Cold Working Process

The working of metals below their recrystallisation temperature is known as cold working. Most of the cold working processes are performed at room temperature. The cold working distorts the grain structure and does not provide an appreciable reduction in size. It requires much higher pressures than hot working. The extent to which a metal can be cold worked depends upon its ductility. The higher the ductility of the metal, the more it can be cold worked. It also increases tensile strength, yield strength and hardness of steel but lowers its ductility. The increase in hardness due to cold working is called work-hardening.

In general, cold working produces the following effects:

1. The stresses are set up in the metal which remain in the metal, unless they are removed by subsequent heat treatment.
2. A distortion of the grain structure is created.
3. The strength and hardness of the metal are increased with a corresponding loss in ductility.
4. The recrystalline temperature for steel is increased.
5. The surface finish is improved.
6. The close dimensional tolerance can be maintained.

Cold Working Processes

The following are the various cold working processes:

l. Cold rolling

It is generally employed for bars of all shapes, rods, sheets and strips, in order to provide a smooth and bright surface finish. It is also used to finish the hot rolled components to close tolerances and improve their toughness and hardness.

2. Cold forging

It is also called swaging. During this method of cold working, the metal is allowed to flow in some pre-determined shape according to design of dies, by a compressive force or impact. It is widely used in forming ductile metals. The commonly used cold forging processes are sizing, cold heading and rotary swaging.

The sizing is the operation of slightly compressing a forging, casting or steel assembly to obtain close tolerance and a flat surface.

The cold heading process is extensively used for making bolts, rivets and other similar headed parts.

The rotary swaging is used for reducing the diameters of round bars and tubes by rotating dies which open and close rapidly on the work.

3. Cold spinning

It is similar to hot spinning except that the metal is worked at room temperature. The process of cold spinning is best suited for aluminum and other soft metals. The commonly used spun articles out of aluminum and its alloys are processing kettles, cooking utensils, liquid containers and light reflectors etc.

4. Cold extrusion

It is similar to hot extrusion. The most common cold extrusion process is impact extrusion. The process of impact extrusion is limited to soft and ductile materials such as lead, tin, aluminum, zinc and some of their alloys. The various items of daily use such as tubes for shaving creams and tooth pastes and such other thin-walled products are made by impact extrusion.

5. Cold drawing

It is generally employed for bars, rods, wires, tubes etc. The important cold drawing processes are bar drawing, wire drawing and tube drawing.

6. Cold bending

The bars, rods, wires, tubes, structural shapes and sheet metal may be bent to many shapes in cold conditions through dies.

7. Cold or shot peening

It is used to improve the fatigue resistance of the metal by setting up compressive stresses in its surface. This is done by blasting or hurling a rai n of small shot at high velocity against the surface to be peened.