production Engineering

2. Fig Iron Carbon Equilibrium Diagram

3. Fig Iron Carbon Equilibrium Diagram

4. Orthorhombic Fe3C. Iron atoms are blue.

5. Fe-C equilibrium diagram
 The structural form of pure iron at room temperature
is called ferrite or -iron.
 Ferrite is soft and ductile.
 Since ferrite has a body-centred cubic structure, the
inter-atomic spaces are small and pronouncedly
oblate, and cannot readily accommodate even a small
carbon atom. Therefore, solubility of carbon in ferrite
is very low, of the order of 0.006% at room
temperature.
 The maximum carbon content in ferrite is 0.05% at 723
°C.

Contd…

6.  In addition to carbon, a certain amount of silicon,
manganese and phosphorous may be found in ferrite.
 The face-centred modification of iron is called
austenite or -iron. It is the stable form of pure iron at
temperatures between 910°C and 1400°C. At its stable
temperature austenite is soft and ductile and
consequently, is well suited for manufacturing
processes.
 The face-centred cubic structure of iron has larger
inter-atomic spacing than in ferrite. Even so, in FCC
structure the interstices are barely large enough to
accommodate carbon atoms, and lattice strains are
produced. As a result, not all the interstitial sites can
be filled at any one time.

Contd…

7.  The maximum solubility is only 2% of carbon at 11
30°C.
 Above 1400°C, austenite is no longer the most stable
form of iron, and the crystal structure changes back to
a body-centred cubic phase called delta iron. This is
the same phase as the -iron except for its
temperature range.
 The solubility of carbon in -ferrite is small, but it is
appreciably larger than In -ferrite, because of higher
temperature. The maximum solubility of carbon in
&iron is 0.1% at 1490°C.



Contd…

8.  In iron-carbon alloys, carbon in excess of the solubility
limit must form a second phase, which is called iron
carbide or cementite.
 Iron carbide has the chemical composition of . This
does not mean that iron carbide forms molecules of
but simply that the crystal lattice contains iron and
carbon atoms in a three -to- one ratio.
 The compound has an orthorhombic unit cell with
twelve iron atoms and four carbon atoms per cell, and thus
has a carbon content of 6.67%.
 As compared to austenite and ferrite, cementite being an
inter-metallic compound, is very hard and brittle.
 The presence of iron carbide with ferrite in steel greatly
increases the strength of steel.
3Fe C
3Fe C
3Fe C
Contd…

9.  In the reaction, the simultaneous formation of ferrite
and cementite from austenite results at the
temperature of 723°C and composition of 0.80%
carbon.
 There are nearly 12% of iron carbide and slightly more
than 88% of ferrite in the resulting mixture.
 Since the ferrite and cementite are formed
simultaneously, they are intimately mixed.
Characteristically, the mixture is lamellar, i.e., it is
composed of alternate layers of ferrite and cementite.
 This micro-structure is called pearlite which is very
important in iron and steel technology, because it can
be formed in almost all steels by means of suitable
heat treatments.
Contd…

10.  The alloy containing 0.80% of carbon is called the
eutectoid steel.
 Upon cooling the eutectoid steel below 723°C, all of
the austenite is transformed into pearlite.
 Alloys with less than 0.80% C are called hypo-
eutectoid steels and those with higher composition are
called hyper-eutectoid steels.
Contd…

11.  Fe-Fe3C phase diagram is characterized by five individual
phases,: α–ferrite (BCC) Fe-C solid solution, γ-austenite (FCC)
Fe-C solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe3C
(iron carbide) or cementite - an inter-metallic compound and
liquid Fe-C solution and four invariant reactions:
 peritectic reaction at 1495 oC and 0.16%C, δ-ferrite + L↔ γ-
iron (austenite)
 monotectic reaction 1495 oC and 0.51%C, L ↔ L + γ-iron
(austenite)
 eutectic reaction at 1147 oC and 4.3 %C, L ↔ γ-iron + Fe3C
(cementite) [ledeburite]
 eutectoid reaction at 723 oC and 0.8%C, γ-iron ↔ α– ferrite +
Fe3C (cementite) [pearlite]
Contd…

12. Three- phase reactions
 Suffix – ic denotes at least one liquid phase is there
 Suffix – oid all phases involve are solid

13. IES-2004
Consider the following temperature ranges:
1. Room temperature 2. 0 to 910°C
3. 910°C to 1400oC 4. 1400oC to below melting
point
In which of the above temperature ranges ferrite with
body centered cubic structure is indicated in, the Fe-
Fe3C phase diagram?
(a)1, 2 and 4 (b)2, 3 and 4 (c)1 and 3 (d)2 and 3

14. IES-1992
The microstructure composition of pearlite for a
Fe3C diagram consists of
(a) Carbon dissolved in alpha iron having a body
cantered cubic structure.
(b) Carbon dissolved in gama iron having a face
cantered cubic structure.
(c) A mixture of body-cantered alpha iron and face-
entered gamma iron
(d) Carbon dissolved in body-cantered alpha iron and
an Fe, Fe3C.

