Page:The New International Encyclopædia 1st ed. v. 10.djvu/888

IRON AND STEEL. 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 heat and adding iron ore in successive small doses, thus forming silica and oxide of manganese, which go into the slag, and carbonic oxide, which escapes with the flame. To determine when this process has proceeded far enough, samples of the molten metal are taken at intervals, cast into iron bars and broken; the carbon content is estimated by the appearance of the fracture, an expert being thus able to determine its amount with much accuracy. When the desired point of carbon content has been reached, as determined by the test, the recarburizer is added in a solid state. This recarburizer is ferromanganese with a very large excess of manganese. In the basic process the problem is the melting and decarburization of the charge, as in the acid process just described, with the additional duty of removing a reasonable quantity of the phosphorus. This is accomplished 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 case were an acid lining used which would take up a portion of the lime.

Recarburization 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 impracticable to stop the operation at any intermediate point. All the carbon content of Bessemer steel has, therefore, to be supplied by the recarburizer, and absolutely perfect homogeneity of product can be secured only by absolutely perfect mixing of the molten metal and the recarburizer. This perfect mixing increases in difficulty as the amount of carbon required in the steel increases. In the open-hearth process the elimination of the carbon can be stopped at any desired point, so that very little carbon is added in the recarburizer, and the necessity of thorough mixing is less imperative. As a result it is generally considered that high carbon steel or hard steel can be produced with a more uniform quality by the open hearth than by the Bessemer process.

An important modification of the open hearth which has been described, and which is intermittent in operation, is the Talbot continuous open-hearth process, now in use in some American steel-works and being installed in several others here and abroad. Mr. Talbot's process consists essentially in working the furnace continuously by tapping off a portion of the molten charge at short intervals, immediately charging an equivalent of pig iron, and again tapping. Several advantages in increased output, economy of maintenance, wider range, etc., are claimed for the process.

The development of the open-hearth process of steel-making was the outgrowth of numerous attempts by inventors to repeat the success of Bessemer. Not much success was realized in these

efforts, however, until 1862, when W. Siemens, a German, applied the regenerative furnace, which he had invented in 1857, to the manufacture of steel. It was not until 1868, when Siemens succeeded in making steel from old iron rails, that the success of the process was fully demonstrated. Meanwhile, P. and E. Martin, of Sireul, France, had succeeded in making steel from a mixture of pig iron and scrap in a Siemens furnace. Thus originated the Siemens-Martin process or open-hearth process of steel-making. At present, this process ranks second in importance only to the Bessemer process. The Bessemer process is practically without rival for the production of steel rails, but the open-hearth process leads in the production of structural steel, ship's plates, and steel for castings.

At first both the Bessemer and the open-hearth process were employed only with acid-lined furnaces, the basic process being a subsequent development. The practical invention of the basic process was due to Sidney G. Thomas and P. C. Gilchrist, and was first made public in 1878. 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 enormous deposits of iron ores high in phosphorus, which had previously been excluded from use in the two great steel-making processes of the world, were rendered available to the steelmakers.

The physical properties of iron and steel which are chiefly useful to the engineer and manufacturer are strength, hardness, weldability, ductility, malleability, elasticity, and homogeneity. With the exception of weldability, all of these properties are exhibited to some extent by every piece of iron or steel. The relative degrees in which these different properties exist in different kinds of iron and steel vary greatly, however. This is a familiar fact to all, and does not need proof. The variation in these properties in different kinds of iron and steel is due partly to variation in the relative amounts of the contained chemical elements, partly to the physical structure, and partly to the method and amount of working to which the metal has been subjected to attain its final useful form. Commercially, iron and steel may be divided into the following general classes: Wrought iron, soft steel, medium steel, hard steel, cast steel, hard cast steel, cast iron, malleable cast iron. The chief physical properties of each of these classes of iron and steel will be referred to after a brief statement of the destructive effects produced by the different chemical elements upon the physical properties of iron and steel generally. The effects of carbon on iron and steel are more pronounced and useful than those of any other known chemical element; it constitutes about 4 per cent. of cast iron, and from ½ to 1½ per cent. of steel, and is nearly entirely absent from wrought iron. The effects of increasing the carbon element in iron or steel are to increase the hardness, strength, and fusibility, and to decrease the ductility, malleability, and weldability. The effect of