Iron Carbon Phase Diagram
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Iron Carbon Phase Diagram, TTT Diagram, CCT Diagram
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Iron Carbon Phase Diagram
- 1. Metallurgy & Material Science Dr.S.Jose Professor, Dept of Mechanical Engg., TKM College of Engineering, Kollam
- 2. Module II Diffusion in crystals Theory of Alloys Equilibrium Diagrams Iron Carbon Phase diagram TTT Diagram Heat Treatment Recovery, Recrystallisation & Grain Growth 2
- 4. Fe – Fe3C Phase Diagram
- 5. Five individual phases a–ferrite (BCC) Fe-C solid solution g-austenite (FCC) Fe-C solid solution d-ferrite (BCC) Fe-C solid solution Fe3C (Iron Carbide) or cementite – an inter-metallic compound Liquid Fe-C solution
- 6. Three invariant reactions A horizontal line always indicates an invariant reaction in binary phase diagrams Peritectic reaction at 1495˚C and 0.18%C, d-ferrite + L↔ g-iron (austenite) Eutectic reaction at 1147˚C and 4.3 %C, L ↔ g-iron + Fe3C (cementite) [ledeburite] Eutectoid reaction at 727˚C and 0.77%C, g-iron ↔ a–ferrite+Fe3C (cementite) [pearlite]
- 9. Fe-C alloy classification Metals Ferrous metals Steels Non-ferrous metals Cast Irons Plain carbon steels Grey Iron Low carbon steels White Iron Medium carbon steels Malleable & Ductile Irons High carbon steels Low alloy steels High alloy steels Stainless & Tool steels
- 10. Fe-C alloy classification Fe-C alloys are classified according to wt.% C present in the alloys Commercial pure irons % C < 0.008 Low-carbon steels 0.008 - %C - 0.3 Medium carbon steels 0.3 - %C - 0.8 High-carbon steels 0.8- %C - 2.14 Cast irons 2.14 < %C
- 11. Cast irons Cast irons that were slowly cooled to room temperature consists of cementite, look whitish – white cast iron. If it contains graphite, look grayish – gray cast iron. It is heat treated to have graphite in form of nodules – malleable cast iron. If inoculants are used in liquid state to have graphite nodules – spheroidal graphite (SG) cast iron.
- 14. Eutectoid steel Eutectoid Reaction 727o C g ¾ cool¾® a + Fe3C ¾ 0.022 6.67 0.77 Pearlite
- 15. Hypoeutectoid steel Proeutectoid Ferrite Pearlite Microstructure of 0.38 wt% C hypoeutectoid steel
- 16. Hypereutectoid steel Pearlite Proeutectoid cementite Microstructure of 1.4 wt% C hypereutectoid steel
- 17. Eutectoid steel Hypoeutectoid steel Hypereutectoid steel a+Fe3C a+Fe3C a+Fe3C Pearlite Pearlite + proeutectoid ferrite Pearlite + proeutectoid cementite
- 18. Phase vs. Microconstituents A phase or a mixture of phases which has a distinct identity in a microstructure is called a microconstituent Pearlite is not a phase. It is a microconstituent and is a mixture of two phases a- Ferrite and Fe3C.
- 19. a-Ferrite Known as a -iron Pure iron at room temperature Body-centered cubic structure Soft & ductile and imparts these properties to the steel. Less than 0.01% carbon will dissolve in ferrite at room temperature High temperature form is d ferrite, but the two forms are identical. Pure ferritic steels are rare
- 20. Austenite Known as g -iron Face-centered cubic Much softer than ferrite Not present at room temperatures. More easily hot worked
- 21. Cementite Iron Carbide - an intermetallic compound Hard, brittle, white melts at 1837 C , density of 7.4 g/cc On the phase diagram, cementite corresponds to a vertical line at 6.7% C Engineers care only about compounds with less carbon Its presence in steels causes an increase in hardness and a reduction in ductility and toughness
- 22. Pearlite A laminated structure formed of alternate layers of ferrite and cementite with average composition 0.83% carbon Pearly lustre in the microscope Interference of light in its regular layers Most common constituent of steel It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels. The laminar structure also acts as a barrier to crack movement as in composites. This gives it toughness
- 24. Phase Transformations Involve some alteration of microstructure 1. No change in number or composition of the phases present, diffusion- dependent. Solidification of pure metals, allotropic transformation. 2. Some alteration in composition and no of phases, diffusion – dependent. Eutectoid reaction 3. A metastable phase is produced, diffusionless. Martensitic transformation.
- 25. Phase Transformations At least one new phase is formed Do not occur instantaneously Begin by the formation of small particles of new phase – nucleation Homogenous – occurs uniformly throughout the parent phase. Hetrogenous – preferentially at grain boundaries, impurities, dislocations Size of these particles increase in size until completion - growth
- 26. Phase Transformations Dependent on Temperature Time Composition Require some finite time for completion Equilibrium is rarely achieved in solids Metastable – intermediate between initial and equilibrium states.
- 31. TTT Diagram
- 32. Transformation of Austenite in Eutectoid steel Pearlite 727 - 540 C Bainite 540 - 210 C Martensite below 210 C
- 34. CCT diagram Usually materials are cooled continuously, thus Continuous Cooling Transformation diagrams are appropriate than TTT diagrams For continuous cooling, the time required for a reaction to begin and end is delayed, thus the isothermal curves are shifted to longer times and lower temperatures. Main difference between TTT and CCT diagrams: no space for bainite in CCT diagram as continuous cooling always results in formation of pearlite.
- 35. CCT diagram
- 36. CCT diagram