Mech 473 Lectures
Professor Rodney Herring
Effect of Alloying Elements on the Eutectoid
Decomposition
Plain Carbon Steels contain:
0.5 – 1.0 % Mn and
0.15 – 0.30 % Si
Low Alloy steeels may also contain additional elements such as:
Co Cr Mo W Ni V Ti – and may have Mn contents up to 5%
When dissolved in austenite, these alloys can affect the ………….
reaction by,
1. Changing the eutectoid temperature – TE
Mn and Ni stabilize ……… and thus ……. the TE to below 727 oC.
Si, Cr, and Mo stabilize ..... and thus ………… the TE above 727 oC.
Note: the effect of alloying elements on the phase stability of ferrite
should not be confused with their effect on the stability of
graphite or cementite, which may be quite different.
Effect of Alloying Elements on the Eutectoid
Decomposition
Alloying additions affecting the eutectoid reaction (cont’d)
2. Lowering the eutectoid composition of the austenite to below
0.77 %C strengthens in the order of Ti>Mo>W>Si>Cr>Mn>Ni.
3. Changing the composition of the ferrite and cementite by
partitioning in the pearlite transformation.
Note that an element in the g-phase upon cooling does not just go
into the a-phase but is ……………. between the ferrite and the
cementite phases. What does this mean?
4. Changing the growth rate of pearlite
Co is the only element that does not retard the growth of pearlite
As, Si, Cr, and Mo raise TE where the degree of undercooling is
increased so at temperatures above the “knee” in the TTT
diagram, the growth rate is increased whereas at temperatures
below the “knee”, the growth rate is decreased.
Effect of Alloying Elements on the Eutectoid Decomposition
What is TE
for 3% Si?
What is %C of
eutectoid for 3% Si?
Effect of Alloying Elements on Pearlite Lamellar Spacing
It was noted earlier that the interlamellar spacing, , varies as:
1  K DT
3
l
Where DT is the …………………….. .
Plots of 1/l versus temperature are thus linear, with a negative slope
as shown by the dashed line for a plain carbon eutectoid steel.
The addition of 0.4-1.8 wt% Cr
displaces the plot to the right, ie.,
to ………….. and upwards to
…………… lamellar spacing.
The addition of 1.08-1.8 %Mn
displaces the plot to the left, ie.,
to lower TE and downwards to
larger lamellar spacing.
What does Mn stand for?
Temperature Dependence of the Pearlite Transformation
At temperatures just below the eutectoid, it can be assumed that:
•
The nuclei are ………….. distributed throughout the austenite
•
The average rate of nucleation, N, is ………………. .
•
The nodules remain ………….. in shape
•
The growth rate, G, is ………………… .
•
The fraction of austenite transformation to pearlite as a function
of time, f(t) is then given by the Johnson-Mehl equation:
NG 3t 4
f(t) 1 e
This equation gives a typical …………… curve for the fraction
transformed as a function of time.
A considerably more complex equation is required at lower reaction
temperatures when the assumption of random nucleation is no
longer valid.
Time-Temperature-Transformation Curves
(TTT Curves)
Transformation-time curves for a given temperature are derived by
examining the …………………….. of samples removed from a
furnace after pre-set times.
The time, which is taken as the …………. , of the reaction is taken as
the time to obtain ……….. of the transformation product.
The time, which is taken as the …………., of the reaction is taken as
the time to obtain ………… of the transformation.
These times are plotted for a series of temperatures to give the TTT
curve.
Time-Temperature-Transformation Curves
(TTT Curves)
The TTT curve is divided into areas, which represent single constituent fields
or two component regions.
To the left of the 1% pearlite “C-curve” the microstructure is austenitic.
To the right of the 99% pearlite “C-curve”, the microstructure is pearlitic.
Between the curves, the microstructure is a mixture of austenite and pearlite.
The amount of the two constituents can be obtained by plotting further
C-curves for 10%P, 20%P, 50%P, etc.
Time-Temperature-Transformation Curves
(TTT Curves)
The TTT curves are only plotted for temperatures above the “knee”
of the temperature dependent growth rate curve for pearlite.
As the temperature is lowered to this knee, the time to start the
reaction, which is the ………………, is drastically lowered and
is a maximum at 600 oC.
In plain carbon steels, an ………… in nucleation and growth rate
with the degree of undercooling can …………. the incubation
period to zero so that the decomposition of austenite cannot be
suppressed by rapid quenching.
At temperatures below the knee of the C-curve, competing
microstructures can become more stable than pearlite.
Below the knee of the C-curve, …………. forms isothermally, while
…………….. forms “athermally” on quenching below 250 oC
as indicated by the horizontal lines at the bottom of the TTT
plot.
The Bainite Reaction
Bainite is formed over a wide range of temperatures from 200-500 oC.
•
………… bainite form in the temperature region 300-500 oC
•
………… bainite forms in the temperature region 200-300 oC
Bainite – like pearlite – is a mixture of ferrite and carbide, but the ……
of the carbide depends on the its temperature of formation.
