TOPICS ON MECHANICS

IRON

This element is grey, malleable and ductile and has exceptional magnetic properties.

It readily oxidizes in moist air and is attacked by many corrosive agents.

It seldom appears pure. It is nearly always found as iron ore which is a compound of iron and oxygen, or of iron, oxygen and carbon joined with other substances such as limestone and clay.

Iron is set free from its ore in a blast furnace which is a tower built of iron plates and lined with firebrick. At the base there is a taphole which can be opened to let the molten iron run off, and above it the blast pipes enter carrying the blast of hot air to keep combustion. The gases are driven off through a taphole and the air blast passes to be heated before it enters the furnace. The charge, which is introduced at the top, consists of alternate layers of iron ore to provide the iron; limestone to act as flux, and coke or coal to supply heat and produce the carbon monoxide which is the active reducing agent.

The iron which is released from the ore, sinks to the bottom of the furnace and is driven off through the taphole into sand moulds outside the furnace where it solidifies giving the blocks known as pig irons, the process being continuous.

Iron was known to the ancient world and probably its first samples examined were of meteoric origin. The Egyptian, Hebrew and Assyrian names for this metal mean, "the metal of heaven" thus indicating that the first known specimens were of heavenly origin. It is said that the working of iron ore was. first practised on a large scale in ancient Egypt.

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Pig iron is hard and brittle and cannot be shaped with a hammer. It is used to make wrought iron and steel and for castings. For these, suitable pig iron is remelted and poured into moulds shaped according to the article required.

Cast iron is brittle. It generally cannot be forged, that is heated and shaped. It resists rusting much better than wrought iron. It is particularly used for railings, gratings, pipes and stoves.

The oldest form of iron used by man is wrought iron which is made by releasing suitable pig iron from most of its impurities.

Pig iron is melted in a puddling furnace with the oxides of iron which oxidize the impurities; the oxygen from the ore combines with the carbon in the pigs, the metal is puddled with long rakes through the furnacedoor, and at a certain stage in the manufacture, the metal begins to set and become sticky; then it is rolled into a ball mixed with slag which is afterwards squeezed out as much as possible by rolling and hammering. This material is malleable, tough and very resistant to shock. Today there are very few uses for this material because commercial mild steel serves the same general purposes and is much cheaper. It has now a few applications such as nails, chains, horse-shoes, anchors and ornamental ironwork.

STEEL

Steel is an alloy of iron and carbon which is made of pig iron by removing the excess of carbon and the undesirable components by means of oxidation reactions. It is hard, strong and resistant.

There are three principal methods to manufacture steel: the Bessemer converter, the open-hearth and the electric process.

For the first, suitable molten pig iron is put into the Bessemer converter, which is a barrel-shaped furnace supported on piers. Compressed air penetrates through the holes in the bottom and oxidizes the impurities which go to form a slag. At the same time, the iron is oxidized and to correct this, an alloy of manganese, iron and carbon is added once the blast is shut off.

The manganese deoxidizes the iron forming manganese oxide which goes into the slag and the carbon dissolves in the iron and converts it into steel. The steel is then put into moulds.

The open-hearth process is very commonly employed. Pig iron and scrap iron are put into a steel furnace heated to a very high temperature by producer gas derived from coal. The impurities of the iron are oxidized; manganese is added to deoxidize the iron and provide the necessary carbon.

The molten steel is put into moulds through a taphole in the bottom of the furnace.

Electric furnaces are used for special types of steel and alloys such as chromium steels, manganese steels and others.

The iron is melted by an electric are and the oxidation of impurities is carried out by iron ore. This process has several advantages: it produces the purest steel; it gives good steel out of impure materials and the composition can be better regulated than by other methods.

A small change in the carbon content may give rise to a large change in the properties of steel which also depend on the method by which it is cooled. Thus, sudden cooling or chilling produces a hard, brittle metal while if it is slowly cooled or annealed, a soft, ductile kind is produced. By tempering the steel we may get properties intermediate to the two extremes.

Stainless steel is an alloy of iron with about 16 % of chromium so as to resist corrosion. It was invented in Sheffield in the year 1913.

The Gordon process for making stainless steel gives ordinary steel a coating of stainless steel which makes the whole absolutely rustless.

Iron and steel and their mixtures are called ferrous metals.

They are given a special name because they constitute the main metals used in our age. All the other metals and their alloys belong to the non-ferrous metals.

Man has been using iron for three thousand years, but pure iron is too soft to use in tools. It is the most important metal of the contemporary world and enormous quantities of it are made each year. For instance, about 25,000,000 tons are produced in Great Britain annually while the production in the United States is of over 100,000,000 per year.

Let us see how iron is treated to make cast iron, wrought iron and steel:

Iron Ore   

 

Pig iron

made in a blast furnace (I ton of pig iron is made with2 tons of ore, 1/2 ton of limestone, 1 ton of coke, 4% tons of air)

  
  • Cast iron : made in a foundry
  • Wrought iron : in a puddling furnace
  • Steel : made by several processes
  • Alloy steels special steels made in electric furnaces

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