What is Cast Iron ?
Cast iron is an iron alloy containing 2 to 4% carbon, as well as varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. Iron ore is reduced in a blast furnace to produce it.
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 the iron is known as white cast iron, 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 is called grey cast iron.
An intermediate stage between these two varieties of iron shows patches of grey in the white structure. This iron is called mottled iron.
Mottled cast iron (MCI) is a material that is used in mill rolling especially in the last mill stands, due to its combination of high: resistance, thermal conductivity and elastic modulus. In MCI the micro-structural areas of white cast iron and grey cast iron result from solidification.
The quality of cast iron thus depends not only upon the absolute amount of carbon it contains, but also on the conditions in which that carbon exists. The varieties of cast iron in common use are :
Types of Cast Iron
The varities of cast iron in common use are:-
- Grey cast iron
- White cast iron
- Malleable cast iron
- Nodular cast iron
- Chilled cast iron
- Alloy cast iron
- 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.
Grey cast iron contains large quantities of carbon and relatively small quantities of the other element, e.g., silicon phosphorus, Sulphur, and manganese .
It presents a dull grey crystalline or granular structure and a strong light will give a glistering effect due to reflection of the free graphite flakes. The presence of this free graphite is also seen when filing or machining cast iron as it makes hands black.
The cast iron is brittle and may easily be broken if a heavy hammer is used. The strength of iron is much greater in compression than in tension. The ultimate tensile strength of cast iron varies between 12 to 13 kgf per mm2 (120 to 300 Newton per mm²) and depends on the composition of the iron. In compression, gray cast iron will withstand about 60 to 75 kgf per mm? (600 to 750 Newton per mm2) before fracturing, whilst in shear its strength is approximately 15 to 22 kgf per mm2 (150 to 225 Newton per mm2). The hardness ranges from 150 to 240 BHN. main advantages in favour of its use are : (1) its cheapness, (2)
The its low melting temperature (1,150°C to 1,200°C), and fluidity when in the molten condition, and (3) it is easily machined. A further good property of cast iron is that the 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 analysis of the iron enables it to be used widely for making castings of parts having intricate shapes and in almost all cast iron forms.
2. 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. The ordinary rate of cooling in sand produces free graphite while rapid cooling helps to produce cementite. Moreover, manganese encourages the formation of carbide. The approximate chemical composition of white cast iron is given in Table 4.2.
The white cast iron is very hard (the hardness ranges from 400 to 600 BHN) and brittle, and its fractured surface has a silvery metallic appearance. From the engineering point of view, white cast iron has limited application. This is due to the difficulty of machining and to its having, in general, relatively poor ability of mechanical processing. This type of iron is widely used to manufacture wrought iron. White cast iron is also cast as the intermediate material for making malleable cast iron. White cast iron does not rust so much as the grey kind.
3. Malleable Cast Iron
The ordinary cast iron is hard and brittle. It is, therefore, unsuitable for articles which are thin, light and subjected to shock and vibration or for small castings used in various machine components.
The application of the term ‘malleable‘ to castings is rather a misnomer because they are not very malleable when compared with the standards of malleability. When compared with grey iron castings, however, which are fairly brittle, malleable castings do possess a degree of toughness, and this is probably why they have been so named.
Malleable castings are first made from an iron having all of its carbon in the combined form, i.e., from white cast iron. Two methods are then used for malleablizing 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. The approximate chemical compositions for both varieties are given it Table 4.3.
4. Ductile Iron
Spheroidal graphite iron, also called nodular cast iron, is of a higher grade because graphite is precipitated not in the form of flakes but in the form of spheroids. This can be achieved by various methods, e.g., by the addition of one of the following elements: magnesium, cerium, calcium, bismuth, zinc, cadmium, titanium or boron.
Ferro-silicon is also used as an inoculant. Spheroidal cast iron can be produced in thicker pieces than those produced by malleable cast iron. Outstanding characteristics of spheroidal cast iron are high fluidity, which permits the castings of intricate shapes, and an excellent combination of strength and ductility. The tensile strength of this iron is about 33 kgf per mm2 (330 Newton pet mm). This material is also known as ductile iron.
