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  1. 1. Introducing steels EF 420 Lecture 5 John Taylor
  2. 2. Pig iron production <ul><li>Blast furnace Charge </li></ul><ul><ul><li>Ore Iron oxide or carbonate + impurity </li></ul></ul><ul><ul><li>Limestone Calcium carbonate, flux. </li></ul></ul><ul><ul><li>Coke </li></ul></ul><ul><ul><li>Charge flows downwards </li></ul></ul><ul><li>Air blown through tuyeres moves up furnace </li></ul><ul><li>Iron oxide reduced to pig iron by coke </li></ul>
  3. 3. Pig iron composition <ul><li>Fe with 4% C, 0.4% Si, 0.3% Mn, 0.025% S, and up to 1.5% P </li></ul><ul><ul><li>Too impure to be of commercial use </li></ul></ul><ul><li>Cast into sand moulds as pigs </li></ul><ul><li>Stored molten in torpedoes and transported to steel works </li></ul>
  4. 4. Steel <ul><li>An alloy of iron with up to 1.5% carbon and other elements </li></ul><ul><li>Extremely wide range of strengths is available (100MPa to 2000MPa) </li></ul><ul><ul><li>Allotropy of iron is the key to producing high strength </li></ul></ul><ul><li>Strength depends mostly on carbon content and heat treatment </li></ul><ul><ul><li>Other alloy elements determine hardenability </li></ul></ul>
  5. 5. Other alloy elements <ul><li>Hardenability increasing </li></ul><ul><ul><li>Mn, Cr, Ni, Mo, V, W, B </li></ul></ul><ul><li>Deoxidants </li></ul><ul><ul><li>Mn, Al, Si, </li></ul></ul><ul><li>Micro alloys - grain boundary pinning </li></ul><ul><ul><li>Ti, V, Nb, Al </li></ul></ul>
  6. 6. Steelmaking <ul><li>Oxidation of carbon in pig iron to CO gas </li></ul><ul><li>Obsolete processes used air </li></ul><ul><ul><li>Bessemer & open hearth </li></ul></ul><ul><li>Modern processes use pure oxygen </li></ul><ul><ul><li>BOS (basic oxygen steelmaking) shown </li></ul></ul><ul><li>Some steel is produced in electric arc furnaces from scrap and DIR </li></ul>Oxygen in Argon in
  7. 7. Ladle metallurgy <ul><li>Ladle treatments used to further refine steel for high quality </li></ul><ul><ul><li>Melt is vigorously stirred by argon </li></ul></ul><ul><ul><li>Powders & gas injected into melt through the lance </li></ul></ul><ul><ul><li>Cored wire is added </li></ul></ul><ul><ul><li>Further decarburisation </li></ul></ul><ul><ul><li>Inclusion shape control </li></ul></ul><ul><ul><li>Alloying </li></ul></ul>Lance Cored Wire Argon stirring
  8. 8. Deoxidation & degassing <ul><li>Steel picks up excess oxygen during steelmaking </li></ul><ul><li>Al, FeMn or FeSi added to form oxides, which float as a slag </li></ul><ul><li>Oxygen, hydrogen and nitrogen can also be removed by vacuum degassing </li></ul><ul><ul><li>Molten steel streams into mould in a vacuum </li></ul></ul><ul><ul><li>Gases evaporate </li></ul></ul>Vacuum Mould
  9. 9. Ingot casting <ul><li>Traditional, batch, low productivity method </li></ul><ul><li>Small quantities </li></ul><ul><li>Special steels </li></ul><ul><ul><li>Magnets, </li></ul></ul><ul><ul><li>Tool steels, </li></ul></ul><ul><ul><li>Stainless </li></ul></ul><ul><li>Non-ferrous metals </li></ul>Killed Semi-killed Rimming
  10. 10. Continuous casting <ul><li>Structural and pressure equipment steel </li></ul><ul><li>Killed steel </li></ul><ul><li>High productivity for large quantities </li></ul><ul><li>Used for all steel production in Australia </li></ul>
  11. 11. Remelting and refining <ul><li>Produces super quality steel </li></ul><ul><li>The impure ingot is used as a consumable electrode in a vacuum electric arc furnace (VAR steel) </li></ul><ul><li>It can also be remelted by induction heating in a vacuum (VIM steel) </li></ul><ul><li>It can also be remelted by passing current through a slag bath ( Electroslag refined or ESR steel) </li></ul>Transformer Consumable electrode Water cooled Moveable mould ESR steel Slag pool
