1. Effect of non equilibrium cooling on microstructure
and properties of steel
TTT diagram for 0.8% carbon steel
Continuous cooling Transformation curves
2. • Phase transformations may be wrought in metal alloy
systems by varying temperature, composition, and
the external pressure;
• however, temperature changes by means of heat
treatments are most conveniently utilized to induce
phase transformations.
• This corresponds to crossing a phase boundary on
the composition-temperature phase diagram as an
alloy of given composition is heated or cooled
3. • Most phase transformations require some finite time
to go to completion, and the speed or rate is often
important in the relationship between the heat
treatment and the development of microstructure.
• One limitation of phase diagrams is their inability to
indicate the time period required for the attainment
of equilibrium.
• It thus becomes imperative to investigate the
influence of time on phase transformations.
5. • Temperature plays an important role in the rate of the
austenite-to-pearlite transformation.
• The temperature dependence for an iron–carbon alloy of
eutectoid composition is indicated in Figure 10.12,
• which plots S-shaped curves of the percentage
transformation versus the logarithm of time at three
different temperatures.
• For each curve, data were collected after rapidly cooling a
specimen composed of 100% austenite to the temperature
indicated; that temperature was maintained constant
throughout the course of the reaction.
6.
7. • A more convenient way of representing both the time and
temperature dependence of this transformation is in the
bottom portion of Figure 10.13.
• Here, the vertical and horizontal axes are, respectively,
temperature and the logarithm of time.
• Two solid curves are plotted; one represents the time
required at each temperature for the initiation or start of the
transformation; the other is for the transformation
conclusion.
• The dashed curve corresponds to 50% of transformation
completion
8. • These curves were generated from a series of
plots of the percentage transformation versus
the logarithm of time taken over a range of
temperatures.
• The S-shaped curve [for 675 C], in the upper
portion of Figure 10.13, illustrates how the
data transfer is made.
9.
10. Interpreting TTT diagram
• In interpreting this diagram, note first that the eutectoid
temperature [727 C] is indicated by a horizontal line; at
temperatures above the eutectoid and for all times, only
austenite will exist, as indicated in the figure.
• The austenite-to-pearlite transformation will occur only if an
alloy is super cooled to below the eutectoid; as indicated
by the curves.
• The time necessary for the transformation to begin and then
end depends on temperature.
11. • To the left of the transformation start curve, only
austenite (which is unstable) will be present.
• whereas to the right of the finish curve, only pearlite
will exist.
• In between, the austenite is in the process of
transforming to pearlite, and thus both
microconstituents will be present.
12. Constraints
• Several constraints are imposed on using diagrams like Figure
10.13.
• First, this particular plot is valid only for an iron–carbon alloy of
eutectoid composition; for other compositions, the curves will
have different configurations.
• In addition, these plots are accurate only for transformations in
which the temperature of the alloy is held constant throughout
the duration of the reaction.
• Conditions of constant temperature are termed isothermal;
thus, plots such as Figure 10.13 are referred to as isothermal
transformation diagrams, or
• sometimes as time–temperature–transformation (or T–T–T)
plots.
13.
14. • An actual isothermal heat treatment curve (ABCD) is
superimposed on the isothermal transformation diagram for a
eutectoid iron–carbon alloy in Figure 10.14.
• Very rapid cooling of austenite to a temperature is indicated
by the near-vertical line AB, and the isothermal treatment at
this temperature is represented by the horizontal segment
BCD.
• Of course, time increases from left to right along this line.
• The transformation of austenite to pearlite begins at the
intersection, point C (after approximately 3.5 s), and has
reached completion by about 15 s, corresponding to point D.
• Figure 10.14 also shows schematic microstructures at various
times duringthe progression of the reaction.
17. Bainite
• In addition to pearlite, other microconstituents that are
products of the austenitic transformation exist; one of these is
called bainite.
• The microstructure of bainite consists of ferrite and
cementite phases.
• Bainite forms as needles or plates, depending on the
temperature of the transformation; the microstructural
details of bainite are so fine that their resolution is possible
only using electron microscopy.
18.
19. • Figure 10.17 is an electron micrograph that shows a
grain of bainite (positioned diagonally from lower left
to upper right); it is composed of a ferrite matrix and
elongated particles of Fe3C;
• The various phases in this micrograph have been
labeled.
• In addition, the phase that surrounds the needle is
martensite, the topic to which a subsequent section
is addressed.
20. • The time–temperature dependence of the bainite
transformation may also be represented on the
isothermal transformation diagram.
