Heat Treatment

Definition
• Heat treatment may be defined as an operation
or combination of operations involving the
heating and cooling of a metal or alloy in solid
state for the purpose of obtaining certain
desirable conditions or properties.
 Types
1) Annealing
(a) Stress-relief
(b) Process annealing
(c) Spheroidising
(d) Full annealing
2) Normalizing
3) Hardening
4) Tempering
5) Martempering
6) Austempering
7) Maraging

Purpose
1) Cause relief of internal stresses developed during production processes.
2) Harden and strengthen Metals
3) Improve machinability
4) Change grain size
5) Soften metals for further working as in wire drawing/cold rolling
6) Improve ductility and toughness
7) Increase heat, wear and corrosion resistance
8) Improve electrical and magnetic properties
9) Homogenise structure
 Stages of Heat treatment

(1) heating to the desired temperature,

(2) holding or “soaking” at that temperature,
(3) cooling, usually to room temperature.

• Important factors
– Solubility
– Cooling rates
– Heating temperature

• A variety of annealing heat treatments are possible; they are characterized

by the changes that are induced, which many times are microstructural and
are responsible for the alteration of the mechanical properties.

Annealing

• The term annealing refers to a heat treatment in which a material is

exposed to an elevated temperature for an extended time period and then
slowly cooled (inside furnace)

Ordinarily, annealing is carried out to

(1) relieve stresses;

(2) increase softness, ductility, and toughness; and/or
(3) produce a specific microstructure.
• Any annealing process consists of three stages:

(1) heating to the desired temperature,

(2) holding or “soaking” at that temperature,
(3) cooling, usually to room temperature.

Time is an important parameter in these procedures

• During heating and cooling, there exist temperature gradients between the
outside and interior portions of the piece; their magnitudes depend on the
size and geometry of the piece.

• If the rate of temperature change is too great, temperature gradients and

internal stresses may be induced that may lead to warping or even cracking.

• Also, the actual annealing time must be long enough to allow for any
necessary transformation reactions.

• Annealing temperature is also an important consideration; annealing

may be accelerated by increasing the temperature, since diffusional
processes are normally involved

Stress Relief
• Internal residual stresses may develop in metal pieces in
response to the following:

(1) Plastic deformation processes such as machining and

grinding;

(2) Nonuniform cooling of a piece that was processed or

fabricated at an elevated temperature, such as a weld or
a casting; and

(3) A phase transformation that is induced upon cooling

wherein parent and product phases have different
densities
• Distortion and warpage may result if these
residual stresses are not removed.

• They may be eliminated by a stress relief

annealing heat treatment in which the piece is
heated to the recommended temperature, held
there long enough to attain a uniform
temperature, and finally cooled to room
temperature in air.

Process annealing
• Process annealing is a heat treatment that is used to
remove the effects of cold work—that is, to soften and
increase the ductility of a previously strain-hardened
metal.

• It is commonly utilized during fabrication procedures that

require extensive plastic deformation, to allow a
continuation of deformation without fracture or excessive
energy consumption.

• Ordinarily a fine-grained microstructure is desired, and

therefore, the heat treatment is terminated before
appreciable grain growth has occurred.

• Surface oxidation or scaling may be prevented or

minimized by annealing at a relatively low temperature
 Normalizing

• Steels that have been plastically deformed by,

for example, a rolling operation, consist of grains
of which are irregularly shaped and relatively
large, but vary substantially in size.

• An annealing heat treatment called normalizing

is used to refine the grains (i.e., to decrease the
average grain size) and produce a more uniform
and desirable size distribution; fine-grained
steels are tougher than coarse-grained ones

Hardening

• Term hardening refers to that process of cooling

by which the steel is made hard.

The steel is quenched in some medium (liquid ;

water, oil., oil-water emulsion, salt bath),
whereby heat is removed from the steel at the
desired rate.

Cooling rate controlled by the selection of the

proper quenching medium.
Quenching characteristics of liquid
Temperature of the medium
Specific heat
Thermal conductivity of medium
Viscosity
Agitation; flow of coolant
 Tempering

• Tempering is the process of heating a hardened

steel to a recommended temperature, then
cooling at a desirable rate.

