S C I E N C E RESEARCH D E V E L O P M E N T

E U R O P E A N C O M M I S S I O N

technical steel research

Properties and in-service performance

The fracture behaviour of girth welds in high strength high yield-to-tensile ratio linepipe steels

h Report

E U R 1 8 4 2 6 EN STEEL ESEIICH

EUROPEAN COMMISSION

Edith CRESSON, Member of the Commission responsible for research, innovation, education, training and youth

DG XII/C.2 RTD actions: Industrial and materials technologies Materials and steel

Contact: Mr H. J.-L. Martin Address: European Commission, rue de la Loi 200 (MO 75 1/10), B-1049 Brussels Tel. (32-2) 29-53453; fax (32-2) 29-65987

European Commission

technical steel research Properties and in-service performance

The fracture behaviour of girth welds in high strength high yield-to-tensile

ratio linepipe steels

A. Correia da Cruz Instituto de Soldadura e Qualidade

Estrade Nacional 249-Km 3 Cabanas-leiao (Tagus Park)

P-2781 Oeiras Codex

T. Lefevre Lab. Soete voor weerstand van materialen en lastechniek

c/o Belgisch Instituut voor Lastechniek St Pietersnieuwstraat 41

B-9000 Gent

F. Santamaria Inasmet

Camino de portuetxe 12 E-20009 San Sebastian

Contract No 7210-MC/202/932/933 1 April 1992 to 31 December 1994

Final report

Directorate-Genera! Science, Research and Development

1998 EUR 18426 EN

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Luxembourg: Office for Official Publications of the European Communities, 1998

ISBN 92-828-4643-1

European Communities, 1998

Reproduction is authorised provided the source is acknowledged.

Printed in Luxembourg

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THE FRACTURE BEHAVIOUR OF GIRTH WELDS

IN HIGH STRENGTH HIGH YS/TS RATIO LINEPIPE STEELS

EXECUTIVE SUMMARY

Improvements in steel making practice and rolling techniques have led to the development of low carbon micro-alloyed (structural and linepipe) steels with increased yield and tensile strength, adequate notch toughness and improved weldability. High strength steel linepipes conforming to API 5L Grade X 80 (SMYS of 551 MPa) are now commercially available. Because of the lack of service data, potential users are reluctant to utilize these steels for onshore and offshore pipeline projects. The major concern is that girth welds in such steels might be less safe in terms of failure avoidance than welds in conventional steel pipes. This is linked with their lower strain hardening capacity (higher yield-to-tensile ratios) and with the fact that, with increasing pipe yield strength, it becomes more difficult to achieve weld metal yield strength (YS) overmatching.

To gain a better understanding into the deformation and failure characteristics of defective girth welds in such micro-alloyed steel pipes, experimental work was conducted on manual girth welds in large diameter (40" O.D. 16,9 mm W.T. and 44" O.D. 16,2 mm W.T.) pipes of API 5L X 70 and X 80 qualities. The work was performed jointly by Instituto de Soldadura e Qualidade (ISQ), Lisboa, Portugal, INASMET, Centro Technologico de Materiales, San Sebastian, Spain, and the Research Centre of the Belgian Welding Institute (BWI), Gent, Belgium.

To incorporate the effects of weld metal YS mismatch (over- / undermatching) girth welds were made in each grade by conventional stick electrode welding with consumables of different strength categories, including cellulosae (E 6010, E 701 OG and E 901 OG) and basic (E 10018G) coated electrodes. Portions of the welds incorporated intentionally introduced defects typical of SMAW welding, i.e. porosity and slag inclusions. Their linear extent was such that they exceeded current workmanship based defect acceptance levels.

The welds were non-destructively inspected by conventional radiography (X-ray) and automated ultrasonics (P-scan). Though both techniques have confirmed the presence of out-of-specification defects, X-ray performed better than P-scan for the detection of small and scattered volumetric defects, such as isolated gas pores. To detect such defects, excessively high magnifications are required which might lead to false (non-significant) indications due to noise.

The welds were subjected to mechanical testing to quantify, in the as-welded condition, their hardness, tensile, Charpy V toughness and CTOD toughness properties. The tests have shown that conventional manual welding procedures can be applied with confidence to produce high-quality girth welds in X 70 and X 80 pipes. Weldability problems, such as poor HAZ toughness or high HAZ hardness, are not to be expected, provided adequate preheating is applied.

Tensile testing has shown that the target levels of weld metal YS mismatch were not achieved. In particular, a situation of undermatching was not formally obtained. This was attributed to the fact that the pipes had yield strengths towards the lower end of the distributions for X 70 and X 80 grades. However, in view of the inherent scatter of the tensile test data, a finite probability of occurrence of weld metal YS undermatching was identified for both pipe grades.

Charpy testing involved the establishment of brittle-to-ductile transition curves, which were subsequently used to define the transition temperatures corresponding with Charpy energies of 40 J (mean) / 30 J (lowest individual) and to select the test temperatures for CTOD and wide plate testing. These were selected such as to produce weld metal notch toughness levels similar to those proposed in the European Pipeline Research Group (EPRG) guidelines.

CTOD toughness testing has demonstrated that, owing to the lack of triaxial crack tip constraint, both "standard" specimens (either through-thickness notched 2B or surface notched ) and specimens with an "alternative" geometry (surface notched 3B testpieces) fail to correctly characterize the fracture behaviour of welds in thin walled pipe : lower bound weld metal CTOD values of the order of 0,12-0,15 mm were measured at maximum load (plastic collapse). Since fitness-for-purpose (Engineering Critical Assessment - ECA) methodologies base the calculation of tolerable defect size on the CTOD toughness and since, moreover, residual stresses of yield point magnitude are to be included as secondary stresses (the YS of X 80 steel is 60 % higher than the YS of normalized CMn steels), calculated defect tolerance levels might be unduly restrictive for high strength pipeline girth welds. Further, the ECA methodo-logies do not take into account the obvious benefits of weld metal YS overmatching. Therefore, the CTOD approach is, in its present form, not suitable to predict the fracture behaviour of defective girth welds in thin walled pipe.

Wide plate testing has shown that girth welds containing gross (out-of-specification) volumetric defects could not be brought to fracture, even when tensile tested at -50 C. Instead, failure was through the onset of necking in the pipe body at stresses approaching the pipe metal tensile strength. Therefore, workmanship based defect tolerance criteria might be too conservative.

The wide plate tests have demonstrated that the fracture behaviour of welds made with cellulosic and basic electrodes differ significantly. For the cellulosic welds, failure was by unstable fracture with only minor tearing, whereas for the basic welds failure was preceded by impressive ductile tearing, yielding, in some cases, a stable pop-through. Since basic electrodes produce welds with a high fracture initiation resistance, it is expected that this type of consumables will overrule the traditional use of cellulosic electrodes for onshore pipeline welding of high strength steel pipes.

None of the welds, produced with cellulosic electrodes in X 70 pipes and incorporating surface notches of up to 180 mm long by 3,0 mm deep, yielded unstable fracture in the Net Section Yielding deformation mode. The welds, produced with either cellulosic or basic coated electro-des in X 80 pipes and provided with sharp surface notches of maximum 150 mm long by 4,0 mm deep, equally yielded failure (either by unstable fracture or by maximum load instability) after Gross Section Yielding. This behaviour was seen for all four weld metal mismatch levels, indicating that a level of overmatching of 5 % is sufficient to induce Gross Section Yielding.

In general terms, the work has demonstrated that the EPRG Tier 2 defect limits, set forth for girth welds in pipes up to X 70, can be applied with confidence for girth welds in higher strength (X 80) steel pipes. The wide plates provided with planar surface breaking root defects of 3,0 mm deep and with a length equal to 7 times the wall thickness invariably produced Gross Section Yielding prior to failure. This implies that the EPRG guidelines yield a conservative upper limit to defect acceptance for girth welds in high strength (up to X 80) steel pipes.

The adequate fracture behaviour, as evidenced by the wide plate tests, also illustrates that current defect assessment procedures are over-conservative for (as-welded) high strength steel pipelines. In particular, the treatment of residual stresses as secondary stresses of yield point magnitude and the fact that the benefits of weld metal YS overmatching are completely ignored drastically reduce tolerable defect sizes predicted by current ECA methodologies.

THE FRACTURE BEHAVIOUR OF GIRTH WELDS

IN HIGH STRENGTH HIGH YS/TS RATIO LINEPIPE STEELS

TABLE OF CONTENTS

1 INTRODUCTION 19

1.1 Backgrounds 19 1.2 Objectives of the research programme 21

2 TEST MATERIALS, EXTENT OF TESTING AND ALLOCATION OF WORK 21

2.1 Test materials 22 2.2 Extent of testing 22

2.2.1 Non-destructive inspection (NDT) of the girth welds 22 2.2.2 Characterisation of the properties of the pipe metals and girth welds 23 2.2.3 Experimental verification - wide plate tensile testing of curved pipe sections 23

2.3 Allocation of work 24

3 PIPE MATERIALS AND GIRTH WELDING 25

3.1 Pipe materials 25 3.2 Girth welding 25

3.2.1 Welding of the API 5L X 70 pipes 25 3.2.2 Welding of the API 5LX 80 pipes 26

4 NON-DESTRUCTIVE TESTING (NDT) OF THE GIRTH WELDS 27

4.1 Radiographic (X-ray) inspection 27 4.2 Ultrasonic (P-scan) inspection 28

4.2.1 General features of the P-scan inspection equipment 28 4.2.2 Ultrasonic inspection results 29 4.2.3 Comparison between radiographic and ultrasonic (P-scan) inspection results 29

