While not as speedy nor as prolific a producer as the Bessemer process and far less spectacular, the open-hearth has several advantages.
The acid Bessemer was always handicapped because pig iron with less than 0.1 per cent of phosphorus was necessary. The majority of ores carry more than this amount. The basic Bessemer requires pig iron containing not less than 2 per cent of phosphorus. The vast quantities of material which contain percentages of phosphorus between these limits are useless as far as the Bessemer process is concerned.
To be successfully used the pig iron must be further limited as to composition. It must have sufficient silicon, manganese, and carbon to give the heat required for Bessemerizing, as the burning of these metalloids has to be depended upon for the conversion to steel and to give proper fluidity to the finished alloy.
Then, too, the large amount of air forced through to a certain extent “over-oxidizes” the bath and some of the gases are mechanically retained by the steel no matter how complete the deoxidation. There also is loss of metal due to unavoidable “spitting,” for the rapid streams of air mechanically carry some metal and slag with the flame out of the vessel.
On the other hand, for the open-hearth process can be used pig iron of widely varied character and composition and, further, large percentages of low-priced steel scrap can be utilized in the charges; as no air is blown through the metal and little comes in contact with it, the conversion takes place quietly and smoothly and with much less loss by oxidation, the yield of steel usually being from 90 to 97 per cent of the metal charged as against 83 to 87 per cent which is the yield by the Bessemer process; besides giving less over-oxidation and gases in the metal, the slowness of the conversion is an advantage, as control is very easy, and, when desired, samples for test may be taken. From his tests the melter can be quite certain when he taps out the steel that it is of the composition desired.
Section Through Typical Stationary Open-Hearth Furnace, Showing Construction of Furnace, Lining, Bath and Air and Gas Ports
The melting in an open-hearth furnace is done largely by indirect or radiated heat, and it is not intended that the flame shall impinge too directly upon the surface of the bath.
Boxes of Steel Scrap and Electric Charging Machine in Front of Charging Doors at Rear of Furnaces