• Objective of tempering is to reduce the hardness

and relives the internal stress of quenched steel
in order to obtained greater ductility than is
associated with the high hardness of the
quenched steel

Recovery, Recrystallization,
and Grain Growth
• These properties and structures may revert back
to the precold-worked states by appropriate heat
treatment (sometimes termed an annealing
treatment).

• Such restoration results from two different

processes that occur at elevated temperatures:

• recovery and recrystallization, which may be

followed by grain growth.

RECOVERY
• During recovery, some of the stored internal strain energy is
relieved by virtue of dislocation motion (in the absence of an
externally applied stress), as a result of enhanced atomic diffusion at
the elevated temperature.

• There is some reduction in the number of dislocations, and

dislocation configurations are produced having low strain energies.

• In addition, physical properties such as electrical and thermal

conductivities and the like are recovered to their precold-worked
states.
 RECRYSTALLIZATION
• Even after recovery is complete, the grains are still in a
relatively high strain energy state.

• Recrystallization is the formation of a new set of

strain-free and equiaxed grains (i.e., having
approximately equal dimensions in all directions) that
have low dislocation densities and are characteristic of
the precold-worked condition.

• The driving force to produce this new grain structure is

the difference in internal energy between the strained
and unstrained material.

• The new grains form as very small nuclei and grow until
they completely consume the parent material, processes
that involve short-range diffusion.

• Several stages in the recrystallization process are

represented in Figures 7.21a to 7.21d; in
 (33%CW) the very small grains are those that have Partial replacement of cold-worked
recrystallized. grains by recrystallized ones

Complete recrystallization Grain growth after 15 min at 580 ̊C ( f ) Grain growth after 10 min 700 C

• Also, during recrystallization, the mechanical properties

that were changed as a result of cold working are
restored to their precold-worked values; that is, the metal
becomes softer, weaker, yet more ductile.

• Some heat treatments are designed to allow

recrystallization to occur with these modifications in the
mechanical characteristics
• Recrystallization is a process the extent of which
depends on both time and temperature.

• The degree (or fraction) of recrystallization

increases with time, as may be noted in the
photomicrographs shown in Figures 7.21a–d.

Influence of temperature
• The influence of temperature is demonstrated in
Figure 7.22, which plots tensile strength and
ductility (at room temperature) of a brass alloy
as a function of the temperature and for a
constant heat treatment time of 1 h.

• The grain structures found at the various stages

of the process are also presented schematically
• Typically, it is between one-third and one-half of the
absolute melting temperature of a metal or alloy and
depends on several factors,

• including the amount of prior cold work and the purity of

the alloy.

• Increasing the percentage of cold work enhances the

rate of recrystallization, with the result that the
recrystallization temperature is lowered, and approaches
a constant or limiting value at high deformations; this
effect is shown in Figure 7.23.
• Recrystallization proceeds more rapidly in pure metals
than in alloys. During recrystallization, grain-boundary
motion occurs as the new grain nuclei form and then
grow.
• It is believed that impurity atoms preferentially segregate
at and interact with these recrystallized grain boundaries
so as to diminish their (i.e., grain boundary) mobilities;
this results in a decrease of the recrystallization rate and
raises the recrystallization temperature, sometimes quite
substantially.
 GRAIN GROWTH
• After recrystallization is complete, the strain-free grains will continue
to grow if the metal specimen is left at the elevated temperature
(Figures 7.21d–f ); this phenomenon is called grain growth.

• Grain growth does not need to be preceded by recovery and

recrystallization; it may occur in all polycrystalline materials, metals
and ceramics alike

• As grains increase in size, the total

boundary area decreases, yielding an
attendant reduction in the total energy; this
is the driving force for grain growth.
• Grain growth occurs by the migration of grain
boundaries. Obviously, not all grains can
enlarge, but large ones grow at the expense of
small ones that shrink.

• Thus, the average grain size increases with

time, and at any particular instant there will exist
a range of grain sizes.
• The dependence of grain size on time and temperature is
demonstrated in Figure 7.25, a plot of the logarithm of grain size as
a function of the logarithm of time for a brass alloy at several
temperatures.

• At lower temperatures the curves are linear.

• Furthermore, grain growth proceeds more rapidly as temperature

increases; that is, the curves are displaced upward to larger grain
sizes.
• This is explained by the enhancement of diffusion rate with rising
temperature
 Summery
• The micro structural and mechanical characteristics of a plastically deformed metal
specimen may be restored to their predeformed states by an appropriate heat
treatment, during which recovery, recrystallization, and grain growth processes are
allowed to occur.

