Metals are good conductors of electricity and heat. They have high luster and density also the ability to be deformed under stress without cleaving. Alkali and Alkaline earth metals have low density, hardness, and melting points, they are extremely reactive, and are rarely encountered in their elemental, metallic form. Metals are obscure, shiny and lustrous. This is due to the fact that visible lightwaves are not readily transmitted through the bulk of their microstructure. The large number of free electrons in any typical metallic solid is accountable for the fact that they can never be classified as transparent materials.
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Some metals and metal alloys possess high structural strength per unit mass, making them useful resources for carrying large loads or defending against impact damage. Metal alloys can be engineered to have high resistance to cut, torque and deformation. Nevertheless the same metal can also be exposed to fatigue damage in the course of repeated use or from sudden stress failure when a load capacity is exceeded. The potency and pliability of metals has led to their frequent use in high-rise building and bridge construction, as well as most vehicles, many appliances, tools, pipes, non-illuminated signs and railroad tracks.
The two most commonly used structural metals are iron and aluminium. They are also the richest metals in the world.
Metals are good conductors, making them important in electrical appliances and for transmitting an electric current over a distance with little lost of energy. Electrical power grids depend on metal cables to dispense electricity.
The thermal conductivity of metal is useful for containers to heat materials over a flame. Metal is also used for heat sinke to protect responsive equipment from overheating.
The high reflectivity of some metals is vital in the construction of mirrors, including precision astronomical instruments. This last property can also make metallic jewelry aesthetically appealing.
Some metals have specialized uses; radioactive metals such as uranium and plutonium are used in nuclear power plants to produce energy by means of nuclear fission.
METALS USED IN CONSTRUCTION INDUSTRY
Metals provide a wide selection of uses in the built landscape, including structural features, such as nails and trusses, as well as decorative features, such as doorknobs and cladding. Metals discovered by early civilizations are still in use today. Scientific study has brought an enormous perceptive of the performance and boundaries of the different types of metals used in structures.
Aluminium is produced by smelting after the extraction of alumina from bauxite, a reddish-brown ore first mined in 1816 at Les Baux in France. The metal is ductile, non-magnetic and can be shaped by stamping, drawing, spinning, forging and extrusion, and it can be rolled into sheets and foil. It can be cast by all known foundry methods and joined by soldering, brazing, welding, adhesive bonding and by such mechanical methods as riveting and bolting. Aluminium are light weight, workability and versatility; its strength when alloyed with other metals; its heat reflectivity and electrical conductivity; and its resistance to corrosion. Aluminium can be mechanically buffed and textured, chemically cleaned, etched and brightened, with finishes ranging from dull to mirror. The main method of finishing is anodizing, an electrolytic process by which the metal is coated with a hard, protective layer of oxide, enhancing its corrosion resistance. Colour can be introduced into this layer by means of dyes, producing a characteristic effect quite unlike paint or lacquer finishes. Typical colours of anodized aluminium include silver, gold, bronze, grey and dead black, which is highly resistant to fading.
Iron has become a significant architectural building component. It is used in four common forms: wrought iron, cast iron, sheet iron, and steel.
Wrought iron was used for negligible structural and ornamental elements in the 18th century. Until the mid-19th century, the use of wrought iron in buildings was generally limited to small items such as tie rods, straps, nails, and hardware, or to decorative ironwork in balconies, railings fences and gates. Around 1850 its structural use became more widespread as iron mills began to roll rails, bulb-tees, and eventually I-beams. It was also used for decorative purposes, such as ornamental balconies or hardware. Since wrought iron is handmade, no two pieces are identical.
Cast iron was a major 19th century building material. Although brittle, it is remarkably strong in compression. It was often used for structural purposes, such as columns, building fronts, domes and light courts. Ornamental uses have included stairs, elevators, lintels, grilles, verandas, balconies, railings, fences, streetlights, and tombs. Today, cast iron is used for plumbing fixtures and piping in new construction, and its structural and decorative purpose is used occasionally through historic preservation practices.
Sheet iron can be exposed to rapid corrosion, forming rust. Sheet iron was used all through the 19th century, even though it is not clear how widespread sheet iron roofs became.
Steel was established to the construction industry at the end of the 19th century. The growth of structural steel in the mid-19th century allowed all buildings to be constructed. Builders and manufacturers used steel which was stronger than cast iron in compression and wrought iron in tension.
Decorative steels used in buildings include:
Stainless steel, a chromium-nickel steel, built up between 1903 and 1912. Its most essential property is its resistance to corrosion. It contains about 18% chromium and 8-12% nickel. Stainless steel is expensive.
Copper-bearing steels, containing from .15% to .25% copper, highly resistant to atmospheric corrosion, when compared to ordinary steel, by forming a protective oxide coating, having a uniform deep brown color and texture
The low melting point of lead permitted its use on a wide scale throughout human history. Water pipes were frequently constructed of lead, until its health hazards were publicized in the late 19th century.
Lead has been a popular roofing material for centuries, being used for roofing, flashing, gutters, downspouts, and conductor heads. Lead was best suited for low-pitched roofs, as steep roofs experienced creep. Lead roofs in regions with large temperature fluctuations. Beginning of the 19th century, a roofing material called “terne” or “terneplate” was used, consisting of sheet iron or sheet steel coated with a lead-tin alloy. It is regularly confused with tinplate.
