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10-MATERIALS

By Don Morales,2014-09-02 08:23
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10-MATERIALS

    MATERIALS

    IRON

    In its various compounds, iron is the fourth most abundant element in the Earths crust.

    The molten core of the Earth is primarily elemental iron. Iron occasionally occurs naturally in its pure or uncombined form, but is abundant in combination with other elements, such as oxides, sulfides, carbonates, and silicates. Iron ores are naturally occurring compounds of iron from which the metal can easily be recovered in significant quantity. Iron pyrite is a yellow, crystalline mineral called fools gold

    because of its gold-like appearance.

    Physical properties

    In its pure form, iron is rather soft and is malleable and ductile at room temperature. It melts at 1535 ? and boils at 3000 ?. Pure iron can exist in two structural types, or

    +++++allotropic forms (Fe and Fe). At room temperature the iron atoms are arranged in

    a body centered cubic lattice called the a-form, which is transformed at 910 ? into a

    cubic, close packed structure called the gamma-form. At 1390 ? iron returns to a

    body centered cubic structure, called the delta-form.

    Iron at room temperature exhibits ferromagnetism, a strong magnetic behavior that the metal may retain even in the absence of an external, applied magnetic field. When iron is heated to 768 ?, it loses this property and exhibits paramagnetism, a weaker attraction to an applied magnetic field. Between 768 and 910 ?, iron is said to be in

    its beta-form, which is not a different allotropic form of iron.

    Although pure iron does conduct electricity, compared to other metals used for that purpose, it is a relatively poor conductor.

    Chemical properties

    Easily oxidized, iron reacts directly with most common nonmetallic elements,

    +++++forming compounds in which iron is in the Fe or Fe oxidation state. At high

    temperatures, iron also absorbs hydrogen and nitrogen and forms phosphides, carbides, and silicides. In the absence of water, and under various conditions, iron reacts with oxygen. Finely divided iron burns in air once it is ignited. Larger pieces of iron react with oxygen in dry air at temperatures above 150 ? to form mixed oxides. At

    temperatures above 575 ? and at low concentrations of oxygen, iron oxide (FeOx) is formed.

    Corrosion

    Perhaps the most important chemical reaction of iron, at least from an economic standpoint, is the least desirable one. The reaction of iron, water, and oxygen to form hydrated iron oxide (rust). The corrosion of iron has been studied carefully, and the formation of rust is known to be an electrochemical reaction.

    For rust to form at room temperature, three components in addition to iron must be present: oxygen, an electrical potential difference and an electrolyte (an ionic

    substance dissolved in water, i.e. a liquid which is electrically conductive). Iron that is partially immersed in salt or fresh water usually rusts more rapidly than does iron that is totally immersed. This is due to the low level of oxygen in the water. In the atmosphere, the formation of rust begins when the relative humidity exceeds 50%, at

    a temperature of 20 ? (lower temperatures will retard the process). The presence of air pollutants, particularly the oxides of sulfur, greatly increases the corrosion rate. In the presence of air and water, sulfur dioxide forms sulfurous acid (and even sulfuric acid) that attacks and oxidizes the iron. Because rust on the surface of iron is porous the iron below the surface layer of rust also reacts.

    Iron alloys

    Iron is abundant and easily obtainable from its ores. Its desirable mechanical and magnetic properties, as well as its resistance to corrosion, may be improved by mixing iron with other elements, frequently metals, to form alloys.

    Perhaps the most important alloy of iron is steel, which contains up to approx. 2%

    carbon. Steels that contain about 0.25% carbon are called mild steels; those with about 0.45% carbon are medium steels; and those with 0.60% to 2% carbon are high-carbon steels. Within this range, the greater the carbon content, the greater the tensile strength of the steel. The hardness of steel may be substantially increased by heating the metal until it is red-hot and then quickly cooling it, a process known as quench hardening. An important component of many steels are cementite, a carbon-iron compound. Mild steels are ductile and are fabricated into sheets, wire, or pipe; the harder medium steels are used to make structural steel. High-carbon steels, which are extremely hard and brittle, are used in tools and cutting instruments. At carbon contents below that of steel is wrought iron, which is nearly pure iron. Because of its low carbon content (usually below 0.035%) it is forge able and no brittle. Iron of high carbon content (3-4%), obtained when pig iron is remelted and cooled, is called cast iron. If cast iron is cooled quickly, hard but brittle white cast iron is formed. If cooled slowly, soft but tough gray cast iron is formed. Because it expands during the cooling process, cast iron is used in moulds.

    The addition of other materials in alloys, for example, manganese or silicon, also increases the hardness of steel. The inclusion of tungsten permits high-speed drills and cutting tools to remain hard even when used at high temperatures. The inclusion of chromium and nickel improves the corrosion resistance of the steel and, within certain limits of composition, is called stainless steel. A common stainless steel contains 0.15% carbon, 18% chromium, and 8% nickel and is used in cooking utensils and food-processing equipment. The inclusion of silicon, ranging from 1 to 5%, results in an alloy that is hard and highly magnetic. An alloy with cobalt is used for permanent magnets.

