Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/357

IRON 341 at a high temperature by an annular fireplace surrounding it, a gentle stream of well-desulphurized coal gas being allowed to pass through the chamber. The expense of the process seems to have been the chief bar to its adoption, as steel of excellent quality can. readily be made by it from good malleable iron. 33. Cast Steel. The crucibles or "pots" used for steel melting are usually made of refractory fireclay, often with an admixture of graphite, which not only communicates a greater degree of infusibility, but also diminishes the decarbonization which partial access of air unavoidably brings about during melting ; they are of such size as to hold from 40 to 60 fi> of steel, and occasionally more, even up to nearly a cwt., especially in certain American steel works. As a rule a pot that has served for three successive meltings in a coke-fired furnace is so much damaged as to be unsafe for a fourth ; and with inferior kinds of clay two meltings or even one only are allowed ; with superior pots and gas-fired furnaces four, five, and even six heats are attainable with ease and safety. After annealing, the crucibles are heated red hot and then placed in the melting furnace on fireclay stands, round which and the pots coke is piled, two pots being usually fixed in the same " melting hole," but sometimes more. When the pots are white hot the steel in small lumps is introduced by lifting up the cover and pouring the pieces down a long iron funnel ; the covers being replaced and the fire made up, after some two or three hours the steel is fluid ; but if cast immediately it is found that a much larger quantity of gas separates during solidification, rendering the steel porous, than is evolved if the metal is dead-melted, i.e., allowed to remain melted for an extra half hour or more, presumably from the reaction of the iron oxide interspersed throughout the steel upon the carbon evolving carbon oxide during the earlier period, this evolution subsequently ceas ing, owing partly to the reduction of the oxide and partly to its floating up to the top of the fused mass as scoriae. According to Bessemer the chief part of the " dead melting" effect of the extra time allowed in fusing steel for the molten metal to stand in the furnace after fusion is brought about is due to the reduction of a little silicon from the crucible materials, &c., the presence of that element greatly diminishing the tendency to evolution of gas during solidification (see 44). When the pots are withdrawn and the casting made (frequently from ths contents of many pots combined together), they are replaced in the melting holes before they cool, and used over again, a somewhat smaller quantity of metal being introduced for the second melting (and less still for the third), in order to prevent the surface of the fused scoria being at the same level as before, the pots being chiefly attacked at that place. About 3 5 parts of coke are required for 1 of rnild steel melted, and somewhat less for harder steels, which melt more easily. The Siemens regenerative furnace ( 10), fed with gas from a producer, can be very advantageously employed instead of the older coal or coke-fired furnaces. In such a steel melting furnace (fig. 59) the fusion chamber generally contains some two dozen pots, and is constructed in the form of a trench with overhanging sides, which are arched both horizontally and vertically to keep them from sinking in whilst in use. The floor is covered with finely ground hard coke, which burns away bvit slowly and does not flux or indurate, thus giving a firm foundation for the pots, which are set in a double row along the centre of the chamber ; the upper roof of the chamber consists of firebrick tiles or frames filled with firebrick capable of being slid off separately by means of levers or handles attached to each, so as to permit of the introduction and withdrawal of the pots. The inventors state that the lining of a furnace of this description will last from fifteen to twenty weeks without repair, working day and night, whilst four to five weeks is the ordinary life of a coke- fired furnace ; that the pots will stand four, five, and sometimes even ten successive meltings instead of two or three ; and that, whilst 3 to 4 tons of hard coke are requisite in coke-fired furnaces per ton of steel melted, 15 to 20 cwts. of much inferior slack burnt in a gas producer will furnish enough fuel to melt a ton of steel on the regenerative principle (Clicm. Soc. Journal, 1868, p. 276). The precise amount of fuel used in actual practice is somewhat variable, but consumptions as low as 64 parts of coal per unit oi steel melted (nearly 20 tons being melted in all during one week) have been recorded. In other works the consumptions were 1 - 1 to