Ferrous metals are iron base metals which include all variety of pig iron, cast iron wrought iron and steels. The ferrous metals are those which have iron as their main constituents. The ferrous metals commonly used in engineering practice are cast iron, wrought iron, steel and alloy steels. The basic principal raw material for all ferrous metals is pig iron which is obtained by smelting iron ore, coke and limestone, in the blast furnace. The principal iron ores with their metallic contents are shown in Table 4.1.
Main Types of Iron
1. Pig iron
2. Cast iron
(A) White cast iron
(B) Gray cast iron
(C) Malleable cast iron
(D) Ductile cast iron
(E) Meehanite cast iron
(F) Alloy cast iron
3. Wrought iron
4. Steel
(A) Plain carbon steels
1. Dead Carbon steels
2. Low Carbon steels
3. Medium Carbon steels
4. High Carbon steels
(B) Alloy steels
1. High speed steel
2. Stainless steel
Some important ferrous metals, their extraction, composition, properties and their common applications are discussed in detail as under.
Pig Iron
Pig iron was originated in the early days by reduction or iron ores in blast furnace and when the total output of the blast furnace was sand cast into pigs which is a mass of iron roughly resembling a reclining pig. It is roughly of 20″ × 9″ × 4″ in size. It is produced in a blast furnace and is the first product in the process of converting iron ore into useful ferrous metal. The iron ore on initial refining and heating in blast furnace becomes pig iron when the impurities are burnt out in a blast furnace. Pig iron acts as the raw material for production of all kinds of cast iron and steel products. It is obtained by smelting (chemical reduction of iron ore in the blast furnace. It is of great importance in the foundry and in steel making processes. It is partly refined in a cupola furnace that produces various grades of cast iron. By puddling processes, wrought iron is produced from pig iron. Steel is produced from pig iron by various steel making processes such as bessemer, open-hearth, oxygen, electric and spray steel making. The charge in the blast furnace for manufacturing pig iron is
(a) Ore Consisting of iron oxide or carbonate associated with earth impurities.
(b) Coke A fuel
(c) Limestone A flux
In addition to iron, pig iron contains various other constituents in varying form of impurity such carbon, silicon, sulphur, manganese and phosphorus etc. It has the following approximate composition which is as given as under.
Carbon — 4 to 4.5% Phosphorus — 0.1 to 2.0%
Silicon — 0.4 to 2.0% Sulphur — 0.4 to 1.0%
Manganese — 0.2 to 1.5 % Iron — Remainder
Carbon exists in iron in free form (graphite) and/or in combined form (cementite and pearlite). Pig iron is classified on the basis of contents of free and combined carbon as follows.
These classifications are also termed as grades.
1. Grey pig iron (Grades 1, 2 and 3)
Grey pig iron contains about 3% carbon in free form (i.e., graphite form) and about 1% carbon in combined form. This is a soft type of pig iron.
2. White pig iron (Grades 4)
White pig iron is hard and strong. It contains almost all of the carbon in the combined form.
3. Mottled pig iron (Grade 5)
This type of pig iron is in between the grey and white variety. It has an average hardness and molted appearance. The free and combined forms of carbon are in almost equal proportion in mottled pig iron.
4.3.3 Cast Iron
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.
4.3.3.1 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.
4.3.3.2 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.
Properties
(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.