Important properties of cast irons

Important Properties of cast Iron
(1) Machinability. Cast iron has broad range of machinability, i.e., from very good to the most unmachinability. Annealed permanent mould iron has got the highest machinability as the carbon in such a cast iron is in the state of finely divided and dispersed graphite flakes and not in the combined state as carbide, and moreover it is free from burned-in sand at the surface. Ductile cast iron has also got very high machinability. Machinability of various types of cast irons in decreasing order are as follows :
Pearlite ferrite irons.
Pearlite irons.
Motteled iron with pearlitic and massive cementite white iron.
Due to the massive carbide structure white iron is particularly very difficult to machine.
(2) Mechanical properties. Cast iron tensile strength varies from 1,350 to 5,350 kg/cm2. Indefinite static load just short of the tensile strength can be sustained by grey iron, without distortion or breakage. Grey iron has low ductility and breaks with perceptible distortion. With static loading the the ultimate strength of cast iron in tension is less than that in compression, the impact strength of most cast irons is low> The damping capacity, or the ability to absorb vibrations, is high. Addition of heat treatment, chilling and alloy reduce the macinability. The white iron or chilled-iron casting is widely used for machinery parts to resist wear. High-alloy cast iron of the chromium, nickel and silicon type is specially resistant to sulphur and acid corrosion. Cast iron is most widely used in engineering and allied industries as it is available in the forms of soft, weak, hard, brittle, and strong irons and has wide range of useful properties.
(3) Weldability. The weldabiltiy of most cast irons are very low. Forge and submerged-melt welding cannot be used fro cast irons. Gas and arc welding can be employed with special rods sections have more than 6mm thickness provided casting is heated red hot before welding and then colled slowly to the room temp. Bronze welding is used mostly for grey & white irons before malleabilizing and without preheat, provided temperatures obtained are 810C to 800C.
(4) Corrosion Resistance. Cast irons with high content of chromium and silicon are quite resistant to acids. Formation of rust is very slow in cast irons in comparison to alloy steels. High nickel austenitic irons are resistant to acids, to stress corrosion in hot to alkalies if stresses are low. High silicon ( 11 to 17% Si) cast irons are remarkably good fro withstanding all acids except hot HCL.
(5) High Temperature Usefulness. Grey cast irons are useful upto 340C for pressure vessels. Lower carbon, lower silicon ad more chromium contents in cast irons increases their permissible temperatures.

Different Types of Castings

1. Chilled Casting. The mould used for chilled casting is either of a metal having high melting point or the mould may be given a lining of such a metal. This results in rapid conduction of heat from the surface of the casting with the result that the casting will have harder outer surface and softer inner core, Chilled castings are very useful in making railway carriage wheels.
2. Centrifugal Casting. The casting produced by pouring the metal in speedily rotating mould is called centrifugal casting. The crystallization of the molten metal takes place from the farthest end unlike the normal casting where the crystallization takes place from sides which are exposed to cooling. Therefore, centrifugal castings are uniform and strong. Even thin castings symmetrical about the axis of a cylinder can be obtained from this method of casting.
3. Malleable Cast Iron. It can be obtained by annealing the castings. The cast product is packed in an oxidizing material such as iron ore or in an inert material such as ground fire-clay. The pack is put into an oven and is heated to a temperature of about 870oC. It is kept at that temperature for about two days and is then allowed to cool at the rate of 5 to 10 degrees per hour. Iron ore acting as an oxidizing agent reacts with carbon and carbon dioxide escapes. The annealed cast product is free from carbon. If the cast product is packed in an inert material, slow cooling will separate out the temper carbon. Malleable cast irons are used for complicated structures.
4. Inoculated Cast Iron. The molten pig iron before casting is inoculated with soluble silicon compounds like calcium silicide. The silicon added in this way has got a better effect. Pearlitic cast iron having microstructure of small flakes of graphite set in pearlite is formed in this way.

