Lecture 37 - Deformation Processing Notes | EduRev

: Lecture 37 - Deformation Processing Notes | EduRev

 Page 1


Lecture 37: Deformation Processing 
Contents 
Introduction 
Hot working 
Cold working 
Spring back phenomenon 
Annealing 
Key words: Hot working, cold working, annealing 
 
Introduction 
The ultimate goal of a manufacturing engineer is to produce steel components with required 
geometrical shape and structurally optimized for a given application. One of the method is the 
deformation processing. Deformation processing exploits the ability of steel to flow plastically 
without altering the other properties. The required forces are often very high. Cast ingots, slabs, 
blooms and billets are reduced in size and converted into plates, sheets, rods and others. These 
forms experience further deformation to produce the desired products formed by processes such as 
forging, extrusion and other sheet metal forming. The deformation may be bulk flow in three 
dimensions, simple shearing, simple bending, or any combination of these and other processes. The 
stresses could either be tensile or compressive or shear or combination of them. In this connection 
the steel chemistry and cleanliness are important factors for deformation processing. 
In the following, some aspect of deformation processing is discussed. This is given to appreciate the 
efforts of steelmakers in producing quality steels. The readers should also understand the reverse 
engineering approach and to appreciate the steelmaking. Deformation processing can be carried out 
either under hot or cold condition. In the following general features of hot and cold working are 
described. Details can be obtained in any text book on deformation processing. 
 
Hot working 
It is plastic deformation of metals above their recrystallization temperatures. Hot working of steel 
requires to heat steel near 1000
o
C for plastic deformation. Hot working of steel involves the 
deformation of fcc austenite. 
? Hot working does not produce strain hardening. Hence no increase in either yield strength or 
hardness occurs. In addition yield strength decreases as temperature increases and the 
ductility improves. 
Page 2


Lecture 37: Deformation Processing 
Contents 
Introduction 
Hot working 
Cold working 
Spring back phenomenon 
Annealing 
Key words: Hot working, cold working, annealing 
 
Introduction 
The ultimate goal of a manufacturing engineer is to produce steel components with required 
geometrical shape and structurally optimized for a given application. One of the method is the 
deformation processing. Deformation processing exploits the ability of steel to flow plastically 
without altering the other properties. The required forces are often very high. Cast ingots, slabs, 
blooms and billets are reduced in size and converted into plates, sheets, rods and others. These 
forms experience further deformation to produce the desired products formed by processes such as 
forging, extrusion and other sheet metal forming. The deformation may be bulk flow in three 
dimensions, simple shearing, simple bending, or any combination of these and other processes. The 
stresses could either be tensile or compressive or shear or combination of them. In this connection 
the steel chemistry and cleanliness are important factors for deformation processing. 
In the following, some aspect of deformation processing is discussed. This is given to appreciate the 
efforts of steelmakers in producing quality steels. The readers should also understand the reverse 
engineering approach and to appreciate the steelmaking. Deformation processing can be carried out 
either under hot or cold condition. In the following general features of hot and cold working are 
described. Details can be obtained in any text book on deformation processing. 
 
Hot working 
It is plastic deformation of metals above their recrystallization temperatures. Hot working of steel 
requires to heat steel near 1000
o
C for plastic deformation. Hot working of steel involves the 
deformation of fcc austenite. 
? Hot working does not produce strain hardening. Hence no increase in either yield strength or 
hardness occurs. In addition yield strength decreases as temperature increases and the 
ductility improves. 
? Hot working can be used to drastically alter the shape of metals without fear of fracture and 
excessively high forces. 
? Elevated temperatures promote diffusion that can remove chemical inhomogeneties; pores 
can be welded or reduced in size during deformation.  
? The dendritic grain structure, small gas cavities and shrinkage porosity formed during 
solidification in large sections can be modified by hot working to produce a fine, randomly 
oriented, spherical-shaped grain structure which results in a net increase in strength, 
ductility and toughness. 
? Hot working results in reorientation of inclusions or impurity particles in the metal with the 
result that an impurity originally oriented so as to aid crack movement through the metal 
can be reoriented into a “crack arrestor” configuration. 
 
The various hot working processes are rolling, extrusion, forging, hot drawing etc. 
 
Cold working   
Cold working is plastic deformation of metals below the recrystallization temperature and is 
generally performed at room temperature. Some advantages are: 
• No heating is required 
• Better surface finish and superior dimensional control are achieved 
• Strength, fatigue, and wear properties are improved 
• Directional properties can be imparted 
 
Disadvantages: 
• Heavier forces are required 
• Strain hardening occurs (may require intermediate annealing treatment to relieve internal 
stresses) 
• Residual stresses may be produced 
For cold working, the ductility and the yield point stress of steel are important. The effect of ductility 
is shown below: 
 
Page 3


Lecture 37: Deformation Processing 
Contents 
Introduction 
Hot working 
Cold working 
Spring back phenomenon 
Annealing 
Key words: Hot working, cold working, annealing 
 
