MSE 2001- Chapter 8 Pt2 Ppt
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Transcript of MSE 2001- Chapter 8 Pt2 Ppt
32Phase Transformation in Steels: Fe-Fe3C System
Eutectoid point
Peritectic isotherm
Eutectic isotherm
Eutectic point
Eutectoid isotherm
Solid phases:
δ-ferrite high temperature
BCC iron
Austenite- FCC iron (γ)
α-ferrite- BCC iron
Cementite-Fe3C
When cooled to just below the
eutectoid temperature, an
alloy containing 0.77 wt % C
transforms from γ-Fe into a
two-phase mixture with a
lamellar (layered) morphology
of α-Fe and Fe3C plates.
� Two-phase lamellar
material: Pearlite
33Important Phases and Structures in Steels
• Hypoeutectoid Steels- Compositions to the left of the eutectoid compositions
• Hypereutectoid Steels- Compositions to the right of the eutectoid compositions
• Proeutectoid Ferrite- Ferrite that forms prior to the eutectoid ferrite
• Proeutectoid Cementite- Cementite that forms prior to the eutectoid cementite
• Decomposition of austenite can lead to:
– Pearlite (γ-Fe → α-Fe +Fe3C; composition: 0.77 wt % C)
– Bainite (Eutectoid of feathery-shaped α-ferrite and Fe3C particles)
– Martensite (Interstitial solid solution of C in a body-centered tetragonal Fe)
– Spheroidite (Spherical particles of cementite in a matrix of α-ferrite)
34
Phase Transformation in Steels: Structure of Pearlite
Alternate lamellae of α-Fe and Fe3C
make up the structure of pearlite.
α-Fe
Fe3C
γ
Advancing interface
γ-Fe → α-Fe +Fe3C with relatively small undercooling
35Fraction of transformed pearlite over time
Isothermal
transformation
diagrams
C curves
36Decomposition of Austenite to Form Pearlite:
Redistribution of C along the austenite-pearlite interface
3
0.77 0.0220.112
6.70 0.022Fe C
f−
= =−
6.70 .770.888
6.70 0.022fα
−= =
−
Pearlite spacing depends on
the diffusion rate of carbon.
37
Microstructure of Formed Pearlite Depends on T
Decreasing the isothermal hold
temperatures decreases the
interlamellar spacing.
655 °°°°C 600 °°°°C534 °°°°C 487 °°°°C
Pearlite formed at higher temperature is coarser
than that formed at lower temperature.
Direction of α+Fe3C growth
At High T, the diffusion of carbon is large. �
Thickness (interlamellar spacing) of structure
is large.
38
Bainite that forms isothermally
γ
Fe3C
α
Schematic of
bainite formation
495 °°°°Cγγγγ
410 °°°°Cγγγγ
As the amount of undercooling (T < 550°C) is increased, an alternative
morphology to pearlite becomes more energetically favorable. � Bainite
γ-Fe → α-Fe +Fe3C with relatively large undercooling
Phase Transformation in Steels: Structure of Bainite
The α-ferrite either has a feathery appearance or occurs as plates. Carbide
particles lie between the α-ferrite regions.
39
If the austenite of composition 0.77 wt % carbon is quenched to <200 °C, it does not form a two-phase mixture of α and Fe3C by diffusion.
An athermal, diffusionless transformation takes place. Carbon atoms in interstitial
position are just trapped. This is known as Martensite.
These are not thermally activated and occur very rapidly and continuously during
cooling. The amount of transformed phase depends solely on temperature, not time.
Decomposition of austenite (γ-Fe) with even large undercooling
Phase Transformation in Steels: Structure of Martensite
10µm
T= 25 °C T= -60 °C T= -100 °CDecreasing temperature, increasing fraction of transformed martensite.
γγγγ
γγγγMartensiteplates
40Phase Transformations in Eutectoid Steel
Austenite to Pearlite-
Diffusional transformation
Austenite to Bainite-
Diffusional transformation
Austenite to Martensite-
Athermal, diffusionless
transformation
(time-independent, so C-curve
becomes horizontal line)
Pro
gre
ssiv
ely
Fin
er
Mic
rostr
uctu
re
Decomposition of austenite (γ-Fe) under different conditions
��: temperature at which martensite
reaction begins; ���: 50% completed
41
Dependence of martensite reaction temperature on %C
Austenite to Martensite
transformation is athermal and
diffusionless.
��: temperature at which
martensite reaction begins;
��: martensite reaction is
finished.
There is a strong dependence of carbon content on the martensitic phase
transformation. As the carbon content increases, more undercooling is required to
induce the martensitic phase transformation.
Unlike pearlite or bainite, in martensite the C atoms remain trapped in interstitial
positions as very effective obstacles to dislocation motion.
Therefore, martensite is the strongest of all common phases found in steels.
42
Structural Transformation: Austenite to Martensite
Austenite (γ, FCC Fe)
c/a = 1.4
Fe
� C
BCT (body-
centered
tetragonal) Fe
crystal with C
interstitial
Martensite
43
1µm
Tempering of Martensite: Martensite → Spheroidite
Increasing the temperature increases the size of the resulting phase. The
driving force for the transformation is the reduction in surface free energy.
426 °C, 1 hour 593 °C, 1 hour 675 °C, 1 hour
Martensite is metastable. When heated, Fe3C begins to form. Carbon diffuses out
of the metastable BCT structure to form the more stable carbide phase.
