Hard metal is understood to be a composite material consisting of a metal matrix and a secondary hard phase. Hard materials are present in hard metals as small particles. These are held together by a metal matrix. Therefore, although hard metals do not have the same hardness as pure hard materials, they are characterized by a significantly higher toughness. Compared to alloys, pure metals or hardened steel, hard metals are significantly harder, but also more fragile.
Hard metal is mainly used in the production of cutting materials for tools such as chisels, drills and milling cutters. Furthermore, hard metal is applied in forming and punching tools. At around 900 °C, the temperature resistance of hard metal significantly exceeds that of other materials such as high-speed steel. As a result, cutting speeds up to three times higher can be achieved.
Different types of hard metal
Hard metal is usually divided into three groups. The decisive factor for the grouping is the composition of the hard metals.
Tungsten carbide cobalt hard metal (WC-Co)
Tungsten carbide cobalt hard metals are the standard among hard metals, measured by the quantity produced. Apart from tungsten carbide, only insignificant quantities of other carbides are used in the production of these materials, not exceeding 0,8 %. WC-Co hard metal is characterized by a large variation in WC grain size, ranging from less than one to up to 20 μm. Thus, the cobalt content is between 3 and 30 %. This allows adjustments to be made for almost any scenario. However, this type of hard metal is little or not at all suitable for machining soft steel, as iron diffusion occurs at elevated temperatures.
Hard metal for steel machining
Hard metal for steel machining is characterized by an increased proportion of carbides or mixed carbides. Compared to WC-Co grades, it contains a higher proportion of titanium carbide, tantalum-niobium carbide or zirconium carbide. Characteristics for this type of hard metal are the high oxidation resistance and the hot hardness/hot strength. Due to an improved diffusion resistance compared to ferrous materials, they are particularly suitable for machining of steel materials. During such operations, temperatures of 1000 °C can occur at the cutting edge. Hard metal grades for steel machining are divided into two groups according to their composition. The proportion of mixed carbides is decisive here: Group A contains more than 10 % of mixed carbides, group B less than 10 %.
The name cermet is a composition of the two English words ceramic and metal. In contrast to WC-Co hard metals, cermet hard metals do not have tungsten carbide, but other hard materials, such as titanium carbide or titanium nitride. Nickel, cobalt and molybdenum are used as binder phase in cermet. On the one hand, cermet hard metals are characterized by high-temperature strength, high hardness and toughness, and on the other hand they have a very low tendency to adhere and diffuse. This enables even higher cutting speeds, which is particularly advantageous when finishing metal. Cutting materials made of cermet hard metals are therefore mainly used in high-speed cutting processes.
How is hard metal composed?
Except in cermets, tungsten carbide is usually used as the hard material for the different types of hard metal. Other hard materials used are titanium carbide, titanium nitride, vanadium carbide or niobium carbide. Cobalt is the most common binder for the matrix. But nickel or a mixture of both elements is also used. The addition of nickel increases the corrosion resistance of the material.
The different properties compared to steel
Compared to steel, hard metals differ significantly in several material properties. Hard metals have a modulus of elasticity of 400 up to 650 GPa and are thus about two to three times higher than steels. As a result, hard metals manage to achieve a significantly stiffer structure while maintaining the same moment of inertia. Hard metals also outperform steel in terms of density. They are around 12.75 to 15 g/cm3, while steel achieves 7.85 g/cm3. Like the modulus of elasticity, the hardness of hard metals increases with decreasing cobalt content. Carbides can reach a hardness of 2200 HV30. The compressive strength of carbides reaches values of up to 8000 MPa. This also correlates negatively with the cobalt content.
The grain size of hard metals has a considerable influence on the properties of the materials. Reducing the grain size can improve hardness and compressive strength, but it also increases the manufacturing costs considerably.
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