Metallographic Studies of Electron Beam Welded Copper Lid - Posiva

Working Report 2014-24

Metallographic Studies of Electron Beam Welded Copper Lid: Macroscopic Studies and Hardness Measurements of the Cross-Section of XK049 at 323deg Taru Karhula

June 2014 POSIVA OY Olkiluoto FI-27160 EURAJOKI, FINLAND Phone (02) 8372 31 (nat.), (+358-2-) 8372 31 (int.) Fax (02) 8372 3809 (nat.), (+358-2-) 8372 3809 (int.)

Working Report 2014-24

Metallographic Studies of Electron Beam Welded Copper Lid: Macroscopic Studies and Hardness Measurements of the Cross-Section of XK049 at 323deg Taru Karhula Tampere of University of Technology Department of Materials Science

June 2014

Working Reports contain information on work in progress or pending completion.

ABSTRACT This work is part of Posiva’s spent nuclear fuel disposal canister sealing development. Posiva has welded a full-scale lid to a canister 450 mm or 890 mm tall at Patria Aviation Facilities. At Tampere University of Technology, Department of Materials Science (TUT DMS), metallographic and electron microscopy studies of electron beam welded copper lid welding have been carried out. At TUT DMS, samples have been ground, etched, photographed and analyzed. During analyzing, different fusion zone measurements and imperfection characterizations were carried out. The methods used in the metallographic studies are presented together with the results. In this report a part of the welding test program is analyzed. The results of the crosssection of the test weld XK049 at 323deg are presented here together with the methods used. The aim of this study was to estimate the residual stresses present in the EB-weld using hardness measurements. In previous study based on reference curves with tensile test samples it was found out that the hardness could be related to the applied strain. Also, the hardness was higher if there was a cosmetic pass on the weld, which was also considered in this report. The average hardness in the weld was 48.8 HV1, 46.6 HV1 and 46.5 HV1 in z=10 mm, 20 mm and 45 mm respectively. The average hardness of the lid material was 48.3 HV1 and 47.7 HV1 in z=20 mm and 45 mm respectively. The average hardness of the tube material was 50.5 HV1 and 48.0 HV1 in z=10 mm and 45 mm respectively. The estimated strain levels in the weld in XK049-323° were 0.015-0.021 mm/mm, when compared directly to the reference curve. When the effect of the cosmetic pass was taken into account, the remaining strain was 0.003-0.008 mm/mm. When these strain ranges were compared with the tensile tests for annealed longitudinal plate weld samples, the estimated residual stresses were in the range 51-59 MPa (the effect of cosmetic pass not taken into account) or 33-42 MPa (the effect of cosmetic pass taken into account). Keywords: EBW, electron beam welding, copper, lid, canister weld, hardness, residual stress