  12. 12. Processing structural steel <ul><li>Slab is hot rolled (over 950˚C) to final shape (plate or section) </li></ul><ul><li>Slow cooling: as-rolled </li></ul><ul><li>Reheat to 910˚C and air cool: normalised </li></ul><ul><li>TMCR (final rolling between 930 and 870˚C) </li></ul><ul><li>Quench and temper </li></ul>
  13. 13. Forms of delivery <ul><li>Steel castings, forgings </li></ul><ul><li>Sheet, strip, bar, billets, ingots </li></ul><ul><li>Plate, tube, pipe, rolled sections </li></ul><ul><li>Reinforcing bar </li></ul><ul><li>Rod, wire, cable, chain </li></ul><ul><li>Springs </li></ul><ul><li>Fasteners - nails, screws, bolts, studs, nuts </li></ul>
  14. 14. Classification by composition <ul><li>Steel - Carbon between 0.05% and 1.5% C </li></ul><ul><li>Soft Iron - Less than 0.05% C </li></ul><ul><li>Cast Iron - More than 1.5% C </li></ul><ul><li>Carbon and carbon-manganese steels </li></ul><ul><li>Low alloy steels (<5% Alloy) </li></ul><ul><li>High alloy steel </li></ul>
  15. 15. Classification by application <ul><li>Structural steel </li></ul><ul><li>Pressure vessel steel </li></ul><ul><li>Piping </li></ul><ul><li>Deep drawing steel </li></ul><ul><li>Engineering steels </li></ul><ul><li>Ultra high strength steels </li></ul><ul><li>Stainless steels </li></ul><ul><li>Tool steels </li></ul>
  16. 16. Allotropy of iron
  17. 17. Phase <ul><li>Physically distinct and mechanically separable portion of a material, often with boundaries </li></ul><ul><li>The atomic structure of a phase is uniform </li></ul><ul><li>If in equilibrium, the composition is uniform </li></ul><ul><ul><li>Gas - only one phase. Different gases mix </li></ul></ul><ul><ul><li>Liquid - multiple phases can occur </li></ul></ul><ul><ul><ul><li>Oil and water separate into 2 phases </li></ul></ul></ul><ul><ul><li>Solids - can have multiple phases in fine mixtures </li></ul></ul>
  18. 18. Equilibrium phase diagram <ul><li>Shows phases present over a range of compositions, temperature and pressure </li></ul><ul><li>A point in the diagram represents the phases present under a set of conditions </li></ul>Temperature Pressure 100 kPa 100˚C 0˚C SOLID LIQUID GAS Isotherm Isobar
  19. 19. Phases in iron 910˚C 1390˚C 1535˚C AUSTENITE (gamma) LIQUID FERRITE (delta) FERRITE (alpha) FERRITE Body centred cubic (bcc) AUSTENITE Face centred cubic (fcc)
  20. 20. Iron-carbon diagram Percentage Carbon Temperature Liquid Austenite + liquid Austenite Austenite + Fe 3 C Ferrite + Fe 3 C Alpha ferrite Delta ferrite A+F 1535˚C 1390˚C 1130˚C 723˚C 4.3% 0.83% 0.02% 1.7% 910˚C
  21. 21. Iron-iron carbide diagram    + Fe 3 C   + Fe 3 C 910°C 723°C 0.02% 0.83% Temperature Percent Carbon
  22. 22. Transformation to pearlite <ul><li>Eutectoid transformation </li></ul><ul><li>Nucleation and growth process </li></ul><ul><li>Controlled by diffusion of carbon </li></ul><ul><li>More undercooling (more rapid cooling) </li></ul><ul><ul><li>Slower diffusion </li></ul></ul><ul><ul><li>More nuclei </li></ul></ul><ul><ul><li>More pearlite colonies of a finer lamellar spacing </li></ul></ul>
  23. 23. Structure of pearlite Ferrite (0.02% C) Cementite (6.67% C) Pearlite (0.8% C average)
  24. 24. Pearlite microstructure Alternate plates of carbide and ferrite. Normally the structure cannot be resolved. Particularly slow cooling has been used. 10  m
