The document discusses the microstructure of austenite: 1. Austenite is the gamma phase of iron that forms above 723°C and has a face-centered cubic crystal structure. It can dissolve up to 2.13% carbon at 1147°C. 2. In carbon steels, austenite forms upon heating by the dissolution of cementite and ferrite. The rate of austenite formation depends on factors like temperature, heating rate, carbon content, and alloying elements. 3. Austenite grain size and morphology affect the properties of the materials that form after austenite transforms upon cooling. Finer austenite grains lead to

1. Bankim Chandra Ray
NIT Rourkela

3. M i c r o s t r u c t u r e o f a u s t e n i t e

4. BCC
FCC
octahedral

5.  Austenite, also known as gamma phase iron (γ-
Fe), is a metallic, non-magnetic allotrope of iron or
a solid solution of carbon in gamma iron (γ-Fe) .
 It forms above 723oC .
 It has a FCC crystal structure.
 The maximum solubility of carbon in austenite is
2.13 % at 1147oC .

6.  Austenite can transform into various products
depending on the composition and cooling rates.
 Morphology of parent austenite(grain size) decides
the morphology of products and thus its
properties.

7.  Austenite is formed on heating an
aggregate of ferrite and pearlite, ferrite
and cementite or cementite and pearlite,
depending on whether the steel is of
hypo-eutectoid, eutectoid or
hypereutectoid type.
 Formation of austenite in eutectoid steel
occurs at a particular temperature ( AC1)
whereas in hypo-eutectoid or
hypereutectoid occurs over a range of
temperature.

8.  The alternate mixture of ferrite and cementite in
eutectoid steel is known as pearlite.
 Cementite has 6.67 wt.% C whereas ferrite is almost
pure iron, free of carbon.
 Inspite of the carbon gradient the structure is
thermodynamically stable at room temperature due to the
low diffusion rate of carbon at low temperatures and
occurs only at sufficiently high temperatures

9.  1st step: ( On heating to eutectoid
temperature)
Lattice changes
BCC iron (α-Fe) FCC iron (γ-Fe)
 2nd step:
Diffusion of carbon from Cementite
(6.67% carbon)
to adjoining regions.

10. α-Fe
α-Fe
Fe3C
Fe3
C
Austeni
te

11.  The maximum diffusion of carbon takes place
from cementite at ferrite –cementite interface.
 Austenite nucleates at interfaces between
ferrite and cementite, specially in between
pearlitic colonies.
 By gradual dissolution of carbon from cementite
austenite is formed.
 The primary austenite formed dissolve the
surrounding ferrite and grow at their expense.
 The growth rate of austenite is higher than the
rate of dissolution of cementite.
 Thus dissolution of ferrite is complete before
that of cementite.

12.  The austenite formed from cementite and
ferrite is generally not homogenous.
 Homogenization requires high
temperature/time , or both.
 High temperatures if the rate of heating is
faster.
 Shorter time spread over a large range of
temperatures if the rate of heating is slower.

13.  Mixture of Pro-eutectoid ferrite and pearlite.
 On slow heating, austenite nuclei are formed
just above the eutectoid temperature.
 More nuclei will form with increase in
temperature.
 At first, the austenitic grains will grow by the
growth of initially formed austenitic grains
and then by the growth of newly formed
austenite nuclei.

14.  The austenite formed is non-homogeneous
due to the presence of embedded cementite
particle within the austenitic grains.
 Thus, for hypo-eutectoid steels, growth of
primary austenitic grains take place at the
expense of pro-eutectoid ferrite.

15.  Mixture of pro-eutectoid cementite and
austenite.
 Cementite dissolves into ferrite which in turn
transforms into austenite.
 Thus, for hyper-eutectoid steels, growth of
primary austenitic grains take place at the
expense of pro-eutectoid cementite.

17.  The formation of austenite on heating occurs by
nucleation and growth
 The factors that affect nucleation rate or growth
rate affect the kinetics of the transformation
 The kinetics depends on:
 Transformation temperature and holding time
 Rate of heating
 Interface between ferrite and cementite
 Grain size
 Nature of the alloying elements present

18.  Transformation Temperature:
 Austenite transformation occurs at a
temperature higher than Ac1 in the Fe-
Cementite phase diagram – Superheating
 Equilibrium temperatures are raised on heating
and lowered on cooling ( free energy should be
negative)
 The rate of austenite formation increases with
increase in temperature as it increases the rate
of carbon diffusion and the free energy is more
negative
 Interdependence of time and temperature :
Transformation takes a shorter time at higher
temperatures of transformation and vice versa

19.  Rate of heating :
 For higher rates of heating, transformation
starts at higher temperatures and for
slower rates, at lower temperatures
 For any rate of heating transformation
occurs over a range of temperature
 For transformation at a constant
temperature, heating rate should
extremely slow
 Special note:
Austenite transformation starts as soon as
the eutectoid temperature is reached, but
the region in between the curves indicates
the majority of the tranformation.

20. Interface between ferrite and cementite:
Higher the interfacial area faster is the transformation
Interfacial area can be increased by:
 Decreasing the inter-lamellar spacing between ferrite and
cementite
The closer the ferrite – cementite lamellae, the higher is the
rate of nucleation.
Carbon atoms have to diffuse to smaller distances from
cementite to low carbon regions to form austenite
 Increasing the cementite or carbon content
This will lead to more pearlite content in steels and thus more
interfaces.
 Examples :
1. High carbon steels austenize faster than low carbon steels
2. Tempered martensite structure austenizes faster than
coarse pearlite
3. Spheroidal pearlite takes longer time to austenize due to
very low interfacial area

21.  Grain size:
 The coarser the parent grain size the slower is
the transformation rate
This is because in larger grains the interfacial
area is lesser
 The smaller is the parent grain size the faster
is the transformation to austenite

22.  Nature of the alloying elements present:
 Alloying elements in steel are present as
alloyed cementite or as alloy carbides.
 Alloy carbides dissolve much more slowly
than alloyed cementite or cementite.
 The stronger the alloy carbide formed the
slower is the rate of formation of
austenization.
 Diffusion of substitutional alloying
elements is much slower than the
interstitial element, carbon.
 Thus the rate of austenization depends on
the amount and nature of alloying element.

23.  In hypoeutectoid steels, austenisation process takes
place rapidly as carbon content increases.
 As carbon percentage increases, the amount of
pearlite increases, which increases the interfacial
area between ferrite and cementite
 Thus Ac3 temperature line decreases continuously
with increasing carbon content

24. In hypereutectoid steels , austenization
process becomes much more difficult as the
amount of carbon increases
 Austenisation of free cementite needs very
high temperature as it involves the
diffusion of large amount of carbon( from
cementite) to become homogenous
 Thus as carbon content increases, amount
of free cementite increases, which needs
higher temperature to austenize.
Thus Acm line is so steep

25.  Original grain size- size of austenite grains
as formed after nucleation and growth
 Actual grain size – size of the austenitic
grains obtained after homogenization at
higher temperatures
 Generally grain size is referred to as actual
grain size
 Depending on the tendency of steel to
grain growth, steels are classified into two
groups:
 Inherently fine grained
 Inherently coarse grained

26.  Inherently fine grain steels resist grain growth with increasing
temperature till 1000oC – 1050oC
 Inherently coarse grain steels grow abruptly on increasing
temperature
 On heating above a certain temperature T1 inherently fine
grain steels give larger grains than inherently coarse grain
steels
Grain
size
Inherently fine grain
Inherently coarse grain

27. Presence of ultramicroscopic particles
like oxides, carbides and nitrides present
at grain boundaries prevent grain growth
in inherently fine grain steels till very
high temperatures.
They act as barriers to grain growth.
Steels deoxidized with Al or treated with
B, Ti and V are inherently fine grained.
At temperatures above T1,dissolution of
ultramicroscopic particles cause sudden
increase in grain size.
Thus inherently fine grain steels can be
hot worked at high temperatures without
getting coarsened.

28.  Austenite grain size plays a very important role in
determining the properties of the steel
 The effect of grain size on different properties are given
below:
 YIELD STRESS
 The dependence is given by Hall-Petch equation :
 Where is the yield stress
 is the frictional stress opposing motion of
dislocation
 K is the extent to which dislocations are piled at
barriers
 D is the average grain diameter

29.  Grain refinement improves the strength and
ductility at the same time.
 IMPACT TRANSITION TEMPERATURE
 Increase in grain size raises the impact transition
temperature, so more prone to failure by brittle
fracture.

30.  CREEP STRENGTH
 Coarse grained steel has better creep strength above
equicohesive temperature.
 Below this fine grain structure have better creep strength.
 FATIGUE STRENGTH
 Fine grained steel have higher fatigue strength.
 HARDENABILITY
 Coarse grained steels have higher hardenability.
 (smaller grain boundary area in coarse grained structure
gives less sites for effective diffusion, so martensite
formation on cooling is favoured ).
 MACHINABILITY
 Coarse grain structure has better machinability due to ease
in discontinuous chip formation(low toughness).

32.  T (Time) T(Temperature)
T(Transformation) diagram is a plot of
temperature versus the logarithm of time
for a steel alloy of definite composition.
 It is used to determine when
transformations begin and end for an
isothermal (constant temperature) heat
treatment of a previously austenitized
alloy
 TTT diagram indicates when a specific
transformation starts and ends and it also
shows what percentage of transformation

33. 33
TTT CURVE
• Transforming one phase into another takes time.
• How does the rate of transformation depend on
time and T?
• How can we slow down the transformation so that
we can engineering non-equilibrium structures?
• Are the mechanical properties of non-equilibrium
structures better?
Fe
g
(Austenite)
Eutectoid
transformation
C FCC
Fe3C
(cementite)
a
(ferrite)
+
(BCC)

34. Time Temperature
Transformation(TTT)
curves

35. 35
• Reaction rate is a result of nucleation and growth
of crystals.
• Examples:
Adapted from
Fig. 10.10, Callister 7e.
% Pearlite
0
50
100
Nucleation
regime
Growth
regime
log(time)
t0.5
Nucleation rate increases with T
Growth rate increases with T
T just below TE
Nucleation rate low
Growth rate high
g
pearlite
colony
T moderately below TE
g
Nucleation rate med .
Growth rate med.
Nucleation rate high
T way below TE
g
Growth rate low

36. Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard
• Transformation of austenite to pearlite:
g
a
a
a
a
a
a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
g
• For this transformation,
rate increases with ( T)
[Teutectoid – T ].
675°C
(T smaller)
0
50
%
pearlite
600°C
(T larger)
650°C
100
Diffusion of C
during transformation
a
a
g
g
a
Carbon
diffusion
Eutectoid Transformation Rate ~ T

37. • The Fe-Fe3C system, for Co = 0.76 wt% C
• A transformation temperature of 675°C.
100
50
0
1 102 104
T = 675°C
%
transformed
time (s)
400
500
600
700
1 10 102 103 104 105
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Consider:

38. Isothermal Transformation Diagrams
2 solid curves are plotted:
 one represents the time
required at each
temperature for the start
of the transformation;
 the other is for
transformation
completion.
 The dashed curve
corresponds to 50%
completion.
The austenite to pearlite
transformation will occur
only if the alloy is
supercooled to below the
eutectoid temperature
(727˚C).
Time for process to
complete depends on the
temperature.

