Heat Treatment

Heat Treatment

R. Manna

Assistant Professor

Centre of Advanced Study

Department of Metallurgical Engineering

Institute of Technology

Banaras Hindu University

Varanasi-221 005, [email protected]

Tata Steel-TRAERF Faculty Fellowship Visiting Scholar

Department of Materials Science and Metallurgy

University of CambridgePembroke Street, Cambridge, CB2 3QZ

[email protected]

HEAT TREATMENT

Fundamentals

Fe-C equilibrium diagram. Isothermal and continuous cooling transformation diagrams for plain carbon and alloy steels. Microstructure and mechanical properties of pearlite, bainite and martensite. Austenitic grain size. Hardenability, its measurement and control.

Processes

Annealing, normalising and hardening of steels, quenching media, tempering. Homogenisation. Dimensional and compositional changes during heat treatment. Residual stresses and decarburisation.

2

Surface Hardening

Case carburising, nitriding, carbonitriding, induction and flame

hardening processes.

Special Grade Steels

Stainless steels, high speed tool steels, maraging steels, high strength

low alloy steels.

Cast irons

White, gray and spheroidal graphitic cast irons

Nonferrous Metals

Annealing of cold worked metals. Recovery, recrystallisation and grain

growth. Heat treatment of aluminum, copper, magnesium, titanium and

nickel alloys. Temper designations for aluminum and magnesium alloys.

Controlled Atmospheres

Oxidizing, reducing and neutral atmospheres. 3

Suggested Reading

R. E. Reed-Hill and R. Abbaschian: Physical Metallurgy Principles, PWS , Publishing Company, Boston, Third Edition.

Vijendra Singh: Heat treatment of Metals, Standard Publishers Distributors, Delhi.

Anil Kumar Sinha: Physical Metallurgy Handbook, McGraw-Hill Publication.

H. K. D. H. Bhadeshia and R. W. K. Honeycombe: Steels-Microstructure and Properties, Butterworth-Heinemann, Third Edition, 2006

R. C. Sharma: Principles of Heat Treatment of Steels, New Age International (P) Ltd. Publisher.

Charlie R. Brooks: Heat Treatment: Structure and Properties of Nonferrous Alloys, A. S. M. Publication. 4

Definition of heat treatment

Heat treatment is an operation or combination of operations

involving heating at a specific rate, soaking at a temperature

for a period of time and cooling at some specified rate. The

aim is to obtain a desired microstructure to achieve certain

predetermined properties (physical, mechanical, magnetic or

electrical).

5

Objectives of heat treatment (heat treatment processes)

The major objectives are

to increase strength, harness and wear resistance (bulk hardening, surface hardening)

to increase ductility and softness (tempering, recrystallizationannealing)

to increase toughness (tempering, recrystallization annealing)

to obtain fine grain size (recrystallization annealing, full annealing, normalising)

to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing)

6

to improve machineability (full annealing and normalising)

to improve cutting properties of tool steels (hardening and tempering)

to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature

resistance-precipitation hardening, surface treatment)

to improve electrical properties (recrystallization, tempering, age hardening)

to improve magnetic properties (hardening, phase transformation)

7

Fe-cementite metastable phase diagram (Fig.1) consists of

phases liquid iron(L), -ferrite, or austenite, -ferrite and Fe3C or cementite and phase mixture of pearlite

(interpenetrating bi-crystals of ferrite and cementite)(P) and

ledeburite (mixture of austenite and cementite)(LB).

Solid phases/phase mixtures are described here.

8

Weight percent carbon

Tem

per

atu

re,

C

Fig.1: Fe-Cementite metastable phase diagram (microstructural)

L

L+CmIL+I

Le

de

burite

=LB

(eu+

Ceu)

I(II+CmII)+LB (eu(II+CmII)+Cmeu)

LB (eu(II+CmII)+Cmeu)

+CmI

LB (P(ed+Cmed)+CmII)+Cmeu)+CmI

LB ((P(ed(ed+CmIII)+Cmed) +CmII)+ Cmeu)+CmI

(P(ed+Cmed)+CmII)+ LB (P(ed+Cmed)+CmII+Cmeu)

(P(ed(ed+CmIII)+Cmed) +CmII)+LB ((P(ed(ed+CmIII)+Cmed)+CmII)+Cmeu)

II+CmIII+

P(ed+Cmed) +CmII

P(ed (ed+CmIII)+Cmed)

+CmII

I+ (P(ed+Cmed)

I(+CmIII)+(P(ed(ed+CmIII)+Cmed)

+CmIII

+

+L

Cm

Ao=210C

A1=727C

A4=1147C

A5=1495C

A2=668/

770C

A3

0.0218

0.77

2.11

4.30

0.090.17

0.53

0.00005

910C

1539C

1394C

1227C

6.67

L=liquid, Cm=cementite, LB=ledeburite, =delta ferrite, =

alpha ferrite, = alpha ferrite(0.00005 wt%C) =austenite,

P=pearlite, eu=eutectic, ed=eutectoid, I=primary,

II=secondary, III=tertiary

Pea

rlit

eI+LB LB+CmI

A

D E F

B C

9

ferrite:

Interstitial solid solution of carbon in iron of body centred

cubic crystal structure (Fig .2(a)) ( iron ) of higher lattice

parameter (2.89) having solubility limit of 0.09 wt% at

1495 C with respect to austenite. The stability of the phase

ranges between 1394-1539 C.

Fig.2(a): Crystal structure of ferrite

This is not stable at room temperature in plain carbon steel.

However it can be present at room temperature in alloy steel

specially duplex stainless steel.

10

phase or austenite:

Interstitial solid solution of carbon in iron of face centred cubic

crystal structure (Fig.3(a)) having solubility limit of 2.11 wt% at

1147 C with respect to cementite. The stability of the phase ranges

between 727-1495 C and solubility ranges 0-0.77 wt%C with respect

to alpha ferrite and 0.77-2.11 wt% C with respect to cementite, at 0

wt%C the stability ranges from 910-1394 C.

Fig.3(a ): Crystal structure of austenite is shown at right

side. 11

Fig. 3(b): Polished sample held at austenitisation temperature.

Grooves develop at the prior austenite grain boundaries due to the

balancing of surface tensions at grain junctions with the free

surface. Micrograph courtesy of Saurabh Chatterjee.

12

-ferrite:

Interstitial solid solution of carbon in iron of body centred

cubic crystal structure ( iron )(same as Fig. 2(a)) having

solubility limit of 0.0218 wt % C at 727 C with respect to

austenite.

The stability of the phase ranges between low temperatures to

910 C, and solubility ranges 0.00005 wt % C at room

temperature to 0.0218 wt%C at 727 C with respect to

cementite.

There are two morphologies can be observed under equilibrium transformation or in low under undercooling condition in low carbon plain carbon steels. These are intergranular allotriomorphs ()(Fig. 4-7) or intragranular idiomorphs(I) (Fig. 4, Fig. 8)

13

Fig. 4: Schematic diagram of grain boundary allotriomoph

ferrite, and intragranular idiomorph ferrite.

14

Fig.5: An allotriomorph of ferrite in a sample which is partially

transformed into and then quenched so that the remaining

undergoes martensitic transformation. The allotriomorph grows

rapidly along the austenite grain boundary (which is an easy

diffusion path) but thickens more slowly. 15

Fig.6: Allotriomorphic ferrite in a Fe-0.4C steel which is slowly

cooled; the remaining dark-etching microstructure is fine

pearlite. Note that although some -particles might be identified

as idiomorphs, they could represent sections of allotriomorphs.

Micrograph courtesy of the DOITPOMS project. 16

Fig.7: The allotriomorphs have in this slowly cooled low-

carbon steel have consumed most of the austenite before the

remainder transforms into a small amount of pearlite.

Micrograph courtesy of the DoITPOMS project. The shape of

the ferrite is now determined by the impingement of particles

which grow from different nucleation sites.17

Fig. 8: An idiomorph of ferrite in a sample which is partially


undergoes martensitic transformation. The idiomorph is

crystallographically facetted.

18

There are three more allotropes for pure iron that form under

different conditions

-iron:

The iron having hexagonal close packed structure. This forms

at extreme pressure,110 kbars and 490C. It exists at the centre

of the Earth in solid state at around 6000C and 3 million

atmosphere pressure.

FCT iron:

This is face centred tetragonal iron. This is coherently

deposited iron grown as thin film on a {100} plane of copper

substrate

Trigonal iron:

Growing iron on misfiting {111} surface of a face centred

cubic copper substrate. 19

Fe3C or cementite:

Interstitial intermetallic compound of C & Fe with a carbon content

of 6.67 wt% and orthorhombic structure consisting of 12 iron atoms

and 4 carbon atoms in the unit cell.

Stability of the phase ranges from low temperatures to 1227 C

Fig.9(a): Orthorhombic crystal structure of cementite. The purple

atoms represent carbon. Each carbon atom is surronded by eight iron

atoms. Each iron atom is connected to three carbon atoms.