  25. 25. Iron-iron carbide diagram    + Fe 3 C   + Fe 3 C A 3 A 1 910°C 723°C 0.02% 0.83% Temperature Percent Carbon Hypoeutectoid Hypereutectoid
  26. 26. Hypoeutectoid steel <ul><li>Ferrite forms in austenite once cooled below A 3 </li></ul><ul><li>Ferrite has low carbon, so as it forms the carbon level of the remaining austenite increases </li></ul><ul><li>When the temperature falls below A 1 </li></ul><ul><ul><li>Carbon content of the remaining austenite is eutectoid level (0.83%) </li></ul></ul><ul><ul><li>The remaining austenite transforms to pearlite </li></ul></ul>
  27. 27. Hypoeutectoid steel microstructure Ferrite + Austenite (yellow) Ferrite (white) + pearlite (black)
  28. 28. 0.5% Carbon Steel Pearlite islands surrounded by ferrite. The ferrite shows the prior austenite grain boundaries
  29. 29. Annealing and normalising <ul><li>About 50°C above Ac 3 for hypoeutectoid steels </li></ul><ul><li>Or 50°C above Ac 1 for hypereutectoid steels </li></ul><ul><li>No cold work necessary </li></ul><ul><li>Full Anneal - furnace cool (pearlitic structure) </li></ul><ul><li>Normalise - air cool (finer pearlitic structure, higher strength) </li></ul>
  30. 30. Sub-critical or process anneal <ul><li>Hypoeutectoid steels only </li></ul><ul><li>Must be cold worked </li></ul><ul><li>Below Ac 1 (500 to 600°C) </li></ul><ul><li>Removes effects of cold work by recrystallisation of ferrite. </li></ul>
  31. 31. Process annealed steel Cold worked steel Process annealed steel
  32. 32. Effect of pearlite on properties 0 0.2 0.4 0.6 0.8 1.0 1.2 % pearlite % ferrite 0% 100% % carbon 300 MPa 900 MPa 500 MPa 700 MPa UTS (R m ) UTS cementite Elongation Elongation (A 5 ) 0% 10% 20% 30% 40%
  33. 33. Effect of rapid cooling of austenite
  34. 34. Isothermal transformation <ul><li>Heating to austenite temperature </li></ul><ul><li>Rapid cooling to transformation temperature </li></ul><ul><ul><li>Tin or lead bath </li></ul></ul><ul><li>Maintaining at transformation temperature </li></ul><ul><ul><li>Some transformations are time dependent </li></ul></ul><ul><ul><li>Others are not </li></ul></ul>Time Temp Deg C
  35. 35. Isothermal transformations <ul><li>If transformation is just below A 3 </li></ul><ul><ul><li>Few nuclei of the transformation products are present </li></ul></ul><ul><ul><li>The energy to transform is low </li></ul></ul><ul><ul><li>Coarse structure forms slowly </li></ul></ul><ul><li>As degree of undercooling increases </li></ul><ul><ul><li>More nuclei are created </li></ul></ul><ul><ul><li>Transformation in more rapid </li></ul></ul><ul><ul><li>Finer structure forms more rapidly </li></ul></ul>
  36. 36. Isothermal transformation diagram Martensite Upper bainite Lower bainite A + F + P A + B Austenite A + M Time (Seconds) Temperature (Deg C) 1 10 100 1000 M s M f Pearlite (+ ferrite) 100% Ac 1 0%
  37. 37. Formation of martensite <ul><li>High cooling rates - quenching </li></ul><ul><ul><li>Rapid cooling with water or oil </li></ul></ul><ul><li>Diffusion of carbon inhibited </li></ul><ul><ul><li>Neither ferrite nor pearlite have sufficient time to form </li></ul></ul><ul><li>Shear transformation </li></ul><ul><ul><li>Body centred tetragonal structure </li></ul></ul><ul><ul><li>Carbon evenly distributed </li></ul></ul>
  38. 38. Martensite Martensite formed by shear transformation in an austenite grain Martensite Retained austenite
  39. 39. Martensite Martensite is created when austenite transforms by a shear mechanism instead of by nucleation and growth. The shear transformation products are acicular. In this case, because of high carbon (1.2%) there is retained austenite.
