Question

Which of the following material is very difficult to be welded?
1. Copper
2. Aluminium
3. White cast iron
4. Silver

Answer 1

Correct Answer - Option 3 : White cast iron

Explanation:

Cast iron can be broadly divided into-

• Grey Cast Iron
• White Cast Iron
• Nodular Cast Iron and
• Malleable Cast Iron

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:

• 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 due to which it is difficult to weld.
• Its hardness ranges between 400 - 500 BHN.

Malleable cast iron:

• Due to the hardness and brittleness of grey cast iron, it is not suitable for making components subjected to shock and impact loads. For such applications malleable cast iron is used.
• White cast iron, which is converted, which is converted into malleable cast iron, has carbon in the range of 2.5% - 3%.
• Percentage of sulphur and phosphorous are kept low.

Nodular cast iron:

• It is also known as high strength cast iron or ductile cast iron.
• It is obtained by adding a small amount of alkali and alkali-earth metals to the alloy in the liquid state.
• The carbon percentage is always less than 3.3%.