Elektronisuihkuhitsatun kuparikannen metallurgiset tutkimukset: Makroskooppiset tutkimukset ja kovuusmittaukset XK049 hitsin poikkileikkauksesta kohdalta 323° TIIVISTELMÄ Tämä työ on osa Posivan käytetyn ydinpolttoaineen kapselin sulkemiskehitystyötä. Posiva Oy on hitsannut täyden mittakaavan kansia 450 mm tai 890 mm pitkiin kanisteriputkiin Patria Aviation Oy:n tiloissa. Tampereen teknillisen yliopiston Materiaaliopin laitoksella (TTY MOL) on tutkittu näitä elektronisuihkuhitsattuja kansihitsejä metallografisin ja elektronimikrosopian menetelmin. TTY:n Materiaaliopin laitoksella näytteet ovat hiottu, syövytetty, kuvattu ja analysoitu. Analysointi sisälsi erilaisia sularajamittauksia ja virheiden karakterisointia. Raportissa esitellään metallografisessa tutkimuksessa käytetyt menetelmät ja tutkimuksen tulokset. Tämän raportin yhteydessä on analysoitu vain osa hitsauskoeohjelman hitseistä. Tässä raportissa esitellään hitsin XK049 kohdan 323° poikkileikkauksen tulokset sekä tutkimuksissa käytetyt menetelmät. Tämän tutkimuksen perimmäinen tarkoitus oli arvioida jäännösjännityksen suuruutta kyseisessä elektronisuihkuhitsatussa kanisterissa kovuusmittausten perusteella. Aikaisemmassa tutkimuksessa vetokoenäytteiden perusteella todettiin, että kovuus oli riippuvainen näytteen venymästä. Ko. raportissa todettiin myös, että kosmeettinen hitsipalko varsinaisen liitoshitsin päällä nosti liitoshitsin kovuutta. Hitsin keskimääräinen kovuus oli 48.8 HV1, 46.6 HV1 ja 46.5 HV1 syvyyksillä z=10 mm, 20 mm ja 45 mm tässä järjestyksessä. Kansimateriaalin keskimääräinen kovuus oli 48.3 HV1 ja 47.7 HV1 syvyyksillä z=20 mm and 45 mm tässä järjestyksessä. Putkimateriaalin keskimääräinen kovuus oli 50.5 HV1 ja 48.0 HV1 syvyyksillä z=10 mm ja 45 mm tässä järjestyksessä. Näytteen XK049-323° hitsin arvioitu venymätaso oli 0.015-0.021 mm/mm, kun verrattiin suoraan referenssikäyrään. Kun kosmeettisen hitsin vaikutus otettiin huomioon, näytteessä olevan venymä oli 0.003-0.008 mm/mm. Verrattaessa näitä venymäalueita levykokeille tehtyjen hehkuttamattomien pitkittäishitsien vetokoetuloksiin, arvioitu jäännösjännitys näytteessä oli 51-59 MPa (ilman kosmeettisen palon huomioimista) tai 33-42 MPa (kosmeettisen palon vaikutus huomioiden). Avainsanat: EBW, elektronisuihkuhitsaus, kupari, kansi, kapselin hitsaus, kovuus, jäännösjännitys.

PREFACE This report is one part of the residual stress evaluation of the EB-welded nuclear waste canister made of copper. Target of the residual stress evaluation is to assess level of the residuals stresses of the weld and also effect of the welding parameters and stress relief on residual stresses. Evaluation can be divided on following parts:

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Evaluation deformations and residual stresses on EB-welds using numerical modeling (finite element method, FEM) Residual stress measurements of the EB-welds using different measurement methods Destructive testing of the weld to assess residual stresses

Macroscopic evaluation and hardness measurement of the lid weld XK049 are reported in this report. Target was to evaluate work hardening of the weld caused by residual stresses and estimate residual stresses using calibration curves defined in Posiva working report WR2013-14. The EBSD analysis of the lid weld XK049 to evaluate plastic deformation is reported in separate report along with this report.

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TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ PREFACE LIST OF SYMBOLS AND ABBREVIATIONS ................................................................. 3 1.

INTRODUCTION .................................................................................................... 5

2.

WELDING PARAMETERS ..................................................................................... 7

3.

RESEARCH METHODS ......................................................................................... 9

4.

5.

6.

3.1.

Identification of sample ................................................................................... 9

3.2.

Sample preparation for macroscopic examination .......................................... 9

3.3.

Weld shape and measures ........................................................................... 11

3.4.

Hardness measurements .............................................................................. 11

MAIN RESULTS ................................................................................................... 13 4.1.

Metallographic measurements ...................................................................... 13

4.2.

Hardness measurements .............................................................................. 14

ANALYSIS AND DISCUSSION ............................................................................ 17 5.1.

The remaining plastic strain in the weld ........................................................ 17

5.2.

Estimated residual stress in the weld ............................................................ 18

CONCLUSIONS.................................................................................................... 21

REFERENCES ............................................................................................................. 23 APPENDICES............................................................................................................... 25

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LIST OF SYMBOLS AND ABBREVIATIONS A

breadth of cross-sectional sample

[mm]

AA

working distance

[mm]

AF

focal length

[mm]

ACX

oscillation input in x-direction for EB-machine

ACY

oscillation input in y-direction for EB-machine

AMIS

average of the mean intra-grain misorientation

av(G)

average of grades

av(s)

average of penetrations

[mm]

α

angle of groove

[º]

β

angle of slot

[º]

B

measure from the weld centerline to lid side of cross-sectional sample [mm]

BSE

backscattered electron

C

distance from inner surface of tube to inner surface of lid

[mm]

D

distance from inner surface of tube to outer surface of tube

[mm]

