Short Notes: Heat Treatment | Short Notes for Mechanical Engineering PDF Download

 Page 1


Heat Treatment 
Heat treatment of a metal or alloy is a technological procedure, including 
controlled heating and cooling operations, conducted for the purpose of changing 
the alloy microstructure and resulting in achieving required properties.
Normalising
• For this process, the metal is placed in the furnace and heated to just above 
its 'Upper Critical Temperature'.
• When the new grain structure is formed it is then removed from the furnace 
and allowed to cool in air as it cools new grains will be formed.
• These grains, although similar to the original ones, will, in fact, be smaller and 
more evenly spaced.
• Normalising is used to relieve stresses and to restore the grain structure to 
normal.
This is particularly useful after heavy machining where grains may have become 
stressed or after the prolonged heating of a forging process has allowed the grains 
to grow large.
Quenching
• It is a heat treatment when metal at a high temperature is rapidly cooled by 
immersion in water or oil.
• Quenching makes steel harder and more brittle, with small grains structure. 
Annealing (Softening)
• Annealing is a heat treatment procedure involving heating the alloy and 
holding it at a certain temperature (annealing temperature), followed by
Page 2


Heat Treatment 
Heat treatment of a metal or alloy is a technological procedure, including 
controlled heating and cooling operations, conducted for the purpose of changing 
the alloy microstructure and resulting in achieving required properties.
Normalising
• For this process, the metal is placed in the furnace and heated to just above 
its 'Upper Critical Temperature'.
• When the new grain structure is formed it is then removed from the furnace 
and allowed to cool in air as it cools new grains will be formed.
• These grains, although similar to the original ones, will, in fact, be smaller and 
more evenly spaced.
• Normalising is used to relieve stresses and to restore the grain structure to 
normal.
This is particularly useful after heavy machining where grains may have become 
stressed or after the prolonged heating of a forging process has allowed the grains 
to grow large.
Quenching
• It is a heat treatment when metal at a high temperature is rapidly cooled by 
immersion in water or oil.
• Quenching makes steel harder and more brittle, with small grains structure. 
Annealing (Softening)
• Annealing is a heat treatment procedure involving heating the alloy and 
holding it at a certain temperature (annealing temperature), followed by
controlled cooling.
• Annealing results in relief of internal stresses, softening, chemical 
homogenizing and transformation of the grain structure into more stable 
state.
• The annealing process is carried out in the same way as normalizing, except 
that the component is cooled very slowly. This is usually done by leaving the 
component to cool down in the furnace for up to 48 hours.
• Annealing leaves the metal in its softest possible state and is usually carried 
out to increase ductility prior to cold working or machining.
• Annealing is carried out in different stages which are classified as:
Stress relief (recovery)
• A relatively low-temperature process of reducing internal mechanical stresses, 
caused by cold-work, casting or welding.
• During this process atoms move to more stable positions in the crystal lattice. 
Vacancies and interstitial defects are eliminated and some dislocations are 
annihilated.
• Recovery heat treatment is used mainly for preventing stress-corrosion 
cracking and decreasing distortions, caused by internal stresses.
Recrystallization
• It can be easily said to be the alteration of the grain structure of the metal.
• If the alloy reaches a particular temperature (recrystallization or annealing 
temperature) new grains start to grow from the nuclei formed in the cold 
worked metal. The new grains absorb imperfections and distortions caused 
by cold deformation. The grains are equiaxed and independent to the old grain 
structure.
• As a result of recrystallization mechanical properties (strength, ductility) of 
the alloy return to the pre-cold-work level.
• The annealing temperature and new grains size are dependent on the degree 
of cold-work which has been conducted. The more is the cold-work degree, the 
lower is the annealing temperature and the fine recrystallization grain 
structure.
• Low degrees of cold-work (less than 5%) may cause the formation of large 
grains. Usually, the annealing temperature of metals is between one-third to 
one-half of the freezing point measured in Kelvin (absolute) temperature 
scale.
