PHASE

TRANSFORMATIONS

in METALS and ALLOYS

IE-114 Materials Science and General Chemistry

Lecture-12-13

Why do we study phase

transformations?

The desirable mechanical properties of a material can be obtained

as a result of phase transformations using the right heat treatment

process.

In order to design a heat treatment for some alloy with desired RT

properties, time and temperature dependencies of some phase

transformations can be represented on modified phase diagrams.

Phase Transformation in Metals

Development of microstructure in both single- and two-phase alloys involves phase transformations-which involves the alteration in the number and character of the phases. Phase transformations take time and this allows the definition of transformation rate or kinetics.

Phase transformations alter the microstructure and there can be three different classes of phase transformations:

a) Diffusion dependent transformations with no change in number of phases and composition ( solidification of a pure metal, allotropic transformations, etc.)

b) Diffusion dependent transformations with change in number of phases and composition (eutectoid reaction)

c) Diffusionless transformations (martensitic transformation in steel alloys)

Steel is heat treated in order to:

Increase/decrease strength (Hardening, normalizing, annealing)

Reduce internal stresses (stress relief annealing)

Adjust grain size (normalizing)

Remove the effects of cold working (normalizing)

Improve machinability (annealing)

Avoid microsegregation (homogenizing)

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)

HEAT TREATMENT: A combination of heating and cooling

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

a way that will produce desired properties (physical and sometimes

chemical properties)

1) Continuous cooling ( very slow, moderate or fast cooling)

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 (isothermal transformation), e.g.room temperature)

Heat treatment of steels

1.Step: Austenitization (heat treatment to obtain ~100% austenite phase)

Cooling;

2.Step:Cooling to low temperatures at different rates

We have to decide the lowest possible temp. in the -region

because of grain coarsening (25oC higher than the -trans. temp.)

For each 1 inch thickness the austenitization time is 45 mins.

Very slow continuous cooling of eutectoid steel

(under equilibrium conditions)

When the carbon content of steel and the temperature are known fraction and

composition of phases can be predicted using Fe-Fe3C diagram.

However, the total transformation time is not known. Transformation time is

missing in Fe-Fe3C diagram

Upon very slow cooling, transformation of austenite to pearlite occurs by diffusion

of carbon atoms(time is required for carbon diffusion). So, this type of

transformation is called DIFFUSIONAL(Time Dependent) TRANSFORMATION.

Fe-Fe3C diagrams are equilibrium phase diagrams and they don’t give

information about non-equilibrium cooling conditions

Cooling rate is so fast that carbon atoms cannot find enough

time to go and locate their equilibrium positions

DIFFUSIONLESS (e.g.martensitic) TRANSFORMATIONS

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)

CT-diagrams (Continuous Cooling Transformation diagrams)

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

Fraction transformed

Phase Transformation involves:

1) Nucleation (the formation of very small particles of the new phase at

some imperfection sites, e.g.,grain boundaries

2) Growth (nuclei increase in size)

Transformation rate:

Fraction of transformation is measured at constant temperatures as a function of time by

either microscopic examination or measurement of some physical property such as

electrical conductivity.Then, the fraction of transformed material versus the logarithm of

time graphs are drawn.

AVRAMI EQUATION

y: fraction of transformation

t : time

k,n : time independent constants

Rate of transformation,r : Reciprocal of time required for 50% transformation

Rate,r : 1/t50%

• Reaction rate increases with T.

TRANSFORMATION RATE ~ UNDERCOOLING( T)

As undercooling is increased finer microstructures are formed

increasing T

ISOTHERMAL TRANSFORMATION(IT) DIAGRAMS

• Fe-C system, Co = 0.77wt%C (Eutectoid steel)

• Transformation at T = 675oC.

logt

IT-diagram of an eutectoid steel

Proeutectoid ferrite

Proeutectoid cementite

Coarse Pearlite

Fine Pearlite

Upper Bainite

Lower Bainite

Martensite

Depending on the alloy composition, amount of undercooling and

isothermal transformation time, one can obtain one of the following

phases or combination of them;

