ELSEVIER PIkSO308.0161(97)00108-7

ht. J. Pm. Ves. & Piping 74 (1997) 129- 133 0 1998 Elsevier Science Limited. All rights reserved

Printed in Northern Ireland 0308-0161/97/$17,00

Cross weld creep testing and its extrapolation to service welds

M. Lawa’* & W. Paytenb ‘University of Technology, Department qf Materials Science, Building 3, ANSTO PMBI, Menia, Sydney NSW 2234, Australia

Australian Nuclear Science and Technology Organisation (ANSTO), Australia

(Received 2 October 1997; accepted 21 October 1997)

Creep testing of welds is often used to predict weld performance. Finite element analysis of cross weld creep tests was undertaken to assess the suitability of using these results to predict service weld performance. Using finite element analysis techniques (FEA). two areas were investigated. The first was how the disposition of the heat affected zone and weld fusion line in the specimen affects the creep behaviour. The second was the effect of specimen radius on the creep behaviour. 0 1998 Elsevier Science Limited. All rights reserved.

1 INTRODUCTION

The heat affected zone (HAZ) and weld fusion line are often obliquely disposed in a cross weld creep specimen, in this case 3D finite element analysis (FEA) is necessary to accurately represent specimen geometry. As an axisymmetric case takes substantially less time than a 3D case, there is a significant saving in analysis time if axisymmetric models can be used. A number of 3D models with the HAZ and weld fusion line disposed at 0, 15 and 30 degrees were compared to a simple axisymmetric model.

The effect of specimen radius on the cross weld creep specimen stress state was also investigated. Axisymmetric models of specimens with HAZ widths of 2 and 4 mm were modelled with radii of 0.5 to 30 mm, and a range of material properties to assess radius effects.

2 MODELLXNG OF HAZ AND WELD ANGLE

A number of specimens were modelled to assess the effect of HAZ angle on specimen stresses. These were 3D speci- mens with the HAZ and weld disposed at 0, 15 and 30 degrees to the specimen axis, and an axisymmetric model at zero degrees. In all analyses the cases were under a

*Corresponding author.

nominal stress of 40 MPa and stresses were taken at same time, 12 years, where the steady-state stress distribution had

been achieved. The specimen was modelled (Table 1) with three material

zones after Tu et al.’ These were (1) the weld metal and the coarse grained heat affected zone (HAZ), (2) the fine and intercritical HAZs, and (3) the parent metal. The creep specimens had a radius of 5 mm and were modelled with 2 and 4 mm width HAZs. The specimen loading was 40 MPa. The materials were assumed to deform in second- ary creep according to a Norton power law: E’ = Ku”, with a power coefficient of 6.0.

The 3D models were created using a computer macro within the FEA program (NISA II) which took in geometry and proposed mesh density. These models had between 2000 and 4000 3D quadratic elements, depending on the specimen geometry (Figs l-5). The axisymmetric model had 304 quadratic elements (Fig. 1).

Table 1. Material properties

Material Coefficient (K) Relative creep rate

Weld A(creep-hard) 1 .O E-22 0.1:1 Weld B(creep-hard) 5.0 E-22 OS:1 HAZ A(creep-soft) 1 .O E-20 1O:l HAZ B(creep-soft) 5.0 E-21 5:l Parent 1.0 E-21 1

129

130 M. Law, W. Payttm

Parent

Weld

Fig. 1. First principal stresses, axisymmetric model.

+I .I D ,9

Fig. 2. First principal stresses, section through 3D model with zero

Fig. 3. First principal stresses, section through 3D 30 degree HAZ model.

Fig. 4. First principal stresses, section through 3D 30 degree HAZ model.

Fig. 5. First principal stresses, front view of 3D 30 degree HAZ model.

3 RESULTS OF HAZ ANGLE MODELLING

The position of the maximum stress in the weld metal (Figs 3-5) compares well with the observed failure position in creep tests. The angle that the HAZ and weld fusion line cuts across the test specimen had a relatively small effect on the maximum first principal stresses with a maximum difference of 9% (Table 2-4).

