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PHASE
TRANSFORMATIONS
in METALS and ALLOYS
IE-114 Materials Science and General Chemistry
Lecture-12-13
Why do we study phase
transformations?
The desirable mechanical properties of a material can be obtained
as a result of phase transformations using the right heat treatment
process.
In order to design a heat treatment for some alloy with desired RT
properties, time and temperature dependencies of some phase
transformations can be represented on modified phase diagrams.
Phase Transformation in Metals
Development of microstructure in both single- and two-phase alloys involves phase transformations-which involves the alteration in the number and character of the phases. Phase transformations take time and this allows the definition of transformation rate or kinetics.
Phase transformations alter the microstructure and there can be three different classes of phase transformations:
a) Diffusion dependent transformations with no change in number of phases and composition ( solidification of a pure metal, allotropic transformations, etc.)
b) Diffusion dependent transformations with change in number of phases and composition (eutectoid reaction)
c) Diffusionless transformations (martensitic transformation in steel alloys)
Steel is heat treated in order to:
Increase/decrease strength (Hardening, normalizing, annealing)
Reduce internal stresses (stress relief annealing)
Adjust grain size (normalizing)
Remove the effects of cold working (normalizing)
Improve machinability (annealing)
Avoid microsegregation (homogenizing)
All of the heat treatment operations conducted on steels based on the
heating of the material to some temperature to form fully austenite and
cooling the material to low temperatures at different rates (formation
of different phases depending on the cooling rate applied)
HEAT TREATMENT: A combination of heating and cooling
operations, timed and applied to a metal or alloy in the solid state in
a way that will produce desired properties (physical and sometimes
chemical properties)
1) Continuous cooling ( very slow, moderate or fast cooling)
2) Interrupted cooling (very fast cooling to a temperature(undercooling) and wait at
that temperature long enough for transformation of austenite to take place, then cooling to
low temperatures (isothermal transformation), e.g.room temperature)
Heat treatment of steels
1.Step: Austenitization (heat treatment to obtain ~100% austenite phase)
Cooling;
2.Step:Cooling to low temperatures at different rates
We have to decide the lowest possible temp. in the -region
because of grain coarsening (25oC higher than the -trans. temp.)
For each 1 inch thickness the austenitization time is 45 mins.
Very slow continuous cooling of eutectoid steel
(under equilibrium conditions)
When the carbon content of steel and the temperature are known fraction and
composition of phases can be predicted using Fe-Fe3C diagram.
However, the total transformation time is not known. Transformation time is
missing in Fe-Fe3C diagram
Upon very slow cooling, transformation of austenite to pearlite occurs by diffusion
of carbon atoms(time is required for carbon diffusion). So, this type of
transformation is called DIFFUSIONAL(Time Dependent) TRANSFORMATION.
Fe-Fe3C diagrams are equilibrium phase diagrams and they don’t give
information about non-equilibrium cooling conditions
Cooling rate is so fast that carbon atoms cannot find enough
time to go and locate their equilibrium positions
DIFFUSIONLESS (e.g.martensitic) TRANSFORMATIONS
Under non-equilibrium conditions TTT-diagrams are used to investigate the
transformation fraction, temperatures and time.
TTT(Time-Temperature-Transformation)-diagrams
IT-diagrams(IsothermalTransformation diagrams)
CT-diagrams (Continuous Cooling Transformation diagrams)
(used in interrupted cooling conditions) (used in continuous cooling conditions)
Fraction transformed
Phase Transformation involves:
1) Nucleation (the formation of very small particles of the new phase at
some imperfection sites, e.g.,grain boundaries
2) Growth (nuclei increase in size)
Transformation rate:
Fraction of transformation is measured at constant temperatures as a function of time by
either microscopic examination or measurement of some physical property such as
electrical conductivity.Then, the fraction of transformed material versus the logarithm of
time graphs are drawn.
AVRAMI EQUATION
y: fraction of transformation
t : time
k,n : time independent constants
Rate of transformation,r : Reciprocal of time required for 50% transformation
Rate,r : 1/t50%
• Reaction rate increases with T.
TRANSFORMATION RATE ~ UNDERCOOLING( T)
As undercooling is increased finer microstructures are formed
increasing T
ISOTHERMAL TRANSFORMATION(IT) DIAGRAMS
• Fe-C system, Co = 0.77wt%C (Eutectoid steel)
• Transformation at T = 675oC.
logt
IT-diagram of an eutectoid steel
Proeutectoid ferrite
Proeutectoid cementite
Coarse Pearlite
Fine Pearlite
Upper Bainite
Lower Bainite
Martensite
Depending on the alloy composition, amount of undercooling and
isothermal transformation time, one can obtain one of the following
phases or combination of them;
IT diagram of a hypoeutectoid steel
EUTECTOID STEEL (0.77 wt.%C)
IT -DIAGRAMS
The position of IT diagrams
Two factors will change the position of
the curves;
1) Chemical composition
2) Austenitic grain size
With few exceptions, an increase in
carbon or alloy content or in grain size
of the austenite always retards
transformation (transformation lines shift
to longer times)
• Eutectoid steel, Co = 0.77wt%C
• What happens if an eutectoid steel is austenitized at T > 727oC then it is
rapidly cooled to 625oC and hold isothermally for about 100 seconds;
PEARLITE TRANSFORMATION in eutectoid steel
FINE and COARSE PEARLITE (Eutectoid steel)
- Smaller T:
colonies are larger
- Larger T:
colonies are smaller
• Ttransf just below TE--Larger T: diffusion is faster
--Pearlite is coarser.
