IRON AND STEEL. Iron is the most useful metal of ma terial civilization; it means plant, tools, machinery. It is princi pally used when alloyed with other elements, notably carbon ; a moderate amount produces steel, an excess produces cast iron. Many special steels for particular uses have been devised in re cent years ; these steels contain a proportion of other metals, such as manganese or chromium (resulting in manganese steel, or chromium steel in these two cases, for instance). About 'co mil lion gross tons of pig iron and 135 million gross tons of steel were made in 1937—a fairly active year.
Outline of Manufacturing Process.—The source of commer cial iron and steel is the oxide ores of iron, occurring in great abun dance in all continents. This is charged into the top of a shaf t like blast furnace together with coke and some limestone. Hot air is blown into this furnace near the bottom, burning the coke and furnishing the gases and heat necessary to reduce the iron oxide to molten iron metal; trickling down over the white hot coke it ab sorbs an excess of carbon ; drained from the bottom is the impure high-carbon alloy pig iron, which is merely remelted and cast into moulds to form the familiar iron castings of commerce. Limestone is charged into the furnace to convert earthy minerals mixed with the iron ore (gangue) and the coke ash into a fusible slag; this is also drained periodically from the furnace hearth. The blast fur nace therefore operates continuously; solids are charged at the top, liquid iron and slag are drawn from the bottom.
To change pig iron into steel two methods are used extensively, the Bessemer converter and the Siemens-Martin open-hearth fur nace. The converter is a brick-lined vessel, with perforations in the bottom through which air is blown. Into this vessel some molten pig iron is charged, and the air blowing through the metal burns out the carbon and some of the other impurities; at these high temperatures there is a pronounced selective action so that relatively little of the iron is burned. Combustion of these impuri ties furnishes the necessary heat to maintain operations. At the end of the blow, a matter of ten to 15 min., the purified metal is dumped into a ladle, and an alloy of manganese and iron known as spiegel is added to degasify the melt, whereupon the finished steel is poured into ingot moulds. Refining pig iron in an open-hearth furnace• is a much more deliberate process, requiring about 15 hours. The furnace is a flat shallow basin with a low roof. At either end are ports through which fuel and hot air enter; the flame sweeps through, out the ports at the other end, and thence into chambers filled with checker brick to absorb some of the waste heat. At intervals the flow of gases is reversed by suitable valves thus returning the recovered heat to the furnace hearth. Scrap and pig iron are placed in the furnace through side doors, and melted down. Iron ore and limestone are also added, and a series of chemical reactions take place between the iron oxide and the impurities in the metal; the latter either vanish as gases or accum ulate in a scum of thin slag. When the reactions are complete the bath is tapped into a ladle from which the slag overflows, and the finished steel is then cast into ingots. Ingots are squat prisms of steel and are given marketable shape in the rolling mills. By suc cessively passing between rolls with steadily diminishing apertures, bulky ingots are converted into long rods, bars, plates, rails or various structural shapes. Rather thicker bars are known as billets and are used in forge shops for hammering out more intricate pieces of non-uniform cross section. Specially shaped rolls and auxiliary machinery also convert bars into sheet-steel, plate into pipe, and rods into wire products.
Antiquity of Iron.—Few implements of iron or steel survive for many years before they rust away, consequently there is little direct evidence to prove the point ; nevertheless the antiquity of iron smelting is great. It doubtless has been discovered and redis covered many times; explorers reaching primitive peoples in many parts of the world find the native blacksmith using methods very similar to those known to other tribes at far distant times and places. An iron blade, probably 5,000 years old, has been found in one of the Egyptian pyramids. Even without this discovery one could plausibly maintain that the ancient Egyptians must have had skilled steel workers in order to have built the great pyramids and other monumental architecture, to say nothing of the statuary and hieroglyphics cut into the hardest rocks. Steel working and hard ening, an advanced stage in the art which doubtless required cen turies to reach, was common 3,000 years ago in Greece, and is mentioned in Homer. It is more probable that iron was first found in the ashes of a big fire built near some red paint-rock than that the first tools were made from meteorites. When paint-rock and fire came to be associated with iron as cause and effect, the next step was to produce it intentionally in fires built against a bank exposed to prevailing winds, or in pits or rude rock furnaces where the fires were fanned by bellows, one of the earliest mechanical devices.
These are the primitive methods which survive to-day in the Catalan forge (q.v.) : a mixture of charcoal and iron ore, selected for its purity, is heated intensely for several hours, with fuel ad ditions made from time to time, and a vigorous blast of air fan ning the fire. The iron ore becomes an incandescent sponge of metal; the clay or other minerals mixed with the iron oxide to gether with the charcoal ash sinter into a slag, permeating the sponge and protecting the freshly reduced iron from further chem ical action. After a given time, the furnace is broken into, the glowing ball of iron pulled out, and immediately, while still white hot, hammered vigorously to expel as much of the slag as possible, and to weld the hot particles of metal into a coherent mass.
As the demand for iron grew, the furnaces became bigger and a stronger blast of air was required to drive the products of corn-. bustion from the bottom up through the mixed charge. It resulted that the iron ore was reduced to spongy metal in the upper part of the furnace, and it then proceeded to absorb more carbon as the charge settled down, thereby converting itself to pig iron, a much more readily fusible alloy. The bigger, taller furnaces introduced in the early 14th century therefore produced liquid pig iron, a complex alloy of iron, carbon and other elements. Subsequent developments have been along three main lines; (a) increasing the size of the furnace and developing mechanical auxiliaries to pro duce pig iron more economically and in greater amounts, (b) in vention of various processes for changing this impure pig iron into the more useful wrought iron, (c) production of steel either by way of wrought iron, or from pig iron direct. Sir Henry Besse mer's process, announced in 1856, for making steel from pig iron in a pneumatic converter, began the steel epoch. Cheap and am ple supplies of steels in a wealth of sizes, shapes and surface fin ishes, grades of hardness, or types of alloy, are now available to meet the special demands of metal workers, machinery builders, or tool users.
Modern metallurgical developments have consequently been away from the simple and direct production of wrought iron to the complex indirect manufacture of steel. The change from the small, intermittent operations controlled by a highly skilled arti san to the large continuous furnaces, machine driven, has been associated with enormous expansion in total output and as great a saving in labour, and a notable conservation of raw materials.