MSE 2001- Chapter 8 Pt2 Ppt

32Phase Transformation in Steels: Fe-Fe3C System

Eutectoid point

Peritectic isotherm

Eutectic isotherm

Eutectic point

Eutectoid isotherm

Solid phases:

δ-ferrite high temperature

BCC iron

Austenite- FCC iron (γ)

α-ferrite- BCC iron

Cementite-Fe3C

When cooled to just below the

eutectoid temperature, an

alloy containing 0.77 wt % C

transforms from γ-Fe into a

two-phase mixture with a

lamellar (layered) morphology

of α-Fe and Fe3C plates.

� Two-phase lamellar

material: Pearlite

33Important Phases and Structures in Steels

• Hypoeutectoid Steels- Compositions to the left of the eutectoid compositions

• Hypereutectoid Steels- Compositions to the right of the eutectoid compositions

• Proeutectoid Ferrite- Ferrite that forms prior to the eutectoid ferrite

• Proeutectoid Cementite- Cementite that forms prior to the eutectoid cementite

• Decomposition of austenite can lead to:

– Pearlite (γ-Fe → α-Fe +Fe3C; composition: 0.77 wt % C)

– Bainite (Eutectoid of feathery-shaped α-ferrite and Fe3C particles)

– Martensite (Interstitial solid solution of C in a body-centered tetragonal Fe)

– Spheroidite (Spherical particles of cementite in a matrix of α-ferrite)

34

Phase Transformation in Steels: Structure of Pearlite

Alternate lamellae of α-Fe and Fe3C

make up the structure of pearlite.

α-Fe

Fe3C

γ

Advancing interface

γ-Fe → α-Fe +Fe3C with relatively small undercooling

35Fraction of transformed pearlite over time

Isothermal

transformation

diagrams

C curves

36Decomposition of Austenite to Form Pearlite:

Redistribution of C along the austenite-pearlite interface

3

0.77 0.0220.112

6.70 0.022Fe C

f−

= =−

6.70 .770.888

6.70 0.022fα

−= =

−

Pearlite spacing depends on

the diffusion rate of carbon.

37

Microstructure of Formed Pearlite Depends on T

Decreasing the isothermal hold

temperatures decreases the

interlamellar spacing.

655 °°°°C 600 °°°°C534 °°°°C 487 °°°°C

Pearlite formed at higher temperature is coarser

than that formed at lower temperature.

Direction of α+Fe3C growth

At High T, the diffusion of carbon is large. �

Thickness (interlamellar spacing) of structure

is large.

38

Bainite that forms isothermally

γ

Fe3C

α

Schematic of

bainite formation

495 °°°°Cγγγγ

410 °°°°Cγγγγ

As the amount of undercooling (T < 550°C) is increased, an alternative

morphology to pearlite becomes more energetically favorable. � Bainite

γ-Fe → α-Fe +Fe3C with relatively large undercooling

Phase Transformation in Steels: Structure of Bainite

The α-ferrite either has a feathery appearance or occurs as plates. Carbide

particles lie between the α-ferrite regions.

39

If the austenite of composition 0.77 wt % carbon is quenched to <200 °C, it does not form a two-phase mixture of α and Fe3C by diffusion.

An athermal, diffusionless transformation takes place. Carbon atoms in interstitial

position are just trapped. This is known as Martensite.

These are not thermally activated and occur very rapidly and continuously during

cooling. The amount of transformed phase depends solely on temperature, not time.

Decomposition of austenite (γ-Fe) with even large undercooling

Phase Transformation in Steels: Structure of Martensite

10µm

T= 25 °C T= -60 °C T= -100 °CDecreasing temperature, increasing fraction of transformed martensite.

γγγγ

γγγγMartensiteplates

40Phase Transformations in Eutectoid Steel

Austenite to Pearlite-

Diffusional transformation

Austenite to Bainite-

Diffusional transformation

Austenite to Martensite-

Athermal, diffusionless

transformation

(time-independent, so C-curve

becomes horizontal line)

Pro

gre

ssiv

ely

Fin

er

Mic

rostr

uctu

re

Decomposition of austenite (γ-Fe) under different conditions

��: temperature at which martensite

reaction begins; ��: 50% completed

41

Dependence of martensite reaction temperature on %C

Austenite to Martensite

transformation is athermal and

diffusionless.

��: temperature at which

martensite reaction begins;

��: martensite reaction is

finished.

There is a strong dependence of carbon content on the martensitic phase

transformation. As the carbon content increases, more undercooling is required to

induce the martensitic phase transformation.

Unlike pearlite or bainite, in martensite the C atoms remain trapped in interstitial

positions as very effective obstacles to dislocation motion.

Therefore, martensite is the strongest of all common phases found in steels.

42

Structural Transformation: Austenite to Martensite

Austenite (γ, FCC Fe)

c/a = 1.4

Fe

� C

BCT (body-

centered

tetragonal) Fe

crystal with C

interstitial

Martensite

43

1µm

Tempering of Martensite: Martensite → Spheroidite

Increasing the temperature increases the size of the resulting phase. The

driving force for the transformation is the reduction in surface free energy.

426 °C, 1 hour 593 °C, 1 hour 675 °C, 1 hour

Martensite is metastable. When heated, Fe3C begins to form. Carbon diffuses out

of the metastable BCT structure to form the more stable carbide phase.

