The charge consisting of alternate layers of coke, pig iron mixed with scrap castings and limestone is fed to the furnace through the charging door. The scrap iron is used in order to improve quality of the cast iron and also to effect economy in the cost of its production.
The air for combustion is forced under pressure through the tuyers. The lime stone is added as a flux, which combines with the impurities left in the pig iron and removes them in the form of slag which floats above the molten iron. The molten iron thus obtained is called cast iron and is cast into moulds. The different steps involved in cupola operation are: 1 Preparation of cupola including repairs 2 Lighting the fire into the coke bed 3 Charging of cupola 4 Melting 5 Slagging and metal tapping 6 Dropping down the cupola bottom 2.
The molten iron collects in this zone before being tapped. The well is situated between the tapered rammed sand bottom and the bottom of the tuyeres. Thus, a lot of heat is supplied from here to other zones. Oxidation of Mn and Si evolve still more heat. It protects from oxidation. The metal charge above and that dropping through it. An endothermic reaction takes place in this zone, in which some of hot CO2 moving upward through hot coke gets reduced.
The temperature in the melting zone is around or above C. As per the following reaction taking place in this zone, the molten iron picks up carbon. This contain cupola charge as alternate layers of coke, limestone and metal. Thus preheated charge gradually moves down in the melting zone. Hot gases from cupola pass through the stack zone and escape to atmosphere. Stack gases i. Cast iron C. Gray cast iron 2.
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White cast iron 3. Malleable cast iron 4. Nodular cast iron 5. Clilled cast iron 6. Alloy cast iron 7. Mechanite cast iron. On solidifying , the iron contain the greater part of carbon 2.
The grey colour is due to the fact that the carbon is present in the form of free graphite. It has a low tensile strength, high compressive strength and no ductility. It can be easily machined. A very good property of gray C. So, it is very suitable for sliding parts. Its melting temperature is C. It possesses machinability better than steel. The slow rate of cooling in sand produces free graphite while rapid cooling helps to produces cementite.
So, it is unsuitable for articles which are thin, light and subjected to shock and vibration or for small casting used in various machine components. The malleable C. The annealing process separates the combined carbon of the white cast iron into nodles of free graphite. Two method in White heart process and Black heart process are used for this purpose. It contain 2. High strength C.
This cast iron is produced by adding magnesium to the molten cast iron.
The magnesium converts the graphite of C. The alloy C.
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These alloying elements give more strength and result in improvement of properties. The carbon occurs in the form of iron carbide Fe3C , because of its ability to increase the hardness and strength of the steel. If, however, the carbon is increased above 1. Steel can be classified as: 1. Carbon Steels 2.
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Alloy Steels. Carbon has a strengthening and hardening effect. It lowers ductility, machineability, weldability, corrosion resistance, thermal and electrical conductivity and magnetic permeability. Carbon steel does not contain more than 0. The plain carbon steels varying from 0. Dead mild Steel 0.
Steel and material subject to drawing and pressing. Mild Steel 0. Uses: Structural steels, universal beams, screw, drop forging,case hardening steel, gears free cutting steel, shaft. Medium Steel 0. High carbon 0. Tool Steel 0. High Carbon 1. The most common alloying elements are chromium, nickel, manganese, silicon, vanadium, molybdenum, tungsten, phosphorous, copper, titanium, zirconium, cobalt, columbium, and aluminium. They are used separately or in combination to produce desired characteristics in the steel.
Silicon: The amount of silicon in the finished steel usually ranges from 0. Silicon is added in low carbon steels to prevent them from becoming porous. It removes the gases and oxides, prevent blow holes and thereby makes the steel tougher and harder. Manganese: It serves as a valuable deoxidising and purifying agent, in steel.
Manganese also combines with sulphur and thereby decreases the harmful effect of this element re- maining in the steel. When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. In high speed steels , it is used to toughen the metal and to increase its critical temperature. The manganese content of carbon steels commonly ranges from 0. Nickel: It improves toughness, tensile strength, ductility and corrosion resistance.
Chromium: It increase strength, hardness, toughness, and corrosion resistance. Cobalt: It improves hardness, toughness, tensile strength, thermal resistance, and magnetic properties.
Molybdenum: It increase wear resistance, thermal resistance, hardness ability to retain mechanical properties as elevated temperature. When added with nickel, it improves corrosion resistance. Tungsten: It increase hardness, toughness, wear resistance, shock resistance, magnetic reluctance and ability to retain mechanical properties at elevated temperature. Vanadium: It improve tensile strength, elastic limit, ductility, shock resistance and also acts as a degausser when added to molten steel.
It is added in low and medium carbon steels in order to increase their yield tensile strength properties. Boron: It increase hardenability and is therefore, very useful when alloyed with low carbon steels. Aluminium: It is basically used as a deoxidiser. It improve the growth of fine grains and helps in providing a high degree of hardness through nitriding by forming aluminium nitrides.
Titanium: It is fairly good deoxidiser and promotes grain growth. Also , it readily forms titanium carbides but has no marked effect on the hardenability of the material. Copper: It increases the strength and improves resistance to corrosion.
Its proportion nor- mally varies from 0. Niobium: It improve ductility, decrease hardenability and substantially increases the impact strength. These steels form a very important groop of alloy steels which have been developed to meet specific requirement in respect of properties under specific situations and special aplications. The most common varities of these steels are: 1 Stainless steel 2 High speed steel 3 Cutting alloys. Their principal alloying element is chromium while some other element like nickel, manganese, etc.
Chromium reacts with the oxygen to form a strong layer of chromium oxide on the surface of the metal which is responsible for offering the resistance to corrosion. As this steel cannot be stained easily, so it is called stainless steel.