Microstructural Characteristics and Sliding Wear Performance of CM550 High-Chromium Surfacing Plate for Scraper Conveyor Mid-Slot

Literature Overview

Published in Lubrication Engineering in 2022, this paper from China University of Mining and Technology and Tianjin Zhongkuang New Materials Technology Co., Ltd. investigates the feasibility of CM550 composite wear-resistant plate for use in coal mine scraper conveyor mid-slots. The research addresses a significant industrial problem: the severe abrasive wear experienced by conveyor mid-slots in underground coal mining operations, which leads to frequent component replacement and production downtime. The study is supported by the Central Universities Basic Research Business Fee Special Fund, reflecting its relevance to coal mining equipment durability.

Core Technical Findings

The investigation combines metallographic examination, X-ray diffraction (XRD), and transmission electron microscopy (TEM) analysis of the CM550 high-chromium surfacing layer deposited on Q235 carbon steel substrate, complemented by sliding wear testing against quartz sand abrasive media. The surfacing layer achieves metallurgical bonding with the base material, and its phase composition consists primarily of α-Fe ferrite matrix and Cr7C3 carbides. The carbides exist in three distinct morphologies: primary rod-shaped, eutectic plate-like, and secondary particulate forms.

Microstructural Analysis and Wear Mechanism

Carbide Morphology and Its Role

The three distinct carbide morphologies identified in the CM550 surfacing layer each contribute differently to wear resistance. Primary rod-shaped carbides, formed during initial solidification, provide continuous reinforcement paths along the solidification direction. Eutectic plate-like carbides, formed during the eutectic reaction, create a lamellar structure that resists crack propagation. Secondary particulate carbides, precipitated during cooling, provide additional hardening throughout the ferrite matrix. The combination of these three morphologies creates a multi-scale reinforcement architecture that is particularly effective against abrasive wear.

Wear Mechanism Analysis

The sliding wear test results reveal a two-stage wear process. In the initial stage, hard quartz sand particles micro-cut the soft ferrite regions of the surfacing layer, while the deformation of the ferrite matrix leads to spalling of material. However, as the ferrite is worn away, the harder Cr7C3 carbides become exposed at the wear surface. These exposed carbides then bear the primary wear load, preventing direct contact between the quartz sand abrasive and the softer ferrite matrix. This self-protecting mechanism effectively reduces the further wear of the ferrite regions and results in a significantly lower overall wear rate.

The relative wear resistance of CM550 exceeding twice that of NM450 wear-resistant steel under identical friction conditions is a substantial improvement for coal mining applications. Furthermore, the friction coefficient of the CM550 surfacing plate is comparable to NM450, indicating that the improved wear resistance does not come at the cost of increased frictional resistance, which is important for conveyor energy efficiency.

Engineering Application Considerations

Suitability for Scraper Conveyor Mid-Slots

The wear motion characteristics of scraper conveyor mid-slots involve a combination of sliding abrasion from coal and rock particles, impact loading from falling material, and periodic mechanical shock from scraper chain engagement. The CM550 surfacing plate's high hardness (from Cr7C3 carbides) addresses the abrasive component, while the metallurgical bond to the Q235 substrate ensures structural integrity under impact loading. The ferrite matrix provides adequate toughness to resist impact cracking, complementing the carbide hardness.

Process and Quality Control Recommendations

For production implementation, the following quality control measures are recommended based on the findings of this study:

1. Metallurgical bond verification: Cross-sectional metallographic examination should confirm complete fusion between the surfacing layer and the Q235 substrate, with no slag inclusions, lack of fusion, or hot cracks at the interface.
2. Carbide morphology assessment: Microstructural examination at appropriate magnification (200× to 500×) should verify the presence of all three carbide morphologies. An absence of primary rod-shaped carbides may indicate improper cooling rates during surfacing.
3. Hardness mapping: Vickers hardness testing across the thickness of the surfacing layer should demonstrate uniform hardness distribution, with no soft zones that could initiate preferential wear.
4. Wear testing protocol: Sliding wear tests against quartz sand abrasive media should be conducted periodically as a lot-acceptance test, with results compared against established baseline data.

Comparative Analysis with Alternative Materials

The CM550 surfacing plate occupies an optimal position in the cost-performance spectrum for coal mining applications, offering substantially improved wear resistance over conventional wear-resistant steels at a reasonable cost premium.

Study Insights and Outlook

This research provides compelling evidence that high-chromium surfacing layers can effectively extend the service life of scraper conveyor mid-slots in underground coal mining. The multi-scale carbide reinforcement architecture and the self-protecting wear mechanism offer a robust solution to the persistent problem of conveyor component wear. For mining engineers, the practical implication is clear: replacing conventional Q235 mid-slots with CM550 surfaced alternatives can significantly reduce maintenance frequency and associated downtime costs. Future research should focus on field validation over extended service periods, investigation of the coating's performance under combined abrasive and impact loading, and optimization of the surfacing process parameters for large-scale production on conveyor mid-slot geometries.