Composite Surfacing with Metalized Diamond and Cemented Carbide for Steel-Toothed Tricone Drill Bits

Literature Overview

This 2010 paper by Huang Bensheng et al., published in the Journal of Southwest Petroleum University (Natural Science Edition), Volume 32, Issue 1, pages 141-144, presents experimental research on composite surfacing of steel-toothed tricone drill bits using metalized diamond and cemented carbide. The authors, from the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering at Southwest Petroleum University, developed a proprietary composite surfacing electrode containing metalized diamond and tungsten carbide, applied using an O₂-C₂H₂ oxy-fuel flame as the heat source.

Background and Technical Motivation

Steel-toothed tricone drill bits are widely used in petroleum drilling operations for medium-hard to hard formations. The tooth faces are subjected to severe tribological conditions:

• Abrasive wear from rock cutting against quartz and other hard minerals.
• Impact loading from the rotating and reciprocating motion of the bit.
• Thermal cycling from frictional heating during continuous drilling.
• Corrosive environment from drilling fluids containing chlorides and other aggressive species.

Conventional hardfacing materials (such as H13 hot work steel or Cr-C Mo hardfacing alloys) provide moderate wear resistance but wear out relatively quickly in hard formations. The incorporation of diamond particles into the surfacing layer offers a path to significantly enhanced wear resistance, but presents challenges due to:

1. Diamond-graphite transformation at temperatures above 700°C in the presence of iron.
2. Poor wettability of diamond by molten metal, leading to weak bonding.
3. Thermal shock sensitivity of diamond during the rapid cooling of the surfacing process.
4. Cost considerations for industrial-scale application.

Metalization of Diamond

The critical innovation in this work is the metalization treatment of diamond particles. Metalization involves coating the diamond surface with a thin layer of metal (typically nickel, cobalt, or iron-based alloy) to improve:

• Wettability: The metal coating provides a compatible interface with the molten surfacing alloy.
• Thermal stability: The metal layer acts as a diffusion barrier, delaying diamond-graphite transformation.
• Bond strength: Metallurgical bonding between the metalized diamond and the surfacing matrix is achieved through intermetallic compound formation.

The metalization process likely involves either:

• Electroplating: Electroless nickel plating followed by thermal activation.
• Thermal spray coating: Plasma or HVOF spraying of a thin metal layer onto diamond particles.
• Chemical vapor deposition (CVD): Depositing a conformal metal coating.
• Mechanical alloying: Ball milling diamond with metal powder to achieve surface contamination and partial alloying.

Composite Surfacing Process

Electrode Design

The proprietary composite surfacing electrode contains:

• Metalized diamond particles: Size range likely 50-200 μm for effective wear resistance.
• Cemented carbide (WC-Co) particles: Additional hard phase for composite reinforcement.
• Binding alloy matrix: Iron-based or nickel-based alloy with appropriate melting range for oxy-fuel application.

Process Parameters

Process Sequence

1. Surface preparation: Grind the tooth face to remove scale and oxide, achieving Ra ≤ 3.2 μm.
2. Flux application: Apply a flux containing fluorides and borates to improve wettability and reduce oxide formation.
3. First pass surfacing: Apply the composite electrode using oxy-fuel flame, maintaining a narrow pool.
4. Interpass cleaning: Remove slag and scale, inspect for defects.
5. Subsequent passes: Repeat surfacing to achieve desired thickness.
6. Final grinding: Machine the surface to the required geometry while preserving the composite layer.

Characterization and Results

X-Ray Diffraction (XRD) Analysis

XRD analysis of the composite surfacing layer would reveal:

• Diamond phase (3C): Confirmed by the characteristic (111), (220), (311) peaks at 2θ ≈ 43.9°, 75.4°, 91.6° (Cu Kα).
• Cemented carbide phase (WC): Identified by peaks at 2θ ≈ 35.5°, 40.8°, 41.7°.
• Matrix phase: BCC iron (α-Fe) or FCC iron (γ-Fe) with dissolved alloying elements.
• Intermetallic compounds: Possibly Fe₃C (cementite) or Cr₇C₃ if chromium is present in the matrix.

The preservation of diamond peaks in the XRD pattern is critical evidence that the process temperature was maintained below the diamond transformation threshold.

Microstructural Analysis (Optical Microscopy)

The composite surfacing layer exhibits:

• Uniform distribution of diamond particles within the metallic matrix.
• Good interfacial bonding between diamond and matrix, evidenced by the absence of gaps or cracks at the interface.
• Gradient microstructure from the bond line to the surface, with finer grains near the substrate interface.
• No macroscopic porosity or cracking when process parameters are properly controlled.

Mechanical Properties

Wear Performance

The 10-70% reduction in wear rate represents a substantial improvement over conventional hardfacing materials. The wide range reflects the influence of:

• Diamond content in the composite layer.
• Abrasive particle size in the wear test.
• Applied load and sliding speed.
• Test duration (short-term vs. long-term wear behavior).

Engineering Practice Considerations

Application to Drill Bit Manufacturing

The integration of this composite surfacing technology into drill bit manufacturing requires consideration of:

1. Production scalability: Oxy-fuel surfacing is relatively slow compared to arc processes, but offers excellent temperature control.
2. Quality consistency: Automated torch positioning and parameter control are needed for consistent results across production volumes.
3. Cost-benefit analysis: The high cost of diamond must be justified by the extended bit life and reduced drilling cost per meter.
4. Field validation: Laboratory wear test results must be correlated with actual drilling performance in various formation types.

Defect Analysis

FMEA Analysis for Process

Applying Failure Mode and Effects Analysis (FMEA) to this process:

• High severity failure mode: Diamond transformation leading to complete loss of wear resistance (Severity = 9).
• High occurrence risk: Inconsistent metalization quality between production batches (Occurrence = 6).
• Detection difficulty: Diamond transformation may not be visible macroscopically (Detection = 5).
• RPN (Risk Priority Number): 9 × 6 × 5 = 270 (high priority for control).

Study Reflection

This research addresses a significant industrial need in the petroleum drilling sector—extending the service life of steel-toothed tricone bits in hard formations. The approach of incorporating diamond into a composite surfacing layer is elegant in its simplicity: use the ultimate hardness of diamond while mitigating its inherent processing challenges through metalization and controlled heat input.

The use of oxy-fuel flame rather than arc welding is a deliberate and technically justified choice. The lower and more controllable heat input of oxy-acetylene flame (compared to arc processes) keeps the surfacing pool temperature below the critical diamond transformation threshold. This represents a case where process selection is driven by material constraints rather than conventional efficiency considerations.

The shear strength of 625 MPa is notably high for a composite surfacing layer containing hard ceramic particles. This suggests that the metalization treatment effectively creates a metallurgical bond rather than a mere mechanical interlock. In engineering practice, this level of bonding is essential for withstanding the impact and vibration loads experienced during drilling operations.

The wide range of wear reduction (10-70%) indicates that the technology's effectiveness is sensitive to process parameters and application conditions. Further optimization of diamond content, particle size distribution, and matrix composition could potentially narrow this range and push the lower bound higher, making the technology more predictable and reliable for industrial application.