5 PIPE METAL, WELD METAL AND HEAT AFFECTED ZONE CHARAC-TERISATION TESTING (SMALL-SCALE MECHANICAL TESTING) 30

5.1 Characterisation testing of the pipe materials 30 5.1.1 Chemical analyses of the pipe metals 30 5.1.2 Optical microscopic examination and Vickers HV 5 hardness testing 30 5.1.3 Pipe metal tensile testing in the longitudinal (pipe axis) direction 31

5.2 Characterisation testing of the deposited weld metals and heat affected zones of the girth welds 31

5.2.1 Chemical analyses of the weld deposits 31 5.2.2 Micro- and macrographic examinations and Vickers HV 5 hardness testing 32 5.2.3 All-weld metal and transverse (cross-weld) tensile testing of the girth welds 33

5.2.3.1 All-weld metal tensile testing 33 5.2.3.2 Transverse (cross-weld) tensile testing 33 5.2.3.3 Levels of weld metal yield strength mismatch 34

5.2.4 Charpy V notch impact testing of the girth welds 35 5.2.4.1 Extent of testing 35 5.2.4.2 Test results 36 5.2.4.3 Discussion and interpretation of the notch toughness test data 36

6 CTOD FRACTURE TOUGHNESS TESTING OF THE GIRTH WELDS BY MEANS OF STANDARD AND ALTERNATIVE SPECIMEN GEOMETRIES 3 8

6.1 Extent of testing and experimental procedures 38 6.2 Test results 39 6.3 Discussion of the weld metal CTOD toughness test data 39

6.3.1 CTOD toughness of the girth welds in API 5L Grade X 70 pipes 39 6.3.2 CTOD toughness of the girth welds in API 5L Grade X 80 pipes 40

6.4 Effect of lateral constraint (triaxiality) on weld metal CTOD toughness 41

7 WIDE PLATE TESTING OF CURVED PIPE SECTIONS EXTRACTED FROM THE GIRTH WELDS - EXPERIMENTAL VERIFICATION 42

7.1 Introduction 42 7.2 Experimental procedures and testing details 43 7.3 Wide plate test results 44

7.3.1 Detailed presentation of the wide plate test data 44 7.3.2 Summary of the wide plate test data 45

7.4 General discussion of the wide plate test data 46 7.4.1 Interpretation of the wide plate test performances in terms of the Gross Section

Yielding (GSY) concept 47 7.4.2 Fracture behaviour of girth welds incorporating intentionally introduced weld defects 48

7.4.3 Assessment of the fracture behaviour of the girth welds in the X 70 pipes 4 y

7.4.3.1 Wide plate tensile testing at -30 C of "overmatching" girth welds (E 7010G / E 901 OG electrodes) in X 70 pipes

7.4.3.2 Wide plate tensile testing at -20 C of "undermatching" girth welds (E 6010 / E 6010 electrodes) in X 70 pipes

7.4.4 Assessment of the fracture behaviour of the girth welds in the X 80 pipes 7.4.4.1 Wide plate tensile testing at -30 C of "overmatching" girth welds (E 6010G / E

10018G electrodes) in X 80 pipes 7.4.4.2 Wide plate tensile testing at -20 C of "undermatching" girth welds (E 6010 / E

9010G electrodes) in X 80 pipes 7.5 Summary and conclusions

8 OVERALL CONCLUSIONS

9 REFERENCES

TABLES 1 TO 16

FIGURES 1 TO 7

ANNEXES I TO VI

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50 51

51

52 53

55

59

65

91

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LIST OF TABLES

Table 1 .a Chemical composition (in weight %) of the API 5L Grade X 70 pipe material.

Table 1 .b Chemical composition (in weight %) of the API 5L Grade X 80 pipe material.

Table 2.a Details of girth welding of the API 5L Grade X 70 pipes.

Table 2.b Details of girth welding of the API 5L Grade X 80 pipes.

Table 3.a Results of pipe metal tensile testing in the longitudinal (pipe axis) direction of the API 5L Grade X 70 pipe material.

Table 3.b Results of pipe metal tensile testing in the longitudinal (pipe axis) direction of the API 5L Grade X 80 pipe material.

Table 4.a Chemical composition (in weight %) of the deposited weld metals of the girth welds in the API 5L Grade X 70 pipes.

Table 4.b Chemical composition (in weight %) of the deposited weld metals of the girth welds in the API 5L Grade X 80 pipes.

Table 5.a Results of all-weld metal tensile testing of the "overmatching" and "undermatching" girth welds in the API 5L Grade X 70 pipes.

Table 5.b Results of all-weld metal tensile testing of the "overmatching" and "undermatching" girth welds in the API 5L Grade X 80 pipes.

Table 6.a Results of transverse (cross weld) tensile testing of the "overmatching" and "undermatching" girth welds in the API 5L Grade X 70 pipes.

Table 6.b Results of transverse (cross weld) tensile testing of the "overmatching" and "undermatching" girth welds in the API 5L Grade X 80 pipes.

Table 7.a Results of Charpy V notch impact testing (transition curves) of the "overmatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 7010G / E 901 OG)

Table 7.b Results of Charpy V notch impact testing (transition curves) of the "undermatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 6010 / E 6010)

i l

Table 8.a Results of Charpy V notch impact testing (transition curves) of the "overmatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 10018G)

Table 8.b Results of Charpy V notch impact testing (transition curves) of the "undermatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 901 OG)

Table 9.a Summary of data of CTOD fracture toughness testing at -30 C of the "overmatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 7010G / E 901 OG)

Table 9.b Summary of data of CTOD fracture toughness testing at -20 C of the "undermatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 6010 / E 6010)

Table 10.a Summary of data of CTOD fracture toughness testing at -30 C of the "overmatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 10018G)

Table 10.b Summary of data of CTOD fracture toughness testing at -20 C of the "undermatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 901 OG)

Table 11 Summary of data of wide plate tensile testing of curved pipe sections incorporating machined surface notches in the weld root.

Pipe grade: API 5LX 70 "Overmatching" girth welds Welding consumables : E 7010G / E 901 OG

Table 12 Summary of data of wide plate tensile testing of curved pipe sections incorporating machined surface notches in the weld root.

Pipe grade : API 5L X 70 "Undermatching" girth welds Welding consumables : E 6010 / E 6010

Table 13 Summary of data of wide plate tensile testing of curved pipe sections incorporating intentionally introduced weld defects.

Pipe grade: API 5LX 70 "Undermatching" girth welds Welding consumables : E 6010 / E 6010

Table 14 Summary of data of wide plate tensile testing of curved pipe sections incorporating machined surface notches in the weld root.

Pipe grade : API 5L X 80 "Overmatching" girth welds Welding consumables : E 6010 / E 10018G

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Table 15 Summary of data of wide plate tensile testing of curved pipe sections incorporating intentionally introduced weld defects.

Pipe grade : API 5L X 80 "Overmatching" girth welds Welding consumables : E 6010/ E 10018 G

Table 16 Summary of data of wide plate tensile testing of curved pipe sections incorporating machined surface notches in the weld root.

Pipe grade : API 5L X 80 "Undermatching" girth welds Welding consumables : E 6010 / E 901 OG

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LIST OF FIGURES

Figure 1 .a Summary of data (transition curves) of Charpy V notch impact testing of the weld metal centreline (cap and root) of the "overmatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 701 OG / E 901 OG)

Figure 1 .b Summary of data (transition curves) of Charpy V notch impact testing of the weld metal centreline (cap and root) of the "undermatching" girth welds in the API 5L Grade X 70 pipes. (Welding consumables : E 6010 / E 6010)

Figure 2.a Summary of data (transition curves) of Charpy V notch impact testing of the weld metal centreline (cap and root) of the "overmatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 10018G)

Figure 2.b Summary of data (transition curves) of Charpy V notch impact testing of the weld metal centreline (cap and root) of the "undermatching" girth welds in the API 5L Grade X 80 pipes. (Welding consumables : E 6010 / E 9010G)

Figure 3.a Summary of data of CTOD fracture toughness testing at -30 C of the weld metal of the "overmatching" girth welds in the API 5L Grade X 70 pipes by means of "standard" and "alternative" specimen geometries. (Welding consumables : E 7010G / E 901 OG)

Figure 3.b Summary of data of CTOD fracture toughness testing at -20 C of the weld metal of the "undermatching" girth welds in the API 5L Grade X 70 pipes by means of "standard" and "alternative" specimen geometries. (Welding consumables : E 6010 / E 6010)

Figure 4.a Summary of data of CTOD fracture toughness testing at -30 C of the weld metal of the "overmatching" girth welds in the API 5L Grade X 80 pipes by means of through-thickness ( 2B) and surface notched ( ) specimens. (Welding consumables : E 6010 / E 10018G)

Figure 4.b Summary of data of CTOD fracture toughness testing at -20 C of the weld metal of the "undermatching" girth welds in the API 5L Grade X 80 pipes by means of through-thickness ( 2B) and surface notched ( ) specimens. (Welding consumables : E 6010 / E 901 OG)

Figure 5 General view of a curved wide plate test panel, photographed upon completion of testing and illustrating the geometry and the instrumentation (moir grid and elongation measuring devices) applied.

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Figure 6.a Summary of data of wide plate tensile testing -20 / -30 C of curved pipe sections extracted from the girth welds in the API 5L Grade X 70 pipes and incorporating machined surface notches in the weld root : Diagramme of the gross failure stress as a function of defect length (defect depth : 3,0 mm).

Figure 6.b Summary of data of wide plate tensile testing -20 / -30 C of curved pipe sections extracted from the girth welds in the API 5L Grade X 70 pipes and incorporating machined surface notches in the weld root : Diagramme of the "corrected" (pipe metal) gross failure strain as a function of defect length (defect depth : 3,0 mm).

