Cast iron and its classification | The Mechanical post

Cast iron may be defined as "an alloy of iron and carbon containing more than 2% carbon".

The minimum carbon content in a cast iron is about 2% while the practical maximum carbon content is about 4.3%. In addition to carbon, cast iron contains other elements such as silicon, sulphur, phosphorous and manganese. Cast iron is a pig iron remelted and thereby refined, together with definite amount of limestone, steel scrap and damaged castings ( foundry returns ), in a cupola or other form of remelting furnace, and poured into suitable moulds of required shape.

Classification of cast irons

Following are the types of cast iron:
  1. White cast iron.
  2. Malleable cast iron.
  3. Gray cast iron.
  4. Nodular cast iron.
  5. Mottled cast iron.
  6. Chilled cast iron.
  7. Alloy cast iron.
  8. Mechanite cast iron.

White cast iron.

White cast iron contains carbon in the form of cementite. This is obtained by rapid cooling. Due to absence of graphite, metal has white colour. It contains 1.75% to 2.3% carbon. The appearance of fractured surface is white.

Properties: 

  1. White cast iron is very hard and brittle.
  2. It is wear resistant and less ductile.
  3. Hardness varies from 400 to 600 B.H.N.
  4. It cannot be machined.
  5. Finishing to final size is done by grinding.

Applications:

  1. Used for manufacturing wrought iron.
  2. For manufacturing those component parts which require a hard and abrasion resistant material.
  3. For producing malleable iron casting.
  4. For weaving plates, road roller surface, industry flooring, crushers, shot blasting nozzles.
  5. For pump lines, mill lines, ball for grinding ball mill.
  6. For dies and extrusion nozzles.
  7. Railway brake blocks.

Gray cast iron.

This type of cast iron is obtained by allowing the molten metal to cool and solidify slowly. Grey cast iron contains 2.5 to 3.75% of carbon. The greater part of carbon is in the form os flakes. The microstructure of grey cast iron depends upon rate of cooling.

 Properties: 

  1. Grey cast iron is brittle and may be broken if a heavy hammer is used.
  2. The ultimate tensile strength varies from 120N/mm^2 to 300N/mm^2.
  3. Hardness of grey cast iron varies from 150 to 240 B.H.N.
  4. Melting point is low.
  5. Good compressive strength.
  6. It is easily machinable.
  7. The graphite present, acts as a lubricant.

Applications:

  1. Used in machine tool structure.
  2. Used in gas or water pipes for underground purposes.
  3. Used in frames for electric motors.
  4. Used in ingot moulds.
  5. Used in piston rings.
  6. Used in sanitary wares.
  7. Used in household appliances etc.

Nodular cast iron.

It is also called spheroidal cast iron because the graphite is precipitated not in the form of flakes but in the form of spheroids. This is a high grade cast iron. It is also called as ductile cast iron.

Properties:

  1. It has very good casting properties as high fluidity and low melting point.
  2. It produces better machined surfaces.
  3. The tensile strength is 330N/mm^2.
  4. High strength and toughness.
  5. Good resistant to wears.
  6. It can be welded nicely.

Applications:

  1. Used in paper industries machinery.
  2. Used in internal combustion engines.
  3. Used in power transmission equipment.
  4. Used in farm implements and tractors.
  5. Used in earth moving machinery.
  6. Used in valves and fitting.
  7. Used in pumps and compressors.
  8. Used in construction machinery.

Malleable cast iron.

Malleable cast iron is obtained by with cast iron by special annealing process. This type of cast iron contains 2 to 3.6% carbon. The microstructure of malleable cast iron contains graphite in pearlitic matrix. The graphite is present in the form of small rounded nodules.


Properties:

  1. It is less brittle.
  2. Tensile strength of malleable cast iron is 180N/mm^2.
  3. Good wear resistant and vibration damping capacity.
  4. Hardness varies from 80 to 275 B.H.N.
  5. These cast irons show some ductility and toughness.

Application:

  1. Used in automobile parts.
  2. Used in sewing machine parts.
  3. Used in bicycle parts.
  4. Used in agricultural equipments.
  5. Used in electrical line hardware parts.
  6. Used in crank shaft.
  7. Used in power train parts.

Also read:

Alloy cast iron.

Because of the cast iron is supposed to be very hard, brittle, lacking in tensile and transverse strength and weak to withstand shocks it is alloyed with other metals to improve its properties. Amongst the alloying metals, nickel is predominating alloying constituent whose addition to the extent of 0.5 to 1.5% avoids the tendency of chilling or hard sports. Two typical e.g. of alloy cast iron are: 
  1. Acicular.
  2. Spheroidal.
Acicular cast iron has nickel and molybdenum as the leading constituents and is employed for crankshaft. Spheroidal cast iron has a graphite content in spheroidal form, which is converted from flaky form by alloying with a small amount of magnesium and cesium. This change in graphite increases and the tensile strength and produces a tough metal which can undergo bending and twisting.

Mottled cast iron.

Composition: 

Iron = 93.5%, Graphite = 1.75%, Combined carbon = 1.75%, remaining are impurities.

Properties:

  1. Mixture of the two states, grey cast iron and white cast iron.
  2. Strength and hardness varies according to the ratio of free carbon to combined carbon.
  3. Less tendency to rust than grey variety.
  4. Fluidity is good.
  5. Hard and brittle.

Application:

Used for manhole covers and pipes.

Mechanite cast iron.

In this type of cast iron, the metal is treated with calcium sillicade. The trade name is 'MECHANITE'. Calcium sillicade acts as a graphister and produces as fine graphite structure, giving a cast iron of excellent mechanical properties. These properties are also due to careful control of melting the iron and also the moulding of the casting.
There are more than 26 types of machanite metals available at present. The board classification of these is:
  1. General engineering.
  2. Heat resisting.
  3. Wear resisting.
  4. Corrosion resisting.
  5. Nodular 'S' type.
All machanite irons have high strength, toughness, ductility and easy machinability. These irons are sort of a bridge between ordinary cast and steel. The casting weigh from 500 to 6000 kg. The metal is close grained.
Machanite irons responds well to heat treatments. It can be hardened wholly or on the surface. It can be toughened by suitable treatment. The metal is very well suitable for machine tool casting.

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