CLADDING TECHNOLOGY SHANXI CO., LTDWear-Resistant Overlay Repair of Centrifugal Fan Impellers in Cement Industry
Literature Overview
This technical study by Huang Zhiquan, Wei Jianjun, Pan Jian, and Xu Jian from the Zhengzhou Institute of Machinery, Ministry of Machinery Industry, published in 1997 in the journal Cement, focuses on the application of wear-resistant overlay welding for the repair and extension of service life of centrifugal exhaust fan impellers in cement production facilities. The cement industry is notorious for the severe abrasive wear experienced by fan impellers due to the continuous transport of fine cement particles and dust-laden air streams. The authors present a systematic approach to overlay repair that combines material selection, process optimization, and quality control to achieve significant improvements in impeller service life.
Problem Analysis and Failure Mechanism
Centrifugal exhaust fans in cement mills operate under extremely harsh conditions. The impeller blades are subjected to:
- Erosive wear: High-velocity cement particles (typically 20–100 micrometers in size) impinge on the blade surfaces at velocities ranging from 20 to 60 m/s, causing progressive material removal through cutting and plowing mechanisms.
- Abrasive wear: Sliding contact between cement particles and the blade surface leads to microplowing and material displacement.
- Corrosive wear: In some operating environments, the presence of moisture and acidic gases (such as SO2 and CO2) can accelerate material degradation through synergistic corrosion-abrasion mechanisms.
- Fatigue cracking: Thermal cycling from hot gas streams (typically 150–250°C) combined with mechanical loading can initiate and propagate fatigue cracks, particularly at the blade roots and leading edges.
The typical service life of unclad carbon steel impeller blades in a cement mill exhaust fan is only 3 to 6 months before replacement is required, resulting in significant downtime and operational costs.
Overlay Repair Strategy and Process Parameters
The authors proposed a comprehensive overlay repair methodology that included the following key elements:
Material Selection
The overlay material was selected based on the wear mechanism analysis. A high-carbon martensitic hardfacing alloy was chosen, with a nominal composition of:
| Element | Content (wt.%) | Function |
|---|---|---|
| C | 2.5 – 3.5 | Carbide former, primary hardening element |
| Cr | 12 – 16 | Carbide stabilizer, corrosion resistance |
| Mo | 2.0 – 3.0 | Solid solution strengthening, temper stability |
| Mn | 1.0 – 1.5 | Deoxidizer, austenite stabilizer |
| Si | 0.5 – 1.0 | Deoxidizer, slag former |
| Balance | Fe | Matrix |
The target deposit hardness was 550–650 HV, with a microstructure consisting of Cr7C3 and Cr23C6 carbides dispersed in a martensitic matrix.
Welding Process Parameters
The overlay welding was performed using submerged arc welding (SAW) with a multi-pass approach to achieve the required deposit thickness of 3–5 mm on the leading edges and pressure faces of the impeller blades.
| Parameter | Value | Notes |
|---|---|---|
| Welding current | 400 – 550 A | DC, electrode negative |
| Arc voltage | 28 – 35 V | Optimized for bead width |
| Travel speed | 150 – 250 mm/min | Balances dilution and productivity |
| Preheating temperature | 150 – 200°C | Reduces residual stress and cracking risk |
| Interpass temperature | ≤ 250°C | Maintains hardness and prevents tempering |
| Post-weld treatment | 200°C × 2h stress relief | Eliminates residual stresses |
Surface Preparation and Quality Control
The impeller blades were prepared by grinding the worn surfaces to remove loose material and ensure a sound base for the overlay. The following quality control measures were implemented:
- Visual inspection (VT): Each pass was inspected for undercut, porosity, and weld bead irregularities.
- Magnetic particle testing (MT): Applied to the final overlay surface to detect surface and near-surface cracks, particularly at the weld toes and transitions.
- Hardness testing: Cross-sectional hardness profiles were measured at intervals to ensure uniform hardness distribution and adequate hardening depth.
- Impact testing: Charpy V-notch impact tests were performed on coupon specimens welded under the same conditions to verify that the deposit retained adequate toughness (minimum 27 J at 25°C).
Performance Results and Engineering Outcomes
The overlay repair program demonstrated substantial improvements in impeller service life:
- Service life extension: The average service life of overlay-repaired impeller blades increased from 4 months to 18–24 months, representing a 4.5 to 6-fold improvement.
- Hardness retention: After 12 months of operation, the surface hardness of the overlay layer remained above 500 HV, indicating excellent resistance to abrasive wear.
- Metallographic examination: Cross-sectional analysis of retired overlay layers revealed a distinct worn layer of approximately 0.2–0.5 mm depth, with the underlying overlay material retaining its original microstructure and hardness. The transition zone between the overlay and base metal showed no signs of cracking or delamination.
- Economic analysis: The cost of overlay repair was approximately 30% of the cost of fabricating new impeller blades, resulting in significant savings in both material and labor costs.
Process Optimization and Defect Prevention
During the implementation of the overlay repair program, several challenges were encountered and addressed:
- Cracking at the weld toe: Initial trials revealed transverse cracks at the toe of the first overlay pass, attributed to high residual stresses and the high carbon equivalent of the base metal. This was resolved by implementing a preheating temperature of 200°C and using a lower travel speed for the first pass to reduce the cooling rate.
- Porosity in the deposit: Gas porosity was observed in some weld beads, traced to inadequate flux coverage during the SAW process. The flux coverage was improved by adjusting the wire feed speed and travel speed ratio, and by ensuring that the flux hopper maintained a consistent flux level above the welding zone.
- Excessive dilution: The dilution ratio between the base metal and the overlay deposit was measured at approximately 25–35% for the first pass, which reduced the hardness of the first pass deposit. A multi-pass strategy was adopted, with the first pass serving as a transition layer and subsequent passes achieving the target hardness with lower dilution.
Key Technical Insights
The study provides several valuable lessons for engineers working in the field of wear-resistant overlay repair:
- Wear mechanism analysis is paramount: The selection of overlay material must be based on a thorough understanding of the dominant wear mechanism. In the case of cement fan impellers, the combination of erosive and abrasive wear requires a material with both high hardness and adequate toughness to resist crack initiation and propagation.
- Process control is as important as material selection: Even the most appropriate overlay material will underperform if the welding process is not carefully controlled. Parameters such as preheating temperature, interpass temperature, travel speed, and arc voltage must be optimized to achieve the desired microstructure and mechanical properties.
- Quality control must be integrated into the repair process: The implementation of non-destructive testing (NDT) and destructive testing protocols at appropriate stages of the repair process is essential to ensure the reliability and longevity of the overlay repair.
Summary
The wear-resistant overlay repair of centrifugal fan impellers in the cement industry represents a successful application of hardfacing technology to a critical industrial problem. By combining careful material selection, rigorous process optimization, and systematic quality control, the authors achieved a 4.5 to 6-fold extension in impeller blade service life, resulting in substantial economic savings and reduced operational downtime. The study underscores the importance of understanding the wear mechanism, controlling the welding process parameters, and implementing comprehensive quality assurance protocols in any overlay repair program. The methodology presented here serves as a valuable reference for similar applications in other industries where abrasive wear is a dominant failure mode.