HOT & COLD WORKING
Q.1. What is hot working ? State its principles,
advantages and disadvantages.
Ans. Mechanical working of metals above its
recrystallisation temperature is known as hot working.
1. Larger deformation can be accomplished and more rapidly
by hot working since the metal is in plastic state.
2. Porosity of the metal is considerably minimised.
3. Concentrated impurities, if any in the metal are
disintegrated and distributed throughout the metal.
4. Grain structure of the metal is refined and physical
1. Due to high temperature a rapid oxidation or scale
formation takes place on the metal surface, leading
to poor surface finish and loss of metal.
2. On account of the lost of carbon from the surface of
the steel piece being worked the surface layer loses
its strength, which is a disadvantage when the part
is put to service.
3. This weakening of the surface layer may give rise to
a latigue crack which may ultimately result in
fatigue failure of the part.
4. Close tolerances cannot be maintained.
5. It involves excessive expenditure on account of high
cost of tooling. This, however, is compensated by the
high production rate and better quality of products.
Q.2. Describe the procedure of hot extrusion of tubes.
Ans. A common method of hot extrusion of tubes is shown in
figure. It is actually a forward extrusion method using a
mandrel to form the bore of the tube. First the mandrel is
pushed through the centre of the billet and the die,
followed by applying pressure on the billet by advancing
the plunger. The metal is forced to flow through the
opening between the die and the mandrel. The operation is
performed quite rapidly. Most of the metals and alloys are
hot extruded, although some of these are cold extruded
also, for production of seamless tubes.
Q.3. Write short notes on
Ans. 1. Hot Spinning :-
The process consist of heating the metal to forging
temperature and then forming it into the desired shape on a
spinning lathe which is similar to an engine lathe.
Usually shapes of circular cross-section which are
symmetrical about the axis of rotation, are formed by this
process. The workpiece is shaped over a formed revolving
metal holding device, called chuck, with the help of
spinning tools. It very well compares with drawing of
stamping in so far as the production in small quantities is
concerned, since the cost of dies for such small quantities
will lead to uneconomical production through the latter
methods. Hot spinning is generally used for thicker plates
and sheets which cannot be shaped through cold spinning.
In operation it is similar to cold spinning and, therefore,
the details of equipment, tools and procedure, etc., will
be given later under 'cold spinning’.
2. Hot Forging
These process basic ally consists of heating the metal
to plastic state and then applying pressure to form it into
different shapes and sizes. Unlike rolling, the pressure
in this case is not continuous but intermittent. The hot
metal piece may be compressed along its length to increase
its cross-section, along its cross-section to increase its
length, within a closed cavity to acquire the shape of that
cavity or in different directions to bend it into different
shapes. The pressure may be applied by hand hammer called
hand or smith forging, by power hammers, called hammer
forging, by presses (press forging) or upset forging
3. Hot drawing
This process is widely used for the production of
thicker walled seamless tubes and cylinders. It is usually
performed in two stages. The first stage consists of
drawing a cup shape out of a hot circular plate with the
help of a die and a punch. The second stage consists of
reheating the drawn cup and drawing is further to the
desired length having the required wall thickness. The
second drawing operation is performed through a number of
dies, which are arranged in a descending order of their
diameters, so that the reduction in wall thickness is
gradual in various stages. The farther end of the drawn
object is always blind, which may be cut off to produce a
through hole, if required.
Q.4. What are the specific advantages and limitations of
Ans. Advantages and limitations:
1.Better dimensional control than hot working is possible
because the reduction in size is not much.
2.Surface finish of the component is better because no
oxidation takes place during the process.
3.Strength and hardness of the metal are increased.
4.It is an ideal method for increasing hardness of those
metals which do not respond to the heat treatment.
5.Only ductile metals can be shaped through cold working.
6.Over-working of metal results in brittleness and it has
to be annealed to remove the same.
7. Subsequent heat treatment is mostly needed to remove the
residual stresses set up during cold working.
Q.5. What are the main characteristics of the hot working
of metals or compared with cold working process?
Ans. (1) Above the re-crystallisation temperature, the
metal becomes plastic and causes the growth of grains. By
hot working, the grains are broken up and their parts are
deformed into small and more numerous crystals or in other
words the refinement of grain occurs. Metals possess
little elasticity and low load is required to shape the
metal as the strength and hardness decrease at elevated
(2)The porosity of the steel ingot can be eliminated to a
(3)Great latitude in shape and size of form is possible due
to reduction of elastic limit.
(4)A uniformity is established either by squeezing other
impurities into fiber sla gs or distributing them throughout
(5)Directional property resulting from a fiber structure is
(6)Due to refinement of grains, mechanical properties such
as toughness, ductility, elongation and reduction in area
(7)The power required to finish the part ingot is less.