Figure 7.a Summary of data of wide plate tensile testing -20 / -30 C of curved pipe sections extracted from the girth welds in the API 5L Grade X 80 pipes and incorporating machined surface notches in the weld root : Diagramme of the gross failure stress as a function of defect length (defect depths : 3,0 and 4,0 mm).

Figure 7.b Summary of data of wide plate tensile testing -20 / -30 C of curved pipe sections extracted from the girth welds in the API 5L Grade X 80 pipes and incorporating machined surface notches in the weld root : Diagramme of the "corrected" (pipe metal) gross failure strain as a function of defect length (defect depths : 3,0 and 4,0 mm).

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LIST OF APPENDICES

Annexe I Selection of microphotographs : Microstructures of the API 5L Grade X 70 & X 80 pipe metals and microstructures of the deposited weld metals of the girth welds made in each of the pipes. (7 pages)

Annexe Summary of data of the non-destructive inspection (radiographic (X-ray) and ultrasonic (P-scan)) of the defective girth welds. (17 pages)

Annexe HI Macrographic examination and Vickers HV 5 hardness testing of the girth welds in the API 5L Grade X 70 & X 80 pipes. (13 pages)

Annexe TV Detailed results of CTOD fracture toughness testing at -20/-30 C of the weld metal centreline (WMC) of the girth welds in the API 5L Grade X 70 & X 80 pipes by means of "standard" and "alternative" specimen geometries. (9 pages)

Annexe V Detailed results of wide plate tensile testing of "overmatching" and "undermatching" girth welds in API 5L Grade X 70 pipes (40" O.D. 16,9 mm W.T.) (20 pages)

Annexe VI Detailed results of wide plate tensile testing of "overmatching" and "undermatching" girth welds in API 5L Grade X 80 pipes (44" O.D. 16,2 mm W.T.) (13 pages)

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THE FRACTURE BEHAVIOUR OF GIRTH WELDS IN HIGH STRENGTH HIGH YS/TS RATIO LINEPIPE STEELS

1 INTRODUCTION

1.1 Backgrounds

Improvements in steel making practice and rolling techniques have led to the development of low to very low carbon, micro alloyed steels with increased yield and tensile strength, adequate notch toughness and improved weldability (as reflected by their low CE and Pcm values). This trend was seen both for structural steels and for steels for gas transmission linepipes. These developments have, however, led to steels which are characterized by higher yield-to-tensile strength (YS/TS) ratios as compared with their more conventional (normalized) counterparts.

Owing to their low strain hardening capacity, steels with a high YS/TS ratio have reduced capabilities to redistribute plastic deformations which might occur in the vicinity of defects in welded joints in such steels when these are subjected to incidental overloads. There are strong indications that this leads to lower defect tolerance levels as compared with more conventional (normalized) steels. This implies that pipeline girth welds in high YS/TS ratio steels could be less safe in terms of structural integrity, because their failure characteristics depend no longer on the strain hardening capacity but solely on toughness.

In addition to this, the deformation capacity and failure behaviour of welded pipelines depend also on the level of mismatch of the weld metal relative to the pipe material. With increasing pipe metal yield strengths, it becomes increasingly difficult to achieve adequate weld metal yield strength overmatching. The possibility of weld metal matching, or even undermatching, can thus not be excluded. In such situations, one does not longer benefit from the shielding effect of overmatching when defects / flaws located in the girth weld or heat affected zone (HAZ) are subjected to high tensile or bending loads. This in turn reduces defect tolerance levels.

Because of the lack of service data and documentation (experience gained from pipelines with conventional YS/TS ratios is no longer applicable), potential users of high YS/TS ratio pipeline steels are somewhat reluctant to modify their specifications. Service related laboratory tests are therefore urgently needed to provide information on the deformation and failure characteristics of "defective" girth welds (incorporating either intentionally introduced weld defects or machined and/or fatigue precracked surface notches) in modern micro alloyed high strength pipeline steels.

A second important aspect to consider is that the integrity of girth welds in gas transmission pipelines is ensured by non-destructive inspection procedures. Advanced non-destructive

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techniques (such as e.g. ultrasonic P-scan, TOFD (time of flight diffraction),...) with increased sensitivity are currently available, which opens new perspectives. Defect acceptance / rejection levels are commonly being based on workmanship quality control procedures. Workmanship defect acceptance levels are, however, based on experience and are, by necessity, both arbitrary and in most instances unduly conservative.

An alternative approach to defect acceptance in pipeline girth welds is based on fitness-for-purpose (ECA - Engineering Critical Assessment) concepts. Modern pipeline codes such as API 1104, BS 4515 and CSA ZI 83 contain a non-mandatory Appendix giving specific procedures for the derivation of tolerable defect sizes. These approaches are based upon optimized analyses ensuring that the girth weld defect will not lead to failure either by brittle fracture or by plastic collapse. For the fracture assessment, a CTOD based design curve is applied to ensure that brittle fracture will not occur for a given applied stress and critical CTOD toughness. The plastic collapse assessment uses a notional flow stress to prevent failure by yielding of the ligament underneath the defect.

Defect assessments based on fitness-for-purpose principles are subjected to much controversy. The different input parameters (i.e. applied stress, fracture toughness, residual stresses,...) and safety factors suggested in the published codes produce significantly different defect acceptance levels. In particular, for the situation of low fracture toughness, the calculated defect sizes might be in conflict with the workmanship defect acceptance levels. Moreover, current defect assess-ment procedures do not account for the effects of weld metal yield strength mismatch on girth weld performance.

An alternative, and much more straightforward, approach for girth weld defect acceptance / rejection has recently been developed by the European Pipeline Research Group (EPRG). Tier 2 of the EPRG Guidelines specifies that, for girth welds made in linepipe steels up to Grade X 70 (SMYS < 482 MPa) and with a YS/TS ratio of maximum 0,85, planar surface breaking defects of 3,0 mm deep (i.e. the height of one weld bead) and with a length equal to 7 times the nominal wall thickness are acceptable, provided the weld metal possesses a Charpy V notch impact energy at the minimum design / operating temperature of 40 Joules (mean) / 30 Joules (minimum).

Hence, experimental work was clearly needed to elucidate some of the outstanding questions rela-ted to workmanship and fitness-for-purpose based girth weld defect acceptance criteria and to verify the degree of conservatism of existing codes and standards. In particular, the effects of higher yield strength (above X 70) and yield-to-tensile ratio (above 0,85) linepipe steels on girth weld performance needed to be addressed.

Upon initiating the project, it was anticipated that the best approach was to conduct a series of wide plate tensile tests on girth welds containing either intentionally introduced workmanship type defects or machined and/or fatigue precracked surface notches. Such experimental information is needed to verify whether existing workmanship criteria and/or fitness-for-purpose (based on CTOD toughness test data) defect assessment criteria are applicable to defective girth welds in high strength high YS/TS ratio steel linepipes without being either unsafe or unduly conservative.

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1.2 Objectives of the research programme

By taking into account the above considerations, the objectives of the research programme were as follows :

(1) To identify the effect of those factors affecting failure behaviour of defective girth welds in large diameter high strength steel linepipes, i.e. Charpy V toughness and/or CTOD fracture toughness, level of weld metal yield strength mismatch and yield-to-tensile ratio, and to investigate to what extent these factors interact.

(2) To produce experimental data which can serve as a basis for simplifying and rationalizing current workmanship and fitness-for-purpose defect acceptance criteria for girth weld defects in high strength steel pipes. In particular, it was aimed at verifying whether the EPRG guidelines are also applicable to higher pipe grades.

The sub-objectives of the research programme were as follows :

(1) To verify whether welding procedures (manual (stick electrode) welding) and consumables currently being applied for pipeline girth welding of steel grades up to X 70 are also suitable for welding of higher grades.

(2) To demonstrate the potentials of advanced non-destructive testing (NDT) techniques, such as e.g. the ultrasonic P-scan technique, and to determine their detection limits.

(3) To determine the mechanical properties, in terms of hardness, tensile and (Charpy V and CTOD) toughness, of the deposited weld metals and heat affected zones of girth welds in high strength steel pipes.

(4) To develop new testing techniques to determine the weld metal CTOD fracture toughness with increased lateral constraint.

(5) To quantify, in terms of tolerable defect size, the effects of weld metal strength mismatch, CTOD toughness and YS/TS ratio of the pipe material on the failure characteristics of defective girth welds.

2 TEST MATERIALS, EXTENT OF TESTING AND ALLOCATION OF WORK

In order to realize the objectives and sub-objectives listed above, experimental work was conducted by each of the laboratories involved in the project, i.e. Instituto de Soldadura e Quali-dade (ISQ), Lisboa, Portugal (main contractor), INASMET, Centro Technologico de Materiales, San Sebastian, Spain (subcontractor), and the Research Centre of the Belgian Welding Institute (BWI), c/o Laboratory Soete, Universiteit Gent, Belgium (subcontractor). For convenience, the test materials, extent of testing and allocation of experimental work between the partners is briefly described below.

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2.1 Test materials

The experimental work was conducted on large diameter high strength steel linepipes of API 5L X 70 / X 80 quality. In order to properly incorporate the effects of the yield strength levels of the pipe material on girth weld performance, two pipe grades were selected, i.e. conforming either to API 5L Grade X 70 (SMYS of 482 MPa) or to Grade X 80 (SMYS of 551 MPa). Their nominal dimensions were as follows :

Grade X 70 : outer diameter (O.D.) of 40" (1.016,0 mm) and wall thickness (W.T.) of 16,9 mm Grade X 80 : outer diameter (O.D.) of 44" (1.117,6 mm) and wall thickness (W.T.) of 16,2 mm

A series of girth welds was made in each of the pipes (6 welds in X 70 pipe and 3 welds in X 80 pipe) by conventional stick electrode (shielded metal arc SMAW) welding. In order to incorporate the effect of weld metal yield strength mismatch, welds were made with different filler metals, including both cellulosic (types E 6010, E 701 OG and E 901 OG) and basic (type E 10018G) coated electrodes. Since girth welds were made in both pipe grades (X 70 and X 80), it was expected that this would result in four distinct weld metal yield strength mismatch levels (under and overmatching).

