Engineering Science 1021A/B Lecture Notes - Lecture 20: Work Hardening, Grain Growth, Ductility
17 May 2018
School
Department
Professor
After a metal has been cold worked:
The density of dislocations has been increased
Up to 10,000 X
§
○
The yield strength has increased because of the increased interaction
between the dislocation strain fields
○
The total energy stored in the metal has increased, again because of the
strain fields
○
Everything strives to minimize its own energy, the structure we have
created by cold working the metal is not stable
○
However, for most metals at room temperature, it is metastable
○
•
Stable VS. Metastable
A system that is metastable cannot change without adding some addition
energy
•
•
3 Stages of Annealing
By increasing temperature, we can provide sufficient thermal energy for the
dislocations to move
•
As a function of temperature and time, the process of annealing goes through 3
phases:
Recovery
○
Recrystallization
○
Grain Growth
○
•
Recovery
Dislocations move to rearrange themselves into a lower energy configuration
and form subgrains
•
Internal strain is relieved because many of the strain fields cancel out each
other out
Process called Stress Relief Anneal
○
•
Only a small % of dislocations are lost•
Yield strength and ductility are largely unchanged•
•
Recrystallization
Longer times at elevated temperature allow the subgrains to recrystallize
Recrystallize: form new small grains
○
•
The dislocation density is drastically reduced•
Yield strength and ductility return to their pre-cold-worked values•
Drastic changes in material properties occur
○
•
Grain Growth
If we continue to hold a recrystallized metal at elevated temperatures, the
grains will begin to grow
Large grains grow at the expense of smaller grains
○
•
It is advantageous to avoid grain growth
Hall-Petch equation
○
•
Design Example
Specify a process by which an annealed brass rod, 10mm in diameter can be
reduced to a final diameter of 6 mm
The finished product must have:
Yield strength of at least 325 Mpa
§
Ductility greater than 20%
§
○
◊
Check Total reduction and resulting properties
○
@ CW=64%:
Yield strength is 425
> 325 Mpa□
§
Ductility is 3%
Too low□
§
○
1.
Determine the required amount of cold work
Yield strength > 325 Mpa =%CW > 17%a.
Ductility > 20% =%CW < 22%b.
Choose a value in the range (20%)
○
2.
Work back from the final dimensions to determine the diameter at the anneal
step
○
○
○
3.
Reduce to 6.7, anneal, reduce to 6 mm4.
Hot Working
Deformation carried out above the recrystallization temperature
This allows you to impose a shape change without increasing the strength
of the material
○
•
Dislocations are still generated and slip still occurs, but the temperature is high
enough that the recovery and recrystallization processes occur immediately
•
A drawback is the surface finish is usually not as good as cold worked material
due to oxidation
•
•
Annealing & Thermochemical Processing
Wednesday, October 18, 2017
8:36 AM
After a metal has been cold worked:
The density of dislocations has been increased
Up to 10,000 X
§
○
The yield strength has increased because of the increased interaction
between the dislocation strain fields
○
The total energy stored in the metal has increased, again because of the
strain fields
○
Everything strives to minimize its own energy, the structure we have
created by cold working the metal is not stable
○
However, for most metals at room temperature, it is metastable
○
•
Stable VS. Metastable
A system that is metastable cannot change without adding some addition
energy
•
•
3 Stages of Annealing
By increasing temperature, we can provide sufficient thermal energy for the
dislocations to move
•
As a function of temperature and time, the process of annealing goes through 3
phases:
Recovery
○
Recrystallization
○
Grain Growth
○
•
Recovery
Dislocations move to rearrange themselves into a lower energy configuration
and form subgrains
•
Internal strain is relieved because many of the strain fields cancel out each
other out
Process called Stress Relief Anneal
○
•
Only a small % of dislocations are lost•
Yield strength and ductility are largely unchanged•
•
Recrystallization
Longer times at elevated temperature allow the subgrains to recrystallize
Recrystallize: form new small grains
○
•
The dislocation density is drastically reduced•
Yield strength and ductility return to their pre-cold-worked values•
Drastic changes in material properties occur
○
•
Grain Growth
If we continue to hold a recrystallized metal at elevated temperatures, the
grains will begin to grow
Large grains grow at the expense of smaller grains
○
•
It is advantageous to avoid grain growth
Hall-Petch equation
○
•
Design Example
Specify a process by which an annealed brass rod, 10mm in diameter can be
reduced to a final diameter of 6 mm
The finished product must have:
Yield strength of at least 325 Mpa
§
Ductility greater than 20%
§
○
◊
Check Total reduction and resulting properties
○
@ CW=64%:
Yield strength is 425
> 325 Mpa□
§
Ductility is 3%
Too low□
§
○
1.
