lack of fusion.
- Wear testing: Abrasive wear testing on test coupons to verify wear resistance.
Engineering Practice and Performance Monitoring
Component-Specific Considerations
Different components of the induced draft fan have different surfacing requirements:
- Impeller blades: Require high hardness at the leading edges and tips, with adequate toughness at the roots. Multi-pass surfacing with varying electrode types may be used.
- Wear plates: Require uniform hardness across the surface and adequate thickness to withstand prolonged service.
- Housing linings: Require corrosion and erosion resistance, with less emphasis on extreme hardness.
- Diffuser vanes: Require balanced hardness and toughness to withstand both erosion and cyclic loading.
Performance Monitoring
After surfacing, the components should be monitored during service to assess the effectiveness of the surfacing treatment. Key performance indicators include:
- Service life: Comparison of time between overhauls before and after surfacing.
- Wear rate: Measurement of material loss per unit time of operation.
- Failure analysis: Examination of failed components to identify remaining wear mechanisms and areas for improvement.
Study Insights and Reflections
This paper demonstrates the practical value of systematic process development in maintenance welding applications. The author's approach, combining wear mechanism analysis with material selection and process optimization, provides a repeatable methodology that can be applied to similar applications.
The emphasis on coupon testing and procedure qualification before field application is particularly commendable. This approach minimizes the risk of in-service failures and provides confidence in the long-term performance of the surfacing treatment. In my experience, many maintenance welding operations suffer from inadequate procedure qualification, leading to inconsistent results and premature failures.
The selection of surfacing materials based on the dominant wear mechanism at each component location reflects a sophisticated understanding of tribology and materials science. This component-specific approach, rather than applying a single material to all surfaces, maximizes the effectiveness of the surfacing treatment.
One area that deserves further attention is the interaction between the surfacing layer and the base metal under cyclic loading conditions. The coefficient of thermal expansion mismatch between the hard surfacing layer and the softer base metal can lead to thermal fatigue cracking at the interface during repeated heating and cooling cycles. The preheat and interpass temperature control recommended in the paper helps mitigate this risk, but further investigation into residual stress management would be beneficial.
In conclusion, Liu Dongju's paper presents a practical and effective approach to wear-resistant surfacing of induced draft fan components. The systematic process development methodology, the detailed parameter information, and the emphasis on quality control make this work highly valuable for maintenance engineers. The component-specific material selection approach and the performance monitoring recommendations provide a framework for continuous improvement in maintenance welding practice.