Name of experiment: cold work Objective of experiment: study the effect of cold work on the grains and mechanical properties

1. Name of experiment: cold work
Objective of experiment: study the effect of cold work on the grains and
mechanical properties
Devices used in Experiment
1. Microscope
2. Brass samples upon which cold work has been performed at various stages.
Theoretical :
This should cover:
1. Definition of cold work, hot working ;
Cold working is the plastic deformation of metals below the recrystallization
temperature. In most cases, such cold forming is done at room temperature.
The major cold-working operations can be classified basically as squeezing, bending,
shearing and drawing.
Cold working is the plastic deformation of metals below the recrystallization
temperature. In most cases of manufacturing, such cold forming is done at room
temperature. Sometimes, however, the working may be done at elevated
temperatures that will provide increased ductility and reduced strength, but will be
below the recrystallization temperature.
When deformation is achieved at a temperature above that at which recrystallization
occurs, the process is termed hot working. With most of the forming techniques,
both hot- and cold-working procedures are possible. For hot-working operations,
large deformations are possible, which may be successively repeated because the
metal remains soft and ductile. Also, deformation energy requirements are less than
for cold working. However, most metals experience some surface oxidation, which
results in material loss and a poor final surface finish

2. 2. Name and discuss briefly the different types of cold work
Forging; is mechanically working or deforming a single piece of a usually hot
metal; this may be accomplished by the application of successive blows or by
continuous squeezing. Forgings are classified as either closed or open die. For
closed die, a force is brought to bear on two or more die halves having the
finished shape such that the metal is deformed in the cavity between them for
open die, two dies having simple geometric shapes (e.g., parallel flat,
semicircular) are employed, normally on large work-pieces. Forged articles have
outstanding grain structures and the best combination of mechanical properties.
Wrenches, automotive crankshafts, and piston connecting rods are typical
articles formed using this technique.
Rolling; the most widely used deformation process, consists of passing a piece of
metal between two rolls; a reduction in thickness results from compressive
stresses exerted by the rolls. Cold rolling may be used in the production of sheet,
strip, and foil with a
Extrusion; a bar of metal is forced through a die orifice by a compressive force
that is applied to a ram; the extruded piece that emerges has the desired shape
and a reduced cross-sectional area. Extrusion products include rods and tubing
that have rather complicated cross-sectional geometries; seamless tubing may
also be extruded.
Drawing
Drawing; is the pulling of a metal piece through a die having a tapered bore by
means of a tensile force that is applied on the exit side. A reduction in cross
section results, with a corresponding increase in length. The total drawing
operation may consist of a number of dies in a series sequence. Rod, wire, and
tubing products are commonly fabricated in this way

3. 3. . The effect of cold work on microscopic structure and mechanical properties
Figures demonstrate how steel, brass, and copper increase in yield
And tensile strength with increasing cold work

4. 4. Can the brasses original mechanical properties returned if yes the how and what
is the process called?
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 pre–cold-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

6. Discussion:
1. Draw a graph showing the change in mechanical properties due to cold work.
Strain hardening is the phenomenon by which a ductile metal becomes
harder and stronger as it is plastically deformed. Sometimes it is also called
work hardening or, because the temperature at which deformation takes place
is “cold” relative to the absolute melting temperature of the metal,
cold working. Most metals strain harden at room temperature. It is
sometimes convenient to express the degree of plastic deformation as
percent cold work rather than as strain. Percent cold work (%CW) is defined
as
Where A0 is the original area of the cross section that experiences
deformation and Ad is the area after deformation

7. 2. Explain in detail how cold work effects the grains
Due to working of the metal there will be changes in the grain structure. The grains
may be elongated in one direction from its equiaxed shape. The pores are reduced
and the inclusions are fragmented and distributed evenly in the metal. In hot
working the coarse equiaxed grains will become fine equiaxed. The changes obtained
in cold working is appreciable. The behavior of the metal with changes in grain size
is shown in the figure. As grain size becomes coarse the strength property comes
down and ductility increases.
As the percentage of cold working increases the material becomes strain hardened,
the hardness and strength properties are increased but the ductility property
decreases as shown in
the figure
It can be summarized as follows. Equiaxed grains will give uniform properties in all
directions. Deformed grains show higher strength properties in the elongated
direction