20

Fig.9(b): The pearlite is resolved in some regions where the

sectioning plane makes a glancing angle to the lamellae. The

lediburite eutectic is highlighted by the arrows. At high temperatures

this is a mixture of austenite and cementite formed from liquid. The

austenite subsequently decomposes to pearlite.

Courtesy of Ben Dennis-Smither, Frank Clarke and Mohamed Sherif

21

Critical temperatures:

A=arret means arrest

A0= a subcritical temperature (

Acm=/+cementite phase field boundary=composition dependent =727-

1147 C

In addition the subscripts c or r are used to indicate that the temperature is

measured during heating or cooling respectively.

c=chaffauge means heating, Acr=refroidissement means cooling, Ar

Types/morphologies of phases in Fe-Fe3C system

Cementite=primary (CmI), eutectic (Cmeu), secondary (CmII)(grain

boundary allotriomophs, idiomorphs), eutectoid (Cmed) and tertiary(CmIII).

Austenite= austenite()(equiaxed), primary (I), eutectic (eu), secondary

(II) (proeutectoid),

-ferrite=ferrite () (equiaxed), proeutectoid or primary (grain boundary

allotriomorphs and idiomorphs)(I), eutectoid(eu) and ferrite (lean in

carbon) ().

Phase mixtures

Pearlite (P) and ledeburite(LB)23

Fig.10: -ferrite in dendrite form in as-cast Fe-0.4C-2Mn-0.5Si-2 Al0.5Cu, Coutesy of S. Chaterjee et al.

M. Muruganath, H. K. D. H. Bhadeshia

Important Reactions

Peritectic reaction

Liquid+Solid1Solid2L(0.53wt%C)+(0.09wt%C)(0.17wt%C) at 1495 C

Liquid-18.18wt% +-ferrite 81.82 wt%100 wt%

24

Fig.11: Microstructure of white cast iron containing

massive cementite (white) and pearlite etched with 4%

nital, 100x. After Mrs. Janina Radzikowska, Foundry

Research lnstitute in Krakw, Poland

Eutectic reactionLiquidSolid1+Solid2Liquid (4.3wt%C) (2.11wt%C) + Fe3C (6.67wt%C) at 1147C

Liquid-100 wt% 51.97wt% +Fe3C (48.11wt%)

The phase mixture of austenite and cementite formed at eutectic

temperature is called ledeburite.

25

Fig. 12: High magnification view (400x) of the white cast iron

specimen shown in Fig. 11, etched with 4% nital. After Mrs.

Janina Radzikowska, Foundry Research lnstitute in Krakw,

Poland

26

Fig. 13: High magnification view (400x) of the white cast

iron specimen shown in Fig. 11, etched with alkaline sodium

picrate. After Mrs. Janina Radzikowska, Foundry Research

lnstitute in Krakw, Poland

27

Eutectoid reaction

Solid1Solid2+Solid3

(0.77wt%C) (0.0218wt%C) + Fe3C(6.67wt%C) at 727 C

(100 wt%) (89 wt% ) +Fe3C(11wt%)

Typical density

ferrite=7.87 gcm-3

Fe3C=7.7 gcm-3

volume ratio of - ferrite:Fe3C=7.9:1

28

Fig. 14: The process by which a colony of pearlite

evolves in a hypoeutectoid steel.

29

Fig. 15 : The appearance of a pearlitic

microstructure under optical microscope.

30

Fig. 16: A cabbage filled with water analogy of the three-

dimensional structure of a single colony of pearlite, an

interpenetrating bi-crystal of ferrite and cementite.

31

Fig. 17: Optical micrograph showing colonies

of pearlite . Courtesy of S. S. Babu.

32

Fig. 18: Transmission electron micrograph of

extremely fine pearlite.

33

Fig.19: Optical micrograph of extremely fine

pearlite from the same sample as used to

create Fig. 18. The individual lamellae cannot

now be resolved.

34

Evolution of microstructure (equilibrium cooling)

Sequence of evolution of microstructure can be described by

the projected cooling on compositions A, B, C, D, E, F.

At composition A

L +L + +I +CmIII

At composition B

L +L L+I I + I+ (P(ed+Cmed)

I(+CmIII)+(P(ed(ed+CmIII)+Cmed)

35

At composition C

L

At composition D

L

L+I II+CmII P(ed+Cmed)+CmII

P(ed (ed+CmIII)+Cmed)+CmII

L+I I+LB I(II+CmII)+LB (eu(II+CmII)+Cmeu)

(P(ed+Cmed)+CmII)+ LB (P(ed+Cmed)+CmII+Cmeu)

(P(ed(ed+CmIII)+Cmed) +CmII)+ LB ((P(ed(ed+CmIII)+Cmed)+CmII)+Cmeu)

36

At composition E

L

At composition F

L Fe3C

L+CmI LB(eu+Cmeu+CmI

LB (eu(II+CmII)+Cmeu)+CmI

LB (P(ed+Cmed)+CmII)+Cmeu)+CmI

LB ((P(ed(ed+CmIII)+Cmed) +CmII)+ Cmeu)+CmI

37

Limitations of equilibrium phase diagram

Fe-Fe3C equilibrium/metastable phase diagram

Stability of the phases under equilibrium condition only.

It does not give any information about other metastable phases.

i.e. bainite, martensite

It does not indicate the possibilities of suppression of

proeutectoid phase separation.

No information about kinetics

No information about size

No information on properties.

38

Time Temperature Transformation

(TTT) Diagrams

R. Manna

Assistant Professor



Institute of Technology, Banaras Hindu UniversityVaranasi-221 005, India

[email protected]



University of Cambridge, Pembroke Street, Cambridge, CB2 [email protected]

TTT diagrams

TTT diagram stands for time-temperature-transformation diagram. It is

also called isothermal transformation diagram

Definition: TTT diagrams give the kinetics of isothermal

transformations.

2

Determination of TTT diagram for eutectoid steel

Davenport and Bain were the first to develop the TTT diagram

of eutectoid steel. They determined pearlite and bainite

portions whereas Cohen later modified and included MS and

MF temperatures for martensite. There are number of methods

used to determine TTT diagrams. These are salt bath (Figs. 1-

2) techniques combined with metallography and hardness

measurement, dilatometry (Fig. 3), electrical resistivity

method, magnetic permeability, in situ diffraction techniques

(X-ray, neutron), acoustic emission, thermal measurement

techniques, density measurement techniques and

thermodynamic predictions. Salt bath technique combined

with metallography and hardness measurements is the most

popular and accurate method to determine TTT diagram.

3

Fig. 2 : Bath II low-temperature

salt-bath for isothermal treatment.

Fig. 1 : Salt bath I -austenitisation

heat treatment.

4

Fig . 3(a): Sample and

fixtures for dilatometric

measurements

Fig. 3(b) : Dilatometer

equipment

5

In molten salt bath technique two salt baths and one water

bath are used. Salt bath I (Fig. 1) is maintained at austenetising

temperature (780C for eutectoid steel). Salt bath II (Fig. 2) is

maintained at specified temperature at which transformation is

to be determined (below Ae1), typically 700-250C for

eutectoid steel. Bath III which is a cold water bath is

maintained at room temperature.

In bath I number of samples are austenitised at AC1+20-40 C

for eutectoid and hypereutectoid steel, AC3+20-40 C for

hypoeutectoid steels for about an hour. Then samples are

removed from bath I and put in bath II and each one is kept for

different specified period of time say t1, t2, t3, t4, tn etc. After

specified times, the samples are removed and quenched in

water. The microstructure of each sample is studied using

metallographic techniques. The type, as well as quantity of

phases, is determined on each sample.6

The time taken to 1% transformation to, say pearlite or bainiteis considered as transformation start time and for 99%transformation represents transformation finish. On quenchingin water austenite transforms to martensite.

But below 230 C it appears that transformation is timeindependent, only function of temperature. Therefore afterkeeping in bath II for a few seconds it is heated to above230 C a few degrees so that initially transformed martensitegets tempered and gives some dark appearance in an opticalmicroscope when etched with nital to distinguish from freshlyformed martensite (white appearance in optical microscope).Followed by heating above 230 C samples are waterquenched. So initially transformed martensite becomes dark inmicrostructure and remaining austenite transform to freshmartensite (white).

7

Quantity of both dark and bright etching martensites are

determined. Here again the temperature of bath II at which 1%

dark martensite is formed upon heating a few degrees above

that temperature (230 C for plain carbon eutectoid steel) is

considered as the martensite start temperature (designated MS).

The temperature of bath II at which 99 % martensite is formed

is called martensite finish temperature ( MF).

Transformation of austenite is plotted against temperature vs

time on a logarithm scale to obtain the TTT diagram. The

shape of diagram looks like either S or like C.

