Unit 4.ppt

Unit 1V:
Heat Treatment of Steels

In this unit we are going to study
Annealing
Normalizing
Hardening
Tempering
Hardenability of steels
Jominey End Quench Test
Unit 4: Heat Treatment of Steels

Effect of Non Equilibrium Cooling on
Microstructure and Properties of Steel
TTT Diagram for 0.8% Carbon Steel Only
Isothermal Treatments
Continuous Cooling Transformation
Curves
CCR

Surface Hardening Treatments
Carburizing
Nitriding
Carbonitriding
Tufftride
Sursulf
Induction Hardening
Flame Hardening

What is Heat Treatment
A combination of heating and cooling
operations, timed and applied to a metal or
alloy in solid state in a way that will produce
desired properties

Objectives of Heat Treatments
To increase hardness, wear and abrasion
resistance and cutting ability of steels
To resoften the steel after it has been hardened by
heat treatment or cold working.
To adjust its other mechanical, physical or
chemical properties such as hardness, T.S.,
ductility, electrical and magnetic properties,
microstructure or corrosion resistance

To reduce or eliminate internal residual stresses.
Internal stresses lead to premature and brittle
failures of the components. They also reduce
corrosion resistance and hence are not desirable
To induce controlled residual stresses; e.g.
compressive stresses on the surface sharply
increase the fatigue life of components
To stabilize the steel so that it does not show
changes in dimensions with time. This property is
highly essential for precision gauges and
measuring instruments

To decrease or increase the grain size of steels
To produce special microstructures to increase
machinability or corrosion resistance
To eliminate gases, particularly hydrogen, which
embrittles the steel. If the steel is held at some
elevated temperature for a short time, these gases
gets diffuse into atmosphere
To change the composition of the surface by
diffusion of C,N etc so as to increase wear
resistance, fatigue life or corrosion resistance

Simple Heat Treatment Cycle
Soaking
Cooling
Heating
Time
Temperature

A
N
AT
T
Q
holding
time
T
Annealing Furnace cooling RC 15
Normalizing Air cooling RC 30
Quenching Water cooling RC 65
Tempering Heating after quench RC 55
Austempering Quench to an inter- RC 45
mediate temp and hold

Types
Conventional Annealing (Full Annealing)
Bright Annealing
Box Annealing
Isothermal Annealing
Spheroidise Annealing
Subcritical Annealing
Stress Relief Annealing
Recrystallization Annealing
Process Annealing
Annealing

Purpose
To relieve the internal stresses induced due to cold
working,welding etc.
(Internal stresses are not desirable because they lead to
premature, sudden, and brittle failures of the components.
They also decrease the corrosion resistance)
To reduce hardness and to increase ductility
Conventional Annealing
(Full Annealing)

Purpose
To increase the uniformity of phase distribution
and to make the material isotropic in respect of
mechanical properties
To refine the grain size
To make the material homogeneous in respect of
chemical composition
To increase machinability
To make the steel suitable for subsequent heat
treatment like hardening
(Full Annealing)

Process
The process consists of heating the steel to above A3
temperature for hypoeutectoid steels and above A1
temperature for hypereutectoid steel by 30-500C,
holding at this temperature for a definite time period
and slow cooling to room temperature in furnace
(Full Annealing)

(Full Annealing)

Why hyperetectoid steels are not annealed
from the temperature above Acm?
 If slowly cooled from above Acm temperature, a
proeutectoid cementite separates along the grain
boundaries of pearlite.
 This forms a brittle cementite network along the grain
boundaries of pearlite.
 Due to this the dislocations get blocked at cementite
regions are not able to move from one pearlite region to
another
 This increases brittleness of steel, departing from the aim
of annealing
(Full Annealing)

Why hyperetectoid steels are not annealed
from the temperature above Acm?
 This is also undesirable condition if machining is to be done
Moreover
 Acm temperature is high and therefore, heating to above
Acm results in more oxidation and decarburization of steel
 Heavy grain coarsening of austenitic grain occurs above
Acm. This leads to deterioration of mechanical properties.
(Full Annealing)

Annealing of steel components is carried out using
some protective medium to prevent oxidation and surface
discolouration.
Such a type of annealing keeps the surface bright a hence
is called bright annealing.
The surface protection is obtain by the use of an inert gas
such as argon or nitrogen or by using reducing
atmospheres like hydrogen gas or dissociated NH3.
Bright Annealing

Box Annealing
Here annealing, is carried out in a sealed
container under conditions that minimise
oxidation.
The components are packed with cast iron
chips, charcoal or clean sand and annealed
in a way similar to full annealing.
It is also called black annealing, close
annealing or pot annealing

Used on steels with carbon contents
above 0.5%
Applied when more softness is
needed for machinability
Cementite transforms into globes, or
spheroids
These spheroids act as chip-breakers
– easy machining
Spheroidize Annealing

A spheroidizing anneal is
designed to improve:
- cold formability
- machinability of
hypereutectoid and tool steels

Holding at just below A1
Thermal cycling around A1
Hardening and high temperature
tempering

Holding at just below A1
1. Heat to just below Lower Critical
Temperature. (about 650-700 deg C)
2. Cool very slowly in the furnace
3. Structure will now be spheroidite, in
which the Iron Carbide has ‘balled
up’
4. Used to improve the properties of
medium and high carbon steels
prior to machining or cold working.

The microstructure of
spheroidite, with Fe3C particles
dispersed in a ferrite matrix

1040 steel - 21 hours @ 700°C 1040 steel - 200 hours @ 700°C

Thermal cycling around A1
A typical heat treatment cycle of thermal cycling

Thermal cycling around A1
Due to thermal cycling in a narrow temperature
interval around A1, cementite lamellae from
pearlite become spheroidal
During heating above A1, cementite or carbides
try to dissolve and during cooling they try to form
This repeated action spheroidises the carbide
particles

Hardening and high temperature
tempering
Due to tempering of hardened steels at 650-700oC
for a long time, cementite globules are formed in
the matrix of ferrite from martensite.
Martensite Cementite (in globuler form) + Ferrite

Subcritical Annealing
Process Annealing
Annealing

Stresses may result from:
Plastic deformation (cold work,
machining)
Non-uniform heating (ex. welding)
Phase transformation (quenching)

In this process, cold worked steel is heated to a
temperature between 500 and 550°C i.e. below its
recrystallization temperature ( 600°C)
Kept at this temperature for 1 - 2 hours and
cooled to room temperature in air.
Due to this, internal stresses are partly relieved
without loss of strength and hardness i.e. without
change of microstructure.

It reduces the risk of distortion in machining, and
also increases corrosion resistance.
Since only low carbon steels can be cold
rolled/worked, the process is applicable to
hypoeutectoid steels containing less than 0.4%
carbon.
Stress relief annealing is also carried out on
components in which internal stresses are
developed from other sources like rapid cooling
and phase changes.

This is done below A1 temperature i.e. at
temperature between 625 and .675°C.
The cold worked ferrite recrystallizes and
cementite tries to spheroidise during this
annealing process.
Not only internal stresses are eliminated but also
the steel becomes soft and ductile.
Refinement in grain size is also possible by
control of degree of cold work prior to annealing or
by control of annealing temperature and time

Process Annealing
In this method, cold worked metal is heated to
above its recrystallization temperature
This is also accomplished by the formation of
strain free equiaxed grains
This is given to metals to soften them during
mechanical processing so as to continue the cold
working process without cracking of metals

Process Annealing
It mayor may not involve full recrystallization of
the cold worked metal.
In principle, process annealing and
recrystallization annealing are same
Both the processes involve recrystallization and
formation of new stress free equiaxed grains from
strained and distorted cold worked grains.

A process anneal is
designed to restore the
ductility of a steel
between processing
steps.
facilitates further cold
working
Prevents cracking
during hot working
Softens for shearing or
straightening
Promotes ease of
machining
Process Annealing

Process:
The normalizing of steel is carried out by
heating approximately 30-50°C above the
upper-critical-temperature (A3 or Acm)line
holding long enough at this temperature
for homogeneous austenitization and
followed by cooling in still air to room
temperature.
Normalising

Normalizing
Normalising
Normalising Temperature Range

Purpose:
The purpose of normalising is almost
same as that of annealing.
However,normalizing is aimed to
produce a harder and stronger steel
than full annealing so that for some
applications normalizing may be a
final heat treatment.
Normalising

Purpose:
 For hypereutectoid steels, the process is used to
eliminate the cementite network that may have
formed due to slow cooling in the temperature range
from Acm to A1
 Used to improve machinability.
 Modify and refine cast dendritic structures
 Refine the grain
 Homogenize the microstructure in order to
improve the response in hardening
operations.
Normalising

 Normalising involves non equilibrium cooling
 The increase in cooling rate due to air cooling as
compared with furnace cooling affects the
transformation of austenite and the resultant
microstructure in several ways.
 There is less time for the formation of the
proeutectoid phase hence there will be less
proeutectoid ferrite in normalized hypoeutectoid
steels and less proeutectoid cementite in
hypereutectoid steels as compare with annealing
Normalising

The faster cooling rate in normalizing will
also affect the temperature of austenite
transformation and the fineness of the
pearlite.
In general,the faster the cooling rate, the
lower the temperature of austenite
transformation and the finer the pearlite.
Normalising

Normalising
The difference in spacing of the cementite plates
in the pearlite obtained after annealing and
normalizing is shown schematically
Ferrite is very soft, while cementite is very hard, with the
cementite plates closer together in the case of normalized
pearlite, they tend to stiffen the ferrite so it will not yield as
easily, thus increasing hardness of steel

Nonequilibrium cooling also shifts the
eutectoid point toward lower carbon
content in hypoeutectoid steels and toward
higher carbon content in hypereutectoid
steels.
The net effect is that normalizing produces
a finer and more abundant pearlite
structure than is obtained by annealing,
which results in a harder and stronger
steel.
Normalising

 Hypereutectoid steels are usually normalised from above
Acm temperature. This is because due to air cooling from
above Acm the proeutectoid Fe3C separates in the form
of needles in the grains of austenite which transform to
pearlite at A1
 The microstructure at room temperature shows
innumerable needles of Fe3C in the matrix of pearlite
(Widmanstatten structure).
 Thus for hypereutectoid steels, normanzing will
reduce the continuity of the proeutectoid
cementite network, and in some cases it may be
suppressed entirely.
 Such structures are less brittle because the dislocation can
move via certain regions avoiding these needles.
Normalising

Normalising
Normalized 0.90 percent carbon steel, 100X. Bright
proeutectoid cementite in dark pearlitic matrix.

