Chapter 2


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Chapter 2

  2. 2. • Most widely used structural metals due to their wide range of mechanical, physical and chemical properties. • Contains iron as their base metal. • Ferrous alloys are produced as o Sheet steels for automobiles and containers o Plates for boilers, ships and bridges o Structural members – I-beam, bar products, crankshafts, railroad rails. o Gears, tools, dies and molds o Fasteners – bolts, rivets and nuts
  3. 3.  Iron-carbon alloy consist 0.005% C called pure iron.  Characteristics : soft, ductile, low strength  Used in : magnetic device and enameling steels.  Invention of blast furnace made production of iron in large quantities.  3 basic materials used in iron production : ◦ Iron ore ◦ Limestone ◦ Coke  Is one of the most abundant element in world, making up about 5% of the earth’s crust ( in the form of various ores)  4 compositions of iron ores :  Magnetite  Hematite ( an iron-oxide mineral)  Limonite (an iron-oxide containing water)  Siderite (Carbonate iron)
  4. 4. Magnetide  Category : Oxide mineral  Colour : black, gray with brownish tint in reflected sun  Hardness : 5 – 6 Hematite  Category : oxide mineral  Colour : metallic gray, dull to bright red  Hardness : 5.5 – 6.5 Limonite  Category : amorphous, mineraloid  Colour : varoius shades of brown and yellow  Hardness : 4 – 5.5 Siderite ( carbonate iron)  Category : carbonte mineral  Colour : yellow, gray, brown, black and sometimes nearly colourless  Hardness : 3.75 – 4.25
  5. 5.  Iron ore processing ◦ Crushing into fine particles ◦ Removal of impurities by various means ( such as magnetic separation) ◦ Pellets, balls or briquettes formation by using water and binders. ( some iron-rich ores used directly without pelletizing)
  6. 6.  Obtained from a soft coal rich in volatile hydrocarbons and tarry matter that are heated in vertical oven and then cooled with water  Functions of coke : ◦ Generating the high level of heat required for the chemical reactions in iron making to take place. ◦ Producing carbon monoxide ( a reducing gas to removes oxygen) which is used to reduce iron oxide to iron.
  7. 7.  Function of limestone: ◦ Remove impurities from the molten iron ◦ Reacts chemically with impurities, acting as flux (flow as fluid) that causes the impurities to melt at a low temperature. ◦ Combine with impurities to form slag, which is light, floats over the molten metal and is removed
  8. 8.  Iron ore, limestone and coke added into the blast furnace. (charging process)  The charge mixture is melted in a reaction at 1650˚C, with the air preheated (to produce sufficient high temperature) to about 1100 ˚C and blasted into furnace through nozzles (tuyeres)  Oxygen reacts with carbon to produce carbon monoxide.  Produced carbon monoxide reacts with iron oxide and reduces it to iron.  The molten metal accumulates at the bottom of the blast furnace, while the impurities floats to the top of the metal.  The molten metal in tapped into ladle cars.  The molten metal at this stage is called pig iron or hot metal.  Composition of pig iron : 4%C, 1.5%Si, 1%Mn, 0.04%S, 0.4%P and the rest is pure iron.
