Transcript Examples of Steel Fractography

Examples of Steel Fractography
Professor M Neil James
[email protected]
Department of Mechanical & Marine Engineering
University of Plymouth
Drake Circus, Plymouth PL4 8AA
ENGLAND
1
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Contents – Use the hyperlinks to navigate around this resource

Fatigue crack growth in moist air

Fatigue crack growth in vacuum

Crack growth by hydrogen embrittlement

Low carbon interstitial-free steels

Charpy impact fracture

Fatigue in a high tensile bolt
2
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – vestigial
striations present
Linear growth rate
regime ~ 10-4
mm/cycle
Original
magnification 2kx
3
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – vestigial
striations present
Linear growth rate
regime ~ 10-4
mm/cycle
Original
magnification 5kx
4
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular with some evidence of underlying structure
Threshold growth
rate regime ~ 10-7
mm/cycle
Original
magnification 2kx
5
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular with some evidence of underlying structure
Threshold growth
rate regime ~ 10-7
mm/cycle
Original
magnification 5kx
6
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular
Threshold growth
rate regime ~ 10-7
mm/cycle
Original
magnification 2kx
7
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular with some environment (moisture) induced IG facets
'Knee' of growth rate
regime ~ 10-6
mm/cycle
Original
magnification 2kx
8
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular with some moisture-induced oxide build-up by fretting
'Knee' of growth rate
regime ~ 10-6
mm/cycle
Original
magnification 1.15kx
Mechanism of oxideinduced fatigue
crack closure
9
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Quenched & Tempered - YS = 653 MPa – Grain size 10m – ductile
transgranular with some moisture-induced oxide build-up by fretting
'Knee' of growth rate
regime ~ 10-6
mm/cycle
Original
magnification 7.6kx
Mechanism of oxideinduced fatigue
crack closure
10
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – microstructure has bigger
influence (e.g. bainite packets) – clear IG facets
Threshold growth
rate regime < 10-6
mm/cycle
Original
magnification 500x
11
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – microstructure has bigger
influence (e.g. bainite packets) – clear IG facets
Threshold growth
rate regime < 10-6
mm/cycle
Original
magnification 500x
12
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Fatigue in Air
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – microstructure has bigger
influence (e.g. bainite packets) – ductile transgranular
Threshold growth
rate regime < 10-6
mm/cycle
Original
magnification 2kx
Back to Contents
13
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – no 'knee' in da/dN curve,
implying no mechanism change over range of growth rate 10-4 to 10-7
mm/cycle. No IG facets in absence of moist air
Growth rate ~ 10-4
mm/cycle
Original
magnification 500x
14
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – no 'knee' in da/dN curve,
implying no mechanism change over range of growth rate 10-4 to 10-7
mm/cycle. No IG facets in absence of moist air
Growth rate ~ 10-4
mm/cycle
Original
magnification 2kx
15
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – no 'knee' in da/dN curve,
implying no mechanism change over range of growth rate 10-4 to 10-7
mm/cycle. No IG facets in absence of moist air
Growth rate ~ 10-7
mm/cycle
Original
magnification 500x
16
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T CGHAZ simulation – Grain size 58m – no 'knee' in da/dN curve,
implying no mechanism change over range of growth rate 10-4 to 10-7
mm/cycle. No IG facets in absence of moist air
Growth rate ~ 10-7
mm/cycle
Original
magnification 2kx
17
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – no 'knee' in da/dN curve, implying no
mechanism change over range of growth rate 10-4 to 10-7 mm/cycle. No
IG facets in absence of moist air
Growth rate ~ 10-7
mm/cycle
Original
magnification 2kx
18
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Fatigue in Vacuum
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – no 'knee' in da/dN curve, implying no
mechanism change over range of growth rate 10-4 to 10-7 mm/cycle. No
IG facets in absence of moist air
Growth rate ~ 10-7
mm/cycle
Original
magnification 5kx
Back to Contents
19
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – Hydrogen can cause cleavage, quasi-cleavage,
MVC or IG fracture, depending on crack tip stress, H2 concentration and
its effect on plasticity
Quasi-cleavage at
initiation site
changes to IG as
crack tip stress
decreases
Bend loading + H2
charging
Original
magnification given
by micron bar
20
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – Hydrogen can cause cleavage, quasi-cleavage,
MVC or IG fracture, depending on crack tip stress, H2 concentration and
its effect on plasticity
Quasi-cleavage at
initiation site shown
at higher
magnification
Bend loading + H2
charging
Original
magnification given
by micron bar
21