Grain growth (over-annealing, secondary recrystallization)
• The growth of the new grains at the expense of their neighbors, occurring at 
temperature, above the recrystallization temperature.
This process results in coarsening grain structure and is undesirable.
Page 3


Heat Treatment 
Heat treatment of a metal or alloy is a technological procedure, including 
controlled heating and cooling operations, conducted for the purpose of changing 
the alloy microstructure and resulting in achieving required properties.
Normalising
• For this process, the metal is placed in the furnace and heated to just above 
its 'Upper Critical Temperature'.
• When the new grain structure is formed it is then removed from the furnace 
and allowed to cool in air as it cools new grains will be formed.
• These grains, although similar to the original ones, will, in fact, be smaller and 
more evenly spaced.
• Normalising is used to relieve stresses and to restore the grain structure to 
normal.
This is particularly useful after heavy machining where grains may have become 
stressed or after the prolonged heating of a forging process has allowed the grains 
to grow large.
Quenching
• It is a heat treatment when metal at a high temperature is rapidly cooled by 
immersion in water or oil.
• Quenching makes steel harder and more brittle, with small grains structure. 
Annealing (Softening)
• Annealing is a heat treatment procedure involving heating the alloy and 
holding it at a certain temperature (annealing temperature), followed by
controlled cooling.
• Annealing results in relief of internal stresses, softening, chemical 
homogenizing and transformation of the grain structure into more stable 
state.
• The annealing process is carried out in the same way as normalizing, except 
that the component is cooled very slowly. This is usually done by leaving the 
component to cool down in the furnace for up to 48 hours.
• Annealing leaves the metal in its softest possible state and is usually carried 
out to increase ductility prior to cold working or machining.
• Annealing is carried out in different stages which are classified as:
Stress relief (recovery)
• A relatively low-temperature process of reducing internal mechanical stresses, 
caused by cold-work, casting or welding.
• During this process atoms move to more stable positions in the crystal lattice. 
Vacancies and interstitial defects are eliminated and some dislocations are 
annihilated.
• Recovery heat treatment is used mainly for preventing stress-corrosion 
cracking and decreasing distortions, caused by internal stresses.
Recrystallization
• It can be easily said to be the alteration of the grain structure of the metal.
• If the alloy reaches a particular temperature (recrystallization or annealing 
temperature) new grains start to grow from the nuclei formed in the cold 
worked metal. The new grains absorb imperfections and distortions caused 
by cold deformation. The grains are equiaxed and independent to the old grain 
structure.
• As a result of recrystallization mechanical properties (strength, ductility) of 
the alloy return to the pre-cold-work level.
• The annealing temperature and new grains size are dependent on the degree 
of cold-work which has been conducted. The more is the cold-work degree, the 
lower is the annealing temperature and the fine recrystallization grain 
structure.
• Low degrees of cold-work (less than 5%) may cause the formation of large 
grains. Usually, the annealing temperature of metals is between one-third to 
one-half of the freezing point measured in Kelvin (absolute) temperature 
scale.
Grain growth (over-annealing, secondary recrystallization)
• The growth of the new grains at the expense of their neighbors, occurring at 
temperature, above the recrystallization temperature.
This process results in coarsening grain structure and is undesirable.
too
-> 8 0 
to
a
» 6 0
i "
20
0
Tipmperarur* °C
60
Hardening
• Hardening also requires the steel to be heated to its upper critical temperature 
(plus 50°C) and then quenched.
• The quenching is to hold the grains in their solid solution state called 
Austenite; cooling at such a rate (called the critical cooling rate) is to prevent 
the grains forming into ferrite and pearlite.
• Hardening is a process of increasing the metal hardness, strength, toughness, 
fatigue resistance.
• The rate of cooling affects the hardness of the metal, in that the faster the 
cooling rate, the greater the hardness.
