10 - Work Hardening & Annealing

7/23/2019 10 - Work Hardening & Annealing

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Mechanical & Aerospace Engineering

West Virginia University

Work Hardening and Annealing


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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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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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Strain "Work% Hardening


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)ensile )est $ Work Hardening

* n is thestrain-hardening e'ponent

+ is thestrength coefficient


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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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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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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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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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Strain "Work% Hardening$ 7islocation M,ltiplication

8rank-5ead So,rce


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7islocation 3nteractions .ake it diffic,lt to .ove


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The intersection of two dislocations creates 1ogs which in

magnitude and direction are e2ual to the other dislocation3s

'urgers vector.


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Microstr,ct,re Change after Cold Work


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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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5ecovery


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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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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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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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E'a.ple2 Design a process to produce .0,in diameter copper

wire.

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10 - Work Hardening & Annealing