Fig. 4 shows the schematic TTT diagram for eutectoid plain

carbon steel

8

Fig.4: Time temperature transformation (schematic) diagram for plain carbon

eutectoid steel

t1 t3t2 t4 t5

MF, Martensite finish temperature

M50,50% Martensite

MS, Martensite start temperature

Metastable austenite +martensite

Martensite

% o

f P

ha

se

0

100

Tem

per

atu

re

Log time

Hard

nes

s

Ae1

T2

T1

50%T2T1

Pearlite

Fine pearlite

Upper bainite

Lower bainite

50% very fine pearlite + 50% upper bainite

At T1, incubation

period for pearlite=t2,

Pearlite finish time

=t4

Minimum incubation

period t0 at the nose

of the TTT diagram,

t0MS=Martensite

start temperature

M50=temperature

for 50%

martensite

formation

MF= martensite

finish temperature

9

At close to Ae1 temperature, coarse pearlite forms at close to

Ae1 temperature due to low driving force or nucleation rate.

At higher under coolings or lower temperature finer pearlite

forms.

At the nose of TTT diagram very fine pearlite forms

Close to the eutectoid temperature, the undercooling is low so

that the driving force for the transformation is small. However,

as the undercooling increases transformation accelerates until

the maximum rate is obtained at the nose of the curve.

Below this temperature the driving force for transformation

continues to increase but the reaction is now impeded by slow

diffusion. This is why TTT curve takes on a C shape with

most rapid overall transformation at some intermediate

temperature.

10

Pearlitic transformation is reconstructive. At a given temperature (say

T1) the transformation starts after an incubation period (t2, at T1).

Locus of t2 for different for different temperature is called

transformation start line. After 50% transformation locus of that time

(t3 at T1)for different temperatures is called 50% transformation line.

While transformation completes that time (t4 at T1) is called

transformation finish, locus of that is called transformation finish line.

Therefore TTT diagram consists of different isopercentage lines of

which 1%, 50% and 99% transformation lines are shown in the

diagram. At high temperature while underlooling is low form coarse

pearlite. At the nose temperature fine pearlite and upper bainite form

simultaneously though the mechanisms of their formation are entirely

different. The nose is the result of superimposition of two

transformation noses that can be shown schematically as below one

for pearlitic reaction other for bainitic reaction (Fig. 6).

Upper bainite forms at high temperature close to the nose of TTT

diagram while the lower bainite forms at lower temperature but above

MS temperature. 11

Fig. 5(a) : The appearance of a (coarse) pearlitic

microstructure under optical microscope.

12

Fig. 5(b): A cabbage filled with water analogy of the three-

dimensional structure of a single colony of pearlite, an

interpenetrating bi-crystal of ferrite and cementite.

13

Fig. 5(c): Optical micrograph showing colonies

of pearlite . Courtesy of S. S. Babu.

14

Fig. 5(d): Transmission electron micrograph

of extremely fine pearlite.

15

Fig. 5(e): Optical micrograph of extremely

fine pearlite from the same sample as used to

create Fig. 5(d). The individual lamellae

cannot now be resolved.

16

Fig. 6: Time Temperature Transformation (schematic) diagram for plain carbon

eutectoid steel

MF

M50

MSMetastable + M

M

Tem

per

atu

re

Log timeH

ard

nes

s

Ae1

P

FP

UB

LB

50% very FP + 50% UB

Metastable

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite

UB=upper bainite

LB=lower bainite

M=martensite

MS=Martensite start

temperature

M50=temperature for

50% martensite

formation

MF= martensite finish

temperature

17

On cooling of metastable austenite 1% martensite forms at

about 230C. The transformation is athermal in nature. i.e.

amount of transformation is time independent (characteristic

amount of transformation completes in a very short time) but

function of temperature alone. This temperature is called the

martensite start temperature or MS.

Below Ms while metastable austenite is quenched at different

temperature amount of martensite increases with decreasing

temperature and does not change with time.

The temperature at which 99% martensite forms is called

martensite finish temperature or MF. Hardness values are

plotted on right Y-axis. Therefore a rough idea about

mechanical properties can be guessed about the phase mix.

18

TTT diagram gives

Nature of transformation-isothermal or athermal (time

independent) or mixed

Type of transformation-reconstructive, or displacive

Rate of transformation

Stability of phases under isothermal transformation conditions

Temperature or time required to start or finish transformation

Qualitative information about size scale of product

Hardness of transformed products

19

Factors affecting TTT diagram

Composition of steel-(a) carbon wt%, (b) alloying element wt%

Grain size of austenite

Heterogeneity of austenite

Carbon wt%-As the carbon percentage increases A3 decreases, similar is the casefor Ar3, i.e. austenite stabilises. So the incubation period for theaustenite to pearlite increases i.e. the C curve moves to right. Howeverafter 0.77 wt%C any increase in C, Acm line goes up, i.e. austenitebecome less stable with respect to cementite precipitation. Sotransformation to pearlite becomes faster. Therefore C curve movestowards left after 0.77%C. The critical cooling rate required to preventdiffusional transformation increases with increasing or decreasingcarbon percentage from 0.77%C and e for eutectoid steel is minimum.Similar is the behaviour for transformation finish time.

20

Pearlite formation is preceeded by ferrite in case of

hypoeutectoid steel and by cementite in hypereutectoid steel.

Schematic TTT diagrams for eutectoid, hypoeutectoid and

hyper eutectoid steel are shown in Fig.4, Figs. 7(a)-(b) and all

of them together along with schematic Fe-Fe3C metastable

equilibrium are shown in Fig. 8.

21

Fig. 7(a) :Schematic TTT diagram for plain carbon hypoeutectoid steel

MF

M50

MS

Metastable + M

M

Tem

per

atu

re

Log timeH

ard

nes

s

Ae1+CP+PFP

UB

LB

FP + UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite

UB=upper Bainite

LB=lower Bainite

M=martensite

MS=Martensite start

temperature

M50=temperature for

50% martensite

formation


temperature

Ae3

t0

Metastable

22

Fig. 7(b): Schematic TTT diagram for plain carbon hypereutectoid

steel

M50

MS

Metastable

Metastable + M

Tem

per

atu

re

Log timeH

ard

nes

s

Ae1

Fe3C+CPFe3C+PFe3C+FP

UB

LB

very FP +UB

Aecm

t0

=austenite

CP=coarse pearlite

P=pearlite

FP=fine pearlite

UB=upper Bainite

LB=lower Bainite

M=martensite

MS=Martensite start

temperature

M50=temperature for

50% martensite

formation

23

Fig. 8: Schematic Fe-Fe3C metastable equilibrium diagram

and TTT diagrams for plain carbon hypoeutectoid, eutectoid

and hypereutectoid steels

MS

(a) Fe-Fe3C

metastable phase

diagram

(b) TTT diagram for

hypoeutectoid steel

(c ) TTT diagram

for eutectoid steel

(d) TTT diagram for

hypereutectoid steel

=austenite

=ferrite

CP=coarse

pearlite

M=martensite

MS=Martensite start temperature

M50=temperature for 50% martensite

formation

MF= martensite finish temperature

P=pearlite

FP=fine pearlite

UB=upper bainite

LB=lower bainite

24

Under isothermal conditions for various compositions

proeutectoid tranformation has been summarised below

(Fig. 9). In hypoeutectoid steel the observable ferrite

morphologies are grain boundary allotriomorph ()(Fig.11(a)-

(d)), Widmansttten plate (W)(Figs. 12-16), and massive (M)

ferrite (Fig.11(f)).

Grain boundary allotriomorphs form at close to Ae3temperature or extension of Aecm line at low undercooling.

Widmansttten plates form at higher undercooling but mainly

bellow Ae1. There are overlap regions where both

allotriomorphs and Widmansttten plates are observed.

Equiaxed ferrite forms at lower carbon composition less than

0.29 wt%C.

25

Weight % carbon

Tem

per

atu

re

Ae3

0.0218 0.77

Austenite

Pearlite

Ae1

Plate martensite

Mix martensite

Lath martensite

MSMF

Volume % of retained

austenite at room

temperature

Volu

me

% o

f re

tain

ed a

ust

enit

e

Fig 9: Temperature versus composition in which various morphologies

are dominant at late reaction time under isothermal condition

W

CmWM

Upper bainite

Lower bainite

W=Widmansttten

plate

M=massive

P=pearlite

ub=upper bainite

lb =lower bainite

26

There are overlapping regions where both equiaxed ferrite

and Widmansttten plates were observed. However at very low

carbon percentage massive ferrite forms. The reconstructive

and displacive mechanisms of various phase formation is

shown in Fig. 10.

In hypereutectoid steel both grain boundary allotriomorph and

Widmanstatten plates were observed. Massive morphology

was not observed in hypereutectoid steel. Grain boundary

allotriomorphs were observed mainly close to Aecm or close to

extension of Ae3 line but Widmansttten plates were observed

at wider temperature range than that of hypoeutectoid steel. In

hypereutectoid steel there are overlapping regions of grain

boundary allotrioorph and Widmansttten cementite.

27

Fig. 10: The reconstructive and displacive mechanisms. 28

Fig. 11(a): schematic diagram of grain boundary allotriomoph

ferrite, and intragranular idiomorph ferrite.

29

Fig.11(b): An allotriomorph of ferrite in a sample which is partially


undergoes martensitic transformation. The allotriomorph grows

rapidly along the austenite grain boundary (which is an easy diffusion

path) but thickens more slowly. 30

Fig. 11(c): Allotriomorphic ferrite in a Fe-0.4C steel which is

slowly cooled; the remaining dark-etching microstructure is fine

pearlite. Note that although some -particles might be identified as

idiomorphs, they could represent sections of allotriomorphs.

