FATIGUE CRACK PROPAGATION BEHAVIOR OF BRAZED STEEL

FATIGUE CRACK PROPAGATION BEHAVIOR OF BRAZED STEEL

FATIGUE CRACK PROPAGATION
BEHAVIOR OF BRAZED STEEL JOINTS
Dr. Tanya Aycan
Aycan Başer
EMPA-Swiss Federal Laboratories for Materials Testing and Research
EMPALaboratory for Joining Interface and Technology, Dübendorf, Switzerland
Materials Science & Technolog y
Outline
 Introduction
•Mechanical properties of materials
• What is fatigue?
• Why brazing?
• Problems occurred during brazing
• Application area of brazing
 Materials and Methods
• Base material and filler metals
• Brazing and heat treatment
• Experimental procedures
 Results and discussion
• Fatigue Crack Propagation (da/dN-ΔK) Curves
• Microstructural Investigations
• Fractographic Investigations
 Brazing Quality
• Effect of shield gas on mechanical properties
• Effect of sample geometry on mechanical properties
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Introduction
MALZEMELERİN MEKANİK ÖZELLİKLERİ
GERILIM-GERINIM DIYAGRAMI
2000
x kopma dayanimi
()
Elastik bolge
Stress / MPa
1500
Cekme dayanimi
GEVREK-SUNEK DAVRANIS
x Akma dayanimi
1000
500
Plastik bolge
0
0
2
4
6
8
10
12
Strain / %
Cekme testi
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14
16
18
20
()
Introduction
YORULMA NEDİR?
Malzemeyi zorlayan gerilmeler zaman icinde degisecek olursa malzeme cekme deneyinde elde
edilen kopma degerinin altinda bir gerilmede sunek de olsa plastik sekil degistirmeden kirilabilir. Bu olaya
yorulma denir.
Yukleme ve bosalmanin periyodik olarak cok sayida tekrari sonucunda malzemede yipranmalar
meydana gelir. Bunun nedeni yukun siddetinden cok onun periyodik olarak uzun bir sure uygulanmasidir.
İc mekanizmasi oldukca karisik olan bu olaya malzemenin yorulmasi denir.
Yorulma 3 asamada gerceklesir:
1-Catlak baslangici
2-Catlak ilerlemesi
3-Kirilma
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Introduction
WHY BRAZING?
Welding fusion takes place with melting of both the base metal and usually a filler metal. To
join metals by applying heat, sometimes with pressure and sometimes with an intermediate or
filler metal having a high melting point.
Brazing is a process for joining similar or dissimilar metals using a filler metal and heating
them the liquidus of the filler metals above 840°F (450°C), and below the solidus of the base
metals
Soldering has the same definition as brazing except for the fact that the filler metal used has
a liquidus below 840°F (450°C) and below the solidus of the base metals.
Brazing process is used because of the compressor
impeller geometry
 Defects such as incomplete gap filling, pores or
cracks may be formed during brazing and they
can act as stress concentration sites in the
brazing zone.
 Under cyclic mechanical loading, fatigue cracks
can initiate and propagate from these defects,
leading to spontaneous failure.
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Brazing
Introduction
Brazing is a quick and low-cost brazing method to produce strong joints and it is used in the aerospace and
other industries as well as for power generation, e.g. turbine parts or compressor impellers.
/www.nasa.org/
/www.airbus.com/
/www.geae.com/
/www.manturbo.com/
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Materials and Method
 Base material and filler metals
 The soft martensitic stainless steel X3CrNiMo13-4 was used as base material.
Chemical composition of X3CrNiMo13-4
Element
Min.
Max.
C
Si
Mn
P
S
0.05
0.70
1.50
0.04
0.01
Cr
Mo
N
Ni
12.00 0.30 0.02 3.50
14.00 0.70
4.50
 As filler metal, foils of the binary alloy Au-18Ni with a thickness of 100 μm were applied (Melting
temperature is around 800 °C).
 Brazing and heat treatment
 Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H2) furnace at a
temperature of 1020°C for 20 minutes.
 After brazing, the specimens were tempered at 520 °C for 5.5 h in nitrogen atmosphere.
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Materials and Method
 Experimental procedures
 Fatigue crack propagation tests were performed on a resonant testing machine.
Geometry of the DCB specimen
(90 x 60 x 8 mm)
Set-up of the fatigue crack propagation test
ASTM E647
 Fractured specimens were investigated by SEM.
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Results
• Fatigue Crack Propagation (da/dN-ΔK) Curves
1E-4
The Paris equation:
da/dN [m/cycle]
1E-5
da
 CK n
dN
1E-6
1E-7
da: difference of crack length
dN: difference of number of cycles
C,n: experimentally measured material constants
1E-8
1E-9
R = 0.1
R = 0.3
R = 0.5
R = 0.7
1E-10
1E-11
1
10
100
1/2
K [MPa m ]
KI 
F 
a
a
8

