strain hardening, cold working, and hot working

1. Strain hardening, three stage
annealing and cold working
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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2. Introduction
• Strain hardening or work hardening is a phenomenon in which the strength
and hardness of material increases
• This phenomenon occurs at a temperature lower than recrystallization range
of metals
• In such case, more stress is required to cause a slip in metals
• In short, an increase in the strength of metals due to mechanical working is
called as strain hardening or work hardening
• Strain hardening occurs due to cold working and the effect of strain
hardening needs to be relived before any material is processed for
manufacturing
• If not done, then there is a chance of failure of material
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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3. Effect of Strain hardening on behavior of materials
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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Strain hardening

4. • When load is gradually increased, Hook’s law is followed i.e. stress is directly
proportional to strain (Path OE)
• If the material is loaded gradually, material will enter into plastic zone by
following the path EA and at the end of plastic region (at point D’) material will
break
• If load is gradually decreased i.e. at point A, instead of following path AEO,
material will follow different path i.e. AB since strain has been induced in the
material which can not be removed
• Upon loading, material will follow a different path i.e. BCD
• If material would have been loaded without intermediate unloading, it would have
followed a path AD’
• Herein, ultimate strength of material is increased due to unloading and re-loading
till fracture occurs
• This increase in the strength of material can be attributed to strain hardening
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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5. Theory of work hardening
• Work hardening effect can be attributed to the increased
resistance to dislocation
• Resistance to dislocation increases due to cold working
• Some dislocations are stuck inside the crystal and act as
source of internal stress which opposes the motion of other
dislocation motion i.e. work hardening occurs due to the
restriction to the motion of dislocations
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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6. Stages of work hardening
• Stage I- Easy glide region
This stage immediately comes after yield point.
Dislocations are able to move over relatively large
distance without encountering barriers. Length of
this region depends upon
Orientation, purity and size of crystals
• Stage II- Linear hardening region
• This region shows a rapid increase in the work
hardening rate the slope of which is
approximately independent of applied stress,
temperature or alloy content. Stage II can be
better explained with the help of dislocation pile-
up theory
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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I
II
III
Strain →
Stress
→

7. Dislocation pile-up theory
• According to this theory, some dislocations are stopped at barriers
• As the deformation progresses, the number of barrier increases until each
source becomes completely surrounded by barriers
• Hardening is principally due to long range internal stresses from piled up
groups interacting with guide dislocations
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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Dislocation pile up

8. Stages of annealing (continued)
• Stage-III- Parabolic hardening region
• It is a region of decreasing rate of strain hardening
• At sufficiently higher values of stress or temperature, dislocations
which are piled up in stage II are able to move by process that has
been suppressed at low stresses and temperatures
• By this mechanism, dislocations are able to by-pass the obstacles
• As a result of this, dislocations do not need to interact strongly with
obstacles
• Eventually, rate of work hardening is low in stage III
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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9. Three stages of annealing
• Cold working is defined as working of metals and alloys below their
recrystallization temperatures
• During cold working, strain hardening occurs and grains are distorted
• Cold working increases hardness, yield strength, internal stresses and
electrical resistance and decreases ductility and corrosion resistance
• As a result of cold working, material becomes excessively hard and brittle
and it is not possible to process the material beyond a certain limit without
cracking
• In order to process the material, effect of strain hardening needs to be
relieved which can be achieved by heat treatment
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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10. Annealing
• It consists of heating the material to a temperature well above
recrystallization temperature, holding for definite time interval and
cooling the material to room temperature slowly in the furnace itself
by switching off the power supply
• Annealing consists of three stages, namely, recovery, recrystallization
and grain growth
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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11. Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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12. 1. Recovery
• No change occurs in microstructure at this stage
• Grains remain distorted and mechanical properties such as hardness, tensile strength and
ductility do not change appreciably
• These properties remain same as that of cold worked metal
• Due to heating, some imperfections such as vacancy and interstitialcy existing in slip may get
eliminated
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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Re-arrangement of dislocations

13. 2. Recrystallization
• Old distorted grains are replaced by new, stress free, strain free equi-axed grains by the process of
nucleation and growth
• Microstructure at the end of recrystallization is much similar to that of the original structure i.e.
microstructure before cold work
• In addition, mechanical properties are also change and become similar to original metal
• Degree of cold work is defined as percentage reduction in cross sectional area during extrusion,
wire drawing, forging, rolling or similar working operations
• Recrystallization does not occur unless the degree of cold work is sufficient and working
temperature is high
• The temperature at which a metal or an alloy with a normal degree of cold work completely ( 95 %
or above) recrystallizes in a reasonable period (usually 1 hour) is called as recrystallization
temperature of a material
• Higher the melting point, higher the recrystallization temperature
• For steel-727°C
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
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14. 3. Grain growth
• If a completely recrystallized material is heated to a higher
temperature or kept for a longer time at the annealing temperature, it
will show grain growth
• Some grains grow abnormally on the expense of surrounding
recrystallized grains
• Grain growth decreases useful properties such as strength of material
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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15. Factors affecting recrystallization temperature
• Melting point of material: higher Tm, higher recrystallization temperature
• Degree of cold work: higher the degree of cold work, lower the
recrystallization temperature
• Grain size: finer the grain size, lower is the recrystallization temperature
• Heating time: longer the heating time, lower is the recrystallization
temperature
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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16. Hot working and cold working
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Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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Cold working Hot working
Working of metals and alloys below their
recrystallization temperature
Working of metals and alloys above their
recrystallization temperature
Microstructure shows distorted grains Microstructure shows equi-axed and
refined grains
Strain hardening occurs that increases
tensile strength and hardness of material
No property change occurs
Energy required for plastic deformation
is more
Energy required for plastic deformation is
less
Surface finish is good Surface finish is not so good due to
oxidation at high temperature
Easy to control dimensional tolerances Difficult to control dimensional tolerances
Cost of cold working is less as compared
to hot working
Cost of hot working is more as compared to
cold working
Examples: Cold rolling, stamping,
bending, wire drawing
Examples : Forging, hot rolling, extrusion,
piercing, spinning

17. Reference
• Chapter 4, Strain Hardening, A Textbook of Materials Technology,
S.B.Barve, Vipul Prakashan, pp. 4-1 to 4-6
Prepared by Dr. Vibhav Ambardekar, School of Mechanical and
Civil Engineering, MIT Academy of Engineering, Alandi, Pune,
412105
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