Bainitic Transformation
Dr. Rampada Manna
Associate Professor
Department of Metallurgical Engineering
Indian Institute of Technology(BHU)
Varanasi-221005
Email: rmanna.met@iitbhu.ac.in
1
Bainitic Transformation
Bainite was coined in 1934 in honor of
Dr. E. C. Bain
Definition: Bainitic transformation is the decomposition of supersaturated
austenite at a temperature below pearlitic transformation and To but above
martensitic transformation (Ms) to a new eutectoid product of non-lamellar
aggregate of two phases (plate shaped ferrite and carbide for steels) through
either isothermal or continuous cooling transformation.
The carbide part of the microstructure is not essential; the carbides form as a
secondary reaction. The ferrite plates are each about 10 μm long and about 0.2
μm thick, making the individual plates invisible in the optical microscope.
At this temperature range there is a possibility of carbon atoms to be mobile.
However completely carbide-free bainite can form in steels containing
minimum amount of about 1.5 wt % Si. If transformation temperature is low the
size of bainite is in nanometric level. Such fine bainite are called nanostructure
bainite. When it is in bulk form it is called bulk nanostructured bainite.
Conventional bainite formed at higher temperature is called upper bainite.
Bainite formed at lower temperature is called lower bainite. 2
3
Diffusionless
Transformation
at T1 is
possible if
composition of
austenite is less
than To
composition
4
5
• In plain carbon steel, or low hardenable steel, fine pearlitic
regions and upper bainite formation temperature overlap due
to inaccuracy in determination, that complicate the study.
• But with alloying addition (Cr, Mo, or Mn, Si usually cause
retardation of proeutectoid ferrite precipitation and pearlitic
reaction as well as depression of the bainitic reaction to lower
temperature that separate the two C curves of reconstructive
transformation (allotriomorphic ferrite, pearlite) and displacive
transformation (widmanstatten ferrite, bainite).
6
7
The most of alloying elements specially Mn lowers free energy
difference between austenite and ferrite therefore it retards feritic
(bainite) transformation however the effect is more for
reconstructive transformation. Transformation kinetics slows
down for reconstructive transformation because the rate is
controlled by Mn partitioning. (Thermodynamic effect and
diffusion effect )
But in case of displacive transformation (bainitic) is influenced by
the lowering of driving force of transformation from austenite to
bainite only (only thermodynamic effect). Therefore, displacive
transformation is less influenced by Mn than reconstructive one.
8
Experimental 9
A further feature to note (Fe–Mn–C) is that the lower C–curve
representing displacive transformations has a flat top. This
represents the highest temperature Th at which displacive
transformations may occur.
The temperature Th may equal to the bainite–start temperature
BS if the hardenability is high enough, Th=Bs, i.e.
Widmanst¨atten ferrite does not form in high–hardenability
steels. but otherwise, for low hardenable steel, i.e. Th = WS
where WS is the Widmanst¨atten ferrite start–temperature.
10
0.002%B in low carbon steel containing 0.5 %
Mo, addition of Si discourage the formation of ε
carbide
11
12
Upper Bainite [Shear mechanism, Bhadeshia]
Nucleation of bainite subunit occurs by equilibrium solute
partitioning. But growth of individual ferrite subunits constituting
the classical sheaf of upper bainite is due to diffusionless
displacive mechanism. The individual supersaturated subunit
ceases growth possibly due to strain energy accumulation,
accumulating defects due to plastic relaxation or a loss of
coherency at the interface.
This is followed by a diffusion-controlled kinetics of secondary
stage where diffusion of carbon from supersaturated ferrite to
surrounding austenite takes place at a slower rate with respect to
the primary stage and cementite precipitation occurs from
austenite. New subunits nucleate and grow on the tips of old
subunits.
13
Austenite
Nucleation by partitioning of carbon and diffusion less growth
Carbon supersaturated plate (ferrite)
Carbon diffusion into Carbon diffusion into
austenite austenite and carbide
precipitation in ferrite
Carbide precipitation
from austenite
UPPER BAINITE LOWER BAINITE
(High Temperature) (Low Temperature)
14
The formation of bainite causes a deformation which is an
invariant–plane strain with a shear component of about 0.26 and a
dilatational strain normal to the habit plane of about 0.03. This is
consistent with a displacive mechanism of transformation.
15
Surface relief for upper bainite at 450°C, in 1.48 %C steel
16
Bainite forms at a relatively higher temperature when
compared with martensite. The parent austenite is weaker at
high temperatures and cannot accommodate the large shape
deformation elastically. It therefore relaxes by plastic
deformation in the region adjacent to the bainite. The effect of
this plastic deformation is to stifle the growth of bainite plates
before they hit any obstacle. This is why each bainite plate
grows to a size which is often smaller than the austenite grain
size and then comes to a halt. Further transformation happens
by the formation of a new plate and this is why the sheaf
morphology arises.
