Metals (2.0) PDF - Past Paper

2.0 METALS  Metals metal, any of a class of substances characterized by high electrical and thermal conductivity, malleability, ductility, and high reflectivity of light. The most abundant varieties in the Earth’s crust are aluminum, iron, calcium, sodium, potassiu...

2.0 METALS  Metals metal, any of a class of substances characterized by high electrical and thermal conductivity, malleability, ductility, and high reflectivity of light. The most abundant varieties in the Earth’s crust are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority of metals are found in ores (mineral- bearing substances), but a few such as copper, gold, platinum, and silver frequently occur in the free state because they do not readily react with other elements.  Physical Properties of Metals good conductors of heat and electricity. Cooking utensils and irons are made up of metals as they are good conductors of heat. Ductile – can be pulled into wire Malleable – able to be pounded into sheets. Aluminium sheets are used in the manufacturing of Aircrafts because of their lightweight and strength. Metals are sonorous because they produces a deep or ringing sound when struck with another hard object. Solid at room temperature (with the exception of mercury) Usually shiny, with metallic luster High melting point High density (exceptions: lithium, potassium and sodium) Corrode in air or seawater Lose electrons in reactions  Chemical properties of Metals Reaction with water: Only highly reactive metals react with water and not all the metals. For example, Sodium reacts vigorously with water and oxygen and gives a large amount of heat in the process. Reaction with acids: Hydrogen gas is produced when metals react with acids. For example, when zinc reacts with hydrochloric acid it produces zinc chloride and hydrogen gas. Reaction with bases: Not all the metals react with bases and when they do react, they produce metal salts and hydrogen gas. When zinc reacts with strong sodium hydroxide it gives sodium zincate and hydrogen gas. Reaction with oxygen: Metal oxides are produced when metals burn in the presence of oxygen. These metal oxides are basic in nature. For example: When a magnesium strip is burned in the presence of oxygen it forms magnesium oxide and when magnesium oxide dissolves in water it forms magnesium hydroxide.  Extractive metallurgy mineral processing, art of treating crude ores and mineral products in order to separate the valuable minerals from the waste rock, or gangue. It is the first process that most ores undergo after mining in order to provide a more concentrated material for the procedures of extractive metallurgy. Following separation and concentration by mineral processing, metallic minerals are subjected to extractive metallurgy, in which their metallic elements are extracted from chemical compound form and refined of impurities. smelting, in which all the constituents of an ore or concentrate are completely melted and separated into two liquid layers, one containing the valuable metals and the other the waste rock.  Extractive metallurgy Extraction is often followed by refining, in which the level of impurities is brought lower or controlled by pyrometallurgical, electrolytic, or chemical means. Pyrometallurgical refining usually consists of the oxidizing of impurities in a high-temperature liquid bath. 2.1 Ferrous Metals  Ferrous metals Ferrous metals includes all forms of iron and steel. Iron in its various forms, including steel, is by far the most important of the metals used in the construction industry. Chemical composition and internal structure of ferrous metals are closely controlled during manufacturing. Therefore, strength and other mechanical properties can be determined with a high degree of reliability. People in the construction field have little control on the quality of iron or steel. Compared to concrete, of which are partially “manufactured” during installation at the construction site, there is little that can be done to improve or harm a ferrous metal product once it leaves the fabrication shop.  Historical Background Cast iron is the first metal as a structural material used on a 30m arch span bridge built in England in 1777-1779. A number of cast-iron bridges were built during 1780- 1820 mostly arch shaped with main girders consisting of individual cast iron pieces forming bars or trusses. Wrought iron began replacing cast iron soon after 1840, the earliest is the Brittania bridge in Wales built in 1846- 1850 made of wrought iron plates and angles. Since 1890, steel has replaced wrought iron as the principal metallic building material. Currently(1989) steels have yield stresses varying from 165 – 690 Mpa. Cast Iron Skillet Wrought Iron  Manufacture of Pig Iron a low grade of iron in a continuously operating furnace called a blast furnace. These furnaces are about 200 ft high and about 50 ft in diameter (see figure). Iron ore, coke, and limestone are loaded continuously at the top. Iron ore is an oxide of iron found in nature mixed with rock or soil called gangue. Coke is produced by heating coal to drive the impurities out. It then burns with greater heat than coal. Pig iron Manufacture of Pig Iron Burning the coke with a strong blast of hot air melt the iron ore and limestone at about 815°C. The heat melts the iron, frees it of oxygen, and forms carbon monoxide gas, which imparts carbon to the liquid iron. Melting permits separation of iron from the gangue, which combines with the molten limestone to form slag.  