. different types of cast iron
. properties of each type cast iron
Cast iron is an alloy of iron, carbon and silicon. The carbon content ranges from 2 to 4%.
Types of cast iron
The following are the types of cast iron.
-Grey cast iron
-White cast iron
-Malleable cast iron
-Nodular cast iron
Grey cast iron
This is widely used for the casting of machinery parts and can be machined easily.
Machine bases, tables, slideways are made of cast iron because it is dimensionally stable after a period of aging.
Because of its graphite content, cast iron provides an excellent bearing and sliding surface.
The melting point is lower than that of steel and as grey cast iron possessses good fluidity intricate casting can be made.
Grey cast iron is widely used for machine tools because of its ability to reduce vibration and minimize tool chatter.
Grey cast iron, when not alloyed, is quite brittle and has relatively low tensile strength. Due to this reason it is not used for making components subjected to high stress or impact loads.
Grey cast iron is often alloyed with nickel, chromium, vanadium or copper to make it tough.
Grey cast iron is weldable but the base metal needs preheating.
White cast iron
This is very hard and is very difficult to machine, and for this reason,, it is used in components which should be abrasion-resistant.
White cast iron is produced by lowering the silicon content and by rapid cooling. When cooled in this manner, it is called chilled cast iron.
White cast iron cannot be welded
Malleable cast iron
Malleable cast iron has increased ductility, tensile strength and toughness when compared with grey cast iron.
Malleable cast iron is produced from White cast iron by a prolonged heat treatment process lasting for about 30 hours.
Nodular cast iron
This is very similar to malleable cast iron. But this is produced without any heat treatment. Nodular cast iron is also known as
NODULAR IORN -DUCTILE IRON- SPHEROIDAL GRAPHITE IRON.
This has good machinability, castability, resistance to wear, low melting point and hardness.
Malleable and nodular casting are used for machine parts where there is a higher tensile stress and moderate impact loading. These casting are less expensive and are an alternative to steel casting.
Cast iron manufacturing process
The pig iron which is tapped from the blast furnace is the crude form of raw material for the cupola, and should be further refined for making castings. This refining is carried out in the cupola furnace which is a small form of a blast furnace.
Generally cupolas are not worked continuously like blast furnaces but are run only as and when required.
Working of a cupola
For starting a cupola, a COKE fire is lit at the bottom of the cupola. When the fire is established, the furnace is charged with alternate layers of coke and pig iron together with limestone. Steel scrap is also sometimes added depending on the required quality of metal. Adding steel scraps with a lower carbon content reduces the carbon content in the metal. During melting, a blast of air is also introduced into the furnace.
The molten limestone combines with sand and other impurities and floats in a liquid form on the top of the molten metal. It is called slag.
The molten metal sinks to the bottom of the furnace from where it is tapped and collected in ladles. The hot metal.
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