Delta

the highest minus the lowest average for each factor

E

distance from weld centerline to inner surface of tube

EBSD

electron backscatter diffraction

EBSP

electron backscatter diffraction pattern

εeng,pl

plastic engineering strain

[mm/mm]

εeng,tot

total engineering strain

[mm/mm]

εtr,pl

plastic true strain

[mm/mm]

F

distance from outer surface of lid to inner surface of lid

[º]

[mm]

[mm]

FESEM Field Emission SEM h

height of an imperfection

[mm]

h1

height of a spike, h1 = S – s

[mm]

hmax

maximum height of welding imperfections in certain welding distance [mm]

4

HAZ

heat affected zone

IB

beam current

[mA]

Ifoc

focus current i.e. lens current

[mA]

L

width of slot

[mm]

L osc

longitudinal oscillation measure

[mm]

MAD

mean angular deviation

s

penetration depth without spiking

[mm]

sintact

clean penetration i.e. minimum intact fusion zone thickness

[mm]

smin

penetration minimum in certain welding distance

[mm]

smax

penetration maximum in certain welding distance

[mm]

Δs

variation in penetration depth in certain welding distance

[mm]

S

penetration depth to the tip of a spike

[mm]

Smax

maximum penetration depth (to the tip of a spike) across the welding distance

[mm]

SE Fit

standard error of fits

SEM

scanning electron microscope



standard deviation



stress

[MPa]

T osc

transversal oscillation measure

[mm]

UB

accelerating voltage

[kV]

v

welding speed

[mm/s]

x

axis in the direction of welding

y

axis in the direction of the side of welding

z

axis in the direction of the weld penetration

w

width of weld or width of an imperfection

[mm]

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1. INTRODUCTION This report contains the methods used at Tampere University of Technology, Department of Materials Science (TUT DMS), in metallographic studies and hardness measurements of electron beam welded copper samples welded by Posiva at Patria Aviation Facilities. The aim of the work was to evaluate the residual stresses in electron beam welded copper canister sample using hardness measurements.

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2. WELDING PARAMETERS The EB welding of the full-scale lid to a 890 mm tall canister was carried out by Posiva at Patria Aviation. In tests XK049 the welding was a circumferential joint of a spent nuclear fuel canister containing slope up, full penetration weld, overlap and slope down. Test XK049 was welded before the repair work and calibration of the used EB welding equipment. Lid weld test XK049 was welded for an extensive residual stress testing program and metallographic studies were conducted after those tests. The section examined at TUT DMS was machined from the area of the full penetration weld, in the weld diameter area of θw1 = 323°. The accelerating voltage UB was 150 kV. The welding of lid weld tests was carried out in a medium vacuum (9.7×10−3 mbar) using a vertical electron beam. The welding parameters, based on the earlier results of full penetration tests, were: 

Welding beam current IB 320 mA



Welding speed v 2.5 mm/s



Lens current IL 2390 mA



Oscillation pattern E32 using the relative set values of ACX 4.2 and ACY 3.8

The welded material was phosphorus micro-alloyed oxygen-free copper Cu−OFP with P= 30−70 ppm, O < 5 ppm, H < 0.6 ppm and S < 8 ppm.

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3. RESEARCH METHODS 3.1.

Identification of sample

The lid welding experiments were marked with running numbers starting from one, and the number of an experiment was indicated with a three-number code, e.g. XK049. For the metallographic studies, the welding samples were cut into smaller pieces which were marked with identification numbers. A sample with parallel surfaces was needed for the planned EBSD studies. The first (front) surface of the cross-section was cut at θw = 323°. The second (back) surface was cut 10 mm after that, which means that this surface was not cut exactly in radial direction. The sample was marked with the angular location of the first surface: XK049-323. The sides of the cross-sectional samples were numbered as “1” and “2”. Side 1 is the front side and side 2 is the back side of the sample when the samples are ordered by welding direction. In the naming of the images, the co-ordinate axis system shown in Figure 2 was used. The zero points of the axes are located at the starting point of the welding. The ‘x’ direction indicates the welding direction. The ‘y’ direction indicates perpendicular welding, i.e. towards the outer side of the tube. The ‘z’ direction points to the depth of welding.

Figure 1. The co-ordinate axis system used.

3.2.

