The Open Hearth Process Of Making Steel

 In the Open hearth process for producing steel, pig iron, steel scrap, and iron oxide in the form of iron ore or scale are melted in a Siemens - Martin open hearth furnace, so called because the molten metal lies in a comparative shallow pool on the furnace bottom or hearth. The hearth is surrounded by the roof and walls of refractory bricks. The charge is fed through a charging door and heated to 1600*C to 1650*C mainly by radiation of heat from the burning of gaseous fuels above it. It is not the amount the heat but rather the high temperature heat that is essential for the purpose. 

In practice, a considerable quantity of steel scrap is previously charged and heated in the furnace and a partly purified molten iron known as blown metal from the Bessemer converter is added to this. Thus the impurities in the Pig Iron are "Diluted" and the refining process does not take so long period as if  the entire charge were pig iron. Very often more than 60 per cent of charge consists of  scrap. The process of making steel in this way is known as the duplex process because it has two stages, viz., 

(1) Blowing Molten Pig Iron in the Bessemer converter, and 

(2) Further purification of the blown metal in the Open hearth furnace.

At the end of process, when the Impurities ( Silicon, Manganese , etc., ) have been brought down to the required level , the metal is tapped off through a tap hole in the furnace. Additions of ferromanganese and ferro silicon are made to bring the steel to the correct composition; later a small addition of aluminum is made to deoxidize the metal further. In some cases , the carbon is reduced practically to zero and then additions are made to bring it to the correct amount, but a more usual method is to catch the carbon at the desired amount.

In acid practice, a lining of silica sand is used on the hearth and an acid slag high in silica is produced on the metal during the refining process. Acid slag cannot remove phosphorous for which basic lining is used. In basic practice, a lining of magnesite is used and limestone is charged with the raw material. A basic slag high in lime is produced on the bath after the metal melts. 

Steel made by the acid open hearth process is considered to be of better quality due to the fact that carefully selected materials low in Sulphur and Phosphorous are used in it. How ever, most of  the steel is now - a - days made by the basic open hearth process as it has got certain advantages. At the present time, about 85 percent of steel is produced in open hearth furnace.

The complete cycle of operation lasts from 5 to 15 hours and open hearth furnaces have been built in capacities of 2 to 700 tons per charge. But the most common types are of 250 to 500 tons per charge.