Open main menu
This page needs to be proofread.
826
IRON AND STEEL


why the molten metal can be freed from mechanically suspended slag more perfectly in them than in the Bessemer converter or the open-hearth furnace. In order that this Enely divided slag shall rise to the surface and there coalesce with the overlying layer, the metal must be tranquil. But tranquillity-is clearly impossible in the Bessemer converter, in which the metal can be kept hot only by being torn into a spray by the blast. It is practically unattainable in the open-hearth furnace, because here the oxygen of the furnace atmosphere indirectly oxidizes the carbon of the metal which is kept boiling by the escape of the resultant carbonic oxide. In short the electric furnaces can be used to improve the molten product. of the Bessemer converter and open-hearth furnace, essentially because their atmosphere is free from sulphur and oxygen, and because they can therefore remove sulphur, iron oxide and mechanically suspended slag, more thoroughly than is possible in these older furnaces. They make a better though a dearer steel. Further, the electric furnaces, e.g. the Kjellin, can be used to replace the crucible melting process (§ ro6), chiefly because their work is cheaper for two reasons. First, they treat a. larger charge, a ton or more, whereas the charge of each crucible is only about 80 pounds. Second, their heat is applied far more economically, directly to the metal itself, whereas in the crucible process the heat is applied most wastefully to the outside of the non-conducting walls of a closed crucible within which the charge to be heated lies. Beyond this sulphur and phosphorus can be removed in the electric furnace, whereas in the crucible process they cannot. In short electric furnaces replace the old crucible furnace primarily because they work more cheaply, though in addition they may be made to yield a better steel than it can. Thus we see that the purification in these electric furnaces has nothing to do with electricity. We still use the old familiar purifying agents, iron oxide, lime and nascent calcium. The electricity is solely a source of heat, free from the faults of the older sources which for certain purposes it now replaces. The electric furnaces are likely to displace the crucible furnaces completely, because they work both more cheaply and better. They are not likely to displace either the open-hearth furnace or the Bessemer converter, because their normal work is only to improve the product of these older furnaces. Here their use is likely to be limited by its costliness, because for the great majority of purposes the superiority of the electrically purified steel is not worth the cost of the electric purification.

109. Electric Ore-smelling Processes.-Though 'the electric processes which have been proposed for extracting the iron from iron ore, with the purpose of displacing the iron blast furnace, have not become important enough to deserve description here, yet it should be possible to devise one which would be useful in a place (if there is one) which has an abundance of water power and iron ore and a local demand for iron, but has not coke, charcoal or bituminous coal suitable for the blast furnace. But this ancient furnace does its fourfold work of deoxidizing, melting, removing the gangue and desulphurizing, so very economically that it is not likely to be driven out in other places until the exhaustion of our coal-fields shall have gone so far as to increase the cost of coke greatly.

rro. Comparison of Steel-making Processes.—When Bessemer discovered that by simply blowing air through molten cast iron rapidly he could make low-carbon steel, which 'is essentially wrought iron greatly improved by being freed from its essential defect, its necessarily weakening and embrittling slag, the very expensive and exhausting puddling process seemed doomed, unable to survive the time when men should have familiarized themselves with the use of Bessemer steel, and should have developed the evident possibilities of cheapness of the Bessemer process. Nevertheless the use of wrought iron actually continued to increase. The first of the United States decennial censuses to show a decrease in the production of wrought iron was that in 1890, 3 5 years after the invention of the Bessemer process. It is still in great demand for certain normal purposes for which either great ease in welding or resistance to corrosion by rusting is of great importance; for purposes requiring special forms of extreme ductility which are not so confidently expected in steel; for miscellaneous needs of many users, some ignorant, some very conservative; and for remelting in the crucible process. All the best cutlery and tool steel is made either by the crucible process or in electric furnaces, and indeed all for which any considerable excellence is claimed is supposed to be so made, though often incorrectly. But the great mass of the steel of commerce is made by the Bessemer and the open-hearth processes. Open-hearth steel is generally thought to be better than Bessemer, and the acid variety of each of these two processes is thought to yield a better product than the basic variety. This may not necessarily be true, but the acid variety lends itself more readily to excellence than the basic. A very large proportion of ores cannot be made to yield cast iron either free enough from phosphorus for the acid Bessemer or the acid open-hearth process, neither of which removes that most injurious element, or rich enough in phosphorus'for the basic Bessemer process, which must rely on that element as its source of heat. But cast iron for the basic open-hearth process can be made from almost any ore, because its requirements, comparative freedom from silicon and sulphur, depend on the management of the blast-furnace rather than on the composition of the ore, whereas the phosphorus-content of the cast iron depends solely on that of the ore, because nearly all the phosphorus of the ore necessarily passes into the cast iron. Thus the basic open-hearth process is the only one which can make steel from cast iron containing more than o-ro % but less than 1-So %of phosphorus. The restriction of the basic Bessemer process to pig iron containing at least I'8O% of phosphorus has prevented it from getting a foothold in the United States; the restriction of the acid Bessemer process to pig iron very low in phosphorus, usually to that containing less than o-10% of that element, has almost driven it out of Germany, has of late retarded, indeed almost stopped, the growth of its use in the United States, and has even caused it to be displaced at the great Duquesne works of the Carnegie Steel Company by the omnivorous basic open-hearth process, the use of which has increased very rapidly. Under most conditions the acid Bessemer process is the cheapest in cost of conversion, the basic Bessemer next, and the acid open hearth next, though the difference between them is not great. But the crucible process is very much more expensive than any of the others.

Until very lately the Bessemer process, in either its acid or its basic form, made all of the world's rail steel; but even for this work it has now begun to be displaced by the basic open-hearth process, partly because of the fast-increasing scarcity of ores which yield pig iron low enough in phosphorus for the acid Bessemer process, and partly because the increase in the speed of trains and in the loads on the individual engine- and car-wheels has made a demand for rails of a material better than Bessemer steel.

rrr. Iron founding, i.e. the manufacture of castings of cast iron, consists essentially in pouring the molten cast iron into moulds, and, as preparatory steps, melting the cast iron itself and preparing the moulds. These are usually made of sand containing enough clay to give it the needed coherence, but of late promising attempts have been made to use permanent iron moulds. In a very few places the molten cast iron as it issues from the blast furnace is cast directly in these moulds, but in general it is allowed to solidify in pigs, and then remelted either in cupola furnaces or in air furnaces. The cupola furnace (fig. 26) is a shaft much like a miniature blast furnace, filled from top to bottom by a column of lumps of coke and of iron. The blast of' air forced in through the tuyeres near the bottom of the furnace burns the coke there, and the intense heat thus caused melts away the surrounding iron, so that this column of coke and iron gradually descends; but it is kept at its full height by feeding more coke and iron at its top, until all the iron needed for the day's work has thus been charged. As the iron melts it runs out through a tap hole and spout at the bottom of the furnace, to be poured into the moulds by means of clay-lined ladles. The air furnace is a reverberatory furnace like that used for puddling (fig. 14), but larger, and in it the pigs of iron, lying on the bottom or hearth, are melted down by the fiame from the coal which burns in the firebox. The iron is then held molten

till it has grown hot enough for casting and till enough of its