IT diagram of a hypoeutectoid steel

EUTECTOID STEEL (0.77 wt.%C)

IT -DIAGRAMS

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)

• Eutectoid steel, Co = 0.77wt%C

• What happens if an eutectoid steel is austenitized at T > 727oC then it is

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

PEARLITE TRANSFORMATION in eutectoid steel

FINE and COARSE PEARLITE (Eutectoid steel)

- 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

Comparison of mechanical properties of

fine and coarse pearlite

BAINITE PHASE (Phase mixture of and Fe3C)

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

the nose region and Ms temperature

Bainite is a phase mixture of 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

Upper bainite

Lower

bainite

Upper(feathery) and Lower(Needlelike) Bainite:

HARDNESSLower bainite > upper bainite > fine pearlite > medium pearlite > coarse pearlite

UPPER BAINITE

LOWER BAINITE

Upper bainite

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

Transformation is rapid!(shear transformation)

% transformation depends on temperature only.

Marte

ntite

needle

sA

uste

nite

60 mEUTECTOID STEEL (Quenching from Austenitization temp.)

MARTENSITE

MARTENSITE

Avrami-type Every steel has a specific TTT-

diagram and on which its Ms

and Mf are indicated

Both Ms and Mf decrease as carbon content increases

xx x

x

x

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

Expansionc

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

MARTENSITE

Altough martensite is very hard unfortunately it is very BRITTLE for industrial use. In

order to toughen the steel and make it more ductile, a heat treatment called

tempering is applied.

Tempering is applied by reheating a martensitic steel to a temperature below

eutectoid temperature and cooling it at any rate to increase the ductility and

toughness.

TEMPERING

1) Conventional Quenching and Tempering

As-quenched

Annealed

TemperingWhile tempering, you are losing

from hardness but if it is applied

correctly it does not reduce

hardness excessively

Strength and reduction in area change with

tempering temperature

Relative hardness values of martensite, tempered

martensite and fine pearlite depending on the

carbon content

SPHEROIDIZING HEAT TREATMENT

If steel alloy with pearlitic or bainitic structure is heated to and left at

a temperature below the eutectoid temperature (such as 7000C) for

18 to 24 hours, another microstructure, called spheroidite, forms.

cementite

ferrite

Spheroidite: has lower strength and hardness than pearlitic microstructures.

Of all the steel alloys, those that are softest and weakest have a spheroidite

microstructure. The spheroidized steels have higher ductility than coarse

pearlite.

Spheroidite structure

Use of IT diagrams

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

Example:

(a)

(b)

(c)

(d)

(e)

(g)(h) (f)

CONTINUOUS COOLING TRANSFORMATION

(CCT) DIAGRAMS:

In industrial heat-treating operations, in most cases a steel is not

isothermally transformed at a temperature above the martensite start

temperature but is continuously cooled from austenitic temperature to room

temperature.

Comparison of IT and CCT diagrams for eutectoid steel

Isothermal transformation

In continuously cooling a plain-carbon steel, the transformation from austenite

to pearlite occurs over a range of temperatures rather than at a single

isothermal temperature

Moderately rapid

cooling

(NORMALIZING)

Slow cooling

(FULL ANNEALING)

Quenching

EFFECT OF ALLOYING on SHAPE of TTT-DIAGRAM

CCT-diagram, 4340 steel

1) Alloying elements shift the austenite-to-pearlite transformation lines to longer times

2) A seperate bainite nose is formed

IT-diagram, 4340 steel (Ni, Cr, Mn, Mo as alloying elements)

Normally, BAINITE will not form (or will form to a small extent) during continuous cooling of plain carbon

steels. By alloying, it becomes possible to obtain bainite phase upon continuous cooling

Use of CCT diagrams

Example:

Which microstructures do you observe upon cooling of 0.35 wt.% C iron-carbon

alloy after austenitization at 850oC at different rates shown below?

(c) Proeutectoid ferrite + martensite

(a) Proeutectoid ferrite+fine pearlite

(b) Martensite

(d) Proeutectoid ferrite + coarse pearlite

(e) Proeutectoid ferrite + pearlite + martensite