Heat Treatment & Microstructure
Evolution in Metals
(MM-504)
Lecture # 3a
Compiled for M.E. (Materials Engg.) by:
Engr. Fawad Tariq
Email: t_fawad@hotmail.com
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Materials Engineering Department, NED University of Engineering and Technology
Crystal Structure
BCC structure of ferrite
FCC structure of
austenite
Cementite –
Orthorhombic structure
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Solubility of C in BCC & FCC
(a) Octahedral &
(b) tetrahedral
interstitial voids
in fcc structure
(a) Octahedral &
(b) tetrahedral
interstitial voids
in bcc structure
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Solubility of C in FCC
There are two types of interstitial voids that
may become sites for carbon atoms in bcc and
fcc structures.
In austenite, assuming spherical Fe atoms in
contact, an octahedral site could accommodate
an atom 0.052 nm (0.52 A°) in radius, but a
tetrahedral site could accommodate an atom
only 0.028 nm (0.28A°) in radius (Ref 3.9).
Carbon atoms have radii of 0.07 nm (0.7 A°),
and are therefore more readily accommodated
in the octahedral voids even though some
lattice expansion is required.
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Solubility of C in BCC
In ferrite the interstitial sites are much smaller,
thus explaining the very limited solubility of C.
A tetrahedral site in ferrite could accommodate
an interstitial atom 0.035 nm (0.35 A°) in
radius and an octahedral site, an atom only
0.019 nm (0.19 A°) in radius.
The octahedral sites in ferrite, however, are not
symmetrical, and a C atom would severely
displace only the two atoms at a distance of
a/2, not those at a distance a/_2.
 C atoms appear to prefer the octahedral sites in
ferrite and do produce a severe distortion of the
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lattice inPowerpoint
<100> directions.
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Solubility of C in BCC
 In ferrite, because of the limited number of
carbon atoms that can be accommodated, the
lattice remains essentially cubic.
If large numbers of carbon atoms present in
austenite are trapped in bcc octahedral sites by
rapid cooling, the cubic structure may actually
become tetragonal.
The latter structure typifies the phase
“martensite”.
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Normalizing
Normalizing is the process of raising the
temperature to over 60ºC, above line A3 (for
hypo) or line ACM (for hyper) fully into the γ
range.
Held at this temperature to fully convert the
structure into γ, and then removed form the
furnace and cooled at room temperature under
natural convection.
This results in a grain structure of fine Pearlite
with excess of α or Fe3C.
 Soaking is usually 1 hour/inch of crosssectional
area but not less than 2 hours at temp.
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Normalizing
 Holding time can also be roughly estimated
by:
T (mins) = 60 + D
Low-carbon steels typically do not require
normalizing.
Castings with relatively uniform wall thickness
and section sizes are usually annealed rather
than normalized.
The structure is more homogenous since the
temp. involve in normalizing is higher than
annealing.
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Normalizing
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Purposes of Normalizing
 For more hardness and strength
 Improve machinability
 Reduces internal stresses induced by
operations (forging, machining, welding,
etc.)
 Improve dimensional stability
 Modify and/or refine the grain structure
 Produce a homogeneous microstructure
 Reduce banding (alternate regions of ferrite
and pearlite due to segregation of Mn in
ingots)
 Improve ductility
 Provide a more consistent response when
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hardening
or case
hardening
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Grain Refinement Benefit
Recall Hall-Petch Equation
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Grain Refinement Benefit
Neighboring grains
interfere dislocation
movement by slip !
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Differnce b/w Normalizing and
Annealing
 Fully annealed parts are uniform in softness
(and machinablilty) throughout the entire part;
since the entire part is exposed to the controlled
furnace cooling.
But in normalized part, depending on the part
geometry, the cooling is non-uniform resulting
in non-uniform material properties across the
part.
Thin pieces cool faster and are harder after
normalizing than thicker ones. By contrast,
after furnace cooling in an annealing process,
the hardness of the thin and thicker sections are
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about the same.
Differnce b/w Normalizing and
Annealing
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Differnce b/w Normalizing and
Annealing
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Differnce b/w Normalizing and
Annealing
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Differnce b/w Normalizing and
Annealing
Fig. – Microstructure after (a) hot rolling 20MnCr5
steel and (b) normalizing at 880°C
 More homogenous γ during heating of steel in
normalizing means more finely dispersed will be
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the α and FePowerpoint
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3C.
Microstructure after Normalizing
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Structure formation during
Normalizing
 At normalizing, the steel is first subjected to
pearlite  γ transformation and then on cooling
transformed to γ  α (+ pearlite)
 We need to understand the decomposition of γ
and nucleation, growth of pearlite
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Fig. - Schematic
representation of grain
size formation after
normalization
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Austenite to Pearlite Reactions
On cooling from Ac3, proeutectoid α forms on
γ GB.
 Remember, GBs are potent sites for nucleation
because of high defect energy + Diffusion rate is
high at GB, so heterogeneous nucleation
occurred!
