THE MANUFACTURE OF IRON AND STEEL The importance of a nation depends upon its agricultural resources, its fuel deposits, and its iron deposits. It is a dif ficult matter to determine which of these resources is the most important or which has contributed most largely to the advance of a country. Undoubtedly the great industrial predominance of the United States is clue to the fact that this country is rich in all three resources. It is, however, possible that industrial prominence depends more upon iron deposits than upon the other two factors, because the foundation of our present industrial structure is steel. Steel is the most important of all manufactured products, and the development of special grades is largely respon sible for the enormous amount of building construction, the great extension of railroads, and the great multiplication and expansion of industry that has occurred in recent years. Steel is a finished product of which iron is the raw material. The ores of iron are red hematite brown hematite, the limonite of the miner alogist (2 and 3 H magnetite and siderite (Fe Co,), these being mixed with more or less silica, clay, etc., besides containing a small percentage of manganese, phosphorus and sulphur.
To extract the metallic content from any ore, it is necessary to get rid of the impurities. With all metals this is done by melt ing the ore by intense heat and adding what is known to the metal lurgist as a flux. A flux is any mineral, usually lime, which unites with the impurities of the ore to form a liquid slag which floats 'upon the molten metal. The metal can then be drawn off from the bottom of the furnace, but is still in a more or less impure state and needs to be refined. This is the case with iron. Crude iron is made in very large circular vertical blast furnaces (see fig. 68), which are lined with refractory fire brick. In the blast furnace ore and limestone, which is used as a flux, together with the coke necessary for providing the intense heat, are raised to the top of the furnace by a hoist (A) and discharged into the hopper (B) and these materials fall into the hopper (D) at the top of the furnace by lowering the bell (C). When the bell (E)
is lowered the materials are dropped into the furnace. The two bells and hoppers are provided to prevent the escape of large volumes of gas from the top of the furnace. In order to provide sufficient air for combustion of the coke enormous volumes heated to 1,100 to 1,500 degrees F. are blown through a set of pipes called "tuyeres" near the bottom of the furnace at a pressure of 12 to 15 pounds per square inch. The burning coke melts the charge, producing intense local heat. About three-quarters of a pound of coke is used per pound of pig iron made. The air blast coming through the tuyeres is heated by paving it through "stoves" which are large cylindrical structures filled with a checker-work of fire brick. One blast furnace usually has three or four "stoves." After the chemical action is completed within the furnace the crude iron is drawn off into moulds called "pigs." Pig iron, however, contains impurities which must be burned away before a good quality of steel is produced. Of the im purities found in iron, graphite is unique, inasmuch as it is rarely found in other metals. It is present in the form of flakes or thin plates in sizes varying from microscopic proportions to approxi mately Y8 sq. in., disseminated throughout the body of the metal and forming an intimate mechanical mixture. It is necessary that the iron from which steel is to be made be low in sulphur and low in phosphorus, but both of these impurities are always pres ent and must be burned away. When sulphur is present in too great quantities in steel, the steel is rendered hot short, that is, when heated, on account of the presence of the sulphur the steel will bend or break. The amount- of sulphur present for good results should not exceed 0.06 percent. It is much better to keep the sulphur content below 0.04 percent, which is the generally accepted specification for open hearth steel. Phosphorus in creases the strength of steel but renders the metal cold short or brittle. For constructional purposes steel should be specified with phosphorus not to exceed 0.04 percent, which is the general specification- for open hearth steel.