Tungsten carbide, formation

Various appHcations such as lubricant additives, dyes, pigments, and catalysts are under investigation. Tungsten can be deposited from tungsten hexacarbonyl, but carbide formation and gas-phase nucleation present serious problems (1,2). As a result, tungsten halides are the preferred starting material. 

The main differences in the SteUite aUoy grades of the 1990s versus those of the 1930s are carbon and tungsten contents, and hence the amount and type of carbide formation in the microstmcture during solidification. Carbon content influences hardness, ductUity, and resistance to abrasive wear. Tungsten also plays an important role in these properties. 

Select a bit with tungsten carbide inserts on gage if sand streaks are expected in the formation. Do not depend on gage hardfacing alone to hold the hole to gauge. 

Select a tungsten carbide bit with chisel crest inserts when drilling a formation that is predominantly shale. Use bit type 4-2, 5-2, 6-1 or 6-2. 

Select a tungsten carbide bit with high offset and chisel inserts if the shale content of the formation increases and/or the mud density is high. Use bit type 5-2 or 5-3. 

Select a tungsten carbide bit with shorter chisel inserts and less offset if the formations become more abrasive and unconsolidated. Use bit type 6-3 or 6-4. 

Select a tungsten carbide bit with projectile or conical inserts if the sand content and abrasiveness of the formation increases. Use bits type 7-1 to 8-3. 

Select a tungsten carbide insert bit with a minimum offset and projectile or conical inserts when drilling limestone, brittle shale, nonporous dolomite and broken formations. Use bit type 6-3 to 7-3. 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

An investigation of bulk (a-WC, a-W2C) and supported tungsten carbide was carried out using the formation of alcohols in Fischer-Tropsch synthesis as a chemical probe. The active phases were characterized by X-ray photoelectron spectroscopy. 

As reviewed by Ponec,18 the formation of alcohols is observed when a metal is promoted by a transition metal oxide. Kiennemann et al,19 has associated the presence of anion vacancies at the metal-support interface with the capability to dissociate CO and allow CO insertion to produce higher alcohols. This model can be used to explain our results on tungsten carbides. 

Cobalt is an essential element that is part of vitamin B12, or cobalamin, a coenzyme that is essential in the formation of proteins, nucleic acids, and red blood cells. Although cobalt poisoning is not common, excessive levels can be harmful. Most cases of human exposure to toxic levels of cobalt have occurred through inhalation in the workplace. Many exposures have been suffered by workers working with hard metal alloys of cobalt and tungsten carbide, where very fine particles... 

When metallic tungsten is heated with carbon in an electric furnace carbide formation takes place, and the carbides WjC, WC, and probably WjC appear capable of existence. A carbide of composition W3C4 has also been described as being formed when finely powdered tungsten is heated in carbon monoxide at 1000° C., but its existence as a chemical entitj is doubted by Ruff. ... 

In general, organic compounds like hydrocarbons react with tungsten at elevated temperature via dissociation of the organic molecule and subsequent carbide formation. 

This section deals primarily with the carburization of tungsten powder, the most widely used process for producing WC on a, technical scale. Also, the carburization performed in melts is discussed briefly owing to historical reasons and the production of hard facing alloys. Finally, the formation of coarse tungsten carbide crystals in auxiliary melts ( Menstruum WC) is discussed. 

FIGURE 8.1. Influence of tungsten on carbide formation in steel [8.3]. 

---