Iron

Iron (Latin ferrum, symbol Fe) is magnetic, malleable, silvery white metallic element. Its' atomic number is 26 and it is one of the transition elements of the periodic table.

Metallic iron was known and used for ornamental purposes and weapons in prehistoric ages; the earliest specimen still extant, a group of oxidized iron beads found in Egypt, dates from about 4000 bc. The archaeological term Iron Age properly applies only to the period when iron was used extensively for utilitarian purposes, as in tools, as well as for ornamentation (see Metalwork). The beginnings of modern processing of iron can be traced back to central Europe in the mid-14th century bc.

Properties of iron

Pure iron has a hardness that ranges from 4 to 5. It is soft, malleable, and ductile.

Iron is easily magnetized at ordinary temperatures; it is difficult to magnetize when heated, and at about 790°C (about 1450°F) the magnetic property disappears.

Pure iron melts at about 1538°C (about 2800°F), boils at 2861°C (5182°F), and has a specific gravity of 7.87.
The atomic weight of iron is 55.845.

The metal exists in three different forms:

• ordinary, or a-iron (alpha-iron);
• g-iron (gamma-iron);
• and δ-iron (delta-iron).

The internal arrangement of the atoms in the crystal lattice changes in the transition from one form to another. The transition from a-iron to g-iron occurs at about 910°C (about 1700°F), and the transition from g-iron to δ-iron occurs at about 1400°C (about 2600°F).

The different physical properties of all allotropic forms and the difference in the amount of carbon taken up by each of the forms play an important part in the formation, hardening, and tempering of steel.

Alloy

Alloy is the substance, which is composed of two or more metals.

Alloys, like pure metals, possess metallic luster and conduct heat and electricity well, although not generally as well as do the pure metals of which they are formed.

Compounds that contain both a metal or metals and certain nonmetals, particularly those containing carbon, are also called alloys.

The most important alloy of iron is steel. Simple carbon steels consist of about 0.5% manganese and up to 0.8% carbon, with the remaining material being iron.

An alloy may consist of an intermetallic compound, a solid solution, an intimate mixture of minute crystals of the constituent metallic elements, or any combination of solutions or mixtures of the foregoing.

Intermetallic compounds, such as NaAu₂, CuSn, and CuAl₂, do not follow the ordinary rules of valency. They are generally hard and brittle; although they have not been important in the past where strength is required, many new developments have made such compounds increasingly important.

Alloys consisting of solutions or mixtures of two metals generally have lower melting points than do the pure constituents. A mixture with a melting point lower than that of any other mixture of the same constituents is called a eutectic. The eutectoid, the solid-phase analog of the eutectic, frequently has better physical characteristics than do alloys of different proportions.

The properties of alloys are frequently far different from those of their constituent elements, and such properties as strength and corrosion resistance may be considerably greater for an alloy than for any of the separate metals. For this reason, alloys are more generally used than pure metals.

Steel is stronger and harder than wrought iron, which is approximately pure iron, and is used in far greater quantities.

The alloy steels, mixtures of steel with such metals as chromium, manganese, molybdenum, nickel, tungsten, and vanadium, are stronger and harder than steel itself, and many of them are also more corrosion-resistant than iron or steel.

An alloy can often be made to match a predetermined set of characteristics. An important case in which particular characteristics are necessary is the design of rockets, spacecraft, and supersonic aircraft. The materials used in these vehicles and their engines must be light in weight, very strong, and able to sustain very high temperatures. To withstand these high temperatures and reduce the overall weight, lightweight, high-strength alloys of aluminum, beryllium, and titanium have been developed. To resist the heat generated during reentry into the atmosphere of the earth, alloys containing heat-resistant metals such as tantalum, niobium, tungsten, cobalt, and nickel are being used in space vehicles.

A wide variety of special alloys containing metals such as beryllium, boron, niobium, hafnium, and zirconium, which have particular nuclear absorption characteristics, are used in nuclear reactors.

Niobium-tin alloys are used as superconductors at extremely low temperatures.

Special copper, nickel, and titanium alloys, designed to resist the corrosive effects of boiling salt water, are used in desalination plants.