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METALS
1. Mankind has used metals for centuries in gradually increasing quantities but only now they are employed in really great quantities. Today we know more than seventy metals, the majority of which are used in industry.
2. Of all the metals iron is the most important one. Absolutely pure iron is never prepared except for laboratory purposes. The irons and steels in use today are really alloys of iron, carbon and other substances. They can be made elastic, tough, hard, or comparatively soft.
3. Mechanical properties of metals are the result of their atomic structure. They include hardness, ductility and malleability which are of special importance in engineering. Ductility is the capacity of a metal to be permanently deformed in tension without breaking. Malleability is the capacity of a metal to be permanently deformed by compression without rupture. The strength of a metal is the property of resistance to external loads and stresses.
4. FERROUS METALS AND STEELS. Ferrous metals consist of iron combined with carbon, silicon and other elements. But carbon is the most important element in ferrous alloys. Ferrous metals are used in industry in two forms: steel and cast iron, which differ in the quantity of carbon content. Alloys consist of a simple metal combined with some other element. Steel is a ferrous material having some carbon content. There are two kinds of steel: carbon steel and alloy steels.
5. Carbon steel should contain only iron and carbon without any other alloying element. Alloy steels are those in which in addition to carbon an alloying element is present. These alloying elements have an effect on the properties of steel. According to their chemical and mechanical properties steels may be used in different branches of industry, for example, in machinebuilding, rocket engineering, automobile industry, etc.
6. METALS AND NONMETALS. There are some distinctions between metals and nonmetals. Metals are distinguished from nonmetals by their high conductivity for heat and electricity, by metallic lustre and by their resistance to electric current. Their use in industry is explained not only by those properties, but also by the fact that their properties, such as strength and hardness, can be greatly improved by alloying them with other metals.
7. There are several important groups of metals and alloys. The common metals such as iron, copper, zinc, etc. are produced in great quantities. The so-called precious metals include silver, gold, platinum and palladium. The light metals are aluminium, berillium and titanium. They are important in aircraft and rocket construction. Many elements are classified as semimetals (bismuth, for example) because they have much poorer conductivity than common metals. Nonmetals (carbon, silicon, sulphur) in the solid state are usually brittle materials without metallic lustre and are usually poor conductors of electricity.
8. Metals can undergo corrosion, changing in this case their chemical and electromechanical properties. In order to protect metals from corrosion the products made of metals and steel are coated by some films (coatings). Organic coatings protect metals and steel from corrosion by forming a corrosion-resistant barrier between metal or steel and the corrosive environment.
NOTES:
pure iron – чистое железо;
alloys– сплавы;
ductility – ковкость;
malleability – тягучесть;
ferrous metals – черные металлы;
conductors – проводник (физ., электр.);
brittle materials – хрупкие материалы;
corrosive environment – коррозионная среда.
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