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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 1
Outline
• Precipitation Hardening
• Dispersion Strengthening
• Martensite
• Mechanical Properties of Steel
• Effect of Pearlite
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 2
• Particles impede dislocations.
• Things that slow down/hinder/impede dislocation movement
will increase, σy and σTS
• And also other phases - especially v. small, well dispersed particles.
In some alloys can get small, uniform particles to precipitate out of (solid)
solution. Hence name “ precipitation hardening ”, also known as " AGE " -
hardening. Examples include:
Al-Cu
Cu-Be
Cu-S
Mg-Al
Some alloy and stainless steels
Precipitation Hardening
• Many Al-alloys are precipitation hardenable
• Al - Cu is best known alloy, e.g. Al - 4%Cu
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 3
Precipitation Hardening
Procedure:
• Heat to To ; Hold until only α - phase
present.
• Quench (rapid cooling) to T1 ; because
rapid, no diffusion occurs - SSSS -Super-saturated solid solution of αformed. B atoms “trapped” in α. Notthermodynamically stable.
• Reheat to T2
; diffusion can occur, smallprecipitates of β-phase form.
M = Max. solubility of
metal B in metal A.
Solid solubility decreases
to N as T↓
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 4
• Ex: Al-Cu system
• Procedure:
- Pt A: solution heat treat
(get a solid solution)
- Pt B: quench to room temp.- Pt C : reheat to nucleate
small θ crystals within α
crystals.
Pt A (sol’n heat treat)
Pt B
Pt C (precipitate θ)
Temp.
Time
Precipitation Hardening
300
400
500
600
700
0 10 20 30 40 50wt%Cu(Al)
Lα+Lα
α+θθ
θ+L
T(°C)
A
B
C
composition rangeneeded for precipitation hardening
CuAl2
α + θ → Heat (~550oC) → Quench (0oC) → α (ssss) → Heat/age (~150oC) α + θppt
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 5
Precipitation Hardening
Initially have asupersaturated solidsolution of Cu in Al
Single phase - α
(SSSS)α + θ”ppt
- form various zonesor small Cu clusters
- upon further aging
form θ'' particles
Lattices are trying to match up with each
other (stay ………….) - large lattice strain -
high resistance to dislocation movement
α + θppt
Can no longer match up
with each other (………….)
- less resistance to
dislocation movement
incoherent
coherent
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 6
Max. strength/hardness
Formation & growth of precipitates.
These are very small (5 x 10-9m)
initially but grow with time.
Too long at temperature and
precipitates get too large and
………… occurs.
Precipitation Hardening
• θ ’ is the optimum for strengthening
• θ is overaged due the precipitate becoming incoherent withthe Al matrix
softening
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 7
• TS peaks with
precipitation time.
• Increasing T ……………
process.
Precipitate Effect On TS and %El
n o n -
e q u i l .
s o l i d
s o l u t
i o n
m a n y
s m a l l
p r e c i p i t a
t e s
“ a g e d
”
precipitation heat treat time (h) t e n s i l e s t r e n g t h ( M P a )
300
400
500
2001min 1h 1day 1mo 1yr
204°C
149 °C
f e w
e r l
a r g e
p r e c i p i t a
t e s
“ o v e
r a g e d
”
• %EL reaches minimum
with precipitation time.
% E L ( 2 i n s a m p l e )
10
20
30
01min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time (h)
• Higher strength achieved at lower
ageing temperatures but for longer times
• Consequently, if cold-working is to be
done, it should be carried out after
……………. and before ………..
accelerates
quenching ageing
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 8
avg. particle size = 64b
avg. particle size = 361b
Effect of Precipitate Size on Dislocation Movement
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 9
• Natural aging occurs at room
temperature
- so will start aging at RT unless
stored at low temperature - freezer !
E.g. some Al-alloys for rivets
• Artificial aging occurs at
elevated temperature and is
accelerated
Precipitation Hardening
2014 Al Alloy:
• Aluminum alloys which show agehardening are:
– Al-Cu– Al-Mg
– Al-Mg-Zn
– combinations of above
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 10
• Adding very small foreign particles (hard + inert /non-reactive) into
matrix metal.
• Particles may be metals/non-metals but oxides are often used -Thorium oxide (Thoria/Th02)
• Like precipitation hardening, particles interfere with dislocation
movement
• But not as large strengthening effect, however, not greatly affected
by temperature - so good for ……..……, …………… applications.
Dispersion Strengthening
long term high temp.
