Surfacing Repair of ZGM95G Coal Mill Roller and Mill Table Liners

Literature Overview

This technical paper, published in "Welding Technology" (2009, Vol. 38, Issue 10, pp. 60-62), authored by Gong Junfeng from Beijing Jingneng Thermal Power Co., Ltd., documents the surfacing repair of ZGM95G coal mill rollers and mill table liners using high-chromium cast iron self-shielded flux-cored wire. The study provides practical engineering guidance for the repair of large coal mill components in thermal power plants, addressing the economic and technical challenges of component refurbishment. The work is directly relevant to power generation engineers responsible for maintaining coal preparation equipment.

Coal Mill Operating Conditions and Wear Mechanism

ZGM95G coal mills operate under severe conditions characterized by:

• Continuous abrasion from coal particles and mineral impurities.
• Impact loading from falling coal and mill components.
• Thermal cycling between ambient and elevated operating temperatures.
• Chemical interaction with coal moisture and sulfur compounds.

The wear mechanism is primarily abrasive, with secondary contributions from impact and possibly adhesive wear. The high-chromium cast iron composition of the original liners provides adequate wear resistance through a combination of hard carbide phases and a tough matrix, but the wear rate eventually necessitates replacement or repair.

Surfacing Repair Process and Material Selection

The selection of high-chromium cast iron self-shielded flux-cored wire is based on several considerations:

1. Metallurgical compatibility: The high-chromium composition provides similar wear resistance to the original ZGM95G material.
2. Weldability: Self-shielded flux-cored wire offers good weldability on cast iron substrates, minimizing the risk of cracking.
3. Deposition rate: Flux-cored wire provides higher deposition rates compared to solid wire, which is important for large component repair.
4. Process flexibility: The self-shielded design allows outdoor and field welding without external shielding gas.
5. Cost-effectiveness: The material cost is significantly lower than complete component replacement.

Welding Process Control and Construction Points

The welding process control for large coal mill component repair requires careful attention to several critical parameters:

1. Preheating: The cast iron substrate should be preheated to 200-300°C to reduce thermal gradients and minimize cracking risk.
2. Interpass temperature: Maintained at 250-350°C to control thermal stresses during multi-pass surfacing.
3. Layer thickness: Typically 3-5 mm per pass, with total overlay thickness of 10-20 mm depending on the wear allowance.
4. Travel speed and heat input: Optimized to ensure complete fusion with the substrate while minimizing dilution and thermal damage.
5. Post-weld cooling: Controlled cooling rate to prevent cracking, often achieved through insulated cooling or controlled air circulation.
6. Surface preparation: Thorough cleaning and grinding of the worn surface to ensure good fusion and remove loose material.

Economic Analysis and Service Life Assessment

The economic benefits of surfacing repair compared to complete replacement are substantial:

• Material cost: Surfacing material represents a fraction of the cost of new rollers and mill table liners.
• Downtime reduction: Surfacing repair can be performed in-situ or with minimal disassembly, reducing plant downtime.
• Service life extension: Properly executed surfacing repair can restore the component to near-original service life.
• Environmental benefit: Reduced material consumption and waste generation.

The study reports that surfacing repair of large coal mill components significantly improves service life and reduces production costs, making it an economically attractive option for power plant maintenance.

Integration with Engineering Practice

For power plant engineers, the practical considerations include:

1. Inspection and assessment: Regular inspection of roller and mill table wear to determine when repair is required.
2. Surface preparation: Thorough cleaning and preparation of the worn surface is critical for successful surfacing.
3. Welding procedure qualification: Development and qualification of welding procedures specific to the component geometry and material.
4. Quality control: Non-destructive testing (MT or PT) of the surfacing layer to detect cracks and lack of fusion.
5. Post-repair inspection: Verification of overlay thickness, hardness, and surface finish after repair.
6. Documentation: Maintenance of repair records for traceability and future maintenance planning.

Key Questions and Reflections

1. The long-term wear performance of the high-chromium cast iron overlay on ZGM95G substrate is not quantified in terms of specific wear rate or service life hours.
2. The effect of thermal cycling during normal coal mill operation on the overlay-substrate interface is not discussed. Thermal fatigue could potentially lead to delamination over time.
3. The dilution rate and its effect on the final overlay composition and properties is not reported. High dilution could reduce the hardness and wear resistance of the overlay.
4. The surface finish and dimensional accuracy of the repaired component is not addressed. For coal mill rollers, the surface profile affects coal grinding efficiency.

Study Insights and Implications

The primary insight from this study is the demonstration that surfacing repair is a technically feasible and economically attractive alternative to complete replacement of coal mill rollers and mill table liners. The use of high-chromium cast iron self-shielded flux-cored wire provides a practical solution that balances wear resistance, weldability, and cost.

For power plant maintenance engineers, this study reinforces the importance of proactive maintenance strategies that include component refurbishment rather than waiting for complete failure. Surfacing repair extends the service life of expensive components while reducing downtime and maintenance costs.

The practical recommendation is to establish a regular inspection and repair program for coal mill components, with surfacing repair performed when wear reaches a predetermined threshold. This approach maximizes the economic benefit of the original component investment while maintaining reliable coal preparation operations.