Annealing Vs Normalising
S.
N
.
Annealing Normalising
1 Furnace Cooling
(Equilibrium Cooling)
Air Cooling
(Non equilibrium Cooling)
2

S.
N.
3 Slightly less hardness,
T.S. and toughness.
Slightly more hardness, T. S.
and
toughness.
4 For plain carbon steels,
microstructure shows
pearlite almost in
accordance with the Fe-C
equilibrium diagram
Microstructure shows more
pearlite than observed in
annealed components
5 Pearlite is coarse and
usually gets resolved by
the optical microscope.
Pearlite is fine and usually
appears unresolved with optical
microscope.

S.
N
.
1 Less internal stresses
are produced
More internal stresses are
produced
2 Cementite network is
produced in
hypereutectiod steels
No Cementite network is
produced in hypereutectiod
steels
3 More time required hence
less economical
Less time required hence more
economical

Heat treatment Temperature Ranges

Hardening
 Under slow or moderate cooling rates, the carbon
atoms are able to diffuse out of the austenite
structure. The iron atoms then move slightly to
become b.c.c. (body-centered cubic).
 This gamma-to-alpha transformation takes place
by a process of nucleation and growth and is time-
dependent.
 With a still further increase in cooling rate,
insufficient time is allowed for the carbon to
diffuse out of solution, and although some
movement of the Iron atoms takes place, the
structure cannot become b.c.c. while the carbon is
trapped In solution.

Hardening
 The resultant structure called martensite,is a
supersaturated solid solution of carbon trapped in
a body-centered tetragonal structure. Two
dimensions of the unit cell are equal, but the third
is slightly expanded because of the trapped
carbon.
 The axial ratio c/a increases with carbon content
to a maximum of 1.08
 This highly distorted lattice structure is the prime
reason forthe high hardness of martensite. Since
the atoms of martensite are less densely packed
than in austenite, an expansion occurs during the
transformation.

Transformations of austenite into pearlite:  → P
Diffusional transformations
1) At slightly lower T below 727 ℃ :
• Coarse pearlite
: nucleation rate is very low.
: diffusion rate is very high.
2) As the Tt (trans. temp.) decreases
to 500 ℃
• Fine pearlite
: nucleation rate increases.
: diffusion rate decreases.
655 ℃ 600 ℃
534 ℃ 487 ℃
pearlite
Hardening

Diffusionless Transformations -
Martensitic transformation
When the austenite is quenched to temp.
below Ms
 → ’ (martensite)
Instead of the diffusional migration of carbon atoms to
produce separate  and Fe3C phases, the matensite
transformation involves the sudden reorientation of C
and Fe atoms from the austenite (FCC) to a body
centered tetragonal (bct) solid solution.
Hardening

BCT unit cell of  (austenite)
414
.
1
2 

a
c
BCT unit cell of ’ (martensite)
08
.
1
00
.
1 

a
c
0% C (BCC) 1.2 % C
Contract
~ 20%
Expand
~ 12%
Martensitic transformation (contd.)

 Austenitizing : heating
the steels to a high
enough temperature until
they convert to austenite.
 Quenching: Media –
brine (salt water), fresh
water, oil,polymers and
air
 Tempering – Reheat to
200 - 700°C, to decrease
internal stresses,
hardness, and regain
ductility and toughness.
Hardening

Purpose
(i) To harden the steel to the maximum
level by austenite to martensite
transformation. (Due to increase in
hardness, brittleness also increases)
(ii) To increase the wear resistance and
cutting ability of steel.
Hardening

Process
The conventional hardening process consists of
heating the steel to above A3 temperature for
hypoeutectoid steels and above A1 temperature
for hypereutectoid steels by 50°C, austenitising
for a sufficient time and cooling with a rate just
exceeding the critical cooling rate of that steel to
room temperature or below room temperature.
Due to this, the usual diffusion transformations
are stopped and the austenite transforms to
martensite by a diffusionless process.
Hardening

Hardening
Hardening
Temperature Range for Hardening

Hypoeutectoid steels are hardened from
above A3 temperature.
 They are not hardened from temperatures between A1
and A3
 This is because the phases which exist at this
temperature are austenite and proeutectoid ferrite and
only austenite gets transformed to martensite with no
change in ferrite.
 Such steels show free ferrite in their microstructures
and since ferrite is a soft phase, the hardness of
hardened steel gets reduced.
Hardening

Hypereutectoid steels are always hardened from
temperatures between A1 and Acm
(i.e. from above A1)
At this temperature, austenitization is not
complete and some proeutectoid Fe3C will exist
along with austenite at the temperature of
heating.
Such steels after hardening show free Fe3C
along with martensite in their microstructures.
Hardening

Since Fe3C being a hard phase, the hardness of
hardened steels does not get reduced
 Moreover, this free Fe3C does not increase the
brittleness of steels because usually it is fine,
well distributed and partially spheroidised. Also,
the grain size remains fine because the Fe3C
particles do not allow to coarsen the austenite.
Hardening

However, if these steels are hardened from
above Acm temperature, the following
drawbacks are observed.
(i) Since Acm line is steep, higher temperatures
are required to cross the Acm line. Due to this and
absence of Fe3C above Acm temperature, heavy
grain coarsening occurs during austenitization and
results in coarse grained martensite which is
extremely brittle.
Hardening

However, if these steels are hardened from
above Acm temperature, the following
drawbacks are observed.
(ii) Quenching from such a high temperature results in
more distortions and may lead to cracking of the
components.
(iii) Due to higher temperatures, oxidation and
decarburization is more.
(iv) The amount of retained austenite increases because of
higher thermal stresses.
Hardening

The needle-like structure of martensite,
the white areas are retained austenite.
Microstructure of Martensite

(a) Lenticular martensite in an Fe–30% Ni alloy.
(b) Lenticular (thermoelastic) martensite in Cu–Al–Ni alloy.

 Lath martensite(strip like or rod like):
Less than 0.6% C
Plate martensite(neddle like): More than
1.0% C


Lath type Plate type

The transformation is diffusion less
There is no change in chemical composition
 Austenite transform to martensite by a
shear mechanism
Salient Features of Martensitic
Transformation

Athermal Transformation
The transformation proceeds only during
cooling and ceases if cooling is interrupted.
Therefore, the transformation depends only
upon the decrease in temperature and is
independent of time.
A transformation of this type is said to be
athermal,in contrast to one that will occur at
constant temperature (isothermal
transformation)
Transformation

Transformation proceeds at a speed close to
the speed of sound
The amount of martensite formed with decreasing temperature
is not linear. The number of martensite needles produced at
first are small; then the number increases,and finally, near the
end, it decreases again .
Transformation

Characteristics Temperatures Ms and Mf
 The temperature of the start of martensite
formation is known as the Ms temperature and
that of the end of martensite formation as the Mf
temperature.
 The Ms and Mf temperatures are function of
chemical composition only.
Ms (OF)= 1,000 - (650 x % C)- (70 x % Mn)- (35 x % Ni)
- (70 x % Cr)- (50 x % Mo)
Transformation

 The influence of carbon on the Ms and Mf temperatures is
shown in Fig
 The Mf temperature line is shown dotted because it is
usually not clearly defined
 Theoretically, the austenite to martensite transformation is
never complete, and small amounts of retained austenite
will remain even at low temperatures
 The transformation of the last traces of austenite becomes
more and more difficult as the amount of austenite
decreases.
Transformation

Transformation

The martensitic transformation of given
alloy cannot be suppressed changing cooling
rate.
Martensite is metastable phase
 Martensite is never in a condition of real
equilibrium
 Although it may persist indefinitely at or near room
temperature.
 The structure can be considered as a transition
between the unstable austenit phase and the
stable ferrite
Transformation

The most significant property of martensite
is its potential of very great hardness.
 Extreme hardness are possible only in steels that
contain sufficient carbon.
 The high hardness of martensite is result of the
severe lattice distortions produced by its
formation, since the amount of carbon present is
many times more than can be held solid solution.
 The maximum hardness obtainable from a steel in
the martensitic condition is function of carbon
content only
Transformation

 The hardness of martensite increases rapidly at
first with increase in carbon content, reaching
about 60 Rockwell C at 0.40 percent carbon
 Beyond that point the curve levels off, and at the
eutectoid composition (0.80.percent carbon), the
hardness is about Rockwell C 65.
 The leveling off Is due to the greater tendency to
retain austenite in higher carbon steels.
Transformation

Transformation

Quenching is done by using the following
mediums:
Brine (cold water + 5 to 10% salt)
(The salt may be sodium chloride, sodium
hydroxide or calcium chloride)
Cold water
Water + soluble oil
Oil
Fused salts
Air
Quenching Media