The carbide precipitates have spherical morphology. So the structure is termed
Spheroidite. Spheroidite is very stable, and has good ductility.
Spheroidite can also be obtained by reheating pearlictic or bainitic phases.
44Example: Thermal treatment of eutectoid steel
100% Austenite(γ)
75% γ, 25% Pearlite
50% γ, 50% Pearlite
25% γ, 75% Pearlite
100%, Pearlite (after 20 seconds)
Case 1: Austenite quenched to 600 °C and held for 20 sec.
45
(x<1 sec) 100% Martensite
(~1.7 sec) 75% Martensite, 25% Pearlite
(~3 sec) 50% Martensite, 50% Pearlite
(~4.5 sec) 25% Martensite, 75% Pearlite
(20 sec) 100%, Pearlite
Example: Thermal treatment of eutectoid steel
Case 2: Austenite quenched to 600 °C, held for x
seconds, and then quenched to room temperature
Example: Phase Transformations in Off-Eutectoid Steels
Proeutectoid α and γ
100% Austenite (γ)
γ and proeutectoid α
γ, proeutectoid α and pearlite
Proeutectoid α and pearlite
Case 1: Austenite quenched to 700 °C and held for different time
47
Proeutectoid α and γ
100% Austenite (γ)
50% γ and 50% Bainite
100% Bainite
Case 2: Austenite quenched to 500 °C and held for different time
Example: Phase Transformations in Off-Eutectoid Steels
48
Proeutectoid α and γ
100% Martensite
50% Bainite and
50% Martensite
100% Bainite
Case 3: Austenite quenched to 500 °C and held for different time,
then quenched to room temperature
Example: Phase Transformations in Off-Eutectoid Steels
49Continuous-Cooling Transformation Diagram
Example: diagram for a eutectoid steelAll previous examples are for
instantaneous quenches
followed by isothermal holds.
When quench rates are not
instantaneous, we need to use
the continuous-cooling
transformation diagram.
In this example:
Path 1: 100% austenite between
O and a1; pearlite develops
between a1 and b1; 100%
pearlite after b1
Path 2: at b2 there are 33%
pearlite and 67% austenite; then
at low temperature 33% pearlite
and 67% martensite
Path 3: transformation to pearlite
is avoided
50
Example of quenching operations for steels
Rapid quenching to avoid
γ-Fe → α-Fe +Fe3C
Temping:
Martensite → spheroidite
for improved ductility
100% MartensiteHold at above �� to minimize thermal
gradients and avoid surface cracks
51
Precipitation from a Supersaturated Solid Solution
X0 at T=T1 for sufficiently long time
A process known as homogenization
or solution heat treatment occurs
� All of the solute dissolves into the
α phase.
If the alloy is rapidly quenched to
temperature T2, the solute is not
immediately able to diffuse out of the
α phase
� the alloy is supersaturated.
The formation of particle from a supersaturated solid solution is a nucleation and
growth process, and the size, number, and spatial distribution of precipitates
(and therefore, the properties) are controlled by temperature and time.
52
Time-temperature process for age-hardened structure
The process of precipitation from a
supersaturated solid solution is called
aging.
Aging at room temperature � natural
aging (too slow)
Aging above room temperature �
artificial aging (what is the best T?)
The first step is a solution heat treatment followed by a rapid quench.
The final step is to reheat the alloy, while remaining well below the solvus
boundary, to accelerate the aging process.
Choosing appropriate aging temperature: two necessary requirements for
commercial application
1. A temperature at which a significant amount of solute can be dissolved.
2. A significant decrease in solubility with decreasing temperature.
53
Example of Aging: The aluminum-copper system
Al-rich portion of the Al-Cu phase diagram
4.5 % Cu at 820 K � Quench to room temperature � Too slow precipitation (too
slow kinetics)
Artificial aging accelerates the precipitation by increasing the atomic diffusion �
the amount of precipitation is small.
We need a balance.
Strength of alloy after aging
54
Solidification and Homogenization of an Alloy
The heat-treating we discussed previously is usually the
final step in material processing. Before that, alloys need to
be homogenized.
Consider the equilibrium cooling (at infinitely slow rate).
Gradually, the alloy is transformed from single liquid
solution (uniform) to L+S (uniform within each phase), then
to single solid solution (uniform).
To
T1
T2
T3
TF
Using tie lines and lever
rule, at � ( � 1, 2, 3…) the
compositions of liquid and
solid are �� and ��,
respectively.
55
T3
T2
To
T1
Non-Equilibrium Solidification
Equilibrium solidification does not generally occur in practice
because of the slow diffusion (almost negligible) in the solid phase.
At any temperature, the composition of the solid and liquid at the
interface is given by the equilibrium phase diagram (tie line).
The composition in each solid grain is not uniform anymore.
For example, at ��the liquid has ��� but
the solid has a center
of ���, surrounded by
a ring of ���, and an
outer ring of ���. The
average composition
is �′��. This process
is called “coring.”
Non-equilibrium
solidus boundary
56Example of Non-Equilibrium Solidification
Al-rich portion of Al-Cu
phase diagram
Effect of Composition
Al-2%Cu
Al-5%Cu
Instead of uniform α-
phase, we see cored
microstructures.
Effect of Cooling Rate
1K/sec 10K/sec
Faster cooling rates lead to greater undercooling, which increases
the nucleation rate and leads to a finer-grained structure.
Al-5%Cu