39.  Think of some austenite, lowered suddenly to a
temperature below 727C and allowed to
transform at that temperature.
 At high temperature, atoms can diffuse rapidly
BUT, nucleation rates are very low due to only
slight undercooling. Therefore, the overall
transformation tends to be lengthy.
 At low temperature, nucleation is very speedy,
but diffusion is slow. Therefore the
transformation tends to be lengthy.
 At some intermediate temperatures there must
be an optimum. Thus we get a C-shaped curve.

40. Iron-carbon alloy
with eutectoid
composition.
 A: Austenite
 P: Pearlite
 B: Bainite
 M: Martensite

42.  Lower half of TTT Diagram (Austenite-Martensite and Bainite
Transformation Areas)

43. Pearlitic Steel partially transformed to
Spheroidite

44. Different types of Time- Temperature-
Transformation (TTT) Curves
 Three types of curves are there depending on the carbon content of steel:
► TTT for hypereutectoid steel
► TTT for eutectoid steel
► TTT for hypo eutectoid steel

45. Hypereutectoid composition – proeutectoid cementite
Consider C0 = 1.13 wt% C
Fe
3
C
(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a+Fe3C
L+Fe3C
d
(Fe)
C, wt%C
T(°C)
727°C
T
0.76
0.022
1.13

46. 46
Adapted from Fig.
10.29, Callister 7e.
(Fig. 10.29 based on
data from Metals
Handbook: Heat
Treating, Vol. 4, 9th
ed., V. Masseria
(Managing Ed.),
American Society for
Metals, 1981, p. 9.)
Adapted from Fig. 9.30,Callister
7e. (Fig. 9.30 courtesy Republic
Steel Corporation.)
Adapted from Fig.
9.33,Callister 7e.
(Fig. 9.33 copyright
1971 by United States
Steel Corporation.)
• More wt% C: TS and YS increase , %EL decreases.
• Effect of wt% C
Co < 0.76 wt% C
Hypoeutectoid
Pearlite (med)
ferrite (soft)
Co > 0.76 wt% C
Hypereutectoid
Pearlite (med)
Cementite
(hard)
300
500
700
900
1100
YS(MPa)
TS(MPa)
wt% C
0 0.5 1
hardness
0.76
Hypo Hyper
wt% C
0 0.5 1
0
50
100
%EL
Impact
energy
(Izod,
ft-lb)
0
40
80
0.76
Hypo Hyper

47. 47
Adapted from Fig. 10.30, Callister 7e.
(Fig. 10.30 based on data from Metals
Handbook: Heat Treating, Vol. 4, 9th
ed., V. Masseria (Managing Ed.),
American Society for Metals, 1981, pp.
9 and 17.)
• Fine vs coarse pearlite vs spheroidite
• Hardness:
• %RA:
fine > coarse > spheroidite
fine < coarse < spheroidite
80
160
240
320
wt%C
0 0.5 1
Brinell
hardness
fine
pearlite
coarse
pearlite
spheroidite
Hypo Hyper
0
30
60
90
wt%C
Ductility
(%AR)
fine
pearlite
coarse
pearlite
spheroidite
Hypo Hyper
0 0.5 1

49. 49
Hypereutectoid composition – proeutectoid cementite
Consider C0 = 1.13 wt% C
a
TE (727°C)
T(°C)
time (s)
A
A
A
+
C
P
1 1
0
102 103 104
50
0
70
0
90
0
60
0
80
0
A
+
P
Adapted from Fig. 11.16,
Callister & Rethwisch 3e.
Adapted from Fig.
10.28, Callister &
Rethwisch 3e.
Fe
3
C
(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a+Fe3C
L+Fe3C
d
(Fe)
C, wt%C
T(°C)
727°C
T
0.76
0.022
1.13

50. TTT curves for hypo,
eutectoid and
hyper-eutectoid steels

51.  Other elements (Cr, Ni, Mo, Si and
W) may cause significant changes
in the positions and shapes of the
TTT curves:
 Change transition temperature;
 Shift the nose of the austenite-to-
pearlite transformation to longer
times;
 Shift the pearlite and bainite noses
to longer times (decrease critical
cooling rate);
 Form a separate bainite nose;
Effect of Adding
Other Elements
4340 Steel
plain
carbon
steel
nose
 Plain carbon steel: primary
alloying element is carbon.

52.  Hardness
 Brinell, Rockwell
 Yield Strength
 Tensile Strength
 Ductility
 % Elongation
 Effect of Carbon Content

53.  Advantages:
 Improved ductility with same hardness
 Elimination of distortion and cracks
 No tempering required
 Impact strength is improved
 Uniformity in properties
 High endurance limit
Austenite
Pearlite
Pearlite + Bainite
Bainite
Martensite
100
200
300
400
600
500
800
723
0.1 1 10 102 103 104
105
Eutectoid temperature
Ms
Mf
t (s) →
T
→
a + Fe3C
Austempering

54.  1015 steel – plain carbon – 0.15%C
 1090 steel – plain carbon – 0.90%C
 What happens as carbon content increases? In
general, we see more and more pearlite in slow
cooled steels. More and more cementite
available in all steels. Strength  up. Ductility
 down.
 BUT, AT A GIVEN CARBON CONTENT, WIDELY
VARYING PROPERTIES ARE AVAILABLE DEPENDING
ON PROCESSING.

55. Mechanical Properties: Influence of Carbon Content
C0 > 0.76 wt% C
Hypereutectoid
Pearlite (med)
Cementite
(hard)
C0 < 0.76 wt% C
Hypoeutectoid
Pearlite (med)
ferrite (soft)

56. Mechanical Properties: Fe-C System

57. Continuous
Cooling
Transformation
Diagrams

58. Continuous Cooling
Transformation Diagrams
 Isothermal heat treatments are
not the most practical due to
rapidly cooling and constant
maintenance at an elevated
temperature.
 Most heat treatments for steels
involve the continuous cooling
of a specimen to room
temperature.
 TTT diagram (dashed curve) is
modified for a CCT diagram
(solid curve).
 For continuous cooling, the time
required for a reaction to begin
and end is delayed.
 The isothermal curves are
shifted to longer times and
lower temperatures.

59.  Moderately rapid and slow
cooling curves are
superimposed on a
continuous cooling
transformation diagram of a
eutectoid iron-carbon alloy.
 The transformation starts
after a time period
corresponding to the
intersection of the cooling
curve with the beginning
reaction curve and ends
upon crossing the completion
transformation curve.
 Normally bainite does not
form when an alloy is
continuously cooled to room
temperature; austenite
transforms to pearlite before
bainite has become possible.

60.  For continuous cooling of a
steel alloy there exists a
critical quenching rate that
represents the minimum
rate of quenching that will
produce a totally
martensitic structure.
 This curve will just miss the
nose where pearlite
transformation begins

61.  Continuous cooling
diagram for a 4340 steel
alloy and several cooling
curves superimposed.
 This demonstrates the
dependence of the final
microstructure on the
transformations that
occur during cooling.
 Alloying elements used to
modify the critical cooling
rate for martensite are
chromium, nickel,
molybdenum,
manganese, silicon and
tungsten.

62. Eutectoid steel (0.8%C)
100
200
300
400
600
500
800
723
0.1 1 10 102 103 104
105
t (s) →
T
→
Different cooling treatments
M = Martensite
P = Pearlite
Coarse P
P
M M + Fine P

63. Austenite (g)
Bainite
(a + Fe3C plates/needles)
Pearlite
(a + Fe3C layers + a
proeutectoid phase)
Martensite
(BCT phase
diffusionless
transformation)
Tempered
Martensite
(a + very fine
Fe3C particles)
slow
cool
moderate
cool
rapid
quench
reheat
Strength
Ductility
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends

65.  Annealing generally involves heating to a
predetermined temperature, holding at this
temperature and finally cooling at a very
slow rate
 The temperature and holding time depend on
a variety of factors such as composition,
size, shape and final properties desired
 Annealing treatment can be classified into
subdivisions based on temperature of
treatment, phase transformation occuring
during the treatment and the purpose of the
treatment

66.  Annealing serves the following purposes:
1. Relieve internal stresses developed during
solidification, machining, forging, rolling
and welding
2. Improve or restore ductility and toughness
3. Enhance machinability
4. Eliminate chemical non uniformity
5. Refine grain size
6. Reduce gas content in steel

67.  Depending on heat treatment temperature,
annealing processes are sub divided as:
1. Full annealing(above upper critical temp A3)
2. Partial annealing(between LCT and UCT)
3. Subcritical annealing(below LCT)
 In subcritical annealing no phase
transformation takes place, only thermally
activated processes such as recovery,
recrystallization and growth takes place

69.  Depending on the specific purpose, annealing
is divided into various types such as:
1. Diffusion annealing
2. Spheroidising annealing
3. Recrystallization annealing, etc.

70. Thus all in all, the various types of annealing processes
are:
 Full annealing
 Homogenising annealing
 Recrystallization annealing
 Spheroidisation annealing
 Stress-relief annealing
 Isothermal annealing

72. Schematic diagram showing approximate temperature ranges superimposed on
the Fe-C diagram for various heat treatments applied to steels. Steels,
Processing, Structure and Properties- George Krauss

73.  Full annealing is the process by which the distorted cold
worked lattice structure is changed back to one which
is strain free through the application of heat. This
process is carried out entirely in the solid state and is
usually followed by slow cooling in the furnace from the
desired temperature.

74.  Full annealing, one of several types of
annealing, is the heat treatment in which
steels are heated just above the Ac3
temperature for low- and medium-carbon
steels and just above the Ac1 temperature
for hypereutectoid steels, and slowly cooled
in furnaces after heating has ceased.
 For hypoeutectoid steels and eutectoid steel
 Ac3+(20-40oC) [to obtain single phase austenite]
 For hypereutectoid steels
 Ac1+(20-40oC) [to obtain austenite+ cementite]

75.  The three important parts of full annealing
are:
 Proper austenitising temperature
 Soaking time
 Very slow cooling through A1(critical temperature)
 Proper Austenitising Temp: the austenitising
temp varies with variation in carbon%. Proper
austenitising temp is required to get fine
grains of austenite

76.  Soaking Time: soaking at the austenitising temp
is of utmost importance as it leads to formation
of homogeneous austenite
 Very Slow Cooling through A1: this is done so
that austenite always transforms at temp just
below A1 to obtain equiaxed and relatively
coarse grained ferrite as well as pearlite with
large interlamellar spacing to induce softness
and ductility.
 The formation of austenite destroys all
structures that have existed before heating.
Slow cooling yields the original phases of ferrite
and pearlite in hypoeuetectoid steels and that of
cementite and pearlite in hypereutectoid steels.

