The document discusses the differences between hot working and cold working of metals. It states that hot working is done above the metal's recrystallization temperature, while cold working is done below it. For hot working, large deformations can be repeated as the metal remains soft, while cold working introduces strain hardening. The document also provides examples of common hot and cold working processes like rolling, forging, extrusion, and describes advantages of each type of working.

1. By,
Satha Siva Pandian.M
2nd
year MECH “B”
Metal Atom Forming

2. The distinction between hot working and cold working does
not depends solely on the temperature, but rather on the
processing temperature with respect to the material
recrystallization temperature. When the processing
temperature of the mechanical deformation of steel is
above the recrystallization temperature, the process is
termed as hot working; otherwise, it is cold working.
For hot working processes, large deformation can be
successively repeated, as the metal remains soft and
ductile. The hardness of the material cannot be controlled
after hot rolling and it is a function of chemical composition
and the rate of cooling after rolling. The hardness is
generally lower than that of cold rolling and the required
deformation energy is lesser as well.
HOT AND COLD PROCESSES (WORKING)

3. Types of Extrusion
Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral;

4. Direct-Extrusion
Schematic illustration of the direct-extrusion process.

5. Cold Working
Plastic deformation which is carried out in a
temperature region and over a time interval
such that the strain hardening is not relieved is
called cold working.

6. Some Cold Working Processes:
Cold rolling
Cold forging
Cold extrusion
Bending
Drawing
Shearing

7. SCHEMATICS
ROLLING SCHEMATICS
FORGING
SCHEMATIC EXTRUSION SCHEMATIC

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10. The advantages of cold working
A better surface finish may be achieved
Dimensional accuracy can be excellent because the
work is not hot so it doesn't shrink on cooling; also the
low temperatures mean the tools such as dies and
rollers can last a long time without wearing out.
Usually there is no problem with oxidative effects such
as scale formation. In fact, cold rolling (for example)
can make such scale come off the surface of a
previously hot-worked object.
Controlled amounts of cold work may be introduced.

11. HOT WORKING
 Hot working refers to the process where metals are defromed
above their recrystallizatıon tempereture and strain hardening
does not occur.Hot working performed at elevated
tempreature.However is hot working at room temperature
because of its low melting temperature.

12. Some Hot Working Processes:
 Rolling
 Forging
 Extrusion
 Hot drawing
 Pipe welding
 Piercing
Reason for Hot Working:
At elevated temperatures, metals weaken and
become more ductile.

13. The advantages of hot working
 Lower working forces to produce a given shape, which means
the machines involved don't have to be as strong, which means
they can be built more cheaply;
 The possibility of producing a very dramatic shape change in a
single working step, without causing large amounts of internal
stress, cracks or cold working;
 Sometimes hot working can be combined with a casting process
so that metal is cast and then immediately hot worked. This
saves money because we don't have to pay for the energy to
reheat the metal.
 Hot working tends to break up large crystals in the metal and
can produce a favourable alignment of elongated crystals
 Hot working can remove some kinds of defects that occur in
cast metals. It can close gas pockets (bubbles) or voids in a cast
billet; and it may also break up non-metallic slag which can
sometimes get caught in the melt (inclusions).

14. Types Of Forging
Hand forging :-Hand forging is emplayed only
to shape a Usually used for small item
forging
Hammer forgings :-small number of light forgings
chiefly in repair shops.
Press forging :- Usually used for heavy item
forging.
Machine forging :- For medium sized and large
articles requiring very heavy blows.
Drop forging :- For mass production of identical
parts.
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17. Rolling
Aluminium is first passed through a hot rolling mill and then transferred to a
cold rolling mill.
 Hot rolling mils
Prior to rolling the aluminium is in the form of an ingot which can be up
to 600mm thick. This ingot is then heated to around 500°C and passed
several times through the hot rolling mill. This gradually reduces the thickness
of the metal to around 6mm.This thinner aluminium is then coiled and
transported to the cold rolling mill for further processing.
 Cold rolling mils:
There are various types of cold rolling mill, and they produce various
types of rolled product, with thicknesses as low as 0.05mm. In general the
type of product depends on the alloy used, the rolling deformation and
thermal treatment used in the process as well as careful adjustments to the
mechanics and chemistry of the process. Rolling mills are controlled by very
precise mechanisms and measuring systems.
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19. Operation on a Rectangular Bar

22.  Direct extrusion: A metal
billet is located into a
container, and a ram
compresses the material,
forcing it to flow through
one or more openings in a
die at the opposite end of
the container.
 Indirect extrusion: The die
is mounted to the ram
rather than at the opposite
end of the container. One
advantage of the indirect
extrusion process is that
there is no friction, during
the process, between the
billet and the container
liner.

23. Cold Extrusion
 Cold extrusion is performed at temperatures significantly below
the melting temperature of the alloy being deformed, and
generally at room temperature.
 The process can be used for most materials, provided that
sufficiently robust machinery can be designed.
 Products of cold extrusion include aluminium cans, collapsible
tubes and gear blanks.

24. The advantages of cold extrusion are:
 No oxidation takes place
 Good mechanical properties due to severe cold working as
long as the temperatures created are below the re-
crystallization temperature
 Good surface finish with the use of proper lubricants

25. So….cheap, convenient way to strengthen mat'ls
Work hardening
On the other hand, work hardening can be problem
Example: cold rolling plate to sheet
plate
sheet
too hard to
continue
rolling
harder harder!!
extremely hard - need backup rolls
to keep work rolls from deforming
fairly soft

27. Press Forging
HYDRAULIC PRESSES:-
– Slower
– More massive
– More costly to operate.
– usually more flexible
– Have greater capacity.
– Can be programmed to have different
strokes for different operations and
even different speeds within a stroke.

28. Thank You!