The carbide precipitates have spherical morphology. So the structure is termed

Spheroidite. Spheroidite is very stable, and has good ductility.

Spheroidite can also be obtained by reheating pearlictic or bainitic phases.

44Example: Thermal treatment of eutectoid steel

100% Austenite(γ)

75% γ, 25% Pearlite



100%, Pearlite (after 20 seconds)

Case 1: Austenite quenched to 600 °C and held for 20 sec.

45

(x<1 sec) 100% Martensite

(~1.7 sec) 75% Martensite, 25% Pearlite

(~3 sec) 50% Martensite, 50% Pearlite

(~4.5 sec) 25% Martensite, 75% Pearlite

(20 sec) 100%, Pearlite

Example: Thermal treatment of eutectoid steel

Case 2: Austenite quenched to 600 °C, held for x

seconds, and then quenched to room temperature

Example: Phase Transformations in Off-Eutectoid Steels

Proeutectoid α and γ

100% Austenite (γ)

γ and proeutectoid α

γ, proeutectoid α and pearlite

Proeutectoid α and pearlite

Case 1: Austenite quenched to 700 °C and held for different time

47


100% Austenite (γ)

50% γ and 50% Bainite

100% Bainite

Case 2: Austenite quenched to 500 °C and held for different time


48


100% Martensite

50% Bainite and

50% Martensite

100% Bainite

Case 3: Austenite quenched to 500 °C and held for different time,

then quenched to room temperature


49Continuous-Cooling Transformation Diagram

Example: diagram for a eutectoid steelAll previous examples are for

instantaneous quenches

followed by isothermal holds.

When quench rates are not

instantaneous, we need to use

the continuous-cooling

transformation diagram.

In this example:

Path 1: 100% austenite between

O and a1; pearlite develops

between a1 and b1; 100%

pearlite after b1

Path 2: at b2 there are 33%

pearlite and 67% austenite; then

at low temperature 33% pearlite

and 67% martensite

Path 3: transformation to pearlite

is avoided

50

Example of quenching operations for steels

Rapid quenching to avoid

γ-Fe → α-Fe +Fe3C

Temping:

Martensite → spheroidite

for improved ductility

100% MartensiteHold at above �� to minimize thermal

gradients and avoid surface cracks

51

Precipitation from a Supersaturated Solid Solution

X0 at T=T1 for sufficiently long time

A process known as homogenization

or solution heat treatment occurs

� All of the solute dissolves into the

α phase.

If the alloy is rapidly quenched to

temperature T2, the solute is not

immediately able to diffuse out of the

α phase

� the alloy is supersaturated.

The formation of particle from a supersaturated solid solution is a nucleation and

growth process, and the size, number, and spatial distribution of precipitates

(and therefore, the properties) are controlled by temperature and time.

52

Time-temperature process for age-hardened structure

The process of precipitation from a

supersaturated solid solution is called

aging.

Aging at room temperature � natural

aging (too slow)

Aging above room temperature �

artificial aging (what is the best T?)

The first step is a solution heat treatment followed by a rapid quench.

The final step is to reheat the alloy, while remaining well below the solvus

boundary, to accelerate the aging process.

Choosing appropriate aging temperature: two necessary requirements for

commercial application

1. A temperature at which a significant amount of solute can be dissolved.

2. A significant decrease in solubility with decreasing temperature.

53

Example of Aging: The aluminum-copper system

Al-rich portion of the Al-Cu phase diagram

4.5 % Cu at 820 K � Quench to room temperature � Too slow precipitation (too

slow kinetics)

Artificial aging accelerates the precipitation by increasing the atomic diffusion �

the amount of precipitation is small.

We need a balance.

Strength of alloy after aging

54

Solidification and Homogenization of an Alloy

The heat-treating we discussed previously is usually the

final step in material processing. Before that, alloys need to

be homogenized.

Consider the equilibrium cooling (at infinitely slow rate).

Gradually, the alloy is transformed from single liquid

solution (uniform) to L+S (uniform within each phase), then

to single solid solution (uniform).

To

T1

T2

T3

TF

Using tie lines and lever

rule, at � ( � 1, 2, 3…) the

compositions of liquid and

solid are �� and ��,

respectively.

55

T3

T2

To

T1

Non-Equilibrium Solidification

Equilibrium solidification does not generally occur in practice

because of the slow diffusion (almost negligible) in the solid phase.

At any temperature, the composition of the solid and liquid at the

interface is given by the equilibrium phase diagram (tie line).

The composition in each solid grain is not uniform anymore.

For example, at ��the liquid has �� but

the solid has a center

of ��, surrounded by

a ring of ��, and an

outer ring of ��. The

average composition

is �′��. This process

is called “coring.”

Non-equilibrium

solidus boundary

56Example of Non-Equilibrium Solidification

Al-rich portion of Al-Cu

phase diagram

Effect of Composition

Al-2%Cu

Al-5%Cu

Instead of uniform α-

phase, we see cored

microstructures.

Effect of Cooling Rate

1K/sec 10K/sec

Faster cooling rates lead to greater undercooling, which increases

the nucleation rate and leads to a finer-grained structure.

Al-5%Cu

MSE 2001- Chapter 8 Pt2 Ppt

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