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HeatTreatment

Dr.

Santosh

S.

Hosmani

Process annealing

It is a heat treatment that is used to negate the effects of

cold workthat is, to soften and increase the ductility of a

previously strain-hardened metal. It is commonly utilizedduring fabrication procedures that require extensive plastic

deformation, to allow a continuation of deformation without

fracture or excessive energy consumption. Recovery and

recrystallization processes are allowed to occur.Ordinarily

- , ,

heat treatment is terminated before appreciable grain growth

has occurred. Surface oxidation or scaling may be prevented

or m n m ze y annea ng a a re a ve y ow empera ure uabove the recrystallization temperature) or in a nonoxidizing

atmosphere.

Figure: influence of

annealing temperature

(for an annealing time

of 1 h) on the tensile

strength and ductility

.

Increasing the percentage of cold work enhances the

,

recrystal lizat ion temperature is lowered, and

approaches a constant or l imiting value at high

deformations

Figure: The variation of

recrystalization

empera ure w percen

cold work for iron. For

deformations less than

cold-working),

recrystallization will notoccur.

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Recrystallization proceeds more rapidly in pure metals than in

alloys.

During recrystallization, grain-boundary motion occurs as the new

grain nuclei form and then grow. It is believed that impurity atoms

grain boundaries so as to diminish their (i.e., grain boundary)mobilities; this results in a decrease of the recrystallization rate

,

substantially.

For ure metals the recr stalization tem erature is normall

0.3*Tm where Tm is the absolute melting temperature; for

some commercial alloys it may run as high as 0.7*Tm.

Plastic deformation operations are often carried out attemperatures above the recrystallization temperature in a process

termed hot working. The material remains relatively soft and

ductile during deformation because it does not strain harden, and

thus large deformations are possible.

Stable

austenite

QUENCHING TTTdiagramforeutectoidsteel

'

coolinra id

Har nessRC65

:martensite M

Extremelyrapid

unstable

cooling

austenite

A+M

MsMs:Martensitestart

temperature

M

MfMf:Martensitefinish

temperature

Martensitictransformation

notdependupontime,onlyon

temperature.

Atomsmoveonlyafractionof

atomicdistanceduringthetransformation:

1.Diffusionless

(nolongrangediffusion)

2.Shear

BCT (onetoonecorrespondence

betweenandatoms)

3.Nocompositionchange

Martensitictransformation

BCTunitcellof(austenite)

414.12 ==

a

c

BCTunitcellof(martensite)

08.100.1 =aExpand~

12%

0%C (BCC) 1.2%CContract

~

20%

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Martensitictransformation

Har nesso martensiteasa unctiono Ccontent

60

s,R

CHardnessof

ardnes

dependsmainly

onCcontentand

20no ono er

alloyingadditions

0.2 0.4 0.6

ar on

Martensitictransformation

a mar ens e

Acicular martensite

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Martensitictransformation

Ref.: V. Kumar and S.S. Hosmani:Journal of Metallurgy and Materials Science, Vol. 53, Pages 393-404 (2011)

TEMPERING

Heating of quenched steel below the eutectoid temperature,

o ng or a spec e me o owe y a r coo ng.

CFetempering

3+ T

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