• The cooling liquid can therefore be selected to suit the hardness required. If a 
steel is quenched too rapidly it may crack, this is especially true with thin 
walled components.
Strain hardening (work hardening)
• Strengthening by cold-work (cold plastic deformation),
• It causes increase of concentration of dislocations, which mutually entangle 
one another, making further dislocation motion difficult and therefore 
resisting the deformation or increasing the metal strength.
Grain size strengthening (hardening)
• Strengthening by grain refining.
• Grain boundaries serve as barriers to dislocations, raising the stress required 
to cause plastic deformation.
Solid solution hardening
• Strengthening by dissolving an alloying element.
• Atoms of solute element distort the crystal lattice, resisting the dislocations 
motion. Interstitial elements are more effective in solid solution hardening, 
than substitution elements.
Dispersion strengthening
• Strengthening by addition of second phase into metal matrix.
• The second phase boundaries resist the dislocations motions, increasing the 
material strength.
Page 4


Heat Treatment 
Heat treatment of a metal or alloy is a technological procedure, including 
controlled heating and cooling operations, conducted for the purpose of changing 
the alloy microstructure and resulting in achieving required properties.
Normalising
• For this process, the metal is placed in the furnace and heated to just above 
its 'Upper Critical Temperature'.
• When the new grain structure is formed it is then removed from the furnace 
and allowed to cool in air as it cools new grains will be formed.
• These grains, although similar to the original ones, will, in fact, be smaller and 
more evenly spaced.
• Normalising is used to relieve stresses and to restore the grain structure to 
normal.
This is particularly useful after heavy machining where grains may have become 
stressed or after the prolonged heating of a forging process has allowed the grains 
to grow large.
Quenching
• It is a heat treatment when metal at a high temperature is rapidly cooled by 
immersion in water or oil.
• Quenching makes steel harder and more brittle, with small grains structure. 
Annealing (Softening)
• Annealing is a heat treatment procedure involving heating the alloy and 
holding it at a certain temperature (annealing temperature), followed by
controlled cooling.
• Annealing results in relief of internal stresses, softening, chemical 
homogenizing and transformation of the grain structure into more stable 
state.
• The annealing process is carried out in the same way as normalizing, except 
that the component is cooled very slowly. This is usually done by leaving the 
component to cool down in the furnace for up to 48 hours.
• Annealing leaves the metal in its softest possible state and is usually carried 
out to increase ductility prior to cold working or machining.
• Annealing is carried out in different stages which are classified as:
Stress relief (recovery)
• A relatively low-temperature process of reducing internal mechanical stresses, 
caused by cold-work, casting or welding.
• During this process atoms move to more stable positions in the crystal lattice. 
Vacancies and interstitial defects are eliminated and some dislocations are 
annihilated.
• Recovery heat treatment is used mainly for preventing stress-corrosion 
cracking and decreasing distortions, caused by internal stresses.
Recrystallization
• It can be easily said to be the alteration of the grain structure of the metal.
• If the alloy reaches a particular temperature (recrystallization or annealing 
temperature) new grains start to grow from the nuclei formed in the cold 
worked metal. The new grains absorb imperfections and distortions caused 
by cold deformation. The grains are equiaxed and independent to the old grain 
structure.
• As a result of recrystallization mechanical properties (strength, ductility) of 
the alloy return to the pre-cold-work level.
• The annealing temperature and new grains size are dependent on the degree 
of cold-work which has been conducted. The more is the cold-work degree, the 
lower is the annealing temperature and the fine recrystallization grain 
structure.
• Low degrees of cold-work (less than 5%) may cause the formation of large 
grains. Usually, the annealing temperature of metals is between one-third to 
one-half of the freezing point measured in Kelvin (absolute) temperature 
scale.
Grain growth (over-annealing, secondary recrystallization)
• The growth of the new grains at the expense of their neighbors, occurring at 
temperature, above the recrystallization temperature.