•
For upper bainite, the carbide is …………. Fe3C with …………. .
•
For lower bainite, the carbide is …….. (Hagg carbide) Fe2.2C with
…………… .
In addition, the physical form of the ferrite and carbide phases are quite
different in upper and lower bainite, giving very distinct
microstructures.
Upper Bainite
Upper bainite formed at 460 oC is composed of a very fine structure
composed of laths of ferrite with layers of cementite between the
laths.
In the electron micrographs taken at 15,000x magnification, the dark
regions are packets of ferrite laths and the light regions are
cementite.
Upper Bainite
Lower bainite formed at 250 oC is much courser and the e-carbide
particles are precipitated with the ferrite.
In the electron micrographs taken at 15,000x magnification, the ferrite
plates are a more regular and needle-like in shape while the
carbide particles are smaller in size and appear as striations at an
angle of 55 degrees to the axis of the plate.
Kinetics of the Bainite Reaction
The isothermal decomposition of austenite to bainite follows a typical ……….. ,
which can be analyzed by the Johnson-Mehl equation.
This equation was used to generate the solid line in the transformation plots for
bainite reaction at 150-390 oC in a 1.1% C hypereutectoid steel.
The experimental data conform closely to the equation at high temperature but
deviate at the lower temperatures, which in fact lie below the Ms of the
steel, ie., the martensitic transformation temperature, at ~180 oC.
On this basis, the bainite reaction can be classified as a …………………………
…………………………………………………………………….. .
TTT Curves of the Bainite Reaction
The isothermal decomposition of austenite to upper and lower bainite can be
separated into ……………………. TTT curves by the addition of alloying
elements such as Si, which slow down the formation of carbides.
This has been demonstrated in a steel containing 0.43 C, 3.0 Mn, and 2.12 Si.
In the plots below, the solid lines refer to 5% transformation, while the dotted
lines indicate the experimental scatter of the data.
Mechanism of the Bainite Reaction
Metallurgists have been arguing for more than 50 years about the mechanism of
the bainite transformation mechanism.
This has arisen because it does not fit precisely under either diffusion controlled
nucleation and growth transformations like pearlite or shear controlled
transformations like martensite.
A solution to this dilemma has been suggested by Ko and Cottrell:
•
Austenite transforms to ferrite by a martensitic shear transformation
•
Substitutional solutes remain in the same positions relative to the Fe atoms
•
In upper bainite, the carbon diffuses in the austenite to form cementite
•
In lower bainite, the carbon diffuses in the ferrite to form e-carbide
Hence the initial phases form by shear and then diffusion of carbon controls the
growth of both the carbide phases.
The Martenisitic Transformation
The horizontal lines on TTT diagrams refer to the formation of martensite.
Ms is the highest temperature at which martensite forms on cooling
•
Martensite can form by deformation at temperatures > Ms and < Md
M50 and M90 are the temperatures at which these percentages of martensite
form.
Mf is the lowest temperature at which martensite forms on cooling although the
transformation may not be 100% completed at Mf
These characteristics are referred to as “……………” as opposed to isothermal.
Athermal martensitic transformation
in 0.40% C low alloy steel
Effect of carbon concentration on Ms
and Mf .
The Martenisitic Phase in Steels
The crystal structure of a-ferrite is ………. with the carbon atoms ………
distributed among the interstitial sites.
As the maximum concentration of C is 0.02 wt%, very few of the available
carbon interstitial sites are actually occupied so the structure remains
cubic.
The Martenisitic Phase in Steels
In a`-matensite, the carbon atoms are restricted to interstitial sites at the centres
of the crystal faces and the unit cell’s edges that lie parallel to the c-axis.
This selective occupation of interstitial sites causes the c-axis to be increased and
a-axis to be contracted so the structure becomes tetragonal, i.e., ……….
………………………………………. , bct.
Lattice Parameters of Austenite and Martensite
The tetragonality, ie., the c/a ratio, of the bct cell of the martensite phase is
governed by the carbon concentration.
For a steel of composition x wt% C, a linear dependency gives the following
relationships.
The ………………………………….. of austenite are given by
a = 0.3555 + 0.0044x
The lattice parameters of martensite are given by
a = 0.2866 – 0.0013x
c = 0.2866 + 0.0106x
Relationship between the Crystal Structures of
Austenite and Martensite
The a lattice parameter of the tetragonal martensite unit cell is given by
a (martensite)  2a (austenite)
While the c lattice parameter of the tetragonal martensite unit cell is given by
c (martensite)  c (austenite)
Volume Changes Accompanying Martensite Formation
Using the equations for the lattice parameter of austenite and martensite, the
volumes of the units cells of the two tetragonal structures can be compared
at a common carbon concentration, e.g., 1 %C.
