“COLD WORKING PROCESS”

SUBMITTED TO:Mr. VIPIN GOYAL

SUBMITTED BY:SHIKHAR SARASWATBRANCH:MECH.(3rdsem)SEC-CROLL NO.:-51

WHAT IS COLD WORKING?

Cold working of metal means the mechanical working at temperature below the re-crystallization temperatures

Normally, it is taken to be working of metals at room temperature.

WHAT WE MEAN BY RECRYSTALLIZATION TEMPERATURE?

When a metal is heated and deformed under mechanical force , an energy level will be reached when the old grain structure (which is coarse due to previous cold working) starts disintegrating.

Simultaneously , an entirely new grain structure (equi- axed, stress free) with reduced grain size starts forming . This phenomenon is known as “ recrystallization ” and the temperature at which the phenomenon starts is called “ Recrystallization temperature” .

According to ASM(AMERICAN SOCIETY OF METALS) “ the approx. temperature at which the complete recrystallization of cold worked metal occurs within a specified period of approx. 1 hour” is recrystallization temperature.

Factors affecting Recrystallization temperature:

Type of metal

Extent of prior cold work

IT IS A FUNCTION OF TIME

AMOUNT OF DEFORMATION IS INVERSELY PROPROTIONAL TO GRAIN SIZE.

Recrystallization temperature is directly proportional to rate of deformation.

Presence of second phase particles decreases recrystallization temperature.

It is clear from the above graphs that due to work hardening , the strength of the metal increases but it’s ability to deform further(formability or ductility ) decreases with cold working . So if the material is excessively deformed , it may fracture before it is formed . To avoid this , large deformations in cold working are obtained in several stages , with intermediate annealing. This will soften the cold worked material & restore it’s ductility & hence the formability.

Characteristics of Cold Working

The Three Stages of Annealing

Recovery - A low-temperature annealing heat treatment designed to eliminate residual stresses introduced during deformation without reducing the strength of the cold-worked material.

Recrystallization - A medium-temperature annealing heat treatment designed to eliminate all of the effects of the strain hardening produced during cold working.

Grain growth - Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area.

1. Annealing effect :

Photomicrographs showing the effect of annealing temperature on grain size in brass. Twin boundaries can also be observed in the structures. (a) Annealed at 400oC, (b) annealed at 650oC, and (c) annealed at 800oC .

The effect of annealing temperature on the microstructure of cold-worked metals. (a) cold-worked, (b) after recovery, (c) after recrystallization, and (d) after grain growth

ADVANTAGES OF COLD WORKING Since cold working is done at room temperature or low temperatures , no

oxidation & scaling of the work material occurs . This results in reduced material loss.

Surface defects are removed. Excellent surface finish which reduces or completely eliminates subsequent

machining resulting in enormous saving in material. High dimensional accuracy. Thin gauge sheets can be produced by cold working. Highly suitable for mass production and automation , because of low working

temperature . The physical properties of metals that do not respond to heat treatment can be

improved by cold working.

DRAWBACKS OF COLD WORKING: At low temperatures , the strength of metal is very high. So ,

large forces are needed for deformation . For this , high capacity equipment is required which is costly.

The ductility / formability of metals is low at low temperatures . Hence for large deformation cold working requires several stages of annealing , which increases the cost of production.

Due to very high forces , the tool pressures & power requirements are high too. So , the tooling must be specially designed , which increases the tool cost.

Due to above factors , normally , only the ductile metals are cold worked.

MATERIALS FOR COLD WORKING:THERE ARE TWO FACTORS BY WHICH THE CHOICE OF MATERIAL IS LIMITED : 1.) The ability of the tool material to withstand the required pressures for cold

working of a material. Obviously , the tool material must have a mechanical strength greater than that of the material to be cold worked . From economic point of view it must have reasonable working life and it must be able to withstand working stress for a particular time.

2.) The economic requirement that the max. possible deformation of the material should be obtained in a single working operation . This will depend upon cold ductility & cold flowability of the material.

In general the requirements of material to be cold worked are:1.) Yield stress curve of gentle slope.2.) Early yield point .3.) Then great elongation with pronounced necking before fracture.

The materials commonly used are :low & medium carbon steel (.25 to .45 % C),low alloy steels , copper & light alloys such as Al , Mg , Ti & Be.

Cold forming is most suitable for axisymmetric (symmetry about an axis or cylindrical symmetry) components like shaft components, flanged(an external/internal rib or rim used to add strength) components , finished gears , & bearing races etc.

PROCESSES UNDER COLD WORKING

COLD ROLLING:

Hot rolled products have a relatively rough surface finish & lack dimensional accuracy & certain physical properties.

However , hot rolled plate , over 6 mm thick & 1800 to 5000 mm wide , is an important starting material for ship building , boiler making .

Thinner gauges , better surface finish & tighter tolerances are obtained by cold rolling . Bars of all shapes , rods , sheets & strips produced are commonly finished by cold rolling.

Cold rolled sheets & strips make up an important part of total steel production . Cold rolling gives springiness to sheets . These sheets are major raw material for some high-production consumer goods industries , such as for household appliances .

Machinability of steel is improved by cold rolling & for this reason , cold rolled stock is widely used for fast automatic machining operations .

Sheet steel less than 1.25 mm thick is cold rolled because it cools too rapidly for practical hot rolling . Prior to cold rolling , steel is pickled to clean the surface and remove scale.

Copper is rolled to various gauges for roofing , containers , cooking vessels & down to 0.075 mm for radiator fin stock.

Cold rolling being carried out:

ANY QUERIES

THANK YOU!