Study on Wear Resistance of Mining Picks Based on Plasma Cladding Technology

Literature Overview

This research investigates the application of plasma transferred arc (PTA) cladding technology for enhancing the wear resistance of mining picks used in continuous mining machines (CMCs). The study evaluates multiple overlay alloy systems, including Ni-Cr-C (Stellite-type), Fe-Cr-C-B, and Co-Cr-W compositions, applied to 40Cr steel pick bodies. Wear performance is evaluated through both laboratory testing (pin-on-disk, sand abrasion) and field trials in underground coal mining operations, providing a comprehensive assessment of the technology's practical viability for severe mining applications.

Core Viewpoints and Application Context

Mining picks experience extremely severe wear conditions characterized by high contact stresses (5-8 GPa), abrasive coal and rock particles (hardness ranging from 200 to 1500 HV), impact loading from coal cutting, and sometimes corrosive environments from water and methane. Conventional hardfacing welding of picks provides limited life extension (typically 2-3 times the base material life), while the PTA cladding approach investigated in this study achieves 5-8 times life extension with improved consistency and reduced residual stresses.

The key technical challenge in mining pick cladding is achieving sufficient hardness (>700 HV) while maintaining adequate fracture toughness (>15 MPa·m^0.5) to resist chipping and spalling during coal cutting. The study demonstrates that a two-layer cladding strategy, combining a Ni-Cr-C bond layer (2 mm thickness) with a Co-Cr-W wear layer (2.5 mm thickness), optimally addresses this challenge by providing both strong metallurgical bonding and superior wear resistance.

Microstructural Analysis and Wear Mechanism

The Co-Cr-W wear layer microstructure consists of a cellular dendrite structure with eutectic M6C carbides (Mo6C, W6C) distributed along cellular boundaries. The carbide morphology is predominantly blocky and plate-like with sizes of 3-8 μm, providing effective resistance to abrasive wear through both matrix deformation resistance and hard particle ploughing resistance. The volume fraction of carbide phases is approximately 25-30%, which represents the optimal balance between hardness and toughness for mining pick applications.

The wear mechanism analysis, conducted through scanning electron microscopy of worn surfaces, identifies three primary wear modes: (1) abrasive wear from hard rock particles ploughing the overlay surface, (2) adhesive wear from coal particle transfer during cutting, and (3) impact fatigue from repetitive loading during coal cutting. The PTA-cladded picks primarily fail through progressive material removal (abrasive wear) rather than catastrophic spalling, indicating that the cladding provides adequate toughness for the impact loading conditions encountered in mining.

Interface Integrity and Bond Strength

The interface between the overlay layer and the 40Cr steel substrate is critical for cladding durability. The study demonstrates that the Ni-Cr-C bond layer provides excellent metallurgical bonding with the 40Cr substrate, with no interfacial defects (cracks, lack of fusion, or excessive porosity) observed in the 2 mm bond layer. The diffusion zone at the interface extends approximately 50-80 μm into the substrate, with carbon and chromium enrichment creating a transition region that provides stress relief and prevents crack initiation at the interface.

The bond strength testing (shear test, ASTM B667) confirms that the interface strength exceeds 350 MPa, which is well above the typical requirement of 250 MPa for mining pick applications. The two-layer approach ensures that the high-stress interface region is protected by the tougher Ni-Cr-C layer, while the wear surface benefits from the harder Co-Cr-W composition.

Process Parameters and Quality Control

The PTA cladding process parameters optimized for mining pick application include: arc current of 280-320 A, arc voltage of 24-28 V, powder feed rate of 250-300 g/min, travel speed of 60-80 mm/min, and shielding gas flow of 20-25 L/min (argon). The 40Cr pick body is preheated to 250-350°C to prevent cold cracking in the bond layer, and the interpass temperature between overlay layers is maintained at 150-250°C.

Quality control measures include: visual inspection for surface defects and undercut, magnetic particle testing (MT) for surface and near-surface cracks in the bond layer, ultrasonic testing (UT) for internal porosity and lack of fusion, and hardness mapping across the overlay cross-section to verify uniform composition and dilution control. The acceptance criteria specify: no cracks longer than 5 mm, porosity less than 2% volume fraction, hardness variation within ±50 HV across the wear surface, and minimum bond strength of 300 MPa.

Field Trial Results and Economic Analysis

Field trials conducted in three underground coal mines over a period of 12 months demonstrate that PTA-cladded picks achieve an average life extension of 6.8 times compared to conventional hardfaced picks. The cladded picks exhibit consistent wear patterns without premature failure, and the failure mode transitions from catastrophic spalling (common with conventional hardfacing) to progressive material removal that can be predicted and scheduled for maintenance.

The economic analysis shows that while PTA cladding increases the initial cost per pick by approximately 45% compared to conventional hardfacing, the extended service life results in a net cost reduction of 35-45% per ton of coal produced. The reduced pick replacement frequency also decreases downtime for pick changes, providing additional productivity benefits that are not captured in the simple cost-per-ton calculation.

Study Insights and Reflections

This research provides compelling evidence that PTA cladding technology offers a significant performance advantage for mining pick applications compared to conventional hardfacing methods. The two-layer approach combining a tough bond layer with a hard wear layer represents an optimal engineering solution that addresses the competing requirements of interface integrity and surface performance.

A particularly valuable insight from the field trials is the improved predictability of pick life with PTA cladding. The progressive wear failure mode allows maintenance planning based on measured wear depth rather than unpredictable sudden failures, which is a significant operational advantage in underground mining environments where safety and productivity are paramount. This predictability also enables optimization of pick change intervals, reducing both unplanned downtime and premature pick replacement.

The research also highlights the importance of considering the complete pick body in the cladding design. The pick body material (40Cr steel in this study) significantly influences the bond layer design and overall cladding performance. Engineers applying this technology to different pick body materials (such as low-alloy steels or high-strength steels) should re-evaluate the bond layer composition and process parameters to ensure adequate metallurgical compatibility and interface integrity.