Transcript Mech 473 Lectures Professor Rodney Herring
Mech 473 Lectures
Professor Rodney Herring
Effect of Alloying Elements on the Eutectoid
Plain Carbon Steels
Decomposition
contain: 0.5 – 1.0 % …… and So are more than just Fe-C.
0.15 – 0.30 % …… .
Low Alloy steeels
may also contain additional elements such as: Co Cr Mo W Ni V Ti – and may have …..
contents up to 5% When dissolved in austenite, these alloys can affect the eutectoid reaction by four (4) notable ways, i.e.,
1.
Changing the eutectoid temperature – T E
……… and Ni stabilize g -Fe and thus lower the T E Si, Cr, and Mo stabilize a -Fe and thus increase to below 727 the T E above 727 o o C.
C.
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
, which 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
partitioned
between the ferrite and the cementite phases. What does partitioning mean?
4.
Changing the growth rate of ……………. .
Co is the only element that does not retard the growth of pearlite As, Si, Cr, and Mo raise T E 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 T E for 3% Si?
Higher Ni will take eutectoid temperature to room temperature to retain
g
and produce stainless steels.
What is %C of eutectoid for 3% Si?
Effect of Alloying Elements on Pearlite Lamellar Spacing
It was noted earlier that the interlamellar spacing, l , varies as: 1 l Where D T is the ……………. .
K 3 D T What does undercooling mean?
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 higher T E to smaller and upwards lamellar spacing
.
The addition of
1.08-1.8 %Mn displaces the plot to the left, ie., to lower T E and downwards to larger lamellar spacing
.
Temperature Dependence of the Pearlite Transformation
• • • • • At temperatures just below the eutectoid, it can be assumed that: The nuclei are randomly distributed throughout the austenite The average rate of nucleation, N, is constant The nodules remain spherical The growth rate, G, is in shape ………….. .
The fraction of austenite transformation to pearlite as a function of time, f(t) is then given by the Johnson-Mehl equation: f(t) 1 e NG 3 t 4 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 start time , of the reaction is taken as the time to obtain ……….
of the transformation product.
The time, which is taken as the finish time , 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 ……………… .
To the right of the 99% pearlite “C-curve”, the microstructure is …………… .
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%Pearlite, 20%Pearlite, 50%Pearlite, etc.
Recall pearlite growth rate as a function of T.
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 incubation period , is drastically lowered and is a maximum at 600 o C.
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.
Time-Temperature-Transformation Curves (TTT Curves)
At temperatures below the knee of the C-curve, competing microstructures can become more stable than pearlite.
Below the knee of the C-curve, bainite martensite forms , while forms “…………..” on quenching below 250 o C as indicated by the horizontal lines at the bottom of the TTT plot.
Bainite region Martensite region
What is the difference between isothermal and athermal?
Isothermal vs Athermal
An isothermal process is a thermodynamic process in which the temperature of the system stays constant: ΔT = 0. Athermal is a reaction that proceeds without thermal activation.
The Bainite Reaction
• •
Bainite is formed over a wide range of temperatures from 200-500 o C.
Upper Lower bainite form in the temperature region 300-500 o bainite forms in the temperature region 200-300 C o C
• •
Bainite – like pearlite – is a mixture of ……………………, but the form of the carbide depends on its temperature of formation.
For upper bainite , the carbide is cementite For lower bainite , the carbide is Fe 2.2
C with 8.4% C .
e
-carbide Fe 3 C with 6.7% C (Hagg carbide) 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 o C 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. Cementite Ferrite
At only 1500x magnification, it is difficult to see the different phases.
Lower Bainite
Lower bainite formed at 250 o C 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 (dark) are a more regular and needle-like in shape while the carbide particles (light) are smaller in size and appear as striations.
Cementite Ferrite
Kinetics of the Bainite Reaction
The isothermal decomposition of austenite to bainite follows a typical S-curve , 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 o C 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 M s of the steel, i.e., the martensitic transformation temperature, at ~180 o C (453 K).
On this basis, the bainite reaction can be classified as a …………………………. nucleation and growth transformation .
TTT Curves of the Bainite Reaction
The isothermal decomposition of austenite to upper and lower bainite can be separated into two distinct 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 ……………………………., while the dotted lines indicate the experimental scatter of the data.
Mechanism of the Bainite Reaction
Metallurgists have been arguing for 50 years or more 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 ……………. and then ……………… 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.
•
M s is the highest temperature at which martensite forms on cooling Martensite can form by ……………… at temperatures > M s and < M f M 50 and M 90 form.
are the temperatures at which these percentages of martensite M f is the lowest temperature at which martensite forms on cooling although the transformation may not be 100% completed at M f.
What is the difference?
These characteristics are referred to as “ athermal ” as opposed to isothermal.
