CAST IRON

CAST IRON

Introduction:-

Cast iron is pig iron remelted and there by refined in a cupola is other form of remelting furnace, and poured into suitable moulds of required shapes. A cupola dose not produces steel. Instead remelting scraps along with pig iron in a furnace called CUPOLA pours iron casting. As the metal is melted in contact with fuel, some elements are picked up while others are lost. This affects the final analysis of the metal and necessitates close regulation of the cupola.

1. Manufacture of Cast Iron

Cast iron is manufactured from Cupola furnace. A Cupola may be described as a smaller version of the Blast furnace. It is a vertical shaft type furnace consisting of a cylindrical steel shell lined with refractory material and equipped with a wind box and tuyeres for admission of air. A charging door is provided at the top for the introduction of melting stock and fuel. Near the bottom are holes and spouts for removal of molten metal and slag.

The charge of Cupola is made of solid pig iron,coke,lime stone, and iron & steel scrap. The iron and steel scrap are very important since they help in reducing the cost of iron and adjusting the carbon content of the whole mass of iron.Coke provides the heat necessary to melt the charge. Flux is a substance which lowers the melting point and improves the fluidity of the slag. Lime stone acts as a flux and separates the slag from the pig iron and ash from the coke to form a liquid slag.

The following are the important zones of Cupola.

1. Preheat zone    2. Reduction zone   3. Oxidation zone      4. Metal/slag zone

Preheat zone: It lies over the melting zone, serves as heat exchanger wherein waste heat is recovered by direct heat exchange between upward flowing gases and downward flowing charges and fuel.

Reduction zone: It is above the combustion zone. In this zone the principal reaction is the combination of carbon dioxide with the fuel to form carbon monoxide. Most of the water vapour present in the blast air also reacts in this zone to form CO & H2. It is a protective zone to prevent oxidation of the metal charge above and while dropping through it, the down coming molten metal and the slag are superheated. Thus the metal drops down more rapidly through the combustion zone.

Oxidation Or Combustion zone: In this zone the principal reaction is the combustion of oxygen with the fuel to form CO2.

Operation of Cupola:

The bottom doors are closed and a sand bed is formed on it. A tap hole is formed at this level in line with tapping spout. Coke, iron, flux  are placed in alternate layers and the coke bed is lighted up by starting the air blow. ( The iron to coke ratio is 10:1 ).  This air rising upwards through the stack furnishes oxygen for combustion of coke. After blowing a few minutes the coke becomes hot enough to cause melting of the metal charge. As the metal melts in the zone above the tuyeres, it trickles down through the coke and accumulates in the well, the cupola is tapped. Liquid slag will continue to flow until the iron level reaches the slag hole, at which time the cupola must be tapped.

2. Composition of Cast iron:-

Cast iron is an alloy of iron, carbon and silicon. Most cast irons contain carbon 2-4%, silicon 0.5 – 3%, manganese 0.2 – 1%, phosphorus 0.05 – 0.8%, sulphur 0.04 – 0.15%.

Carbon in cast iron is found in two forms: in the free state as graphite and in the combined form as cementite. Cementite is very hard and brittle and has little strength.

3. Classification of Cast iron:-

The various types of cast iron are: 

1. Wrought iron [Read Now ##eye##]
2. Grey cast iron [Read Now ##eye##]
3. Malleable cast iron [Read Now ##eye##]
4. Nodular cast iron [Read Now ##eye##]
5. White cast iron [Read Now ##eye##]
6. Meehanite cast iron [Read Now ##eye##]
7. Alloy cast iron  [Read Now ##eye##]