What is Cast Iron?
Cast iron is obtained from the pig-iron which is re-melted with coke and limestone. Pig iron is nothing but impure iron which is obtained from the iron ore. Cast iron has lot of engineering properties so, that it can be used in many ways like for sanitary fittings, rail chairs, casting molds etc.
Manufacturing of Cast Iron
Cast iron in manufactured from re-melting process as mentioned above. This process takes place in a furnace called couple furnace. The furnace is 5 meters in height and cylindrical in shape with 1 m diameter.
The raw materials pig iron, coke and lime stone are entered from the charging door of chamber which provided at the top. Air blast is introduced into the chamber using air blast inlet which removes the impurities present in the pig iron.
Hence, pure cast iron is obtained from the bottom out let and it is poured into molds of required shape. These molds are called as cast iron castings.
Types of Cast Iron used as a Building Material in Construction Works
Following are the types of cast iron used as a building material in construction and their uses:
- Grey cast iron
- Malleable cast iron
- Mottled cast iron
- Toughened cast iron
- White cast iron
- Ductile cast iron
- Chilled cast iron
Grey Cast Iron
As the name suggests, it is grey in color. It has coarse crystalline structure. Its melting point is very low thus it has weak strength and it is only used for casting purposes.
Malleable Cast Iron
Malleability is the property which helps the materials to undergo any shape without breaking or cracking. Hence, malleable cast iron is used for making many types of materials. It has good corrosive resistance.
Its manufacturing process involves two steps. In the first step, it is casted and cooled as ordinary cast iron and then again it is heated to 1050oC and soaked in water for long period (several hours or days).
Hence, carbon content is slightly reduced and graphite content is precipitated as temper carbon. This reduce the brittleness of cast iron. So, it can be worked easily using machines. It is used for making pipe fittings, fastenings, automobiles etc.
Mottled Cast Iron
Mottled iron is the medium stage cast iron which properties are in between grey cast iron and white cast iron. It has Small amount of graphite in its composition. So, mottled type fractures are developed in its micro structure.
Toughened Cast Iron
Toughened cast iron is the combination of cast iron and wrought iron. To obtain this wrought iron scrap and cast iron melted together. The composition of wrought iron is about 0.15 to 0.25 weight of the cast iron.
White Cast Iron
It is in silver color. Its melting point is high so, strength wise it is better but not used for delicate casting purposes. Because of its heavy strength, it cannot be used easily.
Ductile Cast Iron
Ductile cast iron is also called as spheroidal graphite iron. Its manufacturing process is very easy compared to other types.
Its manufacturing process consists manganese treatment which helps to increase the carbon content and opposes the formation of graphite in flaky form. It has very good engineering properties than malleable cast iron.
Ductile cast iron has very good corrosion resistance, high strength and durability. So, usage of ductile iron dominates the other types. It is used for making sewer pipes, water conveying pipes etc.
Chilled Cast Iron
Chilled cast iron consists two layers of which one layer has white cast iron properties and other one has grey cast iron properties. This type of iron is used for casting process in which grey cast iron layer is provided in inner surface and white cast iron layer is provided as outer surface. Hence the casting molds serve longer. Machine parts are also made using chilled cast iron.
Composition of Different Types of Cast Iron
Different types of cast iron are composed of different quantities of constituents shown in table below:
Properties of Cast Iron as a Building Material
The properties of cast iron are as follows
- Good corrosion resistance, so it can be used for water carrying pipes etc.
- Does not get attracted to magnet.
- Specific gravity is 7.5.
- Melting point is about 1250oC.
- Tensile strength is about 150 N/mm2 and compressive strength is about 600 N/mm2.so, it is good in compression.
- It becomes soft when placed in salt water and it shrinks on cooling.
- It cannot be useful for forging work because of lack of plasticity.
Uses of Cast Iron in Building Construction
Cast iron can be used for making different tools, materials etc. as described below.
- Many types of sanitary fittings like manholes, sewer pipes, water pipes, cisterns are manufactured using cast iron.
- Metal columns and column bases can be made using cast iron.
- Casting molds for making metal staircases, lamp posts, gates etc. are made using cast iron.
- Rail chairs and carriage wheels are manufactured.
- Several types of agricultural implements can be made.
- Machinery parts can be manufactured but shock cannot be resisted by cast iron.
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