Introduction 
The ultimate goal of a manufacturing engineer is to produce steel components with required 
geometrical shape and structurally optimized for a given application. One of the method is the 
deformation processing. Deformation processing exploits the ability of steel to flow plastically 
without altering the other properties. The required forces are often very high. Cast ingots, slabs, 
blooms and billets are reduced in size and converted into plates, sheets, rods and others. These 
forms experience further deformation to produce the desired products formed by processes such as 
forging, extrusion and other sheet metal forming. The deformation may be bulk flow in three 
dimensions, simple shearing, simple bending, or any combination of these and other processes. The 
stresses could either be tensile or compressive or shear or combination of them. In this connection 
the steel chemistry and cleanliness are important factors for deformation processing. 
In the following, some aspect of deformation processing is discussed. This is given to appreciate the 
efforts of steelmakers in producing quality steels. The readers should also understand the reverse 
engineering approach and to appreciate the steelmaking. Deformation processing can be carried out 
either under hot or cold condition. In the following general features of hot and cold working are 
described. Details can be obtained in any text book on deformation processing. 
 
Hot working 
It is plastic deformation of metals above their recrystallization temperatures. Hot working of steel 
requires to heat steel near 1000
o
C for plastic deformation. Hot working of steel involves the 
deformation of fcc austenite. 
? Hot working does not produce strain hardening. Hence no increase in either yield strength or 
hardness occurs. In addition yield strength decreases as temperature increases and the 
ductility improves. 
? Hot working can be used to drastically alter the shape of metals without fear of fracture and 
excessively high forces. 
? Elevated temperatures promote diffusion that can remove chemical inhomogeneties; pores 
can be welded or reduced in size during deformation.  
? The dendritic grain structure, small gas cavities and shrinkage porosity formed during 
solidification in large sections can be modified by hot working to produce a fine, randomly 
oriented, spherical-shaped grain structure which results in a net increase in strength, 
ductility and toughness. 
? Hot working results in reorientation of inclusions or impurity particles in the metal with the 
result that an impurity originally oriented so as to aid crack movement through the metal 
can be reoriented into a “crack arrestor” configuration. 
 
The various hot working processes are rolling, extrusion, forging, hot drawing etc. 
 
Cold working   
Cold working is plastic deformation of metals below the recrystallization temperature and is 
generally performed at room temperature. Some advantages are: 
• No heating is required 
• Better surface finish and superior dimensional control are achieved 
• Strength, fatigue, and wear properties are improved 
• Directional properties can be imparted 
 
Disadvantages: 
• Heavier forces are required 
• Strain hardening occurs (may require intermediate annealing treatment to relieve internal 
stresses) 
• Residual stresses may be produced 
For cold working, the ductility and the yield point stress of steel are important. The effect of ductility 
is shown below: 
 
Figure37.1: Stress strain diagram for low carbon and high carbon steel to understand the 
suitability of steel for cold working 
 
In figure 37.1 variation of stress with strain is shown for (A) low carbon steel and (B) high carbon 
steel. Permanent deformation can not occur until strain is greater than X
1
. At the other extreme if 
steel is strained to X
4
, the metal will fracture. From coldworking point of view the following is 
important: 
? The magnitude of yield stress, which indicates the force required to initiate the permanent 
deformation and  
? The extent of region of strain that is 0 to X
4
 which determines the extent of plastic 
deformation 
If considerable deformation is required then the tensile properties of steel should be that depicted 
in figure 37.1A. Greater ductility would be available in the material and less force would be required 
to initiate and continue the deformation. 
 High carbon steel  which shows stress strain behaviour like  figure 37.1B is not suitable for cold 
deformation but may be suitable for shearing operations 
Cold working properties are also affected by the grain size and must be controlled during 
solidification of steel. Too large and too small grain size have undesirable effects. 
Springback  
Springback is also present in cold working operations. In the elastic region, the strained material 
returns to its original size and shape. But removal of load in the plastic region, decreases the strain 
from x
3
 to x
2
 as shown in the figure 37.1 for metal A i. The decrease in strain, x
3
 - x
2
 is elastic 
springback. 
Thus, in cold working the deformation must be carried out beyond the desired point by an amount 
equal to the springback. 
The various cold working processes are squeezing, bending, shearing and drawing 
 
Annealing 
Plastic deformation of polycrystalline material in cold working produces microstructural and 
property changes that include (a) change in grain shape, (b) strain hardening, (c) increase in 
dislocation density. 
Appropriate heat treatment such as annealing reverts back to the pre-cold worked states. The 
purpose of annealing may involve one or more of the following aims: 
? To soften the steel and to improve machinability 
Page 4


Lecture 37: Deformation Processing 
Contents 
Introduction 
Hot working 
Cold working 
Spring back phenomenon 
Annealing 
Key words: Hot working, cold working, annealing 
 
Introduction 
The ultimate goal of a manufacturing engineer is to produce steel components with required 
geometrical shape and structurally optimized for a given application. One of the method is the 
deformation processing. Deformation processing exploits the ability of steel to flow plastically 
without altering the other properties. The required forces are often very high. Cast ingots, slabs, 
blooms and billets are reduced in size and converted into plates, sheets, rods and others. These 
forms experience further deformation to produce the desired products formed by processes such as 
forging, extrusion and other sheet metal forming. The deformation may be bulk flow in three 
dimensions, simple shearing, simple bending, or any combination of these and other processes. The 
stresses could either be tensile or compressive or shear or combination of them. In this connection 
the steel chemistry and cleanliness are important factors for deformation processing. 
In the following, some aspect of deformation processing is discussed. This is given to appreciate the 
efforts of steelmakers in producing quality steels. The readers should also understand the reverse 
engineering approach and to appreciate the steelmaking. Deformation processing can be carried out 
either under hot or cold condition. In the following general features of hot and cold working are 
described. Details can be obtained in any text book on deformation processing. 
 