(8)It can be used on most of the metals, because it is a
rapid and economical process.
Q.6.Why forging generally preferred for those components
which require high strength and resistance to shock?
Ans. Characteristics of the Forged Parts
(1)It refines the structure of metal by closing up the
cavities and by smashing up large grain formations.
(2)Forged parts have directional properties and hence have
(3)Mechanical properties such as percentage elongation,
percentage reduction of area and resistance to shock and
vibration are improved.
(4)Cracks and blow holes are minimised.
Q.7. For various methods of forging known to you give the
(1) Hand forging :- Hand forging is emplayed only to shape
a small number of light forgings chiefly in repair
(2) Hammer forgings :- Usually used for small item forging.
(3) Press forging :- Usually used for heavy item forging.
(4) Machine forging :- For medium sized and large articles
requiring very heavy blows.
(5) Drop forging :- For
mass production of identical parts.
Q.8. Define up setting, edging, drawing, piecing and
punching operations as applied to not working of metals.
Upsetting :- This is just opposite to drawing and involves
increasing of the cross-sectional area usually by pressing
or hammering in a direction parallel to the original ingot
axis. In the process of upsetting, the shaft or rod is
generally gripped in dies, and the head or flange upset
either by a plain flattened ram or with further dies,
shaped to give the desired contour.
Drawing :- Refer – (Hot drawing)
Piercing :- This process is employed for the production of
seamless tubes. It offers the most economical mechanical
working, process for the manufacture of seamless tubes. It
consists of passing the hot rolled billet at 11000C. between
two conical rollers and over a mandrel which helps in
piercing and controlling the size of bore as the billet is
forced over it.
Q.9.Define the following terms related to cold working of
(1) Blanking :- This is the operation carried out on presses
and consists of cutting the outside contour of a
stamping. Production of sheet-metal blanks of flat
shapes requires a single -action press equipped with
tools comprising a punch, a corresponding die, a
stripper to keep the sheet from following the punch on
its upstroke and means for aligning the sheet or strip
of material and for spacing successive cuts. Cutting
inside contours, i.e. holes and slots is called
piercing. All these operations will be dealt in detail
under the chapter of presses. However pressed-metal
parts, or stampings are recommended for mass production.
Stampings combine the virtues of lightness, a high
degree of uniformity, and surfaces well adopted to
receive protective and decorative finishes.
(2) Swaging :- This operation consists of applying
compressive or impact forces on the metal below the
recrystallisation temperature. It causes the metal to
flow in the predetermined shape according to the design
of the dies. Rotary swaging and cold heading are the
two important processes of swaging.
(3) Lancing :- It is a special form of piercing operation in
which the entire contour is not cut, the blanked
material remains attached with the sheet. It is
achieved by bending down one side of partially punched
(4) Embossing :- It is also like a drawing or stretching
operation and does not require much pressure like
drawing and coining. It consists of producing, projected
or raised designs in relief on a surface of sheet. It
is done with the help of two mating dies. The sheet is
first blanked and then little more force is applied by
the punch which forces the metal against a mating die
conforming to the same configuration as the punch. In
this way very little metal is squeezed in the operation
and the words are printed on the sheet in projected
Q.10. Explain extrusion process as used for production of
tubes. Draw sketches for various stages in it.
3.Tube manufacturing by lap welding.
Roll piercing :-
It is a method of producing seamless tubes. The
piercing machine used in the process consists of two
tapered rolls called piercing rolls. Round heated billet
of steel is passed between these rolls over a mandrel.
Both the rolls rotate in the same direction. The billet is
centre punched or provided with a small drilled hole at one
end and heated to proper temperature. It is then pushed
forward into the rolls. The rolls grip the billet and pull
it further into them. The axes of the rolls are crossed,
therefore they revolve the billet as well as draw it
forward to force it on to the mandrel. The mandrel can
also revolve in its own position. This combination of the
revolving motions of the billet, and mandrel, together with
the axial advancement of the billet, provide a helical
rolling effect on the material. Production of a 12 meter
length of upto 150 mm diameter rough tubing will take about
10 to 30 seconds through this method. If tubing of larger
bore (say upto 350 mm) are to be made a second piercing
operation is necessary after the first. Still larger sizes
will need a third piercing operation.
The rough tubing produced as above is further
subjected to rolling, reeling and sizing etc. to bring it
to the correct shape and size and to provide a fine surface
finish. Such tubes are produced in various metals and
alloys like steel alloys, aluminum, brass and copper
Q.13. What do you mean by cold working methods ? Explain
any two cold working process with neat sketch.
