Prediction Of Residual Stresses In Pipe Welds

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



• 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



• 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


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


Residual stresses on inner surface

Residual stresses on outer surface


53


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


54


Comparison of axial residual stresses with literature


Residual stresses

55

Heating

Cooling

56

Residual stresses



57

Residual stressesHoop residual stress on outer

surface


Residual stresses

58


Residual stresses on inner surface

Residual stresses on outer surface


59


Comparison of hoop residual stresses with literature



60

Pipe joint with conventional groove

Comparison of axial residual stresses with literature


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 outer 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.

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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

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Documents

Prediction Of Residual Stresses In Pipe Welds