The Unsung Heroes of Underground Construction: Tungsten Carbide Inserts for Tunnel Boring

Tunnel boring machines (TBMs) are modern marvels of engineering, carving out subterranean pathways for subways, water pipelines, and utility tunnels. These colossal machines rely on a critical, yet often overlooked, component to perform their incredible feats: tungsten carbide inserts. These small, durable cutting tools are the workhorses of the TBM’s cutter head, relentlessly chipping away at rock and soil to create a stable tunnel.

Why Tungsten Carbide?

Tungsten carbide, a compound of tungsten and carbon, is celebrated for its extreme hardness and resistance to wear. On the Mohs Hardness Scale, it ranks higher than 8, making it second only to diamond. This unique combination of properties, along with its high fracture toughness, makes it the ideal material for the demanding conditions of tunnel boring. The inserts are typically a composite material, with tungsten carbide grains held together by a metallic binder, most commonly cobalt. The percentage of cobalt (typically ranging from 6% to 15%) can be adjusted to balance hardness and toughness, depending on the geological formations the TBM will encounter.

The Role and Types of Inserts

The primary function of Tungsten Carbide Inserts for Tunnel Boring Machine is to provide the cutting edge for the TBM’s rotating cutter head. They are designed to withstand significant compressive forces and abrasive wear as they crush and scrape through various rock and soil types. The inserts are often shaped as “buttons” and are strategically placed on the cutter head to optimize cutting efficiency. Different shapes and sizes are used for specific applications, including:

• Chisel Inserts: Used for cutting through soft to medium-hard rock.
• Button Inserts: The most common type, providing a balance of impact resistance and wear resistance. They are often used in a variety of formations, from hard rock to abrasive soil.
• Shield Inserts: These are specifically designed for the shield-type TBMs used in complex urban environments, where the geology can change rapidly.

Manufacturing and Maintenance

The production of these inserts is a precise, multi-step process. It begins with the careful selection and mixing of high-purity tungsten carbide and cobalt powders. These powders are then milled, pressed into the desired shape, and finally sintered (heated to a high temperature below the melting point) to create a solid, durable insert. This process, also known as cemented carbide production, results in a tool with exceptional performance characteristics. Due to the intense wear and tear they experience, these inserts must be regularly inspected and replaced. Their replaceable nature is a key advantage, as it allows for cost-effective maintenance and minimal downtime for the TBM.