Steel

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

 Silicon           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:

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.

Test your understanding and answer these questions:

1. What is steel?
2. How steel is manufactured?
3. What do you understand by quenching, annealing and tempering of steel?