Manufacture of steel.
Steel is made by various methods but (a) Bessemer process, and (b) Open hearth process are important and commonly used.
This process is carried out in a pear-shaped furnace called Bessemer converter (Figure 1.1) The converter is made of steel plates and lined with silicious material (sand and small amount of clay) or bricks. A number of holes are present at the bottom of the converter to admit a blast of air.
In the Bessemer process about 10 tons of molten pig-iron are run into the converter. When the converter has been charged and the blast of air is turned on, the temperature rises due to heat envolved during oxidation and combustion of impurities. Carbon is oxidation to CO, which burns at the mouth of the converter. The carbon monoxide flame gives an indication for the completion of the process. When carbon monoxide flame subsides, calculated amount of spiegeleisen (carbon and manganese or other metals) is added and the blast turned on again for a few minutes.
If Bessemer converter is lined with silica, it is called the Acid Bessemer process. A Basic Bessemer Process involves lining of dolomite or lime or magnesia and is used to convert pig-iron (Containing higher proportions of S, P and silicon) into steel.
The Bessemer process has revolutionized the manufacture of steel because it is cheaper less time consuming.
Open Hearth process (Siemens Martin Method)
This method is now widely used for the manufacture of steel. the open-hearth furnace is shown in figure 18.3 the furnace is charged with a mixture of pig-iron, scrap iron and haematite
ore free from carbon. The mixture is melted in a shallow rectangular
trough or hearth. The furnace is heated by producer gas. The direction of the burning gases is reversed after about ½ hour. In this way the heat of the flue gases is utilized in warming up chambers through which air and unburnt gases are passed. Thus this furnace is called Siemens’ regenerative furnace.
The iron ore and scrap iron help in oxidising impurities. Carbon is oxidised to CO which goes off. Impurities such as P and S from corresponding oxides within 8- 12 hours.
and basic oxides (CaO from limestone added along with the charge in the furnace) act as flux and from slag with oxides of phosphorus and silicon. If sulphur and phosphorus are present in negligibly small quantities, the furnace may be lined with SiO2.
When the required carbon content is reached during the process, the molten steel is removed. Small quantities of other metals such as Mn, Cr or Ni are added to prepare special type of steel .
The steel obtained by open-hearth process is used for making heavy rails, guns, girders and ships, etc.
Advantages of Open-Hearth Process over Bessemer Process.
Scrap iron and the iron ore can be directly converted into steel by open-hearth cess.
The external source of heat used in open-hearth process is more convenient and temperature can be accurately controlled.
The composition of steel is more uniform and accurate in open-hearth process and can be easily controlled.
Low grade cast iron can be used in this process.
Steel obtained by open-hearth process is of high grade and of good quality.
Electric furnaces, often of are type, are used to get high quality steels. As no fuel is used, there are less chances of contamination by impurities, Electrical process are popular for preparing good quality steel, especially when electric power is cheap. A number of steel alloys are made by these met
Various process based upon heat treatment give steel of different quantities Annealing is a process in which steel is heated to redness and allowed to cool slowly quenching refers to the process in which steel is heated to redness and cooled suddenly in water oil tempering means heated steel at constant temperature for some time before cooling.
Many useful alloys of steel are manufactured by adding metals in small amounts. Some important alloys of such type are :
(Fe = 73 – 79%, Cr = 14 – 18%, Ni = 7 – 9%) is a corrosion resistant alloy used in cutlery and industry.
(Fe = 64%, Ni = 36%) has low coefficient of expansion and used in pendulum rods, meter scales, etc.
(Fe = 95 – 99%, Si = 1 – 5%) is hard and highly magnetic and thus used in magnets.