The relatively small difference between stress results for the full 3D oblique models and the simplified axisymmetric models implies that modelling using the simpler axisym- metric method will provide realistic estimates of stresses. These results agree with Sotresund et aL2 who found only small differences between a 0” and a 45” model, based on creep rates of the overall model. However, no details of the modelling or results were given.

Cross weld creep testing and its extrapolation to service welds 131

Table 2. Maximum first principal stresses in cross weld creep specimens (‘A‘ materials, 2 mm HAZ)

Model

Parallel HAZ, Axisymmetric Parallel HAZ, 3D model 15” HAZ 3D model 30” HAZ 3D model

Weld (MPa) HAZ (MPa) Parent (MPa)

61.7 49.5 51.7

63.4 49.7 50.1

65.9 49.7 49.4

66.1 51.2 49.5

Table 3. Maximum first principal stresses in cross weld creep specimens (‘Al materials, 4 mm HAZ)

Model

Parallel HAZ, Axisymmetric Parallel HAZ, 3D model 15” HAZ 3D model 30” HAZ 3D model

Weld (MPa)

73.3

73.4

76.3

75.5

HAZ (MPa) Parent (MPa)

54.8 47.2

55.0 47.4

54.5 52.0

52.8 51.9

Table 4. Maximum first principal stresses in cross weld creep specimens (‘B’ materials, 2 mm HAZ)

Model

Parallel HAZ, Axisymmetric Parallel HAZ, 3D model 15” HAZ 3D model 30” HAZ 3D model

Weld (MPa) HAZ (MPa) Parent (MPa)

47.1 46.0 42.4

47.1 46.0 42.5

48.8 46.5 43.6

49.6 47.1 43.2

4 RADIUS VARIATION

A number of authors have investigated the effect of radius on the stress evolution in a cross-weld creep test. Prager 3 found that there was an increase in specimen life in cross weld creep specimens with increasing specimen size across a range of materials and heat treatments. The specimen sizes ranged from 2.25 to 15.5 mm radius, but the extent of the HAZ was not estimated. Kussmaul et aL4 also found an increase in rupture life with increasing specimen size, this was attributed to slower stress and strain redistribution due to larger constraint effects.

Storesund et aL5 modelled two HAZ widths (3.7 and 4.4 mm) across a range of radii (2 to 8 mm). Specimen life increased with increasing specimen size, and with a decrease in HAZ width. Segel et al6 modelled damage evolution in cross weld creep specimens using FEA. For their HAZ width “3.7 mm), a minimum life occurred at a radius of 5 mm. Specimens were only modelled to a maximum radius of 8 mm. The studies noted above

Table 5. Material properties for radius modelling

Material Relative creep rate

‘A’ Weld(creep-hard) 0.1.1 ‘A’ HAZ(creep-soft) 1O:l Parent 1 ‘C’ Weldccreep-hard) 0.5:1 ‘C’ HAZ(creep-soft) 2:l

demonstrated that specimen size influenced specimen life.

These studies have focussed on the relationship between specimen size and life, rather than specimen size and stresses. Life is a function of stress, and thus modelling in this study has addressed stress to simplify these primary relationships.

5 MODELLING

A range of specimens were analysed with radii from 0.5 mm to 30 mm, with HAZ widths of 2 mm and 4 mm, and with two different sets of material properties (‘A’ and ‘C’) (Table 5). As it has been established above that the axisymmetric models provided a fair assessment of the cross weld creep specimen behaviour, axisymmetric models with the HAZ and weld fusion line normal to the specimen axis were used in this section. The 4 mm HAZ width specimens were modelled with a range of radii from 1 to 30 mm. The 2 mm HAZ models used radii of O-5 to 7-5 mm. These two groups of cases were run with both material properties. A total of 46 cases were modelled. Some results are shown below.