Ans. The processes which are done on the metals by
keeping the temperature below recrystallisation temperature
are known as cold working processes. The force required for
these processes is high as compared to hot working
processes. Cold working processes are also applied for soft
materials otherwise the work piece will crack. In cold
working processes strength & hardness is increased but
grain structure is distorted.
Q.14. Write short notes on
1.Hot spinning (described earlier)
2.Hot drawing (described earlier)
3.Roll piercing (described earlier)
Q.15. Explain principle of rolling showing its effect on
Ans. The process of rolling basically consists of
passing the hot ingot through two rolls rotating in
opposite direction at a uniform peripheral speed. The space
between the rolls is adjusted to conform to the desired
thickness of the rolled section, and the same is always
less than the thickness of the ingot to reduce its crosssection
and increase its length.
The process is illustrated in figure, which shows the
changes that take place in the grain structure of the metal
as it passes through the rolls. As a result of squeezing
the grains are elongated in the direction of rolling and
the velocity of material at exit is higher than that at the
entry. After crossing the stress zone the grains start
refining. But this is the case only in hot rolling. In cold
rolling they tend to retain the shape acquired by them
Q.16.Describe the types of rolling mills. Explain their
arrangements, specific uses in detail.
Ans. The different types of rolling mills are
1)Two high mill-
It consists of two heavy horizontal rolls, placed
exactly one over the other. The rolls are supported on
bearings housed in sturdy upright side frames, called
stands. The space between the rolls can be adjusted by
raising or lowering the upper roll. The position of the
lower roll is fixed. Both the rolls rotate in opposite
directions to one another, as shown in figure. Their
direction of rotation is fixed and cannot be reversed.
Thus, the work can be rolled by feeding from one direction
only. There is another type of two high mill which
incorporates a drive mechanism that can reverse the
workpiece continuously through back-and-forth passes
between the rolls. This type of rolling mill is known as a
reversing mill. They are normally employed for the initial
rolling of an ingot.
2)Three high rolling mills-
It consists of three horizontal rolls, positioned
directly one rotation of the upper and lower rolls are the
same, but the intermediate roll rotates in a direction
opposite to both of these. All the three rolls continuously
revolve in the same fixed directions and are never
reversed. The work piece is fed in one direction between
the upper and middle rolls and in the reverse direction
between the middle and lower rolls. Many pieces may be
passed through the rolls simultaneously. This results in a
higher rates of production than the two-high mill. This
mill may be used for blooming, billet rolling or finish
3)Four-high rolling mills -
It consists four horizontal rolls, two of smaller
diameter and two of larger diameter, arranged directly one
over the other as shown in figure. The larger diameter
rolls are called back-up rolls and their main function is
to prevent the deflection of the smaller rolls, which
otherwise would result in thickening of rolled plates or
sheets at the centre. The smaller rolls are known as
working rolls and they are the rolls which concentrate the
total rolling pressure over the metal. These mills are
generally used for subsequent rolling of slabs. The common
products of these mills are hot or cold rolled sheets and
It consists of two working rolls of smaller
diameter and four or more back-up rolls of larger diameter.
The arrangement of rolls for this mill is shown in figure.
The number of back-up rolls may go up as high as 20 or
more, depending upon the amount of support needed for the
working rolls during the operation. This type of mill is
generally used for cold rolling.
5)Continuous rolling mills-
It consists of a number of nonreversing two-high
mills arranged one after the other, so that the material
can be passed through all of each successively. The
millstand rotate at a faster speed than that of the
preceding rolls in order to accommodate the increasing
length of the metal piece being rolled. This arrangement
facility facilitates a very rapid production, because the
component passes continuously from one stand to the other
until it reaches the final pass. But it is suitable for
mass production work only, because for smaller quantities
quick changes of set-up will be required and they will
consume a lot of time and labour. As the speed of rolls on
each successive stand varies it is necessary that their
respective surface speeds should be properly calculated and
Q.17.Write short notes on
Ans. Cold Spinning-
The process is similar to hot spinning, described
earlier. The metal is pressed on to the surface of a wooden
or metallic form, called chuck, attached to the lathe
spindle. An adapter fitted in the barrel of the tailstock
holds the work against the form. The tools used, called
spinning tools, are provided with blunt edges and are
supported on the tool rest fitted on the cross slide, as
shown in figure.
Aluminum and other soft metals are best suited for cold
spinning. A few commonly used spun articles out of
Aluminum and its alloys are processing kettles, cooking
utensils, liquid containers and light reflectors etc.
