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Proceedings of PVP2005 2005 ASME Pressure Vessels and Piping Division Conference

July 17-21, 2005, Denver, Colorado USA PVP2005-71107

RESIDUAL STRESSES EVALUATION IN WELDS AND IMPLICATIONS FOR DESIGN FOR PRESSURE VESSEL APPLICATIONS

Professor John W.H. Price Monash University, Department of

Mechanical Engineering, PO Box 197, Caulfield East, Vic 3145, Australia

John.Price@eng.monash.edu.au

M.Sc. Anna Pardowska Monash University, Department of

Mechanical Engineering, PO Box 197, Caulfield East, Vic 3145, Australia

Anna.Paradowska@eng.monash.edu.au

Professor Trevor Finlayson, Monash University, School of Physics

and Materials Engineering, Clayton, Victoria 3800 Australia.

Trevor.Finlayson@spme.monash.edu.au

ABSTRACT

Welding residual stresses have important consequences on the performance of engineering components. High residual stresses lead to loss of performance in corrosion, fatigue and fracture but as yet these consequences are poorly quantified. The major cause of this is that residual stress often remains the single largest unknown in industrial damage situations since they are difficult to measure or estimate theoretically.

One of the key issues in the study of residual stress is that the detail of the stress distribution on a small scale (in the order of millimetres) can be important. In this paper, the neutron diffraction technique is used which while it is a very expensive technique, is capable of non-destructively measuring residual stresses at this scale up to a depth of 35mm. The investigation reported compares the residual stress characteristics due to various restraints for a single bead and in fully restrained samples with different numbers of beads.

The findings have important consequences with respect to design of welding procedures and fitness for purpose assessments.

Key words: residual stress, neutron diffraction, welding, bead on plate.

1 INTRODUCTION

Over the last decade, welding residual stresses have received increased attention in pressure vessel and structural applications. The primary driving force can be attributed to the fact that recent advances in structural integrity assessment of weld components demand more accurate information on the weld residual stress state.

Residual stresses arise from misfits in the natural shape between different regions in a component [1,2]. In welding in particular, residual stresses are formed in the structure as the result of differential contractions which

occur as the weld metal solidifies and cools to ambient temperature

The residual stresses can be either beneficial or detrimental depending on their sign, magnitude, and distribution with respect to the service induced stresses. But the detailed assessment of where they are is important in this respect. Imposed mechanical stresses have a significant effect on corrosion, fracture resistance, creep and corrosion/fatigue performance [3] and a reduction of these stresses is normally desirable. Residual stresses in weld joints are mostly reduced by heat treatment [4] or by mechanical stress relieving [5].

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Weld quality is a function of many factors: the type and quality of metal deposited, the heat treatment inevitably experienced by deposited metal and by the parent metal around the weld (the heat-affected zone), and the residual stresses left in both the parent and deposited metal. Temperature and temperature distributions affect expansion and contraction and the relationship between stress and strain and thus, residual stresses [6].

The amount of distortion can be minimized by optimising the welding process. The magnitude of stress is a function of the weld deposit size and the effect of shrinkage [7]. Moreover the distribution of residual stresses along the weld can be influenced by controlling the heat input, shrinkage, and phase transformation [8].

There are various ways of measuring or estimating residual stresses, based on direct measurements or using numerical techniques. The direct measurements can be either semi-destructive (e.g., hole drilling and indenting [9]) or non-destructive (x-ray (laboratory or synchrotron) or neutron diffraction [10] and ultrasonic [11]). However, neutron diffraction is outstanding in its ability to obtain residual stresses non-destructively within the interior of components, in three dimensions, in small test volumes (down to 1x1x1 mm3) and in thick specimens (up to several cm).

Numerical techniques to estimate residual stresses have been developed to a degree of sophistication, which is shown in recent publications. Finite element

approaches have been used for welding but a major review still determined that there remains an “urgent need” [12] to develop the required knowledge. There is a hope that the results from such models will enable the correct welding procedure to be designed. The residual stresses in weld repairs are much more complex, difficult to understand and predict [13] presumably because of the small size of such welds and the high restraint involved.

The aim of our current research is to measure residual stresses using non-destructive methods and to relate this to manufacturing procedures and integrity requirements for various types of steel welds.

In this paper, we report experimental neutron diffraction measurements of weld stresses generated by single and multi beads-on-plate. The focus is on the value of line scans of the stress (strain) variation in the middle of the weldments. The effect of restrain and the influence of multi-passes on the residual stress distribution are discussed.

2 EXPERIMENTAL PROCEDURE

Parent Material

The parent material used in this study was a low-carbon steel [14], the chemical composition of the material and weld metal are shown in Table 1. The dimensions of the plates were 200x100x12mm3. The typical mechanical properties of parent and weld metal are shown in Table 2.