13
.
25

12
 
h
Bh0.5 
h
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2




0.5
Calculated C and n parameters at different R values
R
C
n
0.1
1.309E-22
11.17
0.3
4.071E-23
12.17
0.5
7.234E-22
12.64
0.7
8.489E-21
12.81
da/dN [m/cycle]
Results
The Paris Exponent
1E-5
1E-6
1E-7
1E-8
1E-9
R = 0.1
R = 0.3
R = 0.5
R = 0.7
1E-10
1E-11
1
10
1/2
K [MPa
] 9 (1993) 2765.
R. H. Dauskardt, Acta Metall
Mater. m
Vol 41
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Material
n
Metals
3-4
Ceramics
-50
Brazed components
11-13
?
SEM
Taramali elektron mikroskobu
Çalışma Prensibi
Taramalı Elektron Mikroskobu üç temel kısımdan oluşmaktadır:
Optik Kolonda elektron demetinin kaynağı olan elektron tabancası, elektronları
numuneye doğru hızlandırmak için yüksek gerilimin uygulandığı anot plakası, ince
elektron demeti elde etmek için yoğunlaştırıcı mercekler, demeti numune üzerinde
odaklamak için objektif merceği, bu merceğe bağlı çeşitli çapta apatürler ve
elektron demetinin numune yüzeyini taraması için tarama bobinleri yer almaktadır.
Numune hucresine numune yerlestirilir.
Görüntü sisteminde elektron demeti ile numune girişimi sonucunda oluşan
çeşitli elektron ve ışımaları toplayan dedektörler, bunların sinyal çoğaltıcıları ve
numune yüzeyinde elektron demetini görüntü ekranıyla senkronize tarayan
manyetik bobinler bulunmaktadır.
Nasil goruntu elde edilir?
Taramalı Elektron Mikroskobunda (SEM) görüntü, yüksek gerilim ile
hızlandırılmış elektronların numune üzerine odaklanması, bu elektron
demetinin numune yüzeyinde taratılması sırasında elektron ve numune
atomları arasında oluşan çeşitli girişimler sonucunda meydana gelen
etkilerin uygun algılayıclarda toplanması ve sinyal güçlendiricilerinden
geçirildikten sonra bir katot ışınları tüpünün ekranına aktarılmasıyla elde
edilir.
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Results
•Microstructural Investigations
SEM-cross section
X3CrNiMo13-4
steel
Ni-rich
Solid solution
Au-18Ni
braze
Diffusion zone
100 µm
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Au-rich
Solid solution
20 µm
Results
• Microstructural Investigations
SEM-cross section
20 µm
20 µm
Crack tip
20 µm
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5 µm
Results
•Microstructural Investigations
SEM-cross section
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200 µm
200 µm
100 µm
20 µm
Results
• Microstructural Investigations
SEM-cross section
Pre-damaged zones well
ahead of the crack tip
Crack tip
32 µm
20 µm
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95 µm
20 µm
Results
• Fractographic Investigations
SEM-cross section
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200 µm
20 µm
20 µm
50 µm
Results
• Fractographic Investigations
SEM
stereo
200 µm
pores
3 mm
The stepped nature of the fracture pattern is clearly evident
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200 µm
Fractographic Investigations
Brittle fracture
ductile fracture
transcrystalline steel
Steel wire
Al alloy
1018 steel
BMG
Cu alloy
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Results
• Fractographic Investigations
Brittle or ductile?
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Results
• Fractographic Investigations
50 µm
20 µm
Plastic deformation features
containing ductile dimples
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Discussion
Damage and Fracture Behaviour of Brazed
Joints Under Cyclic Loading
X3CrNiMo13-4
notch
Au-18Ni
X3CrNiMo13-4
•
After crack initiation, high stresses can lead
to the formation of cavities well ahead of the
crack tip.
•
New cavities develop and grow every loading
cycle.
•
The fatigue crack then propagates along
these predamaged zones and coalescence
and final failure occurs.
High Paris exponent, n, was explained by the triaxial stress state in the filler metal, which is a
result of the different elastic-plastic material properties of the filler metal and the base material.
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Brazing quality
 Defects such as incomplete gap filling, pores or cracks may be formed
