Q.1 Classify the metal forming operations and mention their specific application.
(AKTU - 2008-09)
Ans. Metal Forming: -
A process in which the desired size and shape are obtained through the deformation of metals plastically under the action of externally applied forces is known as metal forming.
External forces applied for plastic deformation may be tensile, compressive or shear or a combination of these ones. Metal forming operations are economical because the desired shape, size and finish are obtained without much loss of materials.
Classification of Metal Forming: -
Deformation process can be classified in various groups depending on method and type of operation, temperature, size and shape of the material.
(i) Classification On The Basis Of Temperature: -
Deforming process can be classified as hot working and cold working.
(ii) Classification On The Basis of Type Of Operation : -
(a) Primary Operation : These include operation like forging, rolling, extrusion, etc.
(b) Secondary Operation : These include operation like processing of semi finished products such as bolts, sheet metal parts, etc.
Secondary shaping processes are those processes, which take care of dimensional accuracy during material processing. Sometimes, secondary operations are carried out to improve dimensional accuracy of the produced by primary shaping processes, for example produced casting are subjected to machining to make these castings dimensionally accurate.
Ans. Metal Forming: -
A process in which the desired size and shape are obtained through the deformation of metals plastically under the action of externally applied forces is known as metal forming.
External forces applied for plastic deformation may be tensile, compressive or shear or a combination of these ones. Metal forming operations are economical because the desired shape, size and finish are obtained without much loss of materials.
Classification of Metal Forming: -
Deformation process can be classified in various groups depending on method and type of operation, temperature, size and shape of the material.
(i) Classification On The Basis Of Temperature: -
Deforming process can be classified as hot working and cold working.
(ii) Classification On The Basis of Type Of Operation : -
(a) Primary Operation : These include operation like forging, rolling, extrusion, etc.
(b) Secondary Operation : These include operation like processing of semi finished products such as bolts, sheet metal parts, etc.
Secondary shaping processes are those processes, which take care of dimensional accuracy during material processing. Sometimes, secondary operations are carried out to improve dimensional accuracy of the produced by primary shaping processes, for example produced casting are subjected to machining to make these castings dimensionally accurate.
Q.2 Define the operation of forging. (AKTU - 2008-09)
Ans. Forging: -
Forging is the term for shaping metal by using localized compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is done at a high temperature, which makes metal easier to shape and less likely to fracture. Warm forging is done at intermediate temperature between room temperature and hot forging temperatures. Forged parts can range in weight from less than a kilogram to 170 metric tons. Forged parts usually require further processing to achieve a finished part.
Advantages And Disadvantages: -
A significant advantage of the forging process is that it produces a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics.
Some metals may be forged cold, however iron and steel are almost hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable.
Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high temperature furnace (sometimes referred to as the forge) will be required to heat ingots or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations will require the use of metal-forming dies, which must be precisely machined and carefully heat treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.
Processes: -
A cross-section of a forged connecting rod that has been etched to show the grain flow.
There are many different kinds of forging processes available, however they can be grouped into three main classes:
- Drawn out: length increases, cross-section decreases
- Upset: Length decreases, cross-section increases
- Squeezed in closed compression dies: produces multidirectional flow
Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
Q.3 Describe some forging defects and causes.
Ans. Defects And Causes: -
1. Laps and cracks at corners or surfaces lap is caused due to following over of layer of material over another surface. These defects are caused by improper forging and fault die design.
2. Incomplete forging either due to less material or inadequate or improper flow of material.
3. Mismatched forging due to improperly alegned die halves.
4. Scale pits.
5. Burnt or over heated metal due to improper heating.
Q.4 Give a brief classification of forging operation.
Ans. Forging can be classified as :
1. Open Die Forging Operation: -
Open die forging operation is carried out between flat or simple contoured dies. Dies do not completely cover the work piece. In open die forging operations, metals are worked above their recrystallization temperature. Upsetting operation is a very common example of open die forging operations.
All forgeable ferrous and non-ferrous alloys can be forged by this operations.
Round, square, rectangular, hexagonal objects, etc can be easily produced by this process. Open die forging is further classified into :
(a) Hand Forging: -
It is also called smithy blacksmithy. It is the simplest form of forging. The metal to be forged is first heated to red hot state and then beaten by hammers on a metal anvil to give it a desired shape. In smithy shop, charcoal, coke and coal are generally used as fuels in the forging operations.