Composition C MN Si S P Ni Cr Mo Cu V Ti Co Al

Parent metal .12 .63 .13 .01 .02 .02 .01 .01 .01 <.01 <.01 <.01 .03 Weld metal .10 1.7 .68 .02 .02 .05 .03 .04 - 0.04 - - -

Table 1. Chemical composition of the consumable materials (in wt.%).

Mechanical properties Yield Stress [MPa]

Tensile Strength [MPa]

Elongation [%]

Parent metal (experimental measurements according to AS 1391:1991)

285 429 38

Weld metal (‘as manufactured’ product using

Argoshield 52 shielding gas) 445 550 29

Table.2 . Typical mechanical properties

Figure 1. Illustration of the weldments: a) unrestrained single bead on plate (Sample I); b),c)and d) are fully restrained: b) single bead on plate (Sample II), c) 2 beads on plate 50% overlapping (Sample III), d) 3 beads on plate 50% overlapping

(Sample IV).

1 2 1 3 2 11

0.50

Measurements line

a) d) c) b)

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Electrode Diameter

Current Range

Voltage Range Traverse Speed Wire Feeding

Speed

Electrode Stick-out Distance

Gas Flow Rate

1.6 mm 260-280 28-30 360 mm/min. 3600 mm/min. 20 mm 20 l/min. Table 3. Parameters used in the experimental work

Figure 2. Optical micrographs through welded section

showing: fusion line, FL, parent metal, PM, weld metal, WM, and heat affected zone, HAZ

Figure 3. Hardness profile 1.5 mm below the surface of the parent material. HAZ from -7 to 7 mm from the

center line of the weld x = 0.

2.3 Welding procedure

Experimental work was carried out on four specimens. The difference between the first two specimens was in the restraint: Sample I was unrestrained (Fig. 1a) and Sample II was fully restrained (Fig. 1b). Restraint was achieved by tack welding the sample to a very thick steel plate which was cut off after cooling down to the room temperature. Distortion of Sample I was overall approximately 1º in transverse and 0.5º longitudinal directions, Sample II had no visible distortion. Samples III and IV were fully restrained multi-passes. Sample III had two beads on the plate where the second bead was deposited with the electrode located at the toe of the first bead to achieve 50% overlapping (Fig. 1c) and Sample IV had three beads on the plate with 50% overlapping from both sides (Fig. 1d).

The welds were produced using a flux-cored arc welding (“FCAW”) process. The specimens were mounted under an automatic-speed-controlled welding torch. The welding parameters are shown in Table 3. The width of the single bead was 14 mm. For multi-passes the inter-run temperature was 500C. There was no pre or post-weld heat treatment (“PWHT”).

Transverse sections were taken across the weld for Sample I. The hardness measurements were taken 1.5 mm below the surface. The samples were polished using conventional metallographic techniques for optical microscopy inspection. The hardness profile (HV) was measured using a 5kg indention load and the result is shown in Fig. 3. The typical microstructures of the parent

material, fusion line, weld metal and HAZ are shown in Fig.3.

This information was obtained to establish the hardness of the weld bead and the HAZ. The high hardness indicates quite severe cooling conditions in the heat affected area of the weldment.

3 RESIDUAL STRESS DETERMINATION

Under tensile (or compressive) stress, the lattice spacing, dhkl, for lattice planes, hkl, in individual grains, expands (or contracts). At constant wavelength, this change in lattice spacing is detected as a shift, θ∆hkl, in the hkl diffraction peak. From the Bragg equation the strain, εhkl, is given by

hklhklhkl

hkl ddd θθε ∆−=−= cot

0

0 (1)

where d0 is the strain-free lattice spacing for the hkl planes.

The orientation of the principal strains in any specimen is determined by specimen geometry. The strains (εxx, εyy, εzz) convert to the three-dimensional stress (σxx, σyy, σzz) state. For an isotropic solid, equations of the form:

( )( ) ( ) ( )[ ]zzyyxxxx

E εενεννν

σ ++−−+

= 1211

(2)

WM

HAZ

PM

FL

0.5 mm 120

140

160

180

200

220

240

260

-15 -10 -5 0 5 10 15

Distance from the weld center line (mm)

Har

dn

ess

(H

V)

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give stresses in three directions, using σxx as example, where E is Young’s modulus (207 GPa) and ν is Poisson’s ratio (0.3). The principles of the neutron diffraction technique showing Bragg’s reflection from the crystal plane is shown on Fig.4.

Figure 4. Principles of the neutron diffraction technique showing Bragg’s reflection from the crystal plane d.

(The grain size is greatly exaggerated for clarity- there is normally large number of grains in the gauge volume).

