material engineering notes

1. 1
School of Mechanical Engineering, VIT - Vellore
• 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 part of
the diagram, up to
around 7% Carbon.
The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram
1
• Phases present
 α-ferrite,
 γ-ferrite,
 δ-ferrite,
 Fe3C (iron carbide
or cementite)
 Fe-C liquid solution
School of Mechanical Engineering, VIT - Vellore
 α-ferrite - solid solution of C in BCC Fe
• Stable form of iron at room temperature.
• The maximum solubility of C is 0.022 wt%
• Transforms to FCC γ-austenite at 912 °C
 γ-austenite - solid solution of C in FCC Fe
• The maximum solubility of C is 2.14 wt %.
• Transforms to BCC δ-ferrite at 1395 °C
• Is not stable below the eutectic
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 T, above 1394 °C
• Melts at 1538 °C
Phases in Fe–Fe3C Phase Diagram
2
 Fe3C (iron carbide or cementite)
• This intermetallic compound is
metastable, it remains as a compound
indefinitely at room T, but decomposes
(very slowly, within several years) into α-
Fe and C (graphite) at 650 - 700 °C
 Fe-C liquid solution
α-ferrite austenite

2. 2
School of Mechanical Engineering, VIT - Vellore
• Pure iron when heated experiences two changes in crystal structure before it melts.
• At room temperature the stable form, ferrite (α iron) has a BCC crystal structure.
• Ferrite experiences a polymorphic transformation to FCC austenite (γ iron) at 912 ˚C (1674 ˚F).
• At 1394˚C (2541˚F) austenite reverts back to BCC phase δ ferrite and melts at 1538˚C (2800˚F).
• Iron carbide (cementite or Fe3C) an
intermediate compound is formed
at 6.7 wt% C.
• Typically, all steels and cast irons have
carbon contents less than 6.7 wt% C.
• Carbon is an interstitial impurity in iron
and forms a solid solution with the
α, β, γ phases.
Changes in Crystal Structure
3
School of Mechanical Engineering, VIT - Vellore
• C is an interstitial impurity in Fe. It forms a solid solution with α, β, γ phases of iron
• Maximum solubility in BCC α-ferrite is limited (max. 0.022 wt% at 727 °C) – which can be
explained by the shape and size of the BCC interstitial positions, which make it difficult to
accommodate the carbon atoms. BCC has relatively small interstitial positions. Even though
present in relatively low concentrations, carbon significantly influences the mechanical
properties of ferrite
• Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial
positions
• Mechanical properties: Cementite is very hard and brittle - can strengthen steels.
Mechanical properties also depend on the microstructure, that is, how ferrite and cementite
are mixed.
• Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic
A few comments on Fe–Fe3C system
4

3. 3
School of Mechanical Engineering, VIT - Vellore
• Three types of ferrous alloys:
– Iron:
• less than 0.008 wt % C in α−ferrite at room T
– Steels:
• 0.008 - 2.14 wt % C (usually < 1 wt % );
• α-ferrite + Fe3C at room T
– Cast iron:
• 2.14 - 6.7 wt % (usually < 4.5 wt %)
Classification - Types of ferrous alloys
5
School of Mechanical Engineering, VIT - Vellore
• In binary phase diagrams, a horizontal line always indicates an invariant reaction.
• Three invariant reactions are present in Iron–Iron Carbide (Fe–Fe3C) Phase Diagram.
1. Peritectic reaction
2. Eutectic reaction
3. Eutectoid reaction
Invariant Reactions in Fe–Fe3C System
6
1493 °C
1150 °C
727 °C
γ
δ