Quenching Medium
Vapor Transport
Vapor Blanket
Liquid Cooling

Mechanism of Heat Removal During
Quenching
 The structure, hardness, and strength resulting
from a heat-treating operation are determined by
the actual cooling rating obtained by the quenching
process.
 If the actual cooling rate exceeds the critical
cooling rate, only martensite will result.
 If the actual cooling rate is less than the critical
cooling rate, the part will not completely harden.
Quenching Medium

Mechanism of Heat Removal During
Quenching
 The greater the difference between the two cooling rates
the softer will be the transformation products and the
lower the hardness.
 Liquid cooling media remove the heat from the component
through the following stages
Vapour blanket stage
Vapour transport stage
Liquid cooling stage
Quenching Medium

Vapour blanket stage
 The temperature of the steel is so high that the
quenching medium is vaporized at the surface of
the metal and a thin stable film of vapor surrounds
the hot metal
 This vapour blanket does not allow to extract the
heat and reduce the cooling rate.
 Cooling is by conduction and radiation through the
gaseous film, and since vapor films are poor heat
conductors, the cooling rate is relatively slow
through this stage
Quenching Medium

Vapour transport stage
 This stage starts when the metal has cooled to a
temperature at which the vapor film is no longer
stable
 The vapour blanket breaks and the liquid comes in
contact with the surface of hot component
 Wetting of the metal surface by the quenching
medium take place and violent boiling occur
 Heat is removed from the steel very rapidly as the
latent heat of vaporization
 This is the fastest stage of cooling
Quenching Medium

Liquid cooling stage
This occurs when temperature of the
component reaches boiling point of
quenching medium
Vapor no longer forms, so cooling is by
conduction and convection through the
liquid
The rate of cooling is the lowest in this stage
Quenching Medium

The factors determining the
actual cooling rate are
The type of quenching medium
The temperature of the quenching
medium
 The surface condition of the part
The size and mass of the part
Quenching Medium

Quenching Medium

Brine
 This quenching medium has a very short vapor stage
lasting about 1 s
 And then drops quickly into the boiling stage, where
the cooling rate is very rapid
 It finally goes into the third stage at about 10s
Quenching Medium

Water
The vapor stage is slightly longer than for
brine
It drops into the boiling stage after about 3 s
The cooling rate during this stage, while very
rapid, is not quite so fast as that for brine
The third stage is reached after about 15 s
Quenching Medium

Fused Salt
The fused salt has a very short vapor stage
approximately equal to that of brine.
However,the cooling rate during the boiling
stage is not so rapid as that for brine or water
It reaches the third stage at about 10 s.
Quenching Medium

Oil
They both show a relatively long vapor stage
Enters the boiling stage after about 7 s
The third stage is reached after about 15 s
Air
 Never get out of the vapor stage and therefore
shows a very slow cooling rate over the entire range
Quenching Medium

Temperature of Quenching Medium
 Generally, as the temperature of the medium rises,
the cooling rate decreases
 This is due to the increase in persistence of the
vapor blanket stage
 Since the medium is closer to its boiling point, less
heat is required to form the vapor film
 This is particularly true of water and brine.
Quenching Medium

Quenching Medium

 In the case of oil, there are two opposing factors to
be considered.
 As the temperature of the oiI rises there is a
tendency for the cooling rate to decrease due to the
persistence of the vapor film.
 However, as the temperature of the oil rises it also
becomes more fluid, which increases the rate of
heat conduction through the liquid
Quenching Medium

Surface Condition
 When steel is exposed to an oxidizing atmosphere,
because of the presence of water vapor or oxygen in
the furnace a layer of iron oxide called scale is
formed
 A thin layer of scale has very little effect on the
actual cooling rate, but a thick layer retards the
actual cooling rate
 Many methods are used Industrially to minimize the
formation of scale
• Copper Plating
• Protective Atmosphere
• Cast-iron Chips
Quenching Medium

Size and Mass
 Since it is only the surface of a part which is in
contact with the quenching medium, the ratio of
surface area to mass is an important factor in
determining the actual cooling rate.
 This ratio is a function of the geometric shape of the
part and is largest for a spherical part
 Thin plates and small-diameter wires have a large
ratio of surface area to mass and therefore rapid
cooling rates.
Quenching Medium

Polymer Quenchants
 Polymer quenchants are solution of organic
compounds of high molecular weight in water
 They dissolve in water at room temperature
 But when heated above 770C , they precipitate out
and become insoluble in water.
 Again when solution is cooled below 770C, the
polymer goes back into solution and is fully miscible
Quenching Medium

Why Polymer Quenchants are Ideal
Quenchants
Process of Heat Removal
 Initially the heat is rapidly removed by the water in
the solution. This avoids the nose of the TTT curve
and formation of softer phases
 As temperature of steel compnents is above 770C an
insoluble layer is formed over the components
which reduces the cooling rate in later part of
quenching process. This reduces distortion.
 Thus polymer quenchants are ideal for quenching
Quenching Medium

Quenching Medium
Homogeneous
Quenchant Solution
Seperated Polymer
Water
Heat
Cool

Polymer Quenchants
A few well known organic compounds used as
polymer quenchants are:
Polyvinyl alcohol (PVA)
CH3-CH-(CH2-CH)n
I
OH
Poly-alkylene glycol (PAG)
HO-(CH2-CH2-O)n-(CH2-CH-O)m-H
I
CH3
Quenching Medium

Polymer Quenchants
Sodium polyacrylate (PA)
-CH2-CH-
I
C=O
I
ONa n
Quenching Medium

Advantages of Polymer Quenchants
1. They provide a wide range of cooling rates depending
upon the type of polymer, concentration and
temperature of the solution
2. They reduce distortions and cracking of components
3. They virtually eliminate- smoke, fume and fire hazards
in contrast to oil quenching because they are non-
inflammable
4. They provide uniform cooling rate because of the
deposition of polymer film, which results in uniform
hardening
5. As compared to oils, polymer solution results in
reduced drag out and fluid losses
Quenching Medium

The Isothermal-transformation Diagrams
or Time Temperature Transformation
(TTT) Diagrams
 The iron-iron carbid equilibrium diagram is of little
value in the study of steels cooled under
nonequilibrium conditions
 The time and temperature of austenite
transformation has a profound influence on the
transformation products and the subsequent
properties of the steel
Time Temperature Transformation
(TTT) Diagrams

The Isothermal-transformation Diagrams
or Time Temperature Transformation
(TTT) Diagrams
Since austenite is unstable below the lower critical
temperature A1 it is necessary to know at a particular
subcritical temperature how long it will take for the
austenite to start to transform, how long it will take to
be completely transformed and what will be the
nature of the transformation product
(TTT) Diagrams

 Depending on the temperature of transformation,
austenite may transform to pearlite,bainite, or
martensite.
 The kinetics of the above phase transformations is
indicated on TTT diagrams.
 These diagrams indicate the phases existing in steels at
various temperatures and times.
 They are very much useful in the heat-treatment of
steels.
 With the help of these diagrams, one can choose a
proper cooling cycle to obtain the desired
transformation product (microstructure) so as to obtain
the required properties in the component.
(TTT) Diagrams

Determination of TTT diagram
The best way to understand the TTT diagrams is to
study their derivation
The eutectoid composition of 0.8 percent carbon is the
simplest one to study since there is no proeutectoid
constituent present in the microstructure.
(TTT) Diagrams

The steps usually followed to determine TTT diagram
are:
Step 1:Prepare a large number of samples
cut from the same bar.
One method of handling the small samples during heat
treatment is by means of a wire threaded through a
hole in the sample. The cross section has to be small in
order to react quickly to changes in temperature.
(TTT) Diagrams

A typical sample which is used to determine TTT diagram
(TTT) Diagrams

Step 2: Place the samples in a furnace or
molten salt bath at the proper
austenitizing temperature.
For a 1080 (eutectoid) steel, this temperature is
approximately 750°F. They should be left at the given
temperature long enough to become completely
austenite.
(TTT) Diagrams

Step 3: Place the samples in a molten salt
bath which is held at a constant
subcritical temperature
(A temperature below the A1, line)
Step 4: After varying time intervals in the
salt bath, each sample quenched in cold
water or iced brine
(TTT) Diagrams

(TTT) Diagrams

(TTT) Diagrams
The progress of austenite transformation to coarse pearlite at 7000F as
related to the structure at room temperature; A is austenite,M is
martensite,P is pearlite.

