16.6 Text “Properties of metals and their consequent uses”

Let us consider why metals have come to play so large a part in man's activities. Wood and stone are both older in use, yet to a considerable extent they have been supplanted by the metals. The reason for the increased use of metals is to be found in their characteristic properties. Most important of these properties is their strength, or ability to support weight without bending or breaking, combined with toughness, or the ability to bend rather than break under a sudden blow. Resistance to atmospheric destruction, plasticity, and the ability to be formed into desired shapes add to the remarkable combination of properties possessed by no other class of materials. Some metals also have special properties two of which are the power to conduct electric current and the ability to be magnetized.

Metals can be cast into varied and intricate shapes, weighing from a few ounces to many tons. Their plasticity or ability to deform without rupture makes them safe to use in all types of structures and also allows their formation into required shapes through forging and other operations. Metals also possess the important property of being weldable. Of all engineering materials, only metals are truly weldable and repairable. Other materials used in engineering construction, including glass, stone, and wood, usually are discarded when the structure is no longer usable. On the other hand an obsolete bridge, ship, or boiler made of metal usually is cut into easily handled sections, put into a furnace, remelted, cast, and finally worked into the making of a new ship, bridge, or boiler.

The selection of the proper metal or alloy for a given use is an important part of the practice of metallurgy. Because iron and steel arc used in larger quantities than any of the other metals, it is common practice to divide metallurgical materials into ferrous, or iron-bearing, and nonferrous, of those containing no iron or only small proportions of iron.

Strength, Shaping. Strength, ease of shaping and relatively low cost are of greatest importance for major structural purposes. For these purposes, steel is ideally suited. Steel is used for the structural members of buildings, for rails, locomotives, and ships. For automobile parts, and wherever greater strength and toughness are required, more expensive special steels are used. As a general rule, if strength alone is the main consideration, nonferrous alloys would not be used. However, where the requirement is for strength combined with resistance to rusting, aluminum-bronze or "monel-metal" may be used. There are purposes for which strength is not so important as ease of machining. In making screws, for instance, another kind of steel, or perhaps a brass, may be used. Where the finished form is to be produced by casting, and great strength is not required, cast iron or cast brass are employed.

Metals Light in Weight. For making airplane parts, and in other applications where strength must be combined with light weight, metals such aluminum or magnesium and their alloys are used.

Softness. Ease in Bending. For uses requiring softness and ease in bending, as in cable-sheathing or plumbers' pipe, and where certain chemical properties are needed, lead and it alloy' may be employed.

Electrical Conductivity. Copper and aluminum are used for conducting electricity since they offer less resistance to the passage of the current. Silver offers even less resistance but it is too expensive for commercial use. Copper offers less resistance than aluminum for the same size wire, but aluminum, due to its lighter weight, offers less resistance per unit of weight. Despite this fact, from a cost standpoint the advantage is still with copper.