Both "sound" (virtually defect free) and "defective" (incorporating intentionally introduced defects) girth welds were produced. The "sound" welds were reserved weld metal characterisation testing. The "defective" welds contained deliberately introduced nonplanar (volumetric) defects typical of the manual SMAW process, i.e. porosity and slag inclusions. The linear extent (length) of the defects / flaws was aimed at exceeding the current workmanship limits (based on radiographic inspection) by a factor of approximately two. In other words, the girth welds contained defects which would have required repair on the basis of the currently used standards.

2.2 Extent of testing

2.2.1 Nondestructive inspection (NDT) of the girth welds

Upon completion of welding, the girth welds were radiographically (Xray) inspected by a certified inspection authority ( VINOTTE, Brussels, Belgium). This has confirmed the presence of porosities and slag inclusions over considerable lengths, as planned. In addition, the welds were ultrasonically (US) inspected by ISQ using a computerised ultrasonic (Pscan) technique. This inspection was aimed at detecting the defects, at defining their nature (type) and location (around the pipe circumference) and at accurately sizing their dimensions (length and height).

The findings of the NDT inspection were used as input data for extracting curved pipe sections for wide plate tensile testing : these were taken out at those locations where the defect incidence was the highest.

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2.2.2 Characterisation of the properties of the pipe metals and girth welds

Small-scale mechanical (destructive) tests were conducted by both ISQ and INASMET to determine the hardness, tensile, Charpy V toughness and CTOD toughness properties of the girth welds. Each of the "sound" welds (four distinct strength categories) was subjected to virtually the same test matrix and a sufficient number of repeat tests was performed to account for the variability in properties with the sampling position around the circumference.

A condensed schedule of the experimental work is presented hereinafter :

- Tensile testing of the pipe materials, aimed at quantifying the actual strength properties in the longitudinal (axial) direction.

- All-weld metal tensile testing of the girth welds, aimed at quantifying the actual strength properties of the weld metals and, in conjunction with the pipe metal tensile tests, the actual levels of weld metal yield strength mismatch.

- Macrographic examinations and Vickers HV 5 hardness testing (cap and root side). The variables included were : pipe grade, weld metal strength level and sampling position around the circumference.

- Charpy V notch impact testing of the weld metals and HAZ's of the girth welds :

Brittle-to-ductile transition curves were established for the weld metals and heat affected zones (HAZ's). The variables included were : pipe grade, weld metal strength level, through-thickness sampling location (cap versus root), notch position (WMC and HAZ), sampling position around the circumference and test temperature. The transition temperatures corresponding with Charpy V impact energies of 40 J (mean) / 30 J (lowest individual) were subsequently determined and selected as test temperatures for CTOD and wide plate tensile testing.

- CTOD fracture toughness testing of the girth welds :

The CTOD toughness properties of the weld metals were determined by testing of 2B testpieces through-thickness notched in the weld metal (WMC) and testpieces surface notched in the weld metal from the root side. The variables incorporated were : pipe grade, weld metal strength level, sampling position around the circumference and specimen geometry (crack orientation). In addition, the effect of lateral constraint on CTOD test performance was evaluated by testing of specimens with an "alternative" geometry, i.e. surface notched testpieces width a width equal to 3B.

2.2.3 Experimental verification - wide plate tensile testing of curved pipe sections

Upon initiating this project, defective girth welds in high strength X 70 / X 80 linepipes had not yet been tested in terms of workmanship weld quality. Since girth welds are predominantly

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subjected to bending and tensile loading in the axial direction, wide plate tensile tests on curved pipe sections extracted and tensile loaded in the axial direction are the most representative tests to evaluate full-scale girth weld performance. The curved wide plates were tested at temperatures significantly below the minimum design / operating temperature of gas transmission pipelines. The test temperatures were selected on the basis of Charpy testing (40 J (mean) / 30 J (minimum) requirement).

It was initially planned to extract the curved wide plates (width : 330 mm (arc length)) at those locations where the X-ray and ultrasonic (P-scan) inspection had revealed the highest defect inci-dence. These would be tensile loaded to failure, which would have allowed to accurately measure defect size (length and height) and to compare these with the findings of the NDT inspection. The initial wide plate tests showed that the linear extent of the (non-planar) defects, though exceeding the workmanship based acceptance limits, was too small to produce specimen failure. Therefore, it was decided, for the vast majority of wide plate tests, to introduce additional machined surface notches of nominally 3,0 mm deep (i.e. the height of one weld bead) and with varying lengths in the girth weld metal from the root (to simulate root cracks). The defect lengths were selected on the basis of the EPRG guidelines (7 times the wall thickness).

The wide plate failure stresses were compared against the yield strength of the pipe materials, i.e. it was verified whether Gross Section Yielding (pipe yielding in cross sections remote from the defective girth weld) preceded failure.

By adhering to this procedure, it was believed that the experimental information would be of considerable value to assess the significance of workmanship type defects which would have been rejected because they exceed the acceptance limits. Further, the information generated was believed to be useful to critically assess current assessment procedures for girth weld defects in high strength pipeline steels.

2.3 Allocation of work

The experimental work, as well as the analysis of the test data, was carried out in a joined effort between the three collaborating laboratories, i.e. Instituto de Soldadura e Qualidade (ISQ), INASMET, Centro Technologico de Materiales, and the Research Centre of the Belgian Welding Institute (BWI).

The overall coordination of the research work was with ISQ, which was also responsible for small-scale mechanical testing (weld metal and HAZ characterisation testing) of the girth welds in API 5L X 80 pipes and for the non-destructive ultrasonic inspection. INASMET have focused their efforts on the characterisation in terms of hardness, tensile, notch and fracture toughness properties of the girth welds in API 5L X 70 pipes. In addition, they have looked into the possibilities of implementing a new testing technique (with increased lateral constraint) for characterizing the CTOD toughness of girth welds in thin-walled pipes. The BWI was responsible for the purchase of the test materials, the coordination of the welding activities and for wide plate tensile testing. The draft final report was prepared by the BWI, in close collaboration with ISQ and INASMET.

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3 PIPE MATERIALS AND GIRTH WELDING

3.1 Pipe materials

The materials selected for the project were large diameter submerged arc (SAW) longitudinally welded steel linepipes conforming to Grades X 70 and X 80 of API 5L.

The API 5L X 70 pipes (SMYS = 482 MPa - SMTS = 565 MPa) were stock pipes, which had been produced by GTS INDUSTRIES (formerly VALLOUREC), Dunkerque, France. They had an outer diameter (O.D.) of 40" (1.016,0 mm) and a wall thickness (W.T.) of 16,9 mm nominally. To account for the variability in pipe supply, the selected pipes, further referred to as Pipes "A", "B" and "C", originated from three Heats. Their chemical compositions are listed in Table La. The pipes have a carbon equivalent CE (UW) of 0,35-0,36 and a Pcm value of 0,18-0,19, thus ensuring an excellent weldability and a low hardenability. The pipe metal Charpy V notch toughness (transverse orientation) exceeded 150 Joules at -20 C. Typical optical microphoto-graphs, illustrating the ferritic-pearlitic microstructure at pipe subsurface (1 mm below the outer wall) and at mid-thickness, are presented in Figure 1.1 of Annexe I.

The API 5L X 80 pipes (SMYS = 551 MPa - SMTS = 620 MPa) had on purpose been produced for this research project by EUROPJPE GmbH (formerly MANNESMANN RHRENWERKE), Mlheim, Germany. They had an outer diameter (O.D.) of 44" (1.117,6 mm) and a wall thickness (W.T.) of 16,2 mm nominally. To account for the variability in pipe supply, the selected pipes originated from two Heats, further referred to as Pipes "D" and "E". Their chemical compositions are listed in Table l.b. The pipes have a carbon equivalent CE (UW) of 0,43-0,44 and a Pcm value of 0,20-0,21, thus ensuring an adequate weldability and a moderate hardenability. The pipe metal notch toughness (transverse orientation) exceeded 140 Joules at -10 C. Typical optical micro-photographs, illustrating the low carbon bainitic-ferritic microstructure at pipe subsurface (1 mm below the outer wall) and at mid-thickness, are presented in Figure 1.4 of Annexe I.

3.2 Girth welding

3.2.1 Welding of the API 5L X 70 pipes

Six girth welds have been produced by shielded metal arc welding (SMAW) in the API 5L X 70 pipes by an experienced onshore pipeline contractor (N.V. DENYS, Wondelgem, Belgium). The girth welds were made by means of conventional downhill (5G) welding with cellulosic coated electrodes, using standard procedures (V-bevel preparation with an included angle of 60 and a 2,5 mm root gap). To enable a proper assessment of the effects of weld metal strength mismatch on girth weld performance, the consumables selected had two distinct strength categories, yielding weld deposits with "undermatching" (E 6010 electrodes for the root, hot, fill and cap passes) and "overmatching" (E 7010G for the root pass, E 9010 for the hot, fill and cap passes) yield strength levels.

