UNIT-II

HEAT TREATMENT

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 Heat Treatment of Steels

Heat Treating – defined as the controlled heating and cooling of

metals for the primary purpose of altering their properties
(strength, ductility, hardness, toughness, machinability, etc)
Can be done for Strengthening Purposes (converting structure
to martensite)
Can be done for Softening and Conditioning Purposes
(annealing, tempering, etc.)

First, a basic review of metallurgy!

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 Heat Treatment
• Three reasons for heat treatment

– To soften before shaping

– To relieve the effects of strain hardening

– To acquire the desired strength and toughness in

the finished product.

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 Heat Treatment
• Principal heat treatments
– Annealing
– Martensite formation in steel
– Precipitation hardening
– Surface hardening

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 Annealing
• Process
– Heat the metal to a temperature
– Hold at that temperature
– Slowly cool
• Purpose
– Reduce hardness and brittleness
– Alter the microstructure for a special property
– Soften the metal for better machinability
– Recrystallize cold worked (strain hardened) metals
– Relieve induced residual stresses

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 Spheroidizing
• Strangely, sometimes we would like the steel to be just as soft
and ductile as absolutely possible.
• Why, do you think?
• Pearlite is not the lowest energy arrangement possible
between ferrite and cementite. If heated to just below the
eutectoid temperature, and left for an extended time, the
pearlite layers break down, and spherical clumps of cementite
are found.
• These spherical clumps are hundreds or even thousands of
times larger that those in TM, and spaced much further apart.
 Softest, most duct.
http://info.lu.farmingdale.edu/depts/met/met
205/ANNEALING.html
 Full Anneal
• The idea is to get the soft metal and relieve stresses. We
contrast this anneal with the “process anneal” associated
with CW.
• In the full anneal, we must fully austenitize the steel. This is
followed by a furnace cool, the slowest cooling rate possible.
• The result is coarse Pearlite mixed with primary phase. This
steel will be close to spheroidite in its softness and ductility.
 Normalizing Steel
• Here we austenitize the steel and then air cool as opposed to
furnace cooling.
• The result is a uniform microstructure, very uniformly spaced
pearlite in equal sized grains throughout.
• It is stronger and somewhat less ductile than full anneal.
• Often done after forging to normalize the grain structure.
 Thermal Processing of
Annealing: Heat to T , then cool slowly.
Metals
anneal

• Stress Relief: Reduce • Spheroidize (steels):

stress caused by: Make very soft steels for
-plastic deformation good machining. Heat just
-nonuniform cooling below TE & hold for
-phase transform. 15-25 h.

• Full Anneal (steels):

Types of Make soft steels for
good forming by heating
Annealing to get , then cool in
furnace to get coarse P.
• Process Anneal:
Negate effect of
• Normalize (steels):
cold working by
Deform steel with large
(recovery/
grains, then normalize
recrystallization)
to make grains small.

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 Heat Treatments
800
Austenite (stable)

T(°C) TE
A
a) Annealing P
b) Quenching 600
c) Tempered
Martensite
B
400 A
10
0 %
50
0% %

0%
200 M+A
50%
M+A
90%
b) a)

-1 3 5 c)
10 10 10 10
time (s) 11
 The Iron Carbon System
• Steels, ferrous alloys, cast irons, cast steels
– Versatile and ductile
– Cheap
• Irons (< 0.008% C)
• Steels (< 2.11% C)
• Cast irons (<6.67% [mostly <4.5%]C)

• The material properties are more than

composition – they are dependent on how the
material has been treated.
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 The
Phase
Diagram

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 Fe - C
• Iron melts at 1538°C
– As it cools, it forms in sequence
• Delta ferrite
• Austenite
• Alpha ferrite
• Alpha ferrite
– Solid solution of BCC iron
– Maximum C solubility of 0.022% at 727°C
– Soft and ductile
– Magnetic up to the Curie temperature of 768°C

