Bulk deformation processes

• Those where the surface area of the workpiece changes significantly.

• Thickness or cross sections are reduced or shapes are changed.

• Since the volume remains constant, other dimensions must change in proportion.

Bulk deformation/Hot working

• Rolling• Forging• Extrusion• Wire & Bar Drawing• Piercing• Pipe Welding

Rolling

• Thickness is reduced by compressive forces exerted by two opposing rolls.

• Pulling and Squeezing of work is done by rolls itself.

Rolling

• Most rolling processes are very capital intensive, requires massive pieces of equipments, called roll mills.

• Most rolling is carried out by hot working, called hot rolling, owing to the large amount of deformation required.

• Hot rolled metal is generally free of residual stresses and its properties are isotropic.

Steps in Rolling

• Solidification of metal into an ingot (just solidified).• The ingot is placed in a furnace where it remains for

many hours until it has reached a uniform temperature throughout.

• The heating temperature for steel is around 1200°C.• Heating operation is called as soaking, and the

furnaces in which it is carried out is called as soaking pits.

Rolling Temperature

• Temperature control is a requirement for success.

• If temperature is not uniform, the subsequent deformation will not be uniform.

• If the soaking time is insufficient, the hotter exterior will flow in preference to the cooler, stronger interior.

Contd…

• If a part is removed from the furnace and cools prior to the working, the cooler surfaces will tend to resist deformation.

leads to….

• Cracking and tearing of the surface may result as the hotter, weaker interior tries to deform.

Contd…• Hot rolling is usually terminated when the

temperature falls to about 50 to 100° C above the recrystallization temperature.

Why terminated ?

• To ensure the production of uniform grain size and prevents the possibility of unwanted strain hardening.

Flat Rolling Analysis

PARAMETERS (Flat Rolling)

1) Draft

2) Reduction

3) Slip

4) Average flow stress

5) Coefficient of friction

6) Rolling force

7) Contact length

8) Torque

9) Power

Mechanics (Flat Rolling Analysis)

1) Draft : Amount of reduction in thickness.

d = to – tf ;

d = draft, mm to = starting thickness, mm tf = final thickness, mm

Flat Rolling Analysis

2) Draft is sometimes expressed as a fraction of the starting stock thickness, called the reduction. r = d / to ; r = reduction, d = draft, mm to = starting thickness, mm.

Flat Rolling Analysis

3) The roll contact i.e. the work along a contact arc defined by angle ‘Ɵ’

4) The velocity of roll is greater than the entering speed of the work (vo )and less than the exiting speed of work (vf )

Roll velocity/Surface velocity (vr) = π D N, mm/min