carbide

The chromium-carbon phase diagram shows the existence of three carbide phases: Cr₂₃C₆, Cr₇C₃ and Cr₃C₂. Only Cr₃C₂ has industrial importance and it is manufactured by the carburization of chromium(Ⅲ) oxide under a hydrogen atmosphere.

Chromium carbides are mainly utilized in cemented carbide alloys with a nickel bonding phase. These alloys are notable for their good corrosion and scaling resistance combined with abrasion resistance and are therefore utilized in high temperature applications.

Carbide

Of the different molybdenum-carbon phases only dimolybdenum carbide P-Mo₂C has any industrial importance. It is manufactured by reacting molyb-denum(Ⅵ) oxide or metallic molybdenum with carbon black at 1350 to 1800°C in carbon tube short circuit furnaces under hydrogen atmospheres. Due to its low hardness the use of Mo₂C is virtually limited to TiC-Mo₂C-Ni cutting materials.

Carbide

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 W₂C/WC. Monotungsten carbide is by far the most important metal carbide in cemented carbide metallurgy.

WC is manufactured by first reducing tungstic acid, H₂WO₄, tungsten(Ⅵ) oxide, WO₃, or ammonium paratungstate, 5NH₃^.I₂WO₃^.5H₂O with hydrogen at 700 to 900°C to high purity (> 99.9% W) tungsten powder. Carburization is performed at 1500°C either in vacuum in induction furnaces or under hydrogen in electrical resistance furnaces. The total carbon content of the WC produced is between 6.05 to 6.20% by weight compared with the stoichiometric value of 6.13%, the over-stoichiometric carbon content being present in a free state.
 
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 which are manufactured by reacting very finely divided tungsten(Ⅵ) 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.