Manufacturing Technology, ME-202-E (D-24)
NATURE OF PLASTIC DEFORMAIION
Plastic deformation is the deformation which is permanent and
beyond the elastic range of the material often, metals are worked
by plastic deformation because of the beneficial effect that is
imparted to the mechanical properties by it. The necessary
deformation in a metal can be achieved by application of
large amount of mechanical force only or by heating the metal and
then applying a small force. The deformation of metals which is
caused by the displacement of the atoms is achieved by one or
both of the processes called slip and twinning. On the macroscopic
scale when plastic deformation occurs, the metal appears to flow
in the solid state along specific directions which are dependent on
the type of processing and the direction of applied force. The
crystals or grains of the metal get elongated in the direction of
metal flow. This flow of metal can be seen under microscope after
polishing and suitable etching of the metal surface. These visible
lines are called as “fibre flow lines" .
Since the grains are elongated in the direction of flow, they would
be able to offer more resistance to stresses acting across them. As
a result, the mechanically worked metals called wrought products
would be able to achieve better mechanical strength in specific
orientation, that of the flow direction. Since it is possible to control
these flow lines in any specific direction by careful manipulation of
the applied fibres. It is possible to achieve optimum mechanical
properties. The metal of course, would be weak along the flow
lines. The wastage of material in metal working processes is either
negligible or very small and the production rate is in general very
high. These two factors give rise to the economy in production.
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of the amount of heating applied to the metal before applying the
mechanical force.
Those processes, working above the recrystallisation
temperature, are termed as hot working processes whereas
those below are termed as cold working processes.
Under the action of heat and the force, when the atoms reach a
certain higher energy level, the new crystals start forming which is
termed as recrystallisation. Recrystallisation destroys the old
grain structure deformed by the mechanical working, and entirely
new crystals which are strain free are formed. The grains in fact
start nucleating at the points of severest deformation.
Recrystallisation temperature as defined by American Society of
Metals is "the approximate minimum temperature at which
complete recrystallisation of a cold worked metal occurs within a
specified time'"
The recrystallisation temperature is generally between one-third
to half the melting point of most of the metals. The
recrystallisation temperature also depends on the amount of cold
work a material has already received. Higher the cold work, lower
would be the recrystallisation temperature as shown in Fig.
HOT WORKING AND COLD WORKING
The metal working processes are traditionally divided into hot
working and cold working processes. The division is on the basis
Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Manufacturing Technology, ME-202-E (D-24)
Though cold work affects the recrystallisation temperature to a
great extent, there are other variables which also affect as given in
the Table below. In hot working, the process may be carried above
the recrystallisation temperature with or without actual heating.
For example, for lead and tin the recrystallisation temperature is
below the room temperature and hence working of these metals at
room temperature is always hot working. Similarly for steels, the
recrystallisation temperature is of the order of 1000oC, and
therefore working below that temperature is still cold working
only.
In hot working, the temperature at which the working is
completed is important since any extra heal left after working will
aid in the grain growth, thus giving poor mechanical properties'
The effect of temperature of completion of hot working is shown
schematically. In Fig. A is shown a, simple heating where the grain
start growing after the metal crosses the recrytallizayion
temperature. When it is cooled without any hot working as in B,
the final grain size would be larger than when started in A. After
heating, when the metal is worked, because of recrystallisation,
the grain size is reduced. This is made possible because the
working of metal gives rise to a large number of nucleation sites
for the new crystals to form. But if the hot working is completed
much above the recrystallisation temperature as in C the grain size
start increasing and finally may end up with coarse grain size. This
Notes- Unit- V (Metal Forming Process)
increase the size of the grains occurs by a process of coalescence of
adjoining grains and is a function of time and temperature. This is
not generally desirable. If the hot working is completed just above
the recrystallisation temperature as in D, then the resultant grain
size would be fine. The same is schematically shown for hot rolling
operation.
Hot working
Advantages:
1. As the material is above the recrystallisation temperature, any
amount of working can be imparted since no strain hardening
taking place.
2. At a high temperature the material would have higher amount
of ductility and therefore there is no limit on the amount of hot
working that can be done on a material. Even brittle materials
can be hot worked.
Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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3. Since the shear stress gets reduced at higher temperatures, the
hot working requires much less force to achieve the necessary
deformation.
4. It is possible to continuously deform the grains in metal
working and if the temperature and rate of working are
properly controlled, a very favorable grain size could be
achieved giving rise the better mechanical properties.
Disadvantages:
1. Some metals cannot be hot worked because of their brittleness
at high temperatures.
2. Higher temperature of metal give rise to scaling of the surface
and as a result the surface finish obtained is poor. Although
there is a possibility of the decarburization of skin in steels due
to high temperatures.
3. Because of the thermal expansion of metals, the dimensional
accuracy in hot working is difficult to achieve since it is difficult
to control the temperature of work pieces.
4. Handling and maintaining of hot metal is difficult.
Difference Between Hot & Cold Working:
Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Rolling Principle:
Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Forging:
Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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Mohammad Amir, Lecturer, Department of Mechanical Engineering, BHCET, Dhauj, Faridabad-121004
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