Transcript 32 TTT AND CCT DIAGRAMS

Incubation
Temp.( 0C)
700
650
600
550
500
450
400
350
-------------
Time at which Transformation
Starts/Begins
Ends
S1
E1
S2
E2
S3
E3
S4
E4
S5
E5
S6
E6
S7
E7
S8
E8
S
E
Sn
En
Sr
Er
STABLE AUSTENITE
Near
A1
Sluggish
transformation
Au. to
Coarse Pearlite
Greater time for
diffusion
550
Bianite in
feather
shaped
patches
550-220
Degree of
under cooling
high
Slow rate of
diffusion of C
atoms retards
increased tendency
of Au.
transformation,
1000 sec
1 sec
Temperature
4000C -6000C range
Time (on logarithmic Scale)
Transformation
ends
Transformation
starts/begins
Stable Austenite
Coarse Pearlite
Unstable Austenite
Fine Pearlite
Feathery Bainite
Unstable Austenite
Acicular Bainite
Ms
Austenite + Martensite
Mf
Martensite
Time-Temperature Transformation Curves
STABLE AUSTENITE
Near
A1
Sluggish
transformation
Austenite to
Coarse Pearlite
Greater time for
diffusion
550
Bianite in
feather
550-220
shaped
patches
Degree of
under cooling
high
Slow rate of
diffusion of Carbon
atoms retards
increased tendency
of Austenite
transformation,
Transformation
V/s logarithm
of Time plot
Isothermal
Transformation
diagram plot
Transformation
ends
Transformation
starts/begins
Stable Austenite
Coarse Pearlite
Unstable Austenite
4000C
-6000C
Fine Pearlite
range
Feathery Bainite
Unstable Austenite
Acicular Bainite
Ms ≈ 2500C
M 90% at 1100C
Austenite + Martensite
Mf ≈-500C
Martensite
Time-Temperature Transformation Curves
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
diagramCCT
superimposed
Transformation
starts/begins
Transformation
ends
A- fine Pearlite
Continuous cooling Transformation
EFFECT OF
DIFFERENT
COOLING
RATES
B- M + little Au
A - 50C/sec(normalising)
C
Continuous Cooling
Transformation diagram
D
B - Rapid 4000C/sec (water quench)
C - 1400C/sec- Martensitic
D - 500C/sec- P + M + Au
[2]
Full
Annealing
[1]
Normalising
Moderately
rapid [1] and
slow cooling [2]
curves
superimposed
on continuous
cooling
transformation
diagram for
EUTECTOID
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
COOLING CURVES
LARGE STEEL
PART WHEN
COOLED
BAINITE
SURFACE
CORE
CORE WITH FINE
PEARLITE,
SURFACE
MARTENSITICCALLED
MASS EFFECT OF
HEAT TREATMENT
MARTENSITE
QUENCH CRACKS
MAY FORM, EVEN IF
COOLING RATE
INCREASED
Quenched in bath of
temperature E, kept for
long, removed and cooled
– martensite forms
E
Air Cool
Transformation Complete
MARTEMPERING
Also called
MARQUENCHING
Transformation Complete at G
G
Air Cool
AUSTEMPERING
FINE 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