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ETC 1250 Lesson 05a - Lesson 5a Introduction ETC125O...

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Unformatted text preview: Lesson 5a Introduction ETC125O ' Properties Of Materials All forms of iron and steel are included in the term ferrous metals. . Iron and Steel Ferrous metals are by far the most important ofthe — Chapter # 5 metals used in the construction industry. — P95 229 ' 242 They are manufactured to meet a wide variety of specifications for various uses. Strength and other mechanical properties can be determined with a high degree of reliability. Ferrous products are fabricated in shops to desired size and shape. - The finished products are ordinarily delivered to a construction site ready to be installed. Structure and Composition Structure and Composition Steel is crystalline - Crystals form during cooling to a solid state Properties of steel can be manipulated by: Crystals combine to fom1 an internal structure called grain - Grain is similar to aggregates held together with - Changing the internal structure cement, however the bond is atomic and not — Changing crystalline structure through cohesive. changing the heating l cooling rate - Stronger bonds = stronger tensile strength. — Changing grain structure through the method of imparting the final shape. - Changing chemical composition Structure and Composition Structure and Composition Changing chemical composition Sdinlll ding-n maul-u M an pMIhn Element Common Content Effects Changing crystalline Structure Carbon Up to 0.90% Increases hardness, tensile strength, and responsiveness ' Changing heating if cooling to heat treatment with corresponding increases in . stIength and hardness. rate / Increases hardness and brittleness; over 1.2%, causes loss _ of malleability. Heat Treatment is the Manganese Imparts strength and resoor'siveness to heat treatment; process of heating ferrous / / Swat: hardness, unifon'riity 0f Intemal grain metals u P to S pec itic Up to 2.50% Same general effects as manganese. tem Peratures and 300“” Q at l Up to 0.050% Maintained below this contentto retain malleability at high specific rates and CYCIES to temperatures, which is reduced with increased content. achieve desired changes in " v DDS'M: to 3.0% Improves machinaoility. the molecular (crystalline) ' Phosphorus Up to 0.05% Increases strength and corrosion resistance, but is structura owned-"mm“:- maintained below this content to retain malleaoility and weldability at room temperature. Structure and Composition Changing grain structure. - Any elongation and alignment of grains in one direction will increase the strength of the metal to resist stresses in that direction. Casting : little to no alignment Rolling : slight alignment depending on the extent ofrolling Extrusion = considerable alignment in the direction ofthe extrusion Forging = considerable alignment following the shape of the finished product Production of Ferrous Metals Pig Iron is further refined into Iron products - Gray cast iron - White cast iron . Malleable cast iron - Wrought iron Meta! Si P S Cast Steel 0.2-0.7 0.05 0.05 Gray cast iron 1.1-2.8 0.15 0.1 White cast iron 0.5-2.0 0.1!! 0.1 Malleable cast 0.6-1.3 0.15 0.1 iron Wrought iron 0.075-0.15 0.10-0.15 0.006-0.015 Pig iron 1 4 0.1-2.0 0.04-0.10 Production of Ferrous Metals White cast iron Contains its carbon completely combined with the iron A fractured surface appears bright white. The advantages of white iron over gray iron are: — It is harder and — More resistant to wear from abrasion. The disadvantages of white iron over gray iron are: — It is more difficult to machine, — Less resistant to corrosion, — More brittle, and more difficult to cast. White iron is used in machinery such as crushers, grinders. chutes, and mixers where " resistance to abrasion is critical. Production of Ferrous Metals Pig Iron - Iron ore, coke, and limestone introduced to blast fumace. + - Iron Ore, an oxide of iron found in nature mixed with impurities such A‘ as rock or soil. man Coke, a product made by heating coal to drive impurities out. — Coke is burned with an assist of a blast of hot air. Umestone, a natural rock. — When ore melts, the impurities mix with the limestone to fomi slag. — Slag will float to tire top and is skimmed ofi, sometimes used as aggregate. Pig iron, a low grade of iron which is weak and brilfie, butvery hard, — settles to the bottom and is drained otT. Production of Ferrous Metals Gray cast iron The most widely used type of iron Has a high carbon content Contains large numbers of graphite flakes. (The flakes give