TTT curves by the addition of alloying elements such as Si, which

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Transcript TTT curves by the addition of alloying elements such as Si, which

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 – T E Mn and Ni stabilize ……… and thus …….

the T E Si, Cr, and Mo stabilize .....

and thus ………… 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 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 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?

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 l  K 3 D T Where D T 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 T E 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: 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 …………. , 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 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.

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 o C 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 o C.

………… bainite form in the temperature region 300-500 o C • ………… bainite forms in the temperature region 200-300 o C 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 ………….

Fe 3 C with …………. .

For lower bainite, the carbide is ……..

…………… .

(Hagg carbide) Fe 2.2

C 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 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.

Upper 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 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 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 steel, ie., the martensitic transformation temperature, at ~180 o C.

of the 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.

M s is the highest temperature at which martensite forms on cooling Martensite can form by deformation at temperatures > M s and < M d 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 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 M s and M f .

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

(martensit e)

2

a

(austenite )

While the c lattice parameter of the tetragonal martensite unit cell is given by

c

(martensit e)

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, V at

V at

a

a

2

a

2   0 .

3599  3 4  0 .

0233 nm 3

Volume of martensite 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 martensite transformation

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 ………………………………………………. .

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

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 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 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.

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

In general,

Properties of a Eutectoid Steel

As the transformation temperature is ……………… , a finer microstructure is ………………….

, which causes the tensile strength to be ………………. , while the ductility is …………………… .