Question

Which of the following metals is the purest form of iron?
1. Cast iron
2. Wrought iron
3. Grey cast iron
4. White cast iron

Answer 1

Correct Answer - Option 2 : Wrought iron

Explanation:

Wrought iron

• It is a very pure iron where the iron content is of the order of 99.5%. 
• It is produced by re-melting pig iron and some small amount of silicon, sulphur, or phosphorus may be present.
• It is tough, malleable and ductile and can easily be forged or welded.
• It cannot however take sudden shock.
• Chains, crane hooks, railway couplings and such other components may be made of this iron.

Cast iron 

• It is an alloy of iron, carbon and silicon and it is hard and brittle.
• Carbon content in CI may be within 1.7% to 3% and carbon may be present as free carbon or iron carbide Fe3C.
• In general, the types of cast iron are (a) grey cast iron and (b) white cast iron (c) malleable cast iron, etc.

Grey Cast Iron:

• The presence of graphite gives a grey shade to the fractured surface of cast iron. Therefore, when all the carbon in cast irons is in the form of graphite, it is called Grey cast Iron.
• Grey cast iron with a carbon content of 2.4% - 3.8%.
• In general, a higher percentage of graphite reduces the mechanical properties of cast iron.
• A lower percentage of graphite makes the cast iron difficult to cast.
• Other elements present in grey cast iron are silicon, manganese, sulphur and phosphorous which impart machinability, weldability, and corrosion resistance.

White Cast Iron:

• The presence of cementite gives a shiny light coloured surface to fractured cast iron. Therefore, cast irons which have all the carbon in the form of cementite are called White cast iron.
• It is obtained by the rapid cooling of molten alloy which is also known as chilled cast iron.
• It is extremely hard and brittle.
• Its hardness ranges between 400 - 500 BHN.