Steel is fundamentally an alloy of Iron and Carbon, with the carbon content varying from less than 0-25% to 1.5%. The type of steel is known by its carbon content; for example low carbon steel - upto 0-25% carbon; medium carbon Steel - from 0.25% to 0-75% carbon and high carbon steel from 0-75% to 1-5% carbon.
Under these conditions the carbon gradually diffuses into the iron and dissolves in it, thus raising the carbon content. The steel manufactured by this process is known as “blister steel” since the bars are covered with blisters due to evolution of gas. The amount of carbon introduced into the iron is usually 0.75 — 1.5%.
Cast steel is a perfectly homogeneous product and extremely hard. It is used in making finest cutlery and cutting tools. it is also known as “tool steel.”
Sometimes, molten pig iron from blast furnace is directly used. After the converter is charged with molten pig iron, a strong blast ‘of air is forced through the molten mass for about 20 minutes through the nozzles provided at the bottom of the converter.
The air oxidizes all the carbon and silicon present, leaving pure iron in the converter, The blast is now stopped and the required quantity of ferro-manganese is added in order to give the desired carbon and manganese content to the steel.
The blast is again turned on for a few minutes more to ensure thorough mixing up of the alloy. The converter is then tilted into discharging position and discharged into ladles. From the ladle, the molten metal is poured in rectangular molds where it solidifies into ingots.
The process is conducted in gas-fired regenerativg furnaces, which also effect economy in fuel. The furnace is of the reverbaratory type.
The hearth slopes in all directions towards the taphole at the back of the furnace. 'T' openings are usually provided in the furnace front, pig iron being charged through the outer ones, and the ore or scrap through the central one.
The openings are closed by doors, which are operated by chains. The passage from the regenerators to the furnace is through the end walls of the furnace. Below the furnace are placed the re/generators arranged in two pairs. These pairs are heated alternately by sending through them the hot gases from the furnace on their Way to the chimney.
The ‘heat absorbed this way is afterwards given up again to air and the gas, on their way to the furnace. The direction in which the air and gas pass through the regenerators is reversed about every half hour, thus maintaining the high temperature and retaining most of the heat in the furnace and regenerators .
After pig iron is charged into the furnace, additions of haematite, or other pure oxidised ores are made from time to time, which effect the oxidation and removal of the silicon, carbon and manganese in the pig iron.
Spigel and ferro-manganese are also added to the steel, spigel being added when the carbon is reduced below 0.1% and ferro-manganese after the metal is tapped out into the ladle, the latter being added to restore malleability and to carburise the iron.
Manufacture of Steel
There are a number of methods used for manufacture of Steel. The principle methods are:- Cementation process
- Crucible process,
- Bessemer process,
- Open-hearth process,
- Electric process
Cementation process
In this process, bars of wrought iron are placed in a furnace between layers of powdered charcoal and subjected to high temperature — about 700°C for 5 to 14 days according to the quality of steel required.Under these conditions the carbon gradually diffuses into the iron and dissolves in it, thus raising the carbon content. The steel manufactured by this process is known as “blister steel” since the bars are covered with blisters due to evolution of gas. The amount of carbon introduced into the iron is usually 0.75 — 1.5%.
Crucible process
In this process either fragments of blister steel or short lengths of wrought iron bars mixed with charcoal are heated in fire clay crucibles and the molten product is run into iron molds. The steel so produced is known as “ cast steel”.Cast steel is a perfectly homogeneous product and extremely hard. It is used in making finest cutlery and cutting tools. it is also known as “tool steel.”
Bessemer process
In this process pig iron is melted in a cupola and run into a pear—shaped Bessemer converter which is a steel shell lined with refractory material; and pivoted on trunnions so that it can be tilted for pouring or charging.Sometimes, molten pig iron from blast furnace is directly used. After the converter is charged with molten pig iron, a strong blast ‘of air is forced through the molten mass for about 20 minutes through the nozzles provided at the bottom of the converter.
The air oxidizes all the carbon and silicon present, leaving pure iron in the converter, The blast is now stopped and the required quantity of ferro-manganese is added in order to give the desired carbon and manganese content to the steel.
The blast is again turned on for a few minutes more to ensure thorough mixing up of the alloy. The converter is then tilted into discharging position and discharged into ladles. From the ladle, the molten metal is poured in rectangular molds where it solidifies into ingots.
