Chromium, Molybdenum and Tungsten Carbides

Tungsten carbide occurs in two fundamental compositions, WC and W2C. Less common is the phase of the stoichiometry W3C. The monocarbide has a melting point of 

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Carbides of Group VI Chromium, Molybdenum, and Tungsten Carbides... 

Cyclopentadienyl tricarbonyl hydrides of chromium, molybdenum, and tungsten can be prepared by reaction of Institute for Inorganic Chemistry, The University, Munich, Germany, t Union Carbide Chemicals Company, South Charleston, W.Va. 

Some commercial cobalt-aUoy compositions are listed in Table 24.1. They typically contain chromium as well as molybdenum and/or tungsten, making them relatively resistant to both reducing and oxidizing conditions. High resistance to abrasion results from the precipitation of carbides, which can be carbides of chromium, molybdenum, and tungsten [3]. 

Kuol] Kuo, K., Carbides in Chromium, Molybdenum, and Tungsten Steels , J. Iron Steel Inst., 173, 363-375 (1953) (Crys. Structure, Phase Diagram, Phase Relations, Experimental, 45)... 

This chapter is a review of the characteristics and properties of the interstitial carbides formed by the metals of Group VI chromium, molybdenum, and tungsten. These three carbide systems have similar atomic bonding, composition, and crystallography. Their properties and characteristics are also similar. 

Interstitial nitrides are similar to interstitial carbides in structure and composition, and the two groups of materials closely resemble each other. The nitrides however are not as refractory. In fact, only the nitrides of Group IV and V have melting points above 1800°C. Those of Group VI, i.e., chromium, molybdenum, and tungsten nitrides, have lower melting (or decomposition) points and dissociate rapidly into N2 and the pure element at high temperature ( 1000"C). Their chemical stability is relatively poor and they do not therefore meet the refractory criteria. They are mentioned in this chapter for reference purposes. 

C, Carbide iron complex, 26 246 ruthenium cluster complexes, 26 281-284 CHF,02, Acetic acid, trifluoro-tungsten complex, 26 222 CHFjOjS, Methanesulfonic acid, trifluoro-iridium, manganese, and rhenium complexes, 26 114, 115, 120 platinum complex, 26 126 CH2O2, Formic acid rhenium complex, 26 112 CH, Methyl iridium complex, 26 118 manganese complex, 26 156 rhenium complexes, 26 107 CHjO, Methanol platinum complexes, 26 135 tungsten complex, 26 45 CNajOuRusCn, Ruthenate(2- )ns-carbido-tetradecacarbonyl-disodium, 26 284 CO, Carbonyls chromium, 26 32, 34, 35 chromium, molybdenum, and tungsten, 26 343... 

The four most important carbides for the production of hard metals are tungsten carbide [12070-12-17, WC, titanium carbide [12070-08-5] TiC, tantalum carbide [12070-06-3J, TaC, and niobium carbide [12069-94-2] NbC. The binary and ternary soHd solutions of these carbides such as WC—TiC and WC—TiC—TaC (NbC) are also of great importance. Chromium carbide (3 2) [12012-39-0], molybdenum carbide [12011-97-1], MoC, and... 

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 addition of carbide stabilizers to steel reduces the tendency toward internal Assuring. Elements such as chromium, molybdenum, tungsten, vanadium, titanium, and niobium reduce the number of nucleation sites by forming more stable alloy carbides which resist breakdown by hydrogen and, therefore, decrease the propensity to form methane.9 The solid-line curves in Figure 1 reflect the increased resistance to internal attack when molybdenum and chromium are present. 

In this paper we review the results of our systematic work on the catalytic and adsorptive properties of transition metal carbides (titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and iron). We focus our attention on the oxidation of hydrogen, carbon monoxide, ammonia, and the oxidative coupling of methane. The first two reactions are examples of complete (non-selective) oxidation, while the oxidation of ammonia simulates a selective oxidation process. The reaction of oxidative coupling of methane is being intensively explored at present as a means to produce higher hydrocarbons.5 10... 

The same analysis procedure can be applied to the study of carbides and nitrides of molybdenum and chromium. Three samples are presented in this chapter a W2N film deposited on tungsten, a fi-WCi film deposited on tungsten, and a WC/W2C foil. 

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