17
In case of Martensite
Adjacent austenite elatically deformed, due to
lower temperature
In case of Bainite
It is difficult for austenite to accommodate Invariant plane strain elastically
Adjacent austenite plastically relaxed, slipped, due to higher temperature
Interface –dislocation, kills the movement of interface, grow only 10 µm then stops new
plate nucleate 18
Curvature
Atomic Force Microscopy-image
Surface topology: by maintaining a constant distance with the sample surface
19
by adjusting force
20
Field Ion Microscopy Image Field Ion Microscopy
Image
across an austenite–
Bainite
bainitic ferrite interface
Austenite in a Fe–C–Si–Mn alloy.
(a) Field–ion image; (b)
iron map., (c ) silicon
map; (d) carbon map
The images confirm
quantitative data (Bhadeshia
and Waugh, 1982) showing
the absence of any
substitutional atom diffusion
during transformation.
Growth takes place without diffusion and can only occur if the
carbon concentration of the austenite lies to the left of the21T0
Substitutional Alloying Elements.
These do not redistribute at all during transformation, even
though equilibrium requires them to partition between the
austenite and ferrite. The ratio of substitutional to iron atoms
remains constant everywhere including across the interface.
This is consistent with a displacive mechanism of
transformation and the existence of an atomic correspondence
between the austenite and bainitic ferrite.
22
Interstitial Alloying Elements (C, N)).
The carbon has partitioned into the austenite. It establishes carbon can
escape out of the plate within a fraction of a second. Its original
composition cannot therefore be measured directly.
There are three possibilities. (1)The carbon may partition during
growth. (2) The growth may be diffusionless. (3)There is an
intermediate case in which some carbon may diffuse with the
remainder being diffusionless growth requires that transformation
occurs at a temperature below T0, when the free energy of bainite
becomes less than that of austenite of the same composition.
23
2
Fe-0.4C wt%
Decarburisation time / s
Calculated
time to
move from
bainite to 1
austenite
0
300 400 500
Temperature / °C
24
Diffusionless
Transformation
at T1 is
possible if
composition of
austenite is less
than To
composition
25
If carbon separation takes place during transformation then Bainite can form above
To composition but by experiment it is proven that transformation stops at To
Therefore carbon separation takes place after transformation.
Temperature
Ae3'
T'o
x
Carbon in austenite
Diffusion less growth and then carbon separate for
transformation should stop at To’=To with strain 26
500
T'o To
Ae3'
Temperature / °C 450
400
350
300
250
0.0 0.1 0.2
Carbon in austenite / mole fraction
Experimental evidence transformation stops at To/To’ curve. That
indicates partition takes place after transformation.
If partition takes place during transformation then transformation
should continue at given temperature till composition of austenite
27
touches Ae3’
Cluster of ferrite
28
Dissipation carbon decides the spacing. Diffusion is faster near tip due to
multidimensional diffusion of carbon. Strain field near tip allows a fresh bainite
to nucleate in same orientation. On the other hand away from tip nucleation
can take place with different orientation which is difficult. Nucleation at width
require another orientation that is difficult, therefore fresh bainite platelet nucleate
near tip.
Width is decided by plastic accommodation is sufficient to stop the movement of
interface.
Spacing:
29
50 µm
30
i
4360 steel, at 495oC,Banite
sheaves : upper bainite
415oC
Optical
Microscopy
31
Surface 1 Surface 2
Optical Microscopy
50 µm
Srinivasan & Wayman, 1968 32
Summary
1. The mechanism of transformation is displacive
2. Transformation temperatures is higher than
martensite Ms
3. Bainite grows without diffusion
4. But carbon then escapes into the residual austenite
5. Shape deformation plastically accommodated that
stop growth of bainite plate
6. Sub-unit mechanism of growth
33
There are mainly two types of bainite based on microstructure.
Upper bainite and lower bainite
The ferrite laths are practically free of carbon (<0.03) and do not
themselves contain any carbide precipitate. The ferrite laths
nucleate coherently at austenite grain boundaries.
With the growth and thickening of ferrite laths, the remaining
austenite becomes enriched in carbon and finally reaches such a
level that cementite begins to precipitate at the α lath/γ interfaces.
The formation and growth of cementite depletes the surrounding
austenite region of carbon so that it will transform to ferrite.
34
Upper bainite consists of sheaves in feathery structure. Each sheaf
consists of fine, parallel intragranular ferrite plates called sub unit
with cementite precipitated along the ferrite /austenite boundaries.
The individual platelet or subunit are not completely isolated from
one another but they are generally separated by low angle
boundary. All the subunits in a sheaf have a common orientation.
The cluster of platelets forming a sheaf is also called a packet of
bainite(sheaf).
The width of the sheaf increases with increasing transformation
temperature. Sheaf formation is by sympathetic nucleation of
new ferrite laths at the interphase boundaries of those previously
35
formed.
However the amount of carbide precipitated is low. In this way
side- by -side nucleation of the bainitic ferrite and cementite
is repeated several times in order to produce aggregates of
bainitic plates, called sheaves.
The upper bainite predominates at carbon concentration 0.57
wt% and at temperatures between 350oC and 550oC
In high carbon steels transformation to upper bainite produces
elongated carbide particles (i.e. continuous stringers) along the
lath boundaries.
36
In low carbon steels, these carbides may be present as
discontinuous stringers between the ferrite laths.