Open-hearth process
The outstanding feature of the open-hearth furnace is the intense heat obtainable by its regenerative process. The flame burns above a shallow vessel containing the charge of pig iron, steel scrap, iron ore and flux.The process is conducted in gas-fired regenerativg furnaces, which also effect economy in fuel. The furnace is of the reverbaratory type.
The hearth slopes in all directions towards the taphole at the back of the furnace. 'T' openings are usually provided in the furnace front, pig iron being charged through the outer ones, and the ore or scrap through the central one.
The openings are closed by doors, which are operated by chains. The passage from the regenerators to the furnace is through the end walls of the furnace. Below the furnace are placed the re/generators arranged in two pairs. These pairs are heated alternately by sending through them the hot gases from the furnace on their Way to the chimney.
The ‘heat absorbed this way is afterwards given up again to air and the gas, on their way to the furnace. The direction in which the air and gas pass through the regenerators is reversed about every half hour, thus maintaining the high temperature and retaining most of the heat in the furnace and regenerators .
After pig iron is charged into the furnace, additions of haematite, or other pure oxidised ores are made from time to time, which effect the oxidation and removal of the silicon, carbon and manganese in the pig iron.
Spigel and ferro-manganese are also added to the steel, spigel being added when the carbon is reduced below 0.1% and ferro-manganese after the metal is tapped out into the ladle, the latter being added to restore malleability and to carburise the iron.
Electric Process
The type of electric furnaces used for this process are the High frequency furnace, and the Electric arc furnace.
The principle employed in high frequency furnace is that when a piece of steel is held in a coil of wire and an alternating current is passed through the coil, due to induction this sets an eddy current in the steel. If these eddy currents are powerful enough, their passage through the steel will cause the" steel to be heated up.
In the case of are furnace, which is most commonly used, the heat is obtained from arcs struck between carbon electrodes and the metal of the charge. The charge is taken direct from an open-hearth furnace. The intense heat keeps the metals in fluid state and the impurities are removed as slag. At this stage alloying constituents in required proportions may be added, according to the quality of steel required.
A great advantage of the electric furnace is absence of gas, fume and impurities which are present in fuel-fed furnaces and which may introduce impurities into the melt or oxidise a required constituent out of it. Whatever is put into an electric furnace may be relied upon to stay there, and if well fitting top is kept on the crucible, little oxidation can take place.
Uses: Shafts, rods, bolts, tubes and tools e. g., hacksaw frames, hammer heads, fitter's square etc.
Uses: Cutting tools of ail kinds e. g. chisels, handsaws, drills, reamers, taps, dies, punches, files etc.
The principle employed in high frequency furnace is that when a piece of steel is held in a coil of wire and an alternating current is passed through the coil, due to induction this sets an eddy current in the steel. If these eddy currents are powerful enough, their passage through the steel will cause the" steel to be heated up.
In the case of are furnace, which is most commonly used, the heat is obtained from arcs struck between carbon electrodes and the metal of the charge. The charge is taken direct from an open-hearth furnace. The intense heat keeps the metals in fluid state and the impurities are removed as slag. At this stage alloying constituents in required proportions may be added, according to the quality of steel required.
A great advantage of the electric furnace is absence of gas, fume and impurities which are present in fuel-fed furnaces and which may introduce impurities into the melt or oxidise a required constituent out of it. Whatever is put into an electric furnace may be relied upon to stay there, and if well fitting top is kept on the crucible, little oxidation can take place.
Properties and Uses of Steel
Low carbon (0.25%) or Mild Steel
Properties: Ductile, less malleable, stronger, harder and more uniform than wrought iron. Easily forged, welded, machined or stamped to shape.
Uses: Bolts, tubes, rivets, plates, all parts where great strength or hardness is not required.Medium (0.25% to 0.7%) Carbon Steel
Properties: Rather stronger and harder, but less ductile and malleable than low carbon steel therefore not so easily worked.Uses: Shafts, rods, bolts, tubes and tools e. g., hacksaw frames, hammer heads, fitter's square etc.
High (0.7% to 1.5%) Carbon Steel
Properties: Strong and less ductile. Hardness and toughness depend on heat treatment. Can be made very hard without undue brittleness.Uses: Cutting tools of ail kinds e. g. chisels, handsaws, drills, reamers, taps, dies, punches, files etc.
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