This process results in coarsening grain structure and is undesirable.
too
-> 8 0 
to
a
» 6 0
i "
20
0
Tipmperarur* °C
60
Hardening
• Hardening also requires the steel to be heated to its upper critical temperature 
(plus 50°C) and then quenched.
• The quenching is to hold the grains in their solid solution state called 
Austenite; cooling at such a rate (called the critical cooling rate) is to prevent 
the grains forming into ferrite and pearlite.
• Hardening is a process of increasing the metal hardness, strength, toughness, 
fatigue resistance.
• The rate of cooling affects the hardness of the metal, in that the faster the 
cooling rate, the greater the hardness.
• The cooling liquid can therefore be selected to suit the hardness required. If a 
steel is quenched too rapidly it may crack, this is especially true with thin 
walled components.
Strain hardening (work hardening)
• Strengthening by cold-work (cold plastic deformation),
• It causes increase of concentration of dislocations, which mutually entangle 
one another, making further dislocation motion difficult and therefore 
resisting the deformation or increasing the metal strength.
Grain size strengthening (hardening)
• Strengthening by grain refining.
• Grain boundaries serve as barriers to dislocations, raising the stress required 
to cause plastic deformation.
Solid solution hardening
• Strengthening by dissolving an alloying element.
• Atoms of solute element distort the crystal lattice, resisting the dislocations 
motion. Interstitial elements are more effective in solid solution hardening, 
than substitution elements.
Dispersion strengthening
• Strengthening by addition of second phase into metal matrix.
• The second phase boundaries resist the dislocations motions, increasing the 
material strength.
• The strengthening effect may be significant if fine hard particles are added to 
a soft ductile matrix (composite materials).
Hardening as a result of Spinodal decomposition
• Spinodal structure is characterized by strains on the coherent boundaries 
between the spinodal phases causing hardening of the alloy.
Precipitation hardening (age hardening)
• Strengthening by precipitation of fine particles of a second phase from a 
supersaturated solid solution.
• The second phase boundaries resist the dislocations motions, increasing the 
material strength. The age hardening mechanism in Al-Cu alloys may be 
illustrated by the phase diagram of Al-Cu system.
• When an alloy AI-3%Cu is heated up to the temperature TM , all CuAI2 particles 
are dissolved and the alloy exists in form of single phase solid solution (a- 
phase). This operation is called solution treatment.
• Slow cooling of the alloy will cause formation of relatively coarse particles of 
CuAI2 intermetallic phase, starting from the temperature TN
• However if the the cooling rate is high (quenching), solid solution will retain 
even at room temperature TF . Solid solution in this non-equilibrium state is 
called supersaturated solid.
• Obtaining of supersaturated solid solution is possible when cooling is 
considerably faster, than diffusion processes. As the diffusion coefficient is 
strongly dependent on the temperature, the precipitation of CuAI2 from 
supersaturated solution is much faster at elevated temperatures (lower than 
TN ).This process is called artificial aging. It takes usually a time from several 
hours to one day. When the aging is conducted at the room temperature, it is 
called natural aging
Age hardening in Al -Cu alloys
• As there are very few applications for very hard and brittle steel, the hardness 
and brittleness needs to be reduced. The process for reducing hardness and 
brittleness is called tempering.
• Tempering consists of reheating the previously hardened steel.
• During this heating, small flakes of carbon begin to appear in the needle like 
structure. (See below) This has the effect of reducing the hardness and 
brittleness.
Page 5


Heat Treatment 
Heat treatment of a metal or alloy is a technological procedure, including 
controlled heating and cooling operations, conducted for the purpose of changing 
the alloy microstructure and resulting in achieving required properties.
Normalising
• For this process, the metal is placed in the furnace and heated to just above 
its 'Upper Critical Temperature'.
