fe-c diagram

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  • 1. The Iron–Carbon Phase Diagram
  • 2. Iron–Carbon Phase Diagram • In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase diagram is a fairly complex one, but we will only consider the steel and cast iron part of the diagram, up to 6.67% Carbon.
  • 3. Fe – C Equilibrium Diagram
  • 4. Phases in Fe–C Phase Diagram • α-ferrite - solid solution of C in BCC Fe • Stable form of iron at room temperature. • The maximum solubility of C is 0.025 % at eutectoid temperature • Transforms to FCC γ-austenite at 910°C • γ-austenite - solid solution of C in FCC Fe • The maximum solubility of C is 2.14 % at eutectic temperature • Transforms to BCC δ-ferrite at 1395 °C • Is not stable below the eutectoid temperature(727 ° C) unless cooled rapidly • δ-ferrite solid solution of C in BCC Fe • The same structure as α-ferrite • Stable only at high Temp., above 1394 °C • Melts at 1538 °C
  • 5. Phases in Fe–C Phase Diagram • Fe3C (iron carbide or cementite) It is an intermetallic compound between iron and carbon. It contains 6.67 % carbon and is hard and brittle. This intermetallic compound is a metastablephase and it remains as a compound indefinitely at room temperature.
  • 6. A few comments on Fe–C system • Carbon occupies interstitial positions in Fe. It forms a solid solution with α, γ, δ phases of iron • Maximum solubility in BCC α-ferrite is limited (max. 0.025 % at 727 °C) - BCC has relatively small interstitial positions • Maximum solubility in FCC austenite is 2.14 % at 1147 °C - FCC has larger interstitial positions
  • 7. A few comments on Fe–C system • Mechanical properties – Cementite is very hard and brittle - can strengthen steels. – Mechanical properties depend on the microstructure, that is, how ferrite and cementite are distributed. • Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic
  • 8. Classification • Three types of ferrous alloy • Iron: less than 0.008% C in α−ferrite at room temp. • Steels: 0.008 - 2.14% C (usually < 1%) αferrite + Fe3C at room temp. • Cast iron: 2.14 - 6.7% (usually < 4.5%)
  • 9. Eutectic and eutectoid reactions in Fe–C Eutectic: 4.30 wt% C, 1147 °C L (4.30% C) ↔ γ (2.14% C) + Fe3C Eutectoid: 0.76 wt%C, 727 °C γ(0.76% C) ↔ α (0.022% C) + Fe3C
  • 10. • Eutectic and eutectoid reactions are very important in heat treatment of steels
  • 11. Development of Microstructure in Iron - Carbon alloys • Microstructure depends on composition (carbon content) and heat treatment. In the discussion below we consider slow cooling in which equilibrium is maintained.
  • 12. Iron-Carbon (Fe-C) Phase Diagram T(°C) 1600 - Eutectic (A): L Adapted from Fig. 10.28, Callister & Rethwisch 3e. L 1400 + Fe3C - Eutectoid (B): + Fe3C 1200 +L AA (austenite) 1000 800 +Fe3C B 727°C = T eutectoid 600 120 m Result: Pearlite is alternating layers of 400 0 (Fe) L+Fe3C 1148°C Fe3C (cementite) • 2 points +Fe3C 1 0.76 and Fe3C phases 2 3 4 4.30 5 6 6.7 C, wt% C Fe3C (cementite-hard) (ferrite-soft) 12
  • 13. Microstructure of eutectoid steel
  • 14. Microstructure of eutectoid steel • 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). • The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) by atomic diffusion. • Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard, brittle cementite.
  • 15. Microstructure of eutectoid steel In the micrograph, the dark areas are Fe3C layers, the light phase is αferrite
  • 16. Microstructure of hypoeutectoid steel Compositions to the left of eutectoid (0.022 0.76 wt % C) is hypoeutectoid (less than eutectoid -Greek) alloys. γ → α + γ → α + Fe3C
  • 17. Hypoeutectoid Steel T(°C) 1600 Adapted from Figs. 10.28 and 10.33 L 1400 1000 + Fe3C 800 727°C 600 pearlite + Fe3C 1 0.76 400 0 (Fe)C0 L+Fe3C 1148°C (austenite) 2 3 4 5 6 6.7 C, wt% C 100 m pearlite (Fe-C System) Fe3C (cementite) 1200 +L Hypoeutectoid steel proeutectoid ferrite Adapted from Fig. 10.34, Callister & Rethwisch 3e. 17
  • 18. Hypoeutectoid Steel T(°C) 1600 L 1400 (austenite) 1000 800 600 pearlite Wpearlite = W + Fe3C r s 727°C RS 400 0 (Fe)C0 + Fe3C 1 0.76 W = s/(r + s) W =(1 - W ) L+Fe3C 1148°C W ’ = S/(R + S) WFe3C =(1 – W ’) pearlite 2 3 4 5 6 (Fe-C System) Fe3C (cementite) 1200 +L 6.7 C, wt% C 100 m Hypoeutectoid steel proeutectoid ferrite Adapted from Fig. 10.34, Callister & Rethwisch 3e. 18
  • 19. Microstructure of hypoeutectoid steel Hypoeutectoid alloys contain proeutectoid ferrite (formed above the eutectoid temperature) plus the eutectoid perlite that contain eutectoid ferrite and cementite.
