Hardfacing of Chevron Wear-Resistant Patterns on Scraper Conveyor Middle Troughs

Literature Overview

This 1996 study by Ge Changlu and Ye Rongchang from China University of Mining and Technology, Xuzhou, published in Welding Technology (Vol. 25, No. 4, pp. 24-25), addresses a critical wear problem in underground coal mining equipment. The scraper conveyor middle trough (also known as the center trough or chain trough) is one of the most heavily consumed components in longwall mining systems, where abrasive coal-rock mixtures continuously slide over the trough surface, causing severe material loss. The authors developed a novel boron carbide-fluxed hardfacing electrode and applied a systematic chevron-pattern hardfacing technique to dramatically extend service life without altering the base material or fabrication process of the trough.

Core Technical Approach

The fundamental challenge in scraper conveyor trough hardfacing lies in the enormous surface area that must be covered, the requirement for uniform wear resistance across the entire contact surface, and the need to avoid introducing excessive running resistance that would increase power consumption and accelerate scraper and chain wear. The authors addressed these challenges through three coordinated measures: electrode development, pattern design, and process parameter optimization.

Electrode Development

The newly developed hardfacing electrode employs a boron carbide (B4C) flux coating, which serves a dual purpose. First, B4C is one of the hardest known ceramics (Knoop hardness approximately 2400-2500), and its incorporation into the weld deposit significantly raises the microhardness of the hardfacing layer. Second, the flux coating provides shielding, deoxidation, and alloying functions during the SMAW (Shielded Metal Arc Welding) process. The flux system was specifically formulated to ensure stable arc characteristics, low spatter, and good slag detachability, which are essential for high-productivity field application on large trough surfaces.

Chevron Pattern Design

The chevron (herringbone or "人字形") pattern was selected over conventional cross-hatch or straight-line patterns for several engineering reasons. The V-shaped geometry provides a self-aligning effect that guides coal and debris toward the center of the trough, reducing lateral material accumulation at the trough edges. More importantly, the angled weld beads create a surface profile that increases the effective contact area between the scraper chain and the trough while distributing the sliding friction force more evenly. The pattern dimensions—bead width, bead spacing, and chevron angle—were optimized to balance wear resistance against surface roughness, as excessive roughness would increase the running resistance of the conveyor system.

Process Parameters and Heat Input Control

The hardfacing was performed using SMAW with the B4C-fluxed electrode on the existing Q235 or Q345 carbon steel trough surface. A critical aspect of the process was maintaining controlled heat input to prevent excessive dilution of the hardfacing layer by the base metal. The authors employed the following process measures:

1. Preheating: The trough surface was preheated to 150-200 degrees Celsius to reduce the risk of cracking in the hardfacing layer and to minimize residual stress concentration at the weld-base metal interface.
2. Interpass temperature control: The interpass temperature was maintained below 250 degrees Celsius to limit grain growth in the hardfacing layer and to preserve the integrity of the B4C particles.
3. Travel speed optimization: A moderate travel speed was selected to ensure adequate penetration while limiting the width of the heat-affected zone in the base metal.
4. Direction of welding: The chevron pattern was welded in a specific sequence to minimize cumulative distortion of the trough plate, which is typically 8-12 mm thick.

Performance Results and Engineering Validation

The study reports that the 764-type middle trough, after application of the chevron hardfacing technique, demonstrated a remarkable improvement in service life. The coal throughput capacity increased from approximately 500,000 tonnes to 200,000-250,000 tonnes, representing a threefold or greater increase in operational life. This improvement was achieved without increasing the running resistance of the conveyor system or accelerating the wear of scraper chains and links—a crucial finding that confirms the adequacy of the chevron pattern design.

The wear mechanism in scraper conveyor troughs is predominantly abrasive wear, where hard mineral particles embedded in the coal-rock mixture act as third-body abrasives against the trough surface. The B4C-containing hardfacing layer resists this abrasion through a combination of high hardness (typically 50-60 HRC for the deposit) and the presence of hard ceramic phases that resist micro-cutting and micro-ploughing by abrasive particles.

Critical Analysis and Engineering Insights

Dilution Management

One of the persistent challenges in hardfacing large flat surfaces with SMAW is the high dilution rate, which can range from 20-40% depending on the electrode type, travel speed, and number of passes. Dilution reduces the effective concentration of hardfacing alloy elements in the weld deposit, potentially compromising the wear resistance. The authors mitigated this through the use of a single-layer deposit with high-alloy-content electrode, combined with controlled travel speed to limit heat input. In practice, for trough hardfacing applications, a dilution rate below 30% is generally acceptable when the base metal hardness is below 200 HV.

Residual Stress and Distortion

Hardfacing large flat plates inevitably introduces residual stresses that can lead to warping or distortion of the trough geometry. The chevron pattern, with its alternating weld directions, provides a degree of self-balancing that reduces net distortion compared to unidirectional welding. However, for troughs exceeding 2 meters in length, the cumulative thermal strain can still cause measurable out-of-flatness. In engineering practice, post-hardfacing stress relief at 550-600 degrees Celsius for 1-2 hours is recommended for critical applications where dimensional accuracy affects conveyor alignment.

Field Application Considerations

The study emphasizes that the technique does not require changes to the trough material or manufacturing process, which is a significant advantage for existing mining equipment fleets. Retrofit hardfacing can be performed on worn troughs during scheduled maintenance, avoiding the need for complete trough replacement. The B4C-fluxed electrode was designed for practical field use, with considerations for electrode handling, arc stability in confined mine environments, and slag removal efficiency.

Connection to Pipe and Trough Manufacturing Standards

While this study focuses on mining equipment rather than piping, the hardfacing principles are directly applicable to the wear protection of pipe systems in slurry transport, mining pipelines, and material handling conveyors. The API 5L pipeline standard and ISO 3183 do not specifically address hardfacing, but the welding procedure qualification requirements under ASME Section IX and ISO 15614 provide the framework for qualifying hardfacing procedures. The mechanical property requirements for hardfacing layers are typically specified by the end user rather than by pipe standards, and hardness levels of 40-60 HRC are common for abrasive wear applications.

Study Insights and Implications

This 1996 study remains highly relevant to modern engineering practice, particularly in the context of sustainable mining operations where extending component life reduces waste and operational costs. The systematic approach—combining material development (B4C electrode), pattern design (chevron geometry), and process optimization (SMAW parameters)—provides a model for solving wear problems in other large-surface applications. The threefold improvement in service life with no adverse effect on system performance is a compelling result that validates the engineering approach. For contemporary applications, the same principles can be extended using advanced hardfacing techniques such as submerged arc hardfacing (SAW) or flux-cored wire hardfacing (FCAW) to further increase productivity on large trough surfaces, while the chevron pattern design remains a valuable geometric solution for managing material flow and distributing wear.