(b) Power Forging: -
Power forging is employed to produce identical and standard parts on a large scale. Devices which work on forgings by blow are known as hammers and those which work by pressure are called presses.
2. Closed Die Forging Operation: -
In this forging operation, the work piece is completely surrounded by dies. In this process, a billet is deformed (hot or cold)by a punch in order to fill the die cavity without any wastage of material. The metal enclosed in the die cavity is deformed due to high pressure. In this way, a high precision forged component with close dimensional tolerances is produced. Press used for closed-die forging are of two kinds :
(a) Mechanical Presses: -
Mechanical forging presses of crank type are widely used in forging operations. Mechanical presses are powered from motor mounted on the press frame. These presses are used for the production of rivets, screws, nuts where a high operating speed is desired.
(b) Hydraulic Presses: -
It has the same working principle as that for smithy except for the construction of dies. In smithy, press dies have flat surfaces. While in close die forging, the press dies have shaped impression cut on the dies.
3. Impression Die (Or Precision) Forgings: -
In this type of forging, two or more dies containing impressions of the part shape are brought together, the metal undergoes plastic deformation until its enlarged sides come in contact with the die side walls. During this process, flash is formed become some of the molten metal from the original metal piece flows outside the die impression. On cooling, flash imparts deformation resistance to the metal and thus strengthening the final product. The final product resembles the die impression. Complex shapes can be produced more easily than in open die forging operation e.g. spherical shapes, block-like solids, disc-like objects, etc. Impression die forging can be further classified into following groups :
(a) Drop Forging: -
In this forging, upper half of the die is dropped on to the lower half. The workpiece is divided equally in upper and lower die blocks. When upper die block falls on lower die block, the metal is squeezed in the die cavity because of impact. Die block is made to fall from a height of 3 to 5 metre. A series of punch and die operation are performed to change the shape of a material.
(b) Press Forging: -
In this operation, a slow continuous pressure is applied to the area to be forged. In this forging operation only one blow is given as compared with number of impacts in drop forging. These forgings are more accurate dimensionally, than drop forgings.
(c) Machine Forging: -
In this forging operation, powered hammers or presses are used instead of hand hammers or sledges used in hand forging. Powered hammer or presses can be either mechanical or pneumatic type. These operations are accomplished by the use of series of dies mounted on the same press or hammer.
Q.5 Define the operation of rolling. (AKTU - 2008-09)
Related Questions -
Q. Explain with neat sketch, the basic principle of rolling. Describe its application in industry. (AKTU - 2009-10)
Q. Define rolling process. Using schematic diagrams and state the differences between cluster mills and continuous rolling. (AKTU - 2012 - 13)
Ans. Rolling: -
Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling. In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called “strip”) or plate (typically thickness more than 3 mm). In profile rolling the final product may be a round rod or other shaped bar, such as a structural section (beam, channel, joist etc). Rolling is also classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.
Principle of Rolling: -
In Fig. AB and A'B' are the contact arcs on the rolls. The wedging action on the work is overcome by the frictional forces that act on these arcs and draw the metal through the rolls. In the process of rolling, stock enters the rolls with a speed less than the peripheral roll speed. The metal emerges from the rolls travelling at a higher speed than it enters. At a point midway between A and B, metal speed is the same as the roll peripheral speed. Most deformation takes place in thickness, although there is some increase in width. Temperature uniformity is imortant in all rolling operations. Since it controls metal flow and plasticity.
In rolling, the quantity of metal going into a roll and out of it is the same, but the area and velocity are changed.
Q.3 Describe some forging defects and causes.
Ans. Defects And Causes: -
1. Laps and cracks at corners or surfaces lap is caused due to following over of layer of material over another surface. These defects are caused by improper forging and fault die design.
2. Incomplete forging either due to less material or inadequate or improper flow of material.
3. Mismatched forging due to improperly alegned die halves.
4. Scale pits.
5. Burnt or over heated metal due to improper heating.
Q.4 Give a brief classification of forging operation.
Ans. Forging can be classified as :
1. Open Die Forging Operation: -
Open die forging operation is carried out between flat or simple contoured dies. Dies do not completely cover the work piece. In open die forging operations, metals are worked above their recrystallization temperature. Upsetting operation is a very common example of open die forging operations.