• When the new grain structure is formed it is then removed from the furnace 
and allowed to cool in air as it cools new grains will be formed.
• These grains, although similar to the original ones, will, in fact, be smaller and 
more evenly spaced.
• Normalising is used to relieve stresses and to restore the grain structure to 
normal.
This is particularly useful after heavy machining where grains may have become 
stressed or after the prolonged heating of a forging process has allowed the grains 
to grow large.
Quenching
• It is a heat treatment when metal at a high temperature is rapidly cooled by 
immersion in water or oil.
• Quenching makes steel harder and more brittle, with small grains structure. 
Annealing (Softening)
• Annealing is a heat treatment procedure involving heating the alloy and 
holding it at a certain temperature (annealing temperature), followed by
controlled cooling.
• Annealing results in relief of internal stresses, softening, chemical 
homogenizing and transformation of the grain structure into more stable 
state.
• The annealing process is carried out in the same way as normalizing, except 
that the component is cooled very slowly. This is usually done by leaving the 
component to cool down in the furnace for up to 48 hours.
• Annealing leaves the metal in its softest possible state and is usually carried 
out to increase ductility prior to cold working or machining.
• Annealing is carried out in different stages which are classified as:
Stress relief (recovery)
• A relatively low-temperature process of reducing internal mechanical stresses, 
caused by cold-work, casting or welding.
• During this process atoms move to more stable positions in the crystal lattice. 
Vacancies and interstitial defects are eliminated and some dislocations are 
annihilated.
• Recovery heat treatment is used mainly for preventing stress-corrosion 
cracking and decreasing distortions, caused by internal stresses.
Recrystallization
• It can be easily said to be the alteration of the grain structure of the metal.
• If the alloy reaches a particular temperature (recrystallization or annealing 
temperature) new grains start to grow from the nuclei formed in the cold 
worked metal. The new grains absorb imperfections and distortions caused 
by cold deformation. The grains are equiaxed and independent to the old grain 
structure.
• As a result of recrystallization mechanical properties (strength, ductility) of 
the alloy return to the pre-cold-work level.
• The annealing temperature and new grains size are dependent on the degree 
of cold-work which has been conducted. The more is the cold-work degree, the 
lower is the annealing temperature and the fine recrystallization grain 
structure.
• Low degrees of cold-work (less than 5%) may cause the formation of large 
grains. Usually, the annealing temperature of metals is between one-third to 
one-half of the freezing point measured in Kelvin (absolute) temperature 
scale.
Grain growth (over-annealing, secondary recrystallization)
• The growth of the new grains at the expense of their neighbors, occurring at 
temperature, above the recrystallization temperature.
This process results in coarsening grain structure and is undesirable.
too
-> 8 0 
to
a
» 6 0
i "
20
0
Tipmperarur* °C
60
Hardening
• Hardening also requires the steel to be heated to its upper critical temperature 
(plus 50°C) and then quenched.
• The quenching is to hold the grains in their solid solution state called 
Austenite; cooling at such a rate (called the critical cooling rate) is to prevent 
the grains forming into ferrite and pearlite.
• Hardening is a process of increasing the metal hardness, strength, toughness, 
fatigue resistance.
• The rate of cooling affects the hardness of the metal, in that the faster the 
cooling rate, the greater the hardness.
• The cooling liquid can therefore be selected to suit the hardness required. If a 
steel is quenched too rapidly it may crack, this is especially true with thin 
walled components.
Strain hardening (work hardening)
• Strengthening by cold-work (cold plastic deformation),
• It causes increase of concentration of dislocations, which mutually entangle 
one another, making further dislocation motion difficult and therefore 
resisting the deformation or increasing the metal strength.
Grain size strengthening (hardening)
• Strengthening by grain refining.
• Grain boundaries serve as barriers to dislocations, raising the stress required 
to cause plastic deformation.
Solid solution hardening
• Strengthening by dissolving an alloying element.