Athermal martensitic transformation in 0.40% C low alloy steel Effect of carbon concentration on M s and M f .
The Martenisitic Phase in Steels
The crystal structure of
a
-ferrite is bcc 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.
Which ones are the octahedral and tetrahedral interstitial sites?
The Martenisitic Phase in Steels
In
a
`-martensite, 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 …………….. occupation of interstitial sites causes the c-axis to be increased and a-axis to be contracted so the structure becomes tetragonal, i.e., body centered tetragonal , 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 of x wt% C, a linear dependency gives the following relationships.
The lattice parameters of …………………… are given by a = 0.3555 + 0.0044x
The lattice parameters of …………………... 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
(martensit e)
2
a
(austenite )
While the “c” lattice parameter of the tetragonal martensite unit cell is given by
c
(martensit e)
c
(austenite )
FCC austeniste lattice having two of the three possible orientations of the BCT unit cell showing.
The BCT lattice has an axial ratio =
2 1
Shear Mechanism for converting Austenite to Martensite
Shearing of the FCC austenite on the (111) to the b) BCT and c) BCC structures on the (110).
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 …………………… (FCC) tetragonal cell, V at
V at
a
a
2
a
2 0 .
3599 3 4 0 .
0233 nm 3
Volume of ……………………….. (BCC) tetragonal cell, V mt
V mt
a
a
c
0 .
2853 0 .
2853 0 .
2982 0 .
0243 nm 3
The increase in volume due to the martensite transformation is:
D
V
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 dilatational changes to relax the strain .
Surface Relief Effects of Martensite
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 slip deformation.
Do you remember twins and slip due to dislocations?
What was involved in the movement of a twin boundary?
Lath Martensite
Lath martensite occurs in plain carbon steels with …………………………….. .
Optical micrograph showing surface relief of lath martensite in F-0.2% C steel.
• • •
Note: The laths are typically 0.3 x 4 x 200
m
m 3 .
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 , which is in contrast to lenticular martensite .
Lenticular Martensite
Lenticular martensites occurs in high carbon steels with ………….. and in ………………….. alloys.
Convex lens shape Optical micrograph showing surface relief of Fe-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 habit plane is taken as the plane of the centre-line of the plate or the “mid-rib”.
More than one variant 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 ………………. is required to obtain ………………… 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
Retained austenite giving experimental scatter.
“………………………” of a steel can be defined as the ability of the steel to form martensite.
•
Remember this definition as we will use hardenability to characterize the effects of alloying additions in steels.
Effect of Alloying Elements on Hardness of Martensite
• •
The hardness of martensite is determined essentially by its carbon content because the interstitial carbon atoms: 1) Distort the bcc
a
-Fe structure to bct and thereby build up large elastic stresses, 2) Are very efficient at which is responsible for the sharp yield point in
a
-Fe
•
locking dislocations in place Recall stress-strain curve of BCC steels.
or pinning dislocations ,
•
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 ……………….. total alloy content .
The hardness of 50/50 P/M (pearlite/martensite) can be predicted from the carbon content, which is usually taken as Rockwell C54.
The Complete TTT Diagram for a Eutectoid Steel
• •
The …………………………………………………… 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 overlap so the transition from pearlite to bainite is smooth and continuous .
Above 550 o C, austenite transforms completely to pearlite Between 450-550 o C, both pearlite and bainite are formed Between 450 and 210 o C (the M s ), only bainite is formed On cooling below M s , martensite is formed but in addition lower bainite can also form after long holding times.
Note the hardness on the right side as a function of transformation temperature.
Microstructures from Various Time-Temperature Paths
Path 1 Quench to 160 o C, hold for 20 min and then quench to RT.
The pearlite transformation is suppressed by the quench. 50% martensite is formed at 160 o C but the holding time is not long enough to form bainite so more martensite is formed from 160 o C to RT.
Path 2 Quench to 250 o C and hold for 100 sec, then quench to RT.
The holding time at 250 o C is not long enough to form bainite so the austenite transform to martensite on cooling from 250 o C to RT.
Path 3 Quench to 300 o C, hold for 500 s and quench to RT.
The holding time at 500 o C produces 50% bainite with the remaining austenite tranforming to martensite on cooling to RT.
Path 4 – Quench to 600 o C, hold for 10 4 s and quench to RT.
100% pearlite forms after 8 s at 600 o C with the additional holding time causing no further changes.
Effect of Transformation Temperature on the Mechanical Properties of a Eutectoid Steel
What does all this mean ?
In general, if the transformation temperature is ………………… , a ……….. microstructure is produced , which causes the tensile strength to be increased , while the ductility is reduced .
Martensite has high tensile strength but no ductility. It needs to be tempered. It is rarely used “as quenched”.
Pearlite can be made to have reasonably high tensile strength (600 o C) with reasonably good ductility.
Bainite properties at 400 o C are in between.