stages. Previous to describing these stages it will be necessary to state briefly the manner of preparing the furnace for work. The basin for the charge is made of cast-iron plates. These are covered with a layer of oxidizing material a few inches deep, and the fire is then started and urged until the heat is intense enough to partly fuse this material and the fragments cohere. The sides of the basin are banked up with similar material. Slag is then shoveled in, and the pig iron to be treated is placed in this couch of slag. The lining of the basin lasts for many successive charges, with occasional repairs

as wear occurs.

The furnace having been charged, the door is closed, and rendered as nearly as possible airtight by banking with cinder. Heating is continued until the top of the pig iron is red hot, when it is turned by opening the door, which is afterwards closed until the iron melts, the workmen stirring up the mass with a rod at intervals to hasten the process. This completes the melting-down stage. One of the workmen next introduces a hooked bar and vigorously stirs the molten mass until its appearance to his skilled eye indicates that the silicon has been expelled. This completes the second or clearing stage. The next process is to reduce the temperature, and vigorously continue stirring until the metal and cinder become thoroughly mingled, part of the slag flows off, and the metallic iron and residue of slag form a porous cake in the melting basin. This completes the third stage, known as the boil. The final stage is to break up the porous cake, and form the separate parts into balls by manipulating them with a bar. These balls are taken from the furnace and hammered or squeezed to press out the slag, when the iron is ready to be manufactured. During these various stages of the puddling process the impurities of the cast iron (silicon, manganese, phosphorus, and carbon) are removed. viously stated, various forms of puddling furnace are used, and other variations in apparatus and methods employed; but the general process is always essentially as described.

As pre

The crude puddle ball which is drawn from the furnace is composed of innumerable globules of nearly pure iron, the interstices between which are filled with slag. Much of this slag is removed in squeezing, and each subsequent working removes a further quantity, but it is never all removed. The piece of iron made in the first rolling of the puddle ball is a rough, crude product known as muck bar. To make merchant iron several of these muck bars are bundled together into piles,' so as to give a bloom of proper sectional area, and this after being heated to a welding heat is rolled into the desired shape. See ROLLING-MILL.

ess, by the crucible process, by the acid Bessemer process, by the basic Bessemer process, by the acid open-hearth process, and by the basic open-hearth process. In the following paragraphs each of these processes will be described in outline.

Cement steel is made by placing a bar of soft, pure wrought iron in fine charcoal and exposing it to yellow heat. By a slow process called cementation the carbon penetrates the metal at the rate of about one-eighth inch every twentyfour hours. The process of cementation is carried on in large retorts which handle many tons of bars at one time, so that it will always happen that some parts of the furnace arrive at full heat much sooner than others, and remain longer at that temperature. The consequence is that it is necessary to break all the bars and grade the pieces by fracture according to their degree of carburization. Steel made in this way is commonly known as blister steel. Its use is limited by the fact that it always contains seams or pits of slag, which are present in the wrought iron. To avoid this trouble cement steel may be melted in a crucible out of contact with the air, and, being thus free from the slag, can be cast into ingots and hammered or rolled into any desired shape. This double process is expensive, and a cheaper and more common method of making crucible steel is to place powdered charcoal and crude bar iron in the crucible, the iron absorbing the carbon very rapidly in the molten state. This practice is almost universal in America. Sometimes pig iron and wrought iron are melted together, and in Sweden crucible steel is produced from pig iron and iron ore. Both blister steel and crucible steel belong to the general class known as high-carbon steel. Such steel can be made regularly in open-hearth furnaces; but so far this method, though cheaper, has not replaced the older methods. Blister and crucible steel are chiefly used for high-class edged tools, springs, etc.

BESSEMER PROCESS. The most common steelmaking process is the Bessemer process, which may be subdivided into the acid Bessemer and the basic Bessemer process. The apparatus used is the same for both the acid and the basic process, and the general process is the same up to a certain point. The chemical reactions differ substantially, however. Briefly described, the Bessemer process consists in charging molten pig iron into a vessel called a converter, forcing a blast of air through it until the silicon, manganese, and carbon are burned out, and restoring a definite portion of manganese and carbon by adding a recarburizing material. This is the process in skeleton; it divides itself for the purpose of a detailed consideration into the following divisions: Apparatus and mechanical manipulations and chemical reactions.

VOL. X.-50.

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 so 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

FIG. 2. CROSS-SECTION OF BESSEMER STEEL CONVERTER.

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, how ever, 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