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Element Iron, Fe Ferrum, Transition Metal

About Iron

Metallic iron was not obtained from its naturally occurring compounds at so early a date as some of the other metals, especially copper and tin. This is due to its high point of fusion, and to the much greater difficulty in obtaining it in the metallic state from its compounds. Thus, in prehistoric times iron does not appear till after bronze, i.e. mixtures containing copper as essential constituent, and was apparently at first a great rarity.

Notwithstanding the wide distribution of iron, it scarcely ever occurs in the metallic state on account of its tendency to form compounds with oxygen and sulphur. The chief occurrence of metallic iron, except in some rather accidental cases through the action of chemical processes connected with volcanic activity, is in certain meteorites. These are masses which do not originally belong to the earth, but which, in the course of their flight through space, approach so closely to the earth that, owing to atmospheric friction, they lose their kinetic energy, which is thereby converted into heat, and fall to the earth. Many of these masses consist of iron.

Masses of native iron also occur, although rarely (e.g. at Ofvivak in Greenland), whose meteoric origin is doubtful, although no explanation has been given of any other possible origin.

Iron is a grey, tenacious metal, which fuses with great difficulty, at about 1600°; it combines with free oxygen quickly at high temperatures, slowly at low ones. In the heat essentially compounds of the formula Fe3O4 to Fe2O3 are formed; in the cold, iron hydroxide, Fe(OH)3, is formed. The hydrogen necessary for this is taken up in the form of water; in fact, iron " rusts " or oxidises at a low temperature only in moist, not, or not measurably, in dry air. Since the rust does not cohere, it does not protect the iron against further oxidation.

At all temperatures water is decomposed by iron. The decomposition of water by red-hot iron is a classical experiment. Even at the ordinary temperature decomposition takes place with evolution of hydrogen, but exceedingly slowly, so that the evolution of hydrogen can be observed only by using large surfaces (iron powder). Iron is dissolved even by the weakest acids, thereby passing into divalent diferrion with evolution of hydrogen.

Commercial Iron

Commercial iron is not pure, but contains up to as much as 5 per cent of carbon, which has a very great influence on its properties, and also smaller quantities of other impurities. While pure iron, although very tenacious, is comparatively soft, its hardness increases with the amount of carbon it contains, and its behaviour at moderately high temperatures becomes essentially different.

There are three chief kinds of commercial iron, viz. wrought-iron, steel, and cast-iron; the first contains the smallest, the last the highest, amount of carbon. Wrought-iron approximates most nearly both in composition and in properties to pure iron; it is tough, not very hard, and on being heated first becomes soft like wax or sodium before melting. This property is of the greatest importance for the technical working of iron, as it renders it possible to shape the metal and to unite different pieces without it being necessary to raise the temperature to the melting point of the metal. On the contrary, it is sufficient to heat to the temperature of softening (about 600°), so as to attain the object by pressing, rolling, and forging. The uniting of the two pieces of iron by pressure (hammering) is called welding. The temperature necessary for this is bright red-heat.

The properties of wrought-iron do not undergo essential change when it is heated and suddenly cooled. The character of steel, however, depends in the highest degree on such treatment.

Steel is iron which contains from 0.8 to 2.5 per cent of carbon, but is otherwise as pure as possible. The carbon is chemically combined with the iron, and this carburetted iron or iron carbide, Fe3C, is alloyed with the rest of the iron. The result of the presence of this foreign substance is, in the first place, an appreciable sinking of the melting point; at 1400° steel is liquid and can be cast. Cast-steel is a metal consisting of fine crystalline grains, which, like wrought-iron, softens before melting, and can therefore be forged. By such treatment steel acquires a fibrous or sinewy character, similar to wrought-iron. If the steel is made red hot and then suddenly cooled, it becomes brittle, and at the same time acquires its highest degree of hardness. It is then so hard that it scratches glass, and is hence called glass hard. If this steel is again carefully heated, all degrees of hardness can be imparted to it, for it increases in softness the longer or the higher it is heated. This process is called the tempering of steel.

As an index of the degree of tempering to be attained, use has been made from olden times of the colours which a bright steel surface acquires on being heated. At about 220°, the metal begins to oxidise in the air with a measurable velocity, and the oxide produced forms a thin coating on the metal. If the thickness of this coating is of the order of a wave-length of light, the corresponding interference colours, or the " colours of thin plates," begin to appear. Since the shortest of the visible waves, the violet, is first extinguished, the first tarnish- colour to appear is the complementary colour, pale straw-yellow. This passes through the colours orange, purple, violet, blue, and finally becomes grey. To each of these colours there corresponds a definite degree of hardness of the steel. Steel for tools to work iron is allowed to reach the yellow stage, for brass the purple-red stage, while tools for wood are allowed to become blue. Although colour and hardness do not exactly correspond, still the correspondence is sufficient for an experienced workman.

