Tungsten

The high melting point of tungsten precludes its industrial manufacture in the cast state from the powdered metal, and even if this were possible the product would be of little value as it would be both brittle and difficult to work mechanically. From the researches of Coolidge, who took out his first patent in 1906, a process has been developed which is capable of producing a fine tungsten wire that can be manipulated with little more diffi culty than any other fine wire. In this process the powder is compressed in a hydraulic press into bars which are then heated to a high temperature in a furnace in an atmosphere of hydrogen, whereby a strong but brittle product results. This is now con verted into rods by a mechanical hammering process in which the metal, previously heated to 1,50o° C in an electric furnace in a hydrogen atmosphere, is subjected to 1o,000 blows per minute in a special swaging machine. Further reduction of the diameter of the wire—usually from imm.—is effected by drawing through dies. Tungsten wire can be drawn to o.oi imm. diameter, whilst by immersion for 45 seconds in a fused mixture of sodium nitrate and nitrite at 34o° C a wire of diameter o.o14mm. is uniformly reduced to o•oo7mm.

Physical and Chemical Properties.

Tungsten in the form of wire or sheet is dull white. The physical properties alter with changes in the structure of the metal, so that values for density, tensile strength, specific electrical resistance, etc., vary with the stage of swaging or drawing. The theoretical value for the density of pure tungsten, calculated from the atomic spacings, is 19.32. The probable melting point is 3,382° C, although the value 3,267°±30° C is also quoted. Tungsten has the highest melting point of the metals. X-Ray analysis shows that tungsten has a body-centred cubic lattice, the length of the cube edge being A. For further physical data, see C. J. Smithells,

Tungsten (1926). Tungsten undergoes no appreciable oxidation on heating in air below red heat nor does it react with nitrogen, except when in the state of vapour. It is very resistant to the action of acids, neither aqua regia nor hydrofluoric acid attacking it to any appreciable extent. The best solvent for the fused metal is a mixture of concentrated nitric and hydrofluoric acids. Aque ous alkalis are without action but fused alkalis dissolve the metal. Tungsten in its compounds exhibits valencies of 3, 4, 5 and 6, whilst, in addition, compounds of the empirical formula WR, (where R=C1, Br, or I) are known. In the case of chloride, how ever, the molecular formula is The trioxide is a bright canary-yellow powder which becomes dark orange when heated and fuses between 1,300°-1,400° C. It is insoluble in most acids including aqua regia but hydrofluoric acid dissolves it. With aqueous alkali hydroxides or carbonates it forms tungstates containing varying proportions of to metallic oxide. A hexa chloride and hexafluoride are known. The compounds exhibit tungsten with co-ordination numbers of 4, 6 and 8.

Applications.

Tungsten finds wide application as a filament in electric lamps, and as it has a hardening effect on other metals with which it is alloyed it is a constituent of some of the most important industrial alloys. Tungsten steels are employed in the manufacture of cutting tools and permanent magnets. The stellite alloys containing cobalt, chromium and tungsten are extremely hard and are used for cutting tools and surgical instruments, as they are not affected by organic acids and ordinary antiseptics. Other uses of the metal include X-ray targets, thermionic devices, galvanometer suspensions, electrical contacts, refractory crucibles, etc. (W. WA.)