Cast Iron: a topic asked almost every year in ESE

Cast Iron = Iron +  Carbon (2.1-4.5%)  +  Silicon (up to 3%)

Silicon is added to:

  • Promote graphite formation (graphitization)
  • Improve fluidity of molten metal

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Types of Cast-Iron :

It is classified based upon the silicon percentage, rate of cooling and the microstructure of graphite.

1. Gray Cast-Iron: Silicon (1-3%) + slow cooling rate. It is weak in tension, but strong in compression. Why? In gray cast-iron, graphite exist as flakes (similar to corn flakes), surrounded by α-ferrite. The tips of these graphite flakes are sharp and pointed and serve as point of stress concentration when subjected to tensile stress. So, it is strong in compression.

Applications:-

  • M/C tool bases
  • Machinery beds like in lathe , shaper etc.


2. White Cast-Iron:
Silicon (<1%), rapid cooling rate. Very hard due to rapid cooling. It is a brittle material.

Application:- 

  • Rollers in rolling-mill


3. Ductile (Nodular) Cast-Iron:
Graphite is in nodular or spheroid form. The shape of graphite flakes is changed into nodules (sphere) by small addition of “MAGNESIUM” to the molten metal. This shape permits the material to be ductile and shock resistant.

Applications:-

  • Valves and pump bodies
  • Crankshafts and gears


4. Malleable Cast-Iron:
Obtained by annealing white cast-iron in neutral atmosphere (to control oxidation) between 800-900oC

Graphite exists in the form of “ROSETTES”.

Relatively high strength then white cast-iron.

Applications:-

  • Railroad , marine and heavy duty services
  • Pipe fittings , flanges etc


5. Compacted Cast-Iron(Recent Discovery):
 Silicon(1.7-3%) and Carbon(3.1-4%)

Graphite in short, thick and inter-connected flakes

High thermal conductivity

Applications:-

  • High speed train , flywheels etc

    


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