a gray appearance to a fractured surface.) Properties of gray iron include: — Low viscosity when molten (sothat fairly intricate castings can be made), — Excellent machinability, — High resistanceto abrasion. and — Rather poor ductility and toughness. Production of Ferrous Metals Chiiied iron castings By controlling chemical composition and cooling rate. — Castings with cores of gray iron and surfaces ofwhite iron can be made. — These are called chilled iron castings. - Why would you want to produce Chilled Iron Castings? Production of Ferrous Metals Malleable cast iron - Consists of white iron made tough and ductile by annealing 7 Which consists of heating to about 16DO°F (870°C), holding that temperature for a time, and cooling very slowly to about 1275°F (710°C), over several days. — During the entire process, carbon is precipitated from the solution as small lumps in the metal until there is no combined carbon. Brittleness is eliminated by removal of carbon from solution Machinability is improved by the carbon lumps Malleable iron is used for pipe fittings. guardrail fittings, and other items which require machining and which are subject to shock loads. Production of Ferrous Metals Wrought iron - The fibrous structure of wrought iron resulted in a material with different mechanical properties parallel and perpendicular to the fibers — Tensile strength is 10 to 15% lower across the fibers than parallel — Ductility is only about 20% as much — Shearing strength across the fibers is much greater than shearing strength along the fibers — A rod, twisted until itfails in torsion, comes apart along the axis, separating between fibers — Wrought iron has excellent corrosion resistance which is greater on faces that have been rolled than on sheared or machined laces Production of Ferrous Metals 5:32;: Steel mama" “Durham — Iron is again melted With limestone to reduce impurities Steel — Has a carbon content of less than 2% — Alloying elements are added to improve the properties Steel compared to iron — is stronger, 7 more ductile and — malleable, and — resists shock better Element Production of Ferrous Metals Wrought iron - Was made by refining pig iron in a furnace in a manner similar to the refining of steel — The iron silicate slag is melted, and the relatively pure molten iron is poured into the slag A pasty mixture of the two is formed, with the slag evenly distributed as individual particles "he mixture forms a semisolid ball, which is dumped out and pressed into a rectangular bloc k, squeezing the excess slag out 'n the process "he block is rolledtothe desired shape, aligning the slag as strings or ribbons in the direction of rolling It is then readily shaped further by drawing, bending, orforging, and can be made into thin, intricate shapes It is easily welded and machined. Production of Ferrous Metals Wrought iron - Used — Wrought iron pipe was used extensively where corrosion resistance is needed — Wrought iron was also used extensively for ornamental ironwork, as well as for — miscellaneous ironwork where corrosion resistance, mac hinability, or ductility and malleability were needed Production of Ferrous Metals Steel — Alloying Elements Amount Effect NUI’TIiI'IUlTI Variable Promotes small grain size and uniformity of internal grain stmcture in the ascast metal or during heat treatment Cop per Up to 0.25% Increases strength and corrosion resistance. Lead 0.15% to 0.35% Improves machinability without detrimental effect on mechanical properties. 0.50% to 1.50% In alloy steels, increases responsiveness to heat treatment and nardenablity. In heat—resisting steels. causes retention of mechanical properties at high temperatures. Increases corrosion resistance and hardness. 4.0% to 12% Over 12% 1.0% to 4.0% In alloy steels, increases strength, toughness, and impact resistance. In stainless steels, improves performance at elevated temperatures and prevents work hardening. Up to 27.0% Production of Ferrous Metals Steel — Alloying Elements Element Amount En'eot Molybdenum 0.10% to 0.40% In alloy steels, increases toughness and hardenability Up to 4.0% In stainless steels, increases corrosion resistance and strength retention at high temperatures Tungsten 17% to 20% In tool steels, promotes hardness at high cutting temperatures; In stainless steels, smaller amounts assure strength retention at high temperatures Vanadium 0.15% to 0.20% Promotes small grain size and uniformity of grain structure dunng heat treatment; improves resistance to thermal fatigue and shock Tellurium Up to 0.05% Improves machinability when added to leaded steels. Titanium Variable Prevents