Page:The New International Encyclopædia 1st ed. v. 10.djvu/883

IRON AND STEEL. 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 air-tight 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. As previously 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.

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.

. As has been noted, steel was manufactured at a very early date in the history of civilization. Prior to 1856, however, it was produced in comparatively small amounts, and its use was restricted to the production of cutlery and tools. In 1856 Henry Bessemer made known the process for making steel in large quantities which revolutionized the iron trade of the world. This was followed by the inventions of Siemens, Martin, and Thomas, which gave a further impetus to steel manufacture, and widely extended the use of that material. At present steel is made by the cementation process, 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 twenty- four 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.

. The most common steel-making 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.

The central feature of the plant for making Bessemer steel is the converter. This is a pear-shaped or jug-shaped vessel of steel, lined with a refractory material. Fig. 2 shows a modern American Bessemer converter in cross-section. The vessel is mounted on a horizontal axis, consisting of two hollow gudgeons, through which the air-blast enters the bottom of the converter. An automatic valve shuts off the air when the converter is turned on its side, and admits it when the converter is upright. The blast is furnished by a blowing engine which keeps the