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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 presence of tetracalcium aluminoferrite has to be kept down to about 1 percent. Aluminous cement, made by processing a mixture of bauxite and chalk or limestone, is a very special cement type, used primarily where extremely fast hardening is required, or where heat resistance is a desirable quality (as in flue linings). Composition and strength of concrete

    Concrete is produced by mixing cement, water, fine aggregate (sand), and coarse aggregate (gravel). The mixture is then placed in forms, compacted thoroughly, and allowed to harden. Typically, three-quarters of the volume of hardened concrete is occupied by the aggregate, the maximum size depending on the size of the concrete member and on the steel reinforcement in it. Commonly, a maximum aggregate size of 20 or 40 mm (o.75 or 1.5 in) is used.

    Compaction of the fresh concrete is essential because the strength of hardened concrete depends on the volume of air-voids within it as well as on the water-cement ratio of the cement paste.

    Reinforced and prestressed concrete

    The tensile strength of concrete is relatively low, so concrete structures are designed to exploit the good compressive strength properties of the material, and steel reinforcement is placed where it is necessary for structural members to resist tensile forces. This is called reinforced concrete. The steel reinforcement is bonded to the

    surrounding concrete so that stress is transferred between the two materials. In a further development the steel is stretched before the development of bond between it and the surrounding concrete. When the force that produces the strength is released, the concrete becomes pre-compressed in the part of the structural member that is normally the tensile zone under load. The application of loads when the structure is in service reduces the pre-compression, but generally tensile cracking is avoided. Such concrete is known as prestressed concrete.

    Durability and versatility

    Some concrete structures have satisfactorily stood up for more than a century. However, Portland cement paste is attacked by acids, sulfates and some other salts. Contact with some of these has to be avoided. Contact with others requires the use of particularly well-compacted concrete with a very low water-cement ratio so that the percolation of the attacking medium into the interior of concrete is prevented. Frost can also damage concrete, but this can be prevented by adding an air-entraining agent to the mix.

    Concrete can be formed to obtain any design shape. It can be mixed near the construction site, or it can be mixed at a central plant and then transported by special agitator trucks if the operation can be completed within about 90 minutes. Such concrete is known as ready-mixed.

    Copper

    The chemical element copper is reddish metal at the head of group IB in the periodic table. Its symbol is Cu; atomic number 29; and atomic weight 63.546. Copper follows the first transitional series of elements.

    Copper was the first metal used by humans and is second only to iron in its utility through the ages. The name is derived from the Latin cuprum, copper, from the

    earlier Latin Cyprium, Cyprian metal. The discovery of the metal dates from

    prehistoric times, and it is estimated that copper was first used about 5000 BC or ever earlier.

    Natural occurrence and extraction

    In Roman times much of the copper was obtained from the island of Cyprus, as the name implies. Copper today is mined in many parts of the world, the largest producers at present being Chile, Peru, Poland, the United States, Zaire, and Zambia. More than 160 minerals containing copper are known. Copper constitutes 70 parts per million of the Earths crust and is present to the extent of 0.020-0.001 parts per million in seawater.

    Copper in its native state is often so pure that it requires only melting with a flux to produce lake copper, which for many years was the world standard for pure copper. About 80% of all copper mined today, however, is derived from low-grade ores containing 2% or less of the element.

    Half of the worlds copper deposits are in the form of chalcopyrite ore. All-important copper-bearing ores fall into two main classes: oxidized ores and sulfide ores. Sulfide ores are more important commercially. Ores are removed either by open-pit or by underground mining. Ores containing as little as 0.4% copper can be mined profitably in open-pit mining, but underground mining is profitable only if an ore contains

0.7%-6% copper.