Step 5: After cooling, each sample is
checked for hardness and studied
microscopically
Step 6: The above steps are repeated at
different subcritical temperatures until
sufficient points are determined to plot
the curves on the diagram.
(TTT) Diagrams

We are really interested in knowing what is happening to the
austenite at 700°C, but the samples cannot be studied at that
temperature.
Therefore, we must somehow be able to relate the room-
temperature microscopic examination to what is occurring at the
elevated temperature.
Two facts should be kept in mind:
Martensite is formed only from austenite almost
instantaneously at low temperatures.
If austenite transforms at a higher temperature to a
structure which is stable at room temperature, rapid
cooling will not change the transformation product.
(TTT) Diagrams

 Sample 1 after 30 s at 700°C and quenched, showed only
martensite at room temperature.
 Since martensite is formed only from austenite at low
temperature, it means that at the end of 30 s at 700°C there
was only austenite present and the transformation had not
yet started.
 Sample 2, after 6 h at 700°C and quenched, showed about 95
percent martensite and 5 percent coarse pearlite at room
temperature .
 It means that at the end of 6 hrs at 700°C, there was 95
percent austenite and 5 percent coarse pearlite.
 The transformation of austenite at 700°C has already started,
and the transformation product is coarse pearlite.
(TTT) Diagrams

 Transformation curve at 700°C and several of the
room-temperature microstructures are shown in Fig.
 The light areas are martensite.
 The transformation from austenite to pearlite is not
Iinear.
 Initially the rate of transformation is very slow,then it
increases rapidly, and finally it slows down toward
the end
(TTT) Diagrams

(TTT) Diagrams
Typical IT curve of austenite to pearlite
for 1080 steel

(TTT) Diagrams
 Two points are plotted at 700°C namely, the
time for the beginning and the time for the
end of transformation
 It is also common practice to plot the time
for 50 percent transformed
 The entire steps are repeated at different
subcritical temperatures until sufficient
points are determined to draw one curve
showing the beginning of transformation,
another curve showing the end of
transformation, and dotted curve in
between showing 50 percent transformed

(TTT) Diagrams
 Time is plotted on a logarithmic scale
so that times of 1 min or less as well
as times of 1 day or week, can be
fitted into a reasonable space.
 The diagram is known as an I-T
(isothermal-transformation) diagram
or TTT diagram

(TTT) Diagrams
Diagram showing how measurements of Isothermal
transformation are summarized by the TTT diagram.

(TTT) Diagrams
Isothermal-transformation diagram for a 1080(eutectoid) steel.

Bainite
 The transformation product of austenite at below
5500C is not lamellar but is of different morphology
and is called as bainite
 Bainite is an extremely fine mixture of ferrite and
cementite
 It is formed by a different mechanism from that of
pearlite.
 Bainitic transformation starts by the nucleation of
ferrite, in contrast to the nucleation of cementite in
pearlitic transformation
 Since the transformation occurs at low
temperatures, nucleation rate is very high but the
growth rate is very low due to relatively less
mobility of carbon atoms at lower temperatures.

Bainite
Nucleation and growth of bainite colonies
Ferrite Cementite

Bainite
 This results in a structure with very fine distribution of
ferrite and cementite phases
 The Cementite platelets are usually oriented
at an angle of about 600 to the long axis of the ferrite
needles rather than parallel to this direction
 The bainite formed at higher temperatures is called as
upper bainite and has a feathery appearance, whereas
the bainite formed at lower temperatures is called as
lower bainite and has an acicular (needle like)
appearance.
 The distribution of carbides is finer in lower bainite than
in upper bainite and hence lower bainite is harder,
stronger and tougher than upper bainite.

Bainite
 The hardness of bainite depends upon the
carbon content in the steel and also on the
temperature at which it is formed
 For a given steel, it is intermediate to that of
pearlite and martensite.
 For eutectoid steel, the hardness of upper
bainite is in the range of 40-50 Rc and that of
lower bainite is between 50-60 Rc.

CRITICAL COOLING RATE (CCR)
 The critical cooling rate is a rate which just by
passes the nose of the TTT diagram
 It depends upon the shift of the nose of TTT
diagram to the right side.
 For hardening, steels from austenitic region must
be cooled with such a rate that no transformation
of austenite should occur upto Ms i.e.the
diffusion transformation should be stopped so
that the austenite transforms to martensite by
diffusionless transformation
 The rate of cooling necessary to just suppress the
diffusion transformation or to avoid the nose of
TTT diagram called as the Critical cooling rate.

 The critical cooling rate depends on many factors
but the most important being the content of
carbon and the alloying elements in steel
 With higher carbon and/or alloying elements,
critical cooling rate decreases.
 Most of the alloying elements (except cobalt)
shift the TTT diagram to the right side i.e. retard
the transformation of austenite to pearlite or
bainite decreasing the critical cooling rate.
 The shift of the nose of TTT diagram to the right
side gives an idea about the hardenability of steel
 Less the critical cooling rate, more is the
hardenbility

M
Stable
austenite
unstable
austenite
A+M
Ms
Mf
CCR=Slop of
this curve

 Alloying elements significantly reduce the
critical cooling rate permitting transformation
of austenite to martensite at relatively low
cooling rates.
 A slower cooling rate reduces the danger of
warping and cracking and becomes an advantage
for hardening of complicated shaped components
such as tools and dies
 Low carbon steels of plain type have very high
critical cooling rate and hence rapid cooling is
necessary to suppress the pearlitic or bainitic
transformation.

 In some of the steels, it is not possible to achieve
this even by water or brine quenching.
 Even if the critical cooling rate is exceeded by
certain techniques, the martensite produced is
not so hard because of less carbon in the steel.
 Since such steels are difficult to harden and can
not be effectively hardened, they are called as
non-hardenable steels.

Plain C steel Alloy steel
Alloying shifts the TTT curves to the right.
Lowers Ms and Mf
Effect of Alloying on TTT diagram

Heat Treatments Derived
from Time Temperature
Transformation (TTT)
Diagrams

Isothermal (Cycle)Annealing
In this process
 The components are slightly fast cooled
from the usual austenitizing
temperature of conventional annealing
to a constant temperature just below A1
 Held at this temperature for sufficient
period for the completion of
transformation and
 Then cooled to room temperature in air

Stable austenite
unstable
austenite

Isothermal annealing has distinct advantages over
conventional annealing which are as below:
below:
(i) It reduces the annealing time, especially for alloy
steels which need very slow cooling to obtain the
required reduction in hardness with the conventional
annealing.
(ii) Because of equalization of temperature,
transformation occurs at the same time throughout
the cross-sectction. This leads to more homogeneity
in structure.
(iii) It shows improved machinability, improved
surface finish after machining and less warping
during subsequent hardening process.

Isoforming
 Austenite is worked i.e. rolled or forged
at the isothermal transformation
temperature in the pearlitic region till the
transformation of austenite to pearlite is
complete.
 This results in refinement of structure
with improvement in the fracture
toughness.

A+M
Stable
austenite
unstable
austenite
M
Ms
Mf
Heat Treatment Cycle for Isoforming

The timed quench (Interrupted quench)
 For plain carbon steels of low to medium carbon,
critical cooling rates are high and therefore, very fast
cooling from austenitizing temperature is necessary
to prevent the formation of pearlite or bainite at
temperatures near the nose of the TTT diagram.
 However, once this region of rapid transformation has
been passed, the transfornlation of austenIte
becomes slow.
 Therefore, it is possible to obtain a completely
martensitic structure in a steel of low hardenability
(i.e. of high critical cooling rate) by cooling it rapidly to
a temperature below the nose of the IT diagram and
then cooling it more slowly through the temperature
range in which martensite is formed.

A+M
Stable
austenite
unstable
austenite
M
Ms
Mf
Hardening cycle for the timed quench process

The timed quench (Interrupted quench)
 Since the cooling rate between Ms and Mf is
reduced, the cracking tendency also gets
reduced and This is the chief advantage of the
process.
 The process consists of heating the steel to
the austenitization temperature, quenching a
short period in cold water or brine to a
temperature between the nose and Ms and
then cooling in some other medium like oil to
room temperature.
 Since the time of first quench is very small
(0.1 - 0.3 see.), it is very difficult to control the
process to obtain consistent results.

Martempering (Marquenching)
 In this process, the austenitized steel is cooled
rapidly avoiding the nose of the I.T. diagram to
a temperature between the nose and Ms,
soaked at this temperature for a sufficient
time for the equalization of temperature but
not long enough to permit the formation of
bainite -and then cooled to room temperature
in air or oil
 Since the component has to be held for some
time for equalization of temperature, process
will be applicable to steels of slightly high
hardenability such as high carbon steel and low
alloy steels

A+M
Stable
austenite
unstable
austenite
M
Ms
Mf
Hardening cycle for martempering process

Martempering (Marquenching)
The process produces martensitic structures with
the following advantages:
 It results in less distortions and warping, since
the martensite formation occurs at the same
time throughout the cross section of the
component.
 There is less possibility of quenching cracks
appearing in the component
 This is a hardening process and therefore, the name
martempering(an abbreviation for
"martensite tempering" is a misnomer for the
treatment

Ausforming
 In this process, austenitized steel is cooled with a
rate exceeding the critical cooling rate of that steel
to a temperature between the nose and Ms, forged
or rolled at this temperature and cooled to room
temperature in oil.
 Due to the plastic deformation of austenite, the
martensite formed is fine.
 Also, this results is increased dislocation density in
martensite and a finer distribution of carbides on
tempering.
 Ausformed structures on tempering at low
temperature show better combination of T.S. and
ductility.
 Steels with sufficient hardenability can only be ausformed.

A+M
Stable
austenite
unstable
austenite
M
Ms
Mf
Hardening cycle for Auforming

Retention of Austenite
 The martensitic transformation never goes
to completion i.e. 100% by cooling to any
temperature.
 The amount of retained austenite varies
from surface to center in a hardened steel
component. It is less at or near the surface
and more in the center

 The amount of retained austenite also
depends on the quenching temperature.
 Higher the quenching temperature, more is
the difference in temperature at the surface
and center of a component.
 The greater the temperature difference,
higher will be the thermal stresses
developed and higher will be the
opposition for martensitic transformation
resulting in higher proportion of retained
austenite.

 For some of the steels like high carbon steels containing more
than 0.7% carbon and for some of the alloy steels, Mf
temperature is below room temperature.
 If these steels are quenched from the hardening temperature
to room temperature, all austenite does not transform to
martensite but a part of it remains untransformed.