Sample preparation for macroscopic examination

The transverse cross-section samples were cut by wire electric discharge machining WEDM from the ring containing the weld. The cross-section was ground using P160 and P320 grinding papers to remove the excess oxide layer and to obtain sufficient

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smoothness of the surface. The sample was etched with technical grade nitric acid (HNO3). The sample was first “rinsed” with nitric acid by immersing it for 5−10 s, washed with water, and then actually etched by immersing for 30 s in fresh nitric acid. After etching the sample was washed with water and ethanol and dried with a drier. The etched sample was photographed using a Nikon D70s camera and a Nikon AF Micro-Nikkor 60 mm f/2.8D camera objective. The camera was attached to a Kaiser RS1 (5510) stand. The images were stored in Nikon raw format (.nef, Nikon electronic file format) with Nikon Capture 4 software. The images were edited and saved in .jpeg and .tiff formats. The images from the backside of the cross-section sample were flipped, so that all the cross-section images show the cross-section as viewed from the start of the welding. The cross-sections were examined using a stereomicroscope Leica MZ7.5. The imperfections found are listed in Appendix C. After the macroscopic studies the cross-section sample was cut at the depths z=17 mm, z=35 and z=52 mm. These 17 mm high samples were then prepared using several grinding and polishing steps. First the samples were mechanically wet ground with silicon carbide papers from P320 to P600. Polishing was carried out with 9 μm and 3 μm diamond suspensions. Final polishing was performed with colloidal silica particles. Between the stages the samples were carefully washed with water and cleaning agent. Finally the samples were cleaned in an ultrasonic bath for 10 min in distilled water with cleaning agent and for 10 min in ethanol. The samples were stored in desiccator.

Figure 2. The locations of the hardness measurement samples.

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3.3.

Weld shape and measures

The weld width and depth were measured from the cross-section image using the ImageTool program. Width was measured at several depths to obtain the weld profile. The radius of the root was determined with the software ImageJ. Several points in the fusion line near and at the root were captured and a polynomial curve was fitted.

3.4.

Hardness measurements

The hardness was measured as HV1 values using the hardness tester Struers Duramin A-300. Hardness measurements were carried out in 3 different penetration levels: z=10 mm, z=20 mm and z=45 mm. The hardness was measured 0.5 mm intervals outside the weld and in the weld area. Total of 74-76 points were measured in each depth.

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4. MAIN RESULTS The metallographic images of the cross-section XK049-323 showing the macrostructure of the sample are shown in Appendix B and the few imperfections found are listed in Appendix C. In Appendix B also close-up images of the weld, lid and tube material are presented. 4.1.

Metallographic measurements

The weld penetration depth was 57.3 mm. The measurement points for the determination of the radius of the root are presented in Appendix B. The radius of the root was 1.3 mm. The width of the weld in the cross-section is shown in Figure 3. The width measurement was carried out in 5 mm intervals in “z” direction from top downwards. The weld was at its widest in z = 5 mm and in z = 15 mm and narrowest in z = 25 mm. All measured widths of welds can be found in Appendix B.

Figure 3. Weld width profile.

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4.2.

Hardness measurements

The hardness measurement results are presented in Appendix D and in the Figures 4-6.

Figure 4. Hardness measurement results in z=10 mm.

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From the hardness values measured, 17 measurement points in the weld area were selected to calculate an average hardness for each weld depth. Also for the base material areas, lid and tube, an average hardness was calculated. The average hardness values are shown in Figure 7 and in Table 1.

Figure 7. The average hardness values for the weld and the base materials.

Table 1. Summary of the hardness measurement results for each area in various weld depths. Hardness HV1 average StDev min max

z=10mm WELD 48.8 1.6 45.3 51.1

z=20mm

TUBE 50.5 2.6 48.6 62.5

WELD 46.6 1.0 45.1 48.5

z=45mm

LID 48.3 1.5 45.4 54.0

WELD 46.5 0.7 45.0 47.7

TUBE 48.0 1.1 46.3 50.2

LID 47.7 0.9 46.2 49.2

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5. ANALYSIS AND DISCUSSION The remaining plastic strain in the weld is discussed here. Also the residual stresses are estimated. The estimations are based on the reference curves determined using tensile test samples with known strains and they were reported earlier in the Posiva report 2013-14 [1]. 5.1.