77.  The slow cooling of full annealing causes
austenite transformation to ferrite and
pearlite close to A3 and A1 temperatures,
respectively, and ensures that coarse-grained
equiaxed ferrite and pearlite with coarse
interlamellar spacing will form, producing
microstructures of high ductility and
moderate strength
 Once the austenite has fully transformed to
ferrite and pearlite, the cooling rate can be
increased to reduce processing time and
thereby improve productivity

78.  Although ferrite and pearlite microstructures
are most often produced by full annealing at
the temperatures, microstructures of
spheroidized carbide particles in ferrite may
sometimes form
 Such microstructures are a result of the
divorced eutectoid transformation
 Austenite transforms to spheroidized
carbide/ferrite microstructures instead of
the lamellar ferrite/cementite structure of
classical pearlite

80.  The microstructure consists of cementite
particles dispersed in a matrix of ferrite
 Here, austenite transforms to the dispersed
cementite/ferrite microstructure instead of the
classical lamellar ferrite/cementite
microstructure
 The dispersed cementite/ferrite microstructure
typically forms in high-carbon steels and at
temperatures just below A1; at greater amounts
of undercooling, the transformation of austenite
to the lamellar ferrite/cementite structure of
pearlite is favored

81.  To refine the grain size of steel castings, or
of hot worked steels to improve the
mechanical properties.
 To soften the steel
 To relieve internal stresses
 To improve machinability
 It also reduces some defects like aligned
sulphide inclusions, or bands in steels.

83.  This process, also known as diffusion
annealing, is employed to remove any
structural non uniformity in the sample
 Dendrites, columnar grains, and chemical
inhomogenities are generally seen in ingots,
heavy plain carbon steel an alloy steel
castings, etc and these defects promote
brittleness reducing ductility and toughness
of the steel

84.  The subsequent heating, soaking and hot
working homogenises the structure to a large
extent since diffusion of C is very fast at high
temp and the simultaneous plastic
deformation breaks the dendrites with
different portions moving in relation to each
other which also facilitate diffusion.
 The main aim of homogenising annealing is to
make the composition uniform, i.e to remove
chemical heterogeneity

85.  Here, steel is heated sufficiently above UCT,
at about 1000-1200°C
 The steel is held at this temperature for
prolonged time of 10-20 hours
 This is followed by slow cooling
 This treatment eliminates any sort of
chemical non uniformity in the sample.
Segregated zones are eliminated and
chemically homogeneous steel is obtained

86.  The disadvantages of the process are:
1. Higher temperatures
2. Longer holding time
3. Grain growth
4. Slow cooling rate
5. Excessive cooling
6. Necessity for a second heat treatment
process
7. Highly expensive

88.  All steels that have been heavily cold worked
are subjected to this process of heat
treatment
 The process consists of heating the steel
sample to a temperature above the
recrystallization temperature, holding at this
temperature and cooling thereafter
 This process is used to treat work-hardened
parts made out of low-Carbon steels (< 0.25%
Carbon). This allows the parts to be soft
enough to undergo further cold working
without fracturing.

89.  The temperature for recrystallization
annealing is not fixed, unlike other annealing
processes.
 Recrystallization annealing temperature
depends on amount of prior deformation,
chemical composition, holding time and
initial grain size
 The larger the degree of deformation lower
is the recrystallization temperature
 Increasing holding time allows to
recrystallization to occur at low
temperatures

90.  The main aims of recrystallization annealing
are:
 To restore ductility
 To refine coarse grains
 To improve electrical and magnetic properties in grain-
oriented Si steels.

92.  No phase change takes place and the final structure consists of
strain-free, equiaxed grains of fine ferrite produced at the expense
of deformed elongated ferrite grains.
 Recrystallization temp(Tr) is given by:
 Tr= (0.3-0.5)Tm.p
 As little scaling and decarburisation occurs in recrystallization
annealing, it is preferred over full annealing.
 However It would produce very coarse grains if the steel has
undergone critical amount of deformation. In such cases, full
annealing is preferred.

94.  It is generally used for alloy steels to soften
them
 In this process, hypoeutectoid steel is heated
to a temperature above UCT(20-40°higher)
and held for sometime. This is done so as to
remove any temperature gradient within the
steel component and to get a completely
austenitic structure
 The steel is then rapidly cooled to a
temperature lower than the LCT by
transferring the sample to a furnace
maintained at the desired temperature

95.  The steel is then held at this temperature till
all the austenite gets converted to pearlite.
 Once the transformation is complete, the
steel sample is then cooled in air
 Rate of cooling in air will determine the
amount of residual stresses in the sample
 The microstructure obtained is similar to
that obtained during full annealing.
 The process is generally not applied for
hypereutectoid steels

97.  The advantages of isothermal annealing are:
1. As cooling can be done in air, the time
required for heat treatment process is cut
shirt considerably
2. Shorter heat treatment cycle makes the
process cheaper and also the productivity of
the furnace higher
3. More homogeneous microstructure as the
transformation takes place at constant
temperature
4. Improves machinability and provides better
surface finish

98.  This process cannot be applied for heavy
components as it is not possible to cool them
rapidly and uniformly to holding
temperature. As a result the structure will
not be homogeneous and properties will vary
across the cross section

100.  Spheroidising is a heat treatment process
resulting in a structure consisting of globules
or spheroids of carbides
 In other words, cementite of lamellar
pearlite in case of eutectoid an
hypoeutectoid steels, and both lamellar and
free cementite in case of hypereutectoid
steels coalesce into tiny spheroids
 The degree of spheroidisation depends on
temperature and holding time

101. o Hypereutectoid steels consist of pearlite and
cementite. The cementite forms a brittle
network around the pearlite. This presents
difficulty in machining the hypereutectoid
steels.
o To improve the machinability of the annealed
hypereutectoid steel spheroidize annealing is
applied.

102.  There are various methods of Spheroidising
annealing:
1. Heating steel to a temperature below LCT,
holding at this temperature for prolonged
time, followed by slow cooling

103. 2. It consists of heating and cooling the steel
alternately just above and below the LCT

104.  Improves machinability
 Increases ductility
 Increases softness
 Decreaes hardness and brittleness

106.  Normalizing is a technique used to provide
uniformity in grain size and composition
throughout an alloy.
 The term is often used for ferrous alloys
that have been heated above the upper
critical temperature and then cooled in open
air.
For Steel
• It is a process of heating steel to about 40-
50°C above upper critical temperature (A3 or
Acm),holding for proper time ,then cooling in
still air or slightly agitated air to room
temperature .

109.  To refine the coarse grains of steel castings , forgings , etc. which
have not been worked under high temperatures.
 To improve the mechanical properties of plain carbon steels
particularly forged shafts ,rolled stocks and castings for moderate
load conditions.
 To eliminate , or reduce microstructural irregularities.
 To incresase machinability of low carbon steels.
 To eliminate, or break coarse cementite network in
hypereutectoid steels.
 General refinement of structure prior to hardening of steel.

110.  Normalizing is also used to relieve internal stresses induced by
heat treating, welding, casting, forging, forming, or machining.
 Normalizing also improves the ductility without reducing the
hardness and strength.
 Steel is heated to austenitic temperature and then cooled in air.
Purpose is
• To refine grain structure
• To increase strength of steel
• To reduce segregation in castings or forgings

111.  In special cases cooling rate is controlled
either by air temperature or by changing air
volume.
 Normalizing process consists of three steps.
 The first step involves heating the steel
component above the A3 cm temperature for
hypoeutectoid steels and above A(upper
critical temperature for cementite)
temperature for hypereutectoid steels by
300C to 500C.

112.  The second step involves holding the steel
component long enough at this temperature for
homogeneous austenization.
 The final step involves cooling the hot steel
component to room temperature in still air. Due
to air cooling, normalized components show
slightly different structure and properties than
annealed components.
 Normalizing is used for high-carbon
(hypereutectoid) steels to eliminate the
cementite network that may develop upon slow
cooling in the temperature range from point Acm
to point A1.

113.  During normalising we use grain refinement
which is associated with allotropic
transformation upon heating γ→α .
 Parts that require maximum toughness and
those subjected to impact are often
normalized.
 The microstructure obtained by normalizing
depends on the composition of the castings
(which dictates its hardenability) and the
cooling rate.

114.  By normalizing , an optimum combination of
strength and softness is achieved , which
results in satisfactory level of machinability
in steels.
 This method of improving machinability is
specially applicable to hypoeutectoid steels.
 Normalizing is the very effective process to
eliminate the carbide network form during
annealing of hypereutectoid steels.
 Due to shorter time available during cooling ,
this network does not appear in normalized
structure.

115. • Finer proeutectoid ferrite
grains .
• Much finer pearlite.
• Finer pearlite grains.
• Amount of proeutectoid
ferrite is reduced .In case of
hypereutectoid steels,
proeutectoid cementite is
less than annealing

117. MICROSTRUCTURE AT THE STRIP SURFACE NORMALIZED AT
860oC

118. MICROSTRUCTURE AT THE STRIP SURFACE
NORMALIZED AT 900oC

119. MICROSTRUCTURE AT THE STRIP SURFACE NORMALIZED AT
940oC

120. MICROSTRUCTURE AT THE STRIP
SURFACE NORMALIZED AT 960oC

127.  It induces better mechanical properties ,such as hardness and
strength with slight decreased ductility.
 Mild steels have better machinability in the normalized state ,
but steels having 0.3 to 0.4 % carbon have better
machinability in annealed state.
 Intricate shaped or critical parts, or parts not to have internal
stresses at all are annealed.
 The difference in properties is less in low carbon steel
products . The lower cost and higher productivity favor use of
normalizing.

128.  Normalizing has following advantages from process
point of view :-
-In annealing, parts cool along with the furnace to
room temperature,wheras in normalizing , parts
are taken out of hot furnace. The empty
furnace may be employed for heating
subsequent batch of parts , increasing the
productivity of the furnace ; the time of heat
treatment is less.
-In annealing ,the furnace cools to low temperature ,
and is then heated again for next batch of
parts .So time and consumption of the power
/fuel is much more.

129.  However Normalizing can not substitute for
annealing for
-for greater softness,
-complete absence of internal
stresses particularly so essential in
intricate parts.

131. QUENCHING
Quench hardening is a mechanical process in which steel
and cast iron alloys are strengthened and hardened
The different stages of quenching are as follows:
STAGE 1: VAPOUR BLANKET STAGE.
 Immediately on quenching, coolant gets vapourized as
the steel part is at high temperature, and thus, a
continuous vapour- blanket envelopes the steel part.
 Heat escapes from the hot surface very slowly by
radiation and conduction through the blanket of water
vapour.
 Since the vapour-film is a poor heat conductor, the
cooling rate is relatively low (stage A in fig ). This long
stage is undesirable in most quenching operations.

