10 - work hardening & annealing
TRANSCRIPT
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Mechanical & Aerospace Engineering
West Virginia University
Work Hardening and Annealing
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Mechanical & Aerospace Engineering
West Virginia University
Principal of Strengthening
The ability of a metal to plastically deform
depends on the ability of dislocation to move.
Restricting or hindering dislocation motion
renders a material harder and stronger
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Mechanical & Aerospace Engineering
West Virginia University
Ao
Change in cross-sectional area
CW ! "A#$ A%A#' (##
Forging
Extrusion Drawing
Rolling
Ao
Ao
Ao
A A
A
A
Metal Working Methods
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Mechanical & Aerospace Engineering
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Strain "Work% Hardening
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Mechanical & Aerospace Engineering
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)ensile )est $ Work Hardening
* n is thestrain-hardening e'ponent
+ is thestrength coefficient
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Mechanical & Aerospace Engineering
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)ensile )est $ Work Hardening
log-log c,rve of vs
- Slope of the c,rve is nVario,s hardening co.ponents
- /o0er n.eans poor respond to
0ork hardening
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Mechanical & Aerospace Engineering
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Values for n and K for metals at room temperature
Metal Condition n K, psi
0,05% C steel Annealed 0,26 77000
SAE 4340 steel Annealed 0,15 93000
0,60% C steel Quenched and tempered 1000oF 0,10 228000
0,60% C steel Quenched and tempered 1300oF 0,19 178000
Copper Annealed 0,54 46400
70/30 rass Annealed 0,49 130000
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Mechanical & Aerospace Engineering
West Virginia University
Strain "Work% Hardening
Effect of cold 0ork on .echanical
properties of C,-(1)i alloy
. !A"AR#$!A% &. 'A(A$'RA)A!*A!,
1997
Cold 0ork 0ill lead to2
3ncrease of 4ielding Strength
3ncrease of )ensile Strength
5ed,ction of Elongation
Material 6eco.es stronger
6,t .ore 6rittle
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Mechanical & Aerospace Engineering
West Virginia University
Strain "Work% Hardening
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Mechanical & Aerospace Engineering
West Virginia University
Strain "Work% Hardening
5eason2 *ncreasing dislocation density and the
interaction between dislocations% which
reduces dislocation mobility. As a result%
larger stresses must be applied in order thatadditional deformation may ta+en place.
7islocation M,ltiplication2
, 'efore deformation a metal contains about -/cm of
dislocation line per cubic centimeter of metal. The
number of dislocations may increase to --0cm of
dislocation line per cubic centimeter of metal.
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Mechanical & Aerospace Engineering
West Virginia University
Strain "Work% Hardening$ 7islocation M,ltiplication
8rank-5ead So,rce
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Mechanical & Aerospace Engineering
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Strain "Work% Hardening
7islocation 3nteractions .ake it diffic,lt to .ove
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Mechanical & Aerospace Engineering
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Strain "Work% Hardening
The intersection of two dislocations creates 1ogs which in
magnitude and direction are e2ual to the other dislocation3s
'urgers vector.
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Mechanical & Aerospace Engineering
West Virginia University
Microstr,ct,re Change after Cold Work
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Mechanical & Aerospace Engineering
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4eating a cold,wor+ed metal above a recrystalli5ation temperature 6.78.9 T):
eliminates most of the defects 6dislocations% etc:8 *n - hr% substantial amount of recrystalli5ation occurs
4eating process referred to as annealing
8 Annealing consists of heating to a high enough temperature followed by cooling ata suitable rate
During annealing% metals undergo recovery and recrystalli9ation
8 4ighly,strained grains are replaced by new strain,free grains
Amount of recrystalli5ation is dependent on both time and temperature
Annealing leads to
8 Reduction in yield strength and hardness and increase in ductility as thedislocations are removed
8 *ncrease ductility% softness
8 Development of desired microstructure and properties
;old,wor+ing and annealing are often cycled to assist in production
Annealing
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Mechanical & Aerospace Engineering
West Virginia University
5ecovery
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Mechanical & Aerospace Engineering
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5ecrystalli9ation
Recrystalli5ation=8 !ew grains nucleate and grow at the expense the highly,strained
grains until the whole of the metal consists of strain,free grains
8 !ucleation usually occurs in the most deformed portion of the grain8 boundary or slip plane
> Driving force is the strain energy of the deformed grains8 Dislocation density returns to original value 6before cold wor+ing:
8 4ardness and ductility return to original value
Recrystalli5ation also used to control grain si5e
8 4igh temperatures and long crystalli5ation times can lead to graingrowth of the strain,free grains 6driven by reduction in grainboundary area: tend to produce a large grain si5e
8 "rain growth due to surface tension , big grains eat little grains
8 4igh initial deformation tends to produce small recrystalli5ed grains
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Mechanical & Aerospace Engineering
West Virginia University
Cold Working and Annealing
tarting material with lowdislocation density
;old,wor+ed materialhas greatly increased
dislocation density
Fully recrystalli5ed metal with new
6smaller: strain,free grains
Further annealing leads to
nucleation and growth of new grains
Annealing leads torecovery
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Mechanical & Aerospace Engineering
West Virginia University
5ecrystalli9ation & :ro0th
4 sec 8 sec3 sec0 sec
;old wor+ed brass at recrystali5ation temp 9?o;
;allister% )at ci @ Eng an *ntro% 9 thed.
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Mechanical & Aerospace Engineering
West Virginia University
E'a.ple2 Design a process to produce .0,in diameter copper
wire.