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Prediction of residual stresses in pipe welds using FEM and its effect on crack driving force
1
Niraj DeobhankarJunior Research Fellow
Guide: Shri P. K. SinghReactor Safety Division, BARC
Final M. Tech Viva-voce
Content
• Introduction
• Objectives
• Experimental Details
• Finite Element Analysis
• Results
• Effect of residual stresses on crack driving force
• Conclusions
2
Introduction
• Residual stresses are developed in weld joint due to
expansion during heating and contraction during cooling
along with constraints.
3
Introduction
4
• Due to rapid cooling and solidification of the weld metal
during welding, alloying and impurity elements segregate
extensively in fusion zone and heat affected zone resulting
in inhomogeneous chemical and metallurgical distribution.
• High amount of stresses are consequence of superimposing
of loading and residual stresses
• Residual stresses may lead to loss of performance in
corrosion, fatigue and fracture.
Objectives
• Produce girth welds of 304LN stainless steel pipes using Hot-wire
Gas Tungsten Arc Welding (GTAW) with narrow groove and cold
wire GTAW with conventional groove.
• Measurement and prediction of temperature during welding in the
weld joint and their comparison
• Measurement and prediction of residual stresses during welding in
the weld joint and their comparison
• Quantification of effect of residual stresses on crack driving force
5
Literature Review
6
Heat Transfer in Welding
7
Heat Transfer in Welding
Modelling of heat source depends on :a. Desired accuracy of the heat source model
b. Purpose of prediction
c. Availability of information
8
Residual Stresses
9
Residual Stresses
10
Residual Stresses
11
Summary of Literature Review
European Network on Neutron Techniques Standardizationfor Structural Integrity (NeT) conducted round robinexercise for prediction of temperature and residual stressesin bead on plate (austenitic stainless steel)
12
Experimental Details
13
Chemical Composition
Base Material: SS 312 Type 304LN, Filler Rod Material: ER 308L
14
Composition of Parent Material SS312 Type 304 LN
Compo
sitionC Mn Si S P Cr Ni N
%
Content0.021 0.79 0.33 0.003 0.004 18.26 8.45 0.10
Composition of Filler Rod ER 308 L
Compositi
onC Mn Si S P Cr Ni Mo Cu
% Content 0.017 1.72 0.37 0.011 0.023 19.88 10.02 0.24 0.19
CASE A: Bead on plate
15
CASE B: Hot wire GTAW with narrow groove
16
Distortion Measured Location
Axial DistortionM-M’
N-N’
Thermocouple Positions
Distance
from edge
On Outer
side
On Inner
side
4 mm O1 I1
7mm O2 I2
10mm O3 I3
Residual stress measurement by blind hole drilling technique
Position form weld centre line
Configuration Surface A B C D
Narrow
groove
Inner 0 6 10 16
outer 0 3 7 Nil
CASE C: GTAW with conventional V groove
17
Thermocouple Positions
Distance
from edge
On Outer
side
On Inner
side
4 mm O1 I1
7mm O2 I2
10mm O3 I3
Residual stress measurement by blind hole drilling technique
Position form weld centre line
Configuration Surface A B C D
Conventional
V-groove
Inner 0 3 7 Nil
Outer 0 3 7 Nil
Process ParametersBead on Plate
Pass
NoProcess
Diameter of
filler rod (mm)Voltage
(V)
Current
(A)
Wire
Current
(A)
Velocity
(mm/min)
Heat Input
(J/mm)
1 GTAW 2.4 13.5 160 0 63 2057
18
GTAW with Narrow groove
Pass
NoProcess
Diameter of
filler rod (mm)
Voltage
(V)
Current
(A)
Wire
Current
(A)
Velocity
(mm/min)
Heat Input
(J/mm)
Root GTAW
1.2 8.4
105 0 100 530
2 GTAW 105
15
110 550
3 GTAW 135 110 688
4 GTAW 140 100 782
5 GTAW 150 90 924
6 GTAW 145 90 896
7 GTAW 150 90 924
8 GTAW 145 90 896
9 GTAW 140 90 868
10 GTAW 150 90 924
Process Parameters
GTAW with Conventional groove
Pass
Number
Bead
NumberProcess
Diameter
of filler
rod (mm)
Voltage
(V)
Current
(A)
Velocity
(mm/min)
Heat
Input
(J/mm)
Root 1 GTAW 3.5 12 110 30 2640
2 2 GTAW
2.4
12 110 35 2263
33
GTAW 14 110 38 24324
45
GTAW 14 120 45 22406
57
GTAW 15 130 47 24908
6
9
GTAW 15 130 46 254410
11
7
12
GTAW 16 135 51 254213
14 19
Residual stress Measurement
20
X- ray diffraction method
When a metal is under stress, applied or residual, the resulting elastic strains cause
the atomic planes in the metallic crystal structure to change their spacing.
