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Summary

In general, most alloys consist of two or more metals. Other elements such as carbon, silicon, and sulfur may be present. Industrial alloys are classed as ferrous (iron based) and non-ferrous. Ferrous alloys with less than 0.13 percent carbon by weight are steels. Cast and wrought iron have 2 to 5 percent carbon by weight. Alloy steel and special steel have relatively large amounts of chromium and nickel. The aircraft industry requires lightweight alloys such as aluminum and magnesium; titanium and titanium-based alloys are much used.

Alloys are usually prepared by melting the constituent metals together. The melting can be done in furnaces fired by gas, coke, or oil. Electrical heating is also used. Some alloys, such as pig iron, are prepared directly by the process used to extract the metal from the ore.

A metal can be made harder, stronger, and more resistant to impact by controlled heating and cooling. Heating (annealing) can make a metal softer. Metal properties are often improved at the expense of other properties. Heat-treating processes differ in three important ways: the temperature to which the metal is treated, the rate at which it is cooled, and the properties possessed by the finished metal. The most common forms of metal heat treatment are hardening, tempering, normalizing, case hardening, and hot working. Hardening consists of heating the steel to the appropriate temperature and then cooling it rapidly by quenching. Heating the steel below the critical point and then cooling it in still air tempers it. Annealing requires heating the metal to the proper temperatures, holding that temperature for the required time, and cooling the metal to room temperature. Normalizing consists of heating metal to the appropriate temperature until it is uniformly heated and then cooling it in still air. Only ferrous metals can be normalized, Case hardening consists of carbonizing steel at 1,700° to 1,800° F for several hours and reheating and quenching twice. Hot working consists, in general, of working steel while the core is still hot. Casting is the process of pouring molten metal into a single-purpose or permanent mold. Hot rolling is passing the metal between rollers while it is still hot. Forging is changing a metal's shape by compressive deformation through hammering or pressure. Extrusion is forcing metal to take the die opening's shape. Metal worked at temperatures below the critical range is being cold-worked. Cold rolling, cold drawing, and stamping and pressing are forms of cold working.

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