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) Ductile 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:
chemical composition of grey cast iron
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.
Properties of Grye Cast Iron
- When fractured it gives grey color.
- It can be easily cast.
- 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.
- It can be easily machined and possesses machinability better than steel.
- It possesses lowest melting of ferrous alloys.
- It possesses high vibration damping capacity.
- It has high resistance to wear.
- It possesses high fluidity and hence can be cast into complex shapes and thin sections.
- It possesses high compressive strength.
- It has a low tensile strength.
- It has very low ductility and low impact strength as compared with steel.
Applications of Grye Cast Iron
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
- Machine tool structures such as bed, frames, column etc.
- Household appliances etc.
- Gas or water pipes for under ground purposes.
- Man holes covers.
- Piston rings.
- Rolling mill and general machinery parts.
- Cylinder blocks and heads for I.C. engines.
- Frames of electric motor.
- Ingot mould. And
- General machinery parts.
- Sanitary wares.
- 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.
chemical composition of white cast iron
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
Properties of white cast iron
- Its name is due to the fact that its freshly broken surface shows a bright white fracture.
- It is very hard due to carbon chemically bonded with iron as iron carbide (Fe3C), which is brittle also.
- It possesses excellent abrasive wear resistance.
- Since it is extremely hard, therefore it is very difficult to machine.
- Its solidification range is 2650-2065°F.
- Shrinkage is 1/8 inch per foot.
- The white cast iron has a high tensile strength and a low compressive strength.
Applications of white cast iron
- For producing malleable iron castings.
- For manufacturing those component or parts which require a hard, and abrasion resistant surface such as rim of car.
- 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 figure above. 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:
chemical composition of ductile cast iron
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.
Properties of Ductile cast iron
Ductile iron owes its remarkable mechanical properties to the spheroidal shape of its graphite:
- tensile strength,
- impact resistance,
- high elastic limit,
- good elongation.
These properties are further enhanced by control of the chemical analysis and heat treatment of the metal matrix.
Ductile iron maintains the traditional qualities of cast irons, resulting from the high carbon content:
- compression strength,
- abrasion resistance,
- fatigue strength.
Applications of Ductile cast iron
Ductile iron has greater strength and ductility then gray iron. Those properties allow it to be used effectively in a wide variety of industrial applications, including pipe, automotive components, wheels, gear boxes, pump housings, machine frames for the wind-power industry, agriculture, road and construction applications, paper manufacturing units, aerospace and shipping engineering, infrastructure development companies, mining and transportation industry, flanges and hydro applications and many more.
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.
Chemical composition of malleable cast iron
White heart malleable iron casting
The castings taken out of the mould are put into a drum having sand and powdered slag. The drum is then closed and kept in the air furnace and it is raised to highly temperature slowly. The temperature is raised to 920°C in two days time, kept at this temperature for nearly up to 50 to 80 hours then the drum is allowed to cool in the furnace (generally air furnaces) at the rate 5 to 10°C per hour till it reaches to room temperature. The whole cycle takes about one weak. During this treatment combined carbon separates out and all the carbon does not change into graphite state but change in other form of free carbon called tempered carbon.
Fe3C ——→ 3Fe + C
This makes the casting less brittle and malleable. The fracture portion of such a casting is dark grey or black in appearance. These castings are specially used in automobile industries.
Black heart malleable iron casting
The castings packed in a drum of oxidizing media which is generally powdered iron ore or powered scale (film of Fe3O4 on surface). This close drum is kept in the furnace and heated to 900°C. It is then maintained at this temperature to nearly 40 to 70 hours and allowed to cool slowly in a furnace itself. The castings become malleable like white heart cast iron. The percentage of carbon and silicon should be so selected that it can promote the development of free carbon when these castings are annealed.
Properties of malleable cast iron
- Malleable cast iron is like steel than cast iron.
- It is costly than grey cast iron and cheaper than softer steel.
Applications of malleable cast iron
Malleable cast iron are generally used to form automobile parts, agriculture implementation, hinges, door keys, spanners mountings of all sorts, seat wheels, cranks, levers thin, waned components of sewing machines and textiles machine parts.
Nodular Cast Iron
Nodular Cast Iron, also referred to as ductile iron or spheroidal graphite iron, is a group of irons in which the graphite forms as nodules (spherical) instead of flakes as in normal cast iron. Nodulizing elements, typically magnesium, are used to allow the solidification of the graphite into nodules. Due to different manufacturing processes a wide range of mechanical properties are observalble, care must be taken when comparing stress-strain or fatigue data.
Meehanite cast iron
Meehanite cast iron is an inoculated iron of a specially made white cast iron. The composition of this cast iron is graphitized in the ladle with calcium silicide. There are various types of meehanite cast iron namely heat resisting, wear resisting and corrosion resisting kind. These materials have high strength, toughness, ductility and good machinability. It is highly useful for making castings requiring high temperature applications.
Alloy cast iron
The cast irons as discussed above contain small percentages of other constituents like silicon, manganese, sulphur and phosphorus. These cast irons may be called as plain cast irons. The alloy cast iron is produced by adding alloying elements like nickel, chromium, molybdenum, copper and manganese in sufficient quantities in the molten metal collected in ladles from cupola furnace. These alloying elements give more strength and result in improvement of properties. The alloy cast iron has special properties like increased strength, high wear resistance, corrosion resistance or heat resistance. The alloy cast irons are extensively used for automobile parts like cylinders, pistons, piston rings, crank cases, brake drums, parts of .crushing and grinding machinery etc.