All forgeable ferrous and non-ferrous alloys can be forged by this operations.
Round, square, rectangular, hexagonal objects, etc can be easily produced by this process. Open die forging is further classified into :
(a) Hand Forging: -
It is also called smithy blacksmithy. It is the simplest form of forging. The metal to be forged is first heated to red hot state and then beaten by hammers on a metal anvil to give it a desired shape. In smithy shop, charcoal, coke and coal are generally used as fuels in the forging operations.
(b) Power Forging: -
Power forging is employed to produce identical and standard parts on a large scale. Devices which work on forgings by blow are known as hammers and those which work by pressure are called presses.
2. Closed Die Forging Operation: -
In this forging operation, the work piece is completely surrounded by dies. In this process, a billet is deformed (hot or cold)by a punch in order to fill the die cavity without any wastage of material. The metal enclosed in the die cavity is deformed due to high pressure. In this way, a high precision forged component with close dimensional tolerances is produced. Press used for closed-die forging are of two kinds :
(a) Mechanical Presses: -
Mechanical forging presses of crank type are widely used in forging operations. Mechanical presses are powered from motor mounted on the press frame. These presses are used for the production of rivets, screws, nuts where a high operating speed is desired.
(b) Hydraulic Presses: -
It has the same working principle as that for smithy except for the construction of dies. In smithy, press dies have flat surfaces. While in close die forging, the press dies have shaped impression cut on the dies.
3. Impression Die (Or Precision) Forgings: -
In this type of forging, two or more dies containing impressions of the part shape are brought together, the metal undergoes plastic deformation until its enlarged sides come in contact with the die side walls. During this process, flash is formed become some of the molten metal from the original metal piece flows outside the die impression. On cooling, flash imparts deformation resistance to the metal and thus strengthening the final product. The final product resembles the die impression. Complex shapes can be produced more easily than in open die forging operation e.g. spherical shapes, block-like solids, disc-like objects, etc. Impression die forging can be further classified into following groups :
(a) Drop Forging: -
In this forging, upper half of the die is dropped on to the lower half. The workpiece is divided equally in upper and lower die blocks. When upper die block falls on lower die block, the metal is squeezed in the die cavity because of impact. Die block is made to fall from a height of 3 to 5 metre. A series of punch and die operation are performed to change the shape of a material.
(b) Press Forging: -
In this operation, a slow continuous pressure is applied to the area to be forged. In this forging operation only one blow is given as compared with number of impacts in drop forging. These forgings are more accurate dimensionally, than drop forgings.
(c) Machine Forging: -
In this forging operation, powered hammers or presses are used instead of hand hammers or sledges used in hand forging. Powered hammer or presses can be either mechanical or pneumatic type. These operations are accomplished by the use of series of dies mounted on the same press or hammer.
Q.5 Define the operation of rolling. (AKTU - 2008-09)
Related Questions -
Q. Explain with neat sketch, the basic principle of rolling. Describe its application in industry. (AKTU - 2009-10)
Q. Define rolling process. Using schematic diagrams and state the differences between cluster mills and continuous rolling. (AKTU - 2012 - 13)
Ans. Rolling: -
Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls. There are two types of rolling process, flat and profile rolling. In flat rolling the final shape of the product is either classed as sheet (typically thickness less than 3 mm, also called “strip”) or plate (typically thickness more than 3 mm). In profile rolling the final product may be a round rod or other shaped bar, such as a structural section (beam, channel, joist etc). Rolling is also classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling.
Principle of Rolling: -
In Fig. AB and A'B' are the contact arcs on the rolls. The wedging action on the work is overcome by the frictional forces that act on these arcs and draw the metal through the rolls. In the process of rolling, stock enters the rolls with a speed less than the peripheral roll speed. The metal emerges from the rolls travelling at a higher speed than it enters. At a point midway between A and B, metal speed is the same as the roll peripheral speed. Most deformation takes place in thickness, although there is some increase in width. Temperature uniformity is imortant in all rolling operations. Since it controls metal flow and plasticity.
In rolling, the quantity of metal going into a roll and out of it is the same, but the area and velocity are changed.
