Hot Working Process

The working of metals above the recrystallisation temperature is called hot working. The hot working of metals has its advantages and disadvantages:

Advantages of Hot Working

1. The porosity of the metal is largely eliminated, thus producing strong and uniform structure.
2. The grain structure of the metal is refined i.e. the coarse grains are converted into fine grains which change the properties of the metal.
3. The impurities like slag are squeezed into fibers and are uniformly distributed throughout the metal.
4. The mechanical properties such as toughness, ductility, percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains.
5. The deformation of the metal is easy, with a small pressure applied on it.

Disadvantages of Hot Working

1. It requires expensive tools.
2. It produces poor surface finish, due to the 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

The following are the various hot working processes:

1. Hot rolling

It is one of 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, as shown in figure below. The space between the rolls is adjusted to conform to the desired thickness of the rolled section. The rolls, thus, squeeze the passing ingot to reduce its cross-section and increase its length.

The ingots are casted in moulds of suitable form and are used in rolling mills as raw material for preparing desired sections. The first operation to the ingot is carried out at the blooming mill where it is  rolled to  blooms. The bloom has a square cross-section with a minimum size of 150 mm x 150 mm. The blooms are cut up in lengths convenient for the subsequent reducing process into billets. The billet is smaller than a bloom and has a minimum size of  50,mm x 50 mm.

The following types of rolling mills are used for rolling:

(a) Two-high rolling mill

It consists of two heavy rolls placed exactly one over the other. The metal piece is passed between the two rolls rotating at the same speed but in opposite direction.

(b) Three-high rolling mill

It consists of three rolls. The upper and bottom rolls rotate in the same direction while the middle roll rotates in the opposite direction.

(c) Four-high rolling mill

It consists of four rolls, two of which are working rolls and the other two are back-up rolls. The back-up rolls have larger diameter than working rolls.

2. Hot forging

It consists of heating the metal to plastic state and then the pressure is applied to form it into desired shapes and sizes. The pressure may be applied by hand hammers, power hammers or by forging machines.

3. Hot spinning

It consists of heating the metal to forging temperature and then forming it into the desired shape on a spinning lathe. The parts of circular cross-section which are symmetrical about the axis of rotation, are made by this process.

4. Hot extrusion

It consists of pressing a metal inside a chamber to force it out by high pressure through an orifice which is shaped to provide the desired form of the finished part. The rods, tubes, structural shapes, flooring strips and lead covered cables etc. are the typical products of extrusion.

5. Hot drawing or cupping

It is mostly used for the production of thick-walled seamless tubes and cylinders. It is usually performed in two stages. The first stage consists of drawing a cup out of a hot circular plate with the help of a die and punch. The second stage consists of reheating the drawn cup and drawing it further to the desired length having the required wall thickness.

6. Hot piercing

This process is used for the manufacture of seamless tubes. In its operation. the heated cylindrical billets of steel are passed between two conical shaped rolls operating in the same direction. A mandrel is provided between these rolls which assists in the piercing and controls the size of the hole as the billet is forced over it.