3. Heat Treatment (HT)
• INTRODUCTION:- There are interrelationship among
the structure, properties, processing and performance
of engineering materials.
• In processing structure and companion properties are
manipulated and controlled.
• HT is the term used to describe the controlled heating
& cooling of materials for the primary purpose of
altering their structures and properties.
• Both physical and mechanical properties can be
altered by HT (e.g. strength, ductility, hardness,
toughness, machine ability, wear & corrosion
resistance, etc)
• Can be done for strengthening purpose (Converting
structure to martensite)
• Can be done for softening & conditioning purposes
Annealing, tempering, etc.)
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4. Heat Treatment (HT)
• These changes can be induced with no
concurrent1 change in product shape.
• HT is one of the most important and widely
used manufacturing processes.
• Technically the term heat treatment applies
only to processes where heating & cooling are
performed for the specific purpose of altering
properties; but heating & cooling often occur
as incidental phases of other manufacturing
processes such as hot forming or welding.
• The material properties will be altered,
however, just as though an intentional HT had
been performed, & the result can be either
beneficial or harmful
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5. Heat Treatment (HT)
• For this reason both the individual who selects material &
the person who specifies its processing must be fully
aware of the possible changes that can occur during
heating or cooling activities.
• HT should be fully integrated with other manufacturing
processes if effective results are to be obtained.
• More than 90 % of all HT is performed on steel & other
ferrous metals.
• Steel=0.06% to 1.0 % C, 0.6% ideal content for HT.
• The FCC can hold more carbon in solution and on
rapid cooling the crystal structure wants to return
to its BCC structure. It cannot due to trapped
carbon atoms. The net result is a distorted crystal
structure called body centered tetragonal (BCT)
called martensite.
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6. Heat Treatment (HT)
• 5.2 Processing heat treatments(PHT) The term
HT is often associated with those thermal
processes that increase the strength of a
material, but the broader definition permits
inclusion of an other set of processes that we
will call processing heat treatments.
• These are often performed as a means of
preparing the material for fabrication.
• Specific objectives may be the improvement
of machining characteristics, the reduction of
forming forces or the restoration of ductility
to enable further processing.
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7. Heat Treatment (HT)
• Equilibrium Diagrams as Aids:-Most of the PHTs
involves rather slow cooling or extended times at
elevated temperatures.
• These conditions tend to approximate equilibrium, and
the resulting structures, therefore, can be reasonably
predicted through the use of an equilibrium phase
diagram.
• These diagrams can be used to determine the temp
that must be attained to produce a desired starting
structure, & to describe the changes that will then
occur upon subsequent cooling.
• It should be noted, however, that these diagrams are
for true equilibrium conditions, and any departure
from equilibrium may lead to substantially different
results
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9. Solid Phases in the Iron Carbon Phase Diagram
• α-ferrite, Austenite (γ), Cementite(Fe3C), and , δ-
ferrite
• α-ferrite – solid solution of carbon in α-iron. α-
ferrite has BCC crystal structure and low solubility
of carbon – up to 0.025% at (723ºC). α-ferrite exists
at room temperature. The solubility of carbon in α-
ferrite decreases to 0.005 % at 0ºC
• Austenite (γ)– interstitial solid solution of carbon in
γ-iron. Austenite has FCC crystal structure,
permitting high solubility of carbon – up to 2.06%
at 1147 ºC. Austenite does not exist below 723ºC
and maximum carbon concentration at this
temperature is 0.83%.
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10. Solid Phases in the Iron Carbon Phase Diagram
• Cementite(Fe3C)Unlike ferrite and austenite, cementite
is a very hard inter metallic compound consisting of
6.67% carbon and the remainder (93.3%) iron, but
when mixed with soft ferrite layers its average
hardness is reduced considerably. Slow cooling gives
course perlite; soft easy to machine but poor
toughness. Faster cooling gives very fine layers of
ferrite and cementite; harder and tougher.
• δ-ferrite – The interstitial Solid solution of carbon in
iron. Maximum concentration of carbon in δ-ferrite is
0.09% at 1493ºC– temperature of the peritectic
transformation. The crystal structure of δ-ferrite is like
α-ferrite (BCC) but with a greater lattice constant.
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11. Critical temperatures
• Upper critical temperature (point) A3 is the
temperature, below which ferrite starts to form as a
result of ejection from austenite in the hypoeutectoid
alloys.
• Upper critical temperature (point) ACM is the
temperature, below which cementite starts to form as
a result of ejection from austenite in the
hypereutectoid alloys
• Lower critical temperature (point) A1 is the
temperature of the austenite-to-pearlite eutectoid
transformation. Below this temperature austenite
does not exist.
• Magnetic transformation temperature A2 is the
temperature below which α-ferrite is ferromagnetic.
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12. Transformation on heating cooling
• Hypoeutectoid steels (carbon content from 0 to 0.83%)
consist of primary (proeutectoid) ferrite (according to the
curve A3) and pearlite.
• Eutectoid steel (carbon content 0.83%) entirely consists of
pearlite.
• Hypereutectoid steels (carbon content from 0.83 to 2.06%)
consist of primary (proeutectoid)cementite (according to
the curve ACM) and pearlite.
