The central figure of an open-hearth steel plant is the melting-furnace. This is always a regenerative furnace, for in no other construction is it practicable to obtain the necessary tem perature, but its construction varies greatly in other respects. Fig. 3, showing a furnace de signed for the Illinois Steel Company, of Chicago. may used to explain the general features of construction and operation of an open-hearth steel furnace. In this drawing the checker-work shows time regenerating chambers, and the saucer shaped structure above is the hearth. The two regenerating chambers are separate from each other. but either may be connected with the hearth and with the smokestack. Each cham ber is nearly filled with a sort of mob-house work of brick. In operation. one chamber is heated so that the brick filling is white hot, and then a current of air and gas is admitted at the bottom. As this combustible mixture passes up through the heated brickwork, it is heated so that when it arrives at a point over the hearth it is burning fiercely. A portion of the heat of this combustion is given up to the charge on of a steel and iron plate shell lined with seine refractory material. In hearths for the acid process, the hearth-lining proper consists tirst of a layer of sand fused into a solid mass by heat. In hearths for the basic process the lining consists of a plastic compound made by roasting and grinding dolomite limestone and mixing the powder with tar. The accessories to the hearth consist of charging and tapping appliances, ladles and cranes for handling then', casting molds, etc.
In respect to their chemistry. the acid and the basic open-hearth process can best he considered separately. Referring first to the acid process, the proportions of the constituents of the charge vary in different places. Sometimes pig iron alone is used. but more generally pig iron and scrap wrought iron and steel are mixed. What mer the mixture may be, it is necessary that its contents of phosphorus and sulphur be known. since these are rot eliminated in the process. Broadly considered, the chemical reactions may be divided into three operations: first. the re actions during melting; second, the reactions after melting; and third, the reactions during recarburization. During melting the silicon and manganese are reduced by oxidization to a minute fraction of their original amounts and about one-half of the carbon eliminated. After melting. the remaining silicon, manganese. and carbon are eliminated by keeping the molten metal at a high hent and adding iron ore in suc cessive small does, thus forming silica and oxide of manganese. go into the slag, and carbonic oxide, which escapes with the flame. To determine whoa this process has proeeeded far enough, samples 14 the molten metal are taken at intervals. east into iron Im•s and broken; the carbon content is estimated by the appearance of the fraeture. an expert being thus aide to deter mine its amonnt with much accuracy. \Alen the desired point of earhon content has been reached, as determined by the test. the reea rburizer is added in a solid state. This r•carburizer is fer romanganese with a very large excess of man ganese. In the basic process the problem is the melting and deearburization of the charge, as in the acid process just, described. with the add i tiona I duty of removing a reasonable quantity of the phosphorus. This is aeeomplished by adding lime to the charge. which takes up the phosphorus and confines it. in the slag. The basic lining in the furnace is necessary to leave the lime free to perform useful work, which would not be the ease were an acid lining used which would take up a portion of the lime.
Reearburization is accomplished in much the same way as in the acid process. Summarized,
the chemical problem of the open-hearth process is to eliminate from the crude iron of the charge all the silicon, manganese. carbon, phosphorus, and .sulphur in excess of the amounts required for steel. The problem is practically the same in the Bessemer process, but the method of its solution is different. In the Bessemer process the metal is always blown until nearly all the carbon is eliminated, since it has been found im practicable to stop the operation at any inter mediate point. .11) the carbon content of Bes semer steel has, therefore, to be supplied by the reearburizer, and absolutely perfect homogeneity of product can be secured only by absolutely per fect mixing of the molten metal and the reearbu rizer. This perfect mixing increases in difficulty as the amount. of carbon required in the steel inerea seg. n the open-hearth process the elimination of tlm carbon can lie stopped at any desired point, so that very little carhop is added in the reearburizer, and the necessity of thor ough mixing is less imperative. a result it i3 generally eonsidered that high carbon steel or hard steel can be produced with a more uniform quality by the open hearth than by the Bessemer proses.
An important modification of the open hearth which has been described, and which is intermit tent in operation, is the Talbot continuous open hearth process, now in use in sonic American steel works and being instalhd in several others here and abroad. Mr, Talbot's process consists essen tially in working the furnace continnonsly by tapping off a portion of the molten charge at short intervals, immediately eharging an equiva lent of pig iron, and again tapping. Several ad vantages in increased output, economy of mainte nance, wider range, etc., are claimed for the process-.
The development of the open-hearth process of steel-making was the outgrowth of numerous at tempts by inventors to repeat the success of Bes semer. Not much success was realized in these efforts, however, 11111i1 18(12, when W. Siemens, a Uernian. applied the regenerative furnace. which lie had invented in 1'457, to the manufac ture of steel. It was not until 18118, when Sie mens sueeceded in making steel from old iron rails, that the slave-- of the process was fully demonstrated. Meanwhile, 1). and E. lartin, of Sireul, France, had in making steel from a mixture of pig iron and scrap in a Sie mens furnace. Thins originated the Siemens lartin process or open-hearth process of steel making. At present, this pin ranks seemtd in importance only to the Bessemer proses.. The Be-seiner process is practically without rival for the production of steel rails, hut the open-hearth process leads in the production of structural steel, ship's plates, and steel for castings.
.11.. first both the 13esseme• and the open-hearth process were employed only with acid-lined fur !laces. the basic process being a subsequent de velopment. The practical invention of the basic process was due to Sidney C. Thonms and P. C. t:ilehrist. and was first made public iu ISTS. The essential idea of the invention consisted in the substitution of a basic lining instead of the acid lining previously used in both the Bessemer and open-hearth processes, and the addition of a quantity of quicklime during the process so as to combine with the silicon and phosphorus. and thus to save the lining as much as possible. The success of the invention was not demonstrated until 1879, but since that time the process has developed rapidly. By this invention the enor mous deposits of iron ores high in phosphorus, which had previously been excluded from use in the two great steelmaking processes of the world, were rendered available to the steel-makers.