Determine the required amount of cold work
Yield strength > 325 Mpa =%CW > 17%a.
Ductility > 20% =%CW < 22%b.
Choose a value in the range (20%)
○
2.
Work back from the final dimensions to determine the diameter at the anneal
step
○
○
○
3.
Reduce to 6.7, anneal, reduce to 6 mm4.
Hot Working
Deformation carried out above the recrystallization temperature
This allows you to impose a shape change without increasing the strength
of the material
○
•
Dislocations are still generated and slip still occurs, but the temperature is high
enough that the recovery and recrystallization processes occur immediately
•
A drawback is the surface finish is usually not as good as cold worked material
due to oxidation
•
•
Annealing & Thermochemical Processing
Wednesday, October 18, 2017
8:36 AM
After a metal has been cold worked:
The density of dislocations has been increased
Up to 10,000 X
§
○
The yield strength has increased because of the increased interaction
between the dislocation strain fields
○
The total energy stored in the metal has increased, again because of the
strain fields
○
Everything strives to minimize its own energy, the structure we have
created by cold working the metal is not stable
○
However, for most metals at room temperature, it is metastable
○
•
Stable VS. Metastable
A system that is metastable cannot change without adding some addition
energy
•
•
3 Stages of Annealing
By increasing temperature, we can provide sufficient thermal energy for the
dislocations to move
•
As a function of temperature and time, the process of annealing goes through 3
phases:
Recovery
○
Recrystallization
○
Grain Growth
○
•
Recovery
Dislocations move to rearrange themselves into a lower energy configuration
and form subgrains
•
Internal strain is relieved because many of the strain fields cancel out each
other out
Process called Stress Relief Anneal
○
•
Only a small % of dislocations are lost
•
Yield strength and ductility are largely unchanged
•
•
Recrystallization
Longer times at elevated temperature allow the subgrains to recrystallize
Recrystallize: form new small grains
○
•
The dislocation density is drastically reduced•
Yield strength and ductility return to their pre-cold-worked values•
Drastic changes in material properties occur
○
•
Grain Growth
If we continue to hold a recrystallized metal at elevated temperatures, the
grains will begin to grow
Large grains grow at the expense of smaller grains
○
•
It is advantageous to avoid grain growth
Hall-Petch equation
○
•
Design Example
Specify a process by which an annealed brass rod, 10mm in diameter can be
reduced to a final diameter of 6 mm
The finished product must have:
Yield strength of at least 325 Mpa
§
Ductility greater than 20%
§
○
◊
Check Total reduction and resulting properties
○
@ CW=64%:
Yield strength is 425
> 325 Mpa□
§
Ductility is 3%
Too low□
§
○
1.
Determine the required amount of cold work
Yield strength > 325 Mpa =%CW > 17%a.
Ductility > 20% =%CW < 22%b.
Choose a value in the range (20%)
○
2.
Work back from the final dimensions to determine the diameter at the anneal
step
○
○
○
3.
Reduce to 6.7, anneal, reduce to 6 mm4.
Hot Working
Deformation carried out above the recrystallization temperature
This allows you to impose a shape change without increasing the strength
of the material
○
•
Dislocations are still generated and slip still occurs, but the temperature is high
enough that the recovery and recrystallization processes occur immediately
•
A drawback is the surface finish is usually not as good as cold worked material
due to oxidation
•
•
Annealing & Thermochemical Processing
Wednesday, October 18, 2017 8:36 AM
Document Summary
The yield strength has increased because of the increased interaction between the dislocation strain fields. The total energy stored in the metal has increased, again because of the strain fields. Everything strives to minimize its own energy, the structure we have created by cold working the metal is not stable. However, for most metals at room temperature, it is metastable. A system that is metastable cannot change without adding some addition energy. By increasing temperature, we can provide sufficient thermal energy for the dislocations to move. As a function of temperature and time, the process of annealing goes through 3 phases: Dislocations move to rearrange themselves into a lower energy configuration and form subgrains. Internal strain is relieved because many of the strain fields cancel out each other out. Only a small % of dislocations are lost. Longer times at elevated temperature allow the subgrains to recrystallize. Yield strength and ductility return to their pre-cold-worked values.