  9. 9.  an alloy of iron  Carbon content between 0.002% and 2.1% by weight  Used in : buildings, infrastructure, tools, ships, automobiles, machines and weapons  Steel can be produced by  Basic Oxygen Furnace  Electric Arc Furnace  Vacuum Furnace
  10. 10.  Is the fastest steel making process  Processes in BOF : a) Molten pig iron and scrap charged into vessel b) Pure oxygen in blown into furnace through lance(long tube) c) Fluxing agents (lime) are added through a chute d) Oxygen refines the molten metal by an oxidation process in which iron oxide is produced e) The oxide reacts with the carbon in the molten metal, producing carbon monoxide and carbon dioxide. f) The furnace is tapped by tilting g) The slag is removed by tilting the furnace in opposite direction
  11. 11.  Source of heat : continuous electric arc formed between the electrodes and the charged metal. (temperature generated : 1925˚C)  Steel scrap, a small amount of carbon and limestone dropped into the electric furnace through the open roof  Roof closed and electrodes are lowered  Metal melts after around 2 hours the power has been turned on  Current turned off, and the electrodes raised.  Furnace is tilted and the molten metal is poured into a ladle (used for transferring and pouring molten metal)
  12. 12.  Carbon composition in iron, steel and cast iron :  Pure iron : up to 0.008% C  Steels : 0.008 - 2.14% C  Cast Iron : 2.14 - 6.67% C ( most cast iron contain less than 4.5% C)  Pure iron experiences 2 changes in crystal structure before it melts at a temperature of 1537˚C.  At rt, the stable form called ferrite(α iron) has BCC crystal structure.  Ferrite undergoes polymorphic transformation to FCC austenite (γ iron) at around 912 ˚C.  At 1394 ˚C, austenite reverts back to a BCC phase known as δ ferrite which finally melts at 1537 ˚C
  13. 13. Phase diagram for iron-iron carbide system
  15. 15. IRON-IRON CARBIDE PHASE DIAGRAM Fe-C liquid solution •C is an interstitial impurity in Fe. •It forms a solid solution with α, γ, δ phases of iron •Maximum solubility in BCC α-ferrite is limited (max.0.022 wt% at 727 °C) - BCC has relatively small interstitial positions •Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial positions
  16. 16. γ-austenite - solid solution of C in FCC Fe • The maximum solubility of C is 2.14 wt % at 1148 °C • Transforms to BCC δ-ferrite at 1394 °C • Is not stable below the eutectoid temperature (727° C) unless cooled rapidly α-ferrite - solid solution of C in BCC Fe • Stable form of iron at room temperature. • The maximum solubility of C is 0.022 wt% at 727˚C. • Transforms to FCC γ-austenite at 912 °C •Soft, magnetic at temp below 768 ˚C δ-ferrite solid solution of C in BCC Fe • The same structure as α-ferrite • Stable only at high T, above 1394 °C • Melts at 1538 °C 727˚C
  17. 17. Fe3C (iron carbide or cementite) • This intermetallic compound is metastable • it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into α-Fe and C (graphite)if heated at 650-700°C • hard, brittle Pearlite •When alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms perlite, • a lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C) •Prop intermediate between soft,ductile ferrite and the hard, brittle cementite
  18. 18. Hypoeutectoid At about 875°C microstructure consist entirely of grain of the γ phase At about 775°C, small α particles will form along the original γ grain boundaries At this point, α particles grown larger. As temperature lowered below eutectoid , point d, all γ phase transform to pearlite, but there is no change in α phase. (pearlite is not a phase!!)
  19. 19. Types of carbon steels Carbon content (%) Application Low-carbon Steel (Mild Steel) Less than 0.30% Bolts , nuts, sheets, plate, tubes and for machine components that do not require high strength. Medium-carbon Steel 0.30 – 0.60% Machinery. Automotive, agricultural equipment parts, railroad equipment and parts for metalworking machinery. High-carbon Steel More than 0.60% Cutting tools, cable, springs, cutlery Remember : The higher the carbon content of the steel, the higher is its hardness, strength and wear resistance
  20. 20.  A small increase in carbon has significant impact on properties of the steel. As Carbon increases the steel: ◦ becomes more expensive to produce and less ductile, more brittle ◦ becomes harder and less machinable and harder to weld ◦ has higher tensile strength and a lower melting point
  21. 21. Alloy Steels  A ferrous alloy that contains alloying elements ( other than C and residual amounts of Mn, Si, S and P).  These alloying elements are added to improve mechanical and corrosion resistance properties in steel.  Characteristics : high strength, hardness, creep and fatigue resistance  Alloy steels are widely used in o Construction and transportation industry for their high strength.
