Application of Cladding Technology in Pick Bits

Literature Overview

This 2003 study published in Mining Machinery by researchers from Shandong University of Science and Technology and Tai'an Keda Jina Plasma Technology Co., Ltd. examines the application of weld overlay cladding technology to extend the service life of pick bits used in continuous mining machines. The research addresses a significant economic challenge in underground coal mining operations, where pick bits experience extreme wear conditions and represent a substantial consumable cost. The study evaluates different cladding approaches, including plasma arc welding and gas metal arc welding overlay techniques, applied to tungsten carbide-tipped and solid steel pick bits.

Core Technical Content

Pick bits in continuous mining machines operate under severe tribological conditions characterized by high impact loading, abrasive wear from coal and rock, and cyclic stress from the reciprocating motion of the cutting head. The typical service life of an uncladded pick bit tip ranges from 20 to 80 meters of coal cut, depending on the hardness and abrasiveness of the coal seam. Cladding technology aims to extend this life by applying a wear-resistant overlay layer to the tip surface.

Cladding Process Selection

The study evaluated multiple cladding processes for pick bit application, each with distinct advantages and limitations.

The plasma arc welding process emerged as the preferred method for pick bit cladding due to its low dilution ratio, precise heat input control, and ability to produce dense, well-bonded overlays. The process parameters are optimized to achieve a dilution ratio below 35 percent, ensuring that the overlay retains sufficient wear-resistant phases.

Overlay Material Design

The cladding alloys developed and tested in this study belong to the Cr-B-W-V iron-based system, which produces a microstructure consisting of hard carbide particles dispersed in a tough matrix. The key alloying elements and their roles are as follows:

• Chromium (6-12 wt%): Forms Cr7C3 and Cr23C6 carbides, providing hardness and oxidation resistance
• Boron (0.5-1.5 wt%): Refines grain structure and promotes formation of hard B-rich phases
• Tungsten (3-8 wt%): Forms W2C and WC carbides, contributing to high hardness and wear resistance
• Vanadium (1-3 wt%): Forms VC and V4C3 carbides, enhancing toughness and resistance to thermal cycling

The resulting overlay microstructure typically exhibits hardness values in the range of 800-1100 HV, with a complex carbide morphology that provides excellent resistance to both abrasive and adhesive wear. The matrix hardness is approximately 350-450 HV, providing sufficient toughness to resist chipping under impact loading.

Microstructural Analysis

Metallographic examination of the cladded pick bit tips revealed several important features. The overlay layer exhibits a columnar-to-equiaxed transition in the solidification structure, with the columnar zone adjacent to the substrate and an equiaxed zone at the surface. The columnar zone has a grain size of approximately 50-100 μm, while the equiaxed zone has a grain size of 20-50 μm. The carbide particles are predominantly of the M7C3 type, with sizes ranging from 2 to 10 μm, distributed relatively uniformly throughout the overlay.

At the cladding-bond line, a thin diffusion zone of approximately 20-50 μm is observed, where chromium and carbon have diffused into the base metal. This diffusion zone is critical for bond strength and should be monitored during quality control. Excessive diffusion, which can occur if the interpass temperature is too high, leads to the formation of brittle phases and reduced bond strength.

Engineering Practice Implications

The implementation of pick bit cladding technology in mining operations requires careful consideration of several practical factors:

1. Substrate preparation: The pick bit tip surface must be machined to remove decarburized layers and surface defects. A groove of 1.5-2.0 mm depth and 45-degree included angle is typically prepared to ensure adequate overlap and bond strength.
2. Heat treatment: Post-weld heat treatment at 550-650°C for 1-2 hours is recommended to relieve residual stresses and optimize the carbide morphology. The cooling rate should be controlled to prevent the formation of tempered martensite in the heat-affected zone.
3. Quality control: Each cladded pick bit should undergo visual inspection for surface defects, ultrasonic testing for bond line integrity, and hardness testing at multiple locations across the overlay surface.
4. Application method: The cladding can be applied during pick bit manufacturing or as a field repair process. Manufacturing-stage cladding provides better quality control but requires specialized equipment. Field repair cladding is more flexible but requires skilled welders and portable equipment.

The economic analysis presented in the study demonstrates that cladded pick bits provide a cost reduction of 30-60 percent compared to uncladded bits, even after accounting for the additional cladding process cost. The payback period for cladding equipment investment is typically less than 12 months in high-volume mining operations.

Critical Reflections and Study Insights

This research illustrates the practical value of adapting cladding technology from pressure vessel and process equipment applications to mining machinery. The key insight is that the selection of cladding process and alloy must be driven by the specific wear mechanism encountered in service. For pick bits, the dominant wear mechanisms are abrasive wear from coal and rock particles, impact wear from cyclic loading, and adhesive wear from metal-to-metal contact. The Cr-B-W-V alloy system addresses all three mechanisms simultaneously, making it a well-suited choice for this application.

One important observation from the study is the sensitivity of cladding quality to process parameters. Small variations in welding current, travel speed, and powder feed rate can lead to significant changes in overlay dilution ratio and microstructure. This sensitivity necessitates strict process control and operator training. In practice, the use of semi-automatic or automated cladding equipment with programmable parameters significantly improves consistency and reduces the dependence on individual operator skill.

The study also highlights the importance of understanding the substrate-overlay compatibility. Pick bits are typically made from low-carbon steel or medium-carbon steel, which has different thermal expansion characteristics and thermal conductivity compared to the iron-based overlay alloy. This mismatch can lead to residual stresses at the bond line, which may initiate cracks under cyclic loading. The selection of an appropriate transition layer or the use of low-heat-input welding processes can mitigate this issue.

Reference Value and Outlook

The application of cladding technology to pick bits represents a successful example of technology transfer from the pressure vessel industry to the mining sector. The principles of overlay design, process optimization, and quality control developed in this study are applicable to other mining components such as bucket teeth, conveyor rollers, and crusher jaws. Future developments should focus on the development of multi-layer overlay designs that combine wear resistance with impact toughness, and on the integration of cladding with advanced manufacturing techniques such as additive manufacturing. The economic benefits demonstrated in this study make cladding technology an attractive option for mining companies seeking to reduce consumable costs and improve operational efficiency.