4. CONT’D
• Ferrite is known as α solid solution.
• It is an interstitial solid solution of a small amount of carbon dissolved in α (BCC)
iron.
• stable form of iron below 912°C
• The maximum solubility is 0.025 % C at 723C and it dissolves only 0.008 % C at
room temperature.
• It is the softest structure that appears on the diagram.
• Pearlite is the eutectoid mixture containing 0.80 % C and is formed at 723°C on
very slow cooling.
• It is a very fine plate like or lamellar mixture of ferrite and cementite.
• The white ferritic background or matrix contains thin plates of cementite (dark).
5. CONT’D
• Austenite is an interstitial solid solution of Carbon dissolved in (F.C.C.) iron.
• Maximum solubility is 2.0 % C at 1147°C.
• High formability, most of heat treatments begin with this single phase.
• Cementite or iron carbide, is very hard, brittle intermetallic compound of iron &
carbon, as Fe3C, contains 6.67 % C.
• It is the hardest structure that appears on the diagram.
• Its crystal structure is orthorhombic
• Ledeburite is the eutectic mixture of austenite and cementite.
• It contains 4.3 percent C and is formed at 1147°C.
6. Various Features of Fe-C diagram
Peritectic L + d =
Eutectic L = + Fe3C
Eutectoid = a + Fe3C
Phases present
L
Reactions
d
BCC structure
Paramagnetic
austenite
FCC structure
Non-magnetic
ductile
a ferrite
BCC structure
Ferromagnetic
Fairly ductile
Fe3C cementite
Orthorhombic
Hard
brittle
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%
7.
8. HEAT TREATMENT
It is the heating and cooling of metals to change their physical and mechanical
properties, without letting it change its shape.
Heat Treatment (time and temperature)
Microstructure Mechanical Properties
Four factors which are very effective for heat treatments:
Heat Treatment Process variables :
Temperature
Holding time
Heating rate
Cooling rate
9. 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,
Salt bath technique combined with metallography and hardness measurements is
the most popular and accurate method to determine TTT diagram.
In bath I number of samples are austenitised at AC1+20-40°C for eutectoid steel
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.
10. CONT’D
Fig. 1 : Salt bath I -Austenitisation heat treatment.
Fig. 2 : Bath II low-temperature salt-bath for isothermal
treatment
11. 11
(kinetics time dependence), y=1- exp(-ktn) [Avrami equation]
Transformations do not occur instantaneously
Three categories
Phase transformations: Kinetics
Diffusion-dependent with no change in composition or number of phases present
(melting/solidification of pure metal, allotropic transformations, recrystallization)
Diffusion-dependent but changes in composition or number of phase ( eutectoid
transformations)
Diffusionless metastable phase by small displacements of atoms in structure
(martensitic transformation discussed later)
12. 12
Phase transformation involves:
Kinetics of phase transformations
Nucleation - formation of small particles (nuclei) of the new phase. Often formed at grain
boundaries.
Growth of new phase at the expense of the original phase.
S-shape curve: percent of material
transformed vs. the logarithm of time.
16. Continuous Cooling Transformation (CCT) Curves Eutectoid steel (0.8%C)
Austenite
Martensite
100
200
300
400
600
500
800
723
0.1 1 10 102 103 104
105
Eutectoid temperature
Ms
Mf
t (s) →
T→
Original TTT lines
Cooling curves
Constant rate
Pearlite
1T 2T
17. Liquid → Solid phase transformation
Solid (GS)
Liquid (GL)
Tm T →
G→
T
G
Liquid stableSolid stable
T - Undercooling
↑ t
“For sufficient
Undercooling”
On cooling just below Tm solid becomes stable
But solidification does not start
E.g. liquid Ni can be undercooled 250 K below Tm
G → ve
G → +ve
19. Full Annealing
The purpose of this heat treatment is to obtain a material with high ductility. A microstructure
with coarse pearlite (i.e. pearlite having high interlamellar spacing) is endowed with such
properties.
The range of temperatures used is given in the figure below.
The steel is heated above A3 (for hypo-eutectoid steels) & A1 (for hyper-eutectoid steels) → (hold) → then the
steel is furnace cooled to obtain Coarse Pearlite.
Coarse Pearlite has low (↓) Hardness but high (↑) Ductility.
For hyper-eutectoid steels the heating is not done above Acm to avoid a continuous network of
proeutectoid cementite along prior Austenite grain boundaries (presence of cementite along grain boundaries
provides easy path for crack propagation).
A1
A3
Acm
T
Wt% C
0.8 %
723C
910C
Full Annealing
Full Annealing
20. Recrystallization Annealing
Heat below A1 → Sufficient time → Recrystallization
A1
A3
Acm
T
Wt% C
0.8 %
723C
910C
Recrystallization Annealing
During any cold working operation (say cold rolling), the material becomes harder (due to
work hardening), but loses its ductility. This implies that to continue deformation the material
needs to be recrystallized (wherein strain free grains replace the ‘cold worked grains’).
Hence, recrystallization annealing is used as an intermediate step in (cold) deformation
processing.
To achieve this the sample is heated below A1 and held there for sufficient time for
recrystallization to be completed.
21. Stress Relief Annealing
A1
T
Wt% C
0.8 %
723C
910C
Stress Relief Annealing
Due to various processes like machining, welding, etc. the residual stresses develop in the
sample. Residual stress can lead to undesirable effects like warpage of the component.
The annealing is carried out just below A1 ,
Spheroidization Annealing
This is a very specific heat treatment given to high carbon steel requiring extensive
machining prior to final hardening & tempering. The main purpose of the treatment
is to increase the ductility of the sample.
Like stress relief annealing the treatment is done just below A1.
Long time heating leads cementite plates to form cementite spheroids. The driving
force for this (microstructural) transformation is the reduction in interfacial energy.
22. NORMALIZING
Refine grain structure prior to hardening
To harden the steel slightly
To reduce segregation in casting or forgings
Purposes
The sample is heat above A3 | Acm to complete Austenization. The sample is then air cooled to
obtain Fine pearlite. Fine pearlite has a reasonably good hardness and ductility.
In hypo-eutectoid steels normalizing is done 50C above the annealing temperature.
In hyper-eutectoid steels normalizing done above Acm → due to faster cooling cementite does
not form a continuous film along GB.
The list of uses of normalizing are listed below.
A1
A3
Acm
T
Wt% C
0.8 %
723C
910C
Normalization
Normalization
23. HARDENING
The sample is heated above A3 | Acm to cause Austenization. The sample is then
quenched at a cooling rate higher than the critical cooling rate (i.e. to avoid the nose
of the CCT diagram).
The quenching process produces residual strains (thermal, phase transformation).
The transformation to Martensite is usually not complete and the sample will have
some retained Austenite.
The Martensite produced is hard and brittle and tempering operation usually
follows hardening. This gives a good combination of strength and toughness.
TEMPERING
To remove some of the brittleness from hardened steels, tempering is used. The
metal is heated to the range of 220-300 degrees and cooled.
