Application of Overlay Welding Technology in Rock Bit Cutting Teeth

Literature Overview

This concise technical paper, published in Mining Machinery in 2003, reports on the application of overlay welding technology to enhance the wear resistance of rock bit cutting teeth (also known as drag bits or roller-cone bit teeth). The authors from Shandong University of Science and Technology and Tai'an Keda Jinna Plasma Technology Co., Ltd. describe the use of high-speed steel wear-resistant welding electrodes to deposit a hard, wear-resistant layer onto the cutting tooth substrate, achieving metallurgical bonding between the coating and the base material. While the paper is brief, it addresses a practically significant problem in mining and drilling operations where cutting tooth wear is a major cost driver.

Core Technical Findings

The study demonstrates that overlay welding using high-speed steel (HSS) welding electrodes produces a wear-resistant coating on cutting teeth that is metallurgically bonded to the substrate, with high hardness and excellent wear resistance. The key advantages of the overlay welding approach over alternative surface treatment methods are summarized below.

The metallurgical bonding achieved through overlay welding is a critical advantage, as it ensures that the hardfacing layer does not delaminate under the severe impact and abrasion loads experienced by cutting teeth during drilling and mining operations. The high hardness of the HSS weld metal provides resistance to abrasive wear from rock and ore materials, while the underlying substrate retains its toughness to withstand impact loading.

Interpretation of Technical Points

The selection of high-speed steel welding electrodes for cutting tooth overlay is a well-established practice in the mining and drilling industry. HSS welding electrodes typically contain alloying elements such as tungsten, molybdenum, vanadium, and chromium, which form hard carbides (WC, MoC, VC, Cr7C3) in the weld metal. These carbides provide the primary wear resistance mechanism through their extreme hardness and resistance to abrasion.

The metallurgical bonding between the overlay and substrate is achieved through the melting and re-solidification of both the electrode metal and the substrate surface during welding. This creates a diffusion zone at the interface where the alloying elements from the weld metal intermix with the substrate, producing a gradient in composition and properties. The hardness of the weld metal typically ranges from 55 to 70 HRC, depending on the specific electrode composition and welding parameters.

The repairability of overlay-welded cutting teeth is a significant economic advantage. When the hardfacing layer wears down to a critical thickness, the tooth can be ground back and re-welded, restoring its original geometry and hardness. This is particularly valuable for expensive cutting teeth used in large-diameter drill bits, where replacement costs can be substantial.

Process and Standards Analysis

The overlay welding process for cutting teeth typically involves the following steps:

1. Surface preparation: grinding or machining the tooth surface to remove scale, rust, and contaminated material
2. Preheating: applying localized heat to the substrate to reduce thermal stress and prevent cracking
3. Welding: depositing multiple layers of hardfacing metal using SMAW or FCAW processes
4. Post-weld treatment: grinding the surface to the required geometry and hardness verification

Key process parameters include:

• Welding current: typically 120-200 A for SMAW with HSS electrodes
• Arc voltage: 22-32 V
• Travel speed: 50-100 mm/min
• Layer thickness: 2-4 mm per pass
• Interpass temperature: controlled below 200°C to prevent excessive grain growth

The relevant standards for hardfacing welding include AWS D10.9 for surface preparation and hardfacing welding, ISO 14273 for hardfacing welding, and ASTM A396 for hardfacing alloy welding electrodes. For mining applications, additional qualification requirements may be imposed by the equipment manufacturer or the operating company.

Integration with Engineering Practice

In mining and drilling operations, cutting tooth wear is a major operational cost, often representing 10-20% of total drill bit cost over the life of the bit. The overlay welding approach offers a cost-effective solution by extending tooth life through the application of a hard, wear-resistant surface layer. The following practical considerations are important for field implementation:

• The welding procedure must be qualified on the specific substrate material (typically alloy steel such as 4140, 4340, or 8630)
• Hydrogen-induced cracking must be prevented through proper preheating and post-weld heating
• The overlay layer thickness must be sufficient to withstand the expected wear life but not so thick as to compromise the tooth's structural integrity
• Multiple re-welding cycles can be performed, but each cycle should be monitored for substrate fatigue

The FMEA approach is particularly relevant for cutting tooth overlay welding. The primary failure modes include:

• Overlay delamination due to poor bonding or excessive dilution
• Substrate cracking due to high residual stress or hydrogen pickup
• Overlay cracking due to brittle weld metal or thermal stress
• Excessive dilution reducing overlay hardness below required levels

Each of these failure modes can be mitigated through proper procedure qualification, material selection, and process control.

Key Questions and Reflections

While the paper provides a clear demonstration of the feasibility of overlay welding for cutting tooth hardfacing, it does not address several important practical issues. First, the long-term durability of the overlay under repeated impact and abrasion cycling is not discussed. Second, the effect of multiple re-welding cycles on the substrate's fatigue life is not considered. Third, the paper does not provide quantitative data on wear life extension compared to untreated teeth or teeth with alternative surface treatments.

From a materials science perspective, the microstructural evolution of the HSS weld metal under repeated thermal cycling during multiple re-welding operations is an important consideration. Each re-welding cycle subjects the previously deposited overlay to a thermal cycle that can cause grain coarsening, carbide coarsening, and potential softening. Understanding these effects is essential for determining the maximum number of re-welding cycles before tooth replacement is required.

Study Insights and Implications

This paper, while concise, provides practical confirmation of the effectiveness of overlay welding for cutting tooth wear protection. The key insight for engineers is that overlay welding offers a versatile, field-applicable solution for extending the service life of cutting teeth through the application of hard, wear-resistant layers with metallurgical bonding. The economic benefits are significant in mining operations where drill bit costs are a major operational expense. For practitioners, the paper reinforces the importance of proper welding procedure qualification, material selection, and process control in achieving reliable hardfacing performance. The approach is particularly suited to field repair operations where rapid turnaround and cost-effectiveness are prioritized.