Hypoeutectoid Steels (CC<0.76%) (? hypereutectoid) - PowerPoint PPT Presentation

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Hypoeutectoid Steels (CC<0.76%) (? hypereutectoid)

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... supersaturated solid solution Fe possible sites for C atoms c a ca martensite plates / austenite = high strength = brittle Normalizing (Austenite) slow ... – PowerPoint PPT presentation

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Title: Hypoeutectoid Steels (CC<0.76%) (? hypereutectoid)


1
Hypoeutectoid Steels (CClt0.76)(? hypereutectoid)
Ck45Cc0.45
dark pearlite lamellae of Fe3C and ferrite
light a ferrite
2
Hypoeutectoid Steels (CClt0.76)(? hypereutectoid)
Ck15Cc0.15
dark pearlite
light a ferrite
3
Hypo-Eutectoid Transformation
4
Eutectoid Transformation Pearlite
not instantaneously!! -gt f(time)
5
Phase Transformation
fraction of transformation y
log time t
6
PearliteFormation -IsothermalTransformation
fraction of transformation y
time s
temperature C
Austenite
Pearlite
isothermal transformation diagram / time
temperature transformation TTT plot
time s
7
Alteration in Microstructurecontinuous cooling
transformation (CCT)
equilibrium
Austenite
Pearlite
lower T gt shorter diffusion paths!
8
Bainite Formation
pearlite
bainite
9
Martensite Formation
fcc g
very fast cooling to RT (no intersection with
transformation nose) C diffusion becomes
extremely slow -gt negligible!!
pearlite
bcc a Fe3C
bainite
thermodynamic driving force for fccgt bcc
transformation increases
10
Martensite Formation
bct unit cell of martensite supersaturated solid
solution
Fe
possible sites for C atoms
gt high strength gt brittle
martensite plates / austenite
11
Heat Treatment Mechanical Properties
Normalizing (Austenite)
slow cooling hypo-eutectoid a-ferritepearlite
hyper-eutectoid pearlite Fe3C
moderate cooling bainite
fast cooling martensite
reheat (250C-600C) tempered martensite
12
Mechanical Properties
Brinell hardness (strength)
ductility RA
0
700
100
1
composition C
13
Tempered Martensite
14
Tempered Martensite
good combination of 1 strength and 2 ductility
15
Strengthening Mechanisms in Metals
1 grain size reduction grain boundary acts as
barrier to dislocation motion due to direction
change (misorientation) dicontinuity of slip
planes gt Hall-Petch relationship sYs0kYd-1/2
how can the grain size be modified? control of
solidification rate (fast) avoid grain growth
(high temperatures) plastic deformation heat
treatment (recovery recrystallization)
16
Recovery and Recrystallization
e.g. rolling stored internal strain energy
heat treatment rearrangement of dislocations
nucleation and growth of new grains
in-situ recrystallization in the SEM
17
Recrystallization Temperature
new grain formation (recrystallization)
finished after 1h depends on degree of cold
work in-situ recrystallization during hot working
(e.g. hot rolling)
Ductility
UTS MPa
recrystallization temperature
grain size mm
percent cold work
annealing temperature
18
Strengthening Mechanisms in Metals
3 solid solution strengthening by alloying
elements lattice strains ? restrict dislocation
motion
4 precipitation hardening incoherent
precipitates e.g. carbides in steels,
barriers to dislocation motion/constraints co
herent precipitates e.g. gphase in Ni-base
superalloys or Q phase in Al-Cu alloys
cutting barrier effect by disrupting the
order/new interfaces
19
Precipitation Hardening
cutting coherent gparticles (Ni3Al) in Ni-base
superalloys
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