Materials Engineering and Technology (MEE 1005)
Fall 2021-22 MEE1005
Instructor: Ariful Rahaman
Contact Information
Instructor: Ariful Rahaman
Office: GDN121A
E-mail: arahaman@vit.ac.in
Fe-Fe3C (Fe-C) Phase Diagram
 Pure iron when heated experiences 2 changes in






crystal structure before it melts.
At room temperature the stable form, ferrite (a
iron) has a BCC crystal structure.
Ferrite experiences a polymorphic transformation
to FCC austenite (g iron) at 912 ˚C (1674 ˚F).
At 1394˚C (2541˚F) austenite reverts back to BCC
phase d 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 a, g, d phases.
2
Phases in Fe-C System
Steel: 0.005 to 2 wt% C in Fe
Most carbon steel is less than 1 wt% C
Greatest tonnage produced in the 0.2 to 0.3 wt% C, used for structural steel
in buildings, bridges, ships, etc.
Greater than 1 wt% C is rare, used for razor blades,
cutlery, etc.
Cast iron: >2 wt% C in Fe
Usually has other elements added, such as Si
Steel Phases
Fe-Fe3C (Fe-C) Phase Diagram
Fe-Fe3C (Fe-C) Phase Diagram
Application
Invariant Reactions in Fe-C System
A horizontal line always indicates an invariant reaction
in binary phase diagrams
Peritectic Reaction
a (0.1 wt % C )  L (0.5 wt % C )  d (0.18 wt % C )
1493o C
Eutectic Reaction
L (4.3 wt % C )  g (2.1 wt % C )  Fe3C (6.67 wt % C )
1150o C
Eutectoid Reaction
g (0.8 wt % C ) a (0.02 wt % C )  Fe3C (6.67 wt % C )
725o C
Eutectoid Reactions in Fe-C System
Eutectoid Reaction
g a  Fe3C
0.8
725o C
cool
0.02
6.67
Pearlite
α and Fe3C with a lamellar structure
Grows into the grains from the grain boundaries
Lamellae promoted by crystallography; closepacked planes line up before and after the
eutectoid transition
Eutectoid Reactions in Fe-C System
Eutectoid Reaction
g a  Fe3C
0.8
725o C
cool
0.02
6.67
Pearlite
Ammount of Fe3C in Pearlite
Red Tie Line below eutectoid temp
f
pearlite
F3C
0.8  0.02 0.78


 0.117
6.67  0.02 6.65
Microstructural Development in Fe-C System
fpearlite below TE = faustenite above TE
Tie-Line above the eutectoid temperature TE
0.8  0.38 0.42
f pearlite 

 0.54
0.8  0.02 0.78
Microstructural Development in Fe-C System
Development of
Microstructure
in a
hypereutectoid
steel
Problem
Q1: A eutectoid steel is slowly cooled from 750 degreeC to a temperature just below 723 degreeC .
Calculate the percentage of ferrite and cementite.
Q2: A 0.45 wt% C hypoeutectoid carbon steel is slowly cooled from 950 C to a temperature just slightly
above 723 C. Calculate the weight percent austenite
and weight percent proeutectoid ferrite in this steel
Q3: Consider 10 kg of 99.55wt% Fe- 0.45 wt% C alloy that is cooled to a temperature just below
the eutectoid temperature.
(a) How many kilograms of proeutectoid ferrite (wpro-α) form?
(b) How many kilograms of eutectoid ferrite (wα) form?
(c) How many kilograms of total ferrite (wtotal-α) form?
(d) How many kilograms of cementite (wFe3C) form?
0.45
Problem
0.45
Problem
Q1: Draw and label the microstructures just above eutectoid and just below eutectoid
temperature
0.2
0.8
1.2
Nucleation and Growth Kinetics
Once the embryo exceeds the critical size r*, the growth of the nucleus starts. Nucleation
continues simultaneously.
Nucleation and growth rates are function of temp. Nucleation rate increases with cooling
rate and degree of undercooling (ΔT= Tm – T).
The over all transformation rate is the product of nucleation and growth rates.
high T (close to Tm): low nucleation and high growth rates coarse microstructure with
large grains
low T (strong undercooling): high nucleation and low growth rates fine structure with
small grains
Nucleation and Growth
• Rate is a result of nucleation and growth of crystals.
• Examples:
 2gTm
r* 
Hf T
T-T-T Diagram (Eutectoid Steel)
Martensite Structures
Named after Adolf Martens
M
Diffusionless, military transformation
Same chemical composition as parent
γ
Sharp interfaces
γ(FCC)
M (BCT)
Not a complete transformation
Martensite Structures
Martensite Structures
Problem
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Ductility
Strength
Possible Transformations
Transformation
T-T-T Diagram (Hypo-Eutectoid Steel)
Transformation
T-T-T Diagram (Hyper-Eutectoid Steel)
Problem
Continuous Cooling Transformation (C-C-T) Diagrams
 Isothermal heat treatments are
not the most practical due to
rapidly cooling and constant
maintenance at an elevated
temperature.
 Most heat treatments for steels
involve the continuous cooling
of a specimen to room
temperature.
 TTT diagram (dashed curve) is
modified for a CCT diagram
(solid curve).
 For continuous cooling, the time
required for a reaction to begin
and end is delayed.
 The isothermal curves are
shifted to longer times and
lower temperatures.
C-C-T Diagram
C-C-T Diagram
1
2 3
4
1: 100% martensite (This is called the critical cooling rate. It is the slowest cooling rate that will
produce 100% martensite.)
2: ½ martensite, ½ pearlite (This is called a split transformation.)
3: fine pearlite
4: coarse pearlite
Note: there will be no bainite with a CCT.
C-C-T Diagram
Problem
Q1: Using the following isothermal transformation diagram for eutectoid steel, describe (in words) the resulting
room temperature microstructures produced by the following conditions (draw in the specified cooling path on the
diagram and label as appropriate a, b, c). Indicate what phases you expect in the final product. Assume the material
has been fully austenitized before cooling.
a) Rapidly cool to 630°C, soak for 30 hours, then quench to room temperature:
b) Rapidly cool to 500°C, soak for 30 hours, then quench to room temperature:
c) Rapidly cool to 270°C, soak for 16 minutes, then quench to room temperature:
d) Rapidly cool to 500°C, soak for 6 seconds, then quench to room temperature:
Problem
(i) An Indian steel producer has four “quench baths,” used to quench plates of eutectoid steel to
680˚C, 580˚C, 340˚C, and 30˚C respectively. Using the TTT diagram below, advise the company
on how they can produce steel with the following microstructures. Assume that each bath will
instantaneously allow the steel to reach the bath temperature.
a) 25% fine pearlite, 25% bainite, 50 % martensite.
b) 50% coarse pearlite, 50% martensite.
c) 50% bainite, 50% coarse pearlite
d) 0% coarse pearlite, 100% martensite