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 Fe - C
• Delta ferrite
– exists only at high temperatures and is of little
engineering consequence.
• Note that little carbon can be actually
interstitially dissolved in BCC iron
• Significant amounts of Chromium (Cr),
Manganese (Mn), Nickel (Ni), Molybdenum
(Mb), Tungsten (W), and Silicon (Si) can be
contained in iron in solid solution.
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 Fe - C
• Austenite (gamma  iron)
– Between 1394 and 912°C iron transforms from
the BCC to the FCC crystal structure.
– It can accept carbon in its interstices up to 2.11%
– Denser than ferrite, and the FCC phase is much
more formable at high temperatures.
– Large amounts of Ni and Mn can be dissolved into
this phase
– The phase is non-magnetic.
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 Fe - C
• Cementite
– 100% iron carbide Fe3C
– Very hard
– Very brittle
• Pearlite
– Mixture of ferrite and cementite
– Formed in thin parallel plates
• Bainite
– Alternate mixture of the same phases
– Needle like cementite regions
– Formed by quick cooling
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Martensite formation in Steel
• The diagram at left assumes
slow equilibrium cooling.

• Each phase is allowed to form

• Time is not a variable.

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Martensite formation in Steel
• However
– If cooling is rapid enough
that the equilibrium
reactions do not occur
– Austenite transforms into a
non-equilibrium phase
– Called Martensite.

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 Fe - C
• Martensite
– Hard brittle phase
– Iron carbon solution whose composition is the
same as austenite from which it was derived
– But the FCC structure has been transformed into a
body center tetragonal (BCT)
– The extreme hardness comes from the lattice
strain created by carbon atoms trapped in the BCT

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 The Time – Temperature –
Transformation Curve (TTT)

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 The Time – Temperature –
Transformation Curve (TTT)

• Composition Specific
– Here 0.8% carbon

• At different compositions,
shape is different

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0.8C

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 The Time – Temperature –
Transformation Curve (TTT)
• At slow cooling rates the trajectory
can pass through the Pearlite and
Bainite regions
• Pearlite is formed by slow cooling
– Trajectory passes through Ps
above the nose of the TTT
curve
• Bainite
– Produced by rapid cooling to a
temperature above Ms
– Nose of cooling curve avoided.

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 The Time – Temperature –
Transformation Curve (TTT)
• If cooling is rapid enough
austenite is transformed into
Martensite.
– FCC > BCT
– Time dependent diffusion
separation of ferrite and iron
carbide is not necessary
• Transformation begins at Ms and
ends at Mf.
– If cooling stopped it will
transition into bainite and
Martensite.

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Martensite hardness
– The extreme hardness
comes from the lattice
strain created by carbon
atoms trapped in the BCT

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Tempered Martensite
• Step 1 -- Quench in the
martensitic phase

• Step 2 – soak
– Fine carbide particles
precipitate from the iron –
carbon solution
– Gradually the structure goes
BCT > BCC

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 Quenching Media
• The fluid used for quenching the heated alloy
effects the hardenability.
– Each fluid has its own thermal properties
• Thermal conductivity
• Specific heat
• Heat of vaporization
– These cause rate of cooling differences

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 Other quenching concerns
• Fluid agitation
– Renews the fluid presented to the part
• Surface area to volume ratio
• Vapor blankets
– insulation
• Environmental concerns
– Fumes
– Part corrosion

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 Surface Hardening
• Refers to a “thermo chemical” treatment
whereby the surface is altered by the addition
of carbon, nitrogen, or other elements.
• Sometimes called CASE HARDENING.

• Commonly applied to low carbon steels

– Get a hard wear resistant shell
– Tough inner core

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 Surface Hardening
• The common procedures are:

– Carburizing

– Nitriding

– Carbonnitriding

– Chromizing and boronizing

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 Carburizing
• Heating a low carbon steel in the presence of carbon rich environment at
temperature ~ 900°C
– Carbon diffuses into the surface
– End up with a high carbon steel surface.
• Pack parts in a compartment with coke or charcoal
• Gas carburizing
– Uses propane (C3H8) in a sealed furnace
• Liquid carburizing
– Used NaCN, BaCl2
• Thickness 0.005 in. to 0.030 in.

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 Nitriding
• Nitrogen is diffused in the surface of special
alloy steels at temperatures around ~510°C.
– Steel must contain elements that will form nitride
compounds.
• Aluminum
• Chromium
– Forms a thin hard case without quenching
• Thicknesses 0.001 in – 0.020 in.

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 Chromizing
• Diffuse chromium into the surface 0.001 –
0.002 in.
• Pack the parts in Cr rich powders or dip in a
molten salt bath containing Cr salts.

