mse-226 engineering materials...using the isothermal transformation diagram for a 1.13wt%c steel...

MSE-226 Engineering

Materials

Lecture-3

‘’THERMAL PROCESSING OF METALS-1’’

HEAT TREATMENT of METALS: A combination of heating and cooling

operations, timed and applied to a metal or alloy in the solid state. The heat

treatment is conducted to get desired properties (physical and sometimes

chemical properties) by changing the structure of material (formation of different

phases)

Heat Treatment of Metal Alloys

Heat treatment

Different phases with

different morhologies,

crystal structures and

chemical composition

Desired

properties

All of the heat treatment operations conducted on steels based on the heating of

the material to some temperature to form fully austenite and cooling the

material to low temperatures at different rates (formation of different phases

depending on the cooling rate applied)

Fe 3 C

cementite

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

austenite

g +L

g +Fe3C a

ferrite

a +Fe3C

L+Fe3C

d

(Fe) Co , wt% C

Eutectic:

Eutectoid: 0.76

4.30

727ºC

1148ºC

T(ºC)

HEAT TREATMENT of STEELS

Some of the Heat Treatments Applied to Steels

1)Annealing

Full annealing

Stress relief annealing

Process annealing

Spheroidize annealing

Normalizing

2) Hardening

Conventional quenching and tempering

Martempering

Austempering

Isothermal Quenching and Tempering

Why steels are heat treated?

Aim Type of Heat treatment

Increase strength Hardening, normalizing

Decrease strength, increase ductility Annealing

Reduce internal stress Stress-relief annealing

Improve machinability Annealing

Avoid segregation (i.e.after casting) homogenizing

HEAT TREATMENT of STEELS:

1st

step: Austenitization (obtaining ~100% austenite phase)

We have to decide the lowest possible temp. in the g-region because of grain coarsening

(25oC higher than the g-trans. temp.)

For each 1 inch thickness the austenitization time is 45 mins. Longer times induces grain coarsening!


Fe 3 C

cementite

1600

1400

1200

1000

800

600

400 0 1 2 3 4 5 6 6.7

L

g

austenite

g +L

g +Fe3C a

ferrite a +Fe3C

L+Fe3C

d

(Fe) Co , wt% C

Eutectic:

Eutectoid: 0.76

4.30

727ºC

1148ºC

T(ºC)

Determine the most proper

austenitazing temperatures

for SAE 1020, 1076 and

10100 steels !

Heat treatment of steels are generally carried out in two steps;

1) Continuous cooling

Generally the steel part is cooled down to or below the room temperature continously.

Some types of continous cooling operations are:

Annealing : very slow cooling, i.e. Cooling in furnace

Normalizing : moderate rate of cooling, i.e. Cooling in air

Quenching: very fast cooling, i.e. Cooling in water or oil bath

2) Interrupted cooling

Very fast cooling to a temperature(undercooling) and wait at that temperature

long enough for transformation of austenite to take place, then cooling to low

temperatures ,e.g.room temperature)

2nd

Step: Cooling to low temperatures at different cooling rates


After austenitization, the steel parts are cooled either by continously or by

interrupted cooling

Cooling under equilibrium conditions, i.e. Annealing

Example: Slow cooling transformation of austenite to pearlite

Transformation of austenite pearlite occurs by diffusion of carbon atoms(time is

required for carbon diffusion). So, this type of transformation is called

DIFFUSIONAL(Time Dependent) TRANSFORMATION.



To predict the microstructures (types pf phase and their realtive amounts of

phases) of annealed steels Fe-C equilibrium phase diagram can be used!

Cooling under equilibrium conditions, i.e. Annealing

Fe

3 C

(c

em

en

tite

)

1600

1400

1200

1000

800

6 00

4 00 0 1 2 3 4 5 6 6.7

L

g ( austenite)

g +L

g +Fe 3 C

a +Fe 3 C

L+Fe 3 C

d

(Fe) C o , wt% C 0.77

727°C = T eutectoid

1148°C

T(°C)

A

B S

g g g g

Fe 3 C (cementite-hard) a ( ferrite-soft)

a

For example: Annealing of

eutectoid steel

Fe-Fe3C diagrams are equilibrium phase diagrams and they

don’t give information about non-equilibrium cooling conditions

Under non-equilibrium conditions TTT-diagrams are used to investigate

the transformation fraction, temperatures and time.

TTT(Time-Temperature-Transformation)-diagrams

IT-diagrams (IsothermalTransformation diagrams)

CCT-diagrams (Continuous Cooling Transformation diagrams)

(used in interrupted cooling conditions) (used in continuous cooling conditions)


Cooling under non-equilibrium conditions

TTT-Diagrams


unstable

austenite

A+M

M

Ms

Mf

Ms : Martensite start

temperature

Mf : Martensite finish

temperature

M:Martensite phase

A: austenite

P:pearlite

B:bainite

N: Nose temperature

IT (isothermal transformation) diagram of eutectoid steel

Details of the IT-Diagram of Eutectoid Steel


Eutectoid steel (0.77 wt.%C)

The position of IT diagrams

Two factors will change the position of

the curves;

1) Chemical composition

2) Austenitic grain size

With few exceptions, an increase in

carbon or alloy content or in grain size

of the austenite always retards

transformation (transformation lines shift

to longer times)

IT-Diagrams


What happens if an eutectoid steel (0.77wt%C) is austenitized properly then

rapidly cooled to 625oC and hold isothermally for about 100 seconds;

IT-Diagrams

1) Pearlite Formation

- Smaller T:

colonies are larger

- Larger T:

colonies are smaller

• Ttransf just below TE --Larger T: diffusion is faster

--Pearlite is coarser.