15. IES 2011
Liquid + solid (1) on cooling converting solid (2)
reaction is known as:
(a) Eutectoid reaction
(b) Eutectic reaction
(c) Peritectic reaction
(d) Peritectioid reaction

16. IES 2010
Iron-carbon equilibrium diagram
(a) Correlates the microstructure and properties of
steel and cast iron
(b) Indicates the phase changes occurring during
heating and cooling
(c) Is made by plotting carbon percentage along X-
axis and temperature along Yaxis.
(d) All of the above

17. IES 2010
Pearlite phase in an iron-carbide phase
diagram is
(a) Eutectic phase
(b) Hypoeutectic mixture
(c) Eutectoidal mixture
(d) Hypereutectic phase

18. IES-1995
Match List I with List II and select the correct
answer using the codes given below the lists:
List I (Name of Material) List II (% Carbon Range)
A. Hypo-eutectoid steel 1. 4.3 - 6.67
B. Hyper-eutectoid steel 2. 2.0 - 4.3
C. Hypo-eutectic cast iron 3. 0.8 - 2.0
D. Hyper-eutectic cast iron 4. 0.008 - 0.8
Codes:A B C D A B C D
(a) 4 3 2 1 (b) 1 3 2 4
(c) 4 1 2 3 (d) 1 2 3 4

19. IES-2004
An iron-carbon binary alloy has 0.5% C by weight.
What is this alloy called?
(a) Eutectoid alloy
(b) Eutectic alloy
(c) Hypo-eutectoid alloy
(d) Hypereutectoid alloy

20. IES-1995
Eutectoid reaction occurs at
(a) 600°C
(b) 723°C
(c) 1147°C
(d) 1493°C

21. IES-2005
The eutectoid of carbon in iron, above lower
critical temperature, when cooled, results in:
(a) Ferrite and austenite
(b) Ferrite and cementite
(c) Cementite and austenite
(d) Ferrite, cementite and austenite

22. GATE-1992
Match the terms used in connection with heat-treatment of steel with
the micro structural/physical characteristics:
Terms Characteristics
(A) Pearlite (P)Extremely hard and brittle phase
(B) Martensite (Q)Cementite is finely dispersed in ferrite
(C) Austenite (R)Alternate layers of cementite and ferrite
(D) Eutectoid (S)Can exist only above 723°C
(T)Pertaining to state of equilibrium between three
solid phases
(U)Pertaining to state of equilibrium between one
liquid and two solid phase
Codes: A B C D A B C D
(a) R P S T (b) R S P T
(c) T R P S (d) T R S P

23. IES-2006
Match List-I (Fe-Fe3C Phase Diagram Characteristic) with
List-II (Phase) and select the correct answer using the code
given below the Lists:
List-I List-II
A. Alpha (α) iron 1. iron
B. Iron carbide having crystal lattice 2.Eutectic
with 3 iron and 1 carbon atom
C. BCC pure allotrope of iron is stable 3.Ferrite
between 1388 °C and is melting
point at 1535°C 4.Cementite
Codes:A B C A B C
(a) 4 2 3 (b) 3 4 1
(c) 4 2 1 (d) 3 1 2


24. IES-2002
Match List I with List II and select the correct answer:
List I (Phase diagram) List II (Characteristic)
A. Isomorphous system 1.One liquid decomposes into another
liquid and solid
B.Eutectic system 2.One liquid and another solid combine to
form a new solid
C.Peritectic system 3.Two metals are completely soluble in
liquid state And completely insoluble in
solid state
D.Monotectic system 4.Two metals, soluble in solid and liquid
state
Codes:A B C D A B C D
(a) 2 3 4 1 (b) 4 1 2 3
(c) 2 1 4 3 (d) 4 3 2 1

25. IES-1999
In a eutectic system, two elements are completely
(a) Insoluble in solid and liquid states
(b) Soluble in liquid state
(c) Soluble in solid state
(d) Insoluble in liquid state

26. IES-1993
Eutectic reaction for iron-carbon system occurs at
(a) 600°C
(b) 723°C
(c) 1147°C
(d) 1493°C

27. IES-2000
During peritectic solidification, one liquid
(a) Combines with one solid to form a second new
solid
(b) Solidifies into two different solids
(c) Forms one solid
(d) Forms one solid and another liquid

28. IES 2007
Which one of the following is the correct
statement?
Pearlite in iron-carbon system is a
(a) Phase consisting of ferrite and cementite at room
temperature
(b) Mechanical mixture of ferrite and cementite at
room temperature
(c) Eutectic mixture ferrite and cementite at room
temperature
(d) All the above three are correct

29. IES-2005
Increase of ferrite phase in steel increases:
(a) Strength
(b) Hardness
(c) Ductility
(d) Brittleness

30. IES-2005
A 60 C-plain carbon steel has, approximately:
(a) 75% of pearlite and 25% of ferrite
(b) 25% of pearlite and 75% of ferrite
(c) 75% of cementite and 25% of ferrite
(d) 75% of pearlite and 25% of cementite

