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WEEK 4 PROCESSING of METALS: Specific Objectives: by the End of This Lesson the Students Should Be Able To: I

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WEEK 4 PROCESSING of METALS: Specific Objectives: by the End of This Lesson the Students Should Be Able To: I WEEK 4 PROCESSING OF METALS: Specific Objectives: By the end of this lesson the students should be able to: i. Briefly explain the smelting of iron in a blast furnace. ii. Name the different classifications of iron. iii. State the major difference between iron and steel. iv. Differentiate plain carbon steels from alloyed steels. v. Name different furnaces used in making metals. The basic raw material needed for the production of metals is their metallic ores. Thus, the basic raw material for production of iron and steels is the iron ore. The iron ore is a solid mineral usually dug out from the ground. It contains some unwanted elements (impurities) such as silicon, phosphorus, manganese, sulphur, etc which must be separated before the next grade of iron which is pig iron can be produced. This is done in a blast furnace through a process called smelting. Iron, the most common of all metals is found in large quantities in Itakpe and Ajaokuta areas in Kogi State of Nigeria. SMELTING OF IRON USING A BLAST FURNACE Smelting: Is the process of converting a metallic ore (in this case, iron ore) to its metal by heating with coke or carbon in the blast furnace to remove impurities. Pig iron is the material got after removing the impurities present in iron ore. It is produced from the iron ore using the device called blast furnace. The charge which include iron ore, coke and limestone are fed into the blast furnace through the hopper, and heated at the bottom. Air is pumped into the furnace to help burn the coke to produce heat which melt iron ore. As a result carbon monoxide is formed to react with iron ore to produce pig iron. On melting, the non-metallic part of the ore are combined with limestone which act as flux to form slag-which float to the top of the molten iron. The slag is drawn off the furnace before the molten iron is then cast into moulds and allowed to solidify into ingots. Note the following: Casting: Is the process of melting a metal and pouring it into a mould to take the shape of the mould when it cools down and becomes solid. While, Slag: Is the impurity gotten from a molten metal (e.g. molten iron). FORMS/CLASSIFICATIONS OF IRON: Based on the varying percentages of carbon, iron is broadly classified into the following: I. Pig iron:- This is the first grade of iron gotten after the smelting of iron ore in a blast furnace. It contains up to 5% carbon. This carbon content is quite high and it makes the pig iron (hard and brittle, and thus difficult to work with. Consequently, pig iron is not very useful in technology for fabrication purposes. However, pig iron serves as a raw material for the production of other forms of iron such as cast iron, wrought iron, steels etc. II. Wrought Iron: It is an alloy of iron with a very low carbon (0.1% to 0.25%) content. It has slag inclusions of up to 2% by weight which gives it a “grain”, resembling wood that is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile and easily welded. III. Wrought iron is directly made from iron ore using charcoal by the bloomery process or by simply reprocessing of cast iron or pig iron in a finery forge or reverberatory furnace (puddling furnace) by a process termed “puddling”. Puddling process involved reheating cast iron or pig iron in a puddling furnace and manually mixing air in the molten mass by stirring with a rod which decarburerizes the iron. The material produced this way can resist corrosion and had reasonably high tensile strength and it’s much more ductile and malleable than cast iron. Note, before mild steel become widely available, many items that are now made of mild steel were produced from wrought iron. These include rivets, nails, chains, gates, bolts, nuts, wires, rails, railway couplings, roof trusses, garden furniture etc. However, wrought iron is no longer produced on a commercial scale. So many products described as wrought iron today are actually made of mild steel. IV. Cast Iron: Is a hard, brittle, and non-malleable iron- carbon alloy that contains 2% to 4.5% carbon, along with varying amounts of silicon and manganese and traces of impurities such as sulfur and phosphorus. Unlike wrought iron, cast iron because of its brittleness cannot be worked either hot or cold but must be shaped by casting. Due to its high carbon content which makes it very brittle, cast iron can break easily if struck with a hammer. Cast iron is thus made by