    The oxidized ores, such as cuprite and tenorite, can be reduced directly to metallic copper by heating with carbon in a furnace, but the sulfide ores, such as chalcopyrite and chalcocite, require a more complex treatment in which low-grade ores have to be enriched before smelting begins. This involves the ore-flotation process, in which the ore is crushed and powdered before it is agitated with water containing a foaming agent and an agent to make the copper-bearing particles water-repellent. These particles accumulate in the froth on the surface of the flotation tank, and this froth is skimmed off and heated to about 800 ? to remove some of the water as well as

    antimony, arsenic, and sulfur, which are also present. The residue is then mixed with silica and melted in a furnace at 1400-1500 ?. This produces two liquid layers: a

    lower layer of copper matte (cuprous sulfide mixed with iron sulfide and oxides), and an upper layer of silicate slag, which is drawn off. Silica or siliceous copper ore is added to the liquid matte in a converter, and air under pressure is blown through the liquid.

    Upon removal of the iron slag, the copper sulfide that remains is reduced to copper by heating in a controlled amount of air. The remaining molten copper, which is 98%-99% pure, is either cast into blocks of blister copper or into anodes. The final stage of purification is mainly by electrolytic refining, which yields copper of 99.95%-99.97% purity. The impure copper is made the anode of an electrolytic cell that contains pure strips of copper as the cathode and an electrolyte of aqueous copper (?) sulfate. During electrolysis, copper is transferred from the anode to the cathode. An anode sludge containing silver and gold is produced during this process, and this increases its economic feasibility.

    Physical and chemical properties

    Eleven isotopes of copper are known, two of which are not radioactive and occur with a natural abundance of 69.09% and 30.91%, respectively. Copper melts at 1083.4 ?

    plus or minus 0.2 ? (in a vacuum), boils at 2567 ?, and has a density of 8.96 at

    20 ?. The element has a hardness of 3, takes on a bright metallic luster, has a cubic crystal structure, and is malleable, ductile, and a good conductor of heat and electricity (second only to silver in electrical conductivity).

    Copper exhibits oxidation states of +2 (the most common, forming Cu(?)

    compounds, and +1 Cu(?), stable only in aqueous solution if part of a stable complex ion. A few compounds of copper(?) are also known. Although the electronic

    configuration of copper is formally similar to that of the alkali metals (Group IA) in general and potassium in particular, the behavior of copper is considerably different from that of the alkali metals. The shielding of the outer electron from the attraction of the nucleus is stronger in these than in copper. Thus the outer electron in copper is more tightly bound, resulting in a comparatively high first ionization potential and a relatively small ionic radius for copper.

    The outstanding feature of copper and the other metals of Group IB (gold and silver) is their resistance to chemical attack. Copper is slowly attacked by moist air, and its surface gradually becomes covered with the characteristic green patina that consists of basic sulfate (verdigris). At about 300? copper is attacked by air or oxygen, and a

black coating of copper(?) oxide forms at the surface. At a temperature of 1000?

    copper(?) oxide is formed instead. The metal is attacked by sulfur vapor, with the formation of copper(?) sulfide, and by the halogens, which form copper(?) halides,

    except iodine, which forms copper(?) iodide. Copper is not attacked by water or

    steam, and dilute non-oxidizing acids, such as dilute hydrochloric and dilute sulfuric acids, have no effect in the absence of an oxidizing agent. The metals is attacked by boiling concentrated hydrochloric acid with the evolution of hydrogen, by hot concentrated sulfuric acid, and by dilute or concentrated nitric acid. Alloys of copper

    Copper mixes well with many elements, and more than 1000 different alloys have been formed, several of which are technologically significant. The presence of the other element or elements can modify the hot or cold machining properties, tensile strength, corrosion fatigue, and wear resistance of the copper. It is also possible to create alloys of pleasing colors.

    The best-known alloy of copper is brass, which consists of copper containing between 5% and 40% zinc. It possesses a high tensile strength, hardness, and wear-resistance. The addition of 0.5%-3% lead to a brass alloy (leaded brass) improves the machinability of brass, and brass containing 30%-40% zinc and 1% tin (tin brass) has a high corrosion-resistance.

    Another useful alloy of copper is nickel silver, which consists of copper (55-65%), nickel (10-18%), and zinc (17-27%). It is used as a base for silver-plating items such as costume jewelry and tableware.