    Stainless steel

    Stainless steels are iron-based alloys that have a very high resistance to corrosion because of their chromium content, which is greater than that found in other types of steel. Nickel, molybdenum, and other elements are also used, in addition to chromium, to produce stainless steels with a great range of properties.

    The chromium in the alloy reacts with oxygen, forming a layer of chromium oxide on the stainless steel surface. It is this chromium oxide layer, which provides the resistance to corrosion. Stainless steel with such a chromium oxide layer present on its surface is designated passivated. Removal of this chromium oxide layer will activate

    the stainless steel surface and render it prone to corrosion attacks.

The most widely manufactured stainless steel is the alloy known as 18-8, containing

    18% chromium, 8% nickel, and 0.15% carbon. This alloy can be formed into precisely detailed, complicated shapes and is commonly used for making flatware, cooking utensils, and plumbing fixtures and for a variety of processing and manufacturing machinery and equipment.

    The method of hardening the alloy provides a major distinction between stainless steel formulations. The chromium steels that are classed as Ferritic alloys cannot be hardened by heat but are strengthened as they are formed, or worked. The chromium alloys called Martensitic, in general have higher proportions of carbon than do ferritic alloys, may achieve great strength through heat hardening. The Austenitic group of stainless steels, chromium-nickel alloys such as 18-8, is hardened by cold working.

    Stainless steels are melted almost exclusively in the electric-arc furnace and are semifinished in slab, rod, or tube form. Finished steels are available in many different forms like, e.g. plate, sheet, strip, bar, wire, or tubing. Many stainless steel products are produced by casting.

    Because of their corrosion resistance, stainless steels do (normally) not require protective coatings, and the polish produced by a final finishing is extremely durable. Aluminum

    Aluminum is the third most abundant element in the earths crust, exceeded only by

    oxygen and silicon. Because of its strong affinity to oxygen, aluminum never occurs as a metal in nature but is found only in the form of its compounds, such as alumina.

    This strong affinity to oxygen also explains why it withstood all attempts to prepare it

    thin its elemental form until well into the 19 century.

    Physical properties

    Aluminum (Al) is a silvery-white metal in Group IIIA of the periodic table. Its atomic number is 13, its atomic weight 26.9815. It is ductile, nonmagnetic, and an excellent 3conductor of heat and electricity. The density of aluminum at 20 ? is 2.6999 g/cm.

    It melts at 660.24 ? and boils at 2450?.

    Aluminum is widely used in many kinds of products because a combination of properties gives it special advantages over other materials.

    Lightness and strength

    Perhaps the best-known quality of aluminum is its lightweight. Its specific gravity is only 2.7. It is thus only about one-third as dense as iron, copper, or zinc. Despite its lightweight, it can easily be made strong enough to replace heavier and more costly metals in numerous applications.

    Aluminum alloys have the highest strength-to-weight characteristics of any commercial metal. The strength of the various aluminum alloys increases, with little change in ductility, as temperatures drop into the cryogenic range. At temperatures as high as 200 ?, certain alloys remain remarkably strong.

    Resistance to corrosion

    Aluminum and its various alloys are highly resistant to corrosion. When exposed to air, the metal develops a thin film of aluminum oxide (AlO) almost immediately. 23

    The reaction then slows, however, because the film seals off oxygen, preventing further oxidation or chemical reaction. The film is colorless, tough, and non-flaking.

    Few chemicals can dissolve it. Some aluminum alloys are better suited to certain corrosive environments than others. Many different alloys are available to meet the requirements of specific corrosive environments.

    Electrical and thermal conductivity

    Aluminums electrical alloy has the highest conductivity per weight unit of any commercially sold conductor. Because aluminum is only one third as dense as copper, it supplies about twice the conductivity per weight unit. For this reason more than 90% of the electrical transmission and distribution lines in f.i. the United States is made from aluminum. Aluminum is an excellent conductor of heat as well. It is about 1.8 times as thermally conductive as copper by weight (depending on the alloy) and about 9 times as conductive as stainless steel. For this reason it is widely used in automobile radiators, cooling coils and fins, heat exchangers in the chemical, petroleum, and other industries and heater fins in baseboard and other types of heaters.

    Reflectivity and emissivity

    Aluminum is an excellent reflector of all forms of radiated energy. This characteristic is commonly put to work in building insulation, including roofing materials. Because it reflects about 90% of radiated heat, aluminum is effective at keeping heat out or in. aluminum foil can also be used to jam radar by reflecting it.

    Aluminum gives off as radiant energy only a small percentage (about 7%) of the heat that it does take on. It gives up most of its heat by conduction and convection. This characteristic, known as low emissivity, is especially valuable in installations where aluminum reflects most of any radiant energy rather than absorbing it and radiating it into the interior.

    Workability

    Aluminum has a face-centered cubic crystalline structure similar to that of tin and gold. As a consequence, it is very workable. Aluminum is as weldable as steel, at twice the rate of steel. It is also highly amenable to brazing, soldering, and cold welding.