The Blind Hole Drilling Strain-Gauge
(BHDSG) method
Removal of stressed material results in
the surrounding material readjusting its
stress state to attain equilibrium.
Finite Element Analysis
21
Thermal Analysis
22
Heat transfer to surroundings
by convection and radiation
Heat transfer to surroundings
by convection and radiation
• Quarter three dimensional finite element model
• 37,000 eight noded solid elements
• 34,394 nodes
Heat transfer to
surroundings
by convection
and radiation
Thermal Analysis
23
Heat transfer to surroundings
by convection and radiation
• Half three dimensional finite element model
• 1,29,301 eight noded solid elements
• 1,21,052 nodes
Heat transfer to
surroundings by
convection and
radiation
Thermal Analysis
24
Heat transfer to
surroundings by
convection and radiation
• Half three dimensional finite element model
• 1,52,588 eight noded solid elements
• 1,42,830 nodes
solidus temperature =13600C,
liquidus temperature =14400C
latent heat of fusion=270KJ/Kg
Thermal Properties
25
Heat Source
26
Parameters of double ellipsoidal heat source can
be verified using two criteria:
1. Peak Temperature
2. Weld pool dimensions
Power density distribution in double ellipsoidal heat source
Thermal Analysis
27
Distribution of Temperature
Input to Mechanical Analysis
28
Mechanical Analysis
29
2D finite element model used for Mechanical Analysis
Conventional V- Groove
Narrow Groove
3594 four noded rectangular
elements
3336 number of nodes
4306 four noded rectangular
elements
4012 number of nodes
•Plain strain conditions were assumed.
•The parent and the weld material were assumed to have the same
temperature dependent mechanical and thermal properties.
Mechanical Analysis• Temperature at which elements of the material to be filled gets transformed
to weld material was set to 13000C.
• Analysis was carried out for isotropic and kinematic hardening rule.
• Element Birth Technique:Stresses built up in the supposedly stress-free filler
material and a redistribution of the residual stresses in the previously laid
weld passes
30
low Modulus of Elasticity
Yield Stress same as that of the parent
metal
Coefficient of expansion of filler
material neglected
Transfer of strains from welded material to the
material to be filled without generation of high
stresses.
No thermal stresses are generated in material to
be filled
Mechanical Analysis
31
Mechanical constraints in
case of bead on plate
Mechanical constraints in case
pipe weld joints
Material Properties
32
Material Properties
33
Results
34
Temperature in Bead on Plate
35
0
100
200
300
400
500
600
700
800
0 200 400 600 800 1000
Tem
per
ature
(°
C)
Time (sec)
Temperature
Pipe joint with narrow groove
36
Overall Temperature cycle at 4mm from weld centre line
Temperature cycle at 4mm from weld centre line for first pass
Temperature
Pipe joint with conventional V groove
37
Overall Temperature cycle at 4mm from weld centre line
Temperature cycle at 4mm from weld centre line for first pass
Distortions
Pipe joint with narrow groove
38
Residual stresses
39
Longitudinal stress
Transverse stress
Bead on plate
Residual stresses
40
Residual stresses
41
Residual stresses
42
Residual stresses
43
Residual stresses
44
Residual stresses
45
Residual stresses
46
Residual stresses
47
Residual stresses
48
Residual stresses
49
Residual stresses
50
Hoop residual stress on inner surface
Axial residual stress on inner surface
Residual stresses
51
Hoop residual stress on outer surface
Axial residual stress on outer surface
Comparison of residual stresses
52
Pipe joint with narrow groove
Residual stresses on inner surface
Residual stresses on outer surface
Comparison of residual stresses
53
Pipe joint with narrow groove
Comparison of hoop residual stresses with literature
[5] M. A. Wahab & M. J. Painterb, Numerical models of gas metal arc welds using experimentally determined weld pool shapes as the representation of the welding heat source, International Journal of Pressure Vessels & Piping Vol. 73 (1997) 153-159[11] John W.H. Price. Anna Ziara-Paradowska, Suraj Joshi, Trevor Finlaysonb, Cumali Semetaya, Herman Nied, Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume, International Journal of Mechanical Sciences 50 (2008) 513–521
Comparison of residual stresses
54
Pipe joint with narrow groove
Comparison of axial residual stresses with literature
