Improvement of Hardfacing Electrodes for Drill Bit Tooth Surface Overlay

Literature Overview

This technical paper by Tang Xiaowen, Liu Chengjie, Huang Bensheng, and Yang Mei from the Department of Mechanical Engineering at Southwest Petroleum Institute, published in Petroleum Field Machinery in 2000 (Vol. 29, No. 1, pp. 27-28), addresses a specific and practical problem in oilfield drilling tool maintenance: the improvement of hardfacing electrodes used for overlay welding on tricone drill bit tooth surfaces. The study proposes replacing cast tungsten carbide particles with spheroidal sintered tungsten carbide particles and reducing the tube thickness in tube-packed granular cast tungsten carbide electrodes, resulting in thin-skin tube-packed spheroidal sintered tungsten carbide electrodes.

Background and Problem Statement

Tricone drill bits are critical tools in rotary drilling operations, and their tooth surfaces experience severe abrasive and impact wear during drilling. The tooth surfaces are typically overlay welded with tungsten carbide-containing hardfacing materials to extend service life. Traditional electrodes use cast tungsten carbide particles packed in a metallic tube, which provides good tungsten carbide content in the deposited layer but suffers from several limitations.

The key issues with traditional cast tungsten carbide electrodes include: irregular particle morphology leading to poor packing density and uneven distribution in the weld pool, susceptibility to fracture during welding due to thermal shock, and relatively high tube thickness that contributes excess iron dilution to the deposited layer.

Technical Solution and Design Rationale

The proposed improvement involves two key modifications: replacing cast tungsten carbide particles with spheroidal sintered tungsten carbide particles, and reducing the tube wall thickness. Each modification addresses specific limitations of the traditional design.

Spheroidal Sintered Tungsten Carbide Particles

The use of spheroidal sintered tungsten carbide particles offers several advantages over cast particles:

• Uniform morphology: Spherical particles pack more uniformly, providing consistent tungsten carbide distribution in the weld pool.
• Reduced thermal shock: Spherical geometry eliminates stress concentration points present in irregular cast particles, reducing the likelihood of particle fracture during the rapid heating and cooling cycles of arc welding.
• Improved melting behavior: Spheroidal particles melt more uniformly in the arc pool, promoting better integration with the metallic matrix.
• Higher density: Sintered particles have higher density than cast particles, providing more tungsten carbide content per unit volume.

Reduced Tube Thickness

Reducing the tube wall thickness from the traditional design decreases the amount of iron dilution introduced into the weld pool. The tube material typically melts and dilutes with the tungsten carbide-containing filler, reducing the effective tungsten carbide content in the deposited layer. A thinner tube wall reduces this dilution effect, allowing a higher proportion of tungsten carbide to be retained in the final overlay.

Experimental Results and Performance Evaluation

The indoor testing program demonstrated that the improved electrode with spheroidal sintered tungsten carbide particles and reduced tube thickness achieved significantly enhanced impact abrasion resistance. The study identifies the quantity of spheroidal tungsten carbide particles as a critical factor influencing the wear resistance of the deposited layer.

Key Performance Indicators

The impact abrasion resistance test is particularly relevant to drill bit tooth service conditions, where the tooth surface experiences both abrasive wear from rock fragments and impact loading from the drill string. The combination of these two wear modes creates a demanding service environment that requires the overlay material to possess both hardness and toughness.

The improvement in impact abrasion resistance can be attributed to several factors: the more uniform distribution of tungsten carbide particles in the deposited layer, the reduced likelihood of particle fracture during welding, the higher effective tungsten carbide content due to reduced iron dilution, and the improved particle-matrix bonding facilitated by the more uniform melting behavior of spheroidal particles.

Engineering Practice Integration

For drilling tool maintenance engineers, this study provides a practical and implementable improvement to existing electrode designs. The key implementation considerations include:

• Sourcing of spheroidal sintered WC particles: The particle size distribution and sintering quality must be controlled to ensure consistent performance.
• Tube manufacturing: The reduced tube thickness requires careful manufacturing to maintain tube integrity during electrode production and handling.
• Welding parameter adjustment: The changed electrode geometry and filler composition may require adjustments to welding parameters (current, voltage, travel speed) to achieve optimal deposition.
• Quality control: Inspection of the deposited overlay should verify tungsten carbide content, particle distribution, and hardness profile.

Field Application Considerations

In field applications, the improved electrode should be evaluated through actual drilling performance tests. Key metrics include:

• Number of meters drilled per bit
• Rate of penetration (ROP)
• Bit wear rate
• Cost per meter drilled
• Frequency of bit re-sharpening or replacement

The economic justification for adopting the improved electrode depends on the cost differential between the improved and traditional electrodes versus the extended service life and reduced downtime.

Study Insights and Implications

This paper exemplifies the incremental innovation approach that is often most effective in industrial welding applications. Rather than proposing a revolutionary new technology, the authors identify specific limitations in an existing electrode design and propose targeted modifications that address those limitations. The result is a practical improvement that can be implemented with minimal disruption to existing manufacturing and welding practices.

The emphasis on impact abrasion resistance is particularly relevant to the drilling industry, where the wear environment is a complex combination of abrasive, impact, and adhesive wear. The study's finding that the quantity of spheroidal tungsten carbide particles is a critical factor provides a clear design parameter for further optimization. For the welding consumables industry, this study highlights the importance of filler metal particle morphology and distribution in determining overlay performance, a principle that extends to other hardfacing applications beyond drill bit teeth.