Effects of Retained Austenite
The retained austenite in hardened steels has some advantages as
below:
(i) Austenite reduces the tendency of cracking during hardening
and hence about 10%'retained austenite is desirable for this
purpose.
(ii)If the amount of retained austenite is more such as 30-40%, the
steel can be cold worked to some extent without cracking which
would not have been possible in the absence retained austenite.
In such cases, straightening operation on the components can be
done after hardening. Besides this advantage, the hardness after
straightening operation increases partly due to deformation
induced martensitic transformation and partly due to
strain hardening of austenite.

Retained austenite is not desirable in the finished
components due to the reasons given below:
(i) Austenite is a soft phase
(ii) Small amount of retained austenite does not
decrease the hardness much but it may
increase the brittleness of steel. This is due to
the fact that it is likely to get transformed to
martensite by plastic deformation. This
deformation (strain) induced transformation of
austenite to martensite increases the internal
stresses deteriorating the properties of steel.

Retained austenite is not desirable in the finished
components due to the reasons given below:
(iii) The retained austenite may get slowly
transformed to bainite at room temperature. This
is accompanied by volume expansion and create
trouble in some applications like precision gauges
and test blocks.
(iv)Retained austenite is not at all desirable in
some applications like tool steels for which the
best possible combination of strength, hardness,
toughness, and dimensional stability is essential

Elimination Retained Austenite
 Subzero treatment
(Cold Treatment)
 Plastic Deformation
 Tempering

 Subzero treatment
(Cold Treatment)
 Plastic Deformation
 Tempering

Tempering
Purpose
 To relieve the internal stresses
(Stresses are developed due to rapid cooling of
steels during hardening process (i.e. austenite
to martensite transformation) and due to
volume changes occurring in the above
transformation, to reduce brittleness. )
 To reduce hardness, and to increase
ductility and toughness
 To eliminate retained austenite.
 To obtained spheroidized Cementite

Process
After hardening, heating and holding
steel below A1 line and slow cooling
(usually in air) to room temperature
Done in the range 100-700˚C
Tempering

Tempering
 After hardening heat treatment, steel
contains martensite and retained austenite.
 In some steels like hypereutectoid steels
and alloy steels, carbides are also present.
 Martensite and austenite are not stable
phases and try to transform to more stable
phases during heating.
 Martensite is a supersaturated solid
solution of carbon trapped in a body-
centered tetragonal structure.

C
Fe
tempering
3



 

 

Tempering
 This is a metastable phase, and as energy is
applied by tempering, the carbon will
precipitated as cementite and the
martensite becomes b.c.c aplha ferrite

Tempering is classified in the following
types
Low temperature tempering
(100-200°C)
Medium temperature tempering
(200-500°C)
High temperature tempering
(500-700°C )
9-17
Tempering

Low temperature tempering (100-200°C)
During low temperature tempering the original
as-quenched martensite is beginning to lose its
tetragonal crystal structure by the formation
of a hexagonal close-packed transition carbide
(epsilon Carbide, Fe2.4C) and low-carbon
martensite.
α’ Low carbon α’ + ε-carbide
Tempering

Low temperature tempering (100-200°C)
The precipitation of the transition
carbide may cause a slight increase in
hardness, particularly in high carbon
steels
There is no appreciable change in the
retained austenite.
The brittleness of steel decreases due to
sharp decrease the internal stresses.
Tempering

(200-500°C)
 Heating in the range from 200 to 500°C changes
the epsilon carbide to orthorhombic cementite
(Fe3C) The low-carbon martensite becomes b.c.c.
ferrite
 Retained austenite is transformed to lower bainite
or decomposes and form cementite and
martensite
 This transformation of austenite to martensite is
due to increase Ms temperature because of
decrease in carbon content of austenite
Tempering

(200-500°C)
 These changes in microstructure result in decrease
of hardness with increasing tempering
temperature
 The decrease in hardness is gradual upto 350°C
and rapid thereafter, reaching to almost a
minimum value at about 500°C.
 These changes are accompanied by simultaneous
increase in toughness and ductility.
Tempering

High Temperature tempering
(500-700°C)
 During this stage of tempering, cementite
particles become coarse.
 Except this, there is no other change in the structure.
 When the particles are resolvable by optical
microscope, they appear to be spheroidal in shape
 The structure is called tempered martensite
Tempering

High Temperature tempering
(500-700°C)
 Coarsening of particles results in a slight decrease in
hardness and toughness.
 Since the coarsening rate is very less, the decrease in
properties with time is also very less.
 Spheroidized structures can be machined with high
speeds because of their excellent machinability.
 Some of the steels which are difficult to machine like
high carbon steels and few of the alloy steels can be
machined easily after spheroidising.
Tempering

Tempering
Temperature
100-200 0C
200 – 5000C
500 – 7000C
Structure
Epsilon Carbide+Low carbon α’+RA
Cementite+Ferrite +Cementite+
Bainite/Martensite
Cementite(Spheroidite)+Ferrite
Tempering

Tempering
Variation in properties with tempering temperature

Austempering
 This is a heat-treating process developed from the I-T
diagram to obtain a structure which is 100 percent
bainite.
 It is accomplished by first heating the steel part to the
proper austenitizing temperature followed by cooling
rapidly in a salt bath held in the bainite range (usually
between 200 and 400°C). The piece is left in the bath
until the transformation to bainite is complete
 Depending on the temperature of transformation, the
product may be upper bainite or lower bainite.
 Properties of bainite are intermediate to those of
martensite and pearlite and very much similar to that
of tempered martensite.

End product is 100% bainite
Austempering

Advantages of Austempering:
The most important advantage of
austempering is that it produces structure and
properties very much similar to tempered
martensites without involving martensitic
transformation.
Also,the dimensional stability of components
is more due to the absence of retained
austenite because the austenite to bainite
transformation proceeds to 100% contrast to
austenite to martensite transformation which
never proceeds to 100%.
Austempering

Advantages of Austempering:
 Less Distortion and less chance of quenching
cracks
 Greater Ductility
 Uniform and consistent Hardness
 Tougher and More Wear Resistant
 Higher Impact and Fatigue Strengths
 Resistance to Hydrogen Embrittlement
Austempering

Limitations of Austempering:
 The hardness produced is not so high as that
produced by martensitic transformation. Also,
wide range of property variation can be achieved
by variation of tempering temperature which is
not possible by this process.
 Since the critical cooling rate has to be exceeded
during cooling, the process is applicable only to
slightly high hardenability steels.
 The holding times are long and hence the process
is expensive.
 Only thin sections can be treated
Austempering

Hardening and Tempering
Vs
Austempering
S.
N.
Hardening and
Tempering
Austempering
1 Process: Process:
2 Lower hardness,
ductility and toughness
Higher hardness, ductility
and toughness
3 Less uniform properties More uniform properties

Vs
Austempering
S.
N.
Hardening and
Tempering
Austempering
4 More distortion Less distortion
5 Less dimensional
stability
More dimensional
stability
6 Wide range of property
variation can be
achieved
Wide range of property
variation can’t be
achieved

Vs
Austempering
S.
N.
Hardening and
Tempering
Austempering
7 Process is applicable for
any hardenable steel
Process is applicable only
for steel with higher
hardenability
8 Steel component with
any thickness can be
hardened and tempered
Steel component only
with small thickness can
be austempered
9 Process time is less Process time is more due
to high holding time
10 Less expensive More expensive

Carbon has the following effects on
the TTT diagram
An increase in carbon always retards
transformation of austenite
It moves the curves to the right
Effect of Carbon on TTT diagram

Carbon has the following effects on
the TTT diagram
Carbon content has only a minor effect on the
time required for the pearlitic reaction
 Dissolved carbon greatly retards the initiation
and completion of the bainite reaction
displacing bainitic part of the curve strongly to
the right side.
Dissolved carbon stabilizes austenite and
reduces Ms temperature.
Effect of Carbon on TTT diagram

TTT diagram for eutectoid steel

TTT diagram for hypoeutectoid steel

 TTT diagram for a hypereutectoid
composition (1.13 wt% C)
TTT diagram for hypereutectoid steel

Why low carbon steels can’t be
hardened?

Why low carbon steels can’t be
hardened?
 Low carbon steels have very high critical
cooling rate and hence rapid cooling is
necessary to suppress the pearlitic or bainitic
transformation
 In very low carbon steel, it is not possible to
achieve this even by water or brine quenching
 Even if the critical cooling rate is exceeded by
certain techniques, the martensite produced is
not so hard because of less carbon in the steel
 Since such steels are difficult to harden and can
not be effectively hardened, they are called as
non-hardenable steels

CCT diagram for eutectoid steels
 The TTT diagram shows the time-temperature
relationship for austenite transformation only
as it occurs at constant temperature.
 But most heat treatments involve
transformation on continuous cooling.
 It is possible to derive from the I-T diagram
another diagram which will show the
transformation under continuous cooling.
 This is referred to as the C-T diagram
(cooling-transformation diagram) or CCT
diagram

 Compare to TTT diagram, CCT diagram shows
drop of the "nose" to downward side and to
the right by continuous cooling.
 This means pearlitic transformation occur at
lower temperatures and require a longer time
for their completion
 The critical cooling rate is slower than TTT
diagram
 Absence of an austenite-to-bainite region in
the CCT diagram.
 In this diagram the bainite range is
"sheltered" by the overhanging pearlite nose,
and bainite is not formed.

