Cast iron

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Cast iron is a more purified form of iron, which contains less amount of impurities than in the pig iron. This is obtained by remelting the pig iron in the blast furnace-like structure known as the cupola.

Composition of cast iron

Cast iron is also an impure iron which contains 2-5% carbon. Other impurities present in the cast iron are sulphide and phusphide. As they are found in fewer amounts, they do not affect the property of cast iron.

Formation of Cast Iron

Cast iron is made up or formed by re-melting the pig iron in a furnace known as the cupola furnace. This furnace consists of a cylindrical iron vessel lined inside with refractory fire bricks. It is open at the top. There is a charging door at the upper part of the furnace while at the lower part there are two semicircular gates at different levels. One for skimming the slag and other for trapping molten iron. In between slag skimming gate and charging gate, there is an arrangement to blow air inside the furnace. These are known as tuyers.

Coke is used as fuel in the furnace. First of all the coke at the bottom of the furnace is fired and coke is added to make the strong heat. When sufficient heat is produced, then the alternate layers of pig iron and coke is charged to the furnace. At the same time, the air at the high pressure is blown inside the furnace. This air when it gets high temperature inside goes up and escapes out while doing so, it melts the pig iron charged inside.

Limestone is used as a flux to combine with the impurities and fuel ash. Slag thus formed is skimmed out from the slag-skimming door. When a sufficient amount of molten iron is formed inside the furnace, the trapping door is opened and metal is collected at about 1200-1400 degrees Celsius. In this way cast iron is formed.

Types of cast iron

  1. White cast iron
  2. Molted cast iron
  3. Malleable cast iron
  4. Alloy cast iron etc.

Properties of Cast iron

  1. Cast iron is neither malleable nor ductile.
  2. It is brittle metal and can not resist the impact of the shock.
  3. Cast iron is less corrosive but becomes soft in saline water.
  4. Structure is fibrous crystalline and corrosion.
  5. It is strong in compression but weak in tension.
  6. It can be hardened but can not be tempered.
  7. The melting point is 1200 degrees Celsius.
  8. It cannot be welded generally
  9. And its specific gravity is 7.5.

Uses of Cast iron

  1. Cast iron is used in the sewer pipe, gutters, rainwater pipes, etc.
  2. They are also used to make manhole cover, balustrades, railings, etc.
  3. It may also be used as the raw material to make the wrought iron.

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