    The aluminum industry

    The aluminum industry, founded in 1854, is the newest of the nonferrous metal industries. In the United States, commercial production commenced in 1859 at a cost of USD 17 per pound, thus putting aluminum in the class of precious metals. Not until the late 1880s was a method found to bring prices down and permit aluminum to be used in a wide range of applications. The aluminum industry is now worldwide. Bauxite, the source of aluminum

    Most aluminum produced today is made from bauxite. First discovered in 1821 near Les Baux, France (from which its name is derived), bauxite is an ore rich in hydrated aluminum oxides, formed by the weathering of such siliceous aluminous rocks as feldspars, nepheline, and clays. During weathering the silicates are decomposed and leached out, leaving behind a residue of ores rich in alumina, iron oxide, titanium oxide, and some silica. In general, economically attractive ores contain at least 45% alumina and no more than 5% to 6% silica.

    Most of the large bauxite deposits are found in tropical and subtropical climates,

    where heavy rainfall, warm temperatures, and good drainage combine to encourage the weathering process. Because bauxite is always found at or near the surface, it is mined by open-pit methods. It is then crushed if necessary, screened, dried, milled, and shipped for processing. Countries like, e.g. Guinea, Brazil, Jamaica, Russia, Suriname, lead the world in bauxite production.

    Assuming that 4 tons of bauxite is required to produce 1 ton of aluminum, bauxite reserves known today are large enough to supply the world with aluminum for several hundred years at current production levels. Other ores that might be used when the high-grade bauxite deposits are exhausted include kaolin, anorthosite, and alunite. Although proof of the existence of aluminum as a metal did not exist until the 1800s, clays containing the metallic element were used in Iraq as long ago as 5300 BC to manufacture high-quality pottery. Certain other aluminum compounds such as the alums were used widely by Egyptians and Babylonians as early as 2000 BC. Despite these early uses of the metal of clay, however, it was almost 4000 years

    before the metal was freed from its compounds, which made it a commercially usable metal.

    Aluminum alloys

    Aluminum alloys are generally divided into two basic types, casting alloys and wrought alloys. Aluminum casting alloys most frequently contain silicon, magnesium, copper, zinc, or nickel, alone or in various combinations. Silicon improves the fluidity and castability of molten aluminum. Copper and zinc harden the alloy and increase its strength, magnesium improves corrosion resistance, strength, and machinability and nickel improves dimensional stability and high-temperature strength. The mechanical properties of aluminum casting alloys vary not only with composition but also as a function of casting conditions and subsequent heat treatment, if any. Heat-treated alloys are generally stronger and more ductile than others.

    Wrought alloys are alloys that have been mechanically worked after casting. Working operations include forging, rolling, drawing, and extruding. Alloying elements (magnesium, silicon, copper, and others) usually make pure aluminum stronger and harder but also render it less ductile and more difficult to fabricate. Working and heat treatments change these alloys structure, which in turn determines their corrosion

    resistance and mechanical properties.

    Wrought alloys are divided into two basic classes: non-heat-treatable and heat-treatable alloys. The former rely on the hardening effect of such alloying elements as manganese, silicon, iron, and magnesium for their initial strength. They are further strengthened by various degrees of cold working. Heat-treatable alloys, which contain elements such as copper, magnesium, zinc, and silicon, are strengthened by heat treatment and artificial aging, but they may also be cold worked after an initial heat treatment.

    Cement and concrete

    Cement is a material with adhesive and cohesive properties that make it capable of bonding mineral fragments into a compact whole. The cement most commonly used in civil engineering and building is Portland cement. Concrete is the compact whole achieved by bonding fine and coarse aggregate particles with cement paste, which is a

mixture of cement and water.

    Manufacture of cement

    Portland cement is usually made from a calcareous material, such as limestone or chalk, and from alumina- and silica-bearing material, such as clay or shale. The manufacturing process essentially consists of grinding the raw materials, mixing them intimately in specified proportions, and burning in a large rotary kiln at a temperature of approximately 1350 ?, when the material sinters and partially fuses into balls known as clinker. The clinker is cooled and ground to a fine powder, and gypsum is added to control the speed of setting when the cement is mixed with water. Composition and hydration of cement

    The main compounds in Portland cement and their typical percentage content are as follows:

    Tricalcium silicate: 3CaO.SiO 55% 23

    Dicalcium silicate: 2CaO.SiO 25% 23

    Tricalcium aluminate: 3CaO.AlO 10% 23

    Tetracalcium aluminoferrite: 4CaO.AlO.FeO 8% 2323

    In addition, some trace compounds are present.

    The chemical reaction of the two silicates with water produces calcium silicate hydrates and calcium hydroxide. These hydrates make the largest contribution to the strength of the hydrated cement paste. Tricalcium aluminate also forms a hydrate, but it contributes little to the strength of the paste. Moreover, the reaction of hydration of this compound is so rapid that it has to be controlled by gypsum. The presence of tricalcium aluminate is, however, advantageous in the process of burning in the kiln. Tetracalcium aluminoferrite is not particularly important except that it contributes to the characteristic gray color of Portland cement. If white cement is desired, the