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – Hydrogen can cause cleavage, quasi-cleavage,
MVC or IG fracture, depending on crack tip stress, H2 concentration and
its effect on plasticity
Quasi-cleavage at
initiation site
changes to IG as
crack tip stress
decreases
Bend loading + H2
charging
Original
magnification given
by micron bar
22
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) – 0.2C 2.5Ni 1.5Cr 0.5 Mo
Q&T - Grain size 10m – Comparison between IG and cleavage (induced
by fracture at cryogenic temperatures)
IG region is on the
left, and cleavage is
on the right. Line
demarcates the
boundary.
Original
magnification given
by micron bar
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23
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Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 1 196 172
cycles
Fatigue performance
is no worse than
grades that do not
show IG fatigue
Original
magnification given
by micron bar
24
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Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 1 196 172
cycles
Fatigue striations on
an IG facet
Original
magnification given
by micron bar
25
Fractography Resource - [email protected]
Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 1 196 172
cycles
Fatigue striations on
IG facets
Original
magnification given
by micron bar
26
Fractography Resource - [email protected]
Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 1 196 172
cycles
IG facets at crack
initiation site
Original
magnification given
by micron bar
27
Fractography Resource - [email protected]
Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 24 371 cycles
IG facets at crack
initiation site
(although damaged
by surface contact)
Original
magnification given
by micron bar
28
Fractography Resource - [email protected]
Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 24 371 cycles
Striation growth
once crack is
established and
plasticity levels are
higher at crack tip
Original
magnification given
by micron bar
29
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Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
Some of these steels show IG fatigue at low levels of plasticity (e.g. during
crack initiation, and at long lives)
Nf = 24 371 cycles
Fatigue striations at
higher magnification
Original
magnification given
by micron bar
30
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Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
An extra-low carbon grade does not show IG fatigue
Nf = 37 782 cycles
No IG facets near
crack initiation site
Original
magnification given
by micron bar
31
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Low Carbon Interstitial-Free Steel
Typically 0.002C 0.15Mn + Ti, Nb or B additions - YS = 160-200 MPa
An extra-low carbon grade does not show IG fatigue
Nf = 37 782 cycles
Higher magnification
view of crack
initiation site
Original
magnification given
by micron bar
Back to Contents
32
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Low temperature
fracture showing
cleavage
Original
magnification given
by micron bar
33
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Low temperature
fracture showing
cleavage – twist and
tilt grain boundaries
evident
Original
magnification given
by micron bar
34
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Ductile fracture at
room temperature
showing MVC
Original
magnification given
by micron bar
35
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Room temperature
fracture showing
MVC and regions of
brittle inter-pearlitic
fracture
Original
magnification given
by micron bar
36
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Room temperature
fracture showing
MVC at high
magnification with
inclusion in hole
Original
magnification given
by micron bar
37
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Shear micro-voids
can occur where
plastic constraint is
lower, towards the
specimen edges
Original
magnification given
by micron bar
38
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Charpy Impact Fracture
Plain medium carbon steel (0.4%C) with a normalised microstructure
Smooth featureless
shear can also occur
at specimen edges
Original
magnification given
by micron bar
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39
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Fatigue in a High Tensile Bolt
High tensile bolt with Q&T microstructure
Fatigue in bend with
a slightly reversed
component
Original
magnification given
by micron bar
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40
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Fatigue in a High Tensile Bolt
High tensile bolt with Q&T microstructure
Clear striations are
present in this low
cycle fatigue
situation; this is the
reversed bend area.
Original
magnification given
by micron bar
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41
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Fatigue in a High Tensile Bolt
High tensile bolt with Q&T microstructure
High magnification
view of fatigue
striations in the main
fatigue region.
Original
magnification given
by micron bar
Back to Contents
42
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