Neutron diffraction measurements were undertaken on The Australian Strain Scanner (TASS) at ANSTO, Australia. The same parameters of measurements were specified for both specimens. The neutron wavelength used was 1.40 �. Measurements were made using the (112) reflection, at the detector angle, 2�, of approximately 73.5o. Measurements were made with the scattering vector parallel to the three axes marked transverse, longitudinal and normal as shown on Fig. 5.

Figure 5. The direction of the measurements (transverse x, normal y, longitudinal z) using neutron diffraction on the

single bead-on-plate. The incident and diffracted beam slits for measurements in the transverse and normal directions were 1 mm wide and could be 20 mm high because the stresses do not change significantly in the longitudinal direction. Because the stresses are changing rapidly in the transverse direction; for measurements of strains in the longitudinal direction the slits can only be 1.5 mm in width and 2 mm high. The slits are formed by a “mask” of a neutron absorbing material (Cadmium) in the incident and diffraction beams.

The stress-free parameters for the steel were measured on eight 2x2x2 mm3 cuboids which had been cut and glued together (“the do specimen”). The slits for the stress-free parameters were the same as for longitudinal measurements (1.5x1.5x2 mm3). Scans were made along the transverse line (Fig. 5) from x = 0 (the centre of the weld) to x = 32 mm. The centre of the gauge volume was 1.5 mm below the top surface. The do specimen required three measurements for accuracy and yielded a stress free diffraction angle, 2� = 73.36o.

Counting times are determined by the accuracy required of the measurements. Counts are collected until the peak has been clearly defined. The time required for this varies with each geometry and direction of measurement. The number of counts of neutrons at the collectors used in our experiments was approximately 30,000 for each point. For each data point this required times of approximately 20 minutes for transverse and normal strain measurements and 60 minutes for longitudinal and do parameter measurements.

4 RESULTS

The residual stresses were derived from the elastic strain measurements (Fig.6a, 7a) using Young’s modulus of 207 GPa, and Poisson’s ratio of 0.3. The maximum longitudinal tensile residual stress was found near the middle of the weld and is shown in Fig. 6b and 7b, reaching the approximate value of 350-360 MPa for Sample I and 470-480 MPa for Sample II. The change in residual stress in fully restrained Sample II in comparison to unrestrained Sample I is shown in Fig. 8. The comparison of the residual stress in three directions for three samples: Sample II, Sample III and Sample IV are shown in Fig. 9. Error bars based on uncertainty in the value of the peak diffraction angle have been shown on the pictures.

Direction of the measurements

Longitudinal

Normal

Transverse

1.5 mm

Restraint by tack welding

Slits

Incident beam

2θ

Scattering vector

Slits

Diffracted beam

Transmitted beam

Gauge volume

d Grain

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Figure 6. Unrestrained Sample I. The longitudinal, transverse and normal components of strains (a) and stresses (b) measured by neutron diffraction against distance from the weld center line. Error bars based on uncertainty in the value of the peak

diffraction angle are shown.

Figure 7. Strains and stresses measured in fully Restrained Sample II.

Figure 8. The change in residual stress (RS) in fully restrained Sample I in comparison to unrestrained Sample II.

-50

50

150

250

350

450

550

-5 0 5 10 15 20 25 30 35Distance from the weld center line (mm)

Res

idu

al s

tres

s (M

pa)

Longitudinal

Normal

Transverse

-500

0

500

1000

1500

2000

-5 0 5 10 15 20 25 30 35

Distance from the weld center line (mm)

Mic

rost

rain

LongitudinalNormal

Transverse

Weld Parent metal Weld Parent metal b a

-400

-300

-200

-100

0

100

200

300

400

-5 0 5 10 15 20 25 30 35

Distance from the weld center line (mm)

Res

idua

l str

ess

(Mpa

)

LongitudinalNormal

Transverse

-1500

-1000

-500

0

500

1000

1500

2000

-5 0 5 10 15 20 25 30 35

Distance from the weld center line (mm)

Mic

rost

rain

LongitudinalNormal

Transverse

Weld Parent metal Weld Parent metal b a

-100

0

100

200

300

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Distance from the weld center line (mm)

Cha

nge

in R

S (M

Pa)

Longitudinal Normal Transverse

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.

Figure 9. The change in a) longitudinal, b) normal and c) transverse stress distribution after depositing 1st bead (Sample II),

2nd bead (Sample III) and 3rd bead (Sample IV)

-400

-200

0

200

400

600

-35 -25 -15 -5 5 15 25 35Distance from the 1st w eld center line (mm)

Lo

ng

itu

dio

nal

RS

(M

Pa)

Ist bead2nd bead3rd bead

a 1st bead 2nd bead 3rd bead

-100

0

100

200

300

-35 -25 -15 -5 5 15 25 35

Distance from the 1st weld center line (mm)

Nor

mal

RS

(MP

a)

Ist bead2nd bead3rd bead

b 1st bead 2nd bead 3rd bead

-100

0

100

200

300

-35 -25 -15 -5 5 15 25 35Distance from the 1st weld center line (mm)

Tran

sver

s R

S (M

Pa)

Ist bead2nd bead3rd bead

1st bead 2nd bead 3rd bead

c

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4.1 Restraint

The highest increase in stresses between the unrestrained and restrained specimen was observed in middle of the weld where the normal and transverse stresses change from compressive for unrestrained sample (Fig. 6b) to tensile for the restrained sample (Fig.7b).

