Niobium carbides

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Niobium carbide is used as a component of hard metals, eg, mixtures of metal carbides that are cemented with cobalt, iron, and nickel. Along with tantalum carbide, niobium carbide is added to impart toughness and shock and erosion resistance. The spiraling rise in the price of tantalum has spurred the development of a hafnium carbide—niobium carbide substitute for tantalum carbide (68). These cemented carbides are used for tool bits, drill bits, shovel teeth, and other wear-resistant components turbine blades and as dies in high pressure apparatus (see Carbides). 

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... 

The menstmum niobium—carbide process (7) utilizes either columbite [1310-23-2] mineral concentrates or ferroniobium as starting materials. A low level of TaC in soHd solution with NbC commonly occurs, as Ta and Nb occur together in ores. The properties of NbC are given in Table 1. The grayish brown NbC powder is used in cemented carbides to replace TaC. TaC—NbC soHd solutions that have 3 1, 2 1, 1 1, and 1 2 ratios and the corresponding ternary and quaternary soHd solutions with TiC and WC are common. 

If a graphite substrate is used, Reaction (2) produces niobium carbide which may remain incorporated in the metal deposit. Niobium absorbs hydrogen readily and, forthatreason, an inert atmosphere suchas argon is used in the heating and cooling cycles. 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

Niobium carbide may be obtained by carburization of the metal substrate according to the following reaction at low pressure and at temperatures above 2000°C ... 

Having established the feasibility of niobium metal production by the carbothermic reduction of niobium pentoxide under temperature and pressure conditions readily attainable in the laboratory and in industry, the principles of efficient process execution may now be examined. In a high-temperature vacuum furnace operation, the quantity of gas that is to be pumped off can influence the choice of the vacuum process. When the reduction of niobium pentoxide with either carbon or niobium carbide is attempted according to the following overall equations ... 

Niobium carbide (NbC) is used to make hard-tipped tools and special steels and to coat graphite in nuclear reactors. 

Niobium combines with carbon, boron, silicon and other elements at very high temperatures, forming interstitial binary compounds of varying compositions. With carbon, it forms niobium carbide having compositions varying from NbCo.7 to NbC [12069-94-2]. With boron, the products are orthorhombic niobium boride, NbB [12045-19-1], and the hexagonal diniobium diboride, Nb2B2[12007-29-3]. 

Niobium pentaoxide is used for preparing many niobium salts, including niobium carbide (Balke process). It also is used in the aluminothermic process in the production of niobium metal. 

Also, the pentoxide may be produced by igniting niobium metal powder, niobium carbide, or niobium fluoride in oxygen. 

Niobium pentaoxide undergoes two important types of reactions, one is reduction to niobium metal or to lower oxides, and the other involves conversion of pentoxide to oxide trihalides when treated with halogens or hahdes. These reactions occur mostly at elevated temperatures. Reductions may he carried out hy carhon, hydrogen, niobium carbide, niobium metal, and other reducing agents at elevated temperatures and often in vacuum ... 

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