132. STAGE 2: INTERMITTENT CONTACT STAGE.
• Heat is removed in the form of heat of vaporization in
this stage as is indicated by the steep slope of the
cooling curve.
• During this stage, the vapour-blanket is broken
intermittently allowing the coolant to come in contact
with the hot surface at one instant, but soon being
pushed away by violent boiling action of vapour bubble.
• The rapid cooling in this stage soon brings the metal
surface below the boiling point of the coolant.
• The vaporization then stops. Second stage corresponds
to temperature range of 500◦ to 100◦c , and this refers
to nose of the CCT curve of the steel , when the steel
transforms very rapidly ( to non martensite product ).
• Thus, the rate of cooling in this stage is of great
importance in hardening of steels

133. STAGE 3 : DIRECT CONTACT STAGE
• This stage begins when the temperature of steel
surface Is below the boiling point of coolant.
• Vapours do not form. The cooling is due to convection
and conduction through the liquid. Cooling is slowest
here.

134. QUENCHING MEDIUMS
• As the aim is to get martensite, the coolant should
have quenching power to cool austenite to let it
transform to martensite. The following factors effect
the quenching power of the coolant :
• The cooling rate decreases as the temperature of
water and brine increases, i.e., it increases stage ‘A’,
i.e., helps in persistence of the vapour blanket stage.
• The increased temperature brings it closer to its
boiling point, and thus, requires less heat to form
vapour, specially above 60°C.
• Good range of temperature for water as coolant is 20-
40°C.
• Oils in general, show increased cooling rates with the
rise of temperature, with optimum cooling rates in
range 50°—80°C.

135. • In oils, the increase of temperature increases the
persistence of vapour-blanket, but this resulting
decrease in the cooling rate is more than compensated
by the decrease of viscosity (with the rise in
temperature) to result in increase of rate of heat
removal through the oil.
• If the boiling point of a coolant is low, vapours form
easily to increase the ‘A’ stage of cooling. ¡t is better
to use a coolant with higher boiling point. A coolant
with low specific heat gets heated up at a faster rate
to form vapours easily.
• A coolant with low latent heat of vapourisation
changes into vapour easily to promote ‘A’ stage, i.e.,
decreases the cooling rate.
• A coolant with high thermal conductivity increases the
cooling rate. Coolants with low viscoity provide faster
cooling rates and decrease the ‘A’ stage.

136. • A coolant should be able to Provide rate of cooling fast
enough to avoid transformation of austenite to pearlite
and bainite . Plain carbon steel invariably require
çooling in water or brine. Whereby alloy steels are
quenched normally in oils.
• But milder the cooling medium , lesser the internal
stresses developed , and thus lesser the danger of
distortion , or cracks.
• An ideal quenching medium is one which is able to
provide very fast cooling rate near the nose of the curve
( 650 -550°C)and at the same time it should provide
very considerable slower rate if cooling within the range
of martensitic transformation( 300 - 200°C) to minimize
internal stresses
• Some of the common quenching mediums are as follows:
-water
-brine
-oils
-polymer quenchants.

137. WATER
• The oldest and still the most popular quenching medium,
water meets the requirements of low cost ,general easy
availability, easy handling and safety.
• The cooling characteristics change more than oil with
the rise of temperature, specially there is a rapid fall in
cooling capacity as the temperature rises above 60°C,
because of easy formation of vapour-blanket.
• The optimum cooling power is when water is 2O-4O°C.
• The cooling power of water is between brine and oils.
• Water provides high cooling power to avoid the
transformation of austenite to pearlite/bainite, but the
major draw back is that it also provides high cooling rate
in the temperature range of martensite formation.
• At this stage, the steel is simultaneously under the
influence of structural stresses (non-uniform change in
structure) and thermal stresses which increase the risk
of crack formation.

138. BRINE
• Sodium chloride aqueous solutions of about 10% by
weight are widely used and are called brines.
• The cooling power is between 10% NaOH aqueous
solution and water.
• These are corrosive to appliances.
• The greater cooling efficiency of brines, or other aqueous
solutions is based as :
• In brine heating of the solution at the steel surface
causes the deposition of crystal of the salt on hot steel
surface .
• This layer of solid crystals disrupts with mild explosive
violence, und throws off a cloud of crystals. This action
destroys the vapour-film from the surface, and thus
permits direct contact of the coolant with the steel
surface with an accompanying rapid removal of heat.
• Brines are used where cooling rates faster than water arc
requited.

139. OILS
• Oils have cooling power between water at 40°C to
water at 90°C.
• In oil-quench, considerable variation can be obtained
by the use of animal, vegetable, or mineral oil, or
their blends.
• Oils should be used at 50- 80°C when these are more
fluid, i.e less VISCOUS, which increases the cooling
power.
• As the oils used generally have high boiling points,
moderate increase of temperature of oil does not very
much increase the vapour blanket stage. However, oils
in contrast to water, or brine, have much lower
quenching power .
• Its this relatively slow cooling rate in the range of
martensitic formation is atlvantageous as it helps in
minimsing the danger to crack formation.
• Oils with high viscosity are less volatile, and thus have
decreased vapour-blanket stage (increase thecooling
rate). As lesser volatile matter is lost, their cooling
power is not affected much with use.

140. POLYMERS
• polymer quenchants cool rapidly the heated steel to Ms
temperature, and then rather slowly when martensite is
forming .
• Polymer quenchants are water-soluble organic chemicals
of high ,molecular weights, and are generally
polyalkylene glycol-based, or polyvinyl pyrolidene-
based.
• Widely different cooling rates can be obtained by varying
the concentration of Organic additives in water; higher
the additions, slower is the cooling rate of solution.
• There are little dangers of distortions and cracks.

141. Process of quenching

142. The quenching process
• Internal stresses are produced due to non-uniform
plastic deformation. In quenching of steels ,this may be
caused by thermal stresses, structural stresses, or both,
or even premature failure of part in service.
• Cooling during quenching lakes place non-uniformly,
i.e., causes temperature gradient across the section.
• Surface layers contract more than the central portion.
• Contraction of surface is resisted by the central portion,
and this puts the central portion under the compressive
stresses, and the surface layers in tension .
• If the magnitude of stress becomes more than the yield
stress of steel (at that deformation occurs.
• These stresses that develop in a quenched part as a
result of unequal cooling are called thermal stresses.

143. The quenching process
• Structural stresses are the stresses which develop due to
due to phase change (mainly austenite to martensite),
and at different times.
• Structural stresses are developed due to two main
reasons:
• 1. Austenite and its transformation products have
unequal specific volume i.e. change in volume occurs
when transformation occurs.
• 2. Phase changes occur at different times in the surface
and in centre.
• Under right conditions, both types of stresses get
superimposed to become larger than the yield strength
to cause warping. but when the tensile internal stresses
become larger than the tensile strength cracks appear.
• If an austenitised steel is quenched, it contracts
thermally till Ms temperature is attained .

144. The quenching process
figure(a) illustrates this in stage 1
• As surface cools faster than centre, i.e., contracts more
than centre distribution of stresses across the section is
illustrated in fig (b), i.e, the surface is under tensile
nature of stress, while centre is under compressive
stresses.
• Only thermal Stresses are produced in stage 2 , surface
having attained Ms temperature, transforms to
martensite, and thus expands, while the centre is still
contracting as it is getting cooled.
• In stage Il, centre may get plastically deformed ,as it is
still ductile austenite.
• In stage 3, martensite of surface and austenite of centre
continue contracting leading to slight increase in stress
levels.

146. The quenching process
• In stage IV, centre has attained M5 temperature, and
begins to expand as it forms martensite, while surface is
still contracting.
• The centre, as it expands, puts the surface in higher
stress levels .
• The surface has little deformation as it consists of
brittle martensie.
• It is during this stage, the greatest danger of cracking
exists.
• Thus, stress levels are highest not in the beginning of
the quench, but when the centre is transforming to
martensite.
• However, higher is the Ms temperature of the steel,
lesser is the expansion, there is reduced danger of
quench-cracking.
• Increase of carbon and alloying elements lower the Ms
temperature making the steel more prone to quench
cracking.

148. • Martensite is a very strong phase, but it is
normally very brittle so it is necessary to
modify the mechanical properties by heat,
treatment in the range 150—700°C.
• Essentially, martensite is a highly
Supersaturated solid solution of carbon in iron
which, during tempering, rejects carbon in the
form of finely divided carbide phases.
• The end result of tempering is a fine
dispersion of carbides in an α-iron matrix
which often bears little structural similarity to
the original as-quenched martensite.

149. Tempering of plain carbon steels
• In the as-quenched martensite
structure,the laths or plates are heavily
dislocated to an extent that individual
dislocations are very difficult to observe
in thin-foil electron micrographs.
• A typical dislocation density for a 0.2
wt% carbon steel is between 0.3 and 1.0
x 1012 cm cm-3. As the carbon content
rises above about 0.3 wt%, fine twins
about 5—10 nm wide are also
observed.
• Often carbide particles, usually rods or
small plates, are observed (Fig. 9.1).

150. Tempering of plain carbon steels

151. Tempering of plain carbon steels
• These occur in the first-formed martensite, i.e. the
martensite formed near Ms, which has the
opportunity of tempering during the remainder of the
quench.
• This phenomenon, which is referred to as autó-
tempering, is clearly more likely to occur in steels
with a high Ms.

152. STAGES OF TEMPERING

153. STAGES OF TEMPERING
• On reheating as-quenched martensite, the
tempering takes place in four distinct but
overlapping stages:
• Stage 1, up to 250°C — precipitation of E-
iron carbide; partial loss of tetragonality in
martensite.
• Stage 2, between 200 and 300°C —
decomposition of retained austenite .
• Stage 3, between 200 and 350°C —
replacement of &iron carbide by cementite;
martensite loses tetragonality.
• Stage 4, above 350°C — cementite coarsens
and spheroidizes; recrystallization of ferrite.

154. Tempering — stage 1
• Martensite formed in medium and high carbon steels
(0.3—1.5 wt% C) is not stable at room temperature
because interstitial carbon atoms can diffuse in the
tetragonal martensite lattice at this temperature.
• This instability increases between room temperature
and 250°C, when €-iron carbide precipitates in the
martensite (Fig. 9.2)
• This carbide has a close-packed hexagonal structure,
and precipitates as narrow laths or rodlets on cube
planes of the matrix with a well-defined
orientation relationship .
• At the end of stage 1 the martensite still possesses a
tetragonality, indicating a carbon content of around
0.25 wt%.

155. • It follows that steels with lower carbon
contents are unlikely to precipitate €-
carhide.
• This stage of tempering possess an activation
energy of between 60 and 80 kJ mo1, which
is in the right range for diffusion of carbon in
martensite. The activation energy has been
shown to increase linearly with the carbon
concentration between 0.2 and 1.5 wt% C.
• This would be expected as increasing the
carbon concentration also increases the
occupancy of the preferred
interstitial sites, i.e. the octahedral
interstices at the mid-points of cell edges,
and centres of cell faces, thus reducing the
mobility of C atoms.

