In this article we are going to learn about Cold working processes, Methods, Difference between hot working and cold working, advantages and disadvantages.
What is Cold Working ?
Cold working is the process of metal forming in which deformation of metal occurs below its recrystallization temperature.
Cold forming, also known as cold working, is any metalworking process in which metal is shaped below its recrystallization temperature, which is usually at room temperature. Hot working techniques such as hot rolling, forging, welding, and so on are contrasted with such processes.
This is a technique that is commonly used in non-brittle metals with extremely high melting points. This method can be used to strengthen a variety of polymers. Cold-worked areas of metal, on the other hand, are more prone to corrosion due to increased dislocation concentration.
What is Recrystallisation Temperature ?
The temperature at which new stress-free grains are formed in a metal is known as recrystallisation temperature.
Cold Working Effect
Cold-working is employed chiefly as a finishing operation, following the shaping of the metal by hot-working. It also controls the mechanical properties of steel in a large measure. It increases the tensile strength, yield strength and hardness of steel, but lowers its ductility. The increase in hardness resulting from cold-working is described as work hardening.
In general, cold-working produces the following effects :
- Internal stresses are set up which remain in the metal unless they are removed by proper heat-treatment.
- A distortion of the grain structure is created
- Strength and hardness of the metal are increased but ductility is decreased.
- Smooth surface finish is produced.
- Accurate dimensions of parts can be maintained.
Cold Working Processes
Cold working is the process of working at temperatures lower than the recrystallization temperature or at room temperature.
The metal stock is placed behind the pair of two rollers during this process.
The metal stock is at room temperature or slightly above room temperature but below the recrystallization temperature when it passes through the roller.
Because the metal stock is passed at room temperature, this method generally necessitates a high compression force.
The grain structure of the metal changes and forms a new shape as a result of the high compression force. Brittle metal is cold worked.
Cold Working Processes Methods
The principal methods of cold-working are as follows :-
- Cold rolling
- Cold forging
- Cold drawing
- Cold bending
- Cold spinning
- Cold extruding
- Cold Squeezing
- Cold peening
- Sizing, coining, hobbing
1. Cold Rolling
Bars of all shapes, rods, sheets, and strips are commonly finished by cold rolling. Foil is made of the softer metals in this way. Two main reasons for cold-rolling metals are to get smooth bright surface finish and improved physical properties. If the object is only to give a clean, smooth finish to the metal, only a superficial amount of rolling will be needed.
On the other hand, where it is desirable that the tensile strength, stiffness, and hardness be increased substantially, and that the section thickness be appreciably reduced, then higher roll pressures and deeper kneading are necessary. Cold-rolling also improves machinability by conferring the property of brittleness, a condition which is conducive to smooth tool finishes and well broken chips.
As a preliminary step to the cold-rolling operation, sheets, strips etc. of hot-rolled steel are immersed in an acid solution to remove the scale and washed in water and then dried. The cleaned steel is passed repeatedly through set of rolls thereby producing a slight reduction in each pass until the required thickness is obtained.
2. Cold Forging
Cold forging is the process of deforming a metal material at room temperature using extremely high pressure. The slug is placed in a die and compressed by a press until it fits into the desired shape. Depending on the geometric complexity of the component, several operations may be required to gradually deform the metal.
3. Cold Drawing
Cold drawing operations involve the forcing of metal through a die by means of a tensile force applied to the exit side of the die. Most of the caused by the compressive force which arises from the react with the die. Usually the metal has a circular symmetry, absolute requirement.
Rods, tubes and extrusions are often given a cold-finishing operation to reduce the size, increase the strength, improve the finish, and provide better accuracy.
4. Cold Bending
Bending is the process by which a straight length is transformed into a curved length. It is one of the most widely used forming process for changing plates (or sheets) into drums, curved channels etc. Bars, rods, wires, tubing and structural shapes as well as sheet metal are bent to many shapes in cold conditions through dies.
In all metal bending, the metal stressed beyond the elastic limit is in tension on the outside and in compression on the inside of the bend. Stretching of the metal on the outside makes the stock thinner, Bending a flat strip of metal is commonly done by roll forming.
5. Cold Spinning
Cold-spinning is the operation of shaping very thin metal by pressing it against a form while it is rotating. The method is exactly similar to hot spinning, except the condition of metal. In cold spinning the metal is worked at room temperature.
Equipment required for spinning comprises a lathe, forming tools, and a metal or wood former to suit the shape required. A circular blank is cut and positioned against the former by a follower. A round-nose tool is used and while the lathe rotates the blank is forced to the shape of the former.
6. Cold Extrusion
The principle of cold extrusion is exactly similar to that of hot extrusion. Of the various processes of extrusion, impact extrusion is essentially a cold extrusion method.
6. Impact Extrusion
Impact extrusion is limited primarily for making small workpieces from the more ductile metals. The work material is placed in position into a blind die and a ram punch with clearance is forced into the die, causing the metal to flow plastically around the punch. The outside diameter of the tube is the same as the diameter of the die, and the thickness is controlled by the clearance between the punch and die. The extruding force is usually supplied by a mechanical press. Collapsible medicine tubes and toothpaste tubes are made in a this way.
7. Cold Squeezing
Cold squeezing is a squeezing operation performed at temperatures well below the melting point to finish the surface of a workpiece for improved dimensional accuracy and surface finish.
Squeezing is a quick and widely used way of forming ductile metals. Of the different processes of squeezing, cold heading and rotary swaging are very common.
Cold heading :- Cold heading is a cold forging process, used primarily for the manufacture of bolts, screws, rivets, nails and similar items.
