7 per cent. of the whole, though this percentage may vary considerably. Cast iron melts at about 1200° C., and when cold is hard and brittle. It is not malleable or ductile, and cannot be tempered. Of the several elements named, carbon and silicon are of value in steel-making, which is the chief purpose for which cast iron is produced at the present time; but sulphur and phosphorus are generally highly objectionable. In using the term cast iron here, the chemical nomenclature has been retained. The foundryman generally gives the name pig iron to the direct product of the blast-furnace, and the term cast iron to the form which pig iron assumes after remelting. For commercial purposes pig iron is graded into several classes, according to the appearance of the fracture. This is an unscientific and unsatisfactory method of grading, but it is the one almost always adopted. The different grades are given different names, and these names vary in character and significance in different countries. In the United States the more common grades are: No. 1 Foundry, No. 2 Foundry, No. 3 Foundry, No. 1 Soft, No. 2 Soft, Silver Gray, Gray Forge, Mottled, and White. These grades vary in chemical composition, and it is being urged by many prominent metallurgists that chemical composition be substituted for the appearance of the fracture in distinguishing between different grades of iron.
Wrought Iron. The iron made by the ancients, and, indeed, by all peoples up to 1350, when cast iron was first produced in the German blast-furnaces, was wrought iron, and it was produced from the ore in a single operation. Today wrought iron is nearly always obtained by treating pig or cast iron made in the blast-furnace, and the process is said to be ‘indirect,’ as compared with the ‘direct’ process of the ancients. The direct process is still employed by all savage races who make iron, and is also in use where the character of the ore, the fuel, or other conditions make the adoption of the blast-furnace impracticable. A few forms of the direct process will be described here. The ancients employed small hearths, the fuel used being charcoal and the necessary draught being obtained either by means of rude bellows or by arranging the hearth at the top of a gully or channel in such a manner as to take advantage of the prevailing wind. Such hearths are now used in Africa and some parts of India by the natives. A modification of the hearth process once extensively used in Southern Europe, but now extinct, was the Catalan process, the name being derived from the Province of Catalonia, in Spain, where the process was probably first employed. This process is chiefly notable because of the form of water blower used, which was called a trompe. See Blowing Machines.
A more important hearth process was the American bloomery process, much resembling the Catalan process, but showing an advance in the use of more modern blowing machines with hot blast, and in the provision of means for cooling the furnace walls by water. The three processes described were hearth processes. A more common form of the direct process consists in the use of blast-furnaces. These furnaces are insignificant in size, as compared with modern cast-iron blast-furnaces: the smaller ones are from 4½ to 6 feet in height, while the largest seldom exceed 10 feet in height. These furnaces are extensively used by the natives of India, who produce an excellent quality of iron in them. The two modern forms of the direct process are the retort process and the reverberatory furnace process. In the retort process, of which there are several forms, the ore, reduced to a line division, is placed in the heating chamber with charcoal or other carbonaceous matter, and is heated either by the external fire or by means of gaseous fuel, the products of combination of which are made to pass through the charge. In the reverberatory-furnace process fine ore and coke are reduced in a reverberatory furnace by gaseous fuel. In all of these processes thus far mentioned the direct product of the furnace is a spongy mass of metallic iron and slag, which has to be squeezed or hammered to remove the bulk of the cinder.
The preceding brief review of the direct process of producing wrought iron may be summed up by saying that at present there is no direct process known which is capable of competing for a lengthened period and on a broad scale with the indirect process. The manufacture of wrought iron from cast iron by this process is accomplished by purification. This further purification is always carried out by means of oxidation, though the details of the process vary according to whether the necessary oxygen is supplied chiefly from the atmosphere or from other materials added for the purpose, and also as to whether the iron to be purified is heated in a separate furnace or chamber from that in which the fuel is burned, or in direct contact with the fuel. The furnaces used may be divided into two classes—(1) hearths and (2) reverberatory furnaces. In hearths the fuel is burned in direct contact with the iron, and the chief source of oxygen is the atmosphere; in the reverberatory furnace the oxygen is obtained from special oxidizing materials added for the purpose, and the fuel is burned in a chamber separate from, but communicating with, the chamber in which the charge is placed. With this outline of the methods of producing wrought iron from cast iron, attention will be turned to the process which is chiefly employed.
The chief impurities to be removed from cast iron to make it into wrought iron are silicon, manganese, phosphorus, and carbon. To remove these by the puddling process, a special puddling furnace is employed. These furnaces are of various forms, but the ordinary form is a single-bedded reverberatory furnace. Briefly described, such a furnace consists of a combination chamber of oblong shape. At the front of this chamber are the grate-bar, to the rear of which rises a vertical wall, and back of this, at a higher level than the grate-bars, is a shallow receptacle for the charge. A common roof covers the two chambers. This roof is horizontal over the grate-bars, but curves downward as it extends backward until at the flue-entrance it descends below the level of the charge. There are two doors at the side, one for feeding the fire and the other to give access to the charge. Structurally the furnace consists of cast-iron plates and firebrick. In operation the combustion of the fuel takes place on the grate-bars, and the hot gases and flames rise to the roof and are beaten back or reverberated upon the charge as they move toward the flue. Generally two men work at a furnace. The puddling process consists of four