For each weld metal strength level (E 6010 / E 6010 ("undermatching") and E 7010G / E 9010G ("overmatching")), three girth welds have been produced, i.e. :

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- one "sound" (i.e. virtually defect free) weld reserved for weld metal and HAZ characteri-sation testing and for wide plate tensile testing. These welds were identified as nos. "W 1" (E 7010G / E 9010G) and "W 2" (E 6010 / E 6010)

- two "defective" (i.e. containing intentionally introduced defects) welds, which were reserved for wide plate testing. The defects, typical of stick electrode pipeline welding, were distributed around the circumference as follows :

- 12-3 o'clock : elongated slag inclusions (in the root and hot pass) and clusters of porosities in one of the fill passes (towards the weld cap)

- 3-6 o'clock : clusters of porosities in two successive fill passes (towards the weld cap) - 6-12 o'clock : defect free ("sound")

The defective "overmatching" welds (E 7010G / E 9010G) were identified as nos. "W 3" and "W 4", the "undermatching" ones (E 6010 / E 6010) as nos. "W 5" and "W 6".

The volumetric (non-planar) weld defects were deliberately introduced by either omitting slag removal by brushing and grinding (elongated slag inclusions) or by disturbing the shielding gas of the molten weld pool (cluster porosity). The target defect lengths were equal to twice the API 1104 (workmanship based) acceptance limits.

As is normal practice in the pipeline industry, the welds were tested in the as-welded condition. For convenience, details on girth welding of the API 5L X 70 pipes are gathered in Table 2.a.

3.2.2 Welding of the API 5LX 80 pipes

Three girth welds have been produced by shielded metal arc welding (SMAW) in the API 5L X 80 pipes by the same onshore pipeline contractor. The welds were made by means of downhill (5G) welding using a V-bevel preparation with an included angle of 60 and a 2,5 mm root gap. Since there is a consensus in the pipeline industry that, in general, cellulosic electrodes fail to meet the requirement of weld metal strength overmatching for X 80 pipe and, moreover, have limited (notch and fracture) toughness, it was decided to focus the experimental work on girth welds made with basic coated electrodes of the E 10018G type (for the fill and cap passes). According to experience, these yield weld deposits whose yield strength exceeds (overmatches) the SMYS of API 5L X 80 pipes. In order to avoid root cracking, the root and hot pass were deposited with cellulosic (type E 6010) electrodes.

Two girth welds were produced with this "overmatching" consumable combination (E 6010 / E10018G), i.e. :

- a "sound" (i.e. virtually defect free) weld, identified as weld no. "W 8", was reserved for weld metal and HAZ characterisation testing and for wide plate tensile testing.

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- a partially "defective" (i.e. containing deliberately introduced volumetric defects) weld, identified as weld no. "W 9", was entirely reserved for wide plate tensile testing. Between the 12 and 6 o'clock position, the weld was provided with elongated slag inclusions and porosities in two subsequent fill passes (immediately following the hot pass), whereas between the 6 and 12 o'clock position, the weld was virtually defect free ("sound"). The linear extent and distribution of the weld defects, as well as the techniques applied to introduce them, were similar to those introduced in the X 70 girth welds.

In addition, a third defect free weld was produced with a consumable combination (cellulosic electrodes) customarily being applied in the pipeline industry for the welding of X 70 pipes (type E 6010 for the root and hot pass and type E 901 OG for the fill and cap passes). These should normally yield weld deposits in API 5L X 80 pipes with slightly undermatching strength properties. This "undermatching" weld was identified as weld no. "W 7".

As for the X 70 pipes, all testing was done in the as-welded condition. For convenience, details on girth welding of the API 5L X 80 pipes are gathered in Table 2.b.

4 NON-DESTRUCTIVE TESTING (NDT) OF THE GIRTH WELDS

Upon completion of welding, all nine girth welds have been non-destructively tested. Apart from conventional radiography (X-ray), an advanced ultrasonic inspection technique (P-scan) has been applied to inspect the "defective" welds.

4.1 Radiographic (X-ray) inspection

This activity has been subcontracted to a certified inspection authority, i.e. Affi - VINOTTE, Brussels, Belgium. It is outside the scope of this report to present detailed information. It suffices to mention that the radiographic procedures (panoramic exposure using a G 2 (DIN) film), conformed to Section 8.0, whereas defect acceptance / rejection was based on Section 6.0 of API 1104 (note that API 1104 defect acceptance is entirely based on workmanship rules).

It should further be noted that, apart from the interpretation (in terms of defect acceptability) of Affi - VINOTTE, the radiographs have been interpreted by an independent (third party) expert of ISQ. This included a detailed "mapping" of the distribution of all defects along the circum-ference. These mapping charts were used to select the sampling (o'clock) positions at which the curved wide plate test panels were to be extracted.

The analyses have shown that, as planned, the "defective" welds effectively contained (spherical and cluster) porosity and - to a lesser extent - elongated slag inclusions. The linear extent and distribution of a number of these defects were such that, when the acceptability limits of API 1104 are strictly applied, the girth welds should either have been rejected or repaired.

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4.2 Ultrasonic (P-scan) inspection

4.2.1 General features of the P-scan inspection equipment

The P-Scan, which means Projection View, is an ultrasonic system that produces an image of the reflectors of the ultrasonic beam (i.e. the defects) in a three-dimensional presentation, i.e. C-scan, B-scan and D-scan, corresponding with Top, End and Side views (see Figure . 1 of Annexe ).

All data is recorded by the main unit (PSP-3), into which the essential information, such as probe characteristics and test parameters, are previously introduced. During testing, it is possible to observe a draft real time image in a B&W presentation. This is useful to have a qualitative idea of the type(s) of reflectors present and, eventually, to correct the parameters in order to optimize the final inspection results.

The inspection is performed using an automated scanner (AWS-6), that holds and moves two probes over the pipe and weld area. The probes are fitted face-to-face in the scanner at either side of the girth weld. The ultrasound echoes are automatically recorded and related to the position (coordinates) of the probes. This information allows, together with the test parameters, such as probe angle (60 or 70), probe delay and sound velocity, to determine the position (in three dimensions) of each reflector (discontinuity or defect).

The recorded data is processed and can be printed using a specific software package (PC-PROG), which allows to produce a recorded image of the reflectors, similar to the ones presented in Figures .3 to .8 of Annexe . Each recorded file corresponds with an inspected weld length of 125 mm. It is to be noted that, in some of the files, some recorded images were deleted, in order to avoid the presentation of noise and non-relevant reflectors, such as e.g. the weld root geometry. This judgement is based on the inspector's experience and knowledge.

The software allows to print the Top, End and Side views along with the echo height (amplitude), which is referred to as Echo view and which is similar to an -scan presentation. The printed images are coloured, using four colours (red, yellow, blue and black) which are linked with a particular sound path and a particular probe (see Figure .2 of Annexe ). The red and yellow colour codes mean that the reflectors were detected with probe no. 1, whereas the blue and black colour codes indicate that they were detected with probe no. 2. For each probe, the two colours are linked with a particular sound path (see Figure .2 of Annexe ). The red corresponds to a sound path on the first " 1/2 V path" and selects a range of 20 to 98 % of the thickness, while the yellow corresponds to the second "1/2 V path" and uses a range of 98 to 200 % of the thickness.

The images also include other relevant information (see Figures .3 to .8 of Annexe ). The first set of numbers at the top of the C-scan image are linked with the start and finish of a particular file, corresponding with the length of the inspected zone. Further, the coordinates of the cursor location are also indicated.

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4.2.2 Ultrasonic inspection results

It is beyond the scope of this report to present all detailed information. Therefore, only some typical examples, highlighting the potentials and drawbacks of the system, are presented (see Annexe ).

Prior to the ultrasonic inspection of the project girth welds, a pipe weld of similar geometry was produced for calibration purposes, i.e. to set up and calibrate the P-Scan inspection conditions. The results of the X-ray and US inspection of this weld are presented in Table . 1 of Annexe .

The same information is presented in Tables .2 and .3 of Annexe for the "defective" girth welds nos. W 5 and W 6 (both produced in API 5L X 70 pipe with an E 6010 / E 6010 consumable combination) respectively. Similar information is presented in Tables .4 and .5 of Annexe for girth welds nos. W 7 and W 9 (produced in API 5L X 80 pipe with either an E 6010 / E 9010G or an E 6010 / E 10018G consumable combination).

In Figures .3 to .8 of Annexe a set of typical P-scan images is presented, corresponding to scannings with either 70 and 60 angle probes. It was found that some defects are only detected by one of the angle probes. Hence, to have a more clear picture of the existing defects, it is always advisable to perform scannings with different angle probes.

Tables .2 to .5 of Annexe allow to compare the findings of the X-ray and ultrasonic (P-Scan) inspection. In general, the results agree reasonably well, with the exception of small and scattered porosities, which are better detected by radiography than by ultrasonic P-scan. However, this is not a true lack of detection. To detect small rounded defects, excessively high amplifications are needed, resulting in an increase in noise and signals related to geometric details (such as the root reinforcement). This yields a lack of resolution and, as a consequence, a loss of clarity between relevant and non-relevant indications. Another aspect is the length of the detected defects, which is normally higher on the X-ray film than observed on the P-scan printing. This can be explained by the higher amplifications needed to detect small defects existing at the ends of porosity clusters.

4.2.3 Comparison between radiographic and ultrasonic (P-scan) inspection results

The comparison between X-ray and P-scan inspection allows to conclude as follows :

- Small spherical defects, like scattered porosity, are difficult to detect by ultrasonics. However, average size pores or clusters can be detected by using amplifications higher than the reference level. In this work, a 1,5 mm disk shaped reference reflector was used (1,5 mm DGS).

- For the detection of small rounded and scattered defects, like isolated gas pores, radiography performs better than ultrasonics. The detection by ultrasonics requires excessively high magnifications which might lead to false or an increased number of indications due to noise.

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Pscan gives a threedimensional view presentation of the porosity, but not the actual size. Hence, there is no true correlation between the radiographic and ultrasonic images.