• It occurs at temperatures below those at which
pearlite forms; begin-, end-, and half-reaction
curves are just extensions of those for the
pearlitic transformation, as shown in Figure
10.18,
• The isothermal transformation diagram for an
iron–carbon alloy of eutectoid composition that
has been extended to lower temperatures.
21.
22. • All three curves are C-shaped and have a “nose”
at point N, where the rate of transformation is a
maximum.
• As may be noted, whereas pearlite forms above
the nose [i.e., over the temperature range of
about 540 to 727 C]
• At temperatures between about 215 to 540 C,
bainite is the transformation product
23. Spheroidite
• If a steel alloy having either pearlitic or bainitic
microstructures is heated to, and left at, a
temperature below the eutectoid for a sufficiently
long period of time—for example,
• At about (700C) for between 18 and 24 h—yet
another microstructure will form.
• It is called spheroidite (Figure 10.19).
24.
25. • Instead of the alternating ferrite and
cementite lamellae (pearlite), or
• The microstructure observed for bainite, the
Fe3C phase appears as sphere-like particles
embedded in a continuous α phase matrix.
26. • This transformation has occurred by additional
carbon diffusion with no change in the
compositions or relative amounts of ferrite
and cementite phases.
• The driving force for this transformation is the
reduction in –Fe3C phase boundary area.
27. Martensite
• Martensite is formed when austenitized iron–
carbon alloys are rapidly cooled (or quenched) to
a relatively low temperature (in the vicinity of the
ambient).
• The martensitic transformation occurs when the
quenching rate is rapid enough to prevent carbon
diffusion.
• Any diffusion whatsoever will result in the
formation of ferrite and cementite phases
28. • However, large numbers of atoms experience
cooperative movements, in that there is only a
slight displacement of each atom relative to its
neighbors.
• This occurs in such a way that the FCC
austenite experiences a polymorphic
transformation to a body-centered tetragonal
(BCT) martensite
29. A unit cell of this crystal structure (Figure 10.20) is simply a body-centered cube
that has been elongated along one of its dimensions;
This structure is distinctly different from that for BCC ferrite.
30. • Since the martensitic transformation does not
involve diffusion, it occurs almost
instantaneously;
• The martensite grains nucleate and grow at a very
rapid rate—the velocity of sound within the
austenite matrix.
• Thus the martensitic transformation rate, for all
practical purposes, is time independent
31. • Martensite grains take on a plate-like or needle-
like appearance, as indicated in Figure 10.21.
• The white phase in the micrograph is austenite
(retained austenite) that did not transform during
the rapid quench.
• As already mentioned, martensite as well as
other microconstituents (e.g., pearlite) can
coexist
32.
33. • Being a nonequilibrium phase, martensite does not
appear on the iron–iron carbide phase diagram (Figure
9.24).
• The austenite-to-martensite transformation
is,however, represented on the isothermal
transformation diagram.
• Since the martensitic transformation is diffusionless
and instantaneous, it is not depicted in this diagram as
the pearlitic and bainitic reactions are.
34. • The beginning of this transformation is
represented by a horizontal line designated
M(start) (Figure 10.22).
• Two other horizontal and dashed lines, labeled
M(50%) and M(90%), indicate percentages of
the austenite-to-martensite transformation.
35. • The temperatures at which these lines are
located vary with alloy composition.
• The horizontal and linear character of these lines
indicates that the martensitic transformation is
independent of time; it is a function only of the
temperature to which the alloy is quenched or
rapidly cooled.
• A transformation of this type is termed an
athermal transformation
36.
37. Example
• Consider an alloy of eutectoid composition that is
very rapidly cooled from a temperature above
(727C ) to, say, (165C).
• From the isothermaltransformation diagram
(Figure 10.22) it may be noted that 50% of the
austenite will immediately transform to
martensite; and as long as this temperature is
maintained, there will be no further
transformation
38.
39. Effect of Alloying Elements
• The presence of alloying elements other than carbon (e.g., Cr,
Ni, Mo, and W) may cause significant changes in the positions
and shapes of the curves in the isothermal transformation
diagrams. These include
• (1) Shifting to longer times the nose of the austenite-to-
pearlite transformation (and also a proeutectoid phase nose,
if such exists), and
• (2) The formation of a separate bainite nose.
• These alterations may be observed by comparing Figures
10.22 and 10.23, which are isothermal transformation
diagrams for carbon and alloy steels, respectively.