The remaining plastic strain in the weld

Based on the reference curves determined in the previous study [1], the amount of plastic true strain was estimated. The strain values estimated based on hardness for each weld depth are shown in Table 2. It was discovered in the previous study made for the cross-sections of the plate welding experiments [1] that the hardness is higher in the weld with a cosmetic pass than without a cosmetic pass: with a cosmetic pass the hardness was 3 HV1 higher with welding speed 2.1 mm/s and 5 HV1 higher with the speed 2.5 mm/s. The reference curves were determined with annealed (600°C 2h) samples, in which the effect of a possible cosmetic pass can be considered to have disappeared. In the lid weld XK049 (v=2.5 mm/s) a cosmetic pass has been welded, which may indicate lower strain values than predicted directly based on the tensile test samples from plate welding experiments. The estimated strain, when this difference is taken into account, is also calculated and shown in Table 2 for each weld depth. The plastic strain in the weld estimated based on the average hardness appears to be slightly higher in z=10 mm than deeper in the weld (in z=20 mm and z=45 mm).

Table 2.

The plastic true strain in the weld estimated with the hardness. Average hardness values and average ±StDev compared directly to reference curve True strain, WELD WELD WELD plastic [mm/mm] z=10mm z=20mm z=45mm average 0.021 0.016 0.015 average‐StDev 0.017 0.013 0.013 average+StDev 0.025 0.018 0.017 ± error 0.004 0.003 0.002 The effect of a cosmetic pass (‐5HV) taken into account True strain, WELD WELD WELD plastic [mm/mm] z=10mm z=20mm z=45mm average 0.009 0.003 0.003 average‐StDev 0.005 0.001 0.001 average+StDev 0.012 0.006 0.005 ± error 0.004 0.003 0.002

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Figure 8. The reference curve of hardness values ± standard deviation as a function of strain together with the hardness results in the weld in XK049-323. The effect of the cosmetic pass has not been taken into account here. 5.2.

Estimated residual stress in the weld

The estimation of residual stresses based on strain measurement is not straightforward. According to the microstructural characteristics revealed by the EBSD measurements, it is only possible to say that at some point during the welding process the microstructure has experienced deformation of the indicated strain, which requires a certain stress that depends on the temperature dependent flow stress of the material. It is, however, not known whether relaxation has occurred and to what extent during or after cooling of the welded area. The maximum stress values can be read from the tensile stress-strain – curve after cooling down to room temperature. The actual stress may then be anything between zero and this estimated maximum value. The stress values shown in Table 3 give the maximum stress values estimated in this manner. Also, it is not possible to say based on the microstructure whether the stress is tensile or compressive; although other studies indicate that the stresses are tensile in the fusion zone. The stress values shown in Table 3 were determined from the tensile test data for an annealed plate welding experiment sample, the same as were used for the construction of the reference curves in [1].

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Table 3.

The maximum of the residual true stress in the weld estimated with the hardness. Average hardness values and average ±StDev compared directly to reference curve True stress WELD WELD WELD [MPa] z=10mm z=20mm z=45mm average 59.0 51.4 51.0 average‐StDev 53.0 47.2 47.9 average+StDev 63.8 54.4 52.7 ± error 5.4 3.6 2.4 The effect of a cosmetic pass (‐5HV) taken into account True stress WELD WELD WELD [MPa] z=10mm z=20mm z=45mm average 41.5 33.2 32.6 average‐StDev 35.2 26.9 28.7 average+StDev 46.4 36.7 35.1 ± error 5.6 4.9 3.2

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6. CONCLUSIONS At Tampere University of Technology, Department of Materials Science, metallographic studies and hardness measurements were carried out for the electron beam welded copper sample XK049-323°. The hardness of the weld was higher in z=10 mm (48.8 HV1) than in z=20 mm (46.6 HV1) or z=45 mm (46.5 HV1). The hardness was lower in the weld area than in the tube (48-50.5 HV1) or lid (47.7-48.3 HV1) material. The strain in the weld area was estimated based on the reference curves determined earlier with tensile test samples. The plastic strain in the weld estimated based on the average hardness appeared to be slightly higher in z=10 mm than deeper in the weld (in z=20 mm and z=45 mm). The plastic true strain in the weld in XK049-323 was 0.015-0.021 mm/mm. However, it was found out earlier that the cosmetic pass increases the hardness by ca. 5 HV (with welding speed 2.5 mm/s). When this was subtracted from the hardness results in XK049-323, the remaining strain was 0.003-0.008 mm/mm. The estimated maximum of the residual stress in XK049-323 was 51-59 MPa when the strain values without the effect of a cosmetic pass were fitted in the tensile test curve of an annealed plate welding sample. When the effect of the cosmetic pass was taken into account, the stress was 33-42 MPa. However, it is not known whether relaxation has occurred and to what extent and therefore the true residual stresses are probably somewhat smaller than the estimated maximum values. Also, it is not possible to say based on the applied technique whether the stresses are tensile or compressive.