156. Tempering – stage 1

157. Tempering — stage 2
• During stage 2. austenite retained during
quenching is decomposed usually in the
temperature range 230-300°C.
• In martensitiC plain carbon steels below 0.5
carbon. the retained austenite is often below
2%, rising to around 6 % at 0.8 wt C and over
30 % at 1.25 wt C.
• The little available evidence suggests that in
the range 230-300°C, retained austenite
decomposes to bainitic ferrte and
cementite, but no detailed comparison
between this phase and lower bainite has yet
been made.

158. Tempering — stage 3
• During the third stage of tempering, cementite first
appears in the microstructure as a Widmanstatten
distribution of plates which have a well-defined
orientation relationship with the matrix which has now
lost its tetragonality and become ferrite.
• This reaction commences as low as 100°C and is fully
developed at 300°C, with particles up to 200 nm long
and 15 nm in thickness.
• During tempering, the most likely sites for the
nucleation of the cementite are the €-iron carbide
irterfaces with the matrix (Fig 9.2) and as the Fe3C
particles grow, the €-iron carbide particles gradually
disappear.
• The twins occurring in the higher carbon martensites
are also site for the nucleation and growth of
cementite which tends to grow along.
• the twin boundaries forming colonies of similarly
oriented lath shaped particles (Fig. 9.3) which can be
readily ditinguished from the normal Widmanstatten
habit.

159. Tempering – stage 3

160. • A third site for the nucleation of cementite is the grain
boundary regions (Fig, 9.4)of both the interlath boundaries of
martensite and
the original austenite grain b0unjaries.
• The cementite can form as very thin films which are difficult
to detect but which gradually
sp1eroidise to give rise to welI-defined particles of Fe3C in the
grain boundary regions.
• There is some evidence to show that these.
boundary cementite films can adversely affect
ductility. However it can be modified by addition of alloying
elements.
• During the third stage of tempering , the tetragonality of thc
matrix disappears and it is then, essentially, ferrite, not
supersaturated with
respect to carbon.
• Subsequent changes in the morpriology of
cementite particles occur by process where the smaller
particles dissolve in the matrix providing carbon for the
selective growth of the larger particles.
Tempering-stage 3

161. Tempering-stage 4
• It is useful to define a fourth stage of tempering in
which the cementite particles undergo a coarsening
process and essentially lose their crystallographic
morphology, becoming spheroidized.
• It commences between 300 and 400◦C, while
spheroidizatiun takes place increasingly up to 700◦C.
• At the higher end of this range of tempera.
ture the martensite lath boundaries are replaced by
more equi-axid fèrrite grain boundaries by a process
which is best described as recrystallization.
• The final result is an equi-axed array of ferrite
grains with coarse spheroidized particles of Fe3C (Fig.
9.5), partly, but not exclusively, by the grain
boundaries.

162. Tempering-stage 4
• The spherodisation of the Fe3C is encouraged by the
resulting decrease in surface energy.
• The particles which preferentially grew and spheroidize
are located mainly at interlath boundaries and prior
austenite boundaries, although some particles remain in
the matrix.
• The boundary sites are preferred because of the greater
ease of diffusion in these regions. Also, the growth of
cementite into ferrite is associated with a decrease in
density so vacancies are required to accommodate the
growing cementite.
• Vacancies will diffuse away from cementite particles
which are redissolving in the ferrite and towards
cementite particles which are growing, so that the rate
controlling process is likely to be the diffusion of
vacancies.

163. Tempering-stage 4
• The original martensite lath boundaries remain stable up
to about 600°C, but in the range 350—600°C. there is
considerable rearrangement of the dislocations within the
laths and at those lath boundaries which are essentially
low angle boundaries.
• This leads to a marked reduction in the dislocation density
and to lath-shaped ferritic grains closely related to the
packets of similarly oriented laths in the original
martensite.
• This process, which is essentially one of recovery, is
replaced between 600 and 700°C by recrystallization
which results in the formation of equi-axed ferrite grains
with spheroidal Fe3C particles in the boundaries and
within the grains.
• This process occurs most readily in carbon steels.
• At higher carbon content, the increased
density of cementite particles is much more effective in
pinning the ferrite boundaries, so recrystallisation is much
more sluggish.

164. Tempering-stage 4

165. Mechanical properties of
tempered plain carbon steels
• The absence of other alloying elements means that the
hardenability of the steels is low, so a fully martensitic
structure is only possible in thin sections.
• However, this may not be a disadvantage where shallow
hardened surface layers are all that is required.
• Secondly, at lower carbon levels, the Ms temperature is
rather high, so autotempering is
likely to take place.
• Thirdly, at the higher carbon levels the presence
of retained austenite will influence the results.
• Added to these factors, plain carbon steels can exhibit
quench cracking which makes it difficult to obtain
reliable test results. This is particularly the case at
higher carbon levels, i.e. above 0.5 wt% carbon.

166. Tempering of alloy steels
• The addition of allying elements to a steel has a
substantial effect on the kinetics of the y →α
transformation, and also of the pearlite reaction.
• Most common alloying elements move the TTT
curves to longer times, with the result that it is
much easier to miss the nose of the curve during
quenching.
• This essentially gives higher hardenability, since
martensite structures can be achieved at slower
cooling rates and, in practical terms, thicker
specimens can be made fully martensitic.
• Alloying elements have also been shown to have a
substantial effect in depressing the Ms
temperature.

168.  The strength and hardness of some metal alloys may be
improved with ageing time, by the formation of extremely
small, uniformly dispersed particles (precipitates) of a
second phase within the original phase matrix
 Hardness increases as function of Time
 Some alloys that can be Age-hardened or aged are
 Copper-beryllium (Cu-Be)
 Copper-tin (Cu-Sn)
 Magnesium-aluminum (Mg-Al)
 Aluminum-copper (Al-Cu)
 High-strength aluminum alloys

169.  Higher Cu
contents
result in
higher
maximum
hardnesses
because
larger
volume
fractions of
precipitate
are possible

170. 1. Appreciable maximum solubility of of
component in the other.
2. Solubility limit that rapidly decreases with
decrease in temperature
 Alloys can form Super-Saturated-Solid-Solution
on cooling
 The SSSS can reject fine dispersion of
precipitates on ageing.
3. The precipitates of 2nd phase should be
coherent in nature

171. 1. Solutionizing
 Alloy is heated above solvus or upto T0
temperature to dissolve second phase
particales to form a homogenesous single phase
structure.
 Over heating is avoided as it may lead to:
 Melting
 Oxidation
 Grain growth
 Burning
 Decrease in ductility

172. 2. Quenching:
o rapid cooling to room temperature(T1)
from elevated temperature.
o Single phase solid solution region to form
supersaturated solid solutionin(SSS) two
phase region.
o Hot boiling water or air cooling or cold water
used as required for quenching

173. 3. Ageing:
o The supersaturated a solid solution is usually heated to
an intermediate temperature T2 within the a+b region
(diffusion rates increase)
o The b precipitates begin to form as finely dispersed
particles. This process is referred to as aging.
o After aging at T2, the alloy is cooled to room
temperature
o Strength and hardness of the alloy depend on the
precipitation temperature (T2) and the aging time at this
temperature.
o Ageing for a longer time results in coarsening of the
precipitates- overaging

174. Heat Treating Aluminum
Solution Treat
Quench
Age

175. f11_22_pg403

176. 176
0 10 20 30 40 50
wt% Cu
L
a+L
a
a+q
q
q+L
300
400
500
600
700
(Al)
T(°C)
composition range
needed for precipitation hardening
CuAl2
A
• Particles impede dislocations.
• Ex: Al-Cu system
• Procedure:
--Pt B: quench to room temp.
--Pt C: reheat to nucleate
small q crystals within
a crystals.
Temp.
Time
--Pt A: solution heat treat
(get a solid solution)
Pt A (sol’n heat treat)
B
Pt B
C
Pt C (precipitate q)

178.  The sequence is:
a0  a1 + GP-zones  a2 + q“ a3 + q’
a4 + q
 The phase are:
an - fcc aluminum; nth subscript denotes
each equilibrium
GP zones - mono-atomic layers of Cu on
(001)Al
q“ - thin discs, fully coherent with matrix
q’ - disc-shaped, semi-coherent on (001)q’
bct.
q - incoherent interface, ~spherical,
complex body-centered tetragonal (bct).

179. Al-Cu structures

180.  GP ZONES
 Guinier- Preston Zones also called GP1 Zones
 The first early stage of ageing
 Fully coherent, same lattice structure as
Alluminum with matrix thus nucleation is favored
 Plate-like clusters of Copper atoms segregated
on {100} planes of aluminum lattice
 Diameter – 100Å , Thickness – 3-6Å
 Density 1018 per cm3
 Coherency or elastic strains develop
 Occurs by diffusion of Cu atoms aided by
Quenched-in vacancies over short distances
 Give first peak of hardness

181.  θ’’ (GP2 ZONE)
 Coherent intermediate precipitate
 Composition is CuAl2
 Plate like, Diameter- 1500Å, Thickness- 100Å
 Tetragonal crystal Structure, a= 4.04Å, c
=7.68Å
 Have elastic coherency strains
 Produce greater distortion than any other
transition structure

182.  θ
 Equilibrium precipitate – CuAlu2
Fully incoherent precipitate
 Nucleates heterogeneously
 Tetragonal crystal Structure, a= 6.07Å, c
=4.87Å
 Coherency strains are not present
 Leads to Softening
 Result of Overageing

183.  With increasing time, the hardness
increases, reaching a maximum (peak),
then decreasing in strength.
 The reduction in strength and hardness
after long periods is overaging (continued
particle growth).

