Composite Overlay of Tooth Surface on Steel-Toothed Tooth Wheel Drill Bits

Literature Overview

Steel-toothed tooth wheel drill bits are critical tools used in petroleum drilling operations, particularly for drilling through hard rock formations. The tooth surface of these drill bits is subjected to extreme mechanical and thermal loads during drilling, making wear resistance and toughness the primary performance requirements. The literature on composite overlay welding of the tooth surface addresses the challenge of combining wear-resistant hardfacing materials with the tough structural steel body of the drill bit to create a durable, high-performance drilling tool.

Design Requirements and Material Selection

The tooth surface of a steel-toothed tooth wheel drill bit must withstand:

1. Abrasive wear from contact with hard rock formations
2. Impact loading from drilling vibrations and formation heterogeneity
3. Thermal cycling from frictional heating during drilling
4. Compressive stress from the weight on bit (WOB) applied during drilling

The composite overlay approach involves applying a hard, wear-resistant surface layer over a tough, ductile substrate. This layered structure provides the best of both worlds: the substrate provides structural integrity and resistance to fracture, while the overlay provides the surface hardness and wear resistance required for extended drilling life.

Overlay Material Systems

The literature evaluates several overlay material systems for tooth surface composite overlay:

The selection of overlay material depends on the geological conditions of the drilling formation. For soft to medium formations, high-carbon martensitic overlays are sufficient and cost-effective. For hard, abrasive formations, cemented carbide or ceramic-metal composite overlays are preferred despite their higher cost.

Welding Process and Parameters

The composite overlay of drill bit tooth surfaces typically employs multi-pass welding with a combination of processes. The literature describes a typical approach using submerged arc welding (SAW) for the transition layer and gas metal arc welding (GMAW) or plasma transferred arc (PTA) for the final hardfacing layer.

The SAW transition layer provides good bonding with the structural steel substrate and ensures a gradual transition in composition and properties. The GMAW or PTA final layer deposits the wear-resistant hardfacing material with minimal dilution. The PTA process is particularly advantageous for this application because it allows precise control of the overlay composition through powder feed rate adjustment and offers lower dilution rates compared to conventional arc welding processes.

Process parameters for the composite overlay include:

• SAW transition layer: Current 300-500 A, voltage 25-35 V, travel speed 200-400 mm/min
• GMAW final layer: Current 150-250 A, voltage 20-30 V, travel speed 100-200 mm/min
• PTA final layer: Current 200-400 A, powder feed rate 50-150 g/min, travel speed 100-300 mm/min

Interface Metallurgy and Bond Strength

The interface between the overlay and the substrate is the critical region that determines the overall performance of the composite drill bit. The literature emphasizes the importance of achieving a strong metallurgical bond between the layers, free from defects such as lack of fusion, porosity, and cracking.

Metallographic examination of the overlay-substrate interface reveals several important features:

1. Dilution zone: A region where the substrate composition gradually transitions to the overlay composition
2. Heat-affected zone (HAZ): A region in the substrate that has undergone thermal transformation but not melting
3. Bond line: The actual interface between the overlay and substrate, which must be free from defects

The bond strength of the composite overlay is typically verified through peel testing or shear testing. The literature reports bond strengths exceeding 300 MPa for well-executed composite overlays, which is sufficient for the demanding service conditions of drilling operations.

Performance Testing and Field Results

The performance of composite overlay drill bits is evaluated through laboratory testing and field trials. Laboratory tests include:

• Abrasive wear testing: Using standard methods such as ASTM G65 or internal industry standards
• Impact testing: Drop hammer tests or Charpy impact tests to evaluate toughness
• Hardness profiling: Vickers hardness measurements across the overlay cross-section
• Metallographic examination: Evaluation of microstructure, defects, and phase distribution

Field trials compare the composite overlay drill bits against conventional drill bits in terms of drilling life, rate of penetration (ROP), and cost per meter drilled. The literature reports that composite overlay drill bits can achieve drilling life improvements of 50 to 200 percent compared to conventional bits, depending on the formation conditions and the overlay material system used.

Manufacturing Considerations

The manufacturing of composite overlay drill bits involves several critical steps:

1. Base bit manufacturing: Fabrication of the structural steel body with appropriate geometry
2. Surface preparation: Cleaning, grinding, and preheating of the tooth surface
3. Transition layer deposition: SAW or multi-pass SMAW application of the transition alloy
4. Hardfacing layer deposition: GMAW, PTA, or other process application of the wear-resistant overlay
5. Post-weld treatment: Stress relief, machining, and inspection
6. Quality verification: Non-destructive testing, hardness measurement, and performance testing

The surface preparation step is particularly important for ensuring good bonding. The tooth surface must be free from scale, rust, oil, and other contaminants. Grinding to a smooth surface with a slight concave profile helps to ensure complete fusion and minimize porosity.

Study Insights and Reflections

The composite overlay approach to drill bit tooth surface strengthening represents a sophisticated application of metallurgical engineering principles. The key insight from this literature is that the performance of the composite overlay is determined by the synergy between the substrate, the transition layer, and the hardfacing layer. Each layer must be carefully designed and executed to ensure that the overall structure meets the demanding requirements of drilling operations.

From an engineering practice perspective, the literature highlights several important considerations:

1. Geological assessment: The overlay material system must be selected based on a thorough understanding of the formation being drilled, including hardness, abrasiveness, and heterogeneity
2. Process optimization: The welding parameters must be optimized for each layer to ensure proper bonding, minimal dilution, and consistent microstructure
3. Quality control: Comprehensive inspection at each manufacturing stage is essential to ensure that defects do not propagate into the final product
4. Field feedback: Continuous monitoring of field performance and feedback from drilling operators is essential for continuous improvement of the overlay design and manufacturing process

The composite overlay approach also raises important questions about the cost-benefit analysis of using advanced materials and processes. While the initial cost of a composite overlay drill bit may be higher than a conventional bit, the extended drilling life and improved performance can result in significant cost savings per meter drilled. Engineers must carefully evaluate the total cost of ownership, considering factors such as drilling life, ROP improvement, and reduction in drilling downtime.

This study demonstrates that the application of composite overlay technology to drilling tools is a promising approach to extending the service life of critical equipment and improving drilling efficiency. The key to success lies in the careful integration of materials science, welding engineering, and field experience to create a product that meets the demanding requirements of the drilling industry.