Nickel steel is much used on account of its high strength. The most usual alloy, perhaps, is the one which contains about 3½ per cent of nickel. This is in addition to the carbon which may vary between .15 per cent and ½ per cent. This 3½ per cent of nickel adds several thousand pounds per square inch in strength to the steel, and when tempered, both the strength and toughness are greatly improved. Nickel steels of these compositions can readily be forged and rolled. They are used for drop forgings, machine parts, engine and automobile parts, in seamless tubes and for bridge members of great span. Nickel will not rust so it does not surprise one that with 22 per cent or more of nickel the steel is almost immune from ordinary corrosion. Steels containing from 25 to 46 per cent nickel are variously used for resistance wire, for valve stems, valves for motors, etc. The 36 per cent nickel steel is the alloy, “Invar,” which has such slight expansion and contraction with heat and cold that it is used for clock pendulums, watch parts and for parts of measuring instruments. Forty-six per cent nickel steel is called “Platinite.” It has practically the same rates of expansion and contraction with heat and cold as glass and for this reason it finds extensive use in incandescent electric lamps. Wires of the alloy are fused into the glass bases and connect with the filaments in place of the expensive platinum which formerly was used. As remarked above, the 13 per cent to 15 per cent nickel-iron alloys soften with quenching but not with annealing. The 15 per cent nickel steel has the highest strength of the nickel-iron-carbon series. Though nickel and iron are each strongly magnetic, alloys of the two which contain 24 per cent or more of nickel are not magnetic. Chrome and Nickel-Chrome Steels While “simple” chrome steel is pretty well known as a material for products which require great hardness, such as balls, roller bearings, files, rolls, five-ply safes, stamp shoes, projectiles, etc., and heat-treated chrome-vanadium steels are now extensively used in forged frames and shafts of automobile and other machines, a combination of nickel and chromium gives steels which have been great favorites. With 2 to 3½ per cent of nickel, not over 3 per cent of chromium and ½ per cent of carbon, these steels, when expertly heat-treated, can give “elastic limits” anywhere between 40,000 and 250,000 pounds per square inch, with good freedom from brittleness. They are very largely used for automobile gears, axles, and other parts, for armor plate, for projectiles and for many other purposes. These alloys are also used for castings. Chrome-Vanadium Steels