Page:The New International Encyclopædia 1st ed. v. 10.djvu/884

IRON AND STEEL. pressure at from 25 pounds to 30 pounds per square inch. The converter lining is about one foot thick, and consists of a siliceous composition or stone in the acid process, and of dolomite or limestone in the basic process. The converter is so equipped that it can be rotated from a vertical to a horizontal position and back in either direction. In operation the molten pig iron is charged into the converter when it lies horizontal. When the molten metal is taken directly from the blast-furnace it is usually brought to the converter in ladles; but in case the iron is melted in cupola furnaces these are to placed that they discharge directly into the converter. As soon as the charge has been run into the converter it is turned into an upright position, the operation automatically turning on the blast. The blowing continues from seven to twelve minutes, and then the converter is turned upon its side and the recarburizing material in molten form is added. The charge is then ready for casting. This operation consists first in drawing the contents of the converter into a ladle, which is swung into position under the nose of the converter by a crane, the converter being tipped so as to empty. Sometimes the molds are set in a row around the perimeter of a circular pit, and the ladle is swung around in a circle by the crane to fill one mold after another; but more often, in American practice, the molds are mounted on little platform cars which are hauled past the ladle and filled one at a time, the same cars taking the filled molds to the rolling-mill, where they are stripped from the ingots. Practically all the mechanical operations, like the tilting of the converter and ladles, are performed by power.

The chemical reactions which take place in the converter differ according to whether the converter is acid- or basic- lined. In both cases the object is to burn away the silicon and carbon of the pig iron, and then to add carbon in the proper proportions to make steel. The reason for this seemingly inconsistent practice of first burning away the carbon and then adding the same material is as follows: Pig iron contains varying quantities of carbon, and to burn away just enough would necessitate a different length of the blowing for each charge of iron, thus introducing complications difficult to handle. It was Bessemer's original plan to do this, however, and it delayed the general introduction of the process until the remedy was found by Mushet. This consisted in burning out the impurities and then adding a definite amount of

carbon and manganese in the form of molten spiegeleisen or ferromanganese.

When air is blown through molten pig iron in a Bessemer converter, the first element affected is the silicon, and when the silicon is eliminated the carbon begins to burn and continues until there is only about 0.5 per cent. Up to the point where the carbon content has been reduced to 0.5 per cent., the reactions of the acid and of the basic processes are the same, but at this point the similarity ceases, for here the acid process ends, while the basic process begins its characteristic work of eliminating the phosphorus and sulphur. For practical purposes it may be assumed that neither the phosphorus nor sulphur contents of the original pig iron have altered at the time that the carbon content has reached 0.5 per cent. From that time on the phosphorus seizes the oxygen in the same way as the silicon and carbon had done before, the phosphoric acid immediately uniting with the lime which in the basic process is added to the metal at the beginning of the blow. The basic lining of the furnace is employed so that the lime may do its work without being affected by the lining material; were an acid lining used the silica would combine with the lime, thus rendering much of it incapable of doing useful work on the metal. The decarbonization, dephosphorization, etc., effected by the blowing process require to be supplemented by a further process before the final product of the converter is steel. This process is known as recarburization, and consists in adding carbon and manganese to the molten metal by the use of spiegeleisen or ferromanganese; the manganese promotes the removal of the sulphur with the slag. The amount of these materials to be added varies with the character of steel it is required to produce, and also with the process.

As has been stated, the process of steel-making which has just been described was the invention of Henry Bessemer, an Englishman. It developed from his efforts to produce a stronger metal than cast iron for the manufacture of ordnance. In 1854 James Nasmyth had patented a process for oxidizing the impurities of molten cast iron by introducing steam below the surface of the metal. Bessemer substituted air for steam, and patented the process in 1855. In his first experiments, Bessemer devoted his attention to the production of malleable iron, and such was his success that in 1856 he announced his work to the public. At this time, however, the process was far from perfect. Bessemer soon discovered that, while his process would remove silicon, manganese, and carbon from cast iron, it would not, when conducted in an acid-lined converter, remove phosphorus and sulphur. After some attempts to accomplish the removal of the last two elements, which met with poor success, Bessemer was compelled to revert to the use of iron so low in phosphorus and sulphur that it would make steel without any further diminution of these elements. Thus arose a classification of Bessemer pig, which still exists (see previous section on Cast Iron), and which means simply pig iron so low in phosphorus and sulphur that it may be made into steel without removal of these elements. The question of phosphorus being thus satisfactorily settled, Bessemer was confronted with the difficulty of so regulating the period of blowing that carbon should not be