Special Welding Electrode Overlay Anti-Wear Application on Exhaust Fan Impellers

Literature Overview and Context

The paper by Qiu Hemin, Lai Hui, and Yang Yiyuan, published in China Electric Power (Vol. 28, No. 10, 1995, pp. 70), addresses the persistent and economically significant problem of impeller wear in boiler exhaust fans at power plants. Exhaust fans in coal-fired power stations operate in extremely harsh environments, handling hot flue gas laden with abrasive fly ash particles. The impeller blades of these fans are subjected to continuous high-velocity particle impingement, leading to rapid material loss, progressive loss of aerodynamic efficiency, increased vibration, and ultimately premature failure. The paper reports on the application of special welding electrode overlay technology as an anti-wear solution for exhaust fan impellers at the Pingxiang Power Plant in Jiangxi Province.

This topic is particularly relevant to the power generation industry, where exhaust fan reliability directly impacts plant availability and economic performance. A single exhaust fan failure can force a complete unit shutdown, resulting in significant revenue loss. The cumulative cost of impeller replacement and associated downtime over the lifetime of a power plant unit is substantial, making wear-resistant surface treatments a high-priority area for reliability engineering.

Core Technical Points and Analysis

Wear Environment and Mechanisms

The exhaust fan impeller operates under a uniquely severe combination of wear mechanisms:

Erosive wear: The dominant mechanism, caused by high-velocity fly ash particles (typically 50–200 μm in size) impacting the blade surface at velocities of 30–60 m/s. The angle of impingement, the particle hardness (primarily silica, Mohs hardness 7), and the particle velocity collectively determine the erosion rate.
Abrasive wear: Sliding contact between the blade surface and ash deposits that accumulate on the blade surface.
Thermal degradation: Exposure to flue gas temperatures of 120–250 °C, which can reduce the hardness and strength of the base material.
Corrosive wear: In some cases, acid gases (SO₂, HCl) in the flue gas can contribute to chemical attack of the blade surface.

The base material of exhaust fan impellers is typically a medium-carbon steel (e.g., Q235, 20G) or a low-alloy steel (e.g., 16Mn), which provides adequate structural strength but insufficient resistance to erosive wear.

Overlay Welding Material Selection

The selection of overlay welding material for exhaust fan impellers requires careful consideration of the wear environment. The paper describes the use of special welding electrodes designed specifically for anti-wear overlay applications. Common overlay material systems include:

Material System	Typical Composition	Hardness (HRC)	Key Characteristics
Cr-C-Mo system	Cr 10–20%, C 3–6%, Mo 2–5%	55–65	High hardness, good abrasive wear resistance
Ni-Cr system	Ni base, Cr 20–30%, B, Si	45–55	Good hot hardness, thermal shock resistance
Co-based system	Co base, Cr 25–30%, W 5–10%	40–50	Excellent thermal fatigue resistance
Carbide-containing system	Cr-C with WC or Cr₃C₂ additions	60–70	Very high hardness, superior erosion resistance

For exhaust fan impellers, the Cr-C-Mo system and carbide-containing systems are typically preferred due to their excellent resistance to erosive wear at moderate temperatures.

Welding Process Parameters

The overlay welding process for impeller blades must be carefully controlled to ensure adequate bond strength, minimize dilution, and avoid distortion of the impeller geometry. Key process parameters include:

Parameter	Recommended Value
Welding process	SMAW (shielded metal arc welding)
Electrode type	Low-hydrogen hardfacing electrode
Electrode diameter	3.2–4.0 mm
Welding current	100–160 A
Arc voltage	22–28 V
Travel speed	50–80 mm/min
Preheat temperature	150–250 °C
Interpass temperature	≤ 250 °C
Number of overlay passes	2–3
Post-weld treatment	Stress-relief annealing at 500–600 °C

The multi-pass overlay approach ensures adequate coverage of the blade leading edge and pressure face, which experience the most severe erosion. The overlay thickness is typically maintained at 2–4 mm to provide sufficient wear resistance while minimizing weight and cost.

Distortion Control

Impeller blades are thin-walled components that are highly susceptible to welding distortion. Excessive distortion can alter the blade profile, reduce aerodynamic efficiency, and increase vibration. The paper emphasizes the importance of distortion control measures, which may include:

Sequential welding in a balanced pattern to minimize asymmetric thermal input
Use of backing bars or clamping fixtures to restrain blade movement
Back-step welding technique to reduce longitudinal shrinkage
Post-weld stress-relief annealing to relax residual stresses

Engineering Practice Implications

Maintenance Strategy Integration

The overlay welding approach fits into a comprehensive maintenance strategy for exhaust fan impellers. The typical maintenance cycle includes:

Inspection: Regular ultrasonic thickness measurement of blade surfaces to assess wear progression.
Removal and cleaning: Disassembly of the impeller and removal of ash deposits and corroded surface layers.
Surface preparation: Grinding or machining of the worn surface to expose sound metal.
Overlay welding: Application of the wear-resistant overlay layer.
Post-weld machining: Precision grinding of the blade profile to restore aerodynamic geometry.
Balancing: Dynamic balancing of the impeller to ensure vibration levels are within acceptable limits.

Performance Comparison

The application of overlay welding to exhaust fan impellers typically results in a 3–5 times extension of impeller service life compared to unprotected blades. This translates into significant economic benefits, including reduced replacement frequency, lower spare parts inventory requirements, and decreased unplanned shutdowns. The overlay approach also offers the advantage of being a field-applied solution that can be performed during scheduled maintenance outages, without requiring the complete replacement of the impeller.

Quality Verification

Post-overlay quality verification is essential to ensure the integrity and performance of the repaired impeller. Key verification steps include:

Visual and magnetic particle inspection for surface cracks
Hardness profiling across the overlay thickness
Ultrasonic testing for lack of fusion or delamination at the overlay-base interface
Dimensional inspection of the blade profile after post-weld machining
Dynamic balancing verification

Key Questions and Reflections

Several important considerations arise from this work. First, the long-term performance of the overlay under cyclic thermal and mechanical loading requires continued monitoring — does the overlay maintain its integrity after extended service in the erosive environment? Second, the interaction between the overlay material and the impeller base material during the welding process must be carefully managed to prevent cracking or spalling. Third, the economic optimization of overlay thickness and material selection requires a balance between wear resistance, weight, cost, and aerodynamic performance.

The work by Qiu Hemin and colleagues represents a practical engineering solution to a real-world problem in power plant operations. The approach of using specialized welding consumables to extend component life is a cost-effective alternative to complete component replacement and aligns with modern asset management principles that emphasize reliability and availability. For power plant engineers, this paper provides a clear methodology for implementing overlay welding as a preventive maintenance measure, with detailed process parameters and quality control criteria that can be directly applied to similar applications.

Study Insights and Implications

The application of special welding electrode overlay technology to exhaust fan impellers demonstrates the versatility and economic effectiveness of overlay welding as a surface engineering solution. The key insight from this work is that the selection of appropriate overlay materials, combined with controlled welding parameters and rigorous quality verification, can dramatically extend the service life of wear-critical components in harsh operating environments. For power plant maintenance engineers, this paper provides a practical template for implementing overlay welding programs, with specific recommendations for material selection, process parameters, and quality control procedures. The broader implication is that surface engineering through overlay welding should be considered as a standard preventive maintenance measure for any component subjected to severe erosive or abrasive wear, not only in the power generation industry but across all industrial sectors where equipment reliability is critical to economic performance.