Iron, METALLURGY

OF. Iron forms ab. th of the crust of the earth. Magnetic iron ore (lodestone) carries 72.4 per cent; it occurs in vast deposits in primary or Archæan rocks. Red hematite (specular ore) is an oxide without water, with 70 per cent of iron; it occurs in immense beds in the older geological formations, and is the principal ore of iron. Brown hematite, red ocher, yellow ocher, bog iron-ore, brown clay-iron stone, all impure oxides with 5 to 25 per cent of water and 50 to 65 of iron, frequently impure from admixture with clay, sand, etc., are found in the more recent geological formations. Siderite (spathic ore) is a carbonate, containing 48 percent of iron when pure; frequently impure, with clay or sand (clay-iron stone), or with bituminous matter like soft coal (black-band iron-stone). It occurs in metallic state in meteorites and in microscopic particles in some eruptive rocks. In commerce, iron is found in three distinct forms: (1) wrought iron and so-called soft steel; (2) steel proper; (3) cast-iron or pig-iron. The differences between these correspond closely with differences in the amount of carbon which they contain, as follows: 1. Wrought iron and soft steel. Soft and tough; the former fibrous. Tensile strength 40,000 to 60,000 lbs. per sq. inch. Malleable hot and cold. Not appreciably hardened by heating red-hot and chilling in water. Melt only at 1,600° C. or above. It is found by chemical analysis that iron with these properties contains le than 0.4 per cent of carbon. 2. Steel proper. Malleable hot, elastic cold, and brittle if hardened by quenching from a red-heat. Tensile strength 50,000 to 95,000 lbs. Fracture white and finely granular. Melts at 1,400° C. to 1,500°C. This contains 0.4 to 1.6 per cent of carbon chemically combined with the iron. See STEEL. 3. Cast-iron (called pig-iron in its unmanufactured state as it comes from the blast furnace). Not forgeable hot or cold. Melts at 1,075° C. to 1,300° C. Fracture granular, and either white or gray. White iron has all its carbon chemically combined, as in steel; gray iron has most of its carbon in free scales, as graphite. The former is the cheaper, is very hard, brittle, and used only for conversion into wrought iron: it usually carries considerable sulphur. The latter is soft, dark gray to black, rather tough, and used for castings; it is usually rich in silicon. White iron softens at a lower temperature, but runs thick; gray iron melts at a higher temperature, but is very fluid. The tensile strength of both these varieties is 20,000 to 30,000 lbs., and they carry a total amount of 2.3 to 5 per cent of carbon. When pure iron is heated in contact with carbon to a temperature of 1,800° C., it then quickly absorbs the carbon and pa es into cast-iron, which melts easily at that heat. This principle is of importance in understanding properly the following proce es.