 Slow cooling rate increases the precipitation
of pro- α
 Relatively high cooling rate may results in the
formation of needle like α (Widmannstatten)
 The ejected C from pro- α goes into the γ
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 The C % increases in unstable γ
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Austenite to Pearlite Reactions
 α nucleates in C-lean regions
As the temperature is lowered below Ac1, the
remaining γ (of 0.8% C) transform to pearlite
 Pearlite is a lamellar, regular array of
mechanical mixture of α and Fe3C.
 Grow in colonies till impinge by other colony
 Ferrite etched dark
 % of α and Fe3C can be determine by Lever
rule i.e. approx. 89% α remaining Fe3C.
 Colony of pearlite nucleate from either α or
Fe3C. Powerpoint Templates
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Austenite to Pearlite Reactions
 Diffusion of C is low at lower temps.
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Driving force for Pearlite
 At the Ac1 the free energy of γ is exactly equal
to the free energy of α and Fe3C and there is no
incentive for transformation to occur.
 Interfaces accommodate structural and
chemical discontinuities between phases, and
therefore make positive contributions to raise the
energy of a system.
 Below Ac1, the free energy of a unit volume of
a mixture of α and Fe3C becomes much less than
that of γ.
 Lowering of Gibbs free energy is the driving
force
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Driving force for Pearlite
 This free energy difference is frequently
referred to as the driving force for transformation
and increases with decreasing temperature or
undercooling below the Ac1.
A larger driving force makes possible not only
the development of more colonies of pearlite but
also a finer lamellar spacing within a pearlite
colony, structural changes that result in increased
interfacial area of two types.
A higher density of pearlite colonies results in
increased γ/pearlite interfacial area, and a
reduced interlamellar spacing results in increased
α-Fe3C interfacial
energy within the colonies.
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Driving force for Pearlite
 Many relationships for the change in
interlamellar spacing with undercooling have
been proposed, but the one most closely related
to the above considerations was developed by
Zener and Hillert.
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JML Equation for Pearlite Fraction
Johnson and Mehl, assuming that the pearlite
colonies are spherical and randomly nucleated as
a function of time, developed the following
equation for isothermal pearlite formation:
where f (t) is the volume fraction pearlite formed
at any time t at a given temperature, N is the
nucleation rate of the colonies, and G is the rate
at which the colonies grow into the γ.
 The JML equation describes mathematically
the rate at which austenite is converted to a
pearlitic microstructure by the nucleation and
growth ofPowerpoint
pearlite Templates
colonies.
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JML Equation for Pearlite Fraction
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Nucleation of Pearlite
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Formation of Pearlite
In ternary systems and steels, the effects of
alloying elements must also be considered.
Mn and Ni do not partition themselves
between the α and Fe3C and that, therefore,
pearlite formation in these alloys is dependent
primarily on volume diffusion of C in γ.
Any reduction in the rate of pearlite growth in
these systems is due to the effect of Mn and Ni
on the diffusion of carbon in austenite.
However, Cr & Mo which are strong carbideforming elements, are considered to partition to
the carbide
lamellae by interface diffusion.
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Formation of Pearlite
In the Fe-C-Cr and Fe-C-Mo systems, then,
pearlite growth is retarded because Cr and Mo
atoms must diffuse, a process that is much more
sluggish than the diffusion of C because of the
much larger size of the alloying element atoms co
Steels with < 0.4%C would contain more
proeutectoid α; those with more C would contain
more pearlite.
Depending on the %C of the steel, then, it is
possible to have microstructures consisting of
100% ferrite (if the carbon content is less than or
equal to 0.02%) or 100% pearlite (if the carbon
content is Powerpoint
equal to 0.77%).
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Pearlite in Hypoeutectoid steel
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Pearlite in Hypoeutectoid steel
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Pearlite in Hypereutectoid steel
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Pearlite in Hypereutectoid steel
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Orientation Relationships
 On nucleation, α forms a O.R. with one of the γ
GB.
This relationship is called “Kurdjumov Sachs”
relationship
 Sometime the ferrite may deviate slightly from
having close-packed parallel directions, as shown by
the Nishiyama–Wasserman (NW) orientation
relationship:
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Orientation Relationships
 This habit plane is low
energy habit plane, therefore it
does not move during the
growth of the nucleus
 Therefore, α grain grows
only in one of the two γ grains
associated with the boundary.
 The boundary b/w the α
nucleus and γ grain in which it
grows, is the high energy
boundary and it is non-coherent.
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Orientation Relationships
Ferrite grows in grain with which it does not have
any orientation relationship.
In a similar way, the α component of a pearlite
colony formed at the γ grain boundary should also be
related to one of the austenite grains, irrespective of
whether it is nucleated before or after the cementite.
 Consequently, the pearlite colony will be able to
grow by the movement of an incoherent α - γ
interface i.e. into the γ grain to which the α is
unrelated.
 Orientation relationship between the α and
cementite in the pearlitic colonies is closer to PitschPetch.