  22. 22. ELEMENT INFLUENCE Manganese (Mg) Form stable carbide and increasing hardenability. Silicon (Si) Fluidity and heat resistant. Copper (Cu) Reduce rusting Aluminum (Al) Reducing grain size which adds toughness, increase machinability Boron (B) Increasing hardenability Chromium (Cr) Stabilize α, Corrosion resistant, heat resistant and increases hardenability Cobalt (Co) Permanent magnet Molybdenum (Mo) Increase strength and hardenability. Nickel (Ni) Stabilize , Grain refiner, corrosion,heat resistant, increase toughness, strength and impact resistance Tungsten (W) Stabilize , form very hard carbide, increase toughness and strength and impact resistance Vanadium (V) Increase hardenability, increase toughness and strength and impact resistance
  23. 23. Steel alloys Main class Contents (%) Applications Structural steel Carbon and low alloy 0.55C, 0.70Mn Gears, cylinders and machine-tool parts requiring resistant to wear. Corrosion resistant steel Stainless steel 0.04C, 0.45Mn, 14.00Cr Kitchen tools (forks and spoons) Heat resistant steel Heat resisting steel 0.15C, 20.00Cr, 25.00Ni Conveyers chair and skids, heat treatment box, recuperator valve, and other furnace part. Tool and die steel Alloy tool steel 0.35C, 1.00Si, 5.00Cr, 1.50Mo, 0.40V, 1.35W Extrusion die, mandrels and noses for aluminum and copper alloy. Hot forming, piercing, gripping and heading tools. Magnetic steel Hard magnetic material 1.88Ni Electric motor
  24. 24. 27 Cast Irons •Composed of iron, carbon (2.1% ~ 4.5%), and silicon (1% ~ 3.5%) – ferrous alloy. •Cast irons classification according to the solidification morphology from the eutectic temperature are : o Gray cast iron or gray iron o White cast iron o Black Malleable cast iron o White Malleable cast iron o Nodular Cast Iron ( Ductile Cast Iron)
  25. 25. Gray Cast Iron •Composition of 2.5% to 4% carbon and 1% to 3%) silicon. •Graphite exists largely in the form of flakes. •Properties of gray iron : o Low (negligible) ductility o Weak in tension o Strong in compression o Good vibration damping •Products from gray iron include automotive engine blocks and heads, motor housings, and machine tool bases.
  26. 26. Nodular Cast Iron (Ductile Iron) • This is an iron with the composition of gray iron in which the molten metal is chemically( added with magnesium) treated before pouring to cause the formation of graphite spheroids rather than flakes. • shock resistant , stronger and more ductile iron. • Applications include machinery components requiring high strength and good wear resistance.
  27. 27. White Cast Iron •Due to large amounts of iron carbide presence, the structure of white iron is very hard, wear resistance and brittle. •It is obtained either by cooling gray iron rapidly or by adjusting the composition by keeping the carbon and silicon content low. •Products from white iron include railway brake shoes.
  28. 28. Malleable Iron •Obtained by annealing white iron in an atmosphere of carbon monoxide and carbon dioxide, between 800oC~900oC, for several hours. •2 types of malleable iron : •Pearlite malleable(white malleable) – upon fast cooling of white iron •Ferrite malleable (black malleable) – upon slow cooling of white iron •The structure has good ductility, strength and shock resistance. •Typical products include pipe fittings and flanges, railroad equipment parts.
  29. 29. Cast Irons Structure Properties Application Gray Cast Iron Ferrite and Pearlite with free graphite flakes High strength and hardness. Pipe, engine blocks, machine tools White Cast Iron Pearlite and Cementite Low Machinability and Brittle Wear resistant component such as rolls for steel making. Black Malleable Ferrite and fine Carbon partical Higher in machinability, lower melting point and higher fluidity. Hardware, pipe fitting White Malleable Ferritic structure near the surface and Pearlitic structure near the center. Higher in machinability, lower melting point and higher fluidity. (Higher hardness). Railroad equipment, couplings Nodular Cast Iron Ferrite and Pearlite with Spheroidal Graphite. Higher strength, reduce fatigue failure Pipe, crankshaft, gears, rolls for rolling mills
  30. 30. Process Variable Condition Influences Cooling rate Slow Produced ferrite and large flakes of graphite together with fine flakes of graphite formed by the decomposition of the cementite after solidification. Moderate The structure will consist of flake graphite in a matrix which is entirely pearlitic. Fast (Chilling) The structure will consist of pearlite and cementite. Carbon content Low to high High quantity of graphite will be produced in cast iron by increasing the carbon content. Cross-section size Thin to thick Thin cross section will make the solidification rate faster then the thicker one. Longer solidification time will cause the differences of mechanical properties and shrinkage effect. Element content Silicon Affect the properties in the different of percent in content.  Ferritic gray cast iron: with 3% silicon  Ferritic/pearlitic cast iron: with 2% silicon  Pearlitic cast iron: with 1.5% silicon Sulphur Stabilizing the cementite and preventing the formation of flakes graphite. Thus, sulphur harden the cast iron. Manganese Remove the sulphur. Thus, softens the cast iron. Phosphorus High phosphorus content produced a great fluidity. Cause hardness and embrittlement.