Micrograph courtesy of the DoITPOMS project. 31

Fig. 11(d): The allotriomorphs have in this slowly cooled low-

carbon steel have consumed most of the austenite before the

remainder transforms into a small amount of pearlite.

Micrograph courtesy of the DoItPoms project. The shape of

the ferrite is now determined by the impingement of particles

which grow from different nucleation sites.32

Fig. 11(e): An idiomorph of ferrite in a sample which is partially


undergoes martensitic transformation. The idiomorph is

crystallographically facetted.

33

Fig. 11(f ): Massive ferrite (m) in Fe-0.002 wt%C alloy quenched into ice brine from 1000C. Courtesy of T. B.

Massalski

34

Fig. 12(a): Schematic illustration of primary Widmansttten

ferrite which originates directly from the austenite grain

surfaces, and secondary w which grows from allotriomorphs.

35

Fig. 12(b): Optical micrographs showing white-etching (nital)

wedge-shaped Widmansttten ferrite plates in a matrix quenched to

martensite. The plates are coarse (notice the scale) and etch cleanly

because they contain very little substructure.36

Fig. 13: The simultaneous growth of two self-

accommodating plates and the consequential tent-like

surface relief.

37

Fig.14: Transmission electron micrograph of what optically appears

to be single plate, but is in fact two mutually accommodating plates

with a low-angle grain boundary separating them. Fe-0.41C alloy,

austenitised at 1200 C for 6 hrs, isothermally transformed at 700 C

for 2 min and water quenched. 38

Fig. 15: Mixture of allotriomorphic ferrite, Widmansttten ferrite

and pearlite. Micrograph courtesy of DOITPOMS project.

39

Fig. 16 (a) Surface relief of Widmansttten ferrite Fe-0.41C

alloy, austenitised at 1200 C for 6 hrs, isothermally

transformed at 700 C for 30 min and water quenced, (b) same

field after light polishing and etching with nital.40

For eutectoid steel banitic transformation occurs at 550 to

250C. At higher temperature it is upper bainite and at lower

temperature it is lower bainite. As C increases the austenite to

ferrite decomposition becomes increasingly difficult. As

bainitic transformation proceeds by the nucleation of ferrite,

therefore banitic transformation range moves to higher timing

and lower temperature. With increasing percentage of carbon

the amount of carbide in interlath region in upper bainite

increases and carbides become continuous phase. However at

lower percentage of carbon they are discrete particles and

amount of carbide will be less in both type of bainites. For

start and finish temperatures for both types of bainites go

down significantly with increasing amount of carbon (Figs. 8-

9). However increasing carbon makes it easier to form lower

bainite.

41

Fig 17: Summary of the mechanism of the bainite reaction.

42

Fig. 18: Upper bainite; the

phase between the platelets

of bainitic ferrite is usually

cementite.

43

Fig. 19: Transmission electron micrograph of a sheaf of upper bainite in

a partially transformed Fe-0.43C-2Si-3Mn wt% alloy (a) optical

micrograph, (b, c) bright field and corresponding dark field image of

retained austenite between the sub units, (d) montage showing the

structure of the sheaf. 44

Fig. 20 : Corresponding outline of the sub-units near the sheaf tip

region of Fig. 19 45

Fig. 21 : AFM image showing surface relief due to individual bainite

subunit which all belong to tip of sheaf. The surface relief is

associated with upper bainite (without any carbide ) formed at 350C

for 2000 s in an Fe-0.24C-2.18Si-2.32Mn-1.05Ni (wt% ) alloy

austenitised at 1200C for 120 s alloy. Both austenitisation and

isothermal transformation were performed in vacuum. The

microstructure contains only bainitic ferrite and retained austenite.

The measured shear strain is 0.260.02. 46

Fig. 22: Optical micrograph illustrating the sheaves of lower bainite in

a partially transformed (395C), Fe-0.3C-4Cr wt% ally. The light

etching matrix phase is martensite. (b) Corresponding transmission

electron micrograph illustrating subunits of lower bainite.

a b

47

Fig. 23 : (a) Optical micrograph showing thin and spiny lower

bainite formed at 190C for 5 hours in an Fe-1.1 wt% C steel. (b)

Transmission electron micrograph showing lower bainite midrib in

same steel. Courtesy of M. Oka

48

Fig. 24 : Schematic illustration of

various other morphologies: (a)

Nodular bainite, (b) columnar bainite

along a prior austenite boundary, (c)

grain boundary allotriomorphic

bainite, (d) inverse bainite

a

b

c

d49

Within the bainitic transformation temperature range, austenite of

large grain size with high inclusion density promotes acicular

ferrite formation under isothermal transformation condition. The

morphology is shown schematically (Figs. 25-27 )

Fig. 25 : shows the morphology and nucleation site of

acicular ferrite.

50

Fig . 26: Acicular ferrite

51

Fig. 27: Replica transmission electron micrograph of

acicular ferrire plates in steel weld. Courtesy of Barritte.52

For eutectoid steel martensite forms at around 230C. From

230C to room temperature martensite and retained austenite are

seen. At room temperature about 6% retained austenite can be

there along with martensite in eutectoid steel. At lower carbon

percentage MS temperature goes up and at higher percentage MStemperature goes down (Fig. 4, Figs. 7-8, Fig. 28). Below 0.4

%C there is no retained austenite at room but retained austenite

can go up to more than 30% if carbon percentage is more than

1.2%. Morphology of martensite also changes from lathe at low

percentage of carbon to plate at higher percentage of carbon.

Plate formation start at around 0.6 % C. Therefore below 0.6 %

carbon only lathe martensite can be seen, mixed morphologies

are observed between 0.6%C to 1%C and above 1% it is 100%

plate martensite (Figs. 29-39).

53

Weight % carbon

Tem

per

atu

re

Austenite +cementite

Ae3

0.0218 0.77

Pea

rlit

e

Austenite

Ferrite + pearlite

Pearlite+cementite

Ae1

Plate martensiteMix martensiteLath martensite

MS

MF

Ferrite + austenite

Volume % of retained

austenite at room

temperature

Volu

me

% o

f re

tain

ed a

ust

enit

e

Fig. 28: Effect of carbon on MS, MF temperatures and retained austenite in plain carbon

steel

54

Lath

(Fe-9%Ni-0.15%C)

Lenticular

(Fe-29%Ni-0.26%C)

Thin plate

(Fe-31%Ni-0.23%C)

Substructure DislocationDislocation

Twin (midrib)Twin

Habit plane{111}A{557}A

{259}A{3 10 15}A

{3 10 15}A

O.R. K-SN-W

G-TG-T

Ms high low

Fig. 29: Morphology and crystallography of (bcc or bct) martensite in

ferrous alloys

Courtesy of

T. Maki

55

Fig. 30: Lath martensite

Courtesy of

T. Maki

56

(T. MakiK. Tsuzaki, I. Tamura: Trans. ISIJ, 20(1980), 207.)

Packet: a group of laths

with the same habit plane

( ~{111} )

Block : a group of laths

with the same orientation

(the same K-S variant)

Fig. 31: effect of carbon

on martensite lath size

57

Fig. 33: Fe-31%Ni-0.28%C

(Ms=192K)

Lenticular martensite

(Optical micrograph)

Fig.32: Fe-29%Ni-0.26%C

(Ms=203K)

Fig.34: schematic diagram for

lenticular martensite

Courtesy of

T. Maki

58

cooling

after polished and etched

Fig. 35: Growth behavior of lenticular martensite

in Fe-30.4%Ni-0.4%C alloy

surface relief surface relief

surface relief

(T. Kakeshita, K. Shimizu, T. Maki, I. Tamura, Scripta Metall., 14(1980)1067.)

Courtesy of

T. Maki

59

midrib twinned region

schematic illustration

Fig. 36: Lenticular martensite in Fe-33%Ni alloy

(Ms=171K)

Optical micrograph

Courtesy of

T. Maki

60

Fig. 37: Optical microstructure of lath martensite (Fe-C alloys)

0.0026%C 0.18%C

0.61%C0.38%C

Courtesy of

T. Maki

61

Block structure in a single packet (Fe-0.18%C)

Fig.39 : Orientation

image map

Fig. 38: SEM image

Courtesy of

T. Maki

Alloying elements: Almost all alloying elements(except, Al, Co, Si) increases the stability of supercooled

austenite and retard both proeutectoid and the pearlitic reaction

and then shift TTT curves of start to finish to right or higher

timing. This is due to i) low rate of diffusion of alloying

elements in austenite as they are substitutional elements, ii)

reduced rate of diffusion of carbon as carbide forming

elements strongly hold them. iii) Alloyed solute reduce the rate

of allotropic change, i.e. , by solute drag effect on interface boundary. Additionally those elements (Ni, Mn, Ru,

Rh, Pd, Os, Ir, Pt, Cu, Zn, Au) that expand or stabilise

austenite, depress the position of TTT curves to lower

temperature. In contrast elements (Be, P, Ti, V, Mo, Cr, B, Ta,

Nb, Zr) that favour the ferrite phase can raise the eutectoid

temperature and TTT curves move upward to higher

temperature.63

However Al, Co, and Si increase rate of nucleation and growth

of both ferrite or pearlite and therefore shift TTT diagram to

left. In addition under the complex diffusional effect of various

alloying element the simple C shape behaviour of TTT

diagram get modified and various regions of transformation

get clearly separated. There are separate pearlitic C curves,

ferritic and bainitic C curves and shape of each of them are

distinct and different.