  40. 40. Martensite properties <ul><li>Can be considered to be ferrite supersaturated with carbon </li></ul><ul><ul><li>Body centred tetragonal structure </li></ul></ul><ul><li>High strength (up to 2000 MPa) and hardness (900 HV) </li></ul><ul><li>Can be brittle </li></ul><ul><ul><li>Strength increases & ductility reduces with increasing carbon content </li></ul></ul><ul><li>Effect of other alloy elements on properties is minimal (but not on hardenability) </li></ul>
  41. 41. Factors promoting martensite <ul><li>High cooling rate </li></ul><ul><li>Increasing hardenability </li></ul><ul><li>Hardenability is dependent on carbon and other alloy elements </li></ul><ul><li>The desired cooling rate ensures transformation is to completely to martensite but avoids quench cracking </li></ul>
  42. 42. Cooling rate depends on <ul><li>Component thickness </li></ul><ul><li>Component shape </li></ul><ul><ul><li>Equivalent cooling rates for shape variations </li></ul></ul><ul><li>Quenching medium </li></ul><ul><ul><li>Agitation is important </li></ul></ul><ul><ul><li>Brine quench. Fastest cooling rates </li></ul></ul><ul><ul><li>Water quench. Cheap and fast, spray or immersion </li></ul></ul><ul><ul><li>Oil quench. Intermediate cooling rates. Flammable </li></ul></ul><ul><ul><li>Air cool. Relatively slow, suits thin sections </li></ul></ul>
  43. 43. Hardenability measurement <ul><li>Hardenability - the thickness of steel which will harden to martensite </li></ul><ul><li>Length of an end-quenched bar which is hardened </li></ul><ul><ul><li>Jominy test </li></ul></ul><ul><li>Section thickness that can be through-hardened </li></ul><ul><ul><li>Low hardenability, only thin materials </li></ul></ul>
  44. 44. Hardenability determination <ul><li>Carbon equivalent formulae </li></ul><ul><li>Bar diameter which will through-harden to 50% martensite in centre, D I . </li></ul><ul><ul><li>Depends on cooling rate </li></ul></ul><ul><ul><li>Critical diameter using ideal theoretical fastest possible quench (violently agitated brine) D C </li></ul></ul><ul><ul><li>Depends on austenite grain size </li></ul></ul>
  45. 45. Tempering <ul><li>Heating martensite to between 100 & 600˚C </li></ul><ul><li>Softens & toughens martensite </li></ul><ul><li>Effects dependent on temperature and include </li></ul><ul><ul><li>Stress relief </li></ul></ul><ul><ul><li>Epsilon carbide precipitates from martensite </li></ul></ul><ul><ul><li>Cementite precipitates from martensite </li></ul></ul><ul><ul><li>Epsilon carbide converts to cementite </li></ul></ul><ul><ul><li>Retained austenite transforms to bainite </li></ul></ul>
  46. 46. Effect of tempering <ul><li>Increases ductility and toughness </li></ul><ul><li>Reduces hardness and strength </li></ul><ul><li>Solid solution elements have little effect on tempering </li></ul><ul><ul><li>Ni, Si, Al, Mn </li></ul></ul><ul><li>Strong carbide formers raise tempering temperature for equivalent hardness </li></ul><ul><ul><li>Cr, Mo, V </li></ul></ul>
  47. 47. Constituents of Steel s <ul><li>Ferrite, Alpha (  )iron, Delta iron (  ) </li></ul><ul><li>Ferrite is the body-centred cubic (bcc) phase that pure iron exists as at temperatures up to 910°C. Pure iron can also exist as ferrite at high temperatures, between 1392°C and the melting point of 1536°C. The low temperature form is known as  ferrite and the high temperature form  ferrite, but the two forms are identical. Ferrite is soft and ductile when pure. Alpha ferrite can dissolve only 0.02% carbon, but may contain significant levels of other alloy elements. Pure ferritic steels are rare. </li></ul>