Examples:
Nickel + 3 vol% Thoria (TD Nickel)
SAP (sintered Al. powder )
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 11
Martensite
• Austenite dissolves ~2%Cwhereas ferrite only dissolves0.022% max.
• If a medium to high carbon
steel is quenched then Cremains in solid solutionforming a body-centredtetragonal phase (BCT) called
martensite
carbon sites
martensite is …………… and doesnot appear on the phase diagram
metastable
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 12
Martensite Microstructure
• Diffusionless transformation: γ→ α-martensite (very fast )
• platelike or needle-likeappearance
• microstructure always containsretained austenite
• microstructural developmentof martensite and bainitedefined by transformationcurves,– e.g.: time, temperature,
transformation (TTT)
• alloying element affect theease of martensite formation
martensite
Retainedaustenite
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 13
Bainite
How does the bainite look like?
FIGURE 10.8 Transmissionelectron micrograph (TEM)showing the structure of bainite. A grain of bainite
passes from lower left toupper right-hand corners,which consists of elongated and needle-shaped particlesof Fe3C within a ferrite
matrix. The phasesurrounding the bainite ismartensite.
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 14
Eutectoid steal
Martensite
Note: need to
cool faster than
140oC/s to get
100%martensite!
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 15
Alloy steel; + Ni,
Cr, Mo.
Get martensite at
slower cooling
rates.
Martensite
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 16
Mechanical Properties of Steel
• Depending on microstructure, obtain different mech. properties
• amount of Fe3C (hard phase) in microstructure ……………thehardness and strength
• whereas, the ductility and impact resistance ……………
Effect of
Fe3C and
pearlite
increases
decrease
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 17
Effect of Pearlite
• Fine pearlite is stronger and harder than coarse pearlite
• cementite adheres strongly to ferrite and restricts its deformation(reinforces)
• plastic deformation (dislocations) cannot cross α-Fe3C phaseboundary
Boundaryarea/unit volumeis much greaterin the case of
fine pearlite
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 18
• least boundary area/unit volume
• soft, low strength and ductile structure (easy to machine)
• spheroidized carbide is a low stress raiser
Spheroidite
Spheroidite vs. fine and coarse pearlite
Spheroidite
Fe3C
α
1000X
Fine PearliteCoarse Pearlite
3000X 3000X
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 19
Martensite
• Hardness of martensite isdependant on the carboncontent up to ~0.6%C
• plastic deformation(dislocation motion) isrestricted by interstitialcarbon
• high carbon martensite isthe hardest most brittlemicrostructure in steel
• there is an increase involume upon quenching
which can cause cracking
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 20
Tempered Martensite
• Tempering (heating) at 250-650oC allows diffusion of carbonout of supersaturated martensite:
α-martensite → α-ferrite + Fe3C(bct) (bcc)
• Microstructure is tempered martensite
• Cementite precipitates (hard) arevery fine and dispersed
• Continuous α (ductile) phase
• Very large boundary area/unitvolume
• This microstructure makes TMductile and relatively tough
9300X
When both TM structure and pearlitestructure have the same strength, the
fracture toughness of TM structure willbe much ……….. than that for pearlitestructure.
Cementite precipitates
greater
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Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 21
Effect of Tempering on Mechanical Properties of Steel
• The mechanical properties areaffected by the precipitate size
• Precipitate size ………….. with
tempering time and temperature
up to the eutectoid composition
(carbon diffusion)
1 hour tempered 4340 steel
Note high yield strength:Maximum with fine pearlite was 500MPa.
increases
The microstructure will become Spheriodite
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 22
Austenite ( γ)
Pearlite(α + Fe3C layers + a
proeutectoid phase)
slow
cooling
Bainite(α + Fe3C plates/needles)
moderate
cooling
Martensite(BCT phasediffusionless
transformation)
rapid
quench
Tempered
Martensite(α + very fine
Fe 3C particles)
reheat
S t r e n g t h
D u c t i l i t y
MartensiteT Martensite
bainite
fine pearlitecoarse pearlitespheroidite
General Trends
Summary: Processing Options
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 23
Summary
• The versatility of steel arises
because of the wide range of
microstructures that can beachieved, and the corresponding
wide range of mechanical
properties.
• In general, steel
microstructures in which carbon
is more finely dispersed
have ……….. strength
and ……….. fracture toughness.
S t r e n g t h
D u c t i l i t y
Martensite
T Martensitebainite
fine pearlitecoarse pearlite
spheroidite
General Trends
Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 6/ 24
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Composites
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