64

The effect of alloying elements is less pronounced in bainitic

region as the diffusion of only carbon takes place (either to

neighbouring austenite or within ferrite) in a very short time

(within a few second) after supersaturated ferrite formation by

shear during bainitic transformation and there is no need for

redistribution of mostly substitutional alloying elements.

Therefore bainitic region moves less to higher timing in

comparison to proeutectoid/pearlitic region. Addition of

alloying elements lead to a greater separation of the reactions

and result separate C-curves for pearlitic and bainitic regions

(Fig. 40). Mo encourage bainitic reaction but addition of boron

retard the ferrite reaction. By addition of B in low carbon Mo

steel the bainitic region (almost unaffected by addition of B)

can be separated from the ferritic region.

65

Fig. 40: Effect of boron on TTT diagram of low carbon Mo steel

MS

Metastable austenite + martensite

Tem

per

atu

re

Log time

Ae1

Bainite

Metastable austenite

Ae3

Ferrite C curve in low

carbon Mo steel Ferrite C curve in low

carbon Mo-B steel

Pearlitic C curve in low

carbon Mo steel Pearlitic C curve in low

carbon Mo-B steel

Addition of boron

Addition of boron

Bainite start

66

However bainitic reaction is suppressed by the addition of some

alloying elements. BS temperature (empirical) has been given by

Steven & Haynes

BS( C)=830-270(%C)-90(%Mn)-37(%Ni)-70(%Cr)-83(%Mo)

(elements by wt%)

According to Leslie, B50( C)=BS-60

BF( C)=BS-120

Most alloying elements which are soluble in austenite lower MS, MFtemperature except Al, Co.

Andrews gave best fit equation for MS:

MS(C)=539-423(%C)-30.4Mn-17.7Ni-12.1Cr-7.5Mo+10Co-7.5Si

(concentration of elements are in wt%).

Effect of alloying elements on MF is similar to that of MS. Therefore,

subzero treatment is must for highly alloyed steels to transform

retained austenite to martensite.67

Addition of significant amount of Ni and Mn can change the nature of

martensitic transformation from athermal to isothermal (Fig. 41).

Tem

per

atu

re

Log time

Fig. 41: kinetics of isothermal martensite in an Fe-Ni-Mn alloy68

Effect of grain size of austenite: Fine grain size shifts S curvetowards left side because it helps for nucleation of ferrite,cementite and bainite (Fig. 43). However Yang and Bhadeshiaet al. have shown that martensite start temperature (MS) is

lowered by reduction in austenite grain size (Fig. 42).

Fig. 42: Suppression of Martensite

start temperature as a function

austenite grain size L. MO

S is the

highest temperature at which

martensite can form in large

austenite grain. MS is the observed

martensite start temperature (at 0.01

detectable fraction of martensite).

Circles represent from low alloy data

and crosses from high alloy data.

69

T= MS. a, b are fitting empirial constants,

m =average aspect ratio of martensite=0.05 assumed, V=average volume of austenite. f=detectable fraction of

martensite=0.01 (taken).

It is expected similar effect of grain size on MF as on MS.

Grain size of austenite affects the maximum plate or lath size.

i.e. larger the austenite size the greater the maximum plate size

or lath size

70

Fig. 43 : Effect of austenite grain size on TTT diagram of plain carbon

hypoeutectoid steel

MF

M50

MS

Metastable + M

M

Tem

per

atu

re, T

Log(time, t)H

ard

nes

s

Ae1+CP+PFP

UB

LB

50% FP + 50% UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite

UB=upper Bainite

LB=lower Bainite

M=martensite

MS=Martensite start

temperature

M50=temperature at

which 50% martensite

is obtained


temperature

Ae3

Metastable

For finer austenite

71

Heterogeinity of austenite: Heterogenous austenite increases

transformation time range, start to finish of ferritic, pearlitic

and bainitic range as well as increases the transformation

temperature range in case of martensitic transformation and

bainitic transformation. Undissolved cementite, carbides act

as powerful inocculant for pearlite transformation. Therefeore

heterogeneity in austenite increases the transformation time

range in diffussional transformation and temperature range of

shear transformation products in TTT diagram.

72

Applications of TTT diagrams

Martempering

Austempering

Isothermal Annealing

Patenting

Martempering : This heat treatment is given to oil hardenable

and air hardenable steels and thin section of water hardenable

steel sample to produce martensite with minimal differential

thermal and transformation stress to avoid distortion and

cracking. The steel should have reasonable incubation period

at the nose of its TTT diagram and long bainitic bay. The

sample is quenched above MS temperature in a salt bath to

reduce thermal stress (instead of cooling below MF directly)

(Fig. 44)73

Surface cooling rate is greater than at the centre. The cooling

schedule is such that the cooling curves pass behind without

touching the nose of the TTT diagram. The sample is

isothermally hold at bainitic bay such that differential cooling

rate at centre and surface become equalise after some time.

The sample is allowed to cool by air through MS-MF such

that martensite forms both at the surface and centre at the

same time due to not much temperature difference and thereby

avoid transformation stress because of volume expansion.

The sample is given tempering treatment at suitable

temperature.

74

Fig. 44: Martempering heatreatment superimposed on TTT diagram

for plain carbon hypoeutectoid steel

MF

M50

MS

Metastable + martensite

Martensite

Tem

per

atu

re

Log time

Ae1

+CP

+P

FP

UB

LB

50% FP + 50% UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite

t0=minimum incubation

period

UB=upper bainite

LB=lower bainite

M=martensite

MS=Martensite start

temperature

M50=temperature at which

50% martensite is obtained


temperature

Ae3

t0

Metastable Tempering

Tempered martensite

75

Austempering

Austempering heat treatment is given to steel to produce lower

bainite in high carbon steel without any distortion or cracking to

the sample. The heat treatment is cooling of austenite rapidly in a

bath maintained at lower bainitic temperature (above Ms)

temperature (avoiding the nose of the TTT diagram) and holding

it here to equalise surface and centre temperature (Fig. 45) and .

till bainitic finish time. At the end of bainitic reaction sample is

air cooled. The microstructure contains fully lower bainite. This

heat treatment is given to 0.5-1.2 wt%C steel and low alloy steel.

The product hardness and strength are comparable to hardened

and tempered martensite with improved ductility and toughness

and uniform mechanical properties. Products donot required to

be tempered.

76

Fig. 45: Austempering heatreatment superimposed on TTT diagram

for plain carbon hypoeutectoid steel

MF

M50

MS


Martensite

Tem

per

atu

re

Log time

Ae1

+CP

+P

FP

UB

LB

50% FP + 50% UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite


period

UB=upper bainite

LB=lower bainite

M=martensite

MS=Martensite start

temperature




temperature

Ae3

t0

Metastable Tempering

Lower bainite

77

Isothermal annealing

Isothermal annealing is given to plain carbon and alloy steelsto produce uniform ferritic and pearlitic structures. The

product after austenising taken directly to the annealing

furnace maintained below lower critical temperature and hold

isothermally till the pearlitic reaction completes (Fig. 46). The

initial cooling of the products such that the temperature at the

centre and surface of the material reach the annealing

temperature before incubation period of ferrite. As the

products are hold at constant temperature i.e. constant

undercooling) the grain size of ferrite and interlamellar

spacing of pearlite are uniform. Control on cooling after the

end of pearlite reaction is not essential. The overall cycle time

is lower than that required by full annealing.

78

Fig. 46: Isothermal annealing heat treatment superimposed on TTT

diagram of plain carbon hypoeutectoid steel

MF

M50

MS


Martensite

Tem

per

atu

re

Log time

Ae1

+CP

+P

FP

UB

LB

50% FP + 50% UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite


period

UB=upper bainite

LB=lower bainite

M=martensite

MS=Martensite start

temperature




temperature

Ae3

t0

Metastable

Ferrite and pearlite

79

Patenting

Patenting heat treatment is the isothermal annealing at the nose

temperature of TTT diagram (Fig. 47). Followed by this the

products are air cooled. This treatment is to produce fine

pearlitic and upper bainitic structure for strong rope, spring

products containing carbon percentage 0.45 %C to 1.0%C. The

coiled ropes move through an austenitising furnace and enters

the salt bath maintained at 550C(nose temperature) at end of

salt bath it get recoiled again. The speed of wire and length of

furnace and salt bath such that the austenitisation get over

when the wire reaches to the end of the furnace and the

residency period in the bath is the time span at the nose of the

TTT diagram. At the end of salt bath wire is cleaned by water

jet and coiled.

