MECHANICAL WORKING OF METALS

INTRODUCTION

The Mechanical working of metal is defined as the plastic deformation of metals under the action of externally applied forces. The mechanical working of metals is described as hot working and cold working depending upon whether the metal is worked above or below the recrystallization temperature.

The metal is subjected to mechanical working for the following purpose:-

To reduce the original block or ingot into desired shape. To refine grain size. To control the direction of flow lines.

Hot Working

The working of metals above the recrystallization temperature is called hot working. Recrystallization temperature is the temperature at which new grains are formed in the metal. Hot working of the metals has following advantages and disadvantages.

Advantages:-

1. The porosity of metal is largely eliminated, thus producing strong and uniform structure.2. The grain structure of the metal is refined.3. The impurities like slag are squeezed into fibres and are uniformly distributed throughout the metal.4. Mechanical properties are improved.

Disadvantage:-

1. It requires expensive tools.

2. It produces poor surface finish, due to rapid oxidation and scale formation on the metal surface.

3. Due to the poor surface finish, close tolerances cannot be maintained.

4. The correct temperature range for working is difficult to maintain.

HOT WORKING PROCESSES ARE:-

1. Hot rolling

2. Hot forging

3. Hot Extrusion

4. Hot Drawing

5. Hot spinning

6. Hot piercing or seamless tubing

7. Tube Forming and

8. Hot forming of welded pipes

Cold working

The working of metals below their recrystallization temperature is known as cold working. Most of cold working processes are performed at room temperature. The cold working distorts the grain structure and does not provide an appreciable reduction in size. 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. During cold working severe stresses known as residual stresses are set up. These stresses are undesirable therefore a suitable heat treatment process may be employed to relieve these stresses.

Effects of cold working

The stresses are set up in the metal which remains in the metal, unless they are removed by subsequent heat treatment. A distortion of the grain structure is created.The strength and hardness of the metal are increased with a corresponding loss in ductility.The recrystalline temperature for steel is increased.

Cold working process

1. Cold rolling

2. Cold forging

3. Cold spinning

4. Cold extrusion

5. Cold drawing

6. Cold bending

Difference between Hot & Cold Working:

Rolling

Rolling is one of the most important industrial metal forming operations. Rolling is the plastic deformation of materials caused by compressive force applied through a set of rolls. The cross section of the work piece is reduced by the process. The material gets squeezed between a pair of rolls, as a result of which the thickness gets reduced and the length gets increased. Mostly, rolling is done at high temperature, called hot rolling because of requirement of large deformations. Hot rolling results in residual stress-free product. However, scaling is a major problem, due to which dimensional accuracy is not maintained. Cold rolling of sheets, foils etc are gaining importance, due to high accuracy and lack of oxide scaling. Cold rolling also strengthens the product due to work hardening.

Rolling Principle:

Hot Rolling

The hot rolling is the most rapid method of converting large sections into desired shapes. The forming of bars, plates, sheets, rails, angles, I-beams and other structural sections are made by hot Rolling.

The operation consists of passing the hot ingot through at least two rolls rotating in opposite directions at the same speed. The rolls squeeze the passing ingot to reduce its cross-section and increase its length. The first operation to the ingot is carried out at blooming mill where it is rolled to blooms. The blooms are cut up in lengths for subsequent reducing process into billets.

The materials commonly hot rolled are aluminium, copper, magnesium, their alloys and many grades of steel.

Rolling mills:

Two-High Rolling Mills

A two-high rolling mill as shown in fig. has two horizontal rolls revolving at the same speed but in opposite direction. The rolls are supported on bearings housed in sturdy upright side frames called stands. The space between the rolls can be adjusted by raising or 1owering the upper roll. Their direction of rotation is fixed and cannot be reversed. The reduction in the thickness of work is achieved by feeding from one direction only.

Three-High Rolling Mills

It consists of three parallel rolls, arranged one above the other as shown in Fig. The directions of rotation of the upper and lower rolls are the same but the intermediate roll rotates in a direction opposite to both of these. This type of rolling mill is used for rolling of two continuous passes in a rolling sequence without reversing the drives. This results in a higher rate of production than the two-high rolling mill.

Four-High Rolling Mill

It is essentially a two-high rolling mill, but with small sized rolls. Practically, it consists of four horizontal rolls, the two middle rolls are smaller in size than the top and bottom rolls as shown in Fig. The smaller size rolls are known as working rolls which concentrate the total rolling pressure over the workpiece. The larger diameter rolls are called back-up rolls and their main function is to prevent the deflection of the smaller rolls, which otherwise would result in thickening of rolled plates or sheets at the centre. The common products of these mills are hot or cold rolled plates and sheets.

Cluster Mill

It is a special type of four-high rolling mill in which each of the two smaller working rolls are backed up by two or more of the larger back-up rolls as shown in Fig. For rolling hard thin materials, it may be necessary to employ work rolls of very small diameter but of considerable length. In such cases adequate support of the working rolls can be obtained by using a cluster-mill. This type of mill is generally used for cold rolling work.