Carbide tungsten

Tungsten carbide is of special interest because it retains its hardness to a high temperature compared with several other carbides. Thus titanium carbide is much harder (about 3200VHN) at room temperature (compared with 

The relation of the WC structure to the TiB2 structure is that in the latter boron atoms occupy all of the prismatic interstices instead of half of them. The relation to the TiC structure is that the prismatic interstices become octahedral interstices in the TiC structure and there are half as many. 

The hardness of WC is associated with the fact that the array of W-atoms in the cores of glide dislocations changes from hexagonal prismatic to quasi-octahedral so the coordination number of the C-atoms changes from approximately six to approximately eight. This increases the local electron density so dislocation motion is resisted. 

Two tungsten-carbon compounds exist in the tungsten-carbon system W2C and WC as well as low melting point eutectica in the systems W/W2C and W2CAVC. Monotungsten carbide is by far the most important metal carbide in cemented carbide metallurgy (see Section 5.6.5.4). 

Tungsten carbide is produced by the carburization of high purity metallic tungsten 

The powder properties of WC are influenced by the particle form and particle size of the raw material, the reduction conditions and the carburization conditions. 

There is increasing interest in the hard metal properties of ultrafine tungsten carbide particles 100 nm in diameter (see Section 5.6.5.4), which are manufactured by reacting very finely divided tungsten(Vl) oxide with CO or CH4 at temperatures of ca. 3000°C in a plasma. However, these ultrafine carbide particles contain a relatively high concentration of oxygen, which limits their range of applications. 

Cemented carbides are sintered alloys with one or more hard material phases 

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Besides the material based characteristics, the difference of density of the used particle/substrate combination is a very important criterion. The difference of density influences the contrast of the radiographic tests. Tungsten carbides were used as mechanically resistant particles and titanium based alloys as substrate. The substrate material is marked by an advantageous relation of strength to density. This material is often used in aeronautics, astronautics, and for modification of boundary layers. The density of tungsten carbide (15.7 g/cm ) is about 3.5 times higher than the density of titanium (4.45-4.6 g/cm ). 

The Brinell test range is limited, by the capabUity of the hardened steel baU indenters used, to HBN 444. This range can be extended upward to HBN 500 by using special cold work-hardened steel baUs and to as high as HBN 627 by using special tungsten carbide bads. 

Hydrodynamic principles for gas bearings are similar to those involved with Hquid lubricants except that gas compressibility usually is a significant factor (8,69). With gas employed as a lubricant at high speeds, start—stop wear is minimized by selection of wear-resistant materials for the journal and bearing. This may involve hard coatings such as tungsten carbide or chromium oxide flame plate, or soHd lubricants, eg, PTFE and M0S2. 

A wide apphcation of electrochemical grinding is the production of tungsten carbide [12070-12-1] cutting tools (see Carbides Tool materials). ECG is also useful in the grinding of fragile parts such as hypodermic needles and thinwaH tubes. 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Tungsten—silver contacts are made similarly, but can be operated in air because of the greater stabiUty of silver. The three standard compositions of this class include tungsten—silver, tungsten carbide—silver, and molybdenum—silver. 

Cemented Carbides. Cemented carbides contain mostiy tungsten carbide and lesser amounts of other hard-metal components, embedded in a matrix of cobalt (see Carbides, cemented carbides). 

Processing. Tungsten carbide is made by heating a mixture of lampblack with tungsten powder in such proportions that a compound with a combined carbon of 6.25 wt % is obtained. The ratio of free-to-combined carbon is of extreme importance. Tantalum and titanium carbides are made by heating a mixture of carbon with the metal oxide. Multicarbide powders, such as M02C—WC, TaC—NbC, and TiC—TaC—WC, are made by a variety of methods, the most important of which is carburization of powder mixtures. 

The cermet class of materials contains a large number of compositions (57). Most cermets are carbide-based, eg, WC and titanium carbide [12070-08-5] TiC. Cemented tungsten carbides are widely used for cutting tools and car parts. 

Materials and Process. The steel chosen for tire cord is a eutectoid carbon steel containing 0.7% carbon, 0.5% manganese, 0.2% siUcon, and a very low amount of sulfur and phosphoms (9,48). The steel rod is cleaned with acid, rinsed, drawn through tungsten carbide dies to reduce its diameter from 5.5 to - 3.0 mm, heat treated (patented) to increase ductihty for further drawing to - 1 mm, then patented again. 

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