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Orientation Relationships
Pitsch-Petch Relationship is given as:
Another O.R. reported by Bagaryatski is called
Baaryatski Relationship is:
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Pro-eutectoid Ferrite Growth
There are two types of boundaries which separate
α lath from the γ grain into which the lath grow:
 the parallel side of the lath
 the edge or end of the lath
 The sides of lath are normally low energy or
coherent boundaries and thus immobile.
 The boundary at the end of the lath is high-energy
boundary and thus mobile.
 Lath grow lengthwise more than thickness
 However, high cooling rate in normalizing
reduces the pro-eutectoid phases as compared to
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annealed structures.
Pro-eutectoid Ferrite Growth
Fig. – Growth of proeutectoid α by the movement of ledges (in the
form of small steps) along low-energy γ-α boundary
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Growth of Pearlite
 Growth primarily depends upon diffusion of
C and Fe atoms.
 C atoms moves from α towards Fe3C.
Other factors are degree of supersaturation and
free energy advantage during the α formation.
 Generally carbide plates grow faster than
ferrite laths.
 Normally, compact α grows on γ GBs whereas
Widmannstatten α grows inside the grains.
 Widmannstatten α forms in steels with
<0.4%C. Coarse γ grains also favors formation of
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Widmannstatten
α
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Growth of Pearlite
Diffusion of C
during transformation
a
g
a
a
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g
Carbon
diffusion
Fig.- Schematics of
interface growth
fronts associated
with the
transformation
of γ to (a) pearlite
and (b) dispersed
Fe3C particles
in α.
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Growth of Pearlite
Fig. – The volume and
interface diffusion
process during growth
of pearlite colony
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Growth of Pearlite
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Growth of Pearlite
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Growth of Pearlite
When a pearlite colony
nucleates at a grain boundary, it
grows into only one of the two
grains.
With no Proeuctectoid
cementite the orientation of
pearlite structure is controlled by
orientation of γ1.
Motion is into grain 2 since
opposite end of Pearlite is higher
energy than at GB.
If pearlite nucleates out of
cementite on GB the direction can
be both along
the GB and also
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into grain 2.
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Growth of Pearlite
Nucleation is followed by the growth of
pearlite colonies.
In the Zener-Hillert model which assumes that
volume diffusion of carbon is the ratecontrolling mechanism for the growth of
pearlite.
 The growth rate GV is given by:
Where, Kv = Geometric constant, λα and λθ are lamellar thickness
of ferrite and pearlite, Dcv is the volume diffusion coefficient of C,
Ceq is the equilibrium
C concentration
and λc is the critical spacing
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Branching of Pearlite
Fig. – Branching of
Pearlite
 Yield
strength of pearlite depends upon the
relative vol% of ferrite and cementite in the
mixture.
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Pearlitic Structures
- Smaller T:
colonies are
larger
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- Larger T:
colonies are
smaller
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Pearlitic Structures
Pearlite (med)
Cementite
(hard)
Pearlite (med)
ferrite (soft)
C0 < 0.76 wt% C
Hypoeutectoid
C0 > 0.76 wt% C
Hypereutectoid
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Effects of alloying addition
Alloying elements tends to change the
eutectoid temp. For e.g. Ni and Mn decrease it
whereas Cr, Mo and Si increase it.
 May change the rate of pearlite reaction. All
elements except Co retards the pearlite
transformation.
 Alloying elements also partitioned b/w α and
Fe3C. Elements which are soluble in γ phase do
not remain in the α rather they partitioned into
Fe3C to some extent.
 It means that the cementite is not always
simply Fe3C.
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Effects of alloying addition
 The partitioning of alloying elements b/w α
and Fe3C is required on thermodynamics to
maintain equilibrium.
 In fact, the redistribution of elements is a
growth requirement.
 Elements having strong affinity for C
generally partitioned into Fe3C of pearlite.
 Si, Ni and cobalt are known to partitioned in
the α of pearlite.
 Partitioning can be suppressed by high
undercooling – partitioning is diffusion
controlled.Powerpoint Templates
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Effects of alloying addition
Fig. – Effect of
alloying on
transformation
kinetics of pearlite
- CCT curves shifts
to longer times
Certain alloy steels
transforms to bainite
rather than pearlite
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Properties of Pearlitic steels
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Austenite formation of reheating
On heating steel, with a spheroidized α + Fe3C
mixture, the γ phase nucleates at the α - Fe3C
boundary. With further heating, the γ phase
consumes the Fe3C and then grows into α
through diffusion-controlled growth.
In a pearlitic microstructure, the γ may
nucleate in the Fe3C and grow into the colony by
dissolving both α and Fe3C. A typical micrograph
and schematic illustration of γ growth into a
pearlite colony is shown in Figs.
The rate of γ formation also increases with the
presence of residual γ in the microstructure.
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Austenite formation of reheating
Figs. - (a) Optical
micrograph of a steel
sample with ferrite +
pearlite microstructure
heated to an intercritical
temperature for a short
time, showing the
formation of austenite in
the pearlite colony.
(b) Schematic illustration
of the austenite growth
mechanism that
dissolves the pearlite
colonies and eventually
the allotriomorphic
ferrite to attain an
equilibriumPowerpoint
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fraction.
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Ductility
Strength
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
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