• During recovery there is a reduction in dislocation density and alterations in

dislocation configurations.

• Recrystallization is the formation of a new set of grains that are strain free; in
addition, the material becomes softer and more ductile.

• Grain growth is the increase in average grain size of polycrystalline materials, which
proceeds by grain boundary motion.

Diffusion Treatments
• DT can be applied to add certain
elements/cabron; nitrogen into the surface
that will make
 Effect of non equilibrium cooling on microstructure
and properties of steel

TTT diagram for 0.8% carbon steel

Continuous cooling Transformation curves

• 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.
 TTT diagram

• 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.
 Click to add text

• 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
• 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.
 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.

• 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.
 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.
• 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.

Other micro constitute

• Bainite
• Spheroidite
• Martensite

• CONTINUOUS COOLING TRANSFORMATION DIAGRAMS

 Coarse and Fine Pearlite

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.
• 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.

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
• 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

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.

• 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

• 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
• 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 diffusion

less and instantaneous, it is not depicted in this
diagram as the pearlitic and bainitic reactions are.
• 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.

• 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
 Example

• Consider an alloy of eutectoid composition that is

very rapidly cooled from a temperature above
(727C ) to, say, (165C).

• From the isothermal transformation 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
 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.
Exercise
CONTINUOUS COOLING
TRANSFORMATION
DIAGRAMS
 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.

• 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.
 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.
• 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.

• 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.
• The microstructural products for the
moderately rapid and slow cooling rate curves
in Figure 10.26 are fine and coarse pearlite,
respectively
• 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

• 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.
 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
 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.

• 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.
• 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
• 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
• 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

• 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.
 Summary
• 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.

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.
 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.
• 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.

• 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
 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.
 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.
 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;

• 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.
 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.
 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).

• 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
 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.

• 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.
 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.
• 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
 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.
Click to add text

• 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
• 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.

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 martensite.
• 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%.

TEMPERED MARTENSITE
 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.

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
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

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
 Click to add text

• 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
• 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.
 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.

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.
• 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.
 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
• 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.
 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.

• 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.
REVIEW OF PHASE TRANSFORMATIONS AND
MECHANICAL PROPERTIES FOR IRON–
CARBON ALLOYS
Hardenability

Reed – Hill Ch. 19

Isothermal Transformation Kinetics
T Time-Temperature-Transformation curve
(TTT-curve)

Time to 50% completion

time
Mat E 443 Ferrous Metallurgy
Iowa State University
R.E. Napolitano

Isothermal TTT Curves

T

10% 50% 90%

time

Mat E 443 Ferrous Metallurgy

Iowa State University
R.E. Napolitano
 Isothermal TTT Curves
T

T1

10% 50% 90%

time
1
0.9

f = fraction transformed
f 0.5

0.1 Metallurgy
Mat E 443 Ferrous
0 Iowa State University
R.E. Napolitano

Isothermal TTT Curves

T T4
T3
T2
T1

T5

T6

10% 50% 90%

time
1

T5 T6
T1
T2 T3 T4
f

Mat E 443 Ferrous Metallurgy

0 Iowa State University
R.E. Napolitano
 Phase Prediction from the TTT

CCT from the TTT

cooling curves on log(t) scale

not isothermal

as you are cooling, time is passed at

higher temperatures where nucleation
start is far away

by the time you get to lower T,

nucleation gets “pushed” further right
 Ms and Mf
Alloying additions affect Ms and Mf

determines where you have to cool to for complete martensite

Mf determines retained austenite

machining problems!

Totten, G.E. et. al. Quenchants and Quenching Technology

Retained Austenite Issues

Austenite = soft (decreased part hardness)

subsequent transformation to martensite during machining or end-use

expansion -> internal stresses -> loss of machining tolerance

hard martensite appearing in otherwise tempered microstructure

1% C, 1% Mn, and 0.4% Mo – 40% retained γ

George Langford, Sc.D., Massachusetts Institute of Technology

 Critical cooling rate

What does Hardenability Allow?