4. 4
School of Mechanical Engineering, VIT - Vellore
Peritectic, involves the following phase transformation.
L(0.53% C) + δ (BCC Ferrite of 0.1% C) → γ (FCC Austenite of 0.18% C)
• The maximum solubility of carbon in BCC δ-iron is 0.1% (point M)
whereas in FCC γ-iron, it is greater. The presence of carbon
influences the allotropic changes. As carbon is increased or added
to the iron, the temperature increases from 1400ºC to 1493ºC at
0.1% carbon.
Peritectic reaction - Fe–Fe3C System
7
M
N
P-
B
School of Mechanical Engineering, VIT - Vellore
Consider the portion NMPB in Peritectic Reaction
• On cooling, the portion NM represents the beginning of the crystal
structure change from BCC δ iron to FCC γ iron for alloys
containing less than 0.1% carbon.
• Line MP represents the beginning of crystal structure change by
means of peritectic reaction for the alloys between 0.1 & 0.18%
Carbon.
• Line NP represents the end of crystal structure change for alloys
containing less than 0.18% C.
• Portion PB represents the end of crystal structure by means of
peritectic reaction for the alloys between 0.18- 0.5% carbon. Here
the reaction takes place isothermally (i.e.) at constant temperature.
• At the peritectic reaction point, liquid of 0.53% C combines with δ
ferrite of 0.1% C to form FCC γ austenite of 0.18% C
Peritectic reaction - Fe–Fe3C System
8
M
N
P-
B

5. 5
School of Mechanical Engineering, VIT - Vellore
• Eutectic Point is given by point E (refer fig.2) exists at 4.3% Carbon
and at the temperature of 1147°C.
• Horizontal line represents the eutectic temperature line and
whenever an alloy crosses the line must undergo the eutectic
reaction
• Any liquid that is present when this line is reached must solidify
now into very fine intimate mixture of two phases namely austenite
(γ) and cementite (Fe3C).
• The eutectic mixture has been given with the name LEDEBURITE
and the equation is given as
Eutectic reaction - Fe–Fe3C System
9
L
(4.3% C)
(FCC)
2.14% C
6.67%C
• The eutectic mixture is not usually seen in the microscope because the austenite is not stable at
room temperatures and must undergo another reaction during cooling
School of Mechanical Engineering, VIT - Vellore
• An alloy of eutectoid composition (0.76 wt% C) as it is cooled from a temperature within the phase
region, say, 800 °C—that is, beginning at point a and moving down the vertical line xx`.
• Initially, the alloy is composed entirely of the austenite
phase having a composition of 0.76 wt% C (Figure a).
As the alloy is cooled, no changes will occur until the
eutectoid temperature (727 °C) is reached.
Upon crossing this temperature to point b, the austenite
transforms to α and Fe3C) .
• This microstructure, represented schematically in
point b, is called pearlite (alternating layers or lamellae
of α and Fe3C.
Development of Microstructure - Fe–Fe3C System
10

6. 6
School of Mechanical Engineering, VIT - Vellore
• Formation of pearlite structure
– Nucleating at γ grain boundary,
– growth by diffusion of C to achieve the compositions
of α and Fe3C (with structural changes)
– α lamellae much thick ( relative layer thickness
is approximately 8 to 1)
Pearlite
11
Redistribution of carbon
by diffusion
• Austenite – 0.76 wt% C;
• Ferrite - 0.022 wt% C
• Cementite - 6.70 wt% C
upper-critical-
temperature line
School of Mechanical Engineering, VIT - Vellore
• Composition C0 to the left of the eutectoid, between 0.022 and 0.76 wt% C; is termed a hypoeutectoid
(less than eutectoid) alloy.
• At about 875°C, point c, the microstructure will consist
entirely of grains of the γ phase (Fig c)
• Cooling to point d, at about 775°C, both α phase and
γ phase coexist (Fig d).
• Cooling from point d to e, just above the eutectoid but
still in the α + γ region, will produce an increased fraction
of the α phase and a microstructure similar to fig e,
the particles will have grown larger.
• As the temperature is lowered just below the eutectoid, to
point f, all the phase that was present at temperature Te
(and having the eutectoid composition) will transform
to pearlite,
Hypoeutectoid Alloys
12

