Overlay Repair of ZGM95G Coal Mill Rollers and Grinding Disc Liner Tiles

Literature Overview

This practical engineering paper by Gong Junfeng from Beijing Jingneng Thermal Power Co., Ltd. (Welding Technology, 2009, Vol. 38, No. 10, pp. 60–62) documents the overlay repair methodology for ZGM95G coal mill rollers and grinding disc liner tiles in thermal power generation applications. The study addresses a significant economic and operational challenge in coal preparation systems, where the continuous abrasive wear of grinding components necessitates frequent replacement or repair to maintain mill efficiency and reduce downtime.

Wear Mechanism Analysis

The ZGM95G coal mill operates under conditions of severe abrasive wear involving:

• Sliding abrasion: Coal particles and hard mineral inclusions (quartz, feldspar) slide across the roller and liner surfaces under high contact pressure
• Impact abrasion: Falling coal particles impact the grinding surfaces at velocity, causing micro-fracture and material removal
• Three-body abrasion: Hard particles trapped between the roller and liner surfaces act as grinding agents, accelerating wear on both components

The combination of these wear mechanisms creates a demanding tribological environment that requires overlay materials with high hardness, good fracture toughness, and resistance to both abrasive and impact loading. High-chromium cast iron self-shielded flux-cored wire was selected as the overlay material based on its proven performance in similar applications and its compatibility with the existing mill geometry.

Weldability and Process Considerations

The weldability of high-chromium cast iron presents several challenges that must be managed in the repair process:

1. Carbon reactivity: High carbon content in the base metal promotes carbide formation at the weld interface, potentially creating brittle interfacial zones susceptible to cracking
2. Thermal cracking susceptibility: The combination of high carbon, high chromium, and low ductility in the heat-affected zone increases the risk of hot cracking during solidification
3. Preheating requirements: Adequate preheating is essential to reduce thermal gradients and minimize residual stresses that could cause cracking in the brittle cast iron matrix
4. Post-weld cooling control: Slow cooling rates are necessary to allow graphitization and prevent white cast iron formation in the heat-affected zone

Process Control Points

The implementation of overlay repair on large coal mill components requires careful attention to several process control parameters:

1. Surface preparation: Thorough cleaning of the worn surface to remove loose material, oxidation, and coal residue. The base metal should be ground to a sound, clean surface with adequate mechanical interlock for overlay adhesion.
2. Preheating: Uniform preheating of the component to 200–300°C to reduce thermal stress during welding and minimize the risk of cracking in the heat-affected zone.
3. Layer thickness control: Building up the overlay in multiple thin passes to manage thermal input and reduce residual stresses. Each pass should be allowed to cool to a controlled interpass temperature before the next pass is applied.
4. Welding sequence: Planning the welding sequence to minimize cumulative distortion, particularly important for large roller assemblies where dimensional accuracy affects mill performance.
5. Post-weld inspection: Visual examination, magnetic particle testing, and dimensional verification to ensure complete repair coverage and absence of defects.

Economic and Operational Benefits

The overlay repair approach offers substantial economic advantages over component replacement:

• Extended service life: Properly executed overlay repair can extend the service life of rollers and liner tiles by multiple cycles, significantly reducing the frequency of component replacement
• Reduced downtime: On-site or shop repair of existing components eliminates the lead time associated with procuring and shipping replacement parts
• Cost reduction: Overlay repair costs are typically a fraction of the cost of new component procurement, providing significant savings for power generation operations
• Environmental benefit: Reduced material consumption and waste disposal contribute to sustainability objectives

Quality Assurance and Verification

For reliable long-term performance of overlay-repaired coal mill components, the following quality assurance measures should be implemented:

• Hardness verification: Measure overlay hardness to confirm adequate carbide formation and wear resistance. Target hardness should exceed 500 HV for high-chromium cast iron overlays.
• Bond strength testing: Perform peel or pull-off tests on test coupons to verify adequate metallurgical bonding between the overlay and base metal.
• Crack inspection: Use magnetic particle testing to detect any cracks in the overlay or heat-affected zone that could lead to premature failure.
• Dimensional control: Verify that repaired dimensions conform to mill manufacturer specifications to ensure proper roller-to-disc geometry and coal grinding efficiency.

Engineering Reflections

This practical case study exemplifies the application of overlay welding technology to solve real-world industrial problems with significant economic impact. The selection of self-shielded flux-cored wire for high-chromium cast iron overlay repair reflects a pragmatic approach that balances material performance with process simplicity and field applicability. The success of this approach depends critically on disciplined process control, particularly regarding preheating, interpass temperature management, and welding sequence planning. For engineers managing coal mill maintenance programs, systematic overlay repair programs represent a cost-effective strategy that requires investment in operator training, process documentation, and quality assurance protocols to ensure consistent results across multiple repair cycles.