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Lead was also commonly used for window cames in skylights and stained glass. It was also used for small bits of sculpture and garden ornamentation. Finally, lead was frequently added to paint, with red lead used as an anti-corrosive coloring for iron, and white lead used as paint for wooden houses. Lead-based paint was one of the most durable materials developed as a protective exterior coating. The use of lead paint has been limited on most structures, due to fears of lead poisoning
Lead is very malleable and resistant to corrosion it is extensively used in building construction, e.g., external coverings of roofing joints.
The main architectural uses of tin fall into two types, the alloy of tin with other metals such as copper to form bronze, and the coating of tin for harder metals, such as tinned iron or steel. Architectural bronze usually contains about 90% copper and 10% tin, even though the content may vary widely. The term “tin ceiling” is a misnomer, since these decorative plates were never preserved; they were almost always painted iron or steel.
Tinplate was a kind of architectural material consisting of sheet iron coated with tin. Tin roofs, “a kind of tinplate, was originally used for armor, but ultimately as roofing. Tin was also used for decoration, ornamentation, such as windows and door lintels. Although the tinplates are still available for roofing and flashing, they are widely regarded as expensive as the initial cost is more than that for common types of modern roofing such as asphalt shingles or built-up roofs. However, since a well-kept tinplate roof usually lasts several times longer than any of these types of roofing, it is advantageous if the cost in proportion to the longer span.
Although somewhat rare, there are nickel plating used for architectural details. Nickel is mostly used for building components in the form of alloys: nickel silver, Monel metal and stainless steel.
Nickel Silver was originally called “German Silver,” to World War I. It’s called “white brass”, but probably should be “nickel-brass,” because it generally contains 75% copper, 20% nickel, zinc and 5%. Different rates result in a variety of colors, including silver-white, yellow, light blue, green or pink. Architects and designers prefer new silver, because it could take and retain appropriate finish, and resistance to corrosion.
Monel metal is an alloy of two-third nickel and about one third copper. It is similar to platinum in color. Monel initiated many of the current uses of stainless steel. The first architectural use of Monel was the roofs of the Pennsylvania Railroad Terminal in New York City in 1909. Benefits include as a roofing material its ability to be soldered, welded, bonded or instead to provide a watertight, continuous cover.
THE MANUFACTURING PROCESS OF:
Aluminium production is realized in two phases: the Bayer process of refining the bauxite ore to attain aluminum oxide, and the Hall Héroult process of smelting the aluminum oxide to discharge pure aluminum
THE BEYER PROCESS
First, the bauxite ore is mechanically crushed and the crushed ore is mixed with caustic soda and processed in a mill to produce a slurry or an aqueous suspension with very fine particles of ore. The slurry is pumped into a digester, a tank that serves as a pressure cooker. The slurry is then heated to 230-520°F (110-270°C) under a pressure of 50 lb/in 2 (340 kPa). These conditions are preserved for a time varying from half an hour to quite a few hours. Extra caustic soda may be added to make sure that all aluminum-containing compounds are liquefied. The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process. The slurry is drained into a settling tank. As the slurry rests in the tank, impurities that do not dissolve in the caustic soda settle at the base of the container. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminum in the settling tank remains dissolved in the caustic soda. The residue (called “red mud”) that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.
After the impurities have settled out, the remaining liquid that looks somewhat like coffee, it is drained through a series of cloth filters. Any fine particles of impurities that remain in the result are captured by the filters. This material is washed so as to convalesce alumina and caustic soda that can be reused. The filtered liquid is then pumped through a series of six-story-tall precipitation tanks. Seed crystals of alumina hydrate are added through the top of each tank. The seed crystals rise as they settle through the liquid and dissolved alumina attached to them. The crystals precipitate (settle to the bottom of the tank) and are removed. After washing, they are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor moves a continuous stream of crystals into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) drives off the water molecules, leaving anhydrous (waterless) alumina crystals. After leaving the kiln, the crystals pass through a cooler.
THE HALL-HEROULT PROCESS
The smelting of aluminia into a metallic takes place in a reduction pot. The base of the reduction pot is being lined with carbon which acts as a conductor of an electric current of the system. The opposite electrodes are made of carbon rods suspended above the pot. Reduction pots are placed in rows containing 50-200 pots connected in series to form an electric circuit. Each point like can produce about 60,000-100,000 metric tons of aluminium every year. A smelting plant contains two to three point lines.
Inside that reduction pot is the aluminia crystals which is dissolved in molten cryolite at a temperature of 1.760 to 1.780 ° F (960-970 ° C)to form an electrolyte solution to conduct electricity from carbon rods that carbon-lined bed of the pot. A direct current of (4-6 volts and amps 100000-230000) is passed through the slolution. The resulting reaction breaks ties between aluminum and oxygen atoms in alumina molecules. The oxygen released is attracted to the carbon rods, which form carbon dioxide. The liberated aluminum atoms settle at the base of the pot as molten metal.
With more aluminia added to the cryolite solution to replace the decomposed compound, the smelting process is continuous. The heat gained b the flow of electricity at the base electrode keeps the content of the pot in a liquid form, forming a crust at the top of the molten electrolyte. Basically, the crust is being broken to allow more additional aluminia for processing. The pure molten aluminium accumulates at the base of the reduction pot and it is then drained off. The pots operate non-stop round the clock every day.
A metal pot has been moved down the potline, collected 9000 pounds (4,000 kg) of molten aluminum, which was 99.8% pure. The metal is moved to a holding furnace and then cast (cast in the form) as a mass of metal in a mould. One common technique is to cast molten aluminum into a long, horizontal shape. As the metal moves through the form, the outside is cooled with water, causing the aluminum to solidify. The solid shaft is evident from the far end of the form, where it is appropriate intervals sawed blocks of the desired length to form. As the melting process itself, this casting process is continuous.
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