remelting pig iron along with scraps and alloying elements in cupola furnaces, whilst blowing air into the molten mass until the carbon content is reduced to between 2% and 4.5% and then recasting into already made moulds for producing a variety of products. There are two main types of cast iron namely; (a) grey cast iron (b) white cast iron but four types of cast iron exist in general. a. Grey Cast Iron: Grey cast iron is produced by heating pig iron to a liquid state and allowing it to cool gradually. Grey cast iron has 3% silicon and 2% carbon. This silicon causes the carbon in the grey cast iron to change into graphite causing the iron to have a dark grey or almost black colour, because of the graphite microstructure of grey cast iron, it has a good machinability and good resistances to wear and galling. b. White Cast Iron: White cast iron is produced by heating pig iron with less silicon than in grey cast iron to a molten state and allowing it to cool rapidly. With less silicon, the carbon in the white cast iron changes to cementite or iron carbide. This causes white fracture to form on the surface and thus gives the iron the name white cast iron, due to the carbides prevalence, white cast iron exhibit high compressive strength, hardness and good resistance to wear. c. Ductile Cast Iron: This is produced by adding small amounts of magnesium and cesium to the molten grey cast iron which help nodulates the graphite (make the graphite forms spherical shapes instead of the usual flakes), thereby resulting in high strength and high ductility than grey cast iron. d. Malleable Cast Iron: This type of cast iron is produced by a prolonged heat treatment of white cast iron to improve higher ductility. V. STEEL: Steel is the product obtained when the carbon content of pig iron is reduced to 1.5% or less. Steels are made using the Bessemer converter or the Open-hearth furnace depending on the type of raw material being used. In Bessemer converter, steel is produced from a molten pig iron, while an open-hearth furnace, solid pig iron is mixed with steel scraps and melted together and then molten pig iron is later added to the mixture to produced steel. Note that steel and pig iron are made up of iron and carbon but the main difference between two of them is their carbon content. Whereas pig iron contains 3% to 5% of carbon, steel contains less than 2%. Thus, to make steel from pig iron, this excess carbon in the pig iron is reduced by a process known as oxidation and other impurities burnt out. This means that carbon is made to combine with oxygen and is blown off as slag. Then the amount of carbon needed for a particular grade of steel is now added, and the steel is produced. Based on the carbon content of steel, there are three types of steel, they are: 1. Low or mild carbon steel- this type contains 0.05% to 0.3% carbon. 2. Medium carbon steel- this contains 0.3% to 0.6% carbon. 3. High carbon steel- this contains 0.6% to 1.5% carbon. Note that all the three types of carbon steels listed here above are generally referred to as plain carbon steels since they are solely made up of carbon and iron only. Alloy Steels: Generally speaking all steels are alloys since they contain both iron and carbon. But technically, alloy steel is a combination of plain carbon steel and one or more other element. That is to say that alloy steel contain at least one other element in addition to iron and carbon. Alloy steels and other special purpose steels are produced in an electric arc furnace. Some common examples of allow steels are: Stainless steel- Stainless steel is essentially comprised of iron, carbon and nickel or chromium. Stainless steels are resistant to corrosion and for that reason; they are used where staining is likely to occur if other steels are used. High-speed steels- They are made up of iron, carbon, tungsten and chromium. Some may contain cobalt and vanadium. High speed steels are hard and highly resistant to wear, and as such maintain the same degree of sharpness of their cutting edges even when hot. For this reason they are used as cutting edges in machines (e.g. as milling cutters, twist drills, taps, metal saws etc) where due to continuous cutting, friction is likely to produce excessive heat. High Tensile Steels- High tensile steels or known as nickel- chrome alloys are essentially comprised of iron, carbon, nickel and chromium. They are special constructional steels developed for making machine parts, which require high tensile strength as well as excellent toughness. For example, they are used for shafts, discs, rotors, connecting rods for aircrafts, and for automotive and mechanical engineering applications.
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