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

Literature Overview

This 2007 study by Zhang Xinhua and Li Jing, conducted at the Shandong Jining Mining Group, provides a comprehensive review of weld overlay technology applications across the full spectrum of fully mechanized mining equipment repair. The scope encompasses shearer cutting picks, hydraulic support legs, scraper conveyor chains, transport scraper blades, and various wear components subjected to severe abrasive and impact loading. The literature reflects the maturation of weld overlay as a standard maintenance practice in Chinese coal mining operations, integrating process selection, consumable optimization, and quality control into a systematic repair methodology.

Technical Scope and Equipment Categories

The fully mechanized mining system consists of several major equipment categories, each with distinct wear mechanisms and repair requirements:

Equipment Component	Wear Mechanism	Typical Base Material	Recommended Overlay Material	Target Hardness
Shearer cutting picks	Abrasive (coal + rock)	40Cr, 42CrMo	Cr-based hardfacing (D256/D257)	55-60 HRC
Shearer drum housing	Abrasive + impact	Q345, 16Mn	Ni-Cr alloy (D117) or high-Si cast iron	45-50 HRC
Hydraulic support leg cylinder	Adhesive + abrasive	20# steel, 45# steel	Ni-based (D172) or Cr-based	40-48 HRC
Hydraulic support piston rod	Adhesive wear	40Cr, 27SiMn	Ni-Cr alloy or Stellite-type	42-48 HRC
Scraper conveyor chain link	Abrasive + impact	45# steel, 50Mn	Cr-based hardfacing	50-58 HRC
Scraper conveyor scraper	Abrasive	45# steel	High-Si cast iron or Cr-based	45-55 HRC
Transport scraper blade	Abrasive	Q235, Q345	Cr-based or Ni-based	45-55 HRC
Gate valve seats	Erosion + corrosion	Cast iron, carbon steel	Ni-based or Co-based	40-48 HRC

Process Selection Matrix

The selection of the overlay process depends on the component geometry, required overlay thickness, field availability, and production volume:

Process Comparison

Process	Applicable Components	Overlay Thickness	Field Applicability	Equipment Cost	Production Rate
SMAW (stick welding)	Large structural components, thick overlay	3-15 mm	Excellent	Low	Moderate
SAW (submerged arc)	Flat or slightly curved surfaces, large area	5-20 mm	Limited (requires flux handling)	Moderate	High
GMAW (MIG/MAG)	Curved surfaces, medium sections	2-10 mm	Good	Moderate	High
Oxy-acetylene flame	Field repairs, large components	2-8 mm	Excellent	Low	Low
Plasma transferred arc (PTA)	Precision overlay, thin layers	0.5-3 mm	Limited (requires inert gas)	High	High
Laser cladding	Precision repair, thin layers	0.3-2 mm	Limited	High	Moderate

Quality Control Framework

The study emphasizes a systematic quality control approach based on the PDCA (Plan-Do-Check-Act) cycle:

Plan Phase

Determine the wear mechanism and select the appropriate overlay material
Define the overlay geometry, thickness, and hardness specification
Establish the welding procedure specification (WPS) with approved parameters
Identify critical quality characteristics and inspection requirements

Do Phase

Perform surface preparation to sound metal
Execute the overlay welding according to the WPS
Maintain interpass temperature and control welding sequence
Apply post-weld heat treatment as specified

Check Phase

Visual inspection of the overlay surface for defects
Non-destructive testing (MT for surface cracks, PT for fine cracks)
Hardness testing at multiple locations to verify uniformity
Dimensional inspection after machining
Bond strength testing (peel test or shear test) on coupon samples

Act Phase

Document the repair results and service life performance
Analyze failure modes of repaired components for continuous improvement
Update the WPS based on field performance data

Key Technical Challenges and Solutions

Cracking in Cr-based hardfacing deposits — Cr-based alloys have high carbon equivalent and are prone to cracking, particularly in thick sections. The solution involves preheating to 250-350 °C, controlling interpass temperature below 300 °C, using a slightly lower carbon electrode grade, and applying post-weld stress relief at 580-620 °C.
Dilution effects on hardness — In single-pass overlay on thick base metal, dilution can reduce the overlay hardness by 5-15 HRC. The solution is to use a multi-pass approach with a transition layer, or to select a higher-carbon consumable to compensate for dilution.
Distortion of large structural components — Hydraulic support legs and scraper conveyor chains are particularly susceptible to distortion. The solution involves symmetrical welding sequence, back-plate clamping, and stress relief before machining.
Service life variability — The wear life of overlaid components varies significantly depending on the coal seam geology, equipment operating conditions, and maintenance practices. The solution involves regular inspection and re-overlay before the overlay layer is completely worn through, maintaining a minimum residual thickness of 2-3 mm.

Study Insights

This literature represents a mature engineering practice in Chinese coal mining, where weld overlay has become an integral part of the maintenance strategy for fully mechanized mining equipment. The systematic approach to process selection, consumable optimization, and quality control reflects a level of engineering discipline that is essential for reliable equipment operation. The key insight is that weld overlay repair is not a one-time fix but a recurring maintenance activity that requires ongoing monitoring, documentation, and process improvement. The integration of PDCA methodology into the repair workflow ensures that each repair cycle builds on the experience of the previous one, leading to continuous improvement in repair quality and service life.