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

Directorate-General For Research And Innovation European Commission

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Industrial research and development

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• Sciences et techniques

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 29
• Langue: English
• Poids de l'ouvrage: 5 Mo

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EUROPEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
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
EUR 18426 EN STEEL »ESEÍIICH 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. [phone]; fax [phone] 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 LEGAL NOTICE
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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
PRINTED ON WHITE CHLORINE-FREE PAPER 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. χ 16,9 mm W.T. and 44" O.D. χ 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 Quali­
dade (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 excee­
ded 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, 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 Β χ 2B or surface
notched Β χ Β) and specimens with an "alternative" geometry (surface notched 3B χ Β 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 bym 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