80

Fig. 47: Patenting heat treatment superimposed on TTT diagram of

plain carbon hypoeutectoid steel

MF

M50

MS


Martensite

Tem

per

atu

re

Log time

Ae1

+CP

+P

FP

UB

LB

50% FP + 50% UB

=austenite

=ferrite

CP=coarse pearlite

P=pearlite

FP=fine pearlite


period

UB=upper bainite

LB=lower bainite

M=martensite

MS=Martensite start

temperature




temperature

Ae3

t0

Metastable

fine pearlite and upper bainite

81

Prediction methods

TTT diagrams can be predicted based on thermodynamic

calculations.

MAP_STEEL_MUCG83 program [transformation start

curves for reconstructive and displacive transformations for

low alloy steels, Bhadeshia et al.], was used for the following

TTT curve of Fe-0.4 wt%C-2 wt% Mn alloy (Fig. 48)

Fig. 48: Calculated

transformation start curve

under isothermal

transformation condition

82

The basis of calculating TTT diagram for ferrous sytem

1. Calculation of Ae3 Temperature below which ferrite

formation become thermodynamically possible.

2. Bainite start temperature BS below which bainite

transformation occurs.

3. Martensite start temperature MS below which martensite

transformation occurs

4. A set of C-curves for reconstructive

transformation (allotriomorphic ferrite and pearlite).

A set of C-curves for displacive

transformations (Widmansttten ferrite, bainite)

A set of C-curves for fractional transformation

5. Fraction of martensite as a function of temperature

83

1. Calculation of Ae3 temperature for multicomponent

system. [Method adopted by Bhadeshia et al.](This analysis is based on Kirkaldy and Barganis and is applicable for total alloying

elements of less than 6wt% and Si is less than 1 wt%)

Where Xi=mole fraction of component i, i=activity coefficient of component i, R=universal gas constant, assuming 0 for Fe, 1for C, i=2 to n for Si, Mn, Ni,

Cr, Mo, Cu,V, Nb, Co, W respectively.

for Ae3 temperature, low

temperature phase to be

substituted by and high

temperature phase L to be

substituted by )Where Xo is the mole fraction of iron then

Assume T is the phase boundary temperature at which high

temperature phase L is in equilibrium with low temperature phase .In case of pure iron then T is given by

General procedure of determination of phase boundary

84

The Wagner Taylor expansion for the activity coefficients

are substituted in the above equations.

and 0GL= standard Gibbs free energy of pure high temperature

phase and 0G= Standard Gibbs free energy of pure low

temperature phase

Similarly for carbon ( n=1) or component i

85

The Wagner-Taylor expansions for activity coefficients are

Where =0 (assumed)

are the Wagner interaction parameters i.e. interaction between

solutes are negligible. The substitution of Wagner-Taylor

expansions for activity coefficients gives temperature deviation

T for the phase boundary temperature (due to the addition of

substitutional alloying elements

k=1 to 11 in this case

86

In multicomponent system, the temperature deviations due to

individual alloying additions are additive as long as solute

solute interactions are negligible. Kirkaldy and co-workers

found that this interaction are negiligible as long as total

alloying additions are less than 6wt% and Si is less than

1wt%].

Eventually T takes the following form

Where To is the phase boundary

temperature for pure Fe-C system and To

is given by .

87

And where

for which

and

where n=1 or i and Ho and H1 are standard molar

enthalpy changes corresponding to Go and G1.88

If the relevant free energy changes oG and the interaction parameters are known then T can be calculated for any alloy.

Since all the thermodynamic functions used are dependent on

temperature, T cannot be obtained from single application

of all values (used from various sources) but must be deduced

iteratively. Initially T can be set as To, T is calculated. Then

T=T+ T is used for T and T is found. Iteration can be

repeated for a few times (typically five times) about till T

changes by less than 0.1K.

This method obtains Ae3 temperature with accuracy of 10K.

89

2. Bainite start temperature BS from Steven and Haynes formula

BS( C)=830-270(%C)-90(%Mn)-37(%Ni)-70(%Cr)-83(%Mo)

( % element by wt)

Both bainite and Widmansttten ferrite nucleate by same

mechanism. The nucleus develops into Widmansttten ferrite if

at the transformation temperature the driving force available

cannot sustain diffusionless transformation. By contrast bainite

form from the same nucleus if the transformation can occur

without diffusion. Therefore in principle BS=WS.

Bainite transformation does not reach completion if austenite

enriches with carbon. But in many steels carbide precipitation

from austenite eliminates the enrichment and allow the

austenite to transform completely. In those cases bainite finish

temperature is given (according to Leslie) by

90

3. MS Temperature:

At the MS temperature

91

In the above equation, T refers to MS temperature in absolute

scale, R is universal gas constant, x=mole fraction of carbon,Yi is

the atom fraction of the ith substitutional alloying element, Tmagiand TNMi are the displacement in temperature at which the free

energy change accompanying the transformation in pure

iron (i.e. FFe) is evaluated in order to allow for the changes

(per at%) due to alloying effects on the magnetic and non-

magnetic components of FFe , respectively. These values were

taken from Aaronson, Zenner. FFe value was from Kaufmann.

92

The other parameters are as follows

(i) the partial molar heat of solution of carbon in ferrite,

H=111918 J mol-1 (from Lobo) and

H=35129+169105x J mol-1 (from Lobo)

ii) the excess partial molar non-configurational entropy of

solution of carbon in ferrite S=51.44 J mol-1K-1 (from Lobo)

S=7.639+120.4x J mol-1K-1 (from Lobo)

=the C-C interaction energy in ferrite=48570 J mol-1(average

value) (from Bhadeshia)

=the corresponding C-C interaction energy in austenite values

were derived , as a function of the concentrations of various

alloying elements, using the procedure of Shiflet and Kingman

and optimised activity data of Uhrenius. These results were

plotted as a function of mole fraction of alloying elements and

average interaction parameter was calculated following

Kinsman and Aaronson. 93

f*=Zener ordering term was evaluated by Fisher.

The remaining term, FFe=free energy change from

austenite to martensite as only function of carbon content.

and is identical for Fe-C and Fe-C-Y steels as structure for

both cases are identical (Calculated by Bhadeshia )=-900 to -

1400 J mol-1 (for C 0.01 to 0.06 mole fraction, changes are not

monotonic). However Lacher-Fowler-Guggenheim

extrapolation gives better result of -1100 to -1400 J mol-1

(Carbon mole fraction 0.01-0.06).

The equation was solved iteratatively until the both sides of

the equation balanced with a residual error of

4. Transformation start and finish C curves

The incubation period () can be calculated from the following

equation [Bhadeshia et al.]

Where T is the isothermal transformation temperature in absolute

scale, R is universal gas constant, Gmax is the maximum free energy

change available for nucleation, Q is activation enthalpy for

diffusion, C,p, z=20 are empirical constant obtained by fitting

experimental data of T, Gmax, for each type of transformation

(ferrite start, ferrite finish, bainite start and bainite finish) . By

systematically varying p and plotting ln( Gpmax/ Tz=20) against 1/RT

for each type of transformation (reconstructive and displacive) till

the linear regression coefficient R1 attains an optimum value. Once p

has been determined Q and C follow from respectively the slope

and intercept of the of plot. The same equation can be used to

predict transformation time.95

Table-I: Chemical compositions, in wt% of the steel

chosen to test the model

96

The optimum values of p and corresponding values of C, Q for

different types of steels (Table-I) where concentrations are in wt% are

summarised below [Bhadeshia et.al.] (Table-II).

Table-II: The optimum values of fitting constants

FS=ferrite start, FF=ferrite finish, BS=bainite start and BF=bainite

finish

97

Based on Q, C and Gmax value it can be predicted that Mo

strongly retard the formation of ferrite through its large

influence on Q. however it can promote bainite via the small

negative coefficient that it has for the Q of the bainitic C-

curve. Cr retard both both bainite and ferrite but net effect is

to promote the formation of bainite since the influence on the

bainitic C-curve is relatively small. Ni has a slight retarding

effect on tranformation rate. Mn has also retarding effect on

ferrite as as well as bainitic transformation

The bainite finish C-curve of the experimental TTT diagram

not only shifts to longer time but also but is also shifts to

lower temperature by about 120C. Therefore this is taken

care by plotting against in

order to determine p, Q and C for the bainitic finish curve.

98

Fractional transformation curves

Fractional transformation time can be estimated from the

following Johnson-Mehl-Avrami equation.

X=transformation volume fraction, K1 is rate constant which is

a function of temperature and austenite grain size d, n and m

are empirical constants. By selecting steels of similar grain

size, the austenite grain size can be neglected then the above

equation simplifies to

99

Assuming x=0.01 for transformation start and x=0.99 for

transformation finish. For a given temperature transformation

start time and finish time can be calculated then K1 and n can

be solved for each transformation product an a function of

temperature.

Then fractional transformation curves for arbitary values of x

can therefore be determined using

100

Representation of intermediate state of transformation

between 0% and 100% can be derived by fitting to the

experimental TTT diagram as follows:

Where x refers to the fraction of transformation.