during brazing and they can act as stress concentration sites in the brazing
zone.
 Under cyclic mechanical loading, fatigue cracks can initiate and propagate
from these defects, leading to spontaneous failure. Therefore, defect
assesment of brazed components should be considered.
200 µm
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Brazing quality
Brazed steel plates of different batches
The addition of hydrogen to the argon allows removing the oxide film on the
stainless steel surface, which is essential for filler metal wetting. BUT....
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brazed steel
plates
shielding gas
Batch A
93 vol.-% Ar, 7 vol.-% H2
Batch B, C
93 vol.-% Ar, 7 vol.-% H2
Batch D, E
100 vol.-% H2
Batch F, G
100 vol.-% H2
Brazing quality
(93 vol.-% Ar, 7 vol.-% H2)
100 µm
(100 vol.-% H2)
100 µm
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Materials and Method
 Base material and filler metals
 The soft martensitic stainless steel X3CrNiMo13-4 was used as base material. As filler metal, foils of the
binary alloy Au-18Ni with a thickness of 100 μm were applied.
 Brazing and heat treatment
 Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H2 and 100 vol.-% H2 ) furnace at
a temperature of 1020°C for 20 minutes. After brazing, the specimens were tempered at 520 °C for 5.5 h in
nitrogen atmosphere.
 Experimental procedures
 Fatigue tests were performed on a standard electro-mechanical and servohydrolic testing machine.
 Fractured specimens were investigated by SEM.
Geometry of the t-joint specimen
(t=16mm, W=8 mm)
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standart electro-mechanical
testing machine
Servohydrolic
testing machine
Brazing quality
brazing
σnom = 700 MPa
σnom = 650 MPa
σnom = 700 MPa
N = 8304
N = 11787
N = 8642
crack
standart round specimen
(Ø1=5 mm, Ø2=4 mm)
Brazing zone
!
In general, fracture occured on the base material instead
of the brazing zone
(100 vol.-% H2)
Specimen
geometry
shielding gas
Rm [MPa]
Base material
-
975±25
93 vol.-% Ar, 7 vol.-% H2
882±15.7
100 vol.-% H2
1120±4.8
100 vol.-% H2
1084±3.6
T-joint
Standart round
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The addition of hydrogen to the argon
allows removing the oxide film on the
stainless steel surface
Joint strength was improved under 100% H2
atmosphere
Brazing quality
S-N curves
T-joint-defect free-93 vol.-% Ar, 7 vol.-% H2
T-joint-defect free-100 vol.-% H2
round shape 100 vol.-% H2
1200
1100
1000
nom, max (MPa)
900
800
700
600
500
400
300
200
2
10
3
4
10
10
Nf
5
10
Nu=20 000 cycles
• The only difference in between these specimens is different shielding gases.
• Brazing quality was improved under 100 vol.-% H2
• Fracture occurred on the base material in specimens which were brazed under 100 vol.-% H2
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Results
Fractographic Investigations- Comparison
stereo
SEM
σnom = 400 MPa
N = 5630
(93 vol.-% Ar, 7 vol.-% H2)
The step fractured pattern
3 mm
200 µm
Stronger bonding was
obtained and better
interface reaction
occured under 100% H2
atmosphere
σnom = 650 MPa
N = 11787
The step fractured pattern
could not be observed
3 mm
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200 µm
(100 vol.-% H2)
TEŞEKKÜRLER
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Additional--I
Additional
Table 1. Chemical composition of X3CrNiMo13-4.
Element
C
Si
Mn
P
S
Min.
Max.
0.05
0.70
1.50
0.04
0.01
Cr
Mo
N
Ni
12.00
0.30
0.02
3.50
14.00
0.70
4.50
Table 2. Mechanical properties of X3CrNiMo 13-4 and X3CrNiMo13-4 – Au-18Ni
braze joints.
Rp0.2
[MPa]
X3CrNiMo-13-4
Rm
[MPa]
920 ± 5 975 ± 25 17.5 ± 2.5
X3CrNiMo13-4 -AuNi18 923 ± 7 976 ± 15
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A5
[%]
6 ± 0.5
τe
[MPa]
τmax
[MPa]
KIc
[MPa
m0.5]
620 ± 5
660 ± 10
~270
245 ± 10
539 ± 7
49 ± 1.5
Additional--II
Additional
The stress intensity for mode I loading, KI, as a function of the specimen geometry and the applied
load can be calculated according to;
F 
a
a
K I  0.5 8  13.25  12 
h
Bh 
h
2