44. TTT
• Isothermal heat treatments are not the most
practical to conduct because an alloy must be
rapidly cooled to and maintained at an
elevated temperature from a higher
temperature below the eutectoid.
45. • Most heat treatments for steels involve the
continuous cooling of a specimen to room
temperature.
• An isothermal transformation diagram is valid
only for conditions of constant temperature;
• This diagram must be modified for
Transformations that occur as the temperature is
constantly changing.
46. CCT
• For continuous cooling, the time required for
a reaction to begin and end is delayed.
• Thus the isothermal curves are shifted to
longer times and lower temperatures, as
indicated in Figure 10.25 for an iron–carbon
alloy of eutectoid composition.
47.
48. • A plot containing such modified beginning and
ending reaction curves is termed a continuous
cooling transformation (CCT) diagram.
• Some control may be maintained over the
rate of temperature change depending on the
cooling environment.
49. • Two cooling curves corresponding to
moderately fast and slow rates are
superimposed and labeled in Figure 10.26,
again for a eutectoid steel.
• The transformation starts after a time period
corresponding to the intersection of the
cooling curve with the beginning reaction
curve and concludes upon crossing the
completion transformation curve.
50.
51. • The microstructural products for the
moderately rapid and slow cooling rate curves
in Figure 10.26 are fine and coarse pearlite,
respectively
52. • For any cooling curve passing through AB in
Figure 10.26, the transformation ceases at the
point of intersection;
• with continued cooling, the unreacted
austenite begins transforming to martensite
upon crossing the M(start) line
53. • With regard to the representation of the
martensitic transformation, the M(start),
M(50%), and M(90%) lines occur at identical
temperatures for both isothermal and
continuous cooling transformation diagrams.
54. Critical cooling rate
• For the continuous cooling of a steel alloy, there
exists a critical quenching rate, which represents
the minimum rate of quenching that will produce
a totally martensitic structure.
• This critical cooling rate, when included on the
continuous transformation diagram, will just miss
the nose at which the pearlite transformation
begins, as illustrated in Figure 10.27
55.
56. • As the figure also shows, only martensite will
exist for quenching rates greater than the
critical; in addition,
• There will be a range of rates over which both
pearlite and martensite are produced.
• Finally, a totally pearlitic structure develops
for low cooling rates.
57. Effect of Alloying on CCT
• Carbon and other alloying elements also shift
the pearlite (as well as the proeutectoid
phase) and bainite noses to longer times, thus
decreasing the critical cooling rate.
• In fact, one of the reasons for alloying steels is
to facilitate the formation of martensite so
that totally martensitic structures can develop
in relatively thick cross sections.
58. • Figure 10.28 shows the continuous cooling
transformation diagram for the same alloy
steel for which the isothermal transformation
diagram is presented in Figure 10.23
59.
60. • Several cooling curves superimposed on
Figure 10.28 indicate the critical cooling rate,
and also how the transformation behavior and
final microstructure are influenced by the rate
of cooling
61.
62. • Interestingly enough, the critical cooling rate
is diminished even by the presence of carbon.
• In fact, iron–carbon alloys containing less than
about 0.25 wt% carbon are not normally heat
treated to form martensite because quenching
rates too rapid to be practical are required
63. • Other alloying elements that are particularly
effective in rendering steels heat treatable are
chromium, nickel, molybdenum, manganese,
silicon, and tungsten;
• however, these elements must be in solid
solution with the austenite at the time of
quenching.
64. Summery
• In summary, isothermal and continuous cooling
transformation diagrams are, in a sense, phase
diagrams in which the parameter of time is introduced.
• Each is experimentally determined for an alloy of
specified composition, the variables being temperature
and time.
• These diagrams allow prediction of the microstructure
after some time period for constant temperature and
continuous cooling heat treatments, respectively.
65. MECHANICAL BEHAVIOR OF
IRON–CARBON ALLOYS
Mechanical behavior of iron–carbon alloys having the
microstructures discussed heretofore—namely, fine and coarse
pearlite, spheroidite, bainite, and martensite.
For all but martensite, two phases are present (i.e., ferrite and
cementite), and so an opportunity is provided to explore several
mechanical property-microstructure relationships that exist for
these alloys.
66. Pearlite
• Cementite (Fe3C) is much harder but more
brittle than ferrite.
• Thus, increasing the fraction of (Fe3C) in a
steel alloy while holding other microstructural
elements constant will result in a harder and
stronger material.
67. • This is demonstrated in Figure 10.29a, in
which the tensile and yield strengths as well as
the Brinell hardness number are plotted as a
function of the weight percent carbon
(or equivalently as the percentage of Fe3C) for
steels that are composed of fine pearlite.