In most of TTT diagrams of Russell has a plateau at its

highest temperature. Therefore a horizontal line can be drawn

at BS and joining it to a C-curve calculated for temperatures

below BS.

101

Relation between observed and predicted values for ferrite start

(FS), ferrite finish (FF), bainite start (BS) and bainite finish

(BF) are shown in Fig. 49. Predicted value closely matches the

observed values for selected low alloy steels. Predicted TTT

diagrams are projected on experimental diagrams (Figs. 50-52).

The model correctly predicts bainite bay region in low alloy as

well as in selected high alloy steels. The model reasonably

predicts the fractional C curves (Fig. 52). Mo strongly retard the

formation of ferrite through its large influence on Q. however it

can promote bainite via the small negative coefficient that it

has for the Q of the bainitic C-curve. Cr retard both both bainite

and ferrite but net effect is to promote the formation of bainite

since the influence on the bainitic C-curve is relatively small. Ni

has a slight retarding effect on transformation rate. Mn has also

retarding effect on ferrite as as well as bainitic transformation

The model is impirical in nature but it can nevertheless be

useful in procedure for the calculation of microstructure in steel.102

Fig. 49: Relation between observed and predicted Q(Jmol-1)

value for: (a) FS-ferrite start, (b) FF-ferrite finish, (c) BS-bainite

start and (d) BF-bainite finish curves. 103

Fig. 50: Comparison of experimental and predicted TTT diagram

for BS steel:(a) En14, (b) En 16, (c) En 18 and (d) En 110. 104

Fig. 51: Comparison of experimental and predicted TTT diagram for

US steel:(a) US 4140, (b) US 4150, (c) US 4340 and (d) US 5150

105

Fig. 52: Comparison of the experimental and predicted TTT

diagrams including fractional transformation curves at 0.1, 0.5

and 0.9 transform fractions: (a)En 19 and (b) En24. 106

Limitations of model

The model tends to overestimate the transformation time at

temperature just below Ae3. This is because the driving force

term Gmax is calculated on the basis of paraequilibrium and

becomes zero at some temperature less than Ae3 temperature.

The coefficients utilized in the calculations were derived by

fitting to experimental data, so that the model may not be

suitable for extrapolation outside of that data set. Thus the

calculation should be limited to the following concentration

ranges (in wt%):C 0.15-0.6, Si 0.15-0.35, Mn 0.5-2.0, Ni 0-

2.0, Mo 0-0.8 Cr 0-1.7.

107

5. Fraction of martensite as a function of temperature

Volume fraction of martensite formed at temperature T =f and

f=1-exp[BVpdGv)/dT(MS-T)]

Where, B=constant, Vp=volume of nucleus, Gv=driving force

for nucleation, MS =martensite start temperature. Putting the

measured values

the above equation becomes

f=1-exp[-0.011(MS-T)] [Koistinen and Marburger equation].

The above equation can be used to calculate the fraction of

martensite at various temperature.

108

Continuous Cooling Transformation (CCT)

Diagrams

R. Manna

Assistant Professor



Institute of Technology, Banaras Hindu UniversityVaranasi-221 005, India

[email protected]



University of Cambridge, Pembroke Street, Cambridge, CB2 [email protected]

Continuous cooling transformation (CCT) diagram

There are two types of CCT diagrams

I) Plot of (for each type of transformation) transformation start,

specific fraction of transformation and transformation finish

temperature against transformation time on each cooling curve

II) Plot of (for each type of transformation) transformation start,

specific fraction of transformation and transformation finish

temperature against cooling rate or bar diameter for each type of

cooling medium 2

Definition: Stability of phases during continuous

cooling of austenite

Determination of CCT diagram type I

CCT diagrams are determined by measuring some physical

properties during continuous cooling. Normally these are

specific volume and magnetic permeability. However, the

majority of the work has been done through specific volume

change by dilatometric method. This method is supplemented

by metallography and hardness measurement.

In dilatometry the test sample (Fig. 1) is austenitised in a

specially designed furnace (Fig. 2) and then controlled cooled.

Sample dilation is measured by dial gauge/sensor. Slowest

cooling is controlled by furnace cooling but higher cooling rate

can be controlled by gas quenching.

3

Fig. 1: Sample and fixtures for

dilatometric measurements

Fig. 2 : Dilatometer equipment

4

Cooling data are plotted as temperature versus time (Fig. 3).

Dilation is recorded against temperature (Fig. 4). Any slope

change indicates phase transformation. Fraction of

transformation roughly can be calculated based on the dilation

data as explained below.

III

III

IV

V

Tem

per

atu

re

Time

Dil

ati

on

Temperature

a

c

b

d

For a cooling

schedule

TSTF

Fig. 3: Schematic cooling curvesFig. 4: Dilation-temperature plot

for a cooling curve

X

Y

Z

T

5

In Fig. 3 curves I to V indicate cooling curves at higher

cooling rate to lower cooling rate respectively. Fig. 4 gives the

dilation at different temperatures for a given cooling

rate/schedule. In general slope of dilation curve remains

unchanged while amount of phase or the relative amount of

phases in a phase mixture does not change during cooling (or

heating) however sample shrink or expand i.e. dilation takes

place purely due to thermal specific volume change because of

change in temperature. Therefore in Fig. 4 dilation from a to b

is due to specific volume change of high temperature phase

austenite. But at TS slope of the curve changes. Therefore

transformation starts at TS. Again slope of the curve from c to

d is constant but is different from the slope of the curve from a

to b. This indicates there is no phase transformation between

the temperature from c to d but the phase/phase mixture is

different from the phase at a to b.

6

Slope of the dilation curve from b to c is variable with

temperature. This indicates the change in relative amount of phase

due to cooling. The expansion is due to the formation of low

density phase(s). Some part of dilation is compensated by purely

thermal change due to cooling. Therefore dilation curve takes

complex shape. i.e first slope reduces and reaches to a minimum

value and then increases to the characteristic value of the phase

mixture at c.

Therefore phase transformation start at b i.e. at temperature TSand transformation ends or finishes at c or temperature TF. The

nature of transformation has to be determined by metallography.

When austenite fully transforms to a single product then amount

of transformation is directly proportional to the relative change in

length. For a mixture of products the percentage of austenite

transformed may not be strictly proportional to change in length,

however, it is reasonable and generally is being used.

7

Cumulative percentage of transformation at in between

temperature T is equal to YZ/XZ*100 where X, Y and Z are

intersection point of temperature T line to extended constant

slope curve of austenite (ba), transformation curve (bc) and

extended constant slope curve of low temperature phase (cd)

respectively.

So at each cooling rate transformation start and finish

temperature and transformation temperature for specific

amount (10 %, 20%, 30% etc.) can also be determined. For

every type of transformation, locus of start points,

isopercentage points and finish points give the transformation

start line, isopercentage lines and finish line respectively and

that result CCT diagram. Normally at the end of each cooling

curve hardness value of resultant product at room temperature

and type of phases obtained are shown.

8

Fig. 5 shows the five different cooling curves a to e employedto a hypoeutectoid steel. Fig. 5(a) to (e) show the type of

corresponding dilatometric plots drawn against dilation versus

temperature. Fig. 6 shows the corresponding transformation

temperature and time in a temperature versus log time plot

against each corresponding cooling rate. At the end of each

cooling rate curve normally hardness value and type of phases

obtained at room temperature are shown. Symbols F, P, B, M

stand for ferrite, pearlite, bainite and martensite respectively.

Subscripts S and F stand for reaction start and reaction

finish respectively. In cooling a schedule martensite starts at

MS and finishes at MF and therefore 100% martensite results.

While in cooling schedule b bainite starts at BS but reaction

does not complete and retained austenite enriched in carbon

transforms at lower MS but completes at lower MF. Cooling

schedule b results bainite and martensite.

9

a b c de

Tem

pera

ture

Dila

tion

temperature

Time

Temperature

Temperature

Temperature

Temperature

dila

tion

dila

tion

dila

tion

Dila

tion

Ae3

Ae1

a bc d

e

MS

MF

BS

BF

FS

PS

MS

MF

BSMS

MF

FS

FF

BSMS

MF

FS

FF

BS

BF

FS

PSPF

Tem

pera

ture

Log timeHV

HVHVHVHVM M+B

F+B F+BF+P

Fig. 5: Schematic dilatometric plots for five different cooling rates where F,

P, B and M stands for ferrite, pearlite, bainite and martensite respectively

and subscript S and F stands for transformation start and transformation

finish for respective products for a hypoeutectoid steel

Fig. 6: Schematic CCT diagram constructed

from data of Fig 3(for the hypoeutectoid

steel). Dotted line is 25% of total

transformation.

a

b

c

d

e

10

In cooling schedule c ferrite starts at FS and finishes at FF.

Quantity of ferrite is about 15% but rest of austenite enriched

in carbon transforms to bainite at BS and just finishes at BF.

Therefore cooling c results ferrite and bainite at room

temperature. Similarly cooling schedule d results increased

ferrite and rest bainite. During cooling schedule e ferrite start

at FS and pearlite starts at PS but pearlite reaction finishes at PF.

