Steel is an alloy of iron and carbon. It is prepared by mixing about 0.1 % to 1.5 % of carbon in iron.
Methods of Production of Steel
Steel can be manufactured by using following three methods.
- Bessemer converter
- Open hearth process
- Electric furnace process
But here we will discuss only one method of preparation of steel by using Bessemer converter.
Production of Steel
The raw material used for the production of steel is pig iron or cast iron which contains carbon, Sulphur, manganese, silicon and phosphorous as impurities. Steel is prepared in a pear shaped furnace called as Bessemer converter. Bessemer converter is a specially made furnace which has holes at its bottom through which hot air can be passed into it. The Bessemer converter can also be rotated or tilted in any direction around its axis.
We will now discuss the procedure of production of steel. First of all molten pig iron is put into the Bessemer converter by tilting it sidewise. Some calcium oxide is also put into it which helps in the removal of phosphorus impurities present in the pig iron. Bessemer converter is now brought back into the vertical position and a blast of hot air passed into the Bessemer converter through the holes at its bottom. The oxygen present in the hot air oxidizes the various impurities present the pig iron as follows:
Carbon present as impurity in pig iron is oxidized by the oxygen of hot air to carbon monoxide gas.
2C + O2 2CO
Carbon Oxygen Carbon monoxide
This carbon monoxide gas burns at the mouth of the Bessemer converter to form carbon dioxide gas.
2CO + O2 2CO2
Carbon monoxide Oxygen Carbon dioxide
Sulphur present as impurity in pig iron is oxidized by the oxygen of hot air to Sulphur dioxide gas which escapes from the Bessemer converter.
S + O2 SO2
Sulphur Oxygen Sulphur dioxide
Manganese present as impurity in pig iron is oxidized by the oxygen of hot air to form manganese oxide.
2Mn + O2 2MnO
Manganese Oxygen Manganese oxide
Silicon present in the pig iron is oxidized into silicon dioxide.
Si + O2 SiO2
Oxygen Silicon dioxide
Manganese oxide and silicon dioxide react together to form manganese silicate slag.
MnO + SiO2 MnSiO3
Manganese oxide Silicon dioxide Manganese silicate (slag)
Phosphorous present as impurity in pig iron is oxidized by the oxygen of hot air to form phosphorous pentaoxide.
4P + 5O2 2P2O5
phosphorous Oxygen Phosphorous pentaoxide
This phosphorous pentaoxide reacts with calcium oxide which is added to the Bessemer converter to form calcium phosphate slag.
3CaO + P2O5 Ca3 (PO4)2
Calcium oxide Phosphorous pentaoxide Calcium phosphate (slag)
The slag of magnesium silicate and calcium phosphate formed in the above reacts are light in weight, so these float on the surface of molten iron. The slag in the Bessemer converter can be removed by tilting it to one side.
Properties of Steel
As we have already studied that steel is an alloy of iron with carbon. So its properties depend upon the amount of carbon present in the steel. On the basis of amount of carbon in the steel, it can be categories into following two types:
- Mild steel
- Hard steel
1. Mild Steel
Mild steel is the most common type of steel. It is prepared by mixing 0.1 % to o.4 % of carbon in iron. Mild steel is quite malleable, ductile and elastic. It is tough and not brittle.
Uses:- It is used for making sheets, wires, chains, car bodies, axles, railway lines, wheels, nails, springs, ships, bridges, girders, pipes, cables, and for construction purposes in buildings.
2. Hard Steel
Hard steel is very hard and brittle. It is prepared by mixing 0.5 % to 1.5 % of carbon in iron.
Uses:- It is used for making cutting tools, chisels, razor blades, knives and drill bits
Quenching, Tempering and Annealing of Steel
The processes of quenching, tempering and annealing of steel are done to modify the hardness and brittleness of steel.
Quenching of Steel
The process in which the steel is heated to red heat (8500 C), and then cooled suddenly by putting into cold water is called quenching of steel. By this process steel becomes very hard and brittle.
Tempering of Steel
The process of heating quenched steel or hardened steel to a temperature ranging between 2000 C to 3500 C, and then cooling it slowly, is called tempering of steel. Tempering of steel is done to prepare elastic, less hard and less brittle steel.
Annealing of Steel
The process of annealing of steel is just opposite to that of quenching of steel. The process of heating steel to red heat and then cooling down slowly is called annealing of steel. Annealing of steel is done to make soft steel.
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