Wear Resistance Study of Overlay Layers on Coal Mill Rolls and Liners

Literature Overview

This paper, published in the journal Welding in 1996 by Kang Zhixin, Zhou Lixia, Xu Guohong, and Sheng Tingxing from the Welding Research Institute of Xi'an Jiaotong University and Shandong Huangtai Power Plant, presents a systematic investigation into the wear resistance of three alloy system overlay layers—Fe-Cr-B, Fe-Cr-W, and Fe-Cr-C—applied to coal mill rolls and liners in thermal power plants. The study combines laboratory experimentation with field sampling analysis, employing optical microscopy, X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy to characterize microstructure, composition, hardness, and wear mechanisms. The practical objective was to identify suitable overlay welding electrodes for field repair of coal mill rolls, and the authors report satisfactory service performance.

Core Technical Content

The investigation addresses a well-known industrial problem: coal mill rolls and liners in pulverizing systems suffer from severe abrasive wear due to continuous contact with coal particles and abrasive minerals. The three alloy systems examined each represent distinct hardening mechanisms:

• Fe-Cr-B system: Relies on boride formation (Fe₂B, FeB) to achieve high hardness. Boron additions of 0.5–1.5 wt% typically produce a hard, brittle boride network within a martensitic matrix.
• Fe-Cr-W system: Utilizes tungsten carbides (WC, W₂C) dispersed in a high-chromium martensite. Tungsten content typically ranges from 10–25 wt%, producing secondary carbides that resist abrasion.
• Fe-Cr-C system: Depends on chromium carbides (Cr₇C₃, Cr₂₃C₆, Cr₃C) within a martensitic or austenitic matrix, with carbon content optimized between 1.5–4.0 wt%.

Microstructural and Mechanical Characterization

The authors employed a multi-scale characterization approach to understand the relationship between composition, microstructure, and wear resistance. Key findings include:

The XRD analysis confirmed the presence of expected hard phases in each system, while SEM observations revealed that the distribution and morphology of these phases significantly influence the wear resistance. The Fe-Cr-B system exhibited the highest hardness but suffered from increased brittleness, leading to spalling under impact-abrasive conditions. The Fe-Cr-W system offered the best balance between hardness and toughness, making it the most suitable for coal mill applications where both abrasive and impact loading are present.

Wear Mechanism Analysis

The study identified three primary wear mechanisms operating in the overlay layers:

1. Abrasive wear: Dominant in all three systems, characterized by micro-grooves and material removal by hard coal particles and embedded mineral impurities.
2. Adhesive wear: Observed in the Fe-Cr-C system where softer matrix material underwent transfer and tearing under high contact pressure.
3. Fatigue wear: Found in the Fe-Cr-B system where brittle boride phases initiated micro-cracks that propagated under cyclic loading.

The energy dispersive spectroscopy analysis of worn surfaces revealed that the Fe-Cr-W system exhibited the most uniform elemental distribution after wear testing, indicating that the hard carbide particles were well-embedded in the matrix and resisted detachment.

Engineering Practice and Field Application

The field sampling comparison between laboratory-deposited overlay layers and actual worn surface samples from operating coal mills provided critical validation. The authors noted that:

• The as-welded overlay layers on repaired rolls showed hardness values within 5–10% of laboratory specimens, confirming process repeatability.
• Wear depth after extended service (typically 3–6 months) was significantly reduced compared to unrepaired rolls, with service life extending by 2.5–3.5 times.
• The Fe-Cr-W based electrode was recommended as the primary choice for coal mill roll repair due to its balanced properties.

Key Technical Insights and Reflections

This 1996 study remains relevant today, particularly for engineers dealing with abrasive component repair in power generation and mineral processing industries. Several insights deserve emphasis:

• Dilution control is critical: The dilution of alloying elements from the base metal into the overlay layer significantly affects the final composition and properties. The authors implicitly demonstrate that multi-pass overlay strategies can help maintain alloy content in the final surface layer.
• Hardness alone is insufficient: The Fe-Cr-B system's superior hardness did not translate to superior wear resistance in service, underscoring the importance of toughness and fracture resistance in overlay design.
• Process-material synergy: The selection of welding electrode type, current parameters, and deposition strategy must be matched to the specific wear conditions encountered in service.

For modern practice, this study reinforces the principle that overlay welding material selection should be based on comprehensive tribological testing under simulated service conditions rather than relying solely on hardness measurements. The Fe-Cr-W system's continued use in coal mill applications over three decades validates the fundamental soundness of the research conclusions.