Fe-C diagram

just keep some basic in mind, its give u enough information about this topic.

1. The Iron–Carbon Phase Diagram Prof. H. K. Khaira Professor in MSME Deptt . MANIT, Bhopal

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. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 12.33 The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

5. Phases Observed in Fe-C Diagram - Phases 1. Ferrite 2. Austenite 3. Cementite 4. δ -ferrite And phase mixtures 1 . Pearlite 2. Ledeburite

6. Phases Observed in Fe-C Diagram 1. Ferrite Ferrite is the interstitial solid solution of carbon in alpha iron. It has B.C.C. Structure. It has very limited solubility for carbon (maximum 0.022% at 727°C and 0.008% at room temperature). Ferrite is soft and ductile. 2. Austenite Austenite is the interstitial solid solution of carbon in gamma (γ) iron. It has FCC structure. Austenite can have maximum 2.14% carbon at 1143°C. Austenite is normally not stable at room temperature. Austenite is non-magnetic and soft. 3. Cementite Cementite or iron carbide (Fe 3 C) is an intermetallic compound of iron and carbon. It contains 6.67% carbon. It is very hard and brittle. This intermetallic compound is a metastable phase and it remains as a compound indefinitely at room temperature. 4. δ-ferrite It is a solid solution of carbon in δ-iron. It is stable at high temperatures. It has BCC structure.

7. Phase Mixtures Observed in Fe-C Diagram 1. Pearlite The pearlite consists of alternate layers of ferrite and cementite. It has properties somewhere between ferrite and cementite. The average carbon content in pearlite is 0.76% 2. Ledeburite Ledeburite is an eutetcic mixture of austenite and cementite in the form of alternate layers. The average carbon content in ledeburite is 4.3%.

8. 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) as BCC has relatively small interstitial positions Maximum solubility in FCC austenite is 2.14 % at 1147 °C as FCC has larger interstitial positions

9. 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, amount and distribution of ferrite and cementite . Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic

10. Fe-C Alloys Fe-C alloys can be of two types. 1. Steels Steels are alloys of iron and carbon containing up to 2.14% C. Other alloying elements may also be present in steels. 2. Cast irons Cast irons are alloys of iron and carbon containing more than 2.14% C. Other alloying elements may also be present in cast irons.

11. Important Reactions in Fe-C System There are three important reactions taking place in Fe-C system 1. Eutectic reaction 2. Eutectoid reaction 3. Peritectic Reaction

12. Important Reactions in Fe-C System Eutectic reaction Eutectic: 4.30 wt% C, 1147 °C L ( 4.30 % C) ↔ γ ( 2.14 % C) + Fe 3 C Eutectoid reaction Eutectoid: 0.76 wt%C, 727 °C γ (0.76 % C) ↔ α (0.022 % C) + Fe 3 C Peritectic Reaction Peritectic: 0.16% C, 1493 C δ (0. 11 % C) + L(0.51%C) ↔ γ (0.16%C)

13. Important Reactions in Fe–C System Eutectic: 4.30 wt% C, 1147 °C L ( 4.30 % C) ↔ γ ( 2.14 % C) + Fe 3 C Eutectoid: 0.76 wt%C, 727 °C γ (0.76 % C) ↔ α (0.022 % C) + Fe 3 C Peritectic: 0.16% C, 1493 C δ (0. 11 % C) + L(0.51)%C ↔ γ (0.16%C)

14. Eutectic Reaction Eutectic reaction: at 4.30 % C and 1147 °C L ( 4.30 % C) ↔ γ ( 2.14 % C) + Fe 3 C In eutectic reaction, the liquid solidifies as a phase mixture of austenite (containing 2.14% C) and cementite . This phase mixture is known as ledeburite . The average carbon content in ledeburite is 4.30%. The eutectic reaction occurs at a constant temperature. This is known as eutectic temperature and is 1147 °C.