[5] M. A. Wahab & M. J. Painterb, Numerical models of gas metal arc welds using experimentally determined weld pool shapes as the representation of the welding heat source, International Journal of Pressure Vessels & Piping Vol. 73 (1997) 153-159[11] John W.H. Price. Anna Ziara-Paradowska, Suraj Joshi, Trevor Finlaysonb, Cumali Semetaya, Herman Nied, Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume, International Journal of Mechanical Sciences 50 (2008) 513–521
Residual stresses
55
Heating
Cooling
56
Residual stresses
Axial residual stress on inner surface
Hoop residual stress on inner surface
57
Residual stressesHoop residual stress on outer
surface
Axial residual stress on outer surface
Residual stresses
58
Pipe joint with conventional V groove
Residual stresses on inner surface
Residual stresses on outer surface
Comparison of residual stresses
59
Pipe joint with conventional V groove
Comparison of hoop residual stresses with literature
[5] M. A. Wahab & M. J. Painterb, Numerical models of gas metal arc welds using experimentally determined weld pool shapes as the representation of the welding heat source, International Journal of Pressure Vessels & Piping Vol. 73 (1997) 153-159[11] John W.H. Price. Anna Ziara-Paradowska, Suraj Joshi, Trevor Finlaysonb, Cumali Semetaya, Herman Nied, Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume, International Journal of Mechanical Sciences 50 (2008) 513–521
Comparison of residual stresses
60
Pipe joint with conventional groove
Comparison of axial residual stresses with literature
[5] M. A. Wahab & M. J. Painterb, Numerical models of gas metal arc welds using experimentally determined weld pool shapes as the representation of the welding heat source, International Journal of Pressure Vessels & Piping Vol. 73 (1997) 153-159[11] John W.H. Price. Anna Ziara-Paradowska, Suraj Joshi, Trevor Finlaysonb, Cumali Semetaya, Herman Nied, Comparison of experimental and theoretical residual stresses in welds: The issue of gauge volume, International Journal of Mechanical Sciences 50 (2008) 513–521
Effect of heat input on residual stresses
61
At 4 mm from weld centre line
62
Effect of heat input on residual stresses
Hoop residual stress on inner surface
Hoop residual stress on outer surface
Axial residual stress on outer surface
Axial residual stress on inner surface
Effect of Ri/t ratio on residual stresses
63
Effect of residual stresses on crack driving force
64
Axial defect of finite length
65
Geometry functions at point A for a finite axial external surface crack in a cylinder
Part circumferential external surface crack
66
Geometry functions at point A for a part circumferential external surface crack in a cylinder
Normalised residual stress
67
Effect of residual stress on crack driving force in case of finite axial
defect
68
Effect of residual stress in case of part circumferential crack on external
surface
69
Conclusions
• Thermal cycle matches well with observations in all cases, although peak
temperature is slightly over-predicted. Reason for over-prediction can attributed
to the simplifications considered in heat dissipation in welding process.
• From comparison between residual stresses predicted using various strain
hardening rules, prediction using kinematic strain hardening rule comes close to
measured values.
• In case of bead on plate, residual stresses predicted using available FE code match
well with experimentally measured values. This helps in validation of the code to
be used in further investigation.
70
Conclusions
• In case pipe joints predicted residual stresses on inner surface match well
qualitatively.
• Residual stresses on outer surface follow the trend found in literature.
• Residual stresses in case of pipe joint using conventional groove is more than that
using narrow groove.
• With increase in heat input residual stresses increase in magnitude and hence
excessive heat input is detrimental to the weld joint.
• Ratio of inner radius with thickness does not alter residual stress pattern
drastically. But with increase in Ri/t ratio tensile nature of residual stresses
increases especially on outer surface.
71
Conclusions
• For pipe joints with different thicknesses but same Ri/t ratio and heat
input, residual stresses generated in pipe with larger thickness are low.
• Residual stresses contribute to crack driving force heavily and hence should be
accounted for.
72
Thanks a lot
73