 Bainite is not formed during continuous
cooling because it is hidden below the nose of
CCT diagram.
 Before the steel enters in the bainitic region,
either pearIitic trasformation is
complete or starts and its formation and
growth continues even when the steel passes
through the Bs - Bf region

Carbonitriding
Gas carbonitriding
Gas carbonitriding is almost similar to gas
nitriding or gas carburizing
The gas used is mixture of carburisng and
nitriding gases
However, it produces a case equivalent to
that of cyaniding and hence it is often
known as dry cyaniding, gas cyaniding,
nitro-carburizing,and ni-carbing

Hardenability of a steel is defined as the
property which determines the depth and
distribution of hardness induced by
quenching from the austenite.
It is evaluated by determining the minimum
cooling rate to transform an austenitized steel
to a structure that is predominantly or entirely
martensitic
Hardenability

 Jominy End Quench Test
 Hardenability is most commonly measured by the
Jominy End Quench Test.
 In this test,the specimen dimensions and test
conditions are standardised and are as below:
 The specimen is of cylindrical shape with 25.4 mm (1.0
inch) diameter and approximately 102 mm (4.0 inch) in
length and has a machined shoulder at one end.
 The above specimen is austenitized at a constant
temperature for a fixed time and quickly transferred
to a fixture (quenching jig)
Hardenability

 Water is allowed to flow on the bottom end through a pipe
having inside diameter of 12.7 mm (1/2 inch) for about 20
minutes.
 The distance between the pipe and the bottom end of the
specimen is 12.7 mm (1/2 inch).
 The pressure should be adjusted such that the free
height of water is approximately 64 mm (2.5 inch).
 At this pressure, water forms a complete umbrella over the
bottom surface of the specimen.
 The temperature of water should be between 21 and 27°C.
Hardenability

 The cooling rate is maximum at the quenched end of the
specimen where usually full hardening occurs and
diminishes steadily towards the air cooled end where the
structure is nearly equivalent to that produced by
normalising i.e. all possible rates of cooling, from water
quenching to air cooling are obtained on a single test
piece.
 After quenching, two flat surfaces are ground (about 1.6
mm depth) opposite to each other along
the length of the specimen.
 The hardness (VPN or Rc) is measured at intervals of
1.6mm (1/16 inch) distance from the quenched end.
Hardenability

 The hardness values are plotted as function of
distance from the quenched end and the resulting
curve is called as Jominy hardenability curve.
 The hardness changes most rapidly at a location
where the structure is 50% martensite.
 This distance from the quenched end is reported in
terms of points (1 point = 1/16 inch distance) as
hardenability.
Hardenability

Hardenability
Hardenability is indicated by a dotted line.

Hardenability depends on
 Composition
 Austenitic grain size
 Structure before quenching
Hardenability

Hardenability of a steel increases with an addition
of alloying elements such as Cr, Mo, Ni, W, V .
TTT curve move to the right direction with alloy
addition except Co
This is because substitutional diffusion of alloying
elements is slower than the interstitial diffusion of
C
Alloy steels have higher hardenability than plain
C steels.
Hardenability
Composition

Cr, Mo,
W, Ni
temperature
time
Hardenability

Austenitic grain size
Hardenability also depends on the grain size of
austenite.
Coarse grained austenite has better hardenability
than fine grained austenite.
This is because the grain boundaries
reduce the cooling rate.
Also, since pearlite is nucleated at austenite grain
boundaries, fine grained austenite tends to transform
to pearlite more rapidly than coarse grained
austenite because of more grain boundary area.
Hardenability

Structure before quenching
Inhomogenous austenite shows less hardenability
than homogeneous austenite. The presence of
carbides, nitrides, borides, inclusions, etc. in
austenite will also reduce the hardenability.
Typical jominy curves of high hardenability steels (deep hardening
steels) such as high carbon alloy steels, low hardenability steels
(shallow hardening steels) such as plain carbon steels with more than
0.6% carbon, and non-hardenable steels such as plain carbon steels
with less than 0.2% carbon are shown in fig.
Hardenability

Hardenability
Schematic Jominy Curves of various steels

Hardenability
S.
N.
Hardenability Hardness
1 Hardenability of a steel
determines the depth and
distribution of hardness
induced by quenching from
the austenite.
Resistance to plastic
deformation as measured by
indentation
2 Alloying additions and
carbon both increase the
hardenability of steels but
dominants are alloying
elements
Alloying additions and
carbon both increase the
hardness of steels but
dominants is carbon

Hardenability
S.
N.
Hardenability Hardness
3 As grain size increases
hardenability increases
As grain size increases
hardness decreases
4 Only applicable to steels Applicable for all materials
5 Measured by Jominey
Hardenability test
Measured by Poldi,
Rockwell etc

Surface Heat Treatment
Or
Case Hardening
 Numerous industrial applications require a hard
wear-resistant surface called the case, and a
relatively soft and tough interior called the core.
 This is achieved either by :
1. Increasing the carbon on the surface of a low
carbon (0.1- 0.2% C) or low carbon low
alloy steel and subsequently heat treating the
component in a specific manner to produce
hard and wear resistant surface (i.e. case) and
tough center

Or
Case Hardening
2. Introducing nitrogen in the surface of a tough
steel so as to produce hard nitrided case with no
subsequent heat treatment
3. Introducing carbon and nitrogen in the surface of
a tough steel and subsequently heat treating to
produce hard and wear resistant case
4. Hardening the surface without change of
composition of surface.

Or
Case Hardening
 There are five principal methods of case
hardening:
Carburizing
Nitriding
Carbonitriding
Flame hardening
Induction hardening

Carburizing
 The method of increasing the carbon on the
surface of a steel is called carburizing.
 It consists of heating the steel in the austenitic
region in contact with a carburizing medium,
holding at this temperature for a sufficient
period and cooling to room temperature.
 In the austenitic region, the solubility of
carbon is more and hence the carbon from
medium diffuses e steel i.e. in the austenite.

Carburizing
Depending on the medium used for carburizing, it is
classified into the following types:
 Solid (or Pack or Box) Carburizing
 Gas Carburising
 Liquid Carburizing

Carburizing
 The components to be carburized are packed
with a carbonaceous material in steel or cast
iron boxes and sealed with clay.
 The usual carbonaceous medium consists of
 Hard wood
 Charcoal
 Coke
 Energizer or accelerator such as barium
carbonate, sodium carbonate or calcium
carbonate.

Carburizing

Carburizing
 These boxes are heated to some temperature in the
austenitic region and kept at this temperature until the
desired degree of penetration is obtained.
 Carburizing occurs by the following reactions:

Carburizing
 The maximum carbon at the surface and the case depth depend on
the temperature of carburizing and the time of holding.
 Higher the temperature, higher is the carbon at the
surface and more is the case depth.
 However, higher temperatures lead to excessive grain
coarsening and are not recommended.
 At a given temperature, increase in holding time increases the case
depth without changing the maximum carbon at the surface.
 At the usual temperature of carburizing (925 -
950OC), the case depth varies from 1.0 mm to 2.5
mm for total carburizing times of 6 to 15 hours.

Carburizing
 Gas Carburising
 Here the components are heated in the austenitic region
in the presence of a carbonaceous gas such as methane,
ethane, propane, or butane diluted with a carrier gas such
as flue gas.
 These gases decompose and the carbon diffuses into the
steel components.
 Gas carburizing produces extremely uniform cases with
shorter times

Carburizing
 Gas Carburising
 Also the process can be performed at
somewhat lower temperatures
(900-925°C).
 Gas carburizing is commonly used to
obtain relatively thin cases of high
uniformity. Depths from 0.2 to 0.5 mm can
be obtained in 1 to 2 hours at a
temperature of 900°C.

Carburizing
 Liquid Carburising
 In this method, carburizing is done by immersing the
steel components in a carbonaceous fused salt bath
medium at a temperature in the austenitic region.
 The bath is composed of :
 Sodium cyanide ( 10%)
 Sodium carbonate
 Sodium chloride
 Alkaline earth salts of barium, calcium, or
strontium
(These are usually added to the bath to encourage the
cyanamide shift.)

Carburizing
 In the presence of oxygen, following reactions occur in the
bath

Carburizing
 In the presence of alkaline earth salts, be access of oxygen
to the bath is reduced and if the operating temperature is
kept high, the formation of cyanate (NaCNO) is inhibited.
 Under these conditions, the reaction proceeds chiefly by
the cyanamide shift as:
 This carbon diffuses into the steel and results in
carburizing

Carburizing
 At the usual temperature of liquid carburizing (900-
925°C), some amount of nitrogen also diffuses with
carbon in the steel and results in slight nitriding
which infact helps in increasing the hardness and
wear resistance of the carburized case.
 Use of salt bath offers advantages of rapid and
uniform heat transfer, low distortion, negligible
surface oxidation and rapid absorption of carbon.
 Due to this, highly uniform case depths are obtained
with uniformity of carbon content.

Carburizing
 Case depths from 0.1 to 0.5mm can be obtained
in a period of 1/2 to 1 hour at the usual
carburizing temperature
 Sodium cyanide is highly poisonous and hence
necessary care should be taken during its storage,
use, and disposal.

 Microstructure after carburising
Carburizing

Carbon-concentration gradient in a carburized steel
Carburizing

 Heat Treatment after carburizing
 High carbon content on surface does not mean
high hardness of the surface, unless the carbon
is present in the martensitic form.
 Hence after carburizing, hardening treatment is
necessary to bring the carbon in the martensitic
form.
 The following heat treatments are used:
 Direct Quench
 Double Quench
Carburizing

Carburizing
 Heat Treatment after carburizing

Nitriding
 Nitriding is accomplished by heating the
steel in contact with a source of atomic
nitrogen at a temperature of about 550°C.
 The atomic nitrogen diffuses into the steel
and combines with iron and certain
alloying elements present in the steel and
forms respective nitrides.
 These nitrides increase the hardness and
wear resistance of steels.