15. Eutectoid Reaction Eutectoid reaction: at 0.76 %C and 727 °C γ(0.76 % C) ↔ α (0.022 % C) + Fe 3 C In eutectoid reaction, the austenite transforms into a phase mixture of ferrite (containing 0.76% C) and cementite . This phase mixture is known as pearlite . The average carbon content in pearlite is 0.76%. The eutectoid reaction occurs at a constant temperature. This is known as eutectoid temperature and is 727 °C. Eutectoid reaction is very important in heat treatment of steels.

16. Microstructure of Eutectoid Steel In the micrograph, the dark areas are Fe 3 C layers, the light phase is α- ferrite Pearlite nucleates at austenite grain boundaries and grows into the grain

17. Pearlite Formation Growth direction Pearlite nucleates at austenite grain boundaries and grows into the grain

18. Peritectic Reaction Peritectic reaction: at 0.16% C and 1493 C δ (0. 11 % C) + L(0.51%C) ↔ γ (0.16%C) In peritectic reaction, the liquid and δ iron transforms into austenite (containing 0.16% C). The peritectic reaction occurs at a constant temperature. This is known as peritectic temperature and is 1493 °C.

19. Development of Microstructure in Iron - Carbon alloys

20. 20 Iron-Carbon (Fe-C) Phase Diagram • 2 important points 2. Eutectoid ( B ): g Þ a + Fe 3 C 1. Eutectic ( A ): L Þ g + Fe 3 C Result: Pearlite is alternating layers of and Fe 3 C phases a 120 m m A Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g +Fe 3 C a +Fe 3 C a + g d (Fe) C, wt% C 1148°C T (°C) a 727°C = T eutectoid Adapted from Fig. 10.28, Callister & Rethwisch 3e. 4.30 g g g g A L +Fe 3 C Fe 3 C (cementite-hard) a (ferrite-soft) 0.76 B

21. Microstructure of Eutectoid steel In eutectoid steel, pearlite is formed at eutectoid temperature. The austenite gets converted into pearlite which is a mechanical mixture of ferrite and cementite .. This tranformation occurs at 727 o C (at constant temperature)

22. Microstructure of Eutectoid Steel When alloy of eutectoid composition (0.76 wt % C) is cooled slowly it forms pearlite , a lamellar or layered structure of two phases: α-ferrite and cementite (Fe 3 C). The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution of 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.

23. Microstructure of Hypoeutectoid Steel Compositions to the left of eutectoid (0.022 - 0.76 wt % C) is hypoeutectoid ( less than eutectoid) alloys. Microstructure change is γ → α + γ → α + P 1. First ferrite is formed when temperature comes down below Ae 3 temperature. γ → α + γ 2. The amount of ferrite increases with decrease in temperature till eutectoid temperature. 3. Remaining austenite changes to pearlite at eutectoid temperature. α + γ → α + P

24. 24 Microstructure of Hypo eutectoid Steel Adapted from Fig. 10.34, Callister & Rethwisch 3e. proeutectoid ferrite pearlite 100 m m Hypoeutectoid steel Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g + Fe 3 C a + Fe 3 C L +Fe 3 C d (Fe) C, wt% C 1148°C T (°C) a 727°C ( Fe-C System) C 0.76 a pearlite g g g g a a a g g g g g g g g Adapted from Figs. 10.28 and 10.33

25. Microstructure of Hypoeutectoid Steel Hypoeutectoid steels contain proeutectoid ferrite (formed above the eutectoid temperature) plus the pearlite that contains eutectoid ferrite and cementite.

26. Relative amounts of proeutectoid phase (α or Fe 3 C) and pearlite ? Relative amounts of proeutectoid phase (α or Fe 3 C) and pearlite can be calculated by the lever rule with tie line that extends from the eutectoid composition (0.76 % C) to α – (α + Fe 3 C) boundary (0.022 % C) for hypoeutectoid alloys and to (α + Fe 3 C) – Fe 3 C boundary (6.7 % C) for hypereutectoid alloys. Fraction of total α phase is determined by application of the lever rule across the entire (α + Fe3C) phase field.