Therefore cooling schedule e results increased ferrite and rest

pearlite. The locus of all start points and finish points result the

CCT diagram. This diagram is not a unique diagram like TTT

diagram for a material. It depends on type of cooling. This

diagram can predict phase transformation information if

similar cooling curves had been used during its determination

or if equivalent cooling schedule are used during process of

production.

11

The two cooling curves are considered equivalent if

(i) the times to cool from Ae3 to 500C are same.

(ii) the times to cool from Ae3 to a temperature halfway

between Ae3 and room temperature , are same.

(iii) the cooling rates are same.

(iv) the instant cooling rates at 700C are same.

Therefore to make it useful different types of CCT diagrams

need to be made following any one of the above schedule that

matches with heat treatment cooling schedule.

12

End-quench test method for type I CCT diagram

A number of Jominy end quench samples are first end- quenched

(Fig.7) for a series of different times and then each of them (whole

sample) is quenched by complete immersion in water to freeze the

already transformed structures. Cooling curves are generated putting

thermocouple at different locations and recording temperature

against cooling time during end quenching. Microstructures at the

point where cooling curves are known, are subsequently examined

and measured by quantitative metallography. Hardness

measurement is done at each investigated point. Based on

metallographic information on investigated point the transformation

start and finish temperature and time are determined. The

transformation temperature and time are also determined for specific

amount of transformation. These are located on cooling curves

plotted in a temperature versus time diagram. The locus of

transformation start, finish or specific percentage of transformation

generate CCT diagram (Fig. 8). 13

1(29 mm)

diameter

12(26.2 mm)

1(25.4 mm)

diameter

(3.2 mm)

(12.7 mm)

4(102 mm)

long

2(64 mm)

Free height

of water jet (12.7 mm)

(12.7 mm) diameter

Fig 7(a): Jominy sample with fixture and water jet

Water

umbrellaNozzle

14

Fig.7: Figures show (b) experimental set up, (c ) furnace for

austenitisation, (d) end quenching process. Courtesy of

DOITPoMS of Cambridge University.

dc

b

15

MF, Martensite finish temperature

M50,50% Martensite

MS, Martensite start temperature

Metastable austenite +martensite

Martensite

Ha

rdn

ess,

HR

C

Tem

per

atu

re

Log time

Ae1

to=Minimum

incubation period at

the nose of the TTT

diagram, to=minimum incubation

period at the nose of the

CCT diagram

t0

A

F ED

C

B

Distance from quench end

AB

CDEF

Jominy

sample

Martensite Pearlite+MartensiteFine pearlitepearliteCoarse

pearlite

a

bc

d

Fig. 8: CCT

diagram ( )

projected on

TTT diagram

( ) of eutectoid

steel

t0

16

Fig. 7. shows the Jominy test set up and Fig. 6 shows a schematic

CCT diagram. CCT diagram is projected on corresponding TTT

diagram.

A, B, C, D, E, F are six different locations on the Jominy sample

shown at Fig.8 that gives six different cooling rates. The cooling

rates A, B, C, D, E, F are in increasing order. The corresponding

cooling curves are shown on the temperature log time plot. At the

end of the cooling curve phases are shown at room temperature.

Variation in hardness with distance from Jominy end is also

shown in the diagram.

For cooling curve B, at T1 temperature minimum t1 timing isrequired to nucleate pearlite as per TTT diagram in Fig. 8. Butmaterial has spent t1 timing at higher than T1 temperature incase of continuous cooling and incubation period at highertemperature is much more than t1. The nucleation conditionunder continuous cooling can be explained by the concept ofprogressive nucleation theory of Scheil.

17

Scheils concept of fractional nucleation/progressive

nucleation

Scheil presented a method for calculating the transformation

temperature at which transformation begins during continuous

cooling. The method considers that (1) continuous cooling occurs

through a series of isothermal steps and the time spent at each of

these steps depends on the rate of cooling. The difference between

successive isothermal steps can be considered to approach zero.

(2) The transformation at a temperature is not independent to cooling

above it.

(3) Incubation for the transformation occurs progressively as the

steel cools and at each isothermal step the incubation of

transformation can be expressed as the ratio of cooling time for the

temperature interval to the incubation period given by TTT diagram.

This ratio is called the fractional nucleation time.

18

Scheil and others suggested that the fractional nucleation time are

additive and that transformation begins when the sum of such

fractional nucleation time attains the value of unity.

The criteria for transformation can be expressed

t1/Z1+t2/Z2+t3/Z3+.+tn/Zn=1

Where tn is the time of isothermal hold at Temperature Tn whereincubation period is Zn. This is called additive reaction rule of

Scheil (1935). The reactions for which the additive rule is justifiied

are called isokinetic, implying that the fraction transform at any

temperature depends only on time and a single function of

temperature. This is experimentally verified by Krainer for

pearlitic transformation.

19

Therefore though nucleation has progressed to some fraction of the

event but time is not sufficient for pearlite nucleation at a. If time is

allowed in continuous cooling while summation of fractional

nucleation time becomes unity (at b), pearlite is to nucleate but by

that time temperature drops down as it is continuously cooling.

This concept of progressive nucleation is not strictly valid for

bainite transformation where austenite get enriched with carbon at

higher temperature. As transformation at higher temperature

enriches the austenite by carbon, the transformation characteristic

changes. i.e. transformation slows down at lower temperature.

By continuous cooling transformation temperature moves towards

down and incubation moves toward right. Similar is the case for

pearlite finish temperature and time. Pearlitic region takes the

shape as shown in the diagram. The bainitic region moves so right

that entire region is sheltered by the pearlitic curve.

20

So there is no chance of bainitic tranformation in eutectoid

plain carbon steel under continuous cooling condition. There is

untransformed region where earlier was bainitic region. Under

such circumtances split transformation occurs. However

martensitic region remain unaffected.

Various cooling rates give various combination of phases.

Cooling A indicates very slow cooling rate equivalent to

furnace cooling of full annealing process and that results

coarse pearlite. Cooling B is faster cooling can be obtained

by air cooling. This type of cooling can be obtained by

normalising and that results finer pearlite. Cooling C: just

touches the finishing end of nose that gives fully fine pearlite.

Cooling D is faster cooling that can be obtained by oil

quenching. This is a hardening heat treatment process and that

produces fine pearlite and untransformed austenite transforms

to martensite below MS.21

Cooling curve E just touches the nose of CCT diagram and that

produces almost fully martensite.

Cooling curve F avoid nose of C curve in CCT but touches the

nose of TTT gives entirely martensite. Notice the critical cooling

rate to avoid nose of CCT diagram i.e. diffusional

transformations is lower than that to TTT diagram.

22

General features of CCT diagrams

1. CCT diagram depends on composition of steel, nature of cooling, austenite grain size, extent of austenite homogenising, as well as

austenitising temperature and time.

2. Similar to TTT diagrams there are different regions for different

transformation (i.e. cementite/ferrite, pearlite, bainite and

martensite). There are transformation start and transformation finish

line and isopercentage lines. However depending on factors

mentioned earlier some of the transformation may be absent or some

transformation may be incomplete.

3. In general for ferrite, pearlite and bainite transformation start and

finish temperature moves towards lower temperature and

transformation time towards higher timing in comparison to

isothermal transformation. Transformation curve moves down and

right. 23

4. The bainite reaction can be sufficiently retarded such that

transformation takes shelter completely under pearlitic transformation

in case of eutectoid plain carbon steel and therefore bainite region

vanishes. However in other steel it may be partially sheltered.

Therefore bainitic region observed in non eutectoid plain carbon steel

or alloy steels.

5. C curves nose move to lower temperature and longer time. So actual

critical cooling rate required to avoid diffusional transformation

during continuous cooling is less than as prescribed by TTT diagram.

Actual hardenability is higher than that predicted by TTT.

6. MS temperature is unaffected by the conventional cooling

rate,however, it can be lowered at lower cooling rate if cooling curves

such that austenite enriches with carbon due to bainite or ferrite

formation (in hypoeutectoid steel). On the other hand MS can go up

for lower cooling rate such that austenite become lean in carbon due

to carbide separation (in hypereutectiod steel). 24

7. Large variety of microstructure like ferrite/cementite/carbide

+pearlite+bainite+martensite can be obtained in suitable cooling

rate. It is not feasible or limited in case of isothermal

transformation.

25

Determination of type II CCT diagram

This procedure was developed by Atkins. In this process round

samples of different diameters were quenched in three different

media air, oil and water. The cooling curves were recorded at the

centre of each bar. Later these cooling curves were simulated in

dilatometer test in order to identify the transformation

temperature, microstructure and hardness. The transformation

information is plotted against temperature and bar diameter

cooled in specific medium. These are bar diameter cooled in air,

quenched in oil and quenched in water. A scale cooling rate

(usually at 700C) in C/min is added.

At the bottom of the same diagram another plot is added for

hardness (in HRC) and with same cooling rate axis/bardiameter.

These diagrams have to be read along vertical lines (from top to

bottom), denoting different cooling rates. Fig. 9 shows a

schematic CCT diagram for

Heat Treatment

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