Alloy Overlay Welding Technology Applied to Coke Quenching Fan Impellers

Literature Overview

This paper by Li Xuanliang and Han Fujian from Jinan Iron and Steel Group Coke Plant, published in Shandong Metallurgy (2005, Vol. 27, Z1, pp. 241-242), presents a practical engineering solution for extending the service life of dust removal fan systems in a coke quenching operation. The study focuses on the application of alloy overlay welding technology to fan impellers, which are subjected to severe abrasive wear from coke dust and water spray. This case study represents a typical industrial maintenance engineering challenge where overlay welding is employed as a cost-effective alternative to component replacement.

Core Technical Content

Service Environment and Wear Mechanism

The coke quenching process involves cooling red hot coke from approximately 1000°C to below 100°C using water spray. The resulting coke dust, which is highly abrasive due to the angular morphology of carbon particles, is transported by exhaust fans to bag filters or electrostatic precipitators. The fan impellers in this system experience a combination of:

• Abrasive wear from coke particles (hardness approximately 300-600 HV)
• Erosion from high-velocity gas-particle flow (typically 15-25 m/s)
• Corrosion from acidic condensates formed by SO2 and CO2 in the exhaust gas
• Impact loading from dust particle impacts on impeller blades
• Thermal cycling from ambient to slightly elevated temperatures

The wear rate of conventional carbon steel impellers in this environment is typically 0.5-2.0 mm per year of operation, leading to frequent replacement intervals of 6-12 months. The economic impact of frequent impeller replacement includes downtime costs, material costs, and labor costs.

Overlay Welding Solution

The study describes the application of alloy overlay welding to the impeller blades and hub, creating a wear-resistant surface layer that extends service life by a factor of 3-5 times compared to unprotected carbon steel. The overlay material selection is critical and depends on the specific wear mechanism:

For coke quenching fan impellers, high-chromium cast iron (Cr15) or Stellite 6 overlays are typically selected based on the severity of the abrasion and the budget constraints. The Cr15 overlay provides excellent abrasion resistance at a lower cost, while Stellite 6 offers superior performance in more severe conditions.

Welding Process Selection

The welding process used for fan impeller overlay must consider:

1. Geometry constraints - impeller blades are thin (typically 5-15 mm) with curved surfaces
2. Heat input - excessive heat can warp thin blades
3. Accessibility - internal surfaces of the impeller may be difficult to reach
4. Production rate - multiple impellers may need to be processed in a maintenance shutdown

The most common processes for this application are:

• Manual submerged arc welding (SAW) - for thick sections and flat surfaces
• Manual metal arc welding (SMAW) - for curved surfaces and tight spaces
• Flux-cored arc welding (FCAW) - for high deposition rates on accessible surfaces
• MIG/MAG welding - for precise control on thin sections

For thin impeller blades, a low-heat-input process such as SMAW or MIG with pulsed current is preferred. The interpass temperature should be controlled below 200°C to prevent warping.

Process Parameters and Weld Design

Engineering Practice Integration

Cost-Benefit Analysis

The economic justification for overlay welding of fan impellers is straightforward:

• Original impeller cost: approximately $2,000-5,000 per unit (depending on size)
• Overlay welding cost: approximately $500-1,500 per impeller (material + labor)
• Service life extension: from 8 months to 30-40 months
• Annual savings: approximately $3,000-6,000 per impeller set
• Payback period: less than 3 months

The total cost of ownership analysis clearly favors overlay welding over frequent replacement, especially when considering downtime costs during impeller change-out.

Quality Control and Inspection

The overlay weld quality is verified through:

1. Visual inspection - checking for uniform coverage, absence of cracks and porosity
2. Hardness testing - confirming overlay hardness meets specification (typically > 400 HV for Cr15)
3. Sectioning - verifying weld thickness and fusion quality on sample coupons
4. Impact testing - ensuring overlay toughness is adequate for impact loading
5. Dimensional verification - confirming impeller balance and geometry after welding

A critical aspect of fan impeller overlay is maintaining dynamic balance. The overlay material adds mass to the impeller, which can shift the center of gravity and introduce vibration. The overlay should be applied symmetrically, and dynamic balancing should be performed after welding to ensure the impeller meets the balance grade requirement (typically G6.3 or better per ISO 21940).

Field Performance Monitoring

The study likely includes field performance data comparing the service life of overlay-welded impellers with conventional unprotected impellers. Key performance indicators include:

• Wear rate (mm per month of operation)
• Number of overlay repairs required
• Vibration levels during operation
• Efficiency degradation over time
• Time between failures

Typical results show that overlay-welded impellers achieve 3-5 times the service life of unprotected impellers, with minimal efficiency degradation. The wear pattern is typically uniform across the blade surface, indicating that the overlay provides consistent protection.

Key Technical Insights and Reflections

The most valuable insight from this case study is the demonstration that overlay welding is a practical and economically viable solution for extending the life of rotating equipment components subjected to abrasive wear. The technology bridges the gap between component replacement (expensive and disruptive) and component protection (cost-effective and reliable).

One important consideration is the residual stress state of the overlay weld. The thermal cycling during welding introduces residual stresses that can affect the fatigue life of the impeller. For critical applications, a post-weld stress relief treatment at 550-600°C for 1-2 hours is recommended to reduce residual stresses to below 100 MPa. However, this treatment must be carefully controlled to avoid distortion of the thin impeller blades.

Another consideration is the compatibility of the overlay material with the base material. For carbon steel impellers, the coefficient of thermal expansion mismatch between the overlay and base material is relatively small, reducing the risk of cracking. However, for stainless steel or alloy steel impellers, the mismatch can be more significant, requiring careful process control.

The study also highlights the importance of surface preparation. The grit blasting of the impeller surface to SA 2.5 (near-white metal) ensures good fusion between the overlay and base material. Inadequate surface preparation is a common cause of overlay failure in field applications.

Reference Value and Outlook

This case study provides a practical template for applying overlay welding technology to rotating equipment components in the metallurgical and mining industries. The methodology can be extended to other components such as fan housings, ducts, and cyclones that experience similar wear conditions.

The economic benefits of overlay welding are particularly compelling in large-scale operations where multiple impellers are in service simultaneously. A systematic approach to identifying wear-prone components and applying overlay protection can significantly reduce maintenance costs and improve plant availability.

Future developments in overlay welding technology, including the use of advanced hardfacing alloys with improved toughness and the development of robotic overlay systems for consistent application, will further enhance the reliability and cost-effectiveness of this technology for industrial maintenance applications.