27. Example for hypereutectoid alloy with composition C 1 Fraction of pearlite : W P = X / (V+X) = (6.7 – C 1 ) / (6.7 – 0.76) Fraction of proeutectoid cementite: W Fe3C = V / (V+X) = (C 1 – 0.76) / (6.7 – 0.76)

28. 28 Amount of Phases in Hypo eutectoid Steel Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g + Fe 3 C a + Fe 3 C L +Fe 3 C d (Fe) C, wt% C 1148°C T (°C) a 727°C (Fe-C System) C 0.76 g g g g a a a s r W a = s /( r + s ) W g =(1 - W a ) R S a pearlite W pearlite = W g W a ’ = S /( R + S ) W =(1 – W a ’ ) Fe 3 C Adapted from Fig. 10.34, Callister & Rethwisch 3e. proeutectoid ferrite pearlite 100 m m Hypoeutectoid steel

29. Microstructure of Hypereutectoid Steel Compositions to the right of eutectoid (0.76 - 2.14 wt % C) is hypereutectoid ( more than eutectoid) alloys. γ → γ + Fe3C → P + Fe 3 C 1. First cementite is formed when temperature comes down below Acm temperature. γ → γ + Fe 3 C 2. The amount of cementite increases with decrease in temperature till eutectoid temperature. 3. Remaining austenite changes to pearlite at eutectoid temperature. γ + Fe 3 C → P + Fe 3 C

30. 30 Microstructure of Hyper eutectoid Steel Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g +Fe 3 C a +Fe 3 C L +Fe 3 C d (Fe) C, wt%C 1148°C T (°C) a 0.76 C Fe 3 C g g g g g g g g g g g g Adapted from Fig. 10.37, Callister & Rethwisch 3e . proeutectoid Fe 3 C 60 m m Hypereutectoid steel pearlite pearlite

31. Microstructure of hypereutectoid steel

32. 32 Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g +Fe 3 C a +Fe 3 C L +Fe 3 C d (Fe) C, wt%C 1148°C T (°C) a 0.76 C pearlite Fe 3 C g g g g x v V X W pearlite = W g W a = X /( V + X ) W =(1 - W a ) Fe 3 C’ W = (1- W g ) W g = x /( v + x ) Fe 3 C Adapted from Fig. 10.37, Callister & Rethwisch 3e . proeutectoid Fe 3 C 60 m m Hypereutectoid steel pearlite Amounts of Phases Hyper eutectoid Steel

33. 33 For a 99.6 wt% Fe-0.40 wt% C steel at a temperature just below the eutectoid, determine the following: The compositions of Fe 3 C and ferrite (  ). The amount of cementite (in grams) that forms in 100 g of steel. The amounts of pearlite and proeutectoid ferrite (  ) in the 100 g. Example Problem Steel

34. 34 Solution to Problem Use lever rule with the tie line shown a) Use RS tie line just below Eutectoid C a = 0.022 wt% C C Fe 3 C = 6.70 wt% C Amount of Fe 3 C in 100 g = (100 g) W Fe 3 C = (100 g)(0.057) = 5.7 g Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g + Fe 3 C a + Fe 3 C L +Fe 3 C d C , wt% C 1148°C T (°C) 727°C C R S C Fe C 3 C 

35. 35 Solution to Problem c) Using the VX tie line just above the eutectoid and realizing that C = 0.40 wt% C C a = 0.022 wt% C C pearlite = C  = 0.76 wt% C Amount of pearlite in 100 g = (100 g) W pearlite = (100 g)(0.512) = 51.2 g Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 1 2 3 4 5 6 6.7 L g (austenite) g + L g + Fe 3 C a + Fe 3 C L +Fe 3 C d C, wt% C 1148°C T (°C) 727°C C V X C  C 

36. 36 • Fe – C diagram is 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 steel or cast iron. Summary Fe – C Diagram

37. 37 Alloying Steel With More Elements • T eutectoid changes: 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.) T Eutectoid (°C) wt. % of alloying elements Ti Ni Mo Si W Cr Mn • C eutectoid changes: 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.) wt. % of alloying elements C eutectoid (wt% C) Ni Ti Cr Si Mn W Mo

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