Wear is a constant problem in any process that includes dynamic or static components. Continuous demands for increased productivity and reduced wear cost require constant improvements in design and material properties.

Typically, tools with a high hardness are used to prevent wear. But, high hardness (wear resistance) requires a compromise to toughness (impact resistance). For a long time, therefore, steels and cast irons with alloy additives were applied as standard materials.

Tungsten carbide, a material known for more than 70 years, opens up new possibilities concerning wear minimization in stone applications through both development of new materials and targeted analysis of application technology. Tool life varies from customer to customer and from application to application. In general, however, tungsten carbide provides tool life that is 10 times longer than steels.

Tungsten carbide was originally applied to the stone industry in the form of tipped chisels and drills. This required development of application-specific grades. The success of these grades led to application of tungsten carbide in more difficult wear situations.

Today, it is possible to combine high hardness with the necessary toughness. Precise analysis of the application conditions allows use of the best possible hard metal material for frictional wear and impact applications.

What is hard metal? The name hard metal describes a group of materials characterized by high hardness and metallic properties. Hard metal is a combination of metallic, hard materials - which, due to their high hardness are defined as brittle - with relatively soft, tough metals, such as iron, cobalt and nickel, the so-called binding materials.

Therefore, in hard metal, the high hardness of the metallic material is combined with the toughness of the binding metal. The hard material parts are cemented into the binding metal; this is why they are aptly called cemented carbides.

Although the hard metal developed at Plansee Tizit in 1929 by Paul Schwarzkopf was mainly applied in the field of machining, continuous developments have broadened the application to numerous areas of metal-forming technology and wear protection.

Cobalt (Co) has proven to be the optimal binding metal for tungsten carbide (WC) hard metals. Cobalt overcomes the problem of brittleness. Hardened binding metals, which are similar to steel and have a titanium carbide basis (ferrotic), are used as well as iron and nickel.

In general for WC-Co carbides, with an increased WC content the carbide becomes harder, more brittle and more wear resistant; with an increased binding-metal content it becomes less brittle and tougher (impact resistant). By varying the grain size and the Co content, a wide combination of properties can be obtained, allowing applications ranging from inserts for mining tools, metal working tools and wear parts to the machining of cast iron, non-ferrous metals and non-metals.

Manufacture of hard metal Hard metal is produced by powder metallurgy. Powder metallurgy is the forming of metallic powders by means of high-pressure and subsequent heat-treatment (sintering). This process takes place below the melting point of the metals and guarantees the manufacture of compact, dense workpieces.

The core material for the manufacturing of hard metal is tungsten oxide. Tungsten powder is produced through a hydrogen atmosphere-reduction process. After precise analysis of various parameters, carbon is added and tungsten carbide is produced at a temperature of 1,500 to 1,700C.

Each component is mixed in liquid and the slurry mixture is sprayed under high pressure in a tower. As the slurry liquid evaporates, the carbide powder becomes a granulate.

Parts for crushed stone applications are produced through cold isostatic pressing. During this process, the powder is densified with pressure of approximately 20 kN/cm superscript 2 from all sides. At this stage, the parts have a consistency similar to chalk. The parts then undergo a complex pre-sintering process at about 500 to 700C.

Sintering, the central technological step in powder metallurgy, transforms porous, green compacts of powder into metallic components suitable for further processing, by means of diffusion and surface tension under high temperatures.

The complex sintering process is achieved at a temperature of 1,300 to 1,500C in a liquid phase. During this process, recrystallization takes place. During cool down, the volume shrinks (about 20%) and the cobalt becomes solid, resulting in a compound with the WC crystals. During sintering, the various powder materials form a homogeneous structure.

Exact observation of the sintering cycles, temperatures and duration is critical for carbide quality and consistency.

Application Longer tool life and process efficiencies by way of improved materials and designs are constantly needed to sustain rising demands. Use of tungsten carbide for wear protection in mills and sifters where abrasion is the determining factor in tool life can achieve tremendous results. Tungsten carbide also increases tool life in impact crushers and breakers. The advantage in this area lies in the higher impact energy - hard metal has almost twice the specific weight of steel.

Tungsten carbide provides a wide range of surface protection for rotors, impact mills, strippers and various armor plating. One of tungsten carbide's characteristics is that it can absorb compressive strains, but only with a limited degree of tensile stress. Therefore fixation through brazing, clamping and adhesion must be carried out thoroughly and properly. However, when correctly applied, tungsten carbide's limits have not yet been reached.