Plasma Cladding Repair of Conveyor Scraper Plates in Coal Mining Applications

Literature Overview

This study, authored by Fang Min, Yang Hongfei, Ju Chen, and Yuan Changsuo from ShenDong Coal Group and Xi'an Flare Heat Treatment Co., Ltd., addresses a highly practical engineering problem in coal mining operations: the wear-based failure of scraper plates used in underground conveyor systems. The publication, dated 2020 and categorized under mining machinery, focuses on plasma transferred arc (PTA) cladding as a repair technology for worn scraper plates. Given the harsh operating environment of coal mines—characterized by abrasive coal-rock mixtures, high impact loads, and continuous sliding friction—the scraper plate is one of the most critical consumable components in the mining conveyor chain. The authors propose a systematic PTA cladding repair process that aims to restore functional surface properties while extending service life significantly beyond what conventional welding or simple replacement can achieve.

Core Technical Approach

The fundamental challenge in scraper plate repair lies in achieving a hard, wear-resistant surface layer that maintains adequate bond strength with the base steel substrate under severe mechanical loading. Conventional arc welding repairs often introduce excessive heat input, leading to base metal dilution, microstructural degradation, and residual stress accumulation that can trigger cracking. PTA cladding, by contrast, employs a focused plasma arc with a tungsten electrode and a consumable nozzle to melt a powder feedstock onto the substrate. The plasma arc provides a highly concentrated heat source with a narrow molten pool, which minimizes thermal distortion and dilution while enabling precise control of the overlay composition.

The typical PTA process parameters for scraper plate repair include a plasma current range of 200 to 400 amperes, an arc length of 3 to 5 millimeters, and a travel speed of 200 to 600 millimeters per minute, depending on the desired overlay thickness and powder feed rate. The powder feedstock is commonly a high-chromium alloy (such as Cr15-Cr25 type) or a cobalt-based stellite-type alloy, selected for their exceptional abrasive wear resistance. The powder particle size typically ranges from 45 to 150 micrometers to ensure stable arc operation and uniform melt pool geometry.

Process Design and Engineering Considerations

The repair process typically follows a structured workflow. First, the worn scraper plate is inspected to determine the extent of material loss and any underlying cracks or defects. Surface preparation involves grinding down the worn area to a uniform depth, removing any oxide scale, rust, or embedded foreign material. A preheat temperature of 150 to 250 degrees Celsius is applied to reduce the risk of hydrogen-induced cracking and to moderate the cooling rate of the overlay.

The cladding is applied in multiple passes to build up the required thickness, usually 3 to 6 millimeters total. The interpass temperature is maintained below 300 degrees Celsius to prevent excessive grain growth in the previously deposited layer. Each pass is carefully monitored for uniform width and height to ensure consistent wear performance across the entire scraper surface. After cladding, the component undergoes post-weld heat treatment, typically a stress relief anneal at 550 to 650 degrees Celsius for 1 to 2 hours, to reduce residual stresses and improve the toughness of the overlay-to-base interface.

Defect Analysis and Countermeasures

During PTA cladding repair of scraper plates, several defect types are commonly encountered. Porosity, often caused by inadequate shielding gas flow or contaminated base metal, can be mitigated by ensuring clean surface preparation and maintaining a minimum argon flow rate of 15 to 20 liters per minute. Cracking at the overlay-base interface is another critical concern, particularly when high-carbon or high-chromium powders are used on carbon steel substrates. This can be addressed by selecting a compatible transition layer powder or by optimizing the powder composition to reduce carbon and chromium levels in the first pass. Spatter and incomplete fusion are less common but can occur if the arc length is too long or the travel speed is too fast.

Engineering Practice Integration

In the context of ShenDong Coal Group's operations, which manage some of the world's largest coal mines, the economic incentive for effective scraper plate repair is substantial. Replacing scraper plates on a large-scale conveyor system involves significant downtime, labor, and material costs. A well-executed PTA cladding repair can extend the service life of a scraper plate by 2 to 4 times compared to the original as-manufactured condition, depending on the powder selection and process parameters. The key insight from this literature is that PTA cladding is not merely a surface hardening technique but a comprehensive repair methodology that requires careful process planning, material selection, and quality control to deliver reliable results in the demanding environment of underground coal mining.

Study Insights and Implications

This work underscores the importance of process-structure-property relationships in wear-resistant cladding applications. The selection of powder composition directly governs the microstructure of the overlay layer—for instance, high-chromium powders produce a martensitic structure with carbide networks that provide exceptional abrasion resistance, while cobalt-based powders yield an austenitic or martensitic matrix with Co3W6C6 carbides that offer superior performance under impact-abrasion conditions. Understanding these metallurgical fundamentals enables engineers to tailor the cladding process to the specific wear mechanisms encountered in mining conveyor applications. Furthermore, the study reinforces the principle that repair welding is not simply a reversal of manufacturing defects but a deliberate engineering process that must account for the existing microstructural state of the component, the thermal history, and the operational loading conditions. The practical value of this literature lies in its direct applicability to industrial maintenance operations, where reducing component replacement frequency and minimizing downtime are primary economic drivers.