CAST IRON :

Cast iron is pig iron remelted and thereby refined in a Cupols or other form of remelting furnace and poured into suitable moulds of required shape, it contains about 2 to 4% of carbon, a small percentage of silicon, sulphur, phosphorus and manganese and certain amount of alloying elements e.g. nickel, chromium molybdenum, copper and vanadium.

Carbon in cast iron usually exists in two forms associated together (1) as the compound cementite i.e. in a state of chemical combination and (2) as free carbon i.e. in a state of mechanical mixture. Carbon in the first form is called "Combined Carbon" and in the latter form "graphite" or "graphite Carbon". The total amount of both types in the specimen of iron is called "total Carbon".

The quality of cast iron thus depends not upon the absolute amount ft contain but upon the conditions in which that carbon exists. The varieties of cast iron in common use are :

(1) Grey Cast iron, (2) White Cast iron, (3) Malleable Cast iron, (4) Chilled Cast iron, (5) Alloy Cast iron, (6) Meehanite Cast iron.

1. GREY CAST IRON - This is obtained by allowing the molten metal to cool and
solidify slowly. On solidifying, the iron contains the greater part of carbon in the
form of graphite flakes, and small quantities of the other elements, e.g. silicon,
phosphorus, sulphur and manganese.

The Cast iron is brittle and may easily be broken if a heavy hammer is used, it presents a dull grey crystalline or granular fracture and a strong light will give a glistering effect due to reflection on the free graphite flakes. The ultimate tensile strength of cast iron varies between 120 to 300 N/mm² and compressive strength about 600-750 N/mm² before fracturing, whilst in shear its strength is 150-225 N/mm². Hardness ranges from 150-240 Bhn.

The main advantages in favour of its use are (1) its cheapness, (2) its low melting temperature (11 50°C to 1 200°C) and fluidity when in molten condition, and (3) it is easily machined. A further good property of Cast iron is that of free graphite in its structure seems to act as a lubricant and when large machine slides are made ' of it a very free-working action is obtained. The fluidity enables it for making castings of parts of intricate shapes.

WHITE CAST IRON - White Cast iron contains carbon exclusively in the form of cementite (iron carbide). This is obtained by the presence of relatively large quantities of manganese, a very small amount of silicon and by rapid cooling which, helps to produce cementite.

White Cast iron is very hard (400 -600 Bhn) and brittle and its fractured surface has a silvery metallic appearance. White Cast iron has limited application due to its unmachinability and relatively poor mechanical properties. It is widely used in manufacturing of wrought iron and as a intermediate material for making malleabk cast iron.

White Cast iron may be distinguished from grey Cast iron by dropping nitric acid on the fractured surface and it does not rust so much as the grey Cast iron.

3. MALLEABLE CAST IRON - Malleable Cast irons are made form an iron having all of its carbon in the combined form i.e. from white cast iron. Two methods are used for malleabilizing the castings : (1) White heart and (2) Black heart. The names refer to the colour of the fracture given by castings produced by each method is a highly desirable quality. They include hydraulic cylinders, valves, pipes and pipe fittings, cylinder head for compressors and diesel engines. Rolls for rolling mills and many types of centrifugally cast parts are also made of spheroidal Cast iron.

4. CHILLED CAST IRON - Quick cooling is called 'chilling' and the iron so produced is chilled iron. All castings are chilled at their outer surface by coming in contact with the cool sand in the mould. Since cast iron has a higher conductivity than sand, the chilled portion of the casting undergo rapid solidification and cooling and thereby produce a hard surface. But this hardness only peretrates about 1 to 2 mm in depth.

Chills are used on those castings where some parts are required to have the hardness of white cast iron, while other are required to have relatively soft and tough core of grey cast iron.

5. ALLOY CAST IRON - Alloying elements such as nickel, chromium, molybdenum, vanadium, copper, titanium, silicon etc. are used to improve mechanical properties, by refining grain structure and stabilising cementite.

Nickel- Nickel tends to promote graphitisation, also it has a grain-refining effect, so that whilst nickel will help to prevent chilling in this sections, it will also prevent coarse grain in thick sections. Nickel also reduces the tendency of thick sections to crack. The use of 1 to 2% of nickel ensures machinability and uniformity of structure of cast iron. Use of nickel of the order of 25%, enhanced life can also be obtained in parts subjected to abrasive wear. Cylinders or cylinder liners of all sizes from the smallest to the largest afford outstanding examples. Nickel cast iron is also used in withstanding caustic corrosion. For this reason, it is widely employed for making caustic pots, pipes and other castings. The most useful reason for using nickel in cast iron is to obtain density and pressure tightness in castings with large and varying sections.This has led to the application of nickel cast iron for many parts of steam and hydraulic machinery, compressors and internal combustion engines.

Chromium- Chromium acts as a carbide stabiliser in cast iron and so inhibits the formation of graphite. Moreover, the carbides formed by chromium are more stable and less likely to graphitise under the application of heat than is iron carbide. Thus it intensifies chilling of Cast iron, increases strength, hardness and wear-resistance and is conducive to fine-grain structure. Chromium addition is restricted to 0.15 to 0.90%,1 % or more makes castings hard to machine. With 3% chromium, white Cast iron is formed. Alloy Cast iron containing 10 to 30% chromium and 1 to 3% total carbon also exhibit a high degree of heat resistance combined with strength at high temperature. Disadvantages of either nickel or chromium separately are overcome by using them in conjunction in the ratio of two to three parts of nickel to one part of chromium. Application for pumps of all types in which frictional, as well as erosive wear has to be considered.