Ultrasonic Pscan inspection has shown good detectibility for planar defects for a reference level of 1,5 mm DGS, as well as an accurate sizing of the defects that can be used for fitnessforpurpose assessments.

It is advisable to test with two different angular probes. This leads to an increase of inspection time but increases also the ultrasonic detectibility of small defects.

5 PIPE METAL, WELD METAL AND HEAT AFFECTED ZONE CHARAC

TERISATION TESTING (SMALLSCALE MECHANICAL TESTING)

5.1 Characterisation testing of the pipe materials

5.1.1 Chemical analyses of the pipe metals The chemical compositions (product analyses) of the pipe materials, including their carbon equivalents CE (UW) and their Pcm values, are listed in Tables 1 .a and 1 .b for the API 5L X 70 and X 80 pipes respectively. As a reference, the heat analyses of the pipes (taken from the pipe mill certificates) are also included.

The X 70 pipe material has a low C content (0,08 %), a CE (W) carbon equivalent of 0,350,36 and a Pcm value of 0,180,19, thus ensuring an excellent weldability and a low hardenability. The X 80 pipe material has also a low C content (0,08 %) but the content in alloying elements (Mn, Cr, Ni, Mo) is significantly higher. Its carbon equivalent CE (UW) is 0,430,44 and its Pcm value is 0,200,21, thus ensuring an adequate weldability and a moderate hardenability. Both pipeline steels are very clean, with S and contents of below 0,0015 and 0,015 % respectively.

5.1.2 Optical microscopic examination and Vickers HV 5 hardness testing

Optical microphotographs of the X 70 pipe material, illustrating the ferriticpearlitic microstructure at pipe subsurface (1 mm below the outer wall) and at midthickness, are presented in Figure 1.1 of Annexe I. The average (through the wall thickness) Vickers hardness was 193 HV 5, with the highest hardness (205210 HV 5) being measured at the outer pipe wall. The pipe metal has adequate notch toughness in the transverse orientation with Charpy values exceeding 150 Joules being measured at 20 C.

Optical microphotographs of the X 80 pipe material, illustrating the low carbon bainiticferritic microstructure at pipe subsurface (1 mm below the outer wall) and at midthickness, are presented in Figure 1.4 of Annexe I. The Vickers hardness was 242 HV 5 at the outer pipe wall and 230 HV 5 at the inner wall (average value : 236 HV 5). As the X 70 pipes, the pipe material has adequate notch toughness in the transverse direction with Charpy values exceeding 140 Joules being measured at 10 C.

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5.1.3 Pipe metal tensile testing in the longitudinal (pipe axis) direction

The actual mechanical (strength) properties of the X 70 and X 80 pipe materials were determined at room temperature by tensile testing of fullthickness prismatic tensile testpieces (1,5" wide) extracted in the longitudinal direction (this is the direction of interest for girth welds subjected to bending or tensile loads) at different positions around the pipe circumference. To quantify the variability in tensile properties through the wall thickness, limited tensile testing was also performed on cylindrical ( 5 mm) tensile specimens extracted at subsurface and at pipe midthickness.

The test data are summarized in Tables 3.a and 3.b for the API 5L X 70 and X 80 pipes respectively. Both pipe materials were found to have a continuous yielding behaviour at the onset of yielding, i.e. their stressstrain curves did not exhibit an upper and lower yield point and a Lders elongation (plateau).

Table 3.a reveals that the mechanical properties of all three pipes ("A", "B" and "C") exceed the minimum requirements for API 5L X 70 quality (SMYS : 482 MPa, SMTS : 565 MPa). The actual yield strength (YS) values measured ranged between 472 and 528 MPa, with an average of 498 MPa. This implies that the particular pipes selected for the Project have a yield strength towards the lower end of the distribution for Grade X 70 pipe. The average YS/TS ratio, measured on fullthickness testpieces, was 0,838. In line with experience, the round bar specimens gave a YS/TS ratio which was 0,050,06 lower.

The tensile test data of the X 80 pipes are less conclusive (Table 3.b). Although the strength requirements for API 5L X 80 pipe (SMYS : 551 MPa, SMTS : 620 MPa) were met, the yield strength was found to depend largely on specimen geometry. Opposed to the expectations, the round bar specimens gave significantly higher yield strength values than the fullthickness prismatic ones (594 MPa versus 563 MPa). Further, the YS/TS ratio values measured ranged widely (between 0,80 and 0,88) and were apparently not linked with specimen geometry and throughthickness sampling position. As for the X 70 pipes, the X 80 pipes selected for the project were found to possess a yield strength towards the lower end of the distribution for Grade X 80 pipe.

5.2 Characterisation testing of the deposited weld metals and heat affected zones of the girth welds

5.2.1 Chemical analyses of the weld deposits

The chemical compositions of the deposited weld metals are listed in Table 4.a and 4.b, respectively for the girth welds in X 70 and X 80 pipes. Since the samples for chemical analysis were extracted towards the weld cap, the chemistries listed are representative of the weld deposits of the fill and cap passes (i.e. of highest strength consumables). The type E 6010 (cellulosic) electrodes, used for root pass welding, were virtually unalloyed (apart from Ni 0,25 %). The chemistries of the E 9010G (cellulosic) electrodes, produced by two suppliers (BHLER and THYSSEN), differed significantly, in particular in their C and Mo contents. The E 10018G (basic coated) electrodes yielded weld deposits with a Ni content of nominally 2,0 %, which should give adequate low temperature notch and fracture toughness levels.

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5.2.2 Micro- and macrographic examinations and Vickers HV 5 hardness testing

Micro- and macrographic examinations were performed on a selected number of cross sections. The test matrix included : two pipe grades (X 70 and X 80), two weld metal YS mismatch levels ("overmatching" and "undermatching") and three sampling positions around the pipe circumference.

Typical microphotographs illustrating the coarse (columnar) weld metal microstructures towards the weld cap and the grain refined (by depositing subsequent beads) weld metal microstructures in the weld root area are presented in Figures 1.2 and 1.3 (girth welds in Grade X 70 pipe) and in Figure 1.5 and 1.6 (girth welds in Grade X 80 pipe) of Annexe I.

Macrophotographs of a series of selected polished and etched (5 % nital) cross sections, extracted from the "sound" (virtually defect free) portions of the girth welds, are presented in Annexe HI. These macrographs are typical of manual pipeline girth welding (V bevel with an included angle of 60 and a root gap of 2,5 mm). Further, they illustrate that adequate fusion had been obtained through the wall thickness, including the root area.

On each of the macrographic sections, Vickers HV 5 hardness surveys, encompassing the pipe metal, heat affected zones and weld metal, were performed at three distinct through-thickness locations, i.e. at the weld cap (1 mm below the outer wall), at mid-thickness and at the weld root (1 mm below the inner wall). In addition, a fourth hardness survey was made in the through-thickness direction to characterize the weld metal hardness.

The results of these detailed hardness measurements are, together with the corresponding macrophotographs, presented in tabular form in Annexe HI. It was found that the sampling (o'clock) position did not have a pronounced effect on the weld metal and HAZ hardness values. For convenience, the average pipe and weld metal hardness levels are tabulated below :

Pipe grade

API5LX70

API5LX80

Pipe metal HV 5 hardness

(average)

193

236

Weld metal HV 5 hardness (average of each macrosection)

"Overmatching" electrodes

210 210 224

237 256 255

"Undermatching" electrodes

206 204 203

231 214 233

These hardness data indicate that, for all four pipe metal / weld metal combinations studied, the average hardness of the deposited weld metals is higher than the pipe metal hardness levels. This implies that, in terms of tensile strength, all girth welds have "overmatching" strength properties.

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In line with experience, the maximum HAZ (and also weld metal) hardness values were measured close to the outer pipe wall (underneath the cap passes). Maximum HAZ hardness values of up to 277 HV 5 were measured for the X 70 girth welds and of up to 286 HV 5 for the X 80 welds. These maximum HAZ hardness values, which were virtually identical in the "undermatching" and "over-matching" welds, are typical of as-welded manual pipeline girth welds in X 70 and X 80 pipes (note that, since pipelines are, exception made for special cases, never stress relieved following welding, these values were measured in the as-welded condition).

5.2.3 All-weld metal and transverse (cross-weld) tensile testing of the girth welds

5.2.3.1 All-weld metal tensile testing

The actual mechanical properties of the deposited weld metals were determined by room temperature tensile testing of cylindrical (diameter : either 4, 5 or 6 mm) all-weld metal tensile specimens extracted in the weld axis direction towards the weld cap. The test matrix included : two pipe grades (X 70 and X 80), two weld metal mismatch levels ("overmatching" and "undermatching") and three sampling (o'clock) positions. Duplicate tests were done for each combination of parameters.

The test data are summarized in Tables 5.a and 5.b for the girth welds made in API 5L X 70 and X 80 pipes respectively. The results indicate that the ductility of all weld deposits (as quantified by the percentage elongation and reduction of area at fracture) was adequate. In case somewhat lower values were measured, these were found to be provoked by minor defects (gas pores) contained within the cross sectional area of the testpieces.

Table 5.a shows that the average weld metal yield strength values of the welds made in X 70 pipe were 561 MPa for the E 7010G / E 9010G ("overmatching"), and 512 MPa for the E 6010 / E 6010 ("undermatching") consumable combination. In case of the E 6010/E 6010 welds, discrepancies were observed for the 12 o'clock sampling position, which gave unusually high YS values. A plausible explanation for these anomalous results could not be identified.

The mean weld metal yield strength values of the girth welds in X 80 pipe (Table 5.b) were 699 MPa for the E 6010 / E 10018G ("overmatching"), and 594 MPa for the E 6010 / E 9010G ("under-matching") consumable combination. Further, the scatter between the individual values was low and not directly linked with the sampling position along the circumference.