Various other ductile metals, particularly non-ferrous
ones, are quite successfully spun into various typical
shapes. The thickness of the metal is reduced during
spinning, and proper allowance should be made for this in
the blank to be cut. At the end of the operation the
unwanted material is trimmed off to bring the product to
required size. The form or chuck may be in single piece or
composed of many sections attached together, depending upon
the possibility of the spun part from it. Where the space
of the part does not permit its removal from chuck, off centre
chuck are used.
This process of spinning is generally used for:
2.Keeping the tool costs low.
3.Production such peculiar shapes which are difficult to be
produced through other forming methods.
2) Roll bending-
It is also s kind of roll forming operation
through which metal sheets and bent into cylindrical
shapes. The roll bending machines carries three rolls; two
being fixed and the third adjustable. Diameter of all the
same. By adjusting the position of the adjustable roll the
plates or sheets can be bent in different curvatures.
Q.18. Define cold bending.
Ans. It is employed for bending into desired shapes
various stock materials like rods, wires, bars, pipes,
tubes and various structural shapes. Formed dies are used
for bending these articles and the operation is usually
performed in many stages. Well designed fixtures are also
used where mass bending of such components is required.
Q.20. Sketch and describe the types of rolls used in
Ans. The rolls used in rolling mills essentially
consists of three parts, namely body, neck, and wabbler.
The main rolling operation is performed by the body. It is
therefore made to have different shapes on its periphery,
according to the desired shapes of the rolled products,
such as smooth for flat and grooved for other sections. The
rolls vary in diameters from a few centimeters to about 1.5
meters. The extent to which the reduction in cross-section
can be made in a single pass depends upon the weight and
size of the rolls. The neck is that part of the roll on
which it rotates in the bearing. The wabbler is the starshaped
construction at both ends of the roll which engages
the hollow cylinder to connect it to the driving shaft to
receive power. The advantage of providing the wabblers is
that the main body of the roll is not directly connected
with the driving shaft. Thus, in case of too heavy
loading, if there is any damage, it will occur only in the
wabbler, which is a much weaker section, and the main body
will remain absolutely free from it. The rolls are
generally made from cast or forged steel or cast iron.
Q.21. Describe the process of cold spinning stating its
advantages and specific uses.
Ans. Advantages of cold spinning
(1) This is very cheap process.
(2) It does not require much investment.
(3) The production of in cylindrical shape by spinning is
Specific uses of cold spinning:
Cold-spinning process is frequently used in the
making of bells on musical instruments and also for light
fixtures, kitchen-ware, reflectors, funnels, and large
Q.22. What is continuous rolling mill? What are its
Ans. This consists of several stands of two high mills
arranged one after the other. As the metal comes out of
one set of rolls, it enters second, third and so on and
finally comes out in required size and shape. The speed of
every stage goes on increasing in comparison to preceding
roll in order to accommodate the increasing length of the
metal and thereby making it a continuous process. The
operation is thus very fast and space requirement for mill
is less. The production is economical and at the same time
mass production is possible.
Q.23. Write short notes on
Ans. Cold rolling:
Cold rolling is generally employed for providing
a smooth and bright surface finish to the previously hot
rolled steel. It is also used to finish the hot rolled
components to close tolerances and improve their toughness
and hardness. The items generally subjected to cold
rolling for this purpose are bars, rods, sheet plates,
strips and wires etc. Before being put to cold rolling the
hot rolled articles are cleaned through pickling and other
operations. The same types of rolling mills, described
earlier in connection with hot rolling, are used in cold
rolling. In order to obtain a smooth surface finish the
roll surfaces are polished and scratches, if any, removed.
The part being rolled is usually annealed and pickled
before the final pass is made, so as to bring it to
accurate size and obtain a perfectly clean surface.
Q.24.What is hot working? What are the common hot working
Ans. The principal hot working processes generally
applied to various metals, are the following:
5.Welded pipe and tube manufacturing.
Q.25.Explain the following cold working process.
Ans. Cold hobbing:
It is a process for producing cavities of various
shades in a blank of soft metal by pressing a hardened
steel form into it. This form is known as a hob. The
operation may require several pressings and annealings in
between. Hydraulic presses are used for providing the
required pressure. The main advantages of this process is
the economical production of identical cavities in large
number. Moulds for plastic moulding can be produced
through this method.
Q.26. What principles are involved in hot rolling? Explain.
Q.27. What is hot extrusion? In how many ways it can be
Ans. The process of extrusion consists of compressing
a metal inside a chamber to force it out through a small
opening called die. Any plastic material can be extruded
successfully. Most of the presses used for extruding
metals are hydraulically operated horizontal presses. A
large number of extruded shapes are in common use such as
tubes, rods, structural shapes and lead covered cables