As the transformation temperature is decreased or carbon
increased within upper bainite range, the austenite is enriched with
carbon, ferrite laths become thinner and the volume fraction of
carbide particle density increases.
The orientation relationship between ferrite and its parent
austenite in upper bainite is always close to the classic
Kurdjumov-Sachs relationship.
[111]γ//[110]α and [110]γ//[111]α
Dislocation density in bainite is more than that in allotriomorphic
ferrite. The dislocation density increases with decreasing
transformation temperature.
37
It is difficult to determine habit plane of bainitic ferrite due to its
morphological complexities. However the longitudinal direction
of ferrite laths is parallel to the close packed direction <111>α
The ferrite plates are each about 10μm long and about 0.2μm
thick, making the individual plates invisible in the optical
microscope.
Cementite precipitation from austenite can be prevented by
increasing the silicon concentration to about 1.5 wt%; this works
because silicon is insoluble in cementite. Silicon–rich bainitic
steels can have very good toughness because of the absence of
brittle cementite.
38
Lower bainite [Bhadeshia]
Two steps:
In faster reaction type, Nucleation takes place by carbon
partitioning and growth of ferrite occurs at fast rate by a
diffusionless shear process or displacive mechanism to
supersaturated ferrite plate. The growth is accompanied by a
shape deformation which is an invariant–plane strain with a
large shear component.
In the second stage: Carbon atoms partition into the residual
austenite (or precipitate as carbides), shortly after growth is
arrested. Internal precipitation of carbides within this
supersaturated bainitic ferrite is a subsequent stage of
reaction. More sluggish reaction type carbides are
precipitated between the bainitic ferrite platelets by
decomposition of the carbon rich residual austenite.
39
Lower bainite
Lower bainite is characterized by plate morphology. The carbide
precipitates as small parallel particles making an angle of 55-60o
with the longitudinal axis of the ferrite plate.
40
0.66C, LB, a-OM, b, c-EM
TEM microstructure of thin
spiny lower bainite (with
midrib) formed at 190oC after 5
h in a 1.1%C steel
Optical microstructure
of thin spiny lower
bainite formed at 190oC
after 5 h in a 1.1%C
41
steel
The ferrite plate nucleate at austenite grain boundaries as well as
within the grain.
Carbides in lower bainite is either ε or cementite. As ε carbide
forms at low temperature (275oC), or carbon higher than 0.55%.
Initially ε carbide forms and eventually transforms to cementite
upon further holding at the transformation temperature.
ε carbide formed in lower bainite exhibit OR with ferrite close to
that proposed by Jack
(0001)ε//(011)α’ and (10-11)ε//(101)α
Habit plane is irrational and depends on composition
Fe-C-{456}, Fe-Cr-C= {254}
42
• Lower bainite can form at all carbon composition. The
transition temperature is usually about <350oC. It depends on
composition.
Effect of carbon on
transformation temperature
of UB and LB
43
Kinetic definition :
Bainite has its own C-curve on a TTT diagram whose upper limiting
temperature is termed as kinetic Bs (Bainite start)(100-300oC) lies
much below the eutectoid temperature.
Between the kinetic Bs and some lower temperature, incorrectly
termed bainite finish or kinetic BF temperature, the proportion of
austenite transformed to bainite increases from zero at the kinetic Bs
to unity at the kinetic BF.
Steven & Haynes[0.1-0.55C low alloy steels]
Bs(oC)=830-270(%C)-90(%Mn)-37(%Ni)-70(%Cr)-83(%Mo)
B50(oC)=Bs-60
Bf(oC)=Bs-120
Bodnar et al. [based on continuous cooling, Fe-C-Ni-Cr-Mn steels]
Bs(CCT,oC)=844-597(%C)-63(%Mn)-16(%Ni)-78(%Cr) 44
a) TTT diagram
b) Incomplete
bainite
transformation
c) Bainitic
transformation
temperature
range
45
Above Bs austenite will not transform to bainite. This
characteristic is called incomplete reaction phenomena or the
transformation stasis.
Bainite transformation kinetics is isothermal nature.
Transformation almost ceases at BF.
Upon cessation of bainitic transformation, c redistribution
takes place in untransformed austenite.
After stasis period retained austenite again decomposes to pearlite
or bainite like structure. When at or below Ms, austenite transforms
to bainite, prior formation of martensite catalyses the bainitic
reaction thereby leading to an abrupt deviation of the TTT curve
for initiation of the bainitic reaction.
46
TTT diagram in the vicinity of Ms for 0.75%C steel
47
Effect of alloying elements
• Addition of nickel reduce the growth rate
Chang, Pickering and Llopis[0.095-0.46C, 1.63-2.13Si, 1.99-
2.18Mn, 2-2.07Ni]
LBs (oC)=500-(155±40)C-(38±14)%Si-(17±13)%Mn-
(4±11)%Ni-(10±13)%Cr-(5±20)%Al-(4±56)%Co
48
a: nodular bainite
b: columnar bainite
c: upper bainite sheaf, lath
morphology
d: lower bainite
e: grain boundary allotriomorphic
bainite
f: inverse bainite
49
