Incubation Temp.( 0 C) Time at which Transformation Starts/Begins Ends 700S1E1 650S2E2 600S3E3 550S4E4 500S5E5 450S6E6 400S7E7 350S8E8 -----SE SnEn -----SrEr.

Incubation Temp.( 0C)

Time at which Transformation

Starts/Begins Ends

700 S1 E1

650 S2 E2

600 S3 E3

550 S4 E4

500 S5 E5

450 S6 E6

400 S7 E7

350 S8 E8

----- S E

----- Sn En

----- Sr Er

STABLE AUSTENITE

Bianite in

feather shaped patches

Degree of under cooling high

Sluggish transformation

Au. to

Coarse Pearlite

Greater time for diffusion Slow rate of

diffusion of C atoms retards

increased tendency of Au.

transformation,

550

550-220

Near A1

Te

mp

era

ture

Time (on logarithmic Scale)

4000C -6000C range

1 s

ec

100

0 s

ec

Stable Austenite

Unstable Austenite

Transformation starts/begins

Transformation ends

Coarse Pearlite

Fine Pearlite

Unstable Austenite

Feathery Bainite

Acicular Bainite

Ms

Mf

Austenite + Martensite

Martensite Time-Temperature Transformation Curves

STABLE AUSTENITE

Bianite in feather shaped patches

Degree of under cooling high

Sluggish transformation

Austenite to

Coarse Pearlite

Greater time for diffusion Slow rate of

diffusion of Carbon atoms retards

increased tendency of Austenite

transformation,

550

550-220

Near A1

Transformation V/s logarithm of Time plot

Isothermal Transformation diagram plot

Stable Austenite

Unstable Austenite

Transformation starts/begins

Transformation ends

Coarse Pearlite

Fine Pearlite

Unstable Austenite

Feathery Bainite

Acicular Bainite

Ms ≈ 2500C

Mf ≈-500C

Austenite + Martensite

Martensite Time-Temperature Transformation Curves

4000C -6000C range

M 90% at 1100C

Ferrite nucleates first, over which layer of Cementite (Iron Carbide)- Growth is quick.

Complete Isothermal Transformation

diagram for

EUTECTOID composition

A- Austenite

B- Bainte

M- Martensite

P- Pearlite

Isothermal Transformation Diagram for

alloy steel (with 0.40%C, Ni, Cr & Mo small %)

A- Austenite

B- Bainite

P- Peralite

M- Martensite

F- proeutectoid ferrite

Complete Isothermal Transformation

diagram for

EUTECTOID composition

With

Continuous Cooling Transformation

diagram-

CCT

superimposed

A- fine Pearlite

B- M + little AuC D

Transformation starts/begins

Transformation ends

A - 50C/sec(normalising)

B - Rapid 4000C/sec (water quench)

C - 1400C/sec- Martensitic

D - 500C/sec- P + M + Au

EFFECT OF DIFFERENT COOLING

RATES

Continuous Cooling Transformation diagram

Continuous cooling Transformation

Moderately rapid [1] and slow cooling [2] curves superimposed on continuous cooling transformation diagram for EUTECTOID[1]

Normalising

[2]

Full Annealing

CCT

Dependence of final microstructure on transformations occurring during cooling

Martensite

for quenching rates greater than

the critical

Quenching Medium

Media, arranged In order of quenching speed:

– 5% Caustic Soda - highest– 5-20% Brine– Cold Water– Warm Water– Mineral Oil– Animal Oil– Vegetable Oil - least

LARGE STEEL PART WHEN

COOLED

CORESURFACE

MARTENSITE

COOLING CURVES

BAINITE

CORE WITH FINE PEARLITE, SURFACE

MARTENSITIC-

CALLED

MASS EFFECT OF HEAT TREATMENT

QUENCH CRACKS MAY FORM, EVEN IF COOLING RATE INCREASED

MARTEMPERING

Transformation Complete

Air Cool

E

Quenched in bath of temperature E, kept for long, removed and cooled – martensite forms

Also called MARQUENCHING

AUSTEMPERING

Transformation Complete at G

Air Cool

GFINE BAINITE FORMS, AIR COOLED STRUCTURE AS OF TEMPERED MARTENSITE

GOOD FOR INTRICATE SHAPES. BY REPLACING QUENCHING AND TEMPERING, CRACKS AVOIDED

Classification scheme for various ferrous alloys