0.5
where F is the applied force, B and h the specimen geometry and a the total crack length measured
from the load initiation point.
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Results
• Fatigue Crack Growth Curves
40
ΔF1, ΔF2, ΔF3=constant
ΔF1 = 6.8 kN
ΔK1 = 23 MPa m1/2
30
20
batch 1
batch 2
batch 3
10
a [mm]
0
40
ΔF2= 5.7 kN
ΔK2= 19 MPa m1/2
30
20
batch 1
batch 2
batch 3
10
0
40
ΔF3= 4.6 kN
ΔK3= 16 MPa m1/2
30
20
batch 1
batch 2
batch 3
10
0
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0
75
150
3
N (10 )
225
300
Results
• Average of Fatigue Crack Growth Curves (batch 1, 2 and 3)
35
30
a (mm)
25
20
15
10
F1=6.8 kN
F2=5.7 kN
F3=4.6 kN
5
0
0
50
100
150
3
N [10 ]
200
250
300
The fatigue crack growth rate of brazed components is extremely sensitive to
the load range.
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Additional--III
Additional
Nucleation based theory for ductile fracture
under high triaxial stress
Nucleation based theory for ductile fracture under high triaxial stress
Mechanism of ductile fracture in pure silver under high-triaxial stress states under static loading
(a) A few nanometer-sized cavities nucleate with
small plastic strain.
(b) Additional nucleation occurs with a small
additional macroscopic strain.
(c) At a critical stage, cavities get sufficiently
close, and there is a coalescence between the
small nanometer-sized cavities, and larger
cavities form.
(d) Additional nucleation occurs in regions near
the cavities and further interlinkage occurs.
(e) The interlinkage of larger cavities through
continued nearby nucleation leads to final
failure.
M. C. Tolle, M. E. Kassner, Acta Metall. Mater. Vol 43 (1995) 287.
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Additional--IV
Additional
Damage and Fracture Behaviour of Brazed
Joints Under Cyclic Loading
ΔK2
a1 Δa
BULK MATERIALS
ΔK1
ΔK1 < ΔK2
Δa` >> Δa
X3CrNiMo13-4
BRAZED JOINTS
Au-18Ni
a1
Δa`
ΔK1
ΔK2
X3CrNiMo13-4
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ΔK1 < ΔK2
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