184. Aging and Overaging
• After quenching, there is thermodynamic
motivation for precipitate to form.
• Precipitates initiate and grow due to diffusion,
enhanced by higher temperatures.
• To get significant strengthening the precipitate
should be coherent
• When the precipitates get too large, they lose
coherence and strengthening decreases
(overaging)

186. 186
• 2014 Al Alloy:
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
Precipitate Effect on TS, %EL
precipitation heat treat time
tensile
strength
(MPa)
200
300
400
100
1min 1h 1day 1mo 1yr
204°C
149°C
• %EL reaches minimum
with precipitation time.
%
EL
(2
in
sample) 10
20
30
0
1min 1h 1day 1mo 1yr
204°C 149 °C
precipitation heat treat time

187.  Rate of precipitation is faster at higher
temperatures
 Rate of precipitation is faster in alloys of
widely dissimilar metals
 Rate of precipitation is increased with
presence of impurities
 Rate of precipitation increases with
application of plastic deformation just
before ageing
 Rate of precipitation at a ageing
temperature is faster in a low melting alloy

188. f11_24_pg404

189.  In age hardened alloy, barriers to motion of
dislocation:
 Coherency strains around GP zones
 GP Zones or the precipitates
 Hardening mechanism are:
1. Coherency strain hardening
2. Dispersion hardening
3. Chemical hardening

190. COHERENCY STRAIN-HARDENING
 Coherency strains act as barriers to dislocation
movements
 Higher stress has to be applied to overcome the
barrier
 The internal stress increases on :
 increase in size difference between precipitate and
matrix
 Increase in elastic modulus of matrix
 Increse in surface area of coherent boundary
HARDENING MECHANISMS

191.  DISPERSION HARDENING
 By-pass mechanism
 When precipitates are incoherent and larger in
size
 Stress required to cut through the precipitates is
too high
 The dislocation bows around the precipitate and
meets at the ends X and Y forming a loop
 The nature of dislocation at X and Y are opposite
and so annihilate
 A loop of dislocations is left behind the
precipitate
 This is Orowan Mechanism
HARDENING MECHANISMS

192.  Stress required to Bypass precipitate particles
Where
G is the shear modulus of the matrix
b is the Burgers vector of the dislocation
is the distance between the dislocations
 Every time a dislocation bypasses it leaves behind a
loop of dislocation around the precipitate
 Thus decreases
 Stress needed for next dislocation to bypass increases
In overageing precipitates increases so strength
decreases
HARDENING MECHANISMS

193. HARDENING MECHANISMS
X
Y

194.  CHEMICAL HARDENING
 Dislocation Cut Mechanism
 Precipitates are very fine
 Precipitates are coherent and have common slip
system with the matrix
 The dislocation cuts the precipitate
 Surface imperfections and stacking faults are
created
 The shearing disturbs the atomic arrangement along
the slip plane
 Greater is the disturbance , greater is the stress
required to shear the precipitate
 Thus the dislocations are pinned
HARDENING MECHANISMS

195. HARDENING MECHANISMS

197.  When a piece of steel(large in size) is heated
to austenitising temperature and then
quenched, the cooling rates vary across the
cross section
 The difference in these rates increases with
the severity in quenching
 At the center of the cross section the cooling
rate is the slowest
 This may lead to martensite formation at the
surface and pearlite at the center

199.  Hardenability may be defined as
susceptibility of the steel to hardening when
quenched
 It is related to the depth and distribution of
hardness across a cross section and not to
maximum hardness

200. Steel A has greater hardenability whereas steel B has lower
hardenability but maximum hardness

201.  Maximum hardness depends on the carbon
content in the steel and can be achieved by:
1. All the carbon is in solution in austenite
2. Critical cooling rate is achieved
3. Amount of retained austenite is minimum
4. No autotempering of martensite takes place
 Hardenability, however, depends on addition
of alloying elements and grain size of
austenite

202.  Drastic cooling may lead to formation of
martensite at center of the cross section.
However it is accompanied effects such as
warping or cracking of steel
 Hardenability is thus the ability to harden
throughout the cross section without drastic
quenching

203.  There are two types of steels on the basis of
hardenability:
1. Shallow hardened steel, where the hardness
is limited to a small distance from the
surface of the specimen, eg, carbon steels
2. Deep hardened steel, where hardening is
uniform throughout the cross section of the
specimen, eg, alloy steels

204.  In shallow hardening steels, the narrow zone
near the surface transforms to martensite
wheresas the center to pearlite
 In deep hardening steels, formation of
martensite extends deep into the cross
section
 At the brittle to ductile transition region, the
specimen consists of 50% martensite and 50%
pearlite

205.  Hardenability of steel is determined by the
following methods:
1. Grossman’s critical diameter method
2. Jominy end quench test
3. Estimation of hardenability from chemical
composition
4. Fracture test

206.  The depth at which 50% martensitic and 50%
pearlitic structure is obtained in steel is
dependent on several parameters such as
composition, grain size of austenite, severity
of quench, size of bar.
 M.A. Grossman gave a direct method of
measuring hardenability in terms of critical
diameter

207.  In this method, a no. of steel bars of
different diameters are quenched under
identical conditions
 The length of each bar should at least be 5
times of the diameter to avoid end effects
 The bars with smaller diameters are
effectively hardened throughout the cross
section
 As the diameter increases, cooling rate at
center decreases and soft pearlitic core is
formed

208.  The portions containing martensite more
than 50% are considered hardened

209.  The hardness changes most rapidly at a value
of RC 54 which is the hardness for 50%
martensite and 50% pearlite
 In the example bar having dia 1 inch shows at
its center pearlite 50% and martensite 50%
 This diameter is called critical diameter(D)
 Bars having diameter>D will not harden
throughout the cross section
 Critical diameter is the measure of the
hardenability of steel

210.  Grossman defined an ideal quenching media
and an ideal critical diameter(Di)
corresponding to the ideal quenching media
such that the effect of quenching media can
be eliminated
 The severity of quenching media is indicated
by the heat transfer equivalent H
 H=(heat transfer coefficient between steel
and medium)/(thermal conductivity of steel)

211.  The ideal quenching media removes heat
from the surface of the steel as fast as heat
flows from the interior to the surface of the
steel bar
 Such a cooling media doesn’t exist in
practice and the fastest cooling rate is
possible at H=∞
 Heat flow in actual practice is affected by
factors such as vapour blanket formation,
thermal conductivity of steel, etc.

212.  The relation between critical diameter Dc,
ideal critical diameter Di, and severity of
quench H, are shown in Grossman’s master
graph

213.  It is the most common method of
determining hardenability of steel
 Here a steel bar of 1 inch diameter and 4
inch length is heated to proper austenitising
temperature
 After being soaked for some time, the
specimen is quickly placed in a fixture as
shown
 A Water jet(temp=24°C) comes out at
constant pressure from an orifice of 0.5 inch
diameter

215.  The distance between the orifice and the
bottom of the steel is kept at 0.5 inch
 The stream of water strikes the lower end of
the specimen
 The end quenching is carried on for about 20
minutes till the bar reaches almost ambient
temperature
 The cooling rate is very rapid at the bottom
end and it decreases as the distance from
the bottom end increases

216.  After quenching, two shallow flat surfaces of
.02 inch depth are ground 180° apart on the
test bar
 The hardness is determined at an interval of
1/16 inch which increases to 1/32 inch near
the quench end

217. If 50% martensite be formed in steel bar having 0.8% carbon at
5/32 inch in an ideal quenching medium, then corresponding
value for hardenability is 1.4 inches

218.  There is a contrast in the fracture undergone
by martensitic and pearlitic regions
 Martensite of the case exhibits brittle nature
whereas pearlite of the core is ductile
 So, where there is change from martensitic
to pearlitic structure, brittle to ductile
fracture takes place
 This region of sudden change is the one
containing 50%martensite and 50%pearlite
 This method is successful when the
transformation is quick and sharp boundary
formed

219.  A steel is said to have high hardenability if
austenite of the steel transforms to
martensite at relatively slow cooling rates
 Therefore any factor which shifts the C curve
right makes it easier to form martensitic
structure at slower cooling rate
 The hardenability of steel depends on:
1. Austenitic grain size
2. Carbon content
3. Alloying elements

220.  The size of austenite plays a major role in
determining hardening response of steel
 Fine grained austenite shows lower hardenability
 This is because there are more number of sites
for heterogeneous nucleation of pearlite
 Austenite to pearlite transformation suppresses
austenite to martensite transformation
 Increase in hardenability due to coarse grain size
is not recommendable as it is accompanied by
poor impact properties, quench crack
susceptibility and loss of ductility

223.  An important role of alloying elements is to
shift the nose of the C curve towards right
 Almost all alloying elements except cobalt
shift the curve towards right
 The presence of cobalt helps the N&G of
pearlite. So it is undesirable
 Undissolved inclusions such as carbides and
nitrides, decrease hardenability of steel
 However dissolved elements in austenite
increase hardenability of steel

226.  TOOL STEELS ARE THE STEELS USED TO FORM AND
MACHINE OTHER MATERIALS
 THEY ARE DESIGNED TO HAVE HIGH HARDNESS AND
DURABILITY UNDER SERVICE CONDITIONS
 TOOL STEEL HEAT TREATMENT IS SIMILAR TO THAT OF
HARDENABLE LOW ALLOY STEELS i.e;
FINALPROPERTIES ARE PRODUCED BY
AUSTENIZING,MARTENSITE
 FORMATION AND TEMPERING
 HOWEVER TOOL STEELS ARE HIGH ALLOY
STEELS AND SPECIAL PRECAUTION MUST BE TAKEN TO
ACHIEVE A PROPER BALANCE OF ALLOY CARBIDES

227.  WATER HARDENING-------W
 SHOCK RESISTING -------S
 COLD WORK -------O(OIL HARDENING)
A(MEDIUM ALLOY AIR
HARDENED)
D(HIGH CARBON HIGH
CHROMIUM)
 HOT WORK -------H
 HIGH SPEED -------T(TUNGSTEN BASE)
M(MOLYBDENUM BASE)
 MOLD -------P
 SPECIAL PURPOSE STEELS-------L(LOW ALLOY)
F(CARBON-
TUNGSTEN)

228.  1)HIGH WEAR RESISTANCE
 2)HIGH RED HARDNESS OR HIGH HOT
SHORTNESS
 3)TOUGHNESS TO ABSORB IMPACT LOAD
 4)HIGH HARDENABILITY
 5)NON DEFORMING PROPERTIES
 6)RESISTANCE TO DECARBURISATION
 High toughness and high hardness for better
wear resistance are obtained by hard surface
or case and a soft core inside

229.  THESE ARE ESSENTIALLY PLAIN CARBON TOOL
STEELS,ALTHOUGH SOME HIGH CARBON TOOL
STEELS MAY CONTAIN SMALL AMOUNTS OF
VANADIUM,CHROMIUM TO IMPROVE HARDENABILITY
AND WEAR RESISTANCE
 CARBON CONTENT VARIES BETWEEN 0.60 AND 1.40
PERCENT
 0.60 TO 0.75%-------WHERE TOUGHNESS IS THE
PRIMARY CONSIDERATION
 0.75 TO 0.95%-------WHERE TOUGHNESS AND
HARDNESS ARE EQUALLY IMPORTANT
 0.95 TO 1.40%-------WHERE INCREASED WEAR
RESISTANCE AND RETENTION OF CUTTING EDGE ARE
IMPORTANT

230.  In general the straight carbon tool steels are less
expensive than the alloy tool steels and with
proper heat treatment they yield a hard
martensitic surface with a tough core
 They have the best machinability ratings of all
the tool steels and are best with respect to
decarburisation. But their resistance to heat is
poor
 Because of its low red hardness, carbon steels
cannot be used as cutting tools under conditions
where appreciable amount of heat is generated
at the cutting edge
 Their use as cutting tools is limited to conditions
involving low speed and light cuts on relatively
soft materials