8. 3 What is the difference between cold & hot work?
S.No. Cold working Hot working
1 It is done at a temperature below
the recrystallization temperature.
Hot working is done at a temperature
above recrystallization temperature.
2. It is done below recrystallization
temperature so it is
accomplished by strain
hardening.
Hardening due to plastic deformation
is completely eliminated.
3. Cold working decreases
mechanical properties of metal
like elongation, reduction of area
and impact values.
It increases mechanical properties.
4. Crystallization does not take
place.
Crystallization takes place.
5. Material is not uniform after this
working.
Material is uniform thought.
6. There is more risk of cracks. There is less risk of cracks.
7. Cold working increases ultimate
tensile strength, yield point
hardness and fatigue strength
but decreases resistance to
corrosion.
In hot working, ultimate tensile
strength, yield point, corrosion
resistance are unaffected.
8. Internal and residual stresses
are produced.
Internal and residual stresses are not
produced.
9. Cold working required more
energy for plastic deformation.
It requires less energy for plastic
deformation because at higher
temperature metal become more
ductile and soft.
10. More stress is required. Less stress required.
11. It does not require pickling
because no oxidation of metal
takes place.
Heavy oxidation occurs during hot
working so pickling is required to
remove oxide.
12. Embrittlement does not occur in
cold working due to no reaction
with oxygen at lower
temperature.
There is chance of embrittlement by
oxygen in hot working hence metal
working is done at inert atmosphere
for reactive metals.

9. 4. Which are the metals can form at room temperature (cold work)?
5. Which arc the metals can't form at room temperature and need to heat it for
forming

10. 6. What ate the degree percent of cold, warm and hot working?
There are three temperature ranges-cold, warm & hot working:
Temperature range for different metal forming operations. TA is the ambient
(room) temperature, and Tm is the work metal melting temperature
Metal forming processes can also be classified according to the working
temperature.
The effect of temperature gives the rise to distinctions between cold working,
warm working, and hot working.
Hot and cold working of metals is of great importance in engineering
production. Processes such as forging, rolling, drawing and extrusion
predominate in the primary stages of production and have been perfected
through developments.
Hot Working is the initial step in the mechanical working of most metals and
alloys. Hot working reduces the energy required to deform the metal. It also
increases ability of metals to flow without cracking. However, due to high
temperature, surface oxidation and decarburization can not be prevented.
Cold Working of a metal results in an increase in strength or hardness and a
decrease in ductility. But, when cold working is excessive, the metal will
fracture before final size has been reached (<0.3Tm).
Hot Working of metals takes place above the recrystallization temperature.
Cold Working must be done below the recrystallization range (0.5Tm to
0.75Tm).
Cold working is metal forming performed at room temperature.
Advantages: better accuracy, better surface finish, high
strength and hardness of the part, no heating is required.
Disadvantages: higher forces and power, limitations to the amount of
forming, additional annealing for some material is
required, and some material are not capable of cold
working.
Warm working: is metal forming at temperatures above the room temperature
but below the recrystallize one.

11. Advantages:lower forces and power, more complex part shapes, no
annealing is required.
Disadvantages: some investment in furnaces is needed
7. Write the advantages of cold working
When compared to hot working, cold-working processes have certain distinct
advantages:
A-No heating required
B-Better surface finish obtained
C-Superior dimension control
D-Better reproducibility and interchangeability of parts
E-Improved strength properties
F-Directional properties can be minimized
8. Write the disadvantages of cold working
Some disadvantages associated with cold-working processes include:
A-Higher forces required for deformation
B-Heavier and more powerful equipment required
C-Less ductility available
D-Metal surfaces must be clean and scale-free
E-Strain hardening occurs (may require intermediate anneals)
F-Imparted directional properties may be detrimental
G-May produce undesirable residual stresses
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12. Baghdad University
College of Engineering
Department of Mechanical Engineering
Name of experiment
cold work
Preparation:
Saif al-Din Ali Madi
Group "A"