• Ttransf well below TE--Smaller T: diffusion is slower
--Pearlite is finer.
Interlamellar distance
is very close
Comparison of mechanical properties of
fine and coarse pearlite
BAINITE PHASE (Phase mixture of and Fe3C)
Austenite transforms to -lathes (strips) and rods of Fe3C isothermally between
the nose region and Ms temperature
Bainite is a phase mixture of and Fe3C
Transformation is a diffusion controlled process
***For plain carbon steels bainite is only formed by isothermal
transformation***
Schematic IT diagram for eutectoid steel
Upper bainite
Lower
bainite
Upper(feathery) and Lower(Needlelike) Bainite:
HARDNESSLower bainite > upper bainite > fine pearlite > medium pearlite > coarse pearlite
UPPER BAINITE
LOWER BAINITE
Upper bainite
Transformation of (FCC) to Martensite (BCT, body centered tetragonal)
Transformation is rapid!(shear transformation)
% transformation depends on temperature only.
Marte
ntite
needle
sA
uste
nite
60 mEUTECTOID STEEL (Quenching from Austenitization temp.)
MARTENSITE
MARTENSITE
Avrami-type Every steel has a specific TTT-
diagram and on which its Ms
and Mf are indicated
Both Ms and Mf decrease as carbon content increases
xx x
x
x
xpotential C atom sites
Fe atom sites
(involves single atom jumps)
Expansionc
a
EXPANSION occurs because atoms of martensite are less densely
packed than that of austenite. This expansion during the formation of
martensite produces high localized stress which result in the plastic
deformation of the matrix.
The degree of expansion depends on carbon content
• HARDNESS INCREASES DUE TO HIGHLY DISTORTED LATTICE
MARTENSITE
Altough martensite is very hard unfortunately it is very BRITTLE for industrial use. In
order to toughen the steel and make it more ductile, a heat treatment called
tempering is applied.
Tempering is applied by reheating a martensitic steel to a temperature below
eutectoid temperature and cooling it at any rate to increase the ductility and
toughness.
TEMPERING
1) Conventional Quenching and Tempering
As-quenched
Annealed
TemperingWhile tempering, you are losing
from hardness but if it is applied
correctly it does not reduce
hardness excessively
Strength and reduction in area change with
tempering temperature
Relative hardness values of martensite, tempered
martensite and fine pearlite depending on the
carbon content
SPHEROIDIZING HEAT TREATMENT
If steel alloy with pearlitic or bainitic structure is heated to and left at
a temperature below the eutectoid temperature (such as 7000C) for
18 to 24 hours, another microstructure, called spheroidite, forms.
cementite
ferrite
Spheroidite: has lower strength and hardness than pearlitic microstructures.
Of all the steel alloys, those that are softest and weakest have a spheroidite
microstructure. The spheroidized steels have higher ductility than coarse
pearlite.
Spheroidite structure
Use of IT diagrams
Using the isothermal transformation diagram for a 1.13wt%C steel alloy determine the final microstructure
(in terms of just the microconstituents present) of a small specimen that has been subjected to the
following time-temperature treatments. In each case assume that the specimen begins at 920oC, and that
it has been held at this temperature long enough to have achieved a complete and homogenous structure;
a) Rapidly cool to 250oC, hold for 103 s, then quench to room
temperature
b) Rapidly cool to 775oC, hold for 500 s, then quench to room
temperature
c) Rapidly cool to 400oC, hold for 500 s, then quench to room
temperature
d) Rapidly cool to 700oC, hold for 105 s, then quench to room
temperature
e) Rapidly cool to 650oC, hold for 3 s, rapidly cool to 400oC,
hold for 25 s, then quench to room temperature
f) Rapidly cool to 350oC, hold for 300 s, then quench to room
temperature
g) Rapidly cool to 675oC, hold for 7 s, then quench to room
temperature
h) Rapidly cool to 600oC, hold for 7 s, rapidly cool to 450oC,
hold for 4 s, then quench to room temperature
Example:
(a)
(b)
(c)
(d)
(e)
(g)(h) (f)
CONTINUOUS COOLING TRANSFORMATION
(CCT) DIAGRAMS:
In industrial heat-treating operations, in most cases a steel is not
isothermally transformed at a temperature above the martensite start
temperature but is continuously cooled from austenitic temperature to room
temperature.
Comparison of IT and CCT diagrams for eutectoid steel
Isothermal transformation
In continuously cooling a plain-carbon steel, the transformation from austenite
to pearlite occurs over a range of temperatures rather than at a single
isothermal temperature
Moderately rapid
cooling
(NORMALIZING)
Slow cooling
(FULL ANNEALING)
Quenching
EFFECT OF ALLOYING on SHAPE of TTT-DIAGRAM
CCT-diagram, 4340 steel
1) Alloying elements shift the austenite-to-pearlite transformation lines to longer times
2) A seperate bainite nose is formed
IT-diagram, 4340 steel (Ni, Cr, Mn, Mo as alloying elements)
Normally, BAINITE will not form (or will form to a small extent) during continuous cooling of plain carbon
steels. By alloying, it becomes possible to obtain bainite phase upon continuous cooling
Use of CCT diagrams
Example:
Which microstructures do you observe upon cooling of 0.35 wt.% C iron-carbon
alloy after austenitization at 850oC at different rates shown below?
(c) Proeutectoid ferrite + martensite
(a) Proeutectoid ferrite+fine pearlite
(b) Martensite
(d) Proeutectoid ferrite + coarse pearlite
(e) Proeutectoid ferrite + pearlite + martensite