Spheroidal cast iron is widely used in cast parts where density and pressure tightness 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.
5. 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 thermal conductivity than sand, the chilled portions of the casting undergo rapid solidification and cooling and, thereby produce a herd surface. But this hardness only penetrates about 1 to 2 mm in depth.
Sometimes a casting is chilled intentionally and sometimes becomes chilled accidentally to a small depth. Intentionally chilling is carried out by using cast iron instead of sand for those portions of the mould where hard surfaces are required. Where these are touched by the molten metal, its surface is suddenly cooled and converted into white cast iron. With chill casting permanent moulds made with heat resistant steel or cast iron with flake graphite are used.
Chills are used on those castings where some parts are required to have the hardness of white cast iron, while others are required to have relatively soft and tough core of grey cast iron.
6. Alloy Cast Iron
Alloy cast iron have been developed in recent years to overcome certain inherent deficiencies in ordinary cast iron and to give qualities more suitable for special purposes. The addition of nickel, chromium, molybdenum, titanium, silicon, copper and other alloying elements confer special properties to this cast iron.
The use of 1 or 2 per cent of nickel is suitable good quality iron offers a simple and effective means of improving the properties to this cast iron.
The use of 1 to 2 per cent of nickel is suitable good quality iron offers a simple and effective means of improving the properties and the service given by iron castings. It ensures machinability and uniformity of structure of the cast iron. By the use of nickel of the order of 25 per cent, enhanced life can also be obtained in parts subjected to abrasive wear.
7. Meehanite Cast Iron
Cylinders or cylinder liners of all sizes form 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 in contact with caustic liquor.
The addition of only 1 per cent of nickel gives as all round benefit for this purpose. The most prominent factor 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 for many parts of steam and hydraulic machinery, compressors and internal combustion engines.
Where improvement in the wearing quality is of importance, chromium additions with the nickel are often found useful. Cast iron alloyed in this manner finds wide application for pumps of all types in which frictional, as well as erosive, wear has to be considered. Alloy cast iron containing 10 to 30 per cent chromium and 1 to 3 per cent total carbon, also exhibits a high degree of heat resistance combined with strength at high temperatures.
Cast iron alloyed with 10 per cent nickel and 6 per cent manganese become non-magnetic. A special non-magnetic patented cast iron is known under the name of “Nomag”.
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 qualities of mechanite iron are, however, not solely due to the use of calcium silicide but also due to careful control of all the factors involved in the melting of the iron in the copula or electric furnace and in the moulding of the casting. Very little calcium silicide remains in the iron after solidification.
The metal used is low in silicon, moderately low in carbon which is limited, about 2.5 to 3 per cent. This normally would be white if cast, but graphitising by meehanite system of control provides a range of materials to meet the broad requirements of engineering industries.
There are in all more than twenty six types of meehanite metal available at present under five broad classifications : (a) general engineering, (b) heat resisting, (c) wear resisting, (d) corrosion resisting, (e) nodular ‘S’ type.
All meehanite irons have high strength toughness, ductility and easy machinability. They therefore claim to bridge the gap between ordinary cast iron and steel. The castings weigh form 500 to 6,000 kg. The metal is close grained and shows a Brinell number of 200 to 210. a
Meehanite iron responds to heat treatment unlike ordinary grey cast iron and hardened wholly or on the surface. It can also be toughened by suitable treatment to use for machine tool casting.
Cast Iron Production
Cast iron is made from pig iron, which is the byproduct of melting iron ore in a blast furnace. Cast iron can be made directly from molten pig iron or by re-melting pig iron, often with significant amounts of iron, steel, limestone, carbon (coke), and various steps to remove undesirable contaminants. Phosphorus and sulfur can be burned out of molten iron, but this also burns out the carbon, which must be replaced. Carbon and silicon content are adjusted to the desired levels, which can range from 2–3.5 % and 1–3 %, respectively, depending on the application. Other elements can then be added to the melt if desired before casting the final form.
Cast iron is sometimes melted in a special type of blast furnace known as a cupola, but it is more commonly melted in electric induction furnaces or electric arc furnaces in modern applications. The molten cast iron is poured into a holding furnace or ladle once the melting process is complete.
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