Manufacture of Pig Iron Iron is much heavier than slag, iron flows to the bottom of the furnace and molten slag floats on the iron. Iron is removed from a tap near the bottom and slag from a tap slightly higher. Liquid iron flows into molds and is allowed to solidify into shapes called pigs, or it is taken in a ladle while still liquid to be refined into steel or a better grade of iron. In either case, the product of the blast furnace is called pig iron. Pig iron contains about 4% carbon, 2 % silicon, 1 % manganese, and 0.05% sulfur. Pig iron is not useful for construction because it is weak and brittle, although it is very hard. To produce useful iron or steel, a second melting is needed for further purification.  Manufacture of Cast Iron Cast iron is a general term denoting ferrous metals composed primarily of iron, 2-4% carbon, and silicon, and shaped by being cast in a mold. They are too brittle to be shaped any other way. The brittleness is caused by the large amount of carbon present, which also increases strength. The cast-iron is manufactured by re-melting pig-iron with coke and limestone. This re-melting is done in a furnace known as the cupola furnace. It is more or less same as the blast furnace, but it is smaller in size. Its shape is cylindrical with diameter of about 1 m and height of about 5 m. Figure shows a typical cupola furnace. Chemical composition is controlled by the addition of scrap iron or steel of various kinds and of silicon and manganese as needed. The molten metal flows from the furnace to a ladle from which it is poured into molds to be formed into useful shapes. This operation is called casting. Manufacture of Cast-Iron:  White Cast Iron White cast-produced by rapid cooling of molten pig iron and carbon completely combined with the iron. A fractured surface appears bright white. The advantages of white iron over gray iron Hight strength of 275 Mpa (40 ksi) Very brittle Not easily machined Abrasion resistant Less resistant to corrosion Used in machinery such as crushers, grinders, chutes, and mixers where resistance to abrasion is critical. A532- 86% iron, 3.3% carbon, 4% nickel, 2.5% nickel White Cast Iron  Gray Cast Iron Gray cast – produced by slow cooling of molten pig iron. The most widely used type of iron, has a high carbon content and contains large numbers of graphite flakes. Properties of gray iron include low viscosity when molten (so that fairly intricate castings can be made), excellent machinability, high resistance to abrasion, and rather poor ductility and toughness Tensile strength of 150-400 Mpa ASTM A48 Class 40, 92-94% iron, 3.25-3.5% carbon, 2% silicon, 0.6% Manganese Gray Cast Iron  Ductile Cast Iron Ductile iron, also known as nodular iron or spheroidal graphite iron, is very similar to gray iron in composition, but during solidification the graphite nucleates as spherical particles (nodules) in ductile iron, rather than as flakes. The matrix phase surrounding these particles is either pearlite or ferrite, depending on heat treatment. Ductile iron is stronger and more shock resistant than gray iron, so although it is more expensive due to alloyants, it may be the preferred economical choice because a lighter casting can perform the same function. ASTM A536 86% Iron, 3.5% carbon, 2.2 % silicon, 0.5% magnesium, 0.20% Manganese.  Malleable Cast Iron Malleable cast iron is white cast iron that has been annealed. An annealing heat treatment transforms the brittle structure as the first cast into the malleable form. Therefore, its composition is similar to white cast iron, with slightly higher amounts of carbon and silicon. Malleable iron, like ductile iron, possesses considerable ductility and toughness. Like ductile iron, malleable iron also exhibits high resistance to corrosion and excellent machinability. Used for small castings requiring good tensile strength and the ability to flex without breaking (ductility). Uses include electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm  Manufacture of Wrought Iron Wrought iron is highly refined iron with slag deliberately incorporated but not in chemical union with the iron. The slag forms one-directional fibers uniformly distributed throughout the metal. puddling process, Method of converting pig iron into wrought iron by subjecting it to heat and frequent stirring in a furnace in the presence of oxidizing substances (see oxidation-reduction). Invented by Henry Cort in 1784 (superseding the finery process), it was the first method that allowed wrought iron to be produced on a large scale.  Puddling furnace Cross section of puddling furnace: A the grate ; B the hearth ; C a bridge to separate the fuel from the hearth; D door way through which iron is fed and retrieved by the puddler  Wrought Iron Wrought iron consists primary of iron with 1% to 2% slag, silicon, phosphorus and sulfur. To produce wrought iron, metalworking companies heat and bend or work iron multiple times. After heating the iron, a metalworking company will bend or work it using a hammer. Next, they’ll reheat the iron, followed by performing a second round of bending or working.  Cast Iron vs. Wrought Iron Cast iron is made through casting, whereas wrought iron is made by heating and bending or working iron multiple times. As a result, most metalworking companies will agree that cast iron is easier to produce than its wrought iron counterpart. Wrought iron is also stronger than cast iron. Each time wrought iron is heated and worked, it becomes a little stronger. Because of its strength, wrought iron is often used in commercial applications. Cast iron is harder than its counterpart. It’s able to resist deformation under pressure or stress with greater ease than wrought iron.  Pig Iron 4% C Oxidation using air blast White Cast Iron Wrought iron Cast Iron 2% Grey Cast Iron Carbon Steel 0.04%C C