5.2.3.2 Transverse (cross-weld) tensile testing

The soundness of the girth welds was verified by conducting a limited series of transverse (cross-weld) tensile tests. To account for the variability of the (pipe and weld metal) tensile properties, testpieces were extracted from each weld at three sampling (o'clock) positions. The testpieces were nominally 25 mm wide by full wall thickness and the weld reinforcements were machined flush with the pipe surfaces. Though this is not a standard procedure, an extensometer was mounted straddling the weld, which allowed to determine the yield strength of the "composite" testpieces.

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The test data are summarized in Tables 6.a and 6.b, respectively for the welds made in API 5L X 70 and X 80 pipes. In case of the X 70 welds, fracture was invariably confined to the pipe metal, indicating that, for both electrode combinations, the weld metal tensile strength exceeds ("overmatches") the pipe metal tensile strength. Further, the recorded "pseudo" yield strength values (which were virtually identical to the pipe metal YS) strongly suggest that this is also the case for the yield strength. In case of the welds in X 80 pipe, the "undermatching" electrode combination (E 6010 / E 9010G) produced failure in the weld metal, whereas for the "overmatching" combination (E 6010 / E 10018G) fracture occurred outside the weld area. This strongly suggests that the former combination yields weld deposits which are "softer" than the X 80 pipe material.

5.2.3.3 Levels of weld metal yield strength mismatch

Theoretically, the actual levels of weld metal yield strength (YS) mismatch (over- or undermatching) can be quantified for each combination of pipe grade and consumable combination by comparing the YS values of the pipe materials (Tables 3.a and 3.b) with those of the weld deposits (Tables 5.a and 5.b). Mainly because of the variability of the yield strength properties and the associated scatter band, such an analysis is, however, beset with difficulties.

This is why a pragmatic approach was to be adhered to quantify (with an inherent degree of inaccuracy) the levels of weld metal yield strength mismatch. For each pipe / weld metal combination, three distinct cases were considered, i.e. based on the mean YS values of pipe and weld metal, based on the maximum YS of the pipe and the minimum YS of the weld metal and based on the minimum YS of the pipe and the maximum YS of the weld metal. The results of these comparisons are tabulated below for the X 70 and X 80 welds.

Girth welds in API 5L X 70 pipe

Mean values of pipe and weld metal

Maximum pipe -minimum weld metal

Minimum pipe -maximum weld metal

Pipe metal yield strength

(MPa)

498

528

472

Weld metal yield strength (MPa) Level of weld metal YS mismatch (%)

E 7010/E 9010 weld metal

561 (+12,7 %)

540 (+ 2,3 %)

584 (+ 23,7 %)

E 6010/E 6010 weld metal

512 (+ 2,8 %)

479 (- 9,3 %)

552 (+16,9 %)

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Girth welds in API 5L X 80 pipe

Mean values of pipe and weld metal

Maximum pipe -minimum weld metal

Minimum pipe -maximum weld metal

Pipe metal yield strength

(MPa)

563

630

561

Weld metal yield strength (MPa) Level of weld metal YS mismatch (%)

E6010/E10018G weld metal

699 (+ 24,2 %)

632 (+ 0,3 %)

719 (+ 28,2 %)

E6010/E9010G weld metal

594 (+ 5,5 %)

586 (-7,0%)

598 (+ 6,6 %)

These summary Tables provide evidence that, in case of the "overmatching" welds, the YS of the weld deposits invariably exceeds ("overmatches") the pipe metal YS. In case of the girth welds originally planned to yield undermatching in the X 70 and X 80 pipes, a situation of weld metal YS undermatching was not formally obtained, although the comparisons show this cannot be excluded totally, i.e. that there is a finite probability of occurrence of weld metal YS undermatching. In such case, the remotely applied strains are concentrated in the vicinity of the weld area. Such a situation implies that, in the event plastic strains should - incidently - be applied to such girth welds (e.g. as a result of settlements), the strains would be concentrated (strain accumulation) in the weld region, which is most suspectable (as compared with the pipe metal) to the presence of defects.

The fact that, neither for the X 70 nor for the X 80 pipes, weld metal YS undermatching was formally obtained, as planned, is to a large extent due to the fact that the pipes selected for the project had YS levels which are located towards the lower ends of the distributions for Grades X 70 and X 80 pipes. Further, effects of dilution of the weld metal by the pipe material might have contributed to this.

5.2.4 Charpy V notch impact testing of the girth welds

5.2.4.1 Extent of testing

The notch toughness properties of the girth welds in both pipe grades were determined over a range of temperatures (transition curves) by impact testing of standard Charpy V testpieces (10 mm 10 mm cross section) extracted transverse to the girth welds at the cap and root. Though all variables were not systematically investigated, the test matrix was such that the effects of the following variables on the notch toughness could be quantified :

- two pipe grades (X 70 and X 80) - two levels of weld metal yield strength mismatch ("overmatching" and "undermatching") - one condition : as-welded

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- four sampling (o'clock) positions, to account for the variability of the impact properties around the pipe circumference

- two notch positions, i.e. weld metal centreline (WMC) and heat affected zone HAZ (50 % line, sampling 50 % of weld metal and 50 % of HAZ and base metal)

- two through-thickness sampling locations, i.e. weld root (1 mm below the inner surface) and weld cap (1 mm below the outer surface)

- three test temperatures, ranging from +20 to -50 C, to assess the transitional behaviour - three repeat tests

The test matrix detailed above involved more than 300 Charpy V impact tests in total.

5.2.4.2 Test results

The individual and mean Charpy V impact energy values are summarized in Tables 7.a and 7.b, for the "overmatching" (E 7010G / E 9010G) and "undermatching" (E 6010 / E 6010) welds in API 5L X 70 pipes respectively. The same information is presented in Tables 8.a and 8.b, respectively for the "overmatching" (E 6010 / E 10018G) and "undermatching" (E 6010 / E 9010G) welds in X 80 pipes. It can be appreciated from these Tables that only limited testing was done to characterize the notch toughness of the heat affected zones. This is because the initial series of tests had revealed that the HAZ's of both pipes possess fully adequate low temperature notch toughness properties. Therefore, it was decided to reserve the majority of Charpy V impact bars for the notch toughness characterisation of the deposited weld metals.

The individual and mean weld metal Charpy V impact energy values are plotted versus test temperature (transition curves) in Figures La and l.b, for the "overmatching" (E 7010G / E 9010G) and "undermatching" (E 6010 / E 6010) welds in X 70 pipes respectively. The same information is presented in Figures 2.a and 2.b, respectively for the "overmatching" (E 6010 / E 10018G) and "undermatching" (E 6010 / E 9010G) welds in X 80 pipes. In these graphs, the through-thickness sampling location (cap versus root) and the sampling position around the circumference were used as parameters.

5.2.4.3 Discussion and interpretation of the notch toughness test data

Limited HAZ impact testing has clearly revealed that the heat affected zones (50 % line) of all four pipe / weld metal combinations possess fully adequate notch toughness properties. In case of the girth welds in the X 80 pipes, individual impact energy values exceeding 90 Joules were measured at - 20 C. The heat affected zone of the X 70 pipes gave somewhat lower (50 Joules at - 20 C), though still fully acceptable, impact energy values.

The practical conclusion is that the welding of high strength steel linepipes does not pose any major problem as regards the HAZ notch toughness. It is obvious that these adequate toughness properties are due to the well balanced chemistry (low C, S and content, low CE (IJW) and Pcm values) and the optimized steelmaking route of modem high strength linepipe steels. In view of this, it was logical not to perform CTOD tests to characterize the fracture toughness of the HAZ's in both pipe grades.

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As to the notch toughness of the deposited weld metals (Figures La, Lb, 2.a and 2.b), the experiments have shown that the sampling (o'clock) position of the testpieces does not have a pronounced effect on the impact energy values. Further, in line with experience, the weld cap yielded systematically higher notch toughness levels as compared with the weld root. This is the reason why the wide plate tensile testpieces were provided with machined surface notches introduced in the weld deposit from the root. As to the effect of the weld metal chemistry, it was found that the E 6010 / E 6010 (lowest alloy content) electrodes yielded the lowest notch toughness levels. As expected, the high strength, basic coated E 10018G electrode (alloyed with nominally 2,0 % Ni) gave the highest weld metal Charpy V impact energies.

The transition curves established for the weld root (Figures La, Lb, 2.a and 2.b) were used to determine the transition temperatures corresponding with weld metal impact values of 40 Joules (mean value) and 30 Joules (lowest individual value). Note that these weld metal toughness levels were used by the European Pipeline Research Group to establish their guidelines for defect acceptance in pipeline girth welds. These transition temperatures are, for each pipe / weld metal combination, listed in the Table hereinafter :

Pipe grade

API 5L X 70

API 5L X 80

Consumable combination

E7010G/E9010G ("overmatching")

E 6010/E 6010 ("undermatching")

E6010/E10018G ("overmatching")

E6010/E9010G ("undermatching")

Transition temperatures (C)

Mean value : 40 Joules

(Ttfoj)

-21

-10

-43

-24

Lowest value : 30 Joules

()

-30 < TUOj < -20

-20 < TU0J < -10

-50 < TU0J < -40

-30

6 CTOD FRACTURE TOUGHNESS TESTING OF THE GIRTH WELDS BY MEANS OF STANDARD AND "ALTERNATIVE" SPECIMEN GEOMETRIES

6.1 Extent of testing and experimental procedures

An extensive series of CTOD toughness tests has been performed to characterize, in the as-welded condition, the fracture toughness of the weld deposits (weld metal centreline) of the girth welds made in both high strength steel pipe grades. Testing was conducted at temperatures significantly below the minimum design / operating temperature of gas transmission linepipes, i.e. at either -20 or -30 CC. This was done to be able to correlate the weld metal CTOD toughness test data with the fracture behaviour of the curved pipe sections ("wide plates") incorporating either intentionally introduced weld defects or machined surface notches in the weld root.

