diat_htt_lect-10-11
TRANSCRIPT
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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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