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 Boronizing
• Performed on tool steels, nickel and cobalt
based alloy steels.
• When used on low carbon steels, corrosion
resistance is improved.

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How do Metal
Crystals Fail??
Answer: Slip due
to dislocations

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How do Metal
Crystals Fail??
Answer: Slip due
to dislocations

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How does crystal Structure FCC, BCC, HCP
effect:
• Strength??
• Ductility/Toughness??
• Stiffness??

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 Types of Defects:
• Surface Defects
– Grain boundaries
• Point Defects
– Vacancy, substitutional (atom replaces host),
interstitial (atom squeezes in between host),
impurity
• Line Defects
– Edge dislocations, screw dislocations

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 Little Alloying and
impact on heat treating
strength

Greatest Course GB = weak,

impact on Fine GB = strong and
strength ductile
and
ductility!!

Defects in crystals. (a) Vacancies–missing atoms. (b) Foreign (solute) atom on interstitial and substitutional sites.
(c) Line Defect = A dislocation–an extra half-plane of atoms. (d) Grain boundaries.

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What is the most significant defect?
Answer: The line defect (edge dislocation or screw dislocation)

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 How to Strengthen Metals:
• Key: prevent dislocations from moving through crystal
structure!!!
• Finer grain boundries – can be done by recrystallizing (and
cold working)
• Increase dislocation density via COLD WORKING (strain
hardening)
• Add alloying elements to give –SOLID SOLUTION HARDENING.
• Add alloying elements to give precipitates or dispersed
particles – PRECIPITATION HARDENING (aka Heat Treat)
• DISPERSION HARDENING– fine particles (carbon) impede
dislocation movement.
– Referred to as Quench Hardening, Austenitizing and Quench or simply
“Heat Treat”.
– Generally 3 steps: heat to austenite T, rapid quench, then temper.

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Several cells form a crystal, if many crystals are growing in a melt at
the same time, where they meet = grain boundry as shown below:

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 The Effect of Grain Boundries:
• Dislocations pile up at GB and can’t go further
– this effectively strengthens the crystal!

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 Work Hardening
Work hardening, or strain hardening, results in an increase in
the strength of a material due to plastic deformation.

Plastic deformation = adding dislocations – as dislocation

density increases, they tend to “tie up” and don’t move.

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Hot finishing = 1
benefit
Cold finishing = 2
benefits 47
This is dispersion and precipitate
strengthening

This is solution hardening

(alloying)

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How to strengthen
metals:

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Steel Crystal Structures:
•Ferrite – BCC iron w/
carbon in solid solution
(soft, ductile, magnetic)
•Austenite – FCC iron
with carbon in solid
solution (soft, moderate
strength, non-magnetic)
•Cementite – Compound
of carbon and iron FE3C
(Hard and brittle)
•Pearlite – alternate
layers of ferrite and
cementite.
•Martensite – iron –
carbon w/ body
centered tetragonal –
result of heat treat and
quench

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HT: ferrite then austentite then martensite
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 Heat Treatment of Steels
• Steel = 0.06% to 1.0% carbon
• Must have a carbon content of at least .6% (ideally) to heat
treat.
• Must heat to austenitic temperature range.
• Must rapid quench to prevent formation of equilibrium
products.
• Basically crystal structure changes from BCC to FCC at high
Temp.
• The FCC can hold more carbon in solution and on rapid
cooling the crystal structure wants to return to its BCC
structure. It cannot due to trapped carbon atoms. The net
result is a distorted crystal structure called body centered
tetragonal called martensite.

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 10.4 Direct Hardening – Austenitizing and
quench:
• Austenitizing – again taking a steel with .6%
carbon or greater and heating to the austenite
region.
• Rapid quench to trap the carbon in the crystal
structure – called martensite (BCT)
• Quench requirements determined from
isothermal transformation diagram (IT
diagram).
• Get “Through” Hardness!!!
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 Austenitizing:

Heat to austenite range.

Want to be close to
transformation
temperature to get fine
grain structure.

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For this particular steel want to cool from about 1400 F to <400 F in about 1
second!

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 Quenching:
• Depending on how fast steel must be quenched
(from IT diagram), the heat treater will determine
type of quenching required:
– Water (most severe)
– Oil
– Molten Salt
– Gas/ Air (least severe)
– Many phases in between!!! Ex: add water/polymer to
water reduces quench time! Adding 10% sodium
hydroxide or salt will have twice the cooling rate!

