Examples of Steel Fractography

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Examples of Steel Fractography
Professor M Neil James mjames_at_plymouth.ac.uk Depa
rtment of Mechanical Marine Engineering Universi
ty of Plymouth Drake Circus, Plymouth PL4
8AA ENGLAND
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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

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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
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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
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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
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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
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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
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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
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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 oxide-induced fatigue crack closure
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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 oxide-induced fatigue crack closure
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Fatigue in Air
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
CGHAZ simulation Grain size 58?m
microstructure has bigger influence (e.g. bainite
packets) clear IG facets
Threshold growth rate regime lt 10-6
mm/cycle Original magnification 500x
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Fatigue in Air
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
CGHAZ simulation Grain size 58?m
microstructure has bigger influence (e.g. bainite
packets) clear IG facets
Threshold growth rate regime lt 10-6
mm/cycle Original magnification 500x
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Fatigue in Air
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
CGHAZ simulation Grain size 58?m
microstructure has bigger influence (e.g. bainite
packets) ductile transgranular
Threshold growth rate regime lt 10-6
mm/cycle Original magnification 2kx Back to
Contents
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
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
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
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
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
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
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
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
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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
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Fatigue in Vacuum
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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
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Crack Growth by Hydrogen Embrittlement
Q1N Steel (HY 80) 0.2C 2.5Ni 1.5Cr 0.5 Mo QT
- 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 Back to
Contents
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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
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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
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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 IG
facets Original magnification given by micron bar
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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 IG facets at crack
initiation site Original magnification given by
micron bar
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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 IG facets at crack initiation
site (although damaged by surface
contact) Original magnification given by micron
bar
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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 Striation growth once crack is
established and plasticity levels are higher at
crack tip Original magnification given by micron
bar
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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
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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
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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
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Low temperature fracture showing
cleavage Original magnification given by micron
bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Low temperature fracture showing cleavage twist
and tilt grain boundaries evident Original
magnification given by micron bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Ductile fracture at room temperature showing
MVC Original magnification given by micron bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Room temperature fracture showing MVC and regions
of brittle inter-pearlitic fracture Original
magnification given by micron bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Room temperature fracture showing MVC at high
magnification with inclusion in hole Original
magnification given by micron bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Shear micro-voids can occur where plastic
constraint is lower, towards the specimen
edges Original magnification given by micron bar
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Charpy Impact Fracture
Plain medium carbon steel (0.4C) with a
normalised microstructure
Smooth featureless shear can also occur at
specimen edges Original magnification given by
micron bar Back to Contents
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Fatigue in a High Tensile Bolt
High tensile bolt with QT microstructure
Fatigue in bend with a slightly reversed
component Original magnification given by micron
bar Back to Contents
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Fatigue in a High Tensile Bolt
High tensile bolt with QT microstructure
Clear striations are present in this low cycle
fatigue situation this is the reversed bend
area. Original magnification given by micron
bar Back to Contents
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Fatigue in a High Tensile Bolt
High tensile bolt with QT microstructure
High magnification view of fatigue striations in
the main fatigue region. Original magnification
given by micron bar Back to Contents