231.  THESE STEELS COME INTO PICTURE WHERE
THOUGHNESS AND THE ABILITY TO WITHSTAND
REPEATED SHOCK ARE PARAMOUNT
 THEY ARE GENERALLY LOW IN CARBON,IT VARIES
BETWEEN 0.45 AND 0,65%
 THE PRINCIPAL ALLOYING ELEMENT IN THESE
STEELS ARE SILICON ,CHROMIUM,TUNGSTEN AND
SOMETIMES MOLYBDENUM

232.  SILICON STRENGTHENS FERRITE,WHILE
CHROMIUM INCREASES HARDENABILITY AND
CONTRIBUTES SLIGHTLY TO WEAR
RESISTANCE,MOLYBDENUM AIDS IN INCREASING
HARDENABILITY,WHILE TUNGSTEN IMPARTS SOME
RED HARDNESS TO THESE STEELS
 MOST OF THESE STEELS ARE OIL
HARDENED,ALTHOUGH SOME ARE WATER
QUENCHED TO DEVELOP FULL HARDNESS
 THE HIGH SILICON TENDS TO ACCELERATE
DECARBURIZATION, AND SUITABLE PRECAUTIONS
SHOULD BE TAKEN IN HEAT TREATMENT TO
MINIMIZE THIS

233.  THIS IS CONSIDERED TO BE THE MOST IMPORTANT
GROUP OF TOOL STEELS
 THESE STEELS ARE MAINLY EMPLOYED FOR
MAKING TOOLS INTENDED FOR COLD WORK
APPLICATIONS
 THE CHEMICAL COMPOSITION AND HARDENING
HEAT TREATMENT ARE SO ADJUSTED SO AS TO
PRODUCE MINIMUM POSSIBLE DEFORMATION AND
CONSEQUENTLY, THESE ARE TERMED AS
NON-DEFORMING OR NON-DISTORTING STEELS
 THESE STEELS ARE DIVIDED INTO THREE GROUPS
NAMELY ,oil hardening,air hardening and high
carbon,high chromium type

234.  In contrast to TO COLD WORK TOOL
STEELS,THESE STEELS ARE EMPLOYED FOR
HOT WORKING APPLICATIONS SUCH AS HOT
FORGING OR HOT EXTRUSION and also used
for die casting dies
 Therefore, high temperature properties like
red hardness, wear resistance, erosion
resistance, thermal cracking of reticular
type(heat checking) due to severe thermal
shocks are the main considerations for such
steels

235.  Red hardness is imparted by tungsten, Cr
improves both hardness and oxidation
resistance. Mo and V are also added for
increasing hardness and high temperature
processes
 They are classified as:
1. Cr based
2. Mo based
3. W based

236.  These steels contain chromium and nickel as
the principal alloying elements,with
molybdenum and aluminium as additives
 They are generally characterized by low
hardness in the annealed condition and
resistance to work hardening

238.  As the name indicates this steels are well
suited for manufacturing cutting tools which
can be operated at high speeds
 These steels are amonst the most highly
alloyed of the tool steels and usually contain
large amount of W, Mo along with Cr, V, and
sometimes Co
 The carbon content varies between 0.7-1.5%

239.  The high speed steels are subdivided in two
groups:
1. Molybdenum base (group M)
2. Tungsten base (group T)
 Since there are adequate domestic supplies
of Mo, and most of W has to be imported,
the Mo steels are lower in price and comprise
over 80% of all high speed steels

240.  When better than average red hardness is
required, steels containing cobalt are
recommended
 Higher vanadium content is desirable when the
material being cut is highly abrasive
 Tungsten dissolves in ferrite and austenite but is
a strong carbide former which forms WC and
W6C which increases the wear and abrasion
resistance, apart from maintaining fine grain size
of steel
 Mo is a ferrite stabilizer and relatively strong
carbide forming element and forms (FeMo)3C,
Mo23C6, Mo2C, Mo6C

241.  Vanadium is a ferrite stabiliser and a strong
carbide former forming V4C3 and VC
 Chromium is a strong carbide forming
element and forms carbides like Cr7C3 and
Cr23C6, (FeCr)3C

242.  The service conditions of such cutting tools
demand high red hardness(hardness at high
temperatures), elevated tempertaure wear
resistance and reasonably good shock
resistance
 These tools must have good non deforming
properties
 They must have good wear resistance, fair
machinability and high resistance to
decarburisation

243.  All these characteristics can be imparted in
the steel by alloying it with strong carbide
forming elements such as tungsten, Mo, Cr, V
 Alloying elements should be added in
sufficient amount so that all carbon may
combine with them to form alloy carbides

244.  All high speed steels are heated to the maximum
possible temperature for hardening treatment.
However this temperature should not result in large
scale grain coarsening
 A high hardening temperature ensures dissolution of
all carbon and alloying elements in austenite
 This highly alloyed austenite transforms to
martensite of exactly similar composition on
quenching
 The martensite thus formed, which is highly enriched
in C an alloying elements, has high red hardness and
structural stability
 Generally, the hardening temperature for high speed
steel varies from 1150-1350C.heating to such a high
temperature poses certain problems like oxidation
and decarburization in addition to grain growth

245.  Again on directly heating to 1250c we have
to hold it for longer period for attending
uniform temperature throughout which can
lead to grain growth ,decarburisation
 Since they contain higher amounts of alloying
elements(around 30%) their austenizing
temperature in general are high.
 The exact temperature controls the ultimate
hardness,wear resistance,red hardness and
toughness etc.

246.  Higher the austenizing temperature, more
carbide dissolves in austenite which
ultimately causes increase in amount of
finely dispersed precipitates of carbide
during tempering to result in increased tool
hardness,wear resistance,increasing
tempering temperature as well as heat
resistance during cutting operation
 But the drawback is that higher austenizing
temperature results in lower as quenched
hardness(lower amount of retained
austenite)

248.  Added to these problems is the poor thermal
conductivity of HSS resulting in crack formation
during quenching.
 Simpler shapes and smaller sized tools are heated in
two stages. First it is preheated to about 800C and
then quickly transformed to another furnace
maintained at final hardening temperature
 Larger tools or intricate tools are generally heated
in three stages
1. The first step consists of heating the steel to
about 400C

249. 2. This is followed by second heating upto
800C. The holding time at final hardening is
less and rarely exceeds 5 minutes
3. High speed steels are either quenched in oil
or in salt baths. High speed steel is first cooled
to about 1000C and only then it is quenched in
oil. This avoids formation of quench cracks.
Due to presence of some amount of retained
austenite, sub zero treatment is done

250.  The heating and cooling cycle oh HSS is followed
by tempering which in done in stages
1. In first tempering operation, martensite
decomposes and precipitation occurs of
carbides in it as it SSSS of C and alloying
elements
2. During first tempering operation ret austenite
is said to be conditioned and at least some of it
transforms to martensite on cooling from
tempering temperature
3. During conditioning retained austenite loses
carbon to martensitic region from where
carbon has been depleted because of
precipitation of alloy carbides, thus increasing
the Ms temperature of retained austenite

251. 4. Hence on cooling it transforms to
martensite. This martensite must be
tempered by second haeating to same
tempering temperature
5. double tempering may not be sufficient in
some cases. Thus, 3 to 4 tempering
operations are required to bring down the
retained austenite to acceptable level

252.  Low Hardness:
1. Due to very high austenitising temperature,
which results in grain growth of austenite,
resulting martensite is coarser and thus
decreasing hardness
2. We cannot go for low austenitizing
temperature as less carbides are dissolved
in austenite. The secondary hardening
effect decreases to reduce the hardness
3. Decarburization should be avoided which
can also cause lower surface hardness

253.  Cracks and distortion:
 Rapid quenching of large and intricate parts
due to differential contraction and expansion
may develop cracks and distortion
 Due to stress raiser, faulty design such as
sharp edges
 Decarburized surface layer gets stressed
while central parts get hardened

256. • Addition of micro-alloy (carbide, nitride or carbo-nitride
forming elements) such as Nb, V, Ti in structural steel and
strip steel grades, the materials are known as“High
Strength Low Alloy (HSLA) steel”
 • At slab soaking temperature ~ 1200 oC - undissolved
particles (such as TiN, NbC and AlN) restricts the size of
austenite grain (affect to inhibit recrystallization during
hot rolling → produces fine austenite grain size →
induces fine ferrite grain size)

257. proportion of micro-alloys are dissolved to
solid
solution (affect to precipitate in later process
in form of fine carbide/carbonitride/nitride at
austenite-ferrite interface on cooling to room
temperature).

258. • Hot rolled materials can be strengthened by
separate mechanisms of grain refine &
precipitation strengthening
• Magnitude of effects depend on:
- type and amount of elements added
- base compositions
- soaking temperatures
- finishing and coiling temperatures
- cooling rate to room temperature
• Strength increment up to 300 N/mm2 and Y.S. ~
500-600 N/mm2 can be produced in hot rolled
state
• Y.S. ~ 350 N/mm2 are produced in cold-rolled
strip containing 0.06-0.10 %Nb

261.  Precipitation curves for niobium carbo-nitride in
Austenite

263. 1. Outline Process
SRT ~1200-1250 ºC
FT ~ 1000 ºC
normalizing ~ 920 oC
Normal Rolling and Normalizing

264. Roughing Rolling
Austenite elongated grain

265.  • Importance of slab reheating stage
 - control amount of micro-alloying element
taken into solution
 - starting grain size
 • Re-solution temperature of micro-alloy
precipitates
 - VC: complete solution ~ 920 oC (normalizing
temp.)
 - VN: at somewhat higher temperature
 - Nb(CN), AlN and TiN: around 1150-1300 oC
 - TiN (most stable compound) little dissolution at
normal
 slab reheating temperature (SRT)

266.  • Un-dissolved fine carbo-nitride (CN)
particles
 - maintain fine austenite grain size at slab
reheating stage
 • Micro-alloying elements taken into solution
(which can be influence in later stage in
process)
 - control of recrystallization
 - precipitation strengthening
 • Multiple micro-alloy additions for above
dual requirements

267.  • Three distinct stages during controlled rolling.
 - Deformation in the recrystallization (austenite phase)
 temperature range just below SRT
 - Deformation in temperature range between
 recrystallization temperature and Ar3
 - Deformation in 2 phase (austenite-ferrite) temperature
 range between Ar3 & Ar1
 • At temperature just below SRT
 - rate of recrystallization is rapid
 - provided the strain per pass exceeds a minimum critical
 level
 - recrystallization is retarded by presence of solute atom
Al,
 Nb, Ti, V (solute drag) → strain induced precipitation →
 form fine carbonitride during rolling process

268.  - rolling temperature decrease, recrystallization
more difficult and reach a stage “recrystallization
stop temperature (Trs or No-recrystallization
temperature; Tnr)” (the temperature at which
recrystallization is complete after 15 s. after
particular rolling sequence)
 - Nb is powerfull retardation effect which depend on
solubilities in austenite
 - Nb lease soluble
 - largest driving force for precipitation
 - creating greater effect in increasing of
recrystallization temperature than Al and V
 • At temperature between recrystallization
temperature & Ar3
 temperature below 950 oC Controlled