Hence,
volume of austenite tetragonal cell, Vat
a
a
3
Vat  a 

 0.3599
4  0.0233nm3
2
2
Volume of martensite tetragonal cell, Vmt
Vmt  a  a  c  0.2853 0.2853 0.2982 0.0243nm3
The increase in volume due to martensite transformation
DV  0.0243 0.0233 0.0233 4%
If this volume change is assumed to be isotropic because of the different
orientations of the martensitic plates, then the associated linear expansion
is given by
4.0/3 = 1.3%
The relaxation of the c/a of martensite during tempering due to the reduction of
carbon content caused by the carbide precipitation can thus create
………………………………………………. .
Surface Relief Effects
When we look at the surfaces of these materials using optical microscopy, we see
lines which appear as scratches.
The scratches are continuous but change direction at the interface between the
austenite and martensite phases
Surface relief effects in Fe-5%Pt
Surface Relief Effects
These observations indicate that the surface is tilted in the region of the coherent
interface so that the surface relief is of the form associated with a
mechanical ……………… as opposed to ……………… deformation.
Lath Martensite
Lath martensite occurs in plain carbon steels with < 0.4% C and Fe-Ni-Mn.
Optical micrograph showing surface relief of ……………………. in F-0.2% C
steel.
Note:
•
The laths are typically 0.3 x 4 x 200 mm3.
•
The habit plane of this type of martensite is close to (111)g
•
The laths are usually observed in packets within which each adjacent
lath has the same habit plane variant and shape deformation in contrast
to lenticular martensites.
Lenticular Martensites
These martensites occurs in high carbon steels with > 0.4% C and Fe-Ni-Mn,
Fe, and Pt.
Optical micrograph showing surface relief of F-25% Pt alloy.
The plates have the shape of a convex lens. This is confirmed by examination of
successive sections after repeated removal of a thin surface layer of the
sample.
Since the interface planes are curved, the …………………. is taken as the plane of
the centre-line of the plate or the “mid-rib”.
More than one ……………….. of the habit plane may be observed within a single
austenite grain, which gives different contrast of the light and dark plates.
The martensite plates generally extend across a prior austenite grain and narrow
to a point at the grain boundaries.
Small martensite plates also form between larger prior-formed plates and extend
across the available length between the prior plates.
Effect of Carbon Content on Hardness of Martensite
A minimum of 0.4% C is required to obtain significant hardness.
At carbon contents of < 0.6%, the hardness of martensite decreases
progressively with carbon content. See plot next page.
At carbon contents of > 0.6%, the hardness plots flatten out and there is more
experimental scatter.
This is caused by increasing amounts of retained austenite in the steels so that
the measured hardness is not truly indicative of 100% martensite.
Effect of Carbon Content on Hardness of Martensite
Effect of Alloying Elements on Hardness of Martensite
The hardness of martensite is determined ………………… by its carbon content
because the interstitial carbon atoms:
•
Distort the bcc a-Fe structure to bct and thereby build up large elastic
stresses,
•
Are very efficient at locking dislocations in place or ……………………. ,
which is responsible for the sharp yield point in a-Fe
Substitutional alloying elements, such as Ni, Cr, Pt, etc, do not effect the
hardness of martensite because:
•
They do not contribute to the tetragonality of the structure
•
As point defects they are not very efficient at locking dislocations
Hence low alloy steels contain < 5% total alloy content.
The hardness of 50/50 P/M can be predicted from the carbon content, which is
usually taken as Rockwell C54.
The Complete TTT Diagram for a Eutectoid Steel
The full diagram for a eutectoid steel shows:
•
The start and finishing times for isothermal pearlite and bainite reactions
•
The start and finishing temperatures for athermal martensite formation
In this diagram, the temperature of the pearlite and bainite reactions …………
so the transition from pearlite to bainite is smooth and continuous.
Above 550 oC, austenite
transforms completely to
pearlite
Between 450-550 oC, both pearlite
and bainite are formed
Between 450 and 210 oC (the Ms),
only bainite is formed
On cooling below Ms, martensite is
formed but in addition lower
bainite can also form after
long holding times.
Microstructures from Various Time-Temperature Paths
Path 1 Quench to 160 oC, hold for 20 min and then quench to RT.
The pearlite transformation is suppressed by the quench. 50% martensite is
formed at 160 oC but the holding time is not long enough to form bainite so
more martensite is formed from 160 oC to RT.
Path 2 Quench to 250 oC and hold for 100 sec, then quench to RT.
The holding time at 250 oC is not long enough to form bainite so the austenite
transform to martensite on cooling from 250 oC to RT.
Path 3 Quench to 300 oC, hold for
500 s and quench to RT.
The holding time at 500 oC
produces 50% bainite with the
remaining austenite tranforming
to martensite on cooling to RT.
Path 4 – Quench to 600 oC, hold
for 104 s and quench to RT.
100% pearlite forms after 8 s at
600 oC with the additional holding
time causing no further changes.
Effect of Transformation Temperature on the Mechanical
Properties of a Eutectoid Steel
In general,
As the transformation temperature is ………………, a finer microstructure is
…………………., which causes the tensile strength to be ………………. ,
while the ductility is …………………… .