• Ttransf well below TE --Smaller T: diffusion is slower

--Pearlite is finer.

Interlamellar distance is very close

IT-Diagrams

Comparison of fine and coarse pearlite in eutectoid steel

Comparison of mechanical properties of fine and coarse pearlite

Austenite transforms to a-lathes (strips) and rods of Fe3C isothermally between

the nose region and Ms temperature

Bainite is a phase mixture of a and Fe3C

Transformation is a diffusion controlled process

***For plain carbon steels bainite is only formed by isothermal

transformation***

Schematic IT diagram for eutectoid steel

IT-Diagrams

2) Bainite Formation

Microstructure of bainite

UPPER(Feathery) BAINITE

LOWER (needlelike)BAINITE

Upper bainite

Lower

bainite

IT-Diagrams


The bainitic structure is so fine that Electron microscopy is

needed to resolve the details.

As bainitic steels have finer structure than pearlite (smaller Fe3C

particles) they are stronger and harder than the pearlitic steels

IT-Diagrams


HARDNESS Lower bainite > upper bainite > fine pearlite > medium pearlite > coarse pearlite

Transformation of g(FCC) to Martensite (BCT, body centered tetragonal)

Transformation is rapid!(shear transformation)

% transformation depends on temperature only.

Ma

rten

tite n

ee

dle

sA

us

ten

ite

60 mEUTECTOID STEEL (Quenching from Austenitization temp.)

IT-Diagrams

3) Martensite Formation

Martensite phase

Amount of martensite formed does not depend upon time, only on temperature.

Atoms move only a fraction of atomic distance during the transformation:

1. Diffusionless

(no long-range diffusion)

2. Shear

(one-to-one correspondence

between g and a’ atoms)

3. No composition change

IT-Diagrams


Martensite is the hardest iron-rich phase, the engineers can get

xx x

x

x

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

Expansion

c

a

EXPANSION occurs because atoms of martensite are less densely

packed than that of austenite. This expansion during the formation of

martensite produces high localized stress which result in the plastic

deformation of the matrix.

The degree of expansion depends on carbon content

• HARDNESS INCREASES DUE TO HIGHLY DISTORTED LATTICE

IT-Diagrams


Depending on the carbon concentration various martensitic

microstructures are obtained and the lattice parameter changes such

as the length of the ‘c’ axis is increased and the length of the other

two sides is decreased.

C% 0.3% 0.5% C%

Lath(Needle) martensite Plate(twinned) martensite

Mixed

dislocations

Mid-red

Twin

IT-Diagrams


Both Ms and M

f decrease as carbon content increases

IT-Diagrams


Critical Cooling Rate(CCR)

FULL HARDENING of STEELS

g(austenite, FCC) Martensite(BCT),(~100%)

1) ~100% austenite after

austenitization(temp. and

time)

2) Cooling rate CCR

3) Quenching medium

temperature Mf

Conditions for full hardening;

1)Composition (carbon content and

alloying element)

2) Austenitic grain size of the steel

CCR: is the lowest rate of cooling from

the austenitic region that avoids the

transformation to pearlite or bainite

Depends on

Fully hardened steel is a quenched steel which has only

martensite in the microstructure

e.g. Steel that has 0.75%C, Expected hardness:65 HRc

After quenching if it is 60 HRc, then check your furnace temp. and time

and then control cooling rate, quenching temperature

0.2 0.6 0.8 0.4

55

65

40

62

HRc

Carbon content

The hardness of martensite

increases as the carbon content

increases to a certain value till 0.8

wt.%C. After this composition the

hardness of martensite is not

affected significantly with C

content

Hardness of martensite vs. Carbon content

Use of IT diagrams: Examples

Using the isothermal transformation diagram for a 1.13wt%C steel alloy determine the final microstructure

(in terms of just the microconstituents present) of a small specimen that has been subjected to the

following time-temperature treatments. In each case assume that the specimen begins at 920oC, and that

it has been held at this temperature long enough to have achieved a complete and homogenous structure;

a) Rapidly cool to 250oC, hold for 103 s, then quench to room

temperature

b) Rapidly cool to 775oC, hold for 500 s, then quench to room

temperature

c) Rapidly cool to 400oC, hold for 500 s, then quench to room

temperature

d) Rapidly cool to 700oC, hold for 105 s, then quench to room

temperature

e) Rapidly cool to 650oC, hold for 3 s, rapidly cool to 400oC,

hold for 25 s, then quench to room temperature

f) Rapidly cool to 350oC, hold for 300 s, then quench to room

temperature

g) Rapidly cool to 675oC, hold for 7 s, then quench to room

temperature

h) Rapidly cool to 600oC, hold for 7 s, rapidly cool to 450oC,

hold for 4 s, then quench to room temperature

(a)

(b)

(c)

(d)

(e)

(g) (h) (f)

Quench Cracks !

Reason: surface cool more rapidly than interior so that surface forms martensite

before the interior

Austenite martensite Transformation results in volume expansion

When interior transforms, the hard outer martensitic shell constrains this

expansion leading to residual stresses

Problems in Quenching

xx x

x

x

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

Expansion

c

a

But how to shift the C-curve to higher times?

Solution to quenching cracks

Try to obtain martensite at slower cooling rates by shifting C-curve to the right

(longer times)

By alloying

All alloying elements in steel (Cr, Mn, Mo, Ni, Ti, W, V) etc shift the C-curves to

the right.

(Substitutional diffusion of alloying elements is slower than the interstitial

diffusion of C )

Plain C steel Alloy steel

Alloying shifts the C-curves to the right.

Separate C-curves for pearlite and bainite

Effect of Alloying Elements

mse-226 engineering materials...using the isothermal transformation diagram for a 1.13wt%c steel...

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