• Atoms of solute element distort the crystal lattice, resisting the dislocations 
motion. Interstitial elements are more effective in solid solution hardening, 
than substitution elements.
Dispersion strengthening
• Strengthening by addition of second phase into metal matrix.
• The second phase boundaries resist the dislocations motions, increasing the 
material strength.
• The strengthening effect may be significant if fine hard particles are added to 
a soft ductile matrix (composite materials).
Hardening as a result of Spinodal decomposition
• Spinodal structure is characterized by strains on the coherent boundaries 
between the spinodal phases causing hardening of the alloy.
Precipitation hardening (age hardening)
• Strengthening by precipitation of fine particles of a second phase from a 
supersaturated solid solution.
• The second phase boundaries resist the dislocations motions, increasing the 
material strength. The age hardening mechanism in Al-Cu alloys may be 
illustrated by the phase diagram of Al-Cu system.
• When an alloy AI-3%Cu is heated up to the temperature TM , all CuAI2 particles 
are dissolved and the alloy exists in form of single phase solid solution (a- 
phase). This operation is called solution treatment.
• Slow cooling of the alloy will cause formation of relatively coarse particles of 
CuAI2 intermetallic phase, starting from the temperature TN
• However if the the cooling rate is high (quenching), solid solution will retain 
even at room temperature TF . Solid solution in this non-equilibrium state is 
called supersaturated solid.
• Obtaining of supersaturated solid solution is possible when cooling is 
considerably faster, than diffusion processes. As the diffusion coefficient is 
strongly dependent on the temperature, the precipitation of CuAI2 from 
supersaturated solution is much faster at elevated temperatures (lower than 
TN ).This process is called artificial aging. It takes usually a time from several 
hours to one day. When the aging is conducted at the room temperature, it is 
called natural aging
Age hardening in Al -Cu alloys
• As there are very few applications for very hard and brittle steel, the hardness 
and brittleness needs to be reduced. The process for reducing hardness and 
brittleness is called tempering.
• Tempering consists of reheating the previously hardened steel.
• During this heating, small flakes of carbon begin to appear in the needle like 
structure. (See below) This has the effect of reducing the hardness and 
brittleness.
The temperature to which the steel is reheated depends on the hardness 
required by the application of the component. The higher the tempering 
temperature, the less hard will be the resulting steel.
If the steel is polished before tempering, the range of oxide colours that the steel 
goes through during heating can be used as a guide to its temperature.
Stress Relieving
• When a metal is heated, expansion occurs which is more or less proportional 
to the temperature rise. Upon cooling a metal, the reverse reaction takes 
place. That is, a contraction is observed.
• When a steel bar or plate is heated at one point more than at another, as in 
welding or during forging, internal stresses are set up.
• During heating, expansion of the heated area cannot take place unhindered, 
and it tends to deform. On cooling, contraction is prevented from taking place 
by the unyielding cold metal surrounding the heated area.
• The forces attempting to contract the metal are not relieved, and when the 
metal is cold again, the forces remain as internal stresses. Stresses also 
result from volume changes which accompany metal transformations and 
precipitation.
• The term stress has wide usage in the metallurgical field. It is defined simply 
as bad or force divided by the cross-sectional area of the part to which the 
bad or force is applied.
• Internal, or residual stresses, are bad because they may cause warping of 
steel parts when they are machined.
To relieve these stresses, steel is heated to around 7 7 00 °F (595 °C) assuring that 
the entire part is heated uniformly, then cooled slowly back to room temperature. 
This procedure is called stress relief annealing, or merely stress relieving
Allotropic Forms of Steel
• The temperature 723°C is known as Curie temperature, below it steel shows 
magnetic properties and above it steel becomes non-magnetic.
• In the diagram, the carbon percentage is plotted on X-axis and temperature 
plotted on /-axis.
• The melting point of iron is about 1539°C. The melting temperature of iron 
varies with increasing carbon percentage.