TUNGSTEN STEEL. The earliest alloy of this type is Mushet steel (q.v.). It contains from 5.5 to 9.o% of tungsten and also carries enough manganese to give it air-hardening properties and to class it as a quaternary alloy. This article considers principally commercial ternary alloys of tungsten. High carbon steels containing tungsten up to 6% are essentially water hardening steels. (See IRON AND STEEL : Heat Treatment; METALLOGRAPHY.) The atoms of tungsten, being so much larger and heavier than those of iron, diffuse comparatively slowly in solid metal, so that a high temperature is required for solution before the quench. Accumulation of carbide particles upon tempering is also slow, hence the ability of tungsten steels to hold their hardness at high temperatures. Tungsten carbide also has a very high intrinsic hardness, approaching that of the diamond. For keen edged tools tungsten in the alloy seems to prevent small particles from being torn away during use, and thus retards dulling.
Steel containing 0.9% carbon or more together with i to 2% tungsten is used for hack saw blades and for reamers, broaches and other tools which must show a minimum change in dimension after heat treatment. Steels with I% carbon or more, and 3 to 7% tungsten, are much used for "fast-finishing" tools to cut hard metals or to make a fine smooth cut on softer tough metal. Dies for cold drawing wire and other shapes are made of higher carbon steels (2%) with tungsten ranging from 1 to 12%. Dies with the higher tungsten percentages are used for the harder metals or the finer wires.
Chromium is often added to facilitate the diffusion of tungsten. When 0.5% or more is present the alloy may crack during water hardening. An oil quench, however, will harden it through to the centre. Chromium also causes the useful hardness to be retained at higher working temperatures. For working hot metal, tools
would be made of 0.3% carbon steel with io% tungsten and 3+% chromium (known as "semi-high-speed"). Permanent mag nets for electric meters, magnetos, radio and telephone equipment absorb annually several thousand tons of steel containing 0.7% carbon and 5 to 6% tungsten.
In tungsten-iron alloys, substantially free from carbon, tungsten has the property of lowering the temperature at which delta iron changes to gamma iron and also of raising the temperature at which gamma changes to alpha iron. Consequently the American metallurgist W. P. Sykes was able to show that no gamma iron is formed in carbon-free alloys containing more than 7% tungsten. Such an alloy is permanently austenitic, and cannot be hardened or tempered by heat treatment. In the alloys containing more tungsten the compound appears; its solubility decreases with the temperature, and it is precipitated throughout the metal at a slow rate. To harden such an alloy it is necessary to anneal or "age" it at a moderate temperature for several hours or days. Brinell hardness numbers indicate the extent of this action: Brinell Carbon free iron ...... . . . . 7o Carbon free solid solution alloy, 8o% iron, 2o% tungsten . 16o Above after aging . ..... . 33o Carbon free aged alloy, 78% iron, 22% molybdenum . . 530 Such aged alloys of iron and tungsten and of iron and molybdenum (tungsten and molybdenum being very similar chemically) form excellent wire drawing dies, and have given io to 4o times the service that can be had from best high speed steel before the edge needs re-dressing. (See IRON AND STEEL ; HIGH SPEED