    Phosphor bronze is formed by the addition of up to 0.35% of phosphorus to copper-tin alloys containing up to 10% tin. This alloy has great resilience, fatigue endurance, hardness, and corrosion resistance; these properties make it suitable for use in springs and diaphragms.

    Silicon bronze, consisting of 1-3% silicon, 95-96% copper, and small amounts of other metals-for example lead, tin, zinc, manganese, iron, or nickelis as strong as

    mild steel and has a high resistance to corrosion. It is used in the production of equipment for chemical plants in which corrosive liquids are handled. Aluminum-copper bronzes contain aluminum (5-12%) and sometimes zinc and silicon; they are also corrosion resistant, and have good strength, hardness, and wear resistance. They are used for carrying corrosive liquids such as hot brine in salt refineries.

    Beryllium-copper alloys, containing 2% beryllium, have a high corrosion resistance and high tensile strength, with considerable fatigue and wear resistance. They find wide application where high strength is required and for making non-spark-forming tools.

    Bronze

    Bronze is among the oldest artificially produced alloys. It was of such importance to technological development that the term Bronze Age was coined by archaeologists to characterize the period following the Neolithic when weapons and tools began to be constructed of bronze in a particular area.

    In its narrowest definition, bronze is an alloy of copper and tin, with or without small

    proportions of other elements such as zinc and phosphorus. Certain copper-base alloys containing more manganese, iron, lead, or zinc than tin are also regarded as bronzes. Even some alloys that contain no tin are considered bronzes in modern usage, including aluminum bronze (copper-aluminum), silicon bronze (copper-silicon), and beryllium bronze (copper-beryllium). Some copper-base alloys that are actually brasses have been given bronze trade names, such as architectural bronze (57% copper, 40% zinc, 3% lead) and commercial bronze (90% copper, 10% zinc). Despite the broadened definition, tin is still the principal addition used in most bronzes. Copper-tin alloys are important because of their strength, wear-resistance, and corrosion-resistance in a saltwater environment. The alloys in this system that are most useful from an engineering standpoint are those with less than 20% tin, although other elements are often added to give the best properties for certain applications. Copper-tin bronzes may be categorized readily according to their composition, which affects their machinability. Alloys with up to 8% tin are used mainly for cold-worked applications, such as sheets, wire, and coins; those with 8 to 12% tin are used mainly for gears, bearings, and marine hardware. Bearings are made largely from the 12-to-20% tin-copper alloys, and bells are the principal product made from copper-tin alloys with from 20 to 25% tin. Alloys in this latter group are very hard and extremely brittle compared with other types.

    Beryllium bronzes are in demand for parts that must have good formability in the soft condition, along with high fatigue and yield strengths. They are also used for their resistance to creep in the hardened condition, as in springs. Beryllium bronzes are also used to advantage in firing pins, dies, non-sparking tools, and other hard parts required wearing well when used with hardened steel.

    In general, the substitution of zinc for tin in bronze results in an improvement in workability and the sacrifice of a measure of strength for hardness. When both strength and hardness are essential, it may be desirable to limit the use of zinc. Brass

    Brass is an alloy consisting mainly of copper (over 50%) and zinc, to which smaller

    amounts of other elements may be added. Elements such as tin, lead, and aluminum are added to copper in making brasses, depending upon the color, strength, machinability, corrosion resistance, and ductility desired. The mechanical properties, the tensile strength, and ductility of alloys in the copper-zinc system improve as the zinc content increases (up to 35%).

    The earliest brass, composed only of copper and zinc, was made by the Romans about

    th20 BC, and was later used to make some of their coins. By the 11 century, it was

    being widely produced in Western Europe. Brasses are important partly because they are cheaper than unalloyed copper. In addition, they are more susceptible (up to about 30% zinc) to the important machining process of cold forming.

    Types and machinability

    Some brasses are more susceptible to certain machining processes than others, and these differences result in a wide variety of compositions adaptable to particular uses. Cartridge brass, consisting of 70% copper, is most popular for operations such as cold drawing (successively reducing with dies the diameter of wires, pipes, and tubes) and

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