Iron will alloy with most metals, and some of these alloys have been highly developed for special purposes. These alloys are all alloys of steel and a small per cent of another metal. The metals most commonly used are nickel, chromium, manganese, vanadium, and tungsten, and investigations will possibly develop some surprising results from the use of other metals. Carbon is always present, but apparently as a secondary element.

These steels have several names, as "alloy" steel, "high-speed" steel, "self-hardening" steel and others assigned as commercial or trade names.

The advantages of these alloy steels are numerous, although these advantages are not embodied alike in all of the different compositions. Among the advantages are increased hardness, greater elastic and tensile strength, increased elongation before breaking, resistance to corrosion, and retention of hardness under the influence of high-friction heating. The most important uses for alloy steels are (1) for very hard cutting tools which can stand heavy machine cutting with but little wear, and (2) for high-grade rolled or forged structural material for use where minimum weight and maximum strength of material are required. Much alloy steel is now used in automobiles.

Nickel-steel has a very wide range of use for bridge material, ordnance forgings, wire cables, automobile parts, large axles, engine shafts, and moving parts of marine and other high-grade engines. It contains from 1.5 to 4.5$ of nickel, has a high elastic limit and the tensile strength is between 70,000 and 100,000 lbs. per square inch, according to treatment, while the best low-carbon boiler-plate ranges around 72,000 lbs. The presence of nickel in steel greatly retards corrosion.

Chromium has a more intense effect than nickel, in the same directions. Chrome steel is used for automobile axles and other forgings. Chromium is frequently combined in the alloy with nickel, and nickel-chrome steel is used for toothed wheels requiring great strength and resistance to wear, for very hard steel plates as in plows or burglar-proof safes, for jaws of rock crushers, and for armor and armor-piercing shells. Specimens of chrome steel shows an elastic strength of 180,000 lbs. and a tensile strength of 210,000 lbs.

Manganese steel possesses to a high degree the rare combination of extreme hardness and high ductility in one piece of metal. It is also non-magnetic. Its. uses are restricted because cutting it to shape is extremely difficult. It may be forged, rolled hot, or cast, but any finishing of the shapes so produced must be done by grinding. It is used for burglar-proof safes, rails for curves on railways, jaws for rock crushers, etc. Castings of manganese steel may be battered badly out of shape without breaking, but they are too hard to be machined.

Self-hardening steel is so named because the steel hardens in the air upon cooling, and does not need to be plunged hot into oil or water as is the case in hardening carbon steel. High-speed steel is so named because when used for machine tools, it can cut metal at a very rapid rate without losing its hardness or without wearing away under the friction and resistance of cutting.

The hardest steel so far recorded contains .68% of carbon, 3.01% of chromium, 19.37$ of tungsten, and .04$ of silicon.