• Metal Forming is a process in which shape of the metals are
changed to desired shapes by subjecting them to stresses
greater than yield stress of the metal.
• Metal Forming can be classified into two.
• Plastic Deformation Process – Primary Process : Volume and
mass of the metals are unchanged. E.g.. Rolling, forging,
• Metal Removal or Machining Process –Secondary Process :
Material is removed from the metal. E.g.. Turning, thread
cutting etc. 2
3. ROLLING AND TYPES OF
• The process of plastically deforming (i.e. reducing thickness or
changing cross section) metal by passing it between rolls is
known as Rolling.
• Rolling can be classified into two:
• Hot rolling
• Cold rolling
• A process whereby a distorted grain structure of metals is
replaced by a new, stress-free grain structure as a result of
heating and deforming above a specific minimum temperature
for a specific period of time.
• The temperature at which recrystallization occurs is known as
recrystallization temperature. (Generally, recrystallization
temperature is nearly half of melting point of the metal.)
• During recrystallization, old grain boundaries disappear and
small new grains begin to grow.
• Recrystallization reduces strength and raises the ductility of
5. HOT ROLLING
• Hot rolling is the process of rolling a metal above its recrystallization
• Hot rolling is used for larger reduction of cross section (surface finish
is not considered).
• The hot rolling of most steel products is done in the blooming mills.
• Blooming mills are usually high reversing mills, with each rolls
weighing up to 20 tones.
• They are driven by a reversing electric motor of up to 20 MW
• As a result of squeezing, the grains are elongated in the direction of
rolling and after crossing the stress zone, grains start refining.
• Hot rolling brings homogeneity in rolled components.
• Grain refinement gives optimum mechanical properties to the
• Time taken to produce the component is less compared to
• Surface oxidation takes place.
• Chance of scale inclusion exists.
• Process is more expensive.
• More care is needed to handle the hot part. 7
9. • Cold Rolling is a process of rolling metals and alloys below
their recrystallization temperatures.
• Generally they are worked at room temperatures.
• In Cold Rolling, the grains tend to retain the shape acquired
• Mainly used for surface finishing of hot rolled products.
• Cold rolling is used to produce sheets and strips of fine surface
finish and accuracy.
• Strength of cold rolling will be high because of strain hardening.
• Close dimensional tolerances can be achieved.
• Reduced defects.
• Internal stresses are induced into the cold worked metal thus
making the metal hard and brittle.
• Ductility is loosed to great extent. 10
11. HOT vs COLD ROLLING
SL Hot Working Cold Working
Metal heated above its recrystallization
Metal heated below its recrystallization
Being carried out at recrystallization
temperature, there is no strain hardening.
No recrystallization leading to strain
Co-efficient of friction between rolls and work
4. Heavy reduction in area can be obtained. Heavy reduction in area cannot be obtained.
Mechanical properties are improved. Less
decrease in ductility.
Hardness increases. Brittleness increases.
Blow-holes and other similar defects are
Excessive cold rolling generates cracks.
7. Roll radius is generally larger. Smaller.
Very thin sections are not obtained. Hot rolling
sheets less than 1.25mm is not economical.
Thin sections can be obtained. (0.002 mm)
9. Hot Rolling has scale(metal oxide) on it. Oxide free.
10 Surface finish is not good. Good surface finish.
Used for ferrous and non-ferrous. Steels,
aluminum, copper, brass, bronze, etc. used to
change ingot to billets, slabs, sheets etc.
Hot rolling is the father of cold rolling Cold rolling follows hot rolling