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 10.4 Direct Hardening - Selective
Hardening :
• Same requirements as austenitizing:
– Must have sufficient carbon levels (>0.4%)
– Heat to austenite region and quench
• Why do?
– When only desire a select region to be hardened: Knives,
gears, etc.
– Object to big to heat in furnace! Large casting w/ wear
surface
• Types:
– Flame hardening, induction hardening, laser beam
hardening

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Flame Hardening:

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Induction Hardening

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Diffusion Hardening ( Case Hardening):
• Why do?
– Carbon content to low to through harden with previous
processes.
– Desire hardness only in select area
– More controlled versus flame hardening and induction
hardening.
– Can get VERY hard local areas (i.e. HRC of 60 or greater)
– Interstitial diffusion when tiny solute atoms diffuce into
spaces of host atoms
– Substitiutional diffusion when diffusion atoms to big to
occupy interstitial sites – then must occupy vacancies

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 Diffusion Hardening:
• Requirements:
– High temp (> 900 F)
– Host metal must have low concentration of the
diffusing species
– Must be atomic suitability between diffusing
species and host metal

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 Diffusion Hardening:
• Most Common Types:
– Carburizing
– Nitriding
– Carbonitriding
– Cyaniding

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 Diffusion Hardening - Carburizing:
• Pack carburizing most common:
– Part surrounded by charcoal treated with
activating chemical – then heated to austenite
temperature.
– Charcoal forms CO2 gas which reacts with excess
carbon in charcoal to form CO.
– CO reacts with low-carbon steel surface to form
atomic carbon
– The atomic carbon diffuses into the surface
– Must then be quenched to get hardness!

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 Diffusion Hardening - Nitriding:
• Nitrogen diffused into surface being treated.
Nitrogen reacts with steel to form very hard
iron and alloy nitrogen compounds.
• Process does not require quenching – big
advantage.
• The case can include a white layer which can
be brittle – disadvantage
• More expensive than carburizing

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Source of nitrogen

Reduction process: 2NH3 2N + 3H2

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 10.6 Softening and Conditioning -
• Recrystallization
• Annealing
– Process anneal
– Stress relief anneal
– Normalizing
• Tempering

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 10.6 Softening and Conditioning -
Recrystallization
• Done often with cold working processes
• Limit to how much steel can be cold worked
before it becomes too brittle.
• This process heats steel up so grains return
to their original size prior to subsequent cold
working processes.
• Also done to refine coarse grains

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 10.6 Softening and Conditioning -
Annealing
• Annealing – primary purpose is to soften the
steel and prepare it for additional processing
such as cold forming or machining.
• If already cold worked - allows
recrystallization.

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 10.6 Softening and Conditioning -
Annealing
• What does it do?
1. Reduce hardness
2. Remove residual stress (stress relief)
3. Improve toughness
4. Restore ductility
5. Refine grain size

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 10.6 Softening and Conditioning -
Annealing
• Process Steps:
1. Heat material into the asutenite region (i.e.
above 1600F) – rule of thumb: hold steel for one
hour for each one inch of thickness
2. Slowly furnace cool the steel – DO NOT QUENCH
3. Key slow cooling allows the C to precipitate out
so resulting structure is coarse pearlite with
excess ferrite
4. After annealing steel is quite soft and ductile

77
 Annealing versus Austenitizing:
• End result: One softens and the other hardens!
• Both involve heating steel to austenite region.
• Only difference is cooling time:
– If fast (quenched) C is looked into the structure =
martensite (BCT) = HARD
– If slow C precipates out leading to coarse pearlite (with
excess cementite of ferrite) = SOFT

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 10.6 Softening and Conditioning – Other
forms of Annealing
• Normalizing – use when max softness not
required and cost savings desired (faster
than anneal). Air cooled vs. furnace cooled.
• Process Anneal – not heated as high as full
anneal.
• Stress Relief Anneal – lower temp (1,000F),
slow cooled. Large castings, weldments

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 10.6 Softening and Conditioning - Temper

• Almost always done following heat treat as

part of the austenitizing process!
• Because of lack of adequate toughness and
ductility after heat treat, high carbon
martensite is not a useful material despite
its great strength (too brittle).
• Tempering imparts a desired amount of
toughness and ductility (at the expense of
strength)