• All three parameters increase with increasing
carbon concentration.
68. • Inasmuch as cementite is more brittle,
increasing its content will result in a decrease
in both ductility and toughness (or impact
energy).
• These effects are shown in Figure 10.29b for
the same fine pearlitic steels
69.
70. Effect of Layer thickness
• The layer thickness of each of the ferrite and cementite
phases in the microstructure also influences the
mechanical behavior of the material.
• Fine pearlite is harder and stronger than coarse pearlite, as
demonstrated in
• Coarse pearlite is more ductile than fine pearlite,
as demonstrated in
• Figure 10.30a & b, which plots hardness versus the carbon
concentration.
71.
72. Reason 1
• Phase boundaries serve as barriers to dislocation
motion in much the same way as grain boundaries
• For fine pearlite there are more boundaries
through which a dislocation must pass during
plastic deformation.
• Thus, the greater reinforcement and restriction of
dislocation motion in fine pearlite account for its
greater hardness and strength.
73. Reason 2
• The reasons for this behavior relate to
phenomena that occur at the α–Fe3C phase
boundaries.
• First, there is a large degree of adherence
between the two phases across a boundary.
• Therefore, the strong and rigid cementite phase
severely restricts deformation of the softer ferrite
phase in the regions adjacent to the boundary;
74. • Thus the cementite may be said to reinforce
the ferrite.
• The degree of this reinforcement is
substantially higher in fine pearlite because of
the greater phase boundary area per unit
volume of material.
75. Coarse pearlite
• Coarse pearlite is more ductile than fine pearlite,
as illustrated in Figure 10.30b,
• which plots percentage reduction in area versus
carbon concentration for both microstructure
types.
• This behavior results from the greater restriction
to plastic deformation of the fine pearlite.
76.
77. Spheroidite
• Other elements of the microstructure relate to
the shape and distribution of the phases.
• In this respect, the cementite phase has
distinctly different shapes and arrangements
in the pearlite and spheroidite microstructures
(Figures 10.19).
78. • Alloys containing pearlitic microstructures
have greater strength and hardness than do
those with spheroidite.
• This is demonstrated in Figure 10.30a, which
compares the hardness as a function of the
weight percent carbon for spheroidite with
both the other pearlite structure types
79. Reason
• This behavior is again explained in terms of
reinforcement at, and impedance to, dislocation motion
across the ferrite–cementite boundaries as discussed
above.
• There is less boundary area per unit volume in
spheroidite, and consequently plastic deformation is not
nearly as constrained, which gives rise to a relatively soft
and weak material.
• In fact, of all steel alloys, those that are softest and
weakest have a spheroidite microstructure.
80. • As would be expected, spheroidized steels are
extremely ductile, much more than either fine
or coarse pearlite (Figure 10.30b).
• In addition, they are notably tough because
any crack can encounter only a very small
fraction of the brittle cementite particles as it
propagates through the ductile ferrite matrix.
81.
82. Bainite
• Because bainitic steels have a finer structure
(i.e., smaller α-ferrite and Fe3C particles), they
are generally stronger and harder than
pearlitic ones; yet they exhibit a desirable
combination of strength and ductility.
83. • Figure 10.31 shows the influence of
transformation temperature on the tensile
strength and hardness for an iron–carbon
alloy of eutectoid composition;
• Temperature ranges over which pearlite and
bainite form (consistent with the isothermal
transformation diagram for this alloy, Figure
10.18) are noted at the top of Figure 10.31
84.
85. Martensite
• Of the various microstructures that may be
produced for a given steel alloy, martensite is the
hardest and strongest and, in addition, the most
brittle; it has, in fact, negligible ductility.
• Its hardness is dependent on the carbon content,
up to about 0.6 wt% as demonstrated in
• Figure 10.32, which plots the hardness of
martensite and fine pearlite as a function of
weight percent carbon
86.
87. • In contrast to pearlitic steels, strength and
hardness of martensite are not thought to be
related to microstructure.
• Rather, these properties are attributed to the
effectiveness of the interstitial carbon atoms in
hindering dislocation motion (as a solid-solution
effect), and to the relatively few slip systems
(along which dislocations move) for the BCT
structure.
88. • Austenite is slightly denser than martensite, and
therefore, during the phase transformation upon
quenching, there is a net volume increase.
• Consequently, relatively large pieces that are
rapidly quenched may crack as a result of internal
stresses; this becomes a problem especially when
the carbon content is greater than about 0.5 wt%.
