Tungsten iron carbide

Boj] Bojarski, Z., Leciejewicz, J., Neutron-Diffraction Study of the Crystal Structures of eta-Type Tungsten Iron Carbides (in Polish), Arch. Hutn., 12(3), 255-263 (1967) (Crys. Stracture, Experimental, 9)... 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

Table 16.8 presents experimental data on the effect of temperature, partial pressures of the reactants, and contact time on the oxidative coupling of methane in the presence of iron carbide, Fe3C. Oxidative coupling of methane also occurs if, instead of oxygen, nitrous oxide is used as oxidant. Our data for tungsten carbide are reported in Table 16.9. 

Phase relations in the C-Fe-W system are of great interest above all because of the importance of tungsten as alloying element in steels. Therefore constitution of this system has been studied quite intensively. Other important aspects of this subject, in particular the dissolution of iron carbides during austenitization of tungsten steels, phase transformations of carbides as well as diffusion interaction of carbides with iron and steels also attract attention of investigators widely. Details of experimental studies of phase relations, crystal strac-tures and thermodynamics as well as applied techniques are presented in Table 1. 

Cemented tungsten carbides also find use as a support for polycrystalline diamond (PCD) cutting tips, or as a matrix alloy with cobalt, nickel, copper, and iron, ia which diamond particles are embedded. These tools are employed ia a variety of iadustries including mineral exploration and development oil and gas exploration and production and concrete, asphalt, and dimension stone cutting. 

Applications Ion implantation is widely employed to improve the life of tools. Thus press tools, dies and gear cutters can be treated to increase their durability by three times or more. Nitrogen-implanted tungsten carbide drawing dies for copper and iron wire can be improved up to fivefold. By implanting chromium, aluminium or silicon a considerable increase in the corrosion resistance of steel can be obtained. Implantation of chromium into aircraft bearing alloys has improved their durability in marine environments . 

Iron can be removed by chemical washing with 10 N aq HCl. Cemented carbide balls must be avoided owing to the high contamination level and because the tungsten carbide cannot be removed without attacking the boride. 

CARBIDES. A binary solid compound of carbon and another element. The most familiar carbides are those of calcium, tungsten, silicon, boron, and iron (cemcntitc) Two factors have an important bearing on the properties of carbides (1) the difference in electronegativity between carbon and the second elemenl. and (2) whether the second element is a transition metal. Saltlike carbides of alkali metals are obtained by reaction with acetylene. Those ohlained from silver, copper, and mercury sails are explosive. See also Carbon and Iron Metals, Alloys, and Steels. 

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