Hot working 
It is plastic deformation of metals above their recrystallization temperatures. Hot working of steel 
requires to heat steel near 1000
o
C for plastic deformation. Hot working of steel involves the 
deformation of fcc austenite. 
? Hot working does not produce strain hardening. Hence no increase in either yield strength or 
hardness occurs. In addition yield strength decreases as temperature increases and the 
ductility improves. 
? Hot working can be used to drastically alter the shape of metals without fear of fracture and 
excessively high forces. 
? Elevated temperatures promote diffusion that can remove chemical inhomogeneties; pores 
can be welded or reduced in size during deformation.  
? The dendritic grain structure, small gas cavities and shrinkage porosity formed during 
solidification in large sections can be modified by hot working to produce a fine, randomly 
oriented, spherical-shaped grain structure which results in a net increase in strength, 
ductility and toughness. 
? Hot working results in reorientation of inclusions or impurity particles in the metal with the 
result that an impurity originally oriented so as to aid crack movement through the metal 
can be reoriented into a “crack arrestor” configuration. 
 
The various hot working processes are rolling, extrusion, forging, hot drawing etc. 
 
Cold working   
Cold working is plastic deformation of metals below the recrystallization temperature and is 
generally performed at room temperature. Some advantages are: 
• No heating is required 
• Better surface finish and superior dimensional control are achieved 
• Strength, fatigue, and wear properties are improved 
• Directional properties can be imparted 
 
Disadvantages: 
• Heavier forces are required 
• Strain hardening occurs (may require intermediate annealing treatment to relieve internal 
stresses) 
• Residual stresses may be produced 
For cold working, the ductility and the yield point stress of steel are important. The effect of ductility 
is shown below: 
 
Figure37.1: Stress strain diagram for low carbon and high carbon steel to understand the 
suitability of steel for cold working 
 
In figure 37.1 variation of stress with strain is shown for (A) low carbon steel and (B) high carbon 
steel. Permanent deformation can not occur until strain is greater than X
1
. At the other extreme if 
steel is strained to X
4
, the metal will fracture. From coldworking point of view the following is 
important: 
? The magnitude of yield stress, which indicates the force required to initiate the permanent 
deformation and  
? The extent of region of strain that is 0 to X
4
 which determines the extent of plastic 
deformation 
If considerable deformation is required then the tensile properties of steel should be that depicted 
in figure 37.1A. Greater ductility would be available in the material and less force would be required 
to initiate and continue the deformation. 
 High carbon steel  which shows stress strain behaviour like  figure 37.1B is not suitable for cold 
deformation but may be suitable for shearing operations 
Cold working properties are also affected by the grain size and must be controlled during 
solidification of steel. Too large and too small grain size have undesirable effects. 
Springback  
Springback is also present in cold working operations. In the elastic region, the strained material 
returns to its original size and shape. But removal of load in the plastic region, decreases the strain 
from x
3
 to x
2
 as shown in the figure 37.1 for metal A i. The decrease in strain, x
3
 - x
2
 is elastic 
springback. 
Thus, in cold working the deformation must be carried out beyond the desired point by an amount 
equal to the springback. 
The various cold working processes are squeezing, bending, shearing and drawing 
 
Annealing 
Plastic deformation of polycrystalline material in cold working produces microstructural and 
property changes that include (a) change in grain shape, (b) strain hardening, (c) increase in 
dislocation density. 
Appropriate heat treatment such as annealing reverts back to the pre-cold worked states. The 
purpose of annealing may involve one or more of the following aims: 
? To soften the steel and to improve machinability 
? To relieve internal stresses induced by rolling, forging etc. 
? To remove coarseness of grains 
The annealing consists of  
? Heating the steel to a certain temperature 
? Soaking at this temperature 
? Cooling at a predetermined rate  
 
Such restoration results from recovery, recrystallization, which may be followed by grain growth. 
During recovery some of the stored internal strain energy is relieved by virtue of dislocation motion 
due to atomic diffusion. 
Even after recovery is complete, the grains are still in a relatively high strain energy state. 
Recrystallization is the formation of a new set of strain-free and equixed grains (having 
approximately equal dimensions in all directions).  Strength and hardness decrease, but ductility 
increases. 
After recrystallization is complete, the strain-free grains will continue to grow, if the metal is left at 
the elevated temperature.  
 
References: 
E.P. DeGarmo: Materials and processes in manufacturing 
 
 
 
 
 
 
 
 
 
 
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