Tungsten Carbide Metals: Part One

Abstrakti:

Tungsten Carbide is one of only a few materials which would be considered for some of the toughest tooling jobs in industry.
Formally classified as a refractory carbide Tungsten Carbide has exceptional mechanical and thermal properties with a melting point in excess of 1800°C and extremely high hardness.

High wear resistance is required in many industrial applications to extend the life of parts during production. There are numerous industrial uses of super hard materials such as polycrystalline diamond, natural diamond and Tungsten Carbide. These include tooling for mechanical processing as well as robust substrates for microelectronics in extreme environment applications.

Tungsten is a metal element with exceptional mechanical and thermal properties; it has the highest melt temperature of all metallic elements. In combination with carbon it forms tungsten carbide which counts as a refractory carbide, defined by a melting point over 1800°C, very high hardness and a good chemical resistance. Tungsten carbide can exist in two phases; WC and W2C, and have a hardness of 2000 – 2700 HV. Tungsten carbide powders come as monocristalline WC with a carbon content ≈ 6, 1% or eutectic WC/W2C with a carbon content ≈4% as seen in a carbon – tungsten phase diagram.

Figure 1 shows the tungsten-rich part of the binary W-C equilibrium diagram.

Three W-C stoichiometries have been found, hexagonal W2C crystallizing in three modifications, the PbO2, Fe2N, and CdI2 types, denoted β, β’, and β”, respectively, the cubic sub-carbide WC1-x crystallizing in the NaCl type structure denoted γ, and the hexagonal WC denoted δ. W2C exhibits a comparatively wide homogeneity range of 25.5 to 34 at.% C at 2715°C.

This phase originates from a eutectoidal reaction between elemental W and δ-WC at 1250°C and melts congruently with the W solid solution at 1715 ±5°C and with γ-WC1-x at approximately 2758°C. Phases of W2C stoichiometry are obtained as intermediate products during WC production. The γ-phase results from a eutectoidal reaction between β and δ at 2535°C and melts at approximately 2785°C. It can be obtained at room temperature by extremely rapid cooling, e.g., in plasma sprayed layers. The technically important δ-WC is the only binary phase stable at room temperature and has almost no solid solubility up to 2384°C but may become carbon deficient between this temperature and its incongruent melting point.



Figure 1: The W-C phase diagram

Nominal properties of tungsten and tungsten carbides are presented in Table 1.



Table 1: Tungsten and tungsten carbide properties

Common use for tungsten carbide besides surface coating applications are as cemented carbide in hard metal tools. The tungsten carbides is in this case sintered with a metallic binder, often cobalt and less often nickel, as good wettability is provided with these elements. To cut and grind tungsten carbide a tool of a harder material is needed, for example CBN (Cubic Boron Nitride) or diamond cutting- and grinding discs.

On the other hand, the applications of tungsten carbide metal matrix composites are found in the mining industry to coat different drilling and digging parts, in agriculture for coating on plough shares, tillage equipment and cutters. In the steel industry tungsten carbide coatings are used as coatings for guide rollers. The oil industry also uses this type of coatings in high wear applications. Examples of applications are seen in Figure 2.



Figure 2: Coated plough share, digging equipment and a scraper

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