High Hardenability – Oil Quench

Moderate Hardenability – Oil Quench

Low Hardenability – Water Quench

Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques

Hardenability is an inherent property of the material

Hardening (hardness distribution or depth profile) is a function of processing factors

1. Shape and size of the cross section

2. Hardenability of the material
3. Quenching conditions
 Sample Geometry
Sample geometry plays a large role in the heat treatment of real components

Thermal conductivity of steel is 54 W/mK @ 25C

better than air (0.024) or water (0.58), but <<<<< ∞

Outer surface may cool at “water quench” rate
inside will not

Distortion
Fundamental Causes

1) Residual stresses that cause shape change when they exceed the material yield
strength

2) Stresses caused by differential expansion due to thermal gradients. These stresses

will increase with the thermal gradient and will cause plastic deformation as the yield
strength is exceeded.

3) Volume changes due to transformational phase change. These volume changes will
be contained as residual stress systems until the yield strength is exceeded.

Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques
 Thermal Distortion – Residual Stresses

As heat is applied expansion will occur. The regions of

tension and compression that result are directly related
to this expansion. Upon cooling this situation will reverse
itself.

Constraining the piece can cause residual stresses to

exist.

Question: What residual stresses exist in these figures?

Chumbley, S. Mat E 444 - Corrosion and Failure Analysis. Iowa State University. 2009.

Thermal Distortion – Residual Stresses

Always remember that the last material to cool

will be in tension. Spot heat or partial heating of
a part (e.g. flame cutting a beam, welding) can
cause local stresses and distortions.

Do these residual stresses make sense?

Chumbley, S. Mat E 444 - Corrosion and Failure Analysis. Iowa State University. 2009.
 Transformational Distortion

Vα(BCC)<Vγ(FCC)<Vα’ (BCT)

(well known volume changes)

However, all three events occur simultaneously.

Other factors:
heating/cooling rate
geometry
inconsistent material composition

In steels when hardening using the martensitic transformation the general rule is that
the metal that is the last metal to harden will be in compression

Quench Cracking
Example - cracked replacement retention pin cap for ripper tip wearout

Cracks were noticed after 32 hours in the field

Vetterick. et. al. Retention Pin Cap Failure Analysis. Mat E 444. Iowa State University. 2009
 Quench Cracking
Example - cracked replacement retention pin cap for ripper tip wearout

Vetterick. et. al. Retention Pin Cap Failure Analysis. Mat E 444. Iowa State University. 2009

How do we Prevent Quench Cracking?

Hardenability is an inherent property of the material

Hardening (hardness distribution or depth profile) is a function of processing factors

1. Shape and size of the cross section

2. Hardenability of the material
3. Quenching conditions

Totten, G.E. et. al. Steel Heat Treatment: Metallurgy and Techniques
 Measuring Hardenability
Grossmann’s Hardenability Concept

quench cylinders of a given material into a given quenchant

defined 50% martensite as criteria for hardenability

critical diameter Dcrit defined as the diameter where 50% martensite

Dcrit is valid only for the quenching media used

Measuring Hardenability
Grossmann’s Hardenability Concept

defined 50% martensite as criteria for hardenability

critical diameter Dcrit defined as the diameter where 50% martensite

Why? Very easy to determine using optical microscopy

Brooks, Charlie R. Principles of the heat treatment of plain carbon and low alloy steels. ASM International. 1996.
 Quench Severity
Grossmann’s Hardenability Concept

introduced the quenching intensity (severity) factor H

describes quenching medium and its condition

defined ideal critical diameter DI

diameter of a given steel that would produce 50% martensite at the center
when quenched in a bath of quenching intensity H

H indicates a hypothetical quenching

intensity that reduces the surface temperature
of the heated steel to the bath temperature in
zero time

Quench Severity
Grossmann’s Hardenability Concept

Steel A:
quenched in still water (H=1.0)
Dcrit = 28mm

Steel B:
quenched in oil (H=0.4)
Dcrit = 20mm

However,
Steel A:
DI = 48mm

Steel B:
DI = 52mm

By definition, Steel B has a higher hardenability than Steel A

independent of quenching medium

 Measuring Hardenability (Jominy)
Grossmann’s method
defines useful terms, Dcrit and HI
time consuming

Jominy method

http://www.fagteori.dk/media/8947/jominy-test.gif

Measuring Hardenability (Jominy)

Cooling rate differential along bar

Hardness variation for determination of DI

 Measuring Hardenability (Jominy)
Hardness variability = Hardness bands

What determines Hardenability?