Rotary Swaging :- Rotary swaging is the process used to reduce the cross-sectional area of rods and tubes. Swaging is often accepted of as a cold-forging operation, because the metal-forming takes place under the hammering blows of die sections. The swaging machine consists mainly of a hollow spindle which carries the die sections and rollers.
8. Cold Peening
Cold Peening method is employed to set up a superficial state of surface compressive stress, causing the interior of the member to assume an opposite tensile stress. Because fatigue generally occurs from surface cyclically loaded in tension, the useful lives of such members are frequently extended by shot peening. This method is sometimes employed to achieve an ornamental effect.
Shot peening is done by blasting or hurling a rain of small shot at high velocity against workpieces to cause slight indentations. Figure 7.15 shows shot peenig action on a job.
9. Sizing, Coining and Hobbing
Parts of malleable iron, forged steel, powdered metals, aluminium, and other ductile nonferrous metals are commonly finished to thickness by squeezing an operation called sizing. A special die is needed for almost every job, but each piece can be sized in a fraction of the time of machining. Thus, sizing is economical wherever applicable in high production industries.
Operations like sizing have been called coining, but coining more truly involves the impression and raising of images or characters from a punch and die into the metal. The metal is made to flow, and the designs on opposite sides of a coined piece are not necessarily related as in embossing. Hard money is probably the best known product of coining.
Hobbing or hubbing is a method of making moulds for the plastic and die-casting industries. A punch called the hob or hub, is machined from tool steel to the shape of the cavity, heat treated for hardness, and polished. It is then pressed into a blank of soft steel to form the mould. A prime advantage of this method is that one hob properly applied can make a number of cavities in one mould or in a series of mould. Sizing, coining, hobbling, etc., are illustrated in Fig. 7.16
Difference Between Hot working and Cold Working
|HOT WORKING||COLD WORKING|
|1. The technique of plastically deforming a metal above its recrystallization temperature is known as hot working.||1. The technique of reinforcing a metal through plastic deformation at temperatures below the recrystallization temperature is known as cold working or work hardening.|
|2. Hot working is done at temperatures higher than the metal’s recrystallization temperature.||2. Cold working is done at temperatures lower than the metal’s recrystallization temperature.|
|3. There are no internal or residual tensions in the metal when it is heat worked.||3. Internal and residual tensions develop up in the metal during cold working.|
|4. In hot working, the metal is deformed and recovered at the same time.||4. In cold working, there is no significant metal recovery.|
|5. Hot working can be used to eliminate cracks or pores.||5. Cold working causes cracks to proliferate and new cracks to emerge.|
|6. After hot working, the metal’s homogeneity is extremely high.||6. After cold working, the metal’s homogeneity is low.|
Advantages of Cold Working
- The cold working procedure results in a smooth surface finish.
- This results in an accurate dimension of the parts.
- It improves the metal’s strength and hardness while decreasing its ductility.
- Because this procedure is performed without the use of heat, no oxide is formed on the surface, resulting in a smooth surface.
- This method is primarily used for mass production.
- The defect in this process is easily detectable and repairable.
- Because tensile strength, yield point, and ductility are improved, rust resistance is reduced
- Prevents metal loss due to corrosion.
- There is no need to heat the metal.
- It saves money because it does not require heating.
- Strain hardening takes place.
Disadvantages of Cold Working
- Cold working requires a clean, smooth surface.
- Brittle metals cannot be properly cold worked.
- It takes more force to start and finish the work.
- It necessitates more powerful machinery.
- The maximum amount of deformation that can be given is limited by the capability of the press and hammer due to higher yield strength and low ductility.
Applications of Cold Working
1. Cold working is used to make a variety of products in industries such as large flat sheets, metal tubes, screw heads, riveted joints, and much more.
2. It is used in a variety of industries, including aviation, iron and steel, automobiles, and so on.
3. It is most commonly used with ductile metals.
4. This process is also used after hot working of metals because it provides smooth finishing and strength.
5. Multiple products are produced through the use of various cold working methods such as sheering, cold rolling, and cold extrusion.
What is the Purpose of Cold Working ?
Cold working refers to the process of strengthening metal by changing its shape without the use of heat. Subjecting the metal to this mechanical stress causes a permanent change to the metal’s crystalline structure, causing an increase in strength.
This strengthens the materials through a process called strain hardening (or work hardening). This is when a ductile metal becomes harder and stronger as it is deformed plastically. The cost of this strengthening is a reduction in ductility.
Frequently Asked Questions
Cold working of metal increases
1. Yield strength
2. Tensile strength
4. All of the above
Right Answer is 4. All of the above
Explanation :- Work hardening is that which occurs in metalworking processes that intentionally induce plastic deformation to exact a shape change. These processes are known as cold working or cold forming processes. The cold working of the metal increases the hardness, yield strength, and tensile strength
What cold working increases
2. Impact strength
3. Percentage of elongation
4. Fatigue strength
Right Answer is 4. Fatigue strength
Most of the time cold forming is done at room temperature and materials used are Low and medium carbon steels, low alloy steels, copper and light alloys, Titanium and beryllium.
Cold working of steel is defined as working
1. At its recrystallisation temperature
2. Above its recrystallisation temperature
3. Below its recrystallisation temperature
4. At two thirds of the melting temperature of the metal
Answer: Option 3
Which of the following is not improved by cold working of metals?
3. Surface finish
4. Corrosion resistance
Answer : 4. Corrosion resistance
The Cold working of metal is accomplished at
1. Below re-crystallization temperature
2. Below re-crystallization temperature
3. Re-crystallization temperature
4. Any of the above
Answer : 1. Below re-crystallization temperature
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