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REFERENCES 1. Karhula T, Metallographic studies of electron beam welded copper plates: EBSD studies of the cross-sections and determination of EBSD reference curves by EB-welded tensile test samples. Posiva report 2013-14. 241 p.

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APPENDICES A. Drawings of the test specimens B. Macrostructural images of the sample after light etching and cross-sectional measures of the sample C. List of imperfections found in the cross-section D. Hardness measurement results for the sample E. DVD disc containing macrostructural images in tif-, nef- and jpg-formats

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Drawings

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APPENDIX A

APPENDIX A

Drawings of the test specimens

For the report: Metallographic studies of electron beam welded copper lid: Macroscopic studies and hardness measurements of the cross-section of XK049 at 323deg

APPENDIX A

28

Drawings

Drawings

29

APPENDIX A

APPENDIX A

30

Drawings

Macrostructure of the sample

31

APPENDIX B

APPENDIX B

Macrostructural images of the sample after light etching and cross-sectional measures of the samples


APPENDIX B

Figure 9.

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XK049-323, front side.



Figure 10.

33

XK049-323, front side. close-up of the weld.

APPENDIX B

APPENDIX B

Figure 11.

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XK049-323, back side (image flipped horizontally).


Figure 12.

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APPENDIX B

XK049-323, back side. close-up of the weld (image flipped horizontally).

APPENDIX B

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Figure 13.

Lid in/near the top surface.

Figure 14.

Tube in/near the top.



Figure 15.

Lid in z=13-21 mm.

Figure 16.

Tube in z=13-21 mm.

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APPENDIX B

APPENDIX B

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Figure 17.

Lid in z=42-50 mm.

Figure 18.

Tube in z=42-50 mm



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Figure 19.

Weld in/near the top surface.

Figure 20.

Weld in z=13-21 mm.

APPENDIX B

APPENDIX B

Figure 21.

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Weld in z=42-50 mm.


Macrostructure of the sample Table 4.

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APPENDIX B

Weld width and penetration measurements XK049‐323°

Width

measurement

[mm]

1

10.4

2

11.18

3

10.45

4

9.38

5

9.58

6

9.72

7

10.5

8

10.02

9

10.02

10

10.21

average

StDev

0.524493

Penetration

Width profile

z (distance from surface) [mm]

10.146

57.3 width [mm] 5 11.33

10

10.94

15

11.33

20

10.06

25

9.33

30

10.36

35

10.06

40

10.26

45

10.06

APPENDIX B Table 5.

42 Measurement of the radius of the root.

XK049(323°)

point no

y [mm]

x [mm] 1

19.422

45.523

2

19.857

47.846

3

20.184

51.295

4

20.511

52.856

5

20.801

53.981

6

21.128

55.034

7

21.563

55.76

8

22.253

56.522

9

22.943

57.139

10

23.705

57.611

11

24.685

57.684

12

25.339

57.285

13

25.92

56.776

14

26.537

55.941

15

27.009

55.542

16

27.408

54.526

17

27.88

53.146

18

28.134

51.44

19

28.715

49.915

20

29.042

48.1

21

29.477

46.43

a

r0

R‐radius [mm] ‐0.3817 ‐1.30993 1.31


Imperfections

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APPENDIX C

APPENDIX C

List of imperfections found in the cross-section


APPENDIX C Table 6.

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Imperfections

Imperfections found in the cross-section XK049-323, front side. Cross-section of XK049 323°

Location

Type of imperfection

xstart [mm]

xend [mm]

zstart [mm]

zend [mm] h [mm]

hmax [mm]

XK049 323° surface

0.0

base weld

Pore

5

5

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