Prior austenite grain size
smaller grain size = greater pearlite nucleation

greater pearlite nucleation = lower hardenability

ASTM grain size No. 7 has 4x the grain-boundary area of ASTM No. 3

larger grains = increased brittleness, loss of ductility

increased quench cracking

Carbon content
>%C = increased hardenability

still very low

AISI 1045 ASTM No. 7
DI = 5.6mm (0.22 in)

pearlite and proeutectoid phases

more difficult to form at higher C

austenitized in γ + Fe3C nucleates pearlite

 What determines Hardenability?
Alloying
all alloying elements increase hardenability in steel (to varying degrees)

exception: Cobalt
increases nucleation and growth of pearlite

alter the phase diagram

prevent decomposition of γ -> α + Fe3C (pearlite)

For 4340 – have 90 seconds to cool to Ms

What determines Hardenability?

Alloying
prediction based on multiplying factors
 Surface hardening treatment

Surface Hardening
• Gears, Camshafts, Pistons rings etc.
– Hard wearing surface for constant wear in service.
– Tough core to withstand the shock loads

• Such heat treatments are called Surface hardening heat

treatments.

• 2 Methods
– Whole component is heated
– Only surface is heated.
 Component is heated
• 2 - Step process
– Step 1:- Diffusing the interstial elements (C, N)
– Step 2 :- Heat treatment
• Case Hardening
– Steels with Carbon 0.15%
– Carbon can go up to max 0.2% and Nickel - Chromium may be added
up to 3%.
• Nitriding
– Special alloy steels
• Cyaniding
– For low carbon steels.

Case Hardening
• 3 methods for diffusion
– Solid carburizing (Pack carburizing)
– Liquid carburizing (Salt bath Carburizing)
– Gas Carburizing
• Solid Carburizing
– Packed with 80% Coal and 20% Barium carbonate
– Heated at 930°C for specific time
– 4 to 8 Hours for depth of 1 to 3 mm.

BaCO3  BaO + CO2

CO2 + C  2CO
Carbon reacts with the surface as
2CO + Fe  Fe (C) + CO2
 Case Hardening
• Liquid carburizing (Salt bath Carburizing)
– Due to excessive time and Uniform depth is not assured.
– Molten cyanide is used
– Uses NACN/KCN as a catalyst
– Temperature of about 900 to 910 C
– Shorter heating times – Clean layer up to 3 mm easily achieved.

BaCl2 + NACN  Ba(CN)2 + NACL

Ba(CN)2 + Fe  FE (C) + By products

Case Hardening
• Gas Carburizing
– Most widely used. Time taken is less.
– Methane is used. Decomposition occurs due to which C is added on
surface.
– Thickness depends on flow rate and temperature
– Done in closed furnace.
Click Temp
to add between
text
Click to add text
900°C to 950°C.

2 CO  CO2 + C
C + Fe  Fe (C)
 Case Hardening – Heat treatment
• Carburizing doesn’t harden steel. Only adds additional layers
of C.
• 850°C to 875°C , holding or soaking followed by quenching.
• Again quenching at 775°C. to get fine microstructure.
• Tempering is certain times done at 170°C.

Nitriding
• Special alloys heating in Nitrogen environment.
• Gas is used. Temperature of about 450°C to 540°C.
• Nitrogen is readily absorbed by the steel surface and forms
complex compounds With Fe and other elements in the stee.
• Time is 90 to 110 Hours. Cooled or quenched.
2NH3  3H2 + 2N
• Highest Hardness. Done on majorly finished goods.
• Costly Process. Done for following.
– To get really super hard wearing surface.
– To resist corrosion
– Where resistance to softening of hardened surface is required.
 Cyaniding
• Introducing both carbon and nitrogen on surface of low carbon
steel.
• Carried in liquid bath (NaCN).
• Heating up to 800°C in molten bath for 30 min to 3 hours
depending on thickness required.
2NaCN +O2  2NaCNO
2NaCNO +O2  Na2CO3 + CO + 2N
2CO  CO2 + C
• Followed by quenching. Soft core and hard-wear resistant
surface obtained.
• Depth up to 2.5 mm easily obtained.