To account for the variability of the weld metal CTOD toughness with the sampling position, testpieces were extracted at either two or three positions around the circumference, whereas the degree of scatter was assessed by performing three repeat tests for each combination of parameters.

In case of the girth welds in API 5L X 70 pipes (two consumable combinations), the effect of crack orientation on CTOD toughness was assessed by testing two specimen geometries, i.e 2B specimens, through-thickness notched (a/W = 0,5) at the weld metal centreline (WMC), and specimens, surface notched (a

6.2 Test results

The weld metal CTOD toughness values, measured by means of each of the specimen geometries considered, are presented in detail in Tables TV 1 to TV.4 of Annexe Fv\ according to the following scheme :

- Tables rv.l.a, b & c : Grade X 70 pipe-E 7010G/E 9010G consumables ("overmatching") - Tables IV.2.a, b & c : Grade X 70 pipe - E 6010 / E 6010 consumables ("undermatching") - Table IV.3 : Grade X 80 pipe - E 6010 / E 10018G consumables ("overmatching") - Table IV.4 : Grade X 80 pipe - E 6010 / E 9010G consumables ("undermatching").

For each individual specimen, the sampling (o'clock) position, the geometry and crack orientation ( 2B, B or 3B B), the CTOD fracture toughness value, measured at either -20 or -30 C, and the fracture mode ("c" : pop-in or short arrested brittle fracture, "u" : unstable fracture preceded by slow stable crack extension of minimum 0,20 mm, "m" : maximum force plateau or maximum load instability) are listed in these Tables.

For convenience, the weld metal CTOD toughness values are, along with the essential variables (test temperature, specimen geometry, crack orientation and sampling position), summarized in Tables 9.a and 9.b, for the "overmatching" (E 7010G / E 9010G) and "undermatching" (E 6010 / E 6010) girth welds in API 5L X 70 pipes respectively. The same information is presented in Tables 10.a and lO.b, for the "overmatching" (E 6010 / E 10018G) and "undermatching" (E 6010 / E 9010G) girth welds in X 80 pipes respectively. To facilitate the interpretation, the test data are graphically presented in Figures 3.a and 3.b (girth welds made in Grade X 70 pipes) and in Figures 4.a and 4.b (girth welds made in Grade X 80 pipes).

6.3 Discussion of the weld metal CTOD toughness test data

6.3.1 CTOD toughness of the girth welds in API 5L Grade X 70 pipes

In case of the welds in X 70 pipes, the test matrix was designed such that the effects of welding consumable combination, sampling (o'clock) position and specimen geometry (and associated crack orientation - through-thickness versus surface cracked) on weld metal CTOD toughness could be properly assessed. For obvious reasons, distinction is made between both weld metal strength categories, which are discussed separately.

(1) "Overmatching" welds made with E 7010G / E 9010G electrodes and CTOD tested at -30 C (Table 9.a and Figure 3.a) :

As compared with the other geometries, the through-thickness notched ( 2B) specimens yielded the lowest weld metal CTOD values and, moreover, exhibited the highest degree of scatter. Further, the critical event was either unstable fracture (modes "c" and "u") or maximum load instability. CTOD values of between 0,042 mm and 0,283 mm were measured for the 2B specimens and no distinct effect of the sampling (o'clock) position could be identified.

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The surface notched specimens gave CTOD values which were, on the average, 50 % higher than those measured by means of 2B specimens with CTOD toughness values ranging from 0,132 mm to 0,308 mm being recorded at -30 C. The vast majority of CTOD values was associated with the occurrence of a maximum force plateau, indicating that failure was through plastic collapse. Also here, the CTOD toughness was not affected by the sampling (o'clock) position.

(2) "Undermatching" welds made with E 6010 / E 6010 electrodes and CTOD tested at -20 C (Table 9.b and Figure 3.b) :

The through-thickness notched ( 2B) specimens yielded, as compared with the surface notched ( B) specimens, the lowest (mean and individual) weld metal CTOD values and, moreover, exhibited the highest degree of scatter with CTOD values ranging from 0,112 mm to 0,382 mm being measured at -20 C. The data suggest that testpieces extracted at the 9 o'clock position gave the highest CTOD toughness.

The surface notched specimens gave CTOD values which were, on the average, 35 % higher than those measured by means of 2B specimens. Again, the vast majority of CTOD values was associated with the occurrence of a maximum force plateau, indicating that failure was predominantly through plastic collapse. In line with the 2B specimen geometry, the highest weld metal CTOD toughness was measured at the 9 o'clock position.

6.3.2 CTOD toughness of the girth welds in API 5L Grade X 80 pipes

Testing of the girth welds in X 80 pipes was, for each welding consumable combination, limited to testing of two specimen geometries (and associated crack orientations) and two sampling positions (1 and 6 o'clock).

(1) "Overmatching" welds made with E 6010 / E 10018G electrodes and CTOD tested at -30 C (Table 10.a and Figure 4.a) :

The through-thickness notched ( 2B) specimens gave significantly higher (on the average, 43 % higher) CTOD values than the surface notched ( B) specimens. This is not surprising since, owing to their geometry and type of loading, 2B specimens produce a higher degree of triaxial constraint at the fatigue crack tip. Since all specimens failed by plastic collapse of the ligament underneath the crack (fracture mode "m"), a high triaxial constraint is "beneficial" for obtaining high CTOD toughness values. This is the more true since the specimens were fatigue precracked to % = 0,5.W nominally. The 2B geometry yielded weld metal CTOD values at -30 C ranging from 0,165 mm to 0,201 mm. In case of the specimens, these ranged between 0,113 mm and 0,171 mm.

(2) "Undermatching" welds made with E 6010 / E 901 OG electrodes and CTOD tested at -20 C (Table lO.b and Figure 4.b) :

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Here again, the through-thickness notched ( 2B) specimens yielded, as compared with the surface notched ( ) specimens, significantly higher (mean and individual) weld metal CTOD values : the weld metal CTOD toughness values were, on the average, 30 % higher. Individual values, all being associated with maximum load instability (plastic collapse - mode "m"), ranging from 0,118 mm to 0,206 mm were recorded for the 2B testpieces and ranging from 0,105 mm to 0,154 mm for the specimens. Further, there is a tendency to conclude that the 6 o'clock position gave the lowest CTOD toughness and the highest scatter.

6.4 Effect of lateral constraint (triaxiality) on weld metal CTOD toughness

The CTOD toughness tests have clearly demonstrated that, when the weld metal fracture toughness of girth welds in (thin-walled) linepipes is measured by means of specimens of "standard" geometry (either through-thickness notched 2B testpieces or surface notched testpieces), lower bound CTOD values of the order of 0,12-0,15 mm are not uncommon. These are usually associated with the occurrence of a maximum force plateau in the load versus notch opening displacement diagramme (fracture mode "m"). This type of "failure" at low to moderate CTOD values is due to the lack of triaxial constraint at the fatigue crack tip, typical of CTOD testing of thin walled sections, such as onshore pipeline girth welds (the selected pipes had wall thicknesses of nominally 16,9 mm and 16,2 mm).

The fact that plastic collapse is defined as the critical event penalizes the potentials of pipeline girth welds as to their fracture resistance. This is due to the fact that all engineering critical assessment (ECA) methodologies currently in vigour base the calculation of tolerable defect sizes on the material CTOD toughness ("omar). When CTOD values of the order of 0,12-0,15 mm are used as input data for the calculations, and when, moreover, residual stresses of yield point magnitude are included as secondary stresses (note that pipeline girth welds are seldom stress relieved following welding), the calculated tolerable defect sizes might be unduly restrictive and even in conflict with the workmanship based defect acceptance levels.

Since, moreover, the wide plate tensile tests (see Section 7) have convincingly demonstrated that the pipeline girth welds tested possess an adequate resistance against fracture initiation (Gross Section Yielding (pipe yielding) invariably preceded wide plate specimen failure) in the presence of very severe defects (surface notches), whose dimensions exceed the ECA based tolerable defect lengths by a factor of at least 3, it can be concluded that the CTOD test, in its present form, is not suitable to correctly predict the failure behaviour of defective (thin-walled) pipeline girth welds.

In an attempt to overcome this problem, experimental work was initiated in which it was aimed to increase the lateral constraint at the fatigue crack tip. To that end, an "alternative" specimen geometry was implemented, i.e. the so-called 3B specimen geometry. The testpieces had a width equal to 3. ( is the wall thickness), a height equal to B, were provided with a fatigue extended surface crack (nominal depth : ag = 0,33.B) introduced from the weld root and were subjected to an out-of-plane three-point bending load with a loading span equal to 4.B.

The results of these exploratory CTOD tests, which were performed on the girth welds in API 5L Grade X 70 pipe only, are contained in Tables 9.a and 9.b and in Figures 3.a and 3.b. It can be

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appreciated from these data that, in general, the CTOD values were in between those measured by means of and 2B specimens. In other words, the expectation that, by increasing the specimen width (and, hence, the lateral and triaxial constraint at the fatigue crack tip), higher CTOD values would have been measured, was not confirmed by the experiments. Though this was not further explored, it might, however, be that the irregularity of the fatigue crack fronts (which were, for all testpieces, such that they violated the stringent validity requirements set forth in BS 7448 : Part 1 : 1991) has contributed to this.

In conclusion, it can be stated that the utilisation of an "alternative" specimen geometry, designed such as to increase the lateral constraint, equally fails to correctly characterize the fracture