Q1 = Q2
or A1V1 = A2V2 or
or A1V1 = A2V2 or
where Q1 = Quantity of metal going into roll
Q2 = Quantity of metal leaving the roll
A1 = Area of an element in front of roll, m2
A2 = Area of an element after roll, m2
V1 = Velocity in element before the roll, m/s
V2 = Velocity in element after the roll, m/s
In the process of becoming thinner, the rolled steel becomes longer and may become wider, but it is constrained by vertical rolls set to restrict this sideways growth. As the cross-sectional area is decreased, the velocity increases as does the length of the material. For example, a heated slab 18cm. thick weighing more than 12 tons is reduced to a coil of thin sheet in a matter of minutes.
Rolling mills are classified according to the number and arrangement of rolls in a stand. They are classified as (see Fig.) :
(a) For hot rolling of metals
(1) Two-high rolling mill (2) Three-high rolling mill
(b) For cold rolling of metals
(1) Four high rolling mill (2) Cluster rolling mill
(1) Two-High Rolling Mill: -
It is basically of two types i.e. non-reversing and reversing rolling mill. The two high non-reversing rolling stand arrangement is the most common arrangement. In this the rolls always move in only one direction, while in a two-high reversing rolling stand the direction of roll rotation can be reversed. This type of stand is particularly useful in reducing the handling of the hot metal in between the rolling passes. About 30 passes are required to reduce a large ingot into a bloom. This type is used in blooming and slabbing mills.
(2) Three-High Rolling Mill: -
It is used for rolling of two continuous passes in a rolling sequence without reversing the drives. After all the metal has passed through the bottom roll set, the end of the metal is entered into the other set of the rolls for the next pass. For this purpose a table-tilting arrangement is required to bring the metal to the level with the rolls. Such type of arrangement is used for making plates or sections.
(3) Four-High Rolling Mill: -
It is generally a two-high rolling mill, but with small sized rolls. The other two rolls are the backup rolls for providing the necessary rigidity to the small rolls. It is used for both hot and cold rolling of wide plates and sheets.
(4) Cluster Rolling Mill: -
It uses backup rolls to support the smaller work rolls. In this type of mill, the roll in contact with the work can be as small as in. in diameter. Foil is always rolled on cluster mills since the small thickness requires small-diameter rolls.
Q2 = Quantity of metal leaving the roll
A1 = Area of an element in front of roll, m2
A2 = Area of an element after roll, m2
V1 = Velocity in element before the roll, m/s
V2 = Velocity in element after the roll, m/s
In the process of becoming thinner, the rolled steel becomes longer and may become wider, but it is constrained by vertical rolls set to restrict this sideways growth. As the cross-sectional area is decreased, the velocity increases as does the length of the material. For example, a heated slab 18cm. thick weighing more than 12 tons is reduced to a coil of thin sheet in a matter of minutes.
Rolling mills are classified according to the number and arrangement of rolls in a stand. They are classified as (see Fig.) :
(a) For hot rolling of metals
(1) Two-high rolling mill (2) Three-high rolling mill
(b) For cold rolling of metals
(1) Four high rolling mill (2) Cluster rolling mill
(1) Two-High Rolling Mill: -
It is basically of two types i.e. non-reversing and reversing rolling mill. The two high non-reversing rolling stand arrangement is the most common arrangement. In this the rolls always move in only one direction, while in a two-high reversing rolling stand the direction of roll rotation can be reversed. This type of stand is particularly useful in reducing the handling of the hot metal in between the rolling passes. About 30 passes are required to reduce a large ingot into a bloom. This type is used in blooming and slabbing mills.
(2) Three-High Rolling Mill: -
It is used for rolling of two continuous passes in a rolling sequence without reversing the drives. After all the metal has passed through the bottom roll set, the end of the metal is entered into the other set of the rolls for the next pass. For this purpose a table-tilting arrangement is required to bring the metal to the level with the rolls. Such type of arrangement is used for making plates or sections.
(3) Four-High Rolling Mill: -
It is generally a two-high rolling mill, but with small sized rolls. The other two rolls are the backup rolls for providing the necessary rigidity to the small rolls. It is used for both hot and cold rolling of wide plates and sheets.
(4) Cluster Rolling Mill: -
It uses backup rolls to support the smaller work rolls. In this type of mill, the roll in contact with the work can be as small as in. in diameter. Foil is always rolled on cluster mills since the small thickness requires small-diameter rolls.
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