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 10.6 Softening and Conditioning - Temper
• Typical HT steps:
– Austenize: Heat into stable single phase region and
HOLD for uniform chemistry single phase austenite.
– Quench: Rapid cool – crystal changes from Austenite
FCC to Martensite BCT which is hard but brittle.
– Temper: A controlled reheat (BELOW AUSTENITE
REGION). The material moves toward the formation of a
stable two phase structure – tougher but weaker.
– Quench: The properties are then frozen in by dropping
temperature to stop further diffusion

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The Heat Treat Processes

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Heat Treatment of Plain Carbon
Steels

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 Isothermal Temperature-time Transformation
Diagram
• Let us do an experiment
• Choose an eutectoid steel
• Heat it to the austenite phase region (800˚ C)
• Quench to 727 ˚ C
• Observe transformation   + Fe3C
• Note time when 1% pearlite has formed
• Note time when 99% pearlite has formed
• Plot on Temp.-time transformation diagram
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 727ºC x x
x x
Temp P Pearlite
x x

 x x

x x
Nose B Bainite
x x

x x

x x
Ms
’
Mf
’(marteniste)

time
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 727ºC
coarse
Temp P Pearlite
fine

upper
B Bainite
lower

Ms
’
Mf
’(martensite)

time
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 Properties of phases

Properties of pearlite Properties of bainite

•Fairly tough •Tough
•Fairly ductile •Structure between pearlite
•Depends on size of lamellae and marteniste ( + Fe3C)

Properties of marteniste
•Very hard HRC 45-55
•Brittle
•bc tetragonal structure
•Supersaturated solution of C in Fe
•High residual stresses
•Metastable

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 Microstructures

Coarse pearlite Fine pearlite

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Microstructures

Upper Bainite

Lower Bainite

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Microstructure

Martensite
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 Continuous cooling transformation diagram
727ºC
Coarse:annealing (1ºC/min)
Temp P
in furnace
Pearlite
 Fine:normalizing(10º C/min) I air
upper

isothermal B Bainite

lower

Ms
’
Mf
’(martensite):quenching (100º C/min) in oil or water
time
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 Tempering of martensitic steels
• Martensite is too brittle to serve engineering purpose
• Tempering carried out to increase toughness of
martensite
• Carried out by reheating martenistic steel between 150-
500º C
• 1st stage 150-200º C
’ ”(low C martensite) + carbide (Fe2.3C)
• 2nd stage 200-350º C
 bainite
• 3rd stage 350-500º C
”+ carbide Fe3C(tempered martensite)93
 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.

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 HARDENABILITY
• Can we make steel hard if it is not hard?
• Yes, by quenching process.
• Why steel become hard after quenching
process?
• Because its structure become Martensite
which is very hard and brittle.
• Formation of Martensite depends on cooling
rate.
HARDENABILITY (Cont’d.)
• Faster cooling rate, formation of martensite more
and more hardness.
• Therefore surface is more hard than the core of
steel because of different cooling rates.
• Hardenability is a steel property which descirbes
the depth to which the steel may be hardened
during quenching. It depends mainly on the
composition of steel.
 Hardenability
• We have seen the advantage of getting martensite, M. We
can temper it, getting TM with the best combination of
ductility and strength.
• But the problem is this: getting M in depth, instead of just on
the surface. We want a steel where Pearlite formation is
relatively sluggish so we can get it to the cooler regions where
M forms.
• The ability to get M in depth for low cooling rates is called
hardenability.
• Plain carbon steels have poor hardenability.
 Jominy Test for Hardenability
• Hardenability not the same as hardness!
 Hardenability--Steels
• Ability to form martensite
• Jominy end quench test to measure hardenability.

Adapted from Fig. 11.11,

flat ground Callister 7e. (Fig. 11.11
specimen adapted from A.G. Guy,
Essentials of Materials
(heated to  Science, McGraw-Hill Book
phase field) Rockwell C Company, New York,
1978.)
24°C water hardness tests

• Hardness versus distance from the quenched end.

Hardness, HRC

Adapted from Fig. 11.12,

Callister 7e.

Distance from quenched end

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HARDENABILITY CURVE
 THANK YOU
QUESTIONS IF ANY

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