Hot and cold working of metals

. advantage and disadvantage of hot working metal

. advantage and disadvantage of cold working of metal

Working on metals

This involves the manipulation of metals /materials when in the solid state

Examples of working are forming, rolling, forging etc.

Working processess can be classified

Hot working

Cold working 

Hot working 

Hot working is done at a suitable high temparature so that any damage to the grains is repaired. The temparature required varies from metal to metal.

Examples of hot working

Forging, rolling, extrusion et

Advantage of hot working

-hot working is relatively a low cost process

-refinement of grain from cast structure is possible.

-the scale givea some protection against corrosion during storage.

After the process also the product will be in the fully annealed condition, suitable for further cold working.

Disadvantage of hot working

-poor surface finish is likely

-dimensional inaccuracy is possible due to shrinkage on cooling

-geometrical accuracy is possible due to distortion on cooling

Cold working

Cold working is done at room temaprature. Due to the reason the grains become distorted.

Cold working increases the  hardness and strength of the material at the expense of ductility, and the metal is said to be work hardened. This work hardening increases the strength and rigidity.

Cold Woking must not be used for large deformation unless the metal is frequency heat treated to keep it ductile.

Examples of cold Working

Sheet metal pressing, rivet head forming, cold extrusion etc

Advantage and disadvantage of cold working

-good surface finish, high dimensional and geometrical accuracy is possible

-at times this process is used for finishing a previously hot worked part. Due to this the processing cost may be more

-materials lack ductility due to work hardening and are less suitable for bending etc.

DIFFERENT TYPES OF FRICTION

.meaning of friction

. advantage and disadvantage of friction

. different types of friction

. methods of reducing friction

Friction is the force that offers resistance to relative motion between surfaces in contact.

Friction opposes motion irrespective of the direction. When a book is made to slide on the surface of a table from left to right, the force of friction acts on the lower surface of the book towards the left.

Advantages

Friction plays an important role in our life. Walking would be impossible without friction between the foot and the ground. Unless there is friction, it would not be possible to lean a ladder against a wall, turn the pages of a hook, tie shoelaces etc.

Because of friction

-rolling motion is possible

-sliding motion is possible

-gripping effect is possible

Disadvantage

-reduction in efficiency of machines

-wear and tear of all sliding surfaces in contact

-unwanted heat generation

Types of friction

Different types of friction are as follows

-Limiting friction

-sliding friction

-rolling friction

-solid friction

-liquid friction

-gas friction

-boundary friction

Limiting friction

A rectangular blook of wood is placed on flat surface. One end of a string is attached to the block and the other end to a spring balance so that a horizontal force could be applied. The force applied to the block is increased gradually. It will be noticed that the block remains at rest untill the applied force is large enough to overcome the force which is read on the scale of the balance. This force is called the  limiting friction.

Sliding friction

The block is pulled along so that it slips at a steady speed over the surface. Take the spring balance reading when this happens. This reading will be much less than the limiting friction. This shows that sliding friction is always less than the limiting friction.

Rolling friction

It is a matter of common experience that it is much easier to roll an object on a surface than to slide it. For example, you require less effort to roll a drum full of coal tar than to slide it over the surface of the road. Rolling friction is much less than the sliding friction. It is for this reason that all vehicles used on roads or rails are provided with wheels.

Solid friction

It is the friction offered by one dry solid surface when it moves over another solid surface. This friction is high.

Liquid friction

The resistance offered by a liquid when a body moves in it is known as liquid friction. Eg: a ship sailing in water.

Gas friction

It is the resistance offered by a gas when a body travels in this medium. This is rather low. Eg: aeroplane flying in air

Boundary friction

It is the resistance offered at the boundary. Eg: a shaft is rotating in a bearing lubricated by means of oil. The surface of the shaft and the bearing are only in partial contact as a thin film of oil separates them creating a sort of boundary

Practical methods of reducing friction

-polishing

-use of lubricants

-use of ball and roller bearings

-streamlining the transmission system

-employing low friction material.