Stress was examined rather than specimen life as stress is a primary effect o the strain rate differential, while life is a function of the local stress over time. The case where the parent metal is creep hard to the weld metal may be modelled by changing the labelling on the cases above, so that the weld values are now the parent values.

Effect of radius variation, 4 mm HA2

‘= T

334 - I

0 5 10 15 XI 25 Specimen radius (mm)

Fig. 6. Effect of specimen radius, 4 mm HAZ, ‘A’ material properties.

132 M. Law, W. Payten

Effect of specimen radiuson stresses, 2mmHAZ

354 I

0 2 4 6 a Radius (mm)

Fig. 7. Effect of specimen radius, 2 mm HAZ, ‘C’ material properties.

Parent

HAZ

Weld

Fig. 8. Cross weld creep specimen model, 4 mm HAZ, r = 2 mm, A materials.

6 RESULTS OF RADIUS VARIATION MODELLING

Both the weld and HAZ stresses show maximum values where the radius approximately equals the HAZ width. Representative graphs of some results are shown below (Figs 6 and 7). The highest weld stresses are at the outer wall. The position of the peak HAZ stresses is the specimen centre until the radius is approximately twice the HAZ width, after this point the position of highest stress moves (Figs S-10) towards the parent material (creep-soft material).

The parent metal has peak stresses at the outer wall up to Y = HAZ width, when the stresses become higher at the centre. At large radii (greater than Y = 2 X HAZ width) the parent stresses become continuous with the HAZ stresses (Fig. 10).

7 CONCLUSIONS

The disposition of the HAZ and weld fusion line had a small effect, less than 9% variation, on the maximum stresses in cross weld creep specimens. This is a justification for

Parent _-----_

HA2

_-----_

Weld

--

Fig. 9. Cross weld creep specimen model, 4 mm HAZ, r = 4 mm. A materials.

Parent

Weld

Fig. 10. Cross weld creep specimen model, 4 mm HAZ, r = 7.5 mm, A materials.

modelling cross weld creep specimens with the HAZ and weld fusion line normal to the specimen axis, which allows the use of axisymmetric models which are significantly faster to analyse than full 3D models.

In modelling of cross-weld creep tests, weld and HAZ stresses show maximum values where the specimen radius is approximately equal to the HAZ width. This has implications in testing, particularly with ‘mini’ specimens are used, as this may lead to variable predictions of service performance from different sized weld specimens.

ACKNOWLEDGEMENTS

The authors are grateful to the Australian Institute for Nuclear Science and Engineering (AINSE) and the CRC for Materials Welding and Joining for support for this work.

REFERENCES

1. Tu, S., Wu, R. and Sandstrom, R. Design against creep failure for weldments in 0.5 Cr 0.5 MO 0.25 V pipe. ht. J. Pres. Ves. and Piping, 1994, 58, 345-354.

Cross weld creep testing and its extrapolation to service welds 133

2. Sotresund, J., Tu, S. and Wu, R., A study of the stress Constraint Effects in Fracture, ASTM STP 1171, ed. distribution in uniaxial cross weld creep test specimens. In Hackett, Schwalbe and Dodds, 1993, pp. 341-360. Int. Symp. Life and Performance of High Temperature 5. Storesund, J. and Tu, S., Geometric effect of creep in cross Materials and Structures, ed. P. Auerkari and S. Hietanen, weld specimens. Int. .I. Pres. Vex and Piping, 1995, 179- Tallinn, Estonia, 7-8 October 1992. 193.

3. Prager, M., Issues in life assessment of chrome-moly welds. PVP-Vol. 239/MPC-Vol. 33, Serviceability of petroleum, Process and Power Equipment, 1992, 253-265.

4. Kussmaul, K. ef al., Influence of stress state and specimen size on creep rupture of similar and dissimilar welds. In

6. Segle, P., Tu, S., Storesund and Samuelson, L. Some issues in life assessment of longitudinal seam welds based on creep tests with cross weld specimens. Int. J. Pres. Ves. and Piping, 1996, 66, 179-193