Wear-Resistant Overlay Materials for Coal Mining Machine Picks

Literature Overview

The paper by Ma Shihui and Zhang Jinjuan (2013, Hot Working Technology, Vol. 42, No. 23, pp. 194-196) investigates five different overlay welding materials for the wear-resistant layer of coal mining machine picks (cutting picks). The study evaluates the microstructure, hardness, and wear resistance of the overlay layers produced from each material, with and without post-weld heat treatment. The findings provide technical guidance for selecting appropriate overlay consumables to extend the service life of mining picks, which are subject to extreme abrasive and impact loading during coal cutting operations.

Mining Pick Service Environment and Failure Modes

Coal mining machine picks experience a uniquely severe service environment characterized by:

Abrasive wear: Continuous contact with coal, rock, and coal gangue produces severe abrasive wear, particularly at the pick tip and cutting edge.
Impact loading: Picks encounter sudden impact loads when striking hard rock inclusions or coal seams with variable hardness.
Thermal cycling: Frictional heating during cutting, combined with cooling by water spray or air, creates thermal gradients and cyclic stresses.
Bending and torsional loading: Picks are subjected to complex multiaxial loading during the cutting operation, which can initiate fatigue cracks.

The primary failure modes of mining picks are:

Tip wear and blunting, reducing cutting efficiency
Edge chipping and spalling due to impact
Fatigue cracking at the pick body or weld interface
Delamination of the overlay layer from the base material

Comparative Evaluation of Overlay Materials

The study evaluates five overlay materials, each representing a different metallurgical approach to wear resistance:

Material Type	Typical Composition	Expected Hardness	Microstructure	Wear Mechanism
High-carbon martensitic	High C, Cr, Mo	High (600-700 HV)	Martensite + retained austenite	Hardness-based abrasion resistance
High-chromium white cast iron	Cr > 12%, C > 3%	Very high (800-1000 HV)	M7C3 carbides in martensitic matrix	Carbide-based abrasion resistance
Carbide composite	WC or Cr7C3 particles	High (700-900 HV)	Carbide particles in metallic matrix	Particle-based abrasion resistance
Austenitic	Mn, Ni, Cr	Moderate (300-400 HV)	Austenite + retained carbides	Strain-hardening-based resistance
Hardfacing alloy	Ni-Cr or Co-based	Variable	Complex microstructure	Multi-mechanism resistance

Effect of Heat Treatment on Overlay Layer Properties

Post-weld heat treatment significantly influences the microstructure and mechanical properties of the overlay layers:

Tempering: Reduces hardness slightly but significantly improves toughness and reduces residual stresses. This is particularly beneficial for impact-prone applications.
Annealing: Relieves residual stresses without significant hardness reduction. Suitable for preventing delayed cracking in thick overlay deposits.
Solution treatment and quenching: Can refine the microstructure and redistribute carbides, potentially improving both hardness and toughness.
No heat treatment (as-welded): Maintains maximum hardness but retains high residual stresses, which may lead to cracking under service loading.

The optimal heat treatment depends on the specific overlay material and the service conditions. For example, high-chromium white cast iron overlay layers benefit from tempering to reduce brittleness, while high-carbon martensitic overlays may require no heat treatment to maintain maximum hardness.

Engineering Practice Recommendations

Based on the metallurgical analysis and the severity of mining pick service conditions, the following selection criteria are recommended:

For high-abrasion, moderate-impact conditions: High-chromium white cast iron overlay layers provide the best abrasion resistance due to the hard M7C3 carbide network. Temper after welding to improve toughness.
For moderate-abrasion, high-impact conditions: Carbide composite overlay layers offer a good balance of abrasion resistance and impact toughness. The metallic matrix absorbs impact energy while the carbide particles provide abrasion resistance.
For variable loading conditions: Austenitic overlay layers that strain-harden during service provide adaptive wear resistance. The hardness increases at the wear surface due to strain hardening, while the bulk retains toughness.
For critical applications requiring maximum reliability: Multi-pass overlay welding with a transition layer between the base material and the hardfacing layer can reduce residual stresses and improve metallurgical compatibility.

Key Insights and Reflections

The study underscores the importance of matching overlay material properties to the specific service conditions. A one-size-fits-all approach to overlay material selection is inappropriate because the relative severity of abrasive wear, impact loading, and thermal cycling varies significantly across different mining applications.

The heat treatment findings highlight a critical practical consideration that is often overlooked in field applications. Many mining operations apply overlay welding repairs without post-weld heat treatment due to equipment limitations and time constraints. However, the residual stresses in as-welded overlay layers can lead to premature failure under service loading. Portable induction tempering equipment should be considered for critical pick repairs.

The five-material comparison approach is methodologically sound and provides engineers with a systematic framework for material selection. However, the study would benefit from additional information on the wear test methodology, including the specific wear test apparatus, counterface material, test duration, and wear measurement technique. Standardized wear testing according to ASTM G99 or ISO 9357 would enhance the comparability of results.

The economic aspect of overlay material selection also deserves consideration. While high-performance overlay materials may extend pick life significantly, the total cost of ownership must account for material cost, welding labor, heat treatment, and downtime. A systematic cost-benefit analysis should guide the final selection decision in each specific application.

The findings of this study contribute to the ongoing development of wear-resistant overlay technologies for mining applications, where the relentless demands of the service environment continue to drive innovation in material design and welding process optimization.