89. Concept Check
• Rank the following iron-carbon alloys and
associated microstructures from the highest
to the lowest tensile strength:
0.25 wt%C with spheroidite
0.25 wt%C with coarse pearlite
0.6 wt%C with fine pearlite, and
0.6 wt%C with coarse pearlite.
91. Definition
• In the as-quenched state, martensite, in addition
to being very hard, is so brittle that
• It cannot be used for most applications; also, any
internal stresses that may have been introduced
during quenching have a weakening effect
• The ductility and toughness of martensite may be
enhanced and these internal stresses relieved by
a heat treatment known as tempering.
92. Treatment
• Tempering is accomplished by heating a
martensitic steel to a temperature below the
eutectoid for a specified time period.
• Normally, tempering is carried out at
emperatures between 250 to 650 ̊C.
• Internal stresses, however, may be relieved at
temperatures as low as 200 C
93. Formation of tempered martensite
• This tempering heat treatment allows, by diffusional
processes, the formation of tempered martensite,
according to the reaction.
where the single-phase BCT martensite, which is supersaturated with carbon,
transforms to the tempered martensite, composed of the stable ferrite and
cementite phases, as indicated on the iron–iron carbide phase diagram
94. Microstructure
• The microstructure of tempered martensite consists of
extremely small and uniformly dispersed cementite
particles embedded within a continuous ferrite matrix.
• This is similar to the microstructure of spheroidite
except that the cementite particles are much, much
smaller.
• An electron micrograph showing the microstructure of
tempered martensite at a very high magnification is
presented in Figure 10.33
95.
96. • Tempered martensite may be nearly as hard
and strong as martensite, but with
substantially enhanced ductility and
toughness.
• The hardness versus-weight percent carbon
plot of Figure 10.32 is included a curve for
tempered martensite
97.
98. • The hardness and strength may be explained by
the large ferrite–cementite phase boundary area
per unit volume that exists for the very fine and
numerous cementite particles.
• Again, the hard cementite phase reinforces the
ferrite matrix along the boundaries, and these
boundaries also act as barriers to dislocation
motion during plastic deformation.
• The continuous ferrite phase is also very ductile
and relatively tough, which accounts for the
improvement of these two properties for
tempered martensite.
99. particles influences
• The size of the cementite particles influences
the mechanical behavior of tempered
martensite;
• Increasing the particle size decreases the
ferrite–cementite phase boundary area and,
consequently, results in a softer and weaker
material yet one that is tougher and more
ductile.
100. Heat treatment variables
• Furthermore, the tempering heat treatment
determines the size of the cementite particles.
• Heat treatment variables are temperature and
time, and most treatments are constant-
temperature processes.
101. • Since carbon diffusion is involved in the
martensite-tempered martensite transformation,
• increasing the temperature will accelerate
diffusion, the rate of cementite particle growth,
and, subsequently, the rate of softening.
• The dependence of tensile and yield strength and
ductility on tempering temperature for an alloy
steel is shown in Figure 10.34.
102.
103. Time
• Before tempering, the material was quenched in
oil to produce the martensitic structure; the
tempering time at each temperature was 1 h.
• This type of tempering data is ordinarily provided
by the steel manufacturer.
• The time dependence of hardness at several
different temperatures is presented in Figure
10.35 for a water-quenched steel of eutectoid
composition
104.
105. • With increasing time the hardness decreases, which
corresponds to the growth and coalescence of the
cementite particles.
• At temperatures approaching the eutectoid [ 700 C]
and after several hours, the microstructure will have
become spheroiditic (Figure 10.19),
• with large cementite spheroids embedded within the
continuous ferrite phase. Correspondingly,
overtempered martensite is relatively soft and ductile.
106.
107. Temper Embrittlement
• The tempering of some steels may result in a
reduction of toughness as measured by impact
tests ; this is termed temper embrittlement.
• The phenomenon occurs when the steel is
tempered at a temperature above about 575 C.
followed by slow cooling to room temperature, or
when tempering is carried out at between
approximately 375 TO 575 C.
108. • Steel alloys that are susceptible to temper
embrittlement have been found to contain
appreciable concentrations of the alloying
elements manganese, nickel, or chromium
and, in addition, one or more of antimony,
phosphorus, arsenic, and tin as impurities in
relatively low concentrations.
109. REVIEW OF PHASE TRANSFORMATIONS AND
MECHANICAL PROPERTIES FOR IRON–
CARBON ALLOYS
