INTRODUCTION
Table of Contents
As a mechanical engineer, workshop technology is one of the major sector of working and it is quite necessary for everyone from the chairman of a company to the worker. It is because of its versatility and wide range of services. A formen or an engineer or a worker should definitely be aware of the manufacturing processes and the materials that are used in working.
Here in this article we are going to discuss about the basics of mechanical working of metals which includes Hot working and Cold working.
HOT WORKING:
The working procedure of metals above the *recrystallisation temperature is called Hot working.
* The temperature at which new grains are formed is known as recrystallisation temperature.
Following are the advantages and disadvantages of hot working –
Advantages:-
1. Here the porosity of the natal is eliminated at best, hence it provides uniform and strong structure.
2. The coarse grain are converted into fine grains which change the properties of the metal. Thus we can say that the grain structure of the metal refined.
3. Here the impurities like slag are altered into fibres and are uniformly distributed throughout the metal.
4. Here the mechanical properties such as ductility, toughness, percentage elongation, percentage reduction, resistance to shock and vibration are improved due to the refinement of grains.
5. In hot working the deformation of metal is easy with a small pressure applied on it.
Disadvantages:-
1. It requires quite expensive tools.
2. It produces poor Surface finish due to frequent oxidation and scale formation on the metal surface.
3. Due to poor Surface finish, correct and close tolerance can’t be maintained.
4. The correct temperature range for working is difficult to maintain.
Different hot working processes:
1. Hot rolling
Hot rolling operation consists of passing the hot ingot through at least two rolling rolls rotating in opposite directions at the same speed as shown below. The space between the rolls is adjusted to conform to the desired thickness of the rolled section. By the result of this, the rolls thus suppress or squeeze the passing ingot to reduce its cross section and increase its length.
Following are the types of rolling mills used for roll –
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 directions.
b) Three – high rolling mill : It consists of three heavy rolls. The upper and lower rolls rotate in the same direction while the middle roll rotates in opposite direction.
c) Four – high rolling mill : It consists of four rolls. Out of which two are working rolls and the other two are back up rolls. Here the back up rolls have larger diameter than the working rolls.
2. Hot forging
3. Hot spinning
It consists of heating the metal to the forging temperature and then forming it into the desired shape on a spinning lathe. By the hot spinning process, we generally produce the parts of circular cross section which are symmetrical about the axis of rotation.
4. Hot extrusion
Hot extrusion is the process of suppressing a metal inside a chamber to force it out by the application of pressure through an orifice which is shaped to provide the desired form of the finished part.
The tubes, rods, structural shapes, flooring strips, lead covered cables etc. are the major products of hot extrusion.
5. Hot drawing or cupping
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 comprises of reheating the drawn cup and drawing it further to the desired length having the required wall thickness.
Hot drawing is widely used for the production of thick walled seamless tubes and cylinders.
6. Hot piercing
This procedure is generally used for manufacturing of seamless tubes. In this process, the heated cylindrical billets of steel are passed between two conical shaped rolls operating in same direction. A mandrel is provided between these rolls which helps in the piercing and also controls the size of the hole as the billet is forced over it.
Also read this : Heat treatment of steel – An overview
COLD WORKING:
The working procedure of metals below 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 any appreciable reduction in size. It comparatively requires much higher pressure than hot working. The cold working of any metal depends upon its ductility. The higher the ductility of a metal, the more it can be cold worked. Cold working of a metal also increases tensile strength, yield strength and hardness.
The increase in hardness because of the cold working is called as work – hardening.
Effects of Cold working:
1. Here the stresses that are set up in the metal remains in the metal, unless and until they are removed by subsequent heat treatment.
2. A deformation in the grain structure is developed.
3. Here the strength and hardness of the metal is increased with a corresponding loss in ductility.
4. Surface finish is also improved.
5. The recrystallisation temperature of steel is increased.
6. One of the most important and major effect is that close dimensional tolerance can be maintained.
Different Cold working processes
1. Cold rolling
It is usually applied for bars of all shapes, rods, sheets and strips in order to provide a smooth and bright surface finish. It is also used for the finishing of hot rolled components for close tolerance and improve their toughness and hardness.
2. Cold forging
Cold forging is also known as ‘ swaging ‘. During this method of cold working, the metal is allowed to flow in some predetermined shape according to the design of dies by the application of the compressive force or impact. The commonly used cold forging processes are sizing, cold heading, rotary swaging etc.
• The sizing is the operation of 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 diameter of the round bars and tubes by the rotating dies which open and close rapidly on the work.
3. Cold spinning
Cold spinning is also very much similar to hot spinning except that the metal is performed in room temperature. It is generally suitable for aluminium and other soft metals. It finds major application in the manufacturing of kettles, cooking utensils, liquid containers and light reflectors etc.
4. Cold extrusion
It is also similar to hot extrusion. One of the most common cold extrusion process is impact extrusion. This impact extrusion process is limited to soft and ductile materials such as lead, tin, aluminium, zinc and their alloys. The common examples of products of impact extrusion are tubes of shaving cream and tooth paste tubes.
5. Cold drawing
Cold drawing is usually employed for bars, rods, wires, tubes etc. Some of the important cold working processes are bar drawing, wire drawing and tube drawing.
6. Cold bending
When the bars, rods, tubes, wires, structural shapes and sheet metal are being bent into many shapes in cold conditions through dies then the process is known as cold bending.
7. Cold or shot peening
It is generally used to improve the fatigue resistance of the metal by setting up compressive stress in its surface. This is done by blasting or impinging a rain of small shot at high velocity against the surface to be peened.
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