loss of effective chromium through carbide precipitation in "133" stainless steels. Cobalt 17.0% to 36.0% Increases magnetic properties of alloy steels. In smaller amounts, promotes strength at high temperatures in heat resisting steels. Manufacture of Steel Shapes Rolling - Compressing and shaping an ingot into a useful shape by squeezing it through a succession of rollers - Awide variety of cross-sections can be rolled in long pieces by means of specially shaped rollers. — Flat sheets can be rolled by rollers of a constant diameter. — Corrugated sheets can be rolled from flat sheets by using corrugate. — Structural steel shapes - Hot rolling usually precedes cold rolling until the steel is close to its final shape. - Hot rolling is usually the first step in reducing the size of an ingot prior to extruding, drawing, or forging. Manufacture of Steel Shapes Drawing - pulling steel through a small die to form wire or a small rod of round, square, oval, or other cross section. - Steel is hot rolled to form a rod of a size not much larger than the shape to be drawn. - Seamless steel pipe may be finished by cold drawing over a round, bullet-shaped mandrel to form a hollow center and through a die to form the outside. - The advantages of cold drawing are: — a smootherfinish, — more accurate size, — more strength, and — better machinability. Manufacture of Steel Shapes Hot working Plastic condition at a temperature of about 2000°F — Breaks up coarse grains — Increases density by closingtiny air holes and forcing the grains closer together. Cord working Solid condition at room temperature Elongates grains in the direction of the steel elongation. 7 Increases strength and hardness — Decreases ductility Results in more accurately finished products, because there is no cooling shrinkage to be estimated. The surfaces are smoother, because oxide scale does not form as it does during hot working. Manufacture of Steel Shapes Extrusion forcing a billet of hot, plastic steel through a die of the desired shape to produce a continuous length of material of reduced cross section in the shape of the die. More intricate shapes can be formed by extrusion than by rolling The surface is of higher quality than by rolling. An extrusion is made in one operation rather than repetitive operations as in the case of rolling. Manufacture of Steel Shapes Forging deforming steel by pressure or blows into a desired shape The forging may be made from an ingot or from a rolled shape The steel is usually heated to a semisolid state at a temperature over 2000°F It is forced to fill the shape between dies by pressure or blows of the upper die upon the lower one Forging is preferred if strength of the part is important. — produces a stronger, — more ductile, — more uniform product — with smaller grain size than is produced by casting. Heat Treatment Heat treatment Heating, then holding the metal at the high temperature. and then cooling. The metal is held at the upper temperature so that it can be heated to a uniform temperature throughout. The rate of cooling is very important. Various heat treatment techniques can produce: — Increased uniformity of structure — Stress reduction — Increased hardness — Increased strength — Increased ductility Types of Heat Treatment Quenching Heating the steel to a temperature of about 1500“F and cooling very rapidly in oil, water, or brine. Increases hardness and strength Reduces ductility and toughness. Residual stresses are introduced by quenching, and should be relieved by tempering. Tempering Reheating the quenched steel to a temperature of 300 to 1200°F and cooling in air to: — Reduce the residual stresses — Increase ductility. Heating to the lower end of the temperature range produces: — Greater hardness — Strength — Wear Resistance Heating to the higher end of the temperature range produces: — Greater Toughness Types of Heat Treatment Normalizing Heating the steel to a temperature of about 1500 F or higher, depending on the type of metal, and cooling several hundred degrees slowly in air. Increases Uniformity in sthcture Annealing Heating the steel to a temperature slightly lower than for normalizing and cooling it several hundred degrees very slowly, usually in a furnace. Methods vary somewhat depending on the purpose, which may be to — soften the metal, — produce a special structure, — facilitate machining, — facilitate cold shaping, or — reduce stresses. Assignment Page 263 Review Questions 1, 4, 7, & 8 If you do not understand a question, email me at [email protected] or through myPHCC.edu ...
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