7. 7
School of Mechanical Engineering, VIT - Vellore
• In the austenite range, it is a uniform interstitial solid solution. Upon slow cooling, nothing happens until
the line MO is crossed at point d. This MO line is known as the upper-critical-temperature line on the
hypoeutectoid side.
Hypoeutectoid Alloys
13
• At d, ferrite must begin to form at the austenite grain
boundaries. Since ferrite can dissolve very little carbon,
in those areas that are changing to ferrite the carbon
must come out of solution before the atoms rearrange
themselves to BCC
• The carbon which comes out of solution is dissolved in
the remaining austenite, so that, as cooling progresses
and the amount of ferrite increases, the remaining
austenite becomes richer in carbon.
• Its carbon content is gradually moving down and to the
right along the MO line. Finally, the line NO is reached at
point f.
School of Mechanical Engineering, VIT - Vellore
• The ferrite phase will be present both in the pearlite and also as the phase that formed while
cooling through the α and γ phase region.
• The ferrite that is present in the pearlite is
called eutectoid ferrite,
whereas the other, that formed above Te, is
termed proeutectoid ferrite (meaning “pre- or
before eutectoid”)
Hypoeutectoid Alloys
14

8. 8
School of Mechanical Engineering, VIT - Vellore
• Compositions to the right of eutectoid (0.76 - 2.14 wt % C) hypereutectoid (more than
eutectoid -Greek) alloys.
γ → γ + Fe3C → α + Fe3C
• Hypereutectoid alloys contain proeutectoid cementite
(formed above the eutectoid temperature) plus pearlite
that contain eutectoid ferrite and cementite.
Hypereutectoid Alloys
15
School of Mechanical Engineering, VIT - Vellore
• In the austenite range, this alloy consists of a uniform FCC solid solution with each grain containing 1.0
percent carbon dissolved interstitially.
Hypereutectoid Alloys
16
• Upon slow cooling, nothing happens until the line OP is
crossed at point h. This line is called upper-critical-
temperature line on the hypereutectoid side.
• The OP line shows the maximum amount of carbon that can
be dissolved in austenite as a function of temperature.
• Above the OP line, austenite is an unsaturated solid
solution.
• At h, the austenite is saturated in carbon. As the
temperature is decreased, the carbon content of the
austenite, that is, the maximum amount of carbon that can
be dissolved in austenite, moves down along OP line
towards point O.
• Therefore, as the temperature decreases from h to i, the
excess carbon above the amount required to saturate
austenite is precipitated as cementite primarily along the
grain boundaries.
• Finally, the eutectoid line is reached at i. This line is called
the lower-critical-temperature line on the hypereutectoid side

9. 9
School of Mechanical Engineering, VIT - Vellore
Development of Microstructure - Fe–Fe3C System
17
Eutectoid steel
α+Fe3C
Pearlite
Hypoeutectoid steel
α+Fe3C
Pearlite +
proeutectoid ferrite
Hypereutectoid steel
α+Fe3C
Pearlite +
proeutectoid cementite
School of Mechanical Engineering, VIT - Vellore
How to calculate the 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.
Development of Microstructure - Fe–Fe3C System
18
• Fraction of α phase is determined
by application of the lever rule
across the entire (α + Fe3C) phase.

10. 10
School of Mechanical Engineering, VIT - Vellore
Example for hypoeutectoid alloy with composition
• Fraction of pearlite:
• Fraction of proeutectoid ferrite α’:
Development of Microstructure - Fe–Fe3C System
19
74
.
0
022
.
0
022
.
0
76
.
0
022
.
0 '
0
'
0 −
=
−
−
=
+
=
C
C
U
T
T
WP
74
.
0
76
.
0
022
.
0
76
.
0
76
.
0 '
0
'
0
'
C
C
U
T
U
W
−
=
−
−
=
+
=
α
'
0
C
School of Mechanical Engineering, VIT - Vellore
Example for hypereutectoid alloy with composition
• Fraction of pearlite:
• Fraction of proeutectoid cementite:
Development of Microstructure - Fe–Fe3C System
20
94
.
5
70
.
6
76
.
0
70
.
6
70
.
6 '
1
'
1 C
C
X
V
X
WP
−
=
−
−
=
+
=
94
.
5
76
.
0
76
.
0
70
.
6
76
.
0 '
1
'
1
3
−
=
−
−
=
+
=
C
C
X
V
V
W C
Fe
'
1
C

