Melting metallurgy

Product Most preferable scrap type (according to No. in Table 11.1) 

Any tungsten going in this direction cannot be recycled again by other methods, with the exception of re-melting. It finally ends up in loss by recycling dilution. 

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Carbothermic reduction of WO3 or ore concentrates is of technical importance in melting metallurgy— preparation of ferrotungsten, melting base, and cast carbide (see Chapter 8). 

Besides tungsten metal, only those alloys which were described in Chapter 6 are treated in this chapter. Alloys produced by melting metallurgy as well as the cemented carbides (hardmetals) and their application are treated in Chapters 8 and 9, respectively. 

Refractory metals are associated with powder metallurgy because these metals are not easily melted. Therefore in smelting the ores, the metal is recovered in powder form rather than melted. Refractory metals are used mainly to produce filament wire for incandescent lamps. 

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

Consolidation. Because of its high melting point, tungsten is usually processed by powder metallurgy techniques (see Powder metallurgy). Small quantities of rod are produced by arc or electron-beam melting. 

Sintering is a thermal process through which a loose mass of particles is transformed to a coherent body. It usually takes place at a temperature equal to two-thirds the melting point, or ca 800—1000°C for nickel. The sintered nickel stmcture without active material is called a plaque and it can be prepared by either dry or wet processes (see Metallurgy, powder). 

The preferred method for synthesis of complex carbides is the powder metallurgy technique. Hot-pressed powder mixtures must be subjected to prolonged annealing treatments. If low melting or volatile components are present, autoclaves are used. 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

Consolidation and Fabrication. Chromium metal may be consoHdated by powder metallurgy techniques or by arc melting in an inert atmosphere (8,13,24,25) (see Metallurgy Metallurgy, powder). 

F.D. Richai dson. Physical Chemistry of Melts in Metallurgy, vols I II. Academic Piess, London (1974). 

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