A “hard metal” is a term used in materials science to describe cemented carbide. This material is a composite structure engineered for extreme performance, not a pure metal or standard steel alloy. It is manufactured using powder metallurgy, where microscopic particles of one material are bonded together by a metallic binder. The resulting material exhibits an exceptional combination of hardness and high strength.
Composition and Nature of Hard Metals
Hard metals are composite materials defined by two primary phases: the hard phase and the binder phase. The hard phase is typically composed of refractory metal carbides, most commonly tungsten carbide (WC), which provides the material’s signature hardness and wear resistance. These tungsten carbide particles are extremely fine, often ranging from 0.4 to 10 micrometers in size, and can constitute between 70% and 97% of the composite’s total weight.
The binder phase, which acts as the “cement” in cemented carbide, is usually a softer metal, most often cobalt (Co). This metallic matrix holds the rigid carbide grains together in a structure analogous to aggregate in concrete. The cobalt binder counteracts the inherent brittleness of pure tungsten carbide, providing necessary toughness and resistance to impact. The final properties are determined by the ratio of the carbide to the binder, where a lower cobalt content generally results in higher hardness but reduced impact resistance.
Achieving Extreme Hardness: The Sintering Process
The exceptional properties of hard metals are achieved through powder metallurgy, culminating in the process of sintering. The process begins by precisely mixing fine powders of tungsten carbide and the metallic binder, such as cobalt, often through wet milling to ensure a homogeneous suspension. This mixture is then compacted under high pressure into a “green compact,” which is a fragile, pre-sintered form.
Sintering involves heating the compact in a vacuum furnace to temperatures typically ranging from 1,350°C to 1,650°C. This temperature is high enough to cause the cobalt binder to melt, resulting in liquid-phase sintering, but remains below the melting point of the tungsten carbide particles. The liquid cobalt flows through the structure, dissolving and reprecipitating the carbide grains, fusing them together as the material densifies. This thermal consolidation eliminates nearly all internal porosity, causing the component to shrink significantly and emerge as a dense, ultra-hard body.
Unique Material Properties
The composite structure created by sintering yields a unique set of physical characteristics. Hard metals are second only to diamond in hardness at room temperature, often reaching between 86 and 93 on the Rockwell A scale. This extreme hardness translates directly into outstanding wear resistance against abrasion and erosion.
A primary advantage is the material’s ability to maintain hardness at elevated temperatures, remaining stable up to 900°C or 1,000°C. This property, known as “red hardness,” allows hard metal tools to operate at much higher cutting speeds and temperatures than high-speed steel alternatives. Furthermore, hard metals possess a high compressive strength and a modulus of elasticity that can be three times greater than that of steel alloys, meaning they resist bending under heavy loads.
Common Industrial and Consumer Applications
The unparalleled wear resistance and hot hardness make cemented carbide an indispensable material across diverse industries. The most common application is in metalworking, where it is formed into indexable inserts for turning, milling, and drilling tools. These cutting tools enable the high-speed machining of materials like steel and cast iron that would rapidly dull conventional tools.
In the mining and construction sectors, hard metals are used to create robust components that withstand aggressive impact and abrasion, such as rock drill bits and tunnel boring machine cutters. The material’s durability also makes it ideal for wear parts in high-volume manufacturing, including dies for cold-working, molds, and nozzles that must maintain precise dimensions over a long lifespan. Consumers may encounter hard metal in the form of high-end wood router bits or in specific luxury goods like scratch-resistant watch components.