Nitriding
 Although at suitable temperatures and
with the proper atmosphere all steels are
capable of forming iron nitrides, the best
results are obtained in those steels that
contain one or more of the major nitride-
forming alloying elements.
 These are aluminum, chromium, and
molybdenum.
 Molecular nitrogen does not diffuse into
the steel and hence is completely
ineffective as a nitriding medium

Nitriding
Nitriding Layer
A nitrided case consists of two distinct
zones
In the outer zone the nitride-forming
elements including iron, have been
converted to nitrides
This region is compound zone commonly
known as the "white layer" because of
its appearance under microscope
 Compound zone consists of two phases namely
epsilon nitride (Fe3N) and gamma prime nitride

Nitriding
Nitriding Layer
In the zone beneath this white layer, is
called as Diffusion zone where only alloy
nitrides get precipitated
 The depth of nitride case is determined
by the rate of diffusion of nitrogen from
the white layer to the region beneath.
The white layer is extremely hard and
brittle tend to chip or spall and fracture
during service, which will cause
accelerated wear and premature failure.

Nitriding
Nitriding Layer
White Layer
Difusion
zone
Core
Material

Nitriding
Nitroalloys
 Plain carbon steels produce only white layer and
therefore are not suitable for nitriding.
 In the presence alloying elements such as AI, Cr, Mo, V,
W, Mn and Ti in solid solution, respective nitrides are
formed
 These nitrides are hard and tough and therefore such a
layer of nitrides does not crack or chip.
 During nitriding of alloy steels, first a white layer is
formed on the surface from which nitrogen diffuses
deeper into the steel and selectively precipitates the
alloy nitride
 At the usual temperature of nitriding (500 - 590OC), these
alloy nitrides precipitate out the form of very fine
needles in the matrix of ferrite.

Nitriding
 Nitroalloys
 Due to this, they remarkably increase the hardness
(1000-1200 VPN) without increasing the brittleness
of steel steel.
 This layer of alloy nitrides appears dark under the
microscope and hence it is identified as dark layer.
 This is the useful layer of the nitrided case.
 Therefore, it is clear that only alloy steels are
suitable for nitriding .These alloy steels are called
Nitroalloys
Typical Nitroalloys:25Cr3Mo55,
40Cr2Al1Mo28, En40B, En41B

Nitriding
The source of atomic nitrogen:
 Molten salt bath containing NaCN
(Liquid Nitriding )
 Dissociated ammonia
(Gas Nitriding )
Molten salt bath similar to that used in liquid
carburising without the addition of alkaline earth
salts

Nitriding
 Liquid Nitriding
 In liquid nitriding, nitriding occurs by the formation
and decomposition of cyanate by the same
reactions as given in liquid carburizing i.e.
 Since the temperature of nitriding is less (550°C),
carbon cannot diffuse into the steel because of
absence of austenite and hence only nitrogen
diffuses into the steel.

Nitriding
 Gas Nitriding
 In gas nitriding the atomic nitrogen is
produced due to the dissociation of
ammonia diffuses into the steel
2NH3 2N + 3H2
 For good results, control over
dissociation rate of ammonia and its
circulation is necessary

Nitriding
Gas Nitriding
 Two Types
 Single-stage Nitriding Process
 Double-stage Nitriding Process (Floe process)
 In the single-stage nitriding process dissociation
is held between 15 and 30 percent by adjusting
the rate of flow
 A temperature in the 500 to 525°C range is
employed.

Nitriding
 Double-stage Nitriding Process (Floe process)
 In the first stage of the double-stage process the
ammonia dissociation is held at 20 percent for a period of
5 to10 h at 525°C
 During this period the white layer is established, and the
useful nitride starts to form by diffusion of nitrogen
through it
 In the second stage, the ammonia dissociation is
increased to 83 to 86 percent, and the temperature is
usually raised to 550 to 570°C.
 During this second stage the gas composition is such that
it maintains only a thin white layer on the finished part.
 This process has the advantage of thin white nitride layer

Nitriding
Single-stage process
Double-stage Floe process shows much smaller
white layer compared with single-stage
Double-stage Floe process

Nitriding
Advantages of Nitriding
 Extremely High Hardness
 Nitrided cases have higher hardness (1000 - 1200 VPN)
than the carburized and hardened cases (maximum 830
VPN equivalent to Rc 65) and have tough core.
Resistance to Tempering
 They maintain high hardness upto about 600°C where
most of the hardened steels temper rapidly and become
soft
Wear Resistance
 Extremely high wear resistance is an outstanding
characteristic of the nitrided case
 Wear resistance of the nitrided case is of the order of 10
times more than the carburized case.

Nitriding
 Low Distortion
 Nitriding is carried out at 550°C and the increase in
hardness produced is due to the formation of inherently
hard alloy nitrides.
 It does not require a quenching which is required for
carburized components.
 Therefore, the distortions are minimum.
 It can be applied to finish machine parts requiring close
dimensional tolerances.

Nitriding
 Compressive Stresses
 Nitriding leaves the surface layers of steel parts in high
residual compression
 This effectively reduce the notch sensitivity and sharply
increases the fatigue life of components.
 Corrosion Resistance
 Nitrided cases have better corrosion resistance than the
carburized and hardened components if white layer is
not removed

Nitriding
 Bearing Properties
 Because of non metallic nature of nitrides, nitrided
surfaces have less coefficient of friction.
 They also have exceptionally high resistance to galling
and seizing even under poorly lubricated conditions.
 Due to this, nitrided surfaces have excellent bearing
properties.
Metallic Lustre
 Nitrided surface can be polished to acquire a handsome
metallic lustre

Nitriding
Disadvantages of Nitriding
 Long Cycle
 The nitriding cycle is quite long, depending upon
the case depth desired.
 A case depth from 0.25 to 0.50 mm can be
obtained in a period of 20 to 80 hrs at the usual
temperature ( 550°C) of nitriding
 This makes nitriding expensive
 Thin Cases
 Nitrided cases are relatively thin, usually less than
0.5 mm.

Nitriding
 Long Cycle

Nitriding
 Nitralloy
Use of special alloys called as Nitroalloys steels are required
if maximum hardness is to be obtained
Such steels are costly.
Cyanide Bath and Dissociated Ammonia
 Sodium cyanide is highly poisonous and hence necessary
care should be taken during its storage, use, and disposal.
 Cost of ammonia atmosphere is high and the technical
control required which is difficult

Nitriding
 White Layer
 The presence of white layer on the surface is necessary
for the nitriding reactions and very thin white layer (0.02
to 0.06 mm) remains on the component surfaces when
nitriding is completed.
 The white layer is extremely hard and brittle, tend to chip
or spall and fracture during service, which will cause
accelerated wear and premature failure.
 This white layer has to be removed by precision grinding
or lapping which is difficult and expensive.
Core Properties
 No heat treatment can be done after nitriding
 Therefore, the core properties should adjusted before the

Nitriding
 Applications
 Typical applications are aircraft,
automobile and machine tool components
such as crankshafts, gudgeon pins, shackle
pins, pistons, cylinders liners, shafts
precision gears
 In aircraft it is used extensively for aircraft
engine parts such cylinder liners, valve
stems, shafts, and piston rods.

Carbonitriding
 In this process, both carbon and nitrogen are
diffused into the surface
 The case contains 0.6 to 0.8% carbon and 0.3 to
0.5 % nitrogen
 It has been observed that the nitrided cases
containing more carbon show better behavior
in service
 The source of carbon and nitrogen :
 Fused salt bath
 Gaseous medium containing CH4, C2H6
etc (carburizing gases) with 5 to 10%
ammonia.

Carbonitriding
 The temperature of the process is between A1
and A3 of the steel (i.e. between 750-8500C),
but usually slightly above A1
 The phases present in steel at this temperature
are ferrite and austenite
 Nitrogen diffuses in ferrite and carbon diffuses
in austenite.
 If temperature is lower, nitrogen diffusion is
promoted and the process becomes similar to
nitriding and if temperature is higher, carbon
diffusion is promoted and the process
approaches to that of carburizing

Carbonitriding
 Nitrogen absorption at the surface of steel
retards carbon diffusion so much that, within
hour or so, further increase in case depth
becomes extremely slow.
 Therefore, the treatment times for
carbonitriding are usually less than one hour
and correspondingly the case depth are also
smaller (0.075 to 0.25 mm).
 To transform the carburized areas into fine
martensite the carbonitrided steel is always
quenched from the carbonitriding temperature
in oil or water.

Carbonitriding
 Nitrogen and carbon diffusion increases the
harden ability of surface steel and hence in
many cases oil quenching is sufficient to
produce martensite.
 The components are generally not ground or
lapped after carbonitriding because of very
small case depth.
 Best results from carbonitriding are obtained
when the steel is of an alloy type suitable for
nitriding.

