Application of Weld Overlay Technology in Repair of Fully Mechanized Coal Mining Equipment

Literature Overview

The technical paper by Zhang Xinhua and Li Jing (2007), from Shandong Jining Mining Group, provides a comprehensive overview of weld overlay technology applications in the repair of fully mechanized coal mining equipment. Fully mechanized mining systems include longwall shears, hydraulic supports, armored face conveyors (AFCs), and various auxiliary equipment. These components operate under extreme conditions—high dust, water exposure, impact loading, and abrasive contact with coal and rock. Wear and damage to critical surfaces are inevitable, and weld overlay repair provides an economical and effective solution for extending component life.

Core Technical Points

The paper covers multiple equipment types and repair scenarios, each requiring different overlay materials and processes. The selection of overlay material is driven by the specific wear mechanism and operating environment of each component.

Equipment Component	Wear Mechanism	Overlay Material	Overlay Process	Target Hardness
Shear drum cutting edge	Abrasive (coal + rock)	Cr-Mo-B hardfacing	SAW / FCAW	HRC 55–62
Shear drum body	Impact + abrasion	Ni-Cr alloy	GTAW / SAW	HRC 45–55
AFC chain sprocket teeth	Adhesive + abrasive	Cr-based hardfacing	SAW / PTA	HRC 58–65
Hydraulic support cylinder	Abrasive (dust ingress)	Ni-Cr-Mo alloy	GTAW / PTA	HRC 40–50
Support base plate	Impact + abrasion	Cr-Mo-B alloy	SAW / FCAW	HRC 50–60
Shear drum pins	Impact fatigue	Ni-base alloy	GTAW / laser	HRC 45–55

The diversity of overlay materials reflects the diversity of wear mechanisms encountered in fully mechanized mining. Cr-Mo-B alloys provide the highest hardness and are suitable for pure abrasive wear applications. Ni-Cr alloys offer a good balance of hardness, toughness, and corrosion resistance, making them suitable for mixed wear environments. Ni-base alloys provide excellent toughness and fatigue resistance for impact-loaded components.

Process Analysis and Engineering Practice

The paper emphasizes the importance of process selection based on the specific repair scenario. For large, flat surfaces such as shear drum bodies and support base plates, submerged arc welding (SAW) or flux-cored arc welding (FCAW) is preferred due to high deposition rates and good penetration. For curved surfaces such as hydraulic cylinders and pins, gas tungsten arc welding (GTAW) or plasma transferred arc welding (PTA) provides better control and more uniform bead geometry.

The repair procedure for a typical shear drum cutting edge involves:

Inspection — Visual examination and dimensional measurement to determine the extent of wear. Ultrasonic testing (UT) may be used to detect subsurface cracks.
Surface preparation — Grinding away the worn surface to a depth of 2–3 mm, exposing fresh base material. The surface must be clean and free of contaminants.
Preheating — Local preheating to 200–300 °C to reduce HAZ cracking risk, particularly for high-carbon or high-strength base materials.
Overlay welding — Application of the hardfacing alloy in multiple passes. The first pass may use a transition material to ensure good bond strength, followed by passes with the final hardfacing composition.
Post-weld treatment — Stress relief at 550–650 °C if required by the base material specification.
Machining — Grinding or turning the overlay surface to the required profile and surface finish.

Defect Analysis and FMEA Approach

Failure Mode	Effect	Cause	Detection	Prevention
Overlay cracking	Loss of wear protection	High carbon equivalent; thermal stress	MT / PT	Preheat; low-hydrogen wire; controlled cooling
Poor bond strength	Overlay delamination	Surface contamination; inadequate fusion	Bond strength test	Thorough cleaning; proper first-pass technique
Hardness below specification	Premature wear	Excessive dilution; wrong wire composition	Hardness survey	Multi-pass welding; verify wire composition
Dimensional deviation	Functional failure	Excessive welding distortion; insufficient overlay thickness	CMM measurement	Fixturing; controlled heat input; excess overlay
Residual stress	Late-stage cracking	Rapid cooling; large heat input	MT after stress relief	Stress relief; controlled cooling rate

Study Insights and Implications

The work by Zhang Xinhua and Li Jing is valuable for its comprehensive coverage of weld overlay applications across the full range of fully mechanized mining equipment. The paper demonstrates that weld overlay is not a one-size-fits-all solution but requires careful analysis of the wear mechanism, selection of appropriate materials, and application of the correct process for each specific repair scenario.

A key insight from this literature is the economic argument for overlay repair versus component replacement. For a longwall shear drum, the replacement cost can be tens of thousands of dollars, with a lead time of several weeks. An overlay repair can restore the drum to better-than-original condition in a few days at a fraction of the cost. The cumulative economic benefit across an entire mining operation is substantial.

Another important insight is the role of process parameter control in achieving consistent overlay quality. The paper emphasizes the need for documented welding procedures, qualified welders, and rigorous post-repair inspection. This aligns with the quality management principles outlined in standards such as NB/T 47014 (weld procedure qualification) and ASME Section IX.

The engineering implication is that weld overlay technology should be integrated into the preventive maintenance program of any fully mechanized mining operation. Rather than waiting for components to fail and then performing emergency repairs, scheduled overlay maintenance should be performed at planned intervals to extend component life and minimize unplanned downtime. This approach transforms weld overlay from a reactive repair technique into a proactive asset management strategy.