Molybdenum- When added in small amounts, dissolves in ferrite, but in large amounts forms double carbides. Molybdenum ranges from 0.25 to 1.5%, increases hardness of thick sections and promotes uniformity of microstructure, increases tensile strength and shock resistance of castings. Also improves toughness, fatigue strength, machinability, hardenability and high temperature strength.

Vandium- Addition from 0.10 to 0.50% promotes heat resistance in Cast iron, in so far as the stable carbides which it forms do not break down on heating. Strength, hardness and machinability are increased, particularly when vanadium is used in conjunction with other alloying elements.

Copper- Copper is only sparingly soluble in iron, and has a very slight graphitising effect. It has little influence on the mechanical properties, and its main value is in improving the resistance to atmospheric corrosion.

Silicon- Silicon upto 2.5% promotes the formation of free graphite, which makes the iron soft and easily machinable, Silicon dissolves in the ferrite of a Cast iron and tends to make cementite unstable, so that it decompose, produce graphite, a grey iron. The higher the silicon content, the greater the degree of decomposition of the cementite, and the coarser, the flakes of graphite produced. Thus silicon actually strengthens the ferrite by dissolving in it, at the same time it produces softness by causing the cementite to breakdown to graphite. However, when silicon is present in amounts in excess of that necessary to complete the decomposition of all the cementite, it will again cause hardness and brittleness to increase.

Sulphur- Sulphur usually regarded as harmful in cast iron. It tends to stabilise cementite, inhibits graphitisations and lowers the viscosity, so helps to produce a hard, brittle white iron. Moreover, its presence as the sulphide, FeS, in cast iron will also increase the tendency to brittleness. So it should be kept well below 0.1 % for most foundry purposes.

Manganese - The harmful effect of sulphur is governed, in turn, by the amount of manganese present. Manganese combined with sulphur to form manganese sulphide, MnS, which unlike ferrous sulphide is insoluble in the molten iron and floats to the top to join the slag. The indirect effect of manganese, therefore, is to promote graphitisation because of the reduction of the sulphur content which it causes. Manganese has a stabilising effect on carbides, however, so that this offsets the effect of sulphur reduction in promoting graphitisation. The more direct effect of manganese, include the hardening of the iron, the refinement of grain and an increase in strength. It is often kept below 0.75%.

Phosphorus - It is present in cast iron as the phosphide, Fe₃P, which forms a eutectie with the ferrite in grey irons and with ferrite and cementite in white irons. These eutectics melt at about 950°C, so that high phophorus irons have great fluidity. Cast irons containing 1 % phosphorus are, therefore, very suitable for the production of castings of thin section. Phosphorus has a negligible effect on the stability of cementite, but its direct effect is to promote hardness and brittleness due to the large volume of phosphide eutectie which a comparatively small amount of phosphorus will produce. Phosphorus is rarely allowed to exceed 1 %, in castings where shock-resistance is important.

In wrought iron, the presence of only a very small amount is injurious, only 0.1 % is sufficient to make the iron cold-short, that is, the metal is brittle and liable to crack when cold, but it may be malleable and easily worked at red heat.

6. MEEHANITE CAST IRON- Cast irons in which metal has been treated with calcium silicide are known by the trade name of meehanite. Calcium silicide acts as a graphitiser and produces a fine graphite structure giving a Cast iron of excellent mechanical properties. The high quality of meehanite is not solely due to calcium silicide but careful control in the melting in cupola or electric furnace and in the moulding of the casting. Very little calcium silicide remains in the iron after solidifications.

The metal used is low in silicon, moderately low in carbon, about 2.5 to 3%, and there are more than twenty six types of meehanite metal available, under the five broad classification : a) general engineering, b) heat resisting, c) wear resisting, d) corrosion resisting and e) nodular 'S' type.

All meehanite irons have high strength, toughness ductility and easy machinability. The castings weigh from 500Kg to 6000 kg, metal is closed-grained and shows a Brinell number of 200 to 210. Meehanite iron responds to heat treatment unlike ordinary grey cast iron. It can be hardened either wholly or on the surface, it can also be toughened by suitable heat treatment.

MICROSTRUCTURE OF CAST IRON :

a) Primary Cementite and Pearlite only :

This type of structure is typical of the hard,white,low-silicon, high sulphur irons, similar to those which have been chilled.

b) Primary Cementite, Graphite and Pearlite :

Mottled irons, in which some of the primary cementite has decomposed, forming graphite are of this type.

^c) Graphite and Pearlite :

This structure is typical of a grey high-duty iron in which all of the primary cementite has transformed to graphite.

d) Graphite, Pearlite and Ferrite :

Coarse grey iron which will be weaker and softer.

e) Graphite and Ferrite :

All the pearlitic cementite, as well as the primary cementite, has broken down to graphite. This is usually due to a high silicon content. Such a cast iron will be very soft and easily machined. The ferrite present will contain dissolved silicon and manganese.

THE GROWTH OF CAST IRON :

The so-called "growth" of cast irons is caused by the breakdown of pearlitic cementite to ferrite and graphite when the iron is heated in the region of 700°C. This causes an increase in volume,) which is further amplified as hot gases penetrate into graphite cavities and oxidise the ferrite. Stresses are set up and these lead to warping and the formation of cracks on the surface. Certain alloy cast irons have been developed to resist growth "Silal" is relatively cheap and contains about 5.0% silicon with low carbon, so that its structure consists entirely of ferrite and graphite and no cementite is present which can cause growth. Unfortunately "Silal"is rather brittle,so that where the higher cost is justified, the alloy "Nicrosilal" can be used with advantage. This is an oustentitic nickel-chromium cost iron.

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