The great utility of steel in the arts is due to the diversity in the degrees of hardness which it can acquire. In the soft state it can be shaped to any desired form, and the shaped objects can then be brought to any degree of hardness.

It is only in recent years that the theory of tempering has been made clear. Iron carbide, Fe3C, mentioned above, is not only itself very hard, but it forms with pure iron a homogeneous mixture, a " solid solution," which is also hard; so much the less hard, the less carbide it contains. If, now, such a solid solution, consisting at higher temperatures of carbide and iron, is slowly cooled, it breaks up at about 670° into pure iron and iron carbide, which exist as a conglomerate side by side. Since pure iron is soft, it imparts this property also to the mixture.

If, however, the cooling is performed rapidly, the breaking up of the solid solution does not occur, and the latter therefore preserves its hardness. The solid solution hereby becomes metastable or to a certain extent supersaturated.

This explains, in the first place, why quenched steel is hard, while slowly cooled steel is soft. The tempering of hard steel, now, consists in the separation of the solid solution into its two constituents through elevation of the temperature, the separation occurring all the more rapidly the higher the temperature. By sudden cooling, the state of the mixture attained at any point is preserved, since, at the ordinary temperature, the velocity of change is immeasurably small. The corresponding degree of hardness is then obtained.

These considerations also make clear the fact, learned by experience, that the temper depends not only on the temperature but also on the time, in such a way that a lower temperature for a long period has the same effect as a higher temperature for a shorter time.

The tempering can be carried out in one operation by appropriately heating to above 670° until the desired mixture of iron and solid solution (the equilibrium between which alters with the temperature) is produced, and then fixing this state by suddenly cooling. The temperature necessary for obtaining a definite degree of hardness depends on the amount of carbon present. If this is known, the temperature required to produce a given degree of hardness can be decided beforehand.

If the amount of carbon increases to from 4 to 5 per cent, the melting point of the iron becomes still lower, and the metal loses its toughness and the power of assuming the fibrous condition, but it still retains the power of being tempered to a certain degree. Such iron is called cast-iron.

Two kinds of cast-iron are distinguished, white and grey. The former is obtained by quickly cooling; it is very hard and crystalline, and contains the greater part of its carbon chemically combined as carbide. When the cast-iron is slowly cooled, part of the carbon separates out in fine laminae as graphite, which imparts a grey colour to the iron. At the same time the metal becomes less hard and brittle, and the grain finer. In this condition cast-iron is used for innumerable purposes where ease in the shaping of the object by casting has to be taken into account, and where the smaller resistance of the metal to pulling strain and bending is no essential drawback.

Iron History

Main article History of Iron.

Iron is one of the seven metals of the hoary Ancientry. It is supposed that human beings started to work on meteorite iron earlier than with other metals. From the high antiquity iron was extracted from ores which were deposited everywhere.

The Latin ferrum which has become the international comes from Greek-Latin fars (to be hard) which roots in Sanskrit bhars (to harden). The accordance with ferreus, which means "insensitive, tough, hard, inflexible, heavy" and with ferre (to carry) is also possible. Alchemist used, with Ferrum, also other names, such as Iris, Sarsar, Phaulec and so on.

Iron Occurrence

Meteorite iron is not pure. Usually it contains 30% or less other elements. It may be hammered in the cold state, but becomes brittle when heated.

Iron inclusions may be found in basalt and other igneous rocks. It is supposed that find the Earth's core is predominantly iron metal starting from 2900 km depth (the Earth's equatorial radius is 6377 km) and is composed of iron (91-92%) / nickel (8-9%) alloy.

Iron is most abundant element after oxygen, silicon and aluminium. Iron compounds form a great number of minerals and rocks, soils and living organisms; however the iron ores are few. Iron oxide ores are most valuable among them.

Iron is essential to nearly all known organisms, with concentration approximately 0.02%. It is a very essential element for oxygen exchange and oxidizing processes. Some living organisms, so-called iron concentrators are able to deposit large amounts of iron (until 17-20%) within them like, for instance, iron depositing bacteria. Iron in living organisms is almost entirely involved in protein processes. Iron deficiency, aggravated by high soil pH (alkali soils), brings to plants growth inhibition and chlorosis, a condition in which leaves produce insufficient chlorophyll. Iron abundance in the case of low soil pH (acid soils) is also harmful: it causes flowers sterility. Such plant diseases may occur in large areas.

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