Cast iron is basically an alloy of iron and carbon and is obtained by re-melting pig iron with coke, limestone and steel scrap in a furnace known as cupola. The carbon content in cast iron varies from 1.7% to 6.67%. It also contains small amounts of silicon, manganese, phosphorus and sulphur in form of impurities elements.
General properties of cast iron Cast iron is very brittle and weak in tension and therefore it cannot be used for making bolts and machine parts which are liable to tension. Since the cast iron is a brittle material and therefore, it cannot be used in those parts of machines which are subjected to shocks. It has low cost, good casting characteristics, high compressive strength, high wear resistance and excellent machinability. These properties make it a valuable material for engineering purposes. Its tensile strength varies from 100 to 200 MPa, compressive strength from 400 to 1000 MPa and shear strength is 120 MPa. The compressive strength of cast iron is much greater than the tensile strength. The carbon in cast iron is present either of the following two forms:
1. Free carbon or graphite.
2. Combined carbon or cementite.
The cast iron is classified into seven major kinds as follows:
(a) Grey cast iron,
(b) White cast iron,
(c) Mottled cast iron
(d) Malleable cast iron,
(e) Nodular cast iron,
(f) Meehanite cast iron.
(g) Alloy cast iron and The chemical composition, extraction, properties and general applications of these types of cast iron are discussed as under.
Grey cast iron Grey cast iron is grey in color which is due to the carbon being principally in the form of graphite (C in free form in iron). It contains:
C = 2.5 to 3.8%.
Si = 1.1 to 2.8 %
Mn = 0.4 to 1.0%
P = less than 0.15%
S = less than 0.1%
Fe = Remaining
It is produced in cupola furnace by refining or pig iron.
(i) When fractured it gives grey color.
(ii) It can be easily cast.
(iii) It is marked by presence of flakes of graphite in a matrix of ferrite and pearlite or austenite; graphite flakes occupy 10% of metal volume.
(iv) It can be easily machined and possesses machinability better than steel.
(v) It possesses lowest melting of ferrous alloys.
(vi) It possesses high vibration damping capacity.
(vii) It has high resistance to wear.
(viii) It possesses high fluidity and hence can be cast into complex shapes and thin sections.
(ix) It possesses high compressive strength.
(x) It has a low tensile strength.
(xi) It has very low ductility and low impact strength as compared with steel.
Applications The grey iron castings are mainly used for machine tool bodies, automotive cylinder blocks, pipes and pipe fittings and agricultural implements.
The other applications involved are
(i) Machine tool structures such as bed, frames, column etc.
(ii) Household appliances etc.
(iii) Gas or water pipes for under ground purposes.
(iv) Man holes covers.
(v) Piston rings.
(vi) Rolling mill and general machinery parts.
(vii) Cylinder blocks and heads for I.C. engines.
(viii) Frames of electric motor.
(ix) Ingot mould. And
(x) General machinery parts.
(xi) Sanitary wares.
(xii) Tunnel segment.
White cast iron The white color is due to the fact that the carbon is this iron is in combined form as iron carbide which is commonly specified as cementite. It is the hardest constituent of iron. It is produced in cupola furnace by refining or pig iron. The white cast iron may be produced by casting against metal chills or by regulating analysis. The chills are used when a hard and wear resistance surface is desired for products such as for wheels, rolls crushing jaw, crusher plates.
The chemical composition of white cast iron is given as under.
C = 3.2 to 3.6%
Si = 0.4 to 1.1 %
Mg = 0.1 to 0.4%
P = less than 0.3%
S = less than 0.2%
Fe = Remaining
(i) Its name is due to the fact that its freshly broken surface shows a bright white fracture.
(ii) It is very hard due to carbon chemically bonded with iron as iron carbide (Fe3C), which is brittle also.
(iii) It possesses excellent abrasive wear resistance.
(iv) Since it is extremely hard, therefore it is very difficult to machine.
(v) Its solidification range is 2650-2065°F.
(vi) Shrinkage is 1/8 inch per foot.
(vii) The white cast iron has a high tensile strength and a low compressive strength.
(i) For producing malleable iron castings.
(ii) For manufacturing those component or parts which require a hard, and abrasion resistant surface such as rim of car.
(iii) Railway brake blocks.
Ductile cast iron
When small quantities of magnesium or cerium is added to cast iron, then graphite content is converted into nodular or spheroidal form and it is well dispersed throughout the material. The resulting structure possesses properties more like cast steel than like the other grades of cast iron. A typical structure of spheroidal cast iron is shown in Fig. 2. Graphite is in spheroidal form instead of in flaky form. Its structure may be modified by alloys or heat treatment, as in steel to produce austenite, acicular, martensite, pearlite, and ferrite structure. Compositions of ductile cast iron are as follows:
Carbon = 3.2 to 4.2%
Silicon = 1.0 to 4.0 %
Magnesium = 0.1 to 0.8%
Nickel = 0.0 to 3.5%
Manganese = 0.5 to 0.1%
Iron = Remaining
Silicon is also used as an alloying element since it has no effect on size and distribution of carbon content. The magnesium controls the formation of graphite. But it has little influence on the matrix structure. Nickel and manganese impart strength and ductility. Ductile cast iron has high fluidity, excellent castability, strength, high toughness, excellent wear resistance, pressure tightness, weldability and higher machinability in comparison to grey cast iron.
Malleable cast iron
The ordinary cast iron is very hard and brittle. Malleable cast iron is unsuitable for articles which are thin, light and subjected to shock. It can be flattened under pressure by forging and rolling. It is an alloy in which all combined carbon changed to free form by suitable heat treatment. Graphite originally present in iron in the form of flakes which is the source of weakness and brittleness. Carbon in this cast iron is dispersed as tiny specks instead of being flaky or in combined form. The tiny specks have not such weakening effect and casting would not break when dropped. The tensile strength of this cast iron is usually higher than that of grey cast iron. It has excellent machining quality and is used for making machine parts for which the steel forging and in which the metal should have a fair degree of machining accuracy e.g., hubs of wagon, heels small fittings for railway rolling brake supports, parts of agricultural machinery, pipe fittings, hinges, locks etc.
It can be obtained by annealing the castings. The cast iron castings are packed in an oxidizing material such as iron ore or in an inert material such as ground fire clay depends upon the process used either white heart or black heart. The packed casting is put into an oven and is heated around 900°C temperature and is kept at that temperature for about two days and it is then allowed to cool slowly in the furnace itself. Iron ore acting as an oxidizing agent reacts with C and CO2 escape. Thus annealed cast product is free from carbon. If the castings are packed in an inert material then slow cooling will separate out the combined carbon to temper carbon. To produce malleable casting, first casting is produced which has all combined carbon. The produced castings are then heat-treated in a special manner according to white heart method or black heart method.