  48. 48. <ul><li>Austenite, Gamma (  iron </li></ul><ul><li>Between 910°C and 1392°C pure iron exists as austenite, which has a face-centred cubic crystal structure. Austenite, also known as  , can dissolve much higher levels of carbon than ferrite, up to 2% at 1146°C, so heating a carbon or low alloy steel to temperatures at which it is fully austenitic dissolves all the carbon. This is the key to heat treating these steels. Although this phase is stable only at high temperatures in low alloy and carbon steels, some alloy steels are austenitic at normal temperatures. Austenite is also relatively soft and ductile. </li></ul>
  49. 49. <ul><li>Cementite, Iron carbide, Fe 3 C. </li></ul><ul><li>Cementite is a compound of iron that can form a stable phase in steels and irons. It consists of one atom of carbon with three of iron, and has a carbon content of 6.67%. As a pure phase, cementite is very hard (over 600 HB) and brittle. Ferritic steels contain carbon as cementite, and the strength and ductility of steel is affected by the amount and distribution of the cementite. </li></ul>
  50. 50. <ul><li>Pearlite </li></ul><ul><li>Pearlite is a two-phase mixture of ferrite and cementite arranged as alternating parallel plates. It always contains fixed amount of carbon (0.83% in carbon steel) and forms as a result of an eutectoid reaction when austenite is cooled. The spacing of the plates (lamellae) is very fine and is usually not seen on a light microscope, even with high magnifications. Pearlite is hard and strong, although not as ductile as pure ferrite or austenite, nor as strong as martensite. Its presence in steel confers strength. </li></ul>
  51. 51. <ul><li>Martensite </li></ul><ul><li>Martensite is a metastable phase formed when austenite is cooled so rapidly that carbide precipitation is suppressed. This occurs when carbon or low alloy steel is quenched (cooled rapidly by dipping in a fluid) from high temperature. The rate of cooling is dependent on the type of quenchant (brine, water, oil or air) and on the object’s thickness. The minimum rate of cooling to transform to martensite depends on the alloy content of the steel, and is known as its hardenability. Martensite forms by an instantaneous process of microscopic shear. When steel is quenched to martensite it is in its hardest and strongest condition, although the ductility and toughness is often low. The strength and hardness are determined by the alloy content, in particular the carbon level. The ductility and toughness of martensite can be improved by tempering, which is heating in the range 150°C to 700°C. This allows some stress relief and carbide precipitation. Higher tempering temperatures result in more ductility at the expense of hardness. </li></ul>
  52. 52. <ul><li>Bainite </li></ul><ul><li>Bainite is a two-phase structure that can be formed in carbon steel by rapid cooling austenite to between 400°C and 550°C, followed by a time holding at this temperature. This is generally done by quenching into a bath of liquid tin or lead and holding the steel in this bath. Transformation to bainite is by precipitation of carbides in a very fine feathery or lath configuration. Some alloy steels can transform to bainite on continuous cooling, rather than by the rather impractical process used for carbon steel. The properties of bainite are similar to tempered martensite. Lower bainite (formed at lower temperatures) is extremely tough. Upper bainite (formed at higher temperatures) is a coarser structure and is not as hard or ductile as lower bainite. The matrix is ferrite in steel which is isothermally transformed but may be martensite in alloy steel which is continuously cooled. </li></ul>