Open-Arc Surfacing Repair Technology for Coal Mill Roller

Literature Overview

The paper by Cao Zhaoxia, Ding Zhenbo, and Wang Dong, published in Foundry Technology (2009, Vol. 30, Issue 3, pp. 432-434), addresses the critical issue of coal mill roller wear in thermal power stations. Vertical grinding mills are essential equipment for coal pulverization in power generation, and the coal mill rollers are subjected to severe abrasive and adhesive wear during service. The authors analyzed the wear mechanism of coal mill rollers and proposed a novel open-arc self-shielded surfacing technology for repair and remanufacturing. Practical results demonstrated that the repaired rollers exhibited excellent wear resistance, with service life improved by a factor of two compared to newly cast rollers.

Core Technical Points

Wear Mechanism Analysis

The coal mill roller operates under a complex combination of loading conditions including high contact pressure from coal grinding, sliding friction from coal slurry, and thermal cycling from the hot coal feed. The primary wear mechanisms identified include abrasive wear from hard coal particles embedded in the grinding surface, adhesive wear from metal-to-metal contact under high pressure, and fatigue wear from cyclic contact loading. The base material of the roller is typically a low-alloy cast steel with hardness in the range of 200-250 HB, which is insufficient to resist the severe abrasive conditions encountered during coal grinding.

The open-arc self-shielded surfacing technology utilizes a flux-cored wire or solid wire with an open arc process where the arc is not confined by a nozzle or shield gas. The self-shielded flux provides atmospheric protection and contributes to the weld metal composition. This process is particularly advantageous for large cylindrical components such as coal mill rollers because it allows for efficient multi-pass surfacing with high deposition rates, and the open arc configuration accommodates the large workpiece geometry without requiring complex gas shielding infrastructure.

Surfacing Process and Metallurgical Design

The surfacing alloy was designed to provide a combination of high hardness and adequate toughness to resist both abrasive and impact wear. The typical surfacing composition incorporates Cr, Mo, and C to form fine carbide precipitates that provide primary wear resistance, while maintaining a ductile matrix to prevent brittle fracture under impact loading. The surfacing process parameters included welding current in the range of 180-250 A, arc voltage of 22-28 V, and travel speed of 150-250 mm/min, depending on the wire diameter and pass configuration.

Defect Analysis and Countermeasures

Common defects observed in open-arc surfacing of coal mill rollers include porosity, cracking, and insufficient fusion. Porosity typically arises from inadequate flux coverage or excessive travel speed, leading to gas entrapment in the weld metal. The countermeasure involves ensuring proper flux distribution and optimizing the travel speed to allow sufficient time for gas escape. Cracking is primarily a hot cracking phenomenon associated with sulfur and phosphorus segregation at grain boundaries, and can be mitigated by controlling the surfacing alloy composition to maintain low S and P content. Insufficient fusion occurs when the welding parameters are too low or when the base surface is contaminated, requiring thorough surface preparation and appropriate heat input.

Integration with Engineering Practice

The application of open-arc surfacing for coal mill roller repair offers significant economic advantages over replacement with new cast rollers. The remanufacturing approach reduces material consumption, shortens equipment downtime, and lowers overall maintenance costs for power stations. The technology is particularly suitable for maintenance shops with limited infrastructure because the open-arc self-shielded process does not require external shielding gas supply systems.

In practical implementation, several engineering considerations must be addressed. The roller surface must be thoroughly prepared by machining or grinding to remove the worn layer and provide a clean, sound base for surfacing. The surfacing layer thickness should be designed to provide adequate wear reserve for the expected service interval, typically 3-5 mm of build-up. The multi-pass welding sequence should be planned to minimize angular distortion and to ensure uniform hardness distribution across the roller surface. Post-weld inspection should include hardness mapping and visual examination to verify the quality of the surfacing repair.

Study Insights and Implications

This study demonstrates the practical effectiveness of open-arc self-shielded surfacing technology for large-scale industrial component remanufacturing. The 2x improvement in service life compared to new cast rollers is a remarkable result that validates both the metallurgical design of the surfacing alloy and the process technology. The study highlights an important principle in engineering maintenance: that properly designed surface engineering solutions can outperform original equipment specifications, provided that the failure mechanism is correctly identified and the repair strategy is appropriately matched to the service conditions. The open-arc self-shielded approach represents a pragmatic technology choice that balances performance requirements with practical implementation constraints, making it accessible to maintenance operations with limited resources.