11. 11
School of Mechanical Engineering, VIT - Vellore
Determination of relative amount of ferrite, cementite and pearlite
21
School of Mechanical Engineering, VIT - Vellore
Determination of relative amount of ferrite, cementite and pearlite
22

12. 12
School of Mechanical Engineering, VIT - Vellore
Determination of relative amount of ferrite, cementite and pearlite
23
School of Mechanical Engineering, VIT - Vellore
• The microstructural development of iron–carbon alloys it has been assumed that, upon
cooling, conditions of metastable equilibrium have been continuously maintained; that is,
sufficient time has been allowed at each new temperature for any necessary adjustment in
phase compositions and relative amounts as predicted from the Fe–Fe3C phase diagram.
Influence of other Alloying Elements - Teutectoid changes
24
Fig 1: The dependence of eutectoid
temperature on alloy concentration for
several alloying elements in steel
• These cooling rates are impractically slow and
really unnecessary; in fact, on many occasions
nonequilibrium conditions are desirable. Two
nonequilibrium effects of practical importance are
1. the occurrence of phase changes or
transformations at temperatures other than those
predicted by phase boundary lines on the phase
diagram, and
2. the existence at room temperature of non-
equilibrium phases that do not appear on the
phase diagram.

13. 13
School of Mechanical Engineering, VIT - Vellore
• Additions of other alloying elements (Cr, Ni,Ti, etc.) bring about rather dramatic changes in
the binary iron–iron carbide phase diagram, Fig 1. The extent of these alterations of the
positions of phase boundaries and the shapes of the phase fields depends on the particular
alloying element and its concentration.
Influence of other Alloying Elements - Ceutectoid changes
25
Fig 2: The dependence of eutectoid
composition (wt% C) on alloy concentration
for several alloying elements in steel.
• One of the important changes is the shift in position of
the eutectoid with respect to temperature and to carbon
concentration. Fig 1 and 2, which plot the eutectoid
temperature and eutectoid composition (in wt% C) as a
function of concentration for several other alloying
elements. Thus, other alloy additions alter not only the
temperature of the eutectoid reaction but also the relative
fractions of pearlite and the proeutectoid phase that form.
Steels are normally alloyed for other reasons, however-
usually either to improve their corrosion resistance or to
render them amenable to heat treatment
School of Mechanical Engineering, VIT - Vellore
• Cementite (Fe3C)
– Contains 6.67% wt of Carbon
– Hard, Brittle Interstitial compound
– Tensile strength – 5000 psi approx. and has high compressive strength
– Crystal structure is orthorhombic
• Austenite (γ)
– Interstitial solid solution of carbon
– Has FCC crystal structure – can accommodate more carbon than ferrite
– Max. solubility of carbon in this phase is 2% at 1148 °C and lowers to 0.8% at 723 °C
– Tensile strength – 1,50,000 psi; Elongation – 2% in 2”
– Hardness – 40 HRC
– Not normally stable at room temperatures
Definitions of structures
26

14. 14
School of Mechanical Engineering, VIT - Vellore
• α - Ferrite
– Interstitial solid solution of carbon in BCC crystal lattice
– As indicated in the Iron- Iron carbide equilibrium diagram, carbon is only slightly soluble
in -Ferrite and has the solubility of 0.025% at 723 °C
– Softest structure that appears on the diagram
– Average Props : TS – 40000 psi, Hardness – 90BHN
• Pearlite (α + Fe3C)
– Eutectoid mixture containing 0.8% Carbon and is formed at 723 °C on very slow cooling
– Microstructure has very fine plate like / lamellar structure
– Average Props : TS – 120000 psi, Hardness – 20HRC
Definitions of structures
27