Cupola Furnace - Working principle and Construction

• Cast-iron:-

The cast iron is manufactured by re-melting pig iron with coke and limestone. This re-melting is done in a furnace known as the Cupola furnace. It is more or less the same as the blast furnace, but it is smaller in size. Its shape is cylindrical with a diameter of about 1 m and a height of about 5 m.

       The working of a cupola furnace is also similar to that of a blast furnace. The raw materials are fed from the top. The cupola furnace is worked intermittently and it is open at the top. After the raw materials are placed, the furnace is fired and a blast of air is forced through tuyeres. The blast of air is cold as the impurities in pig iron are removed by oxidation.

       The impurities of pig iron are removed to some extent and comparatively, pure iron is taken out in the molten stage from the bottom of the furnace. The slag is also removed from the top of cast iron at regular intervals. the molten cast-iron is led into moulds of required shapes to form what is known as cast-iron castings.

        Cast iron is a composition of iron-carbon alloys with a carbon content greater than 2%. Its usefulness derives from its relatively low melting temperature. Cast iron tends to be brittle, except for malleable cast irons. With its relatively low melting point, good fluidity, castability, excellent machinability, resistance to deformation and wear resistance, cast iron have become an engineering material with a wide range of applications and are used in pipes, machines and automotive industry parts, such as cylinder heads, cylinder blocks and gearbox cases. It is resistant to destruction and weakening by oxidation.

•        Composition of cast-iron:-

Cast iron contains about 2 to 4 per cent of carbon. In addition, it contains various impurities such as manganese, phosphorus, silicon and sulphur.

The manganese makes cast-iron brittle and hard. Its amount should therefore be kept below 0.75 per cent or so.

The Phosphorus increase fluidity of cast iron. It also makes cast iron brittle and when its amount is more than 0.30 per cent, the resulting cast iron is lacking in toughness and workability. Its percentage is sometimes kept as about 1 to 1.5 to get very thin castings.

Cast iron pan

The silicon combines with part of iron and forms a solid solution. It also removes combined carbon from graphite form. If its amount is less than 2.50 per cent, it decreases shrinkage and ensures softer and better castings.

The sulphur makes cast-iron brittle and hard. It also does not allow smooth cooling in sand moulds. Its presence causes rapid solidification of cast iron and it ultimately results in low-holes and sand-holes. The sulphur content should be kept below 0.10 per cent.

A tool made up of Cast-Iron

• Types of cast-iron:-

Following are the varieties of cast-iron:

1. Grey cast-iron
2. White cast-iron
3. Malleable cast-iron
4. Spheroidal graphite iron or Ductile iron
5. Mottled Cast-iron
6. Toughened cast-iron
7. Chilled cast-iron.

• Properties of cast iron:-

Following are the properties of Cast-iron:

1. If placed in salt water, it becomes soft.
2. It can be hardened by melting and sudden cooling, but it cannot tamper.
3. It can not be Magnetised.
4. It does not rust Easily.
5. It is fusible.
6. It is hard, but it is brittle also.
7. It is not ductile and hence it cannot be adopted to absorb shocks and impact.
8. Its melting temperature is about 1250 degrees centigrade.
9. Ity shrink on cooling. This fact is to be considered while making patterns or moulds for foundry works.
10. Its structure is granular and crystalline with a whitish or greyish tinge.
11. Its specific gravity is 7.5.