31. IES-2000
Pearlite consists of
(a) 6.67% C and 93.33% ferrite
(b) 13% Fe and 87% cementite
(c) 13% C and 87% ferrite
(d) 13% cementite and 87% ferrite

32. IES-1997
A given steel test specimen is studied under
metallurgical microscope. Magnification used is
100 X. In that different phases are observed. One
of them is Fe3C. The observed phase Fe3C is also
known as
(a) Ferrite
(b) Cementite
(c) Austenite
(d) Martensite

33. IES-1995
Which one of the following sets of constituents is
expected in equilibrium cooling of a
hypereutectoid steel from austenitic state?
(a) Ferrite and pearlite
(b) Cementite and pearlite
(c) Ferrite and bainite
(d) Cementite and martensite

34. IES-2001
Martensite is a super-saturated solution of carbon
in
(a) Alpha iron
(b) Beta iron
(c) Gamma iron
(d) Delta iron

35. 1. Eutectic ( Fe-C)
 (L1S1+S2)
 Two materials are completely soluble in liquid state
and completely insoluble in solid state.
L
µ +
µ
µ+L +L

36. 2. Peritectic ( Fe-C)
 (L+S1S2)
 One Liquid and another solid combine to form a new
solid.
L
B
µ
L+µ
µ
µ + 
+L
L

37. 3. Monotectic FeO2-SiO2
 (L1S1+L2)
 One Liquid decomposes into another liquid and solid.
µ 2L
1L 2L1L
1L +
2L
2L+µ
µ

38. 4. Syntectic Na-Zn
 (L1+L2S1)
1
L 2
L
1L 2L
2
L1L +
+µ+µ
µ
µ
LL

39. 5. Eutectoid Fe-C
 (S1S2+S3)
µ

¡µ ¡+
µ +
¡ +
µ

40. 6. Peritectoid Cu-Al
 S1 + S2S3
µ

µ+r
µ + 
+rµ

41.  Isomorphus system: In a binary system when there is
complete inter-solubility between components in all
phases, the system is isomorphus.
Ex: Cu-Ni, Al2O3-Cr2O3, NiO-MgO
 Azeotropic system: Some of isomorphus binary
system, the liquidus touches, the solid tangentially at a
minimum temperature which is lower then milting
temperature of either of the two components.
µ+L µ+L
L
µ
µ+p
A B
½
½
T
Composition B w/o
Au-Cu, Au-Ni

42. IES 2007
Match List I with List II and select the correct answer using the
code given below the Lists:
List I List II
(Name of the Invariant (Invariant Reaction during
Reaction) cooling)
A. Monotectic 1.LIQUID….SOLID1 + SOLID2
B. Eutectic 2.LIQUID1…..LIQUID2 + SOLID
C. Eutectoid 3.SOLID1…..SOLID1 + SOLID2
D. Peritectic 4.LIQUID + SOLID1…..SOLID2
Code: A B C D A B C D
(a) 3 1 2 4 (b) 2 4 3 1
(c) 3 4 2 1 (d) 2 1 3 4

43. IES-2004
Consider the following lead-tin phase diagram
given below:
For which one of the following alloy compositions,
the alloy will have the lowest melting point at 185oC
(a) 20% Sn and 80% Pb by weight
(b) 60% Sn and 40% Pb by weight
(c) 97% Sn and 3% Pb by weight
(d) 40% Sn and 60% Pb byweight

44. Lever Rule
 At a point in a phase diagram, phases present and their composition (tie-line
method) along with relative fraction of phases (lever rule) can be computed.
 Relative amount of liquid and solid phases is given respectively by:
 Therefore it is not restricted to solid phases only.
L S L S
cV Uc
C C
UV UV
= C = C = 1

45. IES-2008
Assertion (A): Lever Rule can be applied to determine
relative amounts of phases present at any
temperature.
Reason (R): Lever Rule is restricted to estimate
relative phases, only if they are solid phases.
(a) Both A and R are true and R is the correct explanation
of A
(b) Both A and R are true but R is NOT the correct
explanation of A
(c) A is true but R is false
(d) A is false but R is true

46. Example
 In a binary system of A and B, if a liquid of 35% A (65%
B) is co-exists with a solid of 75% A (25% B), for an
overall composition of 40% A, the fraction of the liquid
is given by
75 40
0.875
75 35
LC

 


48. GATE-2016
In a binary system of A and B, a liquid of
20% A (80% B) is coexisting with a solid of
70% A (30%B). For an overall composition
having 40% A, the fraction of solid is
(a) 0.40 (b) 0.50
(c) 0.60 (d) 0.75

49. Fe-C alloy classification
 Fe-C alloys are classified according to wt.% C present
in the alloy for technological convenience as follows:
 Commercial pure irons % C < 0.008
 Low-carbon/mild steels 0.008 - %C - 0.3
 Medium carbon steels 0.3 - %C - 0.8
 High-carbon steels 0.8- %C - 2.11
 Cast irons 2.11 < %C

51. IES-2003
According to Gibbs' phase rule, the number of
degrees of freedom of an eutectic point in a binary
system is
(a) 1
(b) 2
(c) 0
(d) 3

52. The End