• Cast irons (carbon content from 2.06% to 4.3%) consist of
proeutectoid cementite C2 ejected from austenite
according to the curve ACM , pearlite and transformed
ledeburite (ledeburite in which austenite transformed to
pearlite).
• Carbon A very small interstitial atom that tends to fit into
clusters of iron atoms. It strengthens steel and gives it the
ability to harden by heat treatment.
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13. Heat Treatment (HT)
• It also causes major problems for welding ,
particularly if it exceeds 0.25% as it creates a hard
microstructure that is susceptible1 to hydrogen
cracking.
• Carbon forms compounds with other elements called
carbides. Iron Carbide, Chrome Carbide etc.
• The diagram shown below is based on the
transformation that occurs as a result of slow
heating. Slow cooling will reduce the transformation
temperatures; for example: the A1 point would be
reduced from 723°C to 690 °C.
• However the fast heating and cooling rates
encountered in welding will have a significant
influence on these temperatures, making the accurate
prediction of weld metallurgy using this diagram
difficult.
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14. Heat Treatment (HT)
• Pearlite A mixture of alternate strips of ferrite
and cementite in a single grain.
• The distance between the plates and their
thickness is dependant on the cooling rate of the
material; fast cooling creates thin plates that are
close together and slow cooling creates a much
coarser structure possessing less toughness.
• The name for this structure is derived from its
mother of pearl appearance under a
microscope.
• A fully pearlitic structure occurs at 0.8% Carbon.
• Further increases in carbon will create cementite
at the grain boundaries, which will start to
weaken the steel.
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15. Heat Treatment (HT)
• Cooling of a steel below 0.8% carbon When a
steel solidifies it forms austenite.
• When the temperature falls below the A3 point,
grains of ferrite start to form.
• As more grains of ferrite start to form the
remaining austenite becomes richer in carbon.
• At about 723°C the remaining austenite, which
now contains 0.8% carbon, changes to pearlite.
• The resulting structure is a mixture consisting of
white grains of ferrite mixed with darker grains
of pearlite.
• Heating is basically the same thing in reverse.
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16. Heat Treatment (HT)
• Martensite If steel is cooled rapidly from austenite, the
F.C.C structure rapidly changes to B.C.C leaving
insufficient time for the carbon to form pearlite.
• This results in a distorted structure that has the
appearance of fine needles.
• There is no partial transformation associated with
martensite, it either forms or it doesn’t.
• However, only the parts of a section that cool fast
enough will form martensite; in a thick section it will
only form to a certain depth, and if the shape is
complex it may only form in small pockets.
• The hardness of martensite is solely dependant on
carbon content, it is normally very high, unless the
carbon content is exceptionally low.
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17. • There are five basic heat treating processes:
hardening, case hardening, annealing,
normalizing, and tempering.
• Although each of these processes bring about
different results in metal, all of them involve
three basic steps: heating, soaking, and cooling.
Heating is the first step in a heat-treating
process. Many alloys change structure when
they are heated to specific temperatures.
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18. Heat Treatment (HT)
• The structure of an alloy at room
temperature can be either a mechanical
mixture, a solid solution, or a combination
solid solution and mechanical mixture.
• A mechanical mixture can be compared to
concrete. Just as the. sand and gravel are
visible and held in place by the cement.
• The elements and compounds in a
mechanical mixture are clearly visible and are
held together by a matrix of base metal.
• A solid solution is when two or more metals
are absorbed, one into the other, and form a
solution.
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19. Heat Treatment (HT)
• When an alloy is in the form of a solid solution,
the elements and compounds forming the metal
are absorbed into each other in much the same
way that salt is dissolved in a glass of water.
• The separate elements forming the metal cannot
be identified even under a microscope.
• A metal in the form of a mechanical mixture at
room temperature often goes into a solid
solution or a partial solution when it is heated.
• Changing the chemical composition in this way
brings about certain predictable changes in grain
size and structure. This leads to the second step
in the heat treating process: soaking.
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20. Heat Treatment (HT)
• SOAKING:- Once a metal part has been
heated to the temperature at which desired
changes in its structure will take place, it
must remain at that temperature until the
entire part has been evenly heated
throughout. This is known as soaking.
• The more mass the part has, the longer it
must be soaked.
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21. Heat Treatment (HT)
• Cooling:- After the part has been properly
soaked, the third step is to cool it.
• Here again, the structure may change from one
chemical composition to another, it may stay
the same, or it may revert to its original form.
For example, a metal that is a solid solution
after heating may stay the same during cooling,
change to a mechanical mixture, or change to a
combination of the two, depending on the type
of metal and the rate of cooling.
• All of these changes are predictable. For that
reason, many metals can be made to conform
to specific structures in order to increase their
hardness, toughness, ductility, tensile strength,
and so forth.
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22. Transformation on heating cooling
• Martensite (FeC):- If a sample of a plain carbon
steel in the austenitic condition is rapidly cooled
to room temp by quenching it in water, its
structure will be changed from austenite to (FeC).
• (FeC). in plain carbon steels is a metastable phase
consisting of a supersaturated interstitial solid
solution of carbon in BCC iron or BCT iron (The
tetragonality is caused by a slight distortion of the
BCC iron unit cell.
• The Ms temp for FeC alloys decreases as the
weight % C increases in these alloys.
• Ms is martensite start temp and Mf is martensite
finish temp
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