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Cast iron bridges
The major use of cast iron for structural purposes began in the late1770 when Abraham Darby III built the Iron Bridge, although short beams had been used prior to the bridge, such as in the blast furnaces at Coalbrookdale. This was followed by others, and cast iron bridges became common as the Industrial Revolution gathered pace. Thomas Telford adopted the material for his bridge upstream at Buildwas, and then for a canal trough aqueduct at London- on - Tetn on the Shewsbury Canal. It was followed by the spectacular Chirk Aqueduct and the breath-taking, which remains in use following recent restorations. Cast iron beam bridges were used widely by the early railways, such as the Water street bridge at the Manchester terminus of the Liverpool and Manchester Railway. However, problems arose when such a bridge collapsed shortly after opening in 1846. The Dee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often in Wrought iron. The bridge had been under-designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. Nevertheless, cast iron continued to be used for structural support, until the Tya Rail Bridge disaster of 1879 created a crisis of confidence in the material. Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents.
Castings
For many purposes, the simplest process of producing metal articles is that of casting the molten metal into a suitable mold. The relatively low melting point of aluminum permits the use of a variety of casting processes that are not suitable for metals like iron and copper. The cheapest type of mold is one made of moist (“green”) sand, which is rammed around a wooden or metallic pattern. Where only a limited number of castings are to be made, or where the casting is very large or intricate, sand molds produce the cheapest castings. If very large numbers of the same castings are to be produced, and if the casting is not too large, a permanent mold (usually iron) may be used, because of the moderate casting temperatures employed, and may produce castings that are both cheaper and metallurgically superior. By rapidly forcing metal under pressure into a suitable permanent molds, “die castings” are produced. They have very high surface smoothness and dimensional accuracy. A special type of plaster used for molds produces castings with surface smoothness and dimensional accuracy comparable with those of die castings, but with somewhat lower mechanical properties.
Producing Iron and Steel
Iron ore. The more common impurities in iron ore is silica, titanium and phosphorus; the ores containing the smallest amounts of these impurities are the most valuable. Much silica and titanium are undesirable because they require extra fluxes to slag in the blast furnace, while phosphorus and sulpher are undesirable because of their adverse effect on iron and steels. The iron ores mined in Sweden are almost entirely free from phosphorus and sulphur, which explains the flame of Swedish iron and steel as highly purity metals.
Coke. – the heat required for melting ore in blast furnaces is obtained from the burning of coke. Coke is the residue left after certain coals have been heated in the absence of air. It is hard, brittle, and porous material containing from 85 to 90% carbon, together with some ash, sulphur, and phosphorus. The strength, brittleness and impurities in the coke are dependent both on the coal used and on the methods of manufacture. There are two ways of making coke. In the older, wasteful process of destructive distillation, it is made in beehive ovens with no by – products obtained distillation. In the newer process it is made in retorts, and many by-products, such as tar, ammonia, and benzol, are obtained from the distillate.
Scrap. – of all the materials used in the field of engineering, only the metals can be used over and over again. Other engineering materials, such as wood, glass, and concrete, become a liability when they have outlived their usefulness. However, the metals from discarded structures, such as boilers, bridges, ships, automobiles, etc. become valuable scrap. The need for scrap metals in the manufacture of both ferrous and nonferrous metals and alloys is one of the major problems facing the manufacturer, particularly in the steel industry where large quantities of selected and segregated scrap are needed. During ordinary periods of productivity, the difficulties of obtaining an amount of good scrap are not serious; however, it remains an important factor in the everyday operation of a steel mill.
Scrap metals receive varying treatments from the melter. A common practice is to place it in the belly of a furnace where it is fused and blended with some primary or virgin metal and then refined and cast into some useful product. Difficulties of controlling the exact composition of the final heat of the metal may be largely due to the improper segregation of the scrap charged into the furnace. With our continuing practice of using the metals over and over again, ultimately all of the scrap used will contain elements that may be undesirable in some finished product.