269.  - strain induced precipitation of Nb(CN) or TiC is
sufficient rapid to prevent recrystallization before
the next pass (deformed-austenite providing
nucleation sites of carbonitride precipitation and pins
the substructure which inhibits recrystallization)
 - finishing rolling below recystallizaion stop
temperature
 - can be obtain elongated-pancake morphology in the
austenite structure
 At temperature between Ar3 & Ar1
 - further grain refinement
 - mixed structures of polygonal-ferrite (transformed
from deformed-austenite) and deformed-austenite
during rolling process

270.  Mean ferrite grain size relate to:
 - thickness of pancake-austenite grain
 - alloying elements depress the austenite to
ferritetransformation which decrease ferrite-
grain size
 - cooling rate from austenite or austenite-ferrite
region(accelerate cooling)
 → increase strength
 → achieve strength level by lower alloy content-
direct quenching
 → refine ferrite-grain
 → formation of bainite and martensite (required
tempering)

273. Gateway Arch in St Louis – 304 series SS
F-35 Joint Strike Fighter (JSF)
Lightning II, built by Lockheed Martin –
airframe 17-7 PH – 600 series SS

274.  Alloy steels containing at least 10% Cr are SS.
 Contain sufficient amount of Cr that they are
NOT considered low alloy.
 Corrosion resistance is imparted by the
formation of a passivation layer characterized
by:
 Insoluble chromium oxide film on the surface of the metal - (Cr2O3) .
 Develops when exposed to oxygen and impervious to water and air.
 Layer is too thin to be visible
 Quickly reforms when damaged
 Susceptible to sensitization, pitting, crevice corrosion and acidic environments.
 Passivation can be improved by adding nickel, molybdenum and vanadium.

275.  Over 150 grades of SS available, usually categorized
into 5 series containing alloys w/ similar properties.
 AISI classes for SS:
 200 series = chromium, nickel,manganese (austenitic)
 300 series = chromium, nickel (austenitic)
 400 series = chromium only (ferritic)
 500 series = low chromium <12% (martensitic)
 600 series = Precipitation hardened series (17-7PH, 17-7 PH,
15-5PH)

276.  Early addition of chromium and nickel in iron resulted
in formation of alloys proved to be more corrosion
resistance than the parent material.
 This property developed in iron lead to the so called
name ‘stainless steel’.
 Various heat treatment process like annealing is used to
alter some property of the stainless steel.

277.  12.5% Cr steels resisted all concentrations of nitric acid
at room temperature while those containing 14 % Cr
withstood such solution to the boiling point
 Addition of Cr reduces corrosion resistance in reducing
acid.
 Addition of Mo enhanced resistance in nitric acid
containing chlorides
 The passivity of stainless steel is contingent on a source
of oxygen.

278.  Electrical Resistivity
 Surface & bulk resistance
is higher than that for
plain-carbon steels
 Thermal Conductivity
 About 40 to 50 percent
that of plain-carbon steel
 Melting Temperature
 Plain-carbon:1480-1540
°C
 Martensitic: 1400-1530
°C
 Ferritic: 1400-1530 °C
 Austenitic: 1370-1450 °C
 Coefficient of Thermal Expansion
 Greater coefficient than
plain-carbon steels
 High Strength
 Exhibit high strength at
room and elevated
temperatures
 Surface Preparation
 Surface films must be
removed prior to welding
 Spot Spacing
 Less shunting is observed
than plain-carbon steels

279. Stainless Steel Types
Austenitic Nitrogen
Strengthened
Austenitic
Martensitic Ferritic
Precipitation Hardened Super Austenitic
Super Ferritic Duplex

280. Castro & Cadenet, Welding Metallurgy of
Stainless and Heat-resisting Steels
Cambridge University Press, 1974
A=Martensitic Alloys
B=Semi-Ferritic
C=Ferritic

281. Austenitic SS
• Contain between 16 and 25 percent chromium, plus
sufficient amount of nickel, manganese and/or nitrogen
• Have a face-centered-cubic (fcc) structure
• Nonmagnetic
• Good toughness
• Spot weldable
• Strengthening can be accomplished by cold work or by
solid-solution strengthening
•Ease of fabrication
•High Temperature Strength
•Good impact resistance down to almost -183 degree celsius

282. C content

283. • Most common SS (roughly 70% of total SS production)
• Used for flatware, cookware, architecture, automotive, etc.
• 0.15% C (max), 16% Cr (min) and Ni or Manganese
• Austenitic, High strength, best corrosion resistance. High temp capability up to 1200 F. non-
magnetic, good ductility and toughness, not hardenable by heat treatment, but they can be
strengthened via cold working, best corrosion resistance but most expensive, corrosive in
hydrochloric acid.
• General use where corrosion resistance is needed.
• Typical alloy 18% Cr and 10% Ni = commonly known as 18/10 stainless
 Also have low carbon version of Austenitic SS (316L or 304L) used to avoid corrosion problem
caused by welding, L = carbon content < 0.03%

284. •300 Series—austenitic chromium-nickel alloys
Type 301—highly ductile, for formed products. Also hardens rapidly during
mechanical working. Good weldability. Better wear resistance and fatigue
strength than 304.
Type 302—same corrosion resistance as 304, with slightly higher strength due
to additional carbon.
Type 303—free machining version of 304 via addition of sulfur and
phosphorus. Also referred to as "A1" in accordance with ISO 3506.[10]
Type 304—the most common grade; the classic 18/8 stainless steel. Also
referred to as "A2" in accordance with ISO 3506.[10]
Type 304L— same as the 304 grade but contains less carbon to increase
weldability. Is slightly weaker than 304.
Type 304LN—same as 304L, but also nitrogen is added to obtain a much
higher yield and tensile strength than 304L.

285. Type 308—used as the filler metal when welding 304
Type 309—better temperature resistance than 304, also sometimes used as
filler metal when welding dissimilar steels, along with inconel.
Type 316—the second most common grade (after 304); for food and surgical
stainless steel uses; alloy addition of molybdenum prevents specific forms of
corrosion. It is also known as marine grade stainless steel due to its increased
resistance to chloride corrosion compared to type 304. 316 is often used for
building steel nuclear reprocessing plants. 316L is an extra low carbon grade of
316, generally used in stainless el watches and marine applications due to its
high resistance to corrosion. Also referred to as "A4" in accordance with ISO
3506.[10] 316Ti includes titanium for heat resistance, therefore it is used in
flexible chimney liners.
Type 321—similar to 304 but lower risk of weld decay due to addition of
titanium. See also 347 with addition of niobium for desensitization during
welding.

286.  This Class of alloy contain 15-18 % Cr, which can
also vary from 11-30 % depending on alloy
composition
 Ferromagnetic in nature
 Cannot be hardened by Heat Treatment
 Poor impact resistance at Low temperature
 Good resistance to HAC
 High temperature oxidation resistance is good
 Weldability is poor.Welding may lead to
brittleness
 This relation ship holds good
 (Cr%-17*(% C))>12.7

287. Fe-Cr-C
Phase
Diagra
m

288. Effect of addition of
C content to 13 % Cr
SS.

289.  Ferritic SS gets corroded in chloride and SO2
solution.
 Due to BCC structure they shoe ductile to
brittle transition
 More prone to Stress Corrosion Cracking
 Intergranular Corrosion is more susceptible in
the heat affected zone
 Grain refinement is Difficult in ferritic SS.

290.  Ferritic, Automotive trim, chemical processing, blades, knives, springs,
ball bearings, surgical instruments. Can be heat treated!
 Contain between 10.5% and 27% Cr, little Ni and usually molybdenum.
 Common grades: 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni
 Magnetic (high in Fe content) and may rust due to iron content.
 Lower strength vs 300 series austenitic grades
 Cheaper in comparsion

291. •400 common alloys
Type 405— ferritic for welding applications
Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).
Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.
Type 416—easy to machine due to additional sulfur
Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent polishability.
Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and
corrosion resistance.
Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when
properly heat-treated. It can be hardened to approximately Rockwell 58 hardness, making it one of the hardest
stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are
made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the
uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant;
440C, having the most, is the strongest and is usually considered more desirable in knifemaking than 440A, except
for diving or other salt-water applications.
Type 446—For elevated temperature service
Common 400 series grades of SS:

292.  Heat Treatable
 12-17 % Cr.
 0.10-1.2 % C
 Follow the relationship:
(% Cr-17 * 5 C)<12.7
• Hardening increases on increasing chromium
content.
• Ferromagnetic in nature

293.  Hardness of martensite depends on %C
• However check 12 %Cr and 18 %Cr diagrams
maximum %C in γ at 1100°C (note high γ-
temperature)
 – 12 %Cr – 0.55 %C
 – 18 %Cr – 0.30 %C
• Therefore can get higher %C martensite with
lower %Cr
 – However higher carbon levels will give
more
 carbides

294.  Low Carbon high strength martensitic SS
 High carbon high hardness martensitic SS

296.  Surgical equipment, knives, razor blades,
scissors, scalpels
 Hardest Rc 60-65
 Difficult to sharpen but maintain edge for a
long time
 Best i.e., Heinkel Knives $100/knife
 Run of the mill Rc 45-55
 Easy to sharpen but don’t maintain edge for
very long
 Continuous sharpening or replacement

297.  Not as corrosion resistant as the other classes but extremely strong and
tough as well as machineable and can be hardened via heat treat.
 High strength structural applications (Su up to 300 ksi) – nuclear plants,
ships, steel turbine blades, tools, etc.
 Magnetic

298.  Structure approximately 50% Ferrite/50 %
austenite,for improved corrosion resistance.
 Ferrite forming element such as Cr,Mo are
present well in excess of the austenitzing
element such as nickel.
 Contains Mo for better CR in chlorodic
environment with less susceptibility than the
single phase SS
 Grain refinement is possible by thermo
mechanical treatment.
 Stronger ,more corrosion resistance than
single phase SS

299.  Attractive combination of property.
 Matrix could be austenite or martensite.
 Precipitation hardeing improves both
strength and wear or galling performance
 Age hardening takes place due to coherency
strain and general dispersion strengthing.

300.  Have corrosion resistance comporable to 300 series austentic grades
but can be precipitation hardened for increased strength!
 Key: High strength + corrosion resistance BOTH.
 Why? Aerospace industry – defense budgets determined 2% of GDP
spent dealing with corrosion so developed high strength corrosion
resistant steel to replace alloy steels.
 Lockheed-Martin Joint Striker Fighter – 1st aircraft to use PH SS for
entire airframe.
 Common Grades:
 630 grade = 17-4 PH (17% Cr, 4% Ni),
 17-4 PH,
 15-5 PH

301.  Food industry (cookware, flatware, food
transport and storage tankers) due to its
corrosion resistance and antibacterial
properties.
 Surgical equipment
 Aerospace
 High end automotive, industrial, etc.