The open hearth furnace

 

 

The open hearth process is one of the most important methods of making steel. It is much slower than the Besнsemer but it is easier to control, and for that reason it is more frequently used. The open hearth process takes from eight to twelve hours, so that there is plenty of time for accurate analysis. The quality of steel can be carefully conнtrolled.

The name "open hearth" is given to it because the hearth of the furnace is exposed to the sweep of the flames which melt the steel.

The capacity of the open hearth furnace varies, at an average, from 50 to 400 tons of metal. The furnace is lined with firebrick to withstand the very high temperatures used (up to 3000░ F). The charge consists of molten pig iron, scrap iron and steel and some hematite. Lime is added to the charge to take out the phosphorus and sulphur as slag. Manganese, carbon, nickel, vanadium, or other mateнrials are added to make the kind of steel desired.

The fuel, which may be natural or artificial gas, powнdered coal, oil, tar, or two or more of them in combination, is blown into the furnace through one of the two large openнings, or ports, located on each end of the furnace. To facilнitate combustion previously heated air is blown through the port along with the fuel. At the present time oxygen is used for the intensification of the open hearth process. It is introduced into the flame to accelerate the burning of the fuel and into the molten bath to oxidize the impurities. Combustion occurs above the hearth, and the smoke and other products of combustion escape through the ports at the other end of the furnace.

Beneath the furnace there are two large chambers containing a checkerboard arrangement of firebricks through which air or gas flows freely. As the hot products of combustion pass out through one of the ports and through the checker-work, they heat the bricks.

At the same time the gas and air entering through the other side of the furnace are being heated by the bricks in the corresponding checker chamber which has previously been heated in the same way.

When the cold air and gas cool the bricks to a point where they no longer give up enough heat, the direction of flow is reversed so that the chamber which has been heatнing the incoming fuel now becomes reheated by the prodнucts of combustion, and vice versa. Thus a great deal of heat is saved and temperatures can be reached which would otherwise be impossible without great waste and consumpнtion of enormous amounts of fuel.

About four hundred tons of steel, or even more, may be refined at each melt or heat. A heat is the amount of steel that is made from one furnace charge of raw materials. Two or three heats of steel are usually refined a day in the open hearth furnace. To-day the number of heats can be increased depending upon the capacity of the furnace and the method of work.

There are three stages in the operation of this furnace. The first is known as the process of charging; the secondЧ as the melting down process; the thirdЧthe period of refinнing. The period of refining is especially important and requires the constant supervision of the operator. The refinнing consists first in removing objectionable impurities and then controlling the elements other than iron which the final product must contain. Alloying elements are added to the steel before it is tapped or when it is in the ladle.