  • 20. Microstructure of hypereutectoid steel Compositions to the right of eutectoid (0.76 - 2.14 wt % C) is hypereutectoid (more than eutectoid -Greek) alloys. γ → γ + Fe3C → α + Fe3C
  • 21. Microstructure of hypereutectoid steel
  • 22. T(°C) Hypereutectoid Steel 1600 L 1400 +L 1000 +Fe3C 800 600 400 0 (Fe) pearlite +Fe3C 0.76 Fe3C L+Fe3C 1148°C (austenite) 1 C0 2 3 4 5 6 Fe3C (cementite) 1200 6.7 C, wt%C 60 mHypereutectoid steel pearlite proeutectoid Fe3C Adapted from Fig. 10.37, Callister & Rethwisch 3e. 22
  • 23. T(°C) Hypereutectoid Steel 1600 L 1400 1200 +L 1000 +Fe3C W =x/(v + x) v x 800 600 pearlite 400 0 (Fe) Wpearlite = W V X +Fe3C 0.76 WFe3C =(1-W ) 1 C0 W = X/(V + X) WFe 3C’ L+Fe3C 1148°C (austenite) 2 3 4 5 6 Fe3C (cementite) Fe3C 6.7 C, wt%C 60 mHypereutectoid steel =(1 - W ) pearlite proeutectoid Fe3C Adapted from Fig. 10.37, Callister & Rethwisch 3e. 23
  • 24. Relative amounts of proeutectoid phase (α or Fe3C) and pearlite? Application of the lever rule with tie line that extends from the eutectoid composition (0.76 wt% C) to α – (α + Fe3C) boundary (0.022 wt% C) for hypoeutectoid alloys and to (α + Fe3C) – Fe3C boundary (6.7 wt% C) for hypereutectoid alloys. Fraction of α phase is determined by application of the lever rule across the entire (α + Fe3C) phase field.
  • 25. Example for hypereutectoid alloy with composition C1 Fraction of pearlite: WP = X / (V+X) = (6.7 – C1) / (6.7 – 0.76) Fraction of proeutectoid cementite: WFe3C = V / (V+X) = (C1 – 0.76) / (6.7 – 0.76)
  • 26. Example Problem Steel For a 99.6 wt% Fe-0.40 wt% C steel at a temperature just below the eutectoid, determine the following: a) The compositions of Fe3C and ferrite ( ). b) The amount of cementite (in grams) that forms in 100 g of steel. c) The amounts of pearlite and proeutectoid ferrite ( ) in the 100 g. 26
  • 27. Solution to Problem a) Use RS tie line just below Eutectoid b) Use lever rule with the tie line shown WFe 3C R R S 1600 T(°C) 1200 C0 C CFe 3C C 0.40 0.022 6.70 0.022 L 1400 +L 1000 + Fe3C 800 727°C R 0.057 S + Fe3C 600 400 0 Amount of Fe3C in 100 g L+Fe3C 1148°C (austenite) Fe3C (cementite) C = 0.022 wt% C CFe3C = 6.70 wt% C C C0 1 2 3 4 C, wt% C 5 6 6.7 CFe 3C = (100 g)WFe3C = (100 g)(0.057) = 5.7 g 27
  • 28. Solution to Problem c) Using the VX tie line just above the eutectoid and realizing that C0 = 0.40 wt% C C = 0.022 wt% C Cpearlite = C = 0.76 wt% C V X T(°C) C0 C C C 0.40 0.022 0.76 0.022 L 1400 1200 +L L+Fe3C 1148°C (austenite) 1000 + Fe3C 0.512 800 727°C VX Amount of pearlite in 100 g = (100 g)Wpearlite = (100 g)(0.512) = 51.2 g 600 400 0 + Fe3C 1 C C0 C 2 3 4 5 6 Fe C (cementite) Wpearlite V 1600 6.7 C, wt% C 28
  • 29. Summary Phase Diagrams and Microstructures • Phase (T vs c) diagrams are useful to determine: - the number and types of phases, - the wt% of each phase, - and the composition of each phase for a given T and composition of the system. • Alloying to produce a solid solution usually - increases the tensile strength (TS) - decreases the ductility. • Binary eutectics and binary eutectoids allow for a range of microstructures. - Alloy composition and annealing temperature and time determine possible microstructure(s). Slow vs. Fast. - In 2-phase regions, use “lever rule” to get wt% of phases. - Varying microstructure affects mechanical properties. 29
  • 30. Alloying Steel With More Elements Ti Mo • Ceutectoid changes: Si W Cr Mn Ni wt. % of alloying elements Adapted from Fig. 10.38,Callister & Rethwisch 3e. (Fig. 10.38 from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 127.) Ceutectoid (wt% C) T Eutectoid (°C) • Teutectoid changes: Ni Cr Si Ti Mo W Mn wt. % of alloying elements Adapted from Fig. 10.39,Callister & Rethwisch 3e. (Fig. 10.39 from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 127.) 30
  • 31. THANKS