Transverse and normal stresses are low in the unrestrained Sample I, because the sample deformed during welding. However for Sample II transverse and normal stresses are raised at all points of measurements as shown on Fig. 8. Longitudinal stress also increases but there are some reductions around the toe of the weld.

The peak stress in the weld (which is in the longitudinal direction) is significantly higher in the weld area in both samples, than the specified yield stress of the parent metal in question (250 MPa). Hardness tests (Fig. 3) indicate significant increases in hardness in the HAZ reflecting higher yield stresses in this region. The peak stress in this case does not occur at the toe of the weld but in the middle of weld. The maximum longitudinal stress at the toe is around 150 MPa which will be approximately 60 % of the minimum yield strength (250 MPa) of the parent material.

4.2 Multi-passes

A change in longitudinal stress (Fig. 9a) after deposition of the second bead is observed. Residual stress under the bead is increasing (from approximately 150 MPa to 450 MPa), but the values of the residual stress on the opposite side of the first weld bead do not increase significantly. After deposition of the third bead an increase under the bead is observed, but a decrease on opposite side of the weld is observed. The value of the stress at the toe increases to nearly 270±20 MPa on both sides where the second bead was deposited, but after the third bead the value of the stress on the side of the new deposited bead remains 260±25 MPa but on the opposite side decreases to low values, 15±12 MPa.

The value of the residual stress for the normal (Fig. 9b.) and transverse (Fig. 9c) directions also decreases with every bead, especially in the toes of the new weldments and outside the weld region. The value at the toe drops to nearly 0 MPa or even becomes compressive for transverse residual stress.

5 CONCLUSIONS

The use of a neutron beam as a non-destructive method of measuring residual stress due to welding has been explored.

Key findings are:

• Additional restraint on the weld increases the value of residual stress.

• The peak stress in all four samples (which is in the longitudinal direction) exceeded the minimum yield stress, for this steel (250 MPa) but does not occur at the toe, but in the middle of the weld bead (where the yield stress is higher).

• In transverse and normal direction maximum residual stress of around half of the maximum value of longitudinal stress have been observed.

• Additional welding beads tend to reduce the stress in previous beads and the peak stress tends to be in the middle of the last bead - the last point of solidification.

Implications for pressure vessel design

• Higher residual stresses occur in the longitudinal directions of welding. his is probably a major factor in causing transverse cracking due to hydrogen.

• The transverse stresses at the toe of welds have implications in terms of fatigue crack initiation. Any in service fatigue cracks start at weld toes but the data in this paper suggest that the transverse residual stress is much less then yield stress. Even so, the transverse stresses cannot be ignored.

• In multi-bead welding appear to release residual stresses in both adjacent and distant weld beads.

Fitness for purpose assessments

An important feature of the stresses measured in this work is that the stresses encountered are much lower and more favourably oriented than those required to be assumed in fatigue and fitness-for-purpose analyses such as those included in standard BS7910 [15].

According to BS7910 7.2.4 in “a structure in the as–welded condition, with a flaw lying in a plane transverse to the welding direction the tensile longitudinal residual stress is to be assumed to be a uniform membrane stress … equal to the room temperature yield strength of the material in which the flaw is located“. “For a flaw parallel to the welding direction” the transverse residual stress “should be assumed to be the lesser of the room temperature yield strength of the weld or parent material”.

From the measurements reported in this paper the transverse stress which needs to be considered is significantly lower than yield. The stresses in both longitudinal and transverse directions tend to be high in the areas which have been molten or close by and assumption of uniform membrane stress is highly conservative. Stresses are also reduced by subsequent weld beads.

Future studies

The full exploitation of this kind of information is one key issue in future studies. Our work is currently developing methods of cross checking these results with finite element modelling of the welding process.

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6 ACKNOWLEDGEMENTS

This work was conducted with the assistance of an Australian Research Council grant supported by the Welding Technology Institute of Australia (“WTIA”). Other assistance has been received from the Monash University Research Fund, the Australian Nuclear Science and Technology Organisation (“ANSTO”) and an Australian Institute of Nuclear Science and Engineering. (“AINSE”) grant.

7 REFERENCES

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15 British Standards. BS 7910: 1999, “Guide on methods for assessing the acceptability of flaws in metallic structures”, London: BSI.