Carbonitriding
Depending on the medium used in this process, the
process is called
Liquid carbonitriding
Fused salt bath
 Gas carbonitriding
Gaseous medium containing CH4, C2H6
etc (carburizing gases) with 5 to 10%
ammonia

Carbonitriding
Liquid carbonitriding (Cyaniding)
Liquid carbonitriding is very much similar to liquid
nitriding and is done in a similar salt bath
containing higher amount of sodium cyanide (20-
30%).
This is also called cyaniding (or cyanide
hardening) because of the use of cyanide salt bath.
Cyaniding and liquid carburizing are also almost
similar processes

Carbonitriding
Liquid carbonitriding (Cyaniding)
S.
N.
Cyaniding Liquid Carburising
1 The salt bath used in
cyaniding does not
contain alkaline earth
salts
These salts are present
in liquid carburizing
bath.
2 Cyaniding is
performed in a bath
containing a higher
percentage of sodium
cyanide
(20 to 30%)
Cyaniding is performed
in a bath containing a
lower percentage of
sodium cyanide (10%)

Carbonitriding
S.
N.
3 The case produced by
cyaniding is higher in
nitrogen and lower in
carbon content
The case produced by
carburising is higher in
carbon and lower in
nitrogen content
4 Cyanided case depths are
less
(usually 0.075 to 0.25
mm )
Liquid carburizing
permits thick cases
(usually 0.1 to 0.5 mm)

Carbonitriding
S.
N.
5 Time required for
process is less
Time required for process
is bit more
6 Temperature:
750 to 850 0C
Temperature:
900 to 925 0C

Tufftride
 Tufftride is a salt bath nitrocarburising process
patented by M/s Degussa, West Germany, developed
for the improvement of surface properties like wear
resistance, anti-seizing and anti-galling properties,
fatigue strength and corrosion resistance of
components made from any steel, cast iron and
sintered iron.
 This process does not make use of poisonous salts like
sodium or potassium cyanide
 The compound zone formed in this process mainly
consists of a ductile carbon bearing epsilon iron nitride
which is responsible for the wear and galling
resistance of tufftrided components
 Hardness of case varies from 400 to 1200 VHN
depending on steel

Tufftride
Time
Temperature
Air
3500C
580-6100C
AB1
330-4000C
Heat Treatment Cycle

Tufftride
 Process
1.Pre-treatment:
 The compound to be treated should be free
from oxides, grease, oil, etc before being
inserted into the bath
 This is achieved by degreasing process.
 After this the components are preheated in a
pre-heating furnace in air at about 3500C
 This ensures that the components are dry and
temperature of tuffride bath will not fall below
5400C since ductile carbon bearing epsilon iron
nitride can’t be formed below 5400C

Tufftride
 Process
2.Nitriding Treatment:
 After pre-treatment the components are
charged into the tufftride bath maintained at
5800C to 6100C usually in the range of 10 to 180
min
 Bath contains only 3 to 4% cyanide
3. Quenching in AB1 salt bath:
 Components after treatment quenched in hydroxide salt
bath AB1 maintained at 330 to 4000C and after that
quenched in water.
 Quenching in AB1 reduces distortion and
oxidized the cyanides to convert them into carbonates
 AB1 salt bath also impart cosmetic black finish on parts

Tufftride
Advantages:
 Cyanide free process
 Since case is not very hard drawing, rolling, revetting can
be possible
 Compare to nitriding time required is very less
 Tufftriding yields outstanding improvements in wear and
fatigue resistance compare to conventional case
hardening methods
 Nitoalloys are not necessary
 Remarkable improvement in corrosion resistance
 Very very less distortion
 Grinding operation is not required to eliminate white
layer
 Cosmetric black colour add to the asthetics

Sursulf Process
 An almost similar process like Tufftide is
developed in France by M/s Hydromecanique et
Frottement and is called the SURSULF process.
 SURSULF process is also a non-polluting,
nonpoisonous liquid bath nitriding process in
which the bath consists of alkaline cyanates and
carbonates stabilized by the addition of lithium
compounds and very small quantity of sulphur
compounds.

Sursulf Process
 The treatment consists of immersing the steel
components in the above salt bath at 560
570°C for about 1.5 hours.
 This forms a compound zone rich in nitrogen and
containing sulphur which sharply improves many
properties such as wear resistance, corrosion
resistance, fatigue resistance, scuffing resistance
and resistance to seizure.
 The compressive stresses produced on the surface
due to Sursulf process are more than produced by
Tufftrdeing process and hence the improvement in
fatigue resistance is better than any other existing
similar surface hardening process.

Flame Hardening
The remaining two methods, flame hardening and
induction hardening do not change the chemical
composition of the steel
They are essentially shallow hardening methods.
Selected areas of the surface of a steel are heated
into the austenite range and then quenched to form
martensite.
Therefore, it is necessary to start with a steel which
is capable of being hardened.
Generally, this is in the range of 0.30 to 0.60 percent
carbon.

Flame Hardening
 Flame Hardening is a process of heating the surface layer
of a hardenable steel to above its upper critical temperature
by means of oxyacetylene flames followed by water spray
quenching or immersion quenching to transform austenite to
martensite.
 Depth of the hardened zone is controlled by
Adjustment of the flame intensity
Distance between the gas flames and the
component surface
Heating time
Speed of travel.
Skill is required in adjusting and handling manually
operated equipment to avoid overheating the work because
of high flame temperature.

Flame Hardening
Flame Head with Integral Quenching

Flame Hardening
Four methods are in general use for
flame hardening:
(1)Stationary
(2)Progressive
(3)Spinning
(4)Progressive-spinning

Flame Hardening
In all above methods, provision is made for
rapid quenching after the surface has been
heated to the required temperature.
This is accomplished by the use of water
sprays, or by quenching the entire piece in
water or oil
 After quenching, the part should be stress-
relieved by heating in the range of 100 to
200°C and then air cooled.

Flame Hardening
Advantages of flame hardening
Adaptability & Portability
The equipment can be taken to the job and adjusted to
treat only the area which requires hardening. Parts too
large to be placed in a furnace can be handled easily and
quickly with the torch.
The ability to treat components after surface
finishing, since there is little scaling, decarburization or
distortion.
Flame hardening causes less distortion than conventional
hardening and due to high heating rate, oxidation and
decarburization are minimum.

Flame Hardening
Disadvantages
(1) The possibility of overheating and thus damaging the part
Overheating can result in cracking after quenching and
excessive grain growth
(2) Difficulty in producing hardened cases less than 1mm depth
The depth of hardened layer can be varied from 1mm to a
maximum of about 5 mm.
(3) Difficult to control the case depth

Induction Hardening
Here heating is done within thin layer surface
metal by using high frequency induced currents.
The component is heated by means of an inductor
coil which consists of one or several turns of water-
cooled copper tube.
High frequency alternating currents flowing
through the inductor generate alternating magnetic
field.
This electromagnetic field induces eddy currents
of the same frequency in the surface layers which
rapidly heat the surface of the component.

Induction Hardening
Within a short period of 2 to 5 minutes, the
temperature of surface layer comes to above the
upper critical temperature that steel.
The high frequency induced currents chiefly flow
through the surface layer (phenomenon known as
Shin effect)
The layer through which these currents flow is
inverse proportional to the square root of
frequency of induced currents and hence, the
depth of hardened layer can be controlled by
controlling the frequency of supply voltage.

Typical induction heating setup. High frequency
alternating current in a coil induces current in the
workpart to effect heating
Induction Hardening

Induction Hardening
The usual range of frequency is from 1000 Hz to
1,00,000 Hz and the hardened depths obtained at
from 0.5 to 6 mm.
 After the necessary temperature is attained, the
component is quenched by water spray usually
without removing from the inductor coil.
Due to very fast heating and no holding time, the
austenitic grain size is very fine which results in fine
grained martensite.
Induction hardening is commonly followed by low
temperature tempering at 160 to 200°C.

Induction Hardening
Steels with carbon content from 0.4 to 0.5% are
most suitable for induction hardening.
However, case carburized components can also be
hardened.
Some of the examples are crank-shafts, camshafts,
axles, gears, rolls of rolling mills, boring bars, brake
drums, overhead travelling crane wheels, etc.

Induction Hardening
Advantages
1. The special advantage is rapid heating of surface
without an appreciable rise in the temperature
of core
2. Fast heating and no holding time leads to
increase in production rates.
3. No scaling and decarburization
4. Less distortion because of heating of only
surface.
5. Easy control over the depth of hardening by
control of frequency of supply voltage and/or
time of holding.

Induction Hardening
Drawbacks
1. Irregular shaped parts are not suitable for
induction hardening.
2. Because of high cost of induction hardening
unit, the process is not economical for small
scale production

Heat Treatment of High Speed Steel
(HSS)
 These steels are used for cutting of metals at high
speeds.
 They maintain their high hardness upto a temperature
of 5500C
 They also have high wear resistance.
 The most widely used HSS is T1 which cantains 0.7%C,
18% W, 4% Cr and 1% V (IS : T 70W18Cr4 V1) famously
known as 18-4-1 HSS
 W, Mo, Cr and V are carbide formers and hence form
carbides.
 These alloy carbides increase red hardness, wear,
and cutting ability at high temperatures.
 In this respect vanadium is most powerful.
 It also increases the resistance to grain coarsening

(HSS)
Pseudo-binary phase diagram for alloys of iron plus 18% W 4%
Cr and 1%V with varying carbon content

(HSS)
A pseudo-binary phase diagram for 18-4-1
HSS shows following changes as compare to
Fe-C diagram:
 A1 (eutectoid temperature) rises from 727°C to
840°C.
 Eutectic temperature rises from 1147°C to
1330° C
 Eutectoid carbon decreases from 0.8% to 0.25%
 Maximum solubility of carbon in austenite
reduces from 2.0% to 0.7%

(HSS)
Following heat treatment sequence is
followed for 18-4-1 HSS
 Preheating
 Soaking
 Quenching
 Multiple
Tempering
 Preheating
 Soaking
 Quenching
 Sub-Zero
 Tempering
or

Hardening and tempering cycle of 18-4-1 HSS

Unit 4.ppt

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Unit 4.ppt