Here is a list of commonly used cast iron: 1. Grey Cast Iron 2. White Cast Iron 3. Mottled Cast Iron 4. Nodular Cast Iron 5. Malleable Cast Iron 6. Alloy Cast Iron.
1. Grey Cast Iron:
Composition:
It is basically an alloy of carbon and silicon with iron. It contains C = 2.5- 3.8%; Si = 1.1-2.8%; Mn = 0.4-1%; P = 0.15% and S = 0.10%.
Properties:
ADVERTISEMENTS:
(i) It is characterised by presence of a large portion of its carbon in the form of graphite flakes. Graphite flakes occupy about 10% of the metal volume.
(ii) When fractured, a bar of grey cast iron gives grey appearance.
(iii) It is readily cast into a desired shape in a sand mould.
(iv) It possesses lowest melting point of the ferrous alloys.
ADVERTISEMENTS:
(v) It possesses machinability better than steel.
(vi) It possesses high fluidity and hence it can be cast into complex shapes and thin sections.
(vii) It has high resistance to wear.
(viii) It possesses high vibration damping capacity.
ADVERTISEMENTS:
(ix) It has low ductility and low impact strength as compared with steel.
(x) It associates low cost combined with hardness and rigidity.
(xi) It possesses high compressive strength.
(xii) It possesses excellent casting qualities for producing simple and complex shapes.
ADVERTISEMENTS:
Uses:
(i) Manhole covers.
(ii) Machine tool structures, e.g., bed, frame and details.
(iii) Frames for electric motors.
ADVERTISEMENTS:
(iv) Rolling mill and general machinery parts.
(v) Cylinder blocks and heads for I.C. engines.
(vi) Gas or water pipes for underground purposes.
(vii) Ingot moulds.
(viii) Sanitary works.
(ix) Household appliances.
(x) Tunnel segment etc.
2. White Cast Iron:
Composition:
Iron = 94%; Graphite carbon = 0.5%; Combined carbon = 3.5% and the remainder other impurities.
Properties:
(i) Hard, brittle and cannot be machined.
(ii) So called because the carbon is mostly in combined iron.
(iii) Highly resistant to wear.
(iv) Tensile strength is good.
(v) Obtained by rapid cooling of metal.
(vi) Due to its poor fluidity it does not fill the mould freely.
(vii) White iron castings can be made in sand moulds.
(viii) White iron can also be made on the surface of a grey iron casting provided the material is of special composition.
(ix) White iron of a particular composition is the first step in the production of malleable iron castings.
Uses:
(i) Used for parts subjected to excessive wear, e.g., in rim of freight car wheel or railway brake blocks.
(ii) Used for inferior castings and does not rust so much as grey variety.
(iii) Used for making malleable castings also.
3. Mottled Cast Iron:
Composition:
Iron = 93.5%, Graphite = 1.75%, Combined carbon = 1.75%; the remainder impurities.
Properties:
(i) Mixture of the two states, grey cast iron and white cast iron.
(ii) Strength and hardness varies according to the ratio of free carbon to combined carbon.
(iii) Less tendency to rust than grey variety.
(iv) Fluidity is good.
(v) Hard and brittle.
Uses:
(i) Used for manhole covers and pipes.
(ii) Also employed for making fire plugs and lamp posts.
4. Nodular Cast Iron:
Composition:
Nodular or ductile cast iron contains- C = 3.2- 4.2%; Si = 1.1 – 3.5%; Mn = 0.3-0.8%; P = 0.08% and S = 0.2%. It is produced by ladle addition of nickel magnesium alloy. It is cheaper to make and behaves like steel.
Properties:
(i) In nodular cast iron graphite appears as rounded particles, or nodules or spheroids (unlike long flakes as in grey cast iron).
(ii) It possesses very good machinability.
(iii) Soft annealed grades of nodular cast iron can be turned at very high feeds and speeds.
(iv) It possesses damping capacity intermediate between cast iron and steel.
(v) It possesses excellent castability and wear resistance.
(vi) The properties of nodular cat iron depend upon metal composition and cooling rate.
Uses:
(I) Steel mill rolls and mill equipment.
(ii) Power transmission equipment.
(iii) Valves and fittings.
(iv) Internal combustion engines,
(v) Pumps and compressors.
(vi) Paper industries and machinery.
(vii) Construction machinery.
(viii) Farm implements and tractors.
5. Malleable Cast Iron:
It is obtained by giving a long time annealing to cast iron. To get high duty cast irons, steel scrap 10 to 40% may be added to lower the total carbon contents.
Composition:
Malleable cast iron contains- C = 2-3%, Si = 0.6 – 1.3%, Mn = 0.2-0.6%, P = 0.15% and S = 0.10%.
The procedure of obtaining malleable cast iron is as follows:
The white cast iron casting along with silica is packed in a steel pot in heated ruffle oven or continuous type thermal kiln. The temperature is maintained at 870°C for 60 hours and casting is cooled slowly in the furnace, iron carbide dissociates as Fe3C → Fe + C and thus reduces to malleable cast iron.
Malleable cast iron is of two types:
(i) Black heart,
(ii) White heart.
White heart malleable cast iron can be obtained by keeping temperature and time comparatively of high values.
Properties:
(i) It possesses high yield strength.
(ii) It can be hammered and rolled to different shapes.
(iii) It has high Young’s modulus and low coefficient of thermal expansion.
(iv) It possesses good wear resistance and vibration damping capacity.
(v) It has shrinkage of 1.5 mm/100 mm.
(vi) It has low to moderate cost.
(vii) It is soft, tough and easily machined.
Uses:
(i) Differential and steering gear housing.
(ii) Brake pedals.
(iii) Tractor springs.
(iv) Hangers.
(v) Washing machine parts.
(vi) Agriculture implements.
(vii) Universal joint yoke.
(viii) Automotive crankshafts.
(ix) Rail road, etc.
6. Alloy Cast Iron:
Because 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 spots.
Two typical examples of alloy cast iron are:
(i) Acicular,
(ii) Spheroidal.
(i) Acicular cast iron has nickel and molybdenum as the leading constituents and is employed for crankshafts.
(ii) Spheroidal cast iron has 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 the tensile strength and produces a tough metal which can undergo bending and twisting.
Wrought Iron:
A. Wrought iron is virtually pure iron, containing a large number of minute threads of slag lying parallel to each other, thereby giving the metal a fibrous appearance when broken.
B. It contains practically no carbon and therefore, does not harden when quenched in water (this property distinguishes it from steels).
C. Chemical Composition:
A representative analysis range of wrought iron would be: C = 0.02-0.03%, Si = 002-0.10%. S = 0.008-0.02%, Mn = Nil-0.02%, P = 0.05-0.25%, slag = 0.15 – 1.50%, Iron = balance.