Impurities in Pig Iron and Their Effects

1. Phosphorous. The quantity of phosphorous present in pig iron varies from 0.1 to 2.0%. It combines with iron to form Fe3P which embrittles cast iron. Phosphorous is introduced into iron from phosphate in the ore. Phosphates get reduced in the reducing atmosphere in the blast furnace and phosphorous is formed. This phosphorous combines with iron to form Fe3P. Some of the phosphorous may get oxidized too into P2O5. This oxidation of phosphorous to P2O5 is an exothermic reaction and takes place at lower temperature
, if formed, is removed by the slag which is basic in character. In this way the quantity of phosphorous in iron is reduced. Its amount is kept as low as possible.
Phosphorous when present in pig iron increases the fluidity of molten iron and thus makes the filling of the moulds in a better way. However, wrought iron made from phosphatic iron is brittle when cold.
2. Sulphur. The quantity of sulphur present in ores varies from 0.4% to 1.0%. Presense of manganese in iron lowers the percentage of sulphur by combining with sulphur in iron and forming MnS. MnS formed thus combines with CaO and is removed. In this way the percentage of sulphur in iron is reduced. The reaction of sulphur with manganese is as follows:
FeS + Mn → Fe + MnS
MnS + CaO → MnO + CaS (Slag)
MnO + C → Mn + CO
It may be noted that FeS has a low melting point and it forms between the grains that make up the alloy. Iron sulphide being very brittle, whole alloy becomes brittle.
Presense of sulphur tends to make iron hard and produces unsound castings. Wrought iron and steel produced from iron containing sulpher makes wrought iron and steel to be brittle when heated.
3. Silicon. Percentage of silicon present in pig iron varies from 1.0 to 4.0%. Source of silicon in iron is from its presence in any of the raw materials. At low temperatures some silicon is oxidized into silica and basic slag removes it. In this way there may occur some elimination of silicon from iron.
Silicon affects the hardness and strength of iron. Both these properties worsen by presence of silicon in iron. Presence of silicon in steel increases its electrical resistance. Silicon when present in steel promotes the decomposition of cementite to form graphite.
4. Manganese. The quantity of manganese varies from 0.2 to 1.5%. The source of this impurity in iron is from the ore. Manganese dioxide in ore is reduced at higher temperature as the reduction reaction is endothermic in nature. Manganese thus formed gets mixed up with iron. Some of the manganese is removed by acid slag.
Presence of manganese in iron reduces the sulphur content by forming MnS thereby improving the quality of alloy since MnS is not harmgul as harmful as FeS. Manganese increases the tensile strength of iron. Since Mn promotes combined carbon, it increases hardness of cast iron.
5. Carbon. The quantity of carbon in pig iron varies from 4 to 4.5%. Source of carbon in steel is coal. Carbon is present in pig iron either in free state as graphite or in combined state as iron carbide. Carbon in pig iron increases its hardness. Due to presence of impurities in pig iron, pig iron is too brittle and possesses very little strength and ductility. So most of pig iron is converted into steel.
Pig iron is the direct metallic product from the blast furnace without any change in it while cast iron is prepared from pig iron with or without refining or alloying treatments, or it may be prepared from mixtures of pig iron with steel or scrap or alloying agents.

Classification of Iron and Steel

Most important commercial form of iron are :

• Pig iron. It is the product of  the blast furnace and is made by the reduction of iron ore.
• Cast iron. It is an alloy of iron containing so much carbon that , as cast, it is not appreciably malleable at any temperature.
• Malleable cast iron. It is made by changing all the combined carbon in a special white cast iron to free or  temper carbon by suitable heat-treatment.
• White cast iron. It contains carbon in the combined form which makes the metal hard  and bittle, and the absence of graphite  gives the fracture  a white color.
• Grey cast iron. This one as cast, has combined  or cementitic  carbon not in a excess of a eutectoid percentage, the balance of the carbon occurring as graphite flakes.
• Ingot iron. It is an open-hearth iron very low in carbon, manganese and other impurities.
• Wrought Iron. It is ferrous material aggregated from a solidifying mass of pasty particles of highly refined metallic iron with which is incorporated, without subsequent, fusion, a minutely and uniformly distributed quantity of slag.
• Puddled iron.  It is wrought iron made by the pudding process.
• Steel. It is a malleable alloy of iron and carbon, usually containing substantial quantities of manganese.
• Carbon steel. It owes its distinctive properties chiefly to the carbon that it contains.
• Alloy steel. It owes its distinctive properties chiefly to some element or elements other than carbon, or jointly to such other elements and carbon.
• Bessemer steel. open hearth steel, cruicible steel, and electric-firnace steel. These are names given according to the process from which steel is made, irrespective of carbon content.
• Electrolytic iron. It is produced in the form of thin-wall large-diameter tubes by employing large revolving mandrels as cathodes and ferrous-chloride as electrolyte. It is extremely brittle and can therefore be readily pulverised to a fine powder.  

Allotropic Forms of Iron

Iron has three allotropic forms of crystal at different temperature. It undergoes all the allotropic forms when it is heated  from normal temperature to high temperature (molten state).
1. Alpha Iron. It occurs from normal temperature to 910 degree centigrade and has got body-centered cubic (b.c.c.)lattice crystals.
(i) Ferromagnetic alpha iron which occurs from normal temperature to 770 degree centigrade.
(ii) Paramagnetic  alpha iron which occurs from 770 degree centigrade to 910 degree centigrade.
2. Gamma Iron. This occurs from 910 degree centigrade to 1400 degree centigrade, and has got crystal structure of face-centered cubic (f.c.c.) lattice.
3. Delta Iron. This occurs from 1400 degree centigrade to 1539 degree centigrade (molten state), and has got crystal structure of body-centered lattice.
 Pure Iron is soft and has got silvery white color.It is strongly magnetic in presence of a magnetic field or electric current. When inducing field is removed the induced magnetism is not retained by pure iron. This power of retentivity of magnetism of pure iron is improved by the addition of other elements such as carbon, cobalt, or nickel. Iron loses its magnetic properties when heated to 770 degree centigrade.