Quality Characteristics of Hardfacing Repair on Scraper Conveyor Middle Trough

Literature Overview

This paper, published in Coal Mine Machinery (Vol. 33, No. 9, 2012, pp. 206-207) by researchers from Henan University of Technology and China Pingmei Shenma Group Equipment Leasing Branch, investigates the quality characteristics of hardfacing repair on scraper conveyor middle troughs. The study employs optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness testing, and impact toughness testing to characterize the overlay weld quality. The research aims to improve the working reliability and safety production cycle of scraper conveyors in coal mining operations.

Background and Problem Statement

Scraper conveyor middle troughs are critical components in underground coal mining operations. They are subjected to severe abrasive wear from coal, rock fragments, and water, leading to progressive material loss and eventual failure. Hardfacing repair is a common maintenance practice to restore wear surfaces, but the research indicates that repaired troughs often have low service life in actual operating conditions.

The primary quality concerns for hardfacing repair of conveyor troughs include:

Insufficient bond strength - Delamination between overlay and base metal
Inconsistent overlay thickness - Leading to premature wear through thin areas
Poor hardness uniformity - Soft spots that wear preferentially
Cracking - Thermal cracks in the overlay or base metal
Porosity - Gas pockets that reduce effective load-bearing area

Quality Analysis Methodology

Analysis Method	Purpose	Information Obtained
Optical microscopy	Microstructure characterization	Grain structure, phase distribution, defects
Scanning electron microscopy (SEM)	Fine-scale microstructure	Phase morphology, inclusion distribution, fracture surfaces
X-ray diffraction (XRD)	Phase identification	Carbide types, matrix phases, phase fractions
Hardness testing	Mechanical property evaluation	Hardness distribution, gradient, uniformity
Impact toughness	Fracture resistance	Energy absorption, ductility

Key Quality Determinants

The research identifies four critical factors that determine hardfacing repair quality:

Base metal condition - The condition of the original trough material, including any pre-existing defects, hardness variations, and residual stresses from previous service and repairs.
Welding material selection - The choice of hardfacing electrode or wire must be appropriate for the specific wear mechanism (abrasive, impact-abrasive, erosive) and must provide adequate bond strength to the base metal.
Repair process parameters - Welding current, voltage, travel speed, interpass temperature, and layer sequence all affect the final quality.
Welding method - The selection of welding process (SMAW, FCAW, GMAW, plasma arc, etc.) affects heat input, dilution, and process control.

Microstructural Characteristics

The overlay weld microstructure in scraper conveyor hardfacing typically consists of:

Matrix: Martensitic or austenitic matrix depending on the hardfacing alloy composition
Hard phases: Carbides (WC, Cr7C3, Fe3C, etc.) that provide wear resistance
Bond zone: Transition region between base metal and overlay showing dilution effects

The XRD analysis would identify the specific carbide phases present and their relative abundance. Common hardfacing alloys for conveyor troughs include:

High-carbon martensitic (e.g., D2, D3 type) - Good for moderate abrasion
Austenitic with carbides (e.g., D3 type) - Good for impact-abrasion
Carbide-containing (e.g., Stellite, tungsten carbide) - For severe abrasion

Hardness Distribution Analysis

The hardness profile across the overlay weld typically shows:

Base metal hardness - Usually 150-200 HB for structural steel troughs
Dilution zone - Transition from base metal hardness to overlay hardness
Overlay hardness - Target hardness of 40-60 HRC or higher for wear resistance
Surface hardness - May differ from bulk hardness due to cooling rate effects

Non-uniform hardness distribution is a common quality issue. Soft spots (below 35 HRC) in the overlay can lead to preferential wear, creating uneven surface profiles that accelerate subsequent wear.

Impact Toughness Considerations

Impact toughness is critical for conveyor trough applications because the trough is subjected to impact loads from falling material. The overlay weld must maintain adequate toughness to resist cracking under impact-abrasive conditions.

Key factors affecting overlay toughness:

Carbon content - Higher carbon increases hardness but decreases toughness
Carbide morphology - Fine, uniformly distributed carbides maintain toughness better than coarse, network-forming carbides
Matrix structure - Austenitic matrix provides better toughness than martensitic matrix
Residual stress - High tensile residual stress reduces effective toughness

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Delamination	Poor base metal preparation, excessive dilution	Thorough surface cleaning, controlled heat input
Cracking	High carbon content, rapid cooling, restraint stress	Preheat, controlled interpass temperature, post-weld stress relief
Porosity	Moisture in flux, contaminated base metal	Dry electrodes, clean base metal, proper shielding
Excessive dilution	High heat input, thin overlay layers	Reduce current, increase travel speed, build up in thin layers
Inconsistent thickness	Poor welder technique, inadequate inspection	Use multi-pass technique, measure thickness regularly

Engineering Practice Recommendations

Based on the quality analysis, the following improvements are recommended for hardfacing repair of scraper conveyor middle troughs:

Base metal preparation - Remove all existing worn material, rust, and contaminants. Grind to bare metal with adequate overlap into sound material.
Process parameter optimization - Use moderate heat input to minimize dilution while ensuring adequate penetration. Build up the overlay in multiple thin passes.
Filler material selection - Select hardfacing alloy based on the specific wear mechanism. For impact-abrasive conditions, consider austenitic alloys with carbides for better toughness.
Quality inspection - Implement systematic inspection including visual examination, thickness measurement, hardness testing, and magnetic particle testing for surface cracks.
Post-weld treatment - Consider controlled cooling or low-temperature stress relief to reduce residual stresses without affecting overlay hardness.

Quality Control Framework

Applying a PDCA (Plan-Do-Check-Act) approach to hardfacing repair quality management:

Plan - Define quality requirements, select appropriate materials and procedures, train welders
Do - Execute hardfacing repair following the qualified procedure with proper documentation
Check - Inspect completed repairs using appropriate NDT methods and mechanical testing
Act - Analyze quality data, identify trends, implement improvements to procedures and training

Study Insights and Reflections

This research addresses a practical and economically significant problem in coal mining operations. The quality of hardfacing repairs directly impacts equipment availability, maintenance costs, and safety. The study's identification of the four critical quality determinants provides a clear framework for improving repair quality.

The key insight is that hardfacing repair quality is not determined by any single factor but by the integrated performance of base metal condition, material selection, process parameters, and welding method. Improvements must be systematic rather than addressing individual factors in isolation.

The research also highlights the importance of characterizing the overlay weld using multiple analytical techniques. Optical microscopy provides general microstructural information, SEM reveals fine-scale features, XRD identifies specific phases, and mechanical testing quantifies performance. Only through this comprehensive approach can the true quality of the repair be assessed.

For practitioners in the mining industry, the study reinforces the need for disciplined quality management in hardfacing repair operations. The implementation of systematic inspection, documentation, and continuous improvement processes is essential for achieving reliable repair quality that translates into extended equipment service life and improved operational safety.