The effect of the application of very low yield strength consumables on the fabrication procedures and the yield and fracture behaviour of welded joints

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: 24
• Langue: English
• Poids de l'ouvrage: 4 Mo

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EURO PEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
Properties and in-service performance
The effect of the
application of very
low yield strength
consumables on the
fabrication procedures
and the yield
and fracture behaviour
of welded joints
h
Report
EUR 18777 EN STEEL RESEARCH 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
SS S SS «¡w ■ IB
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Properties and in-service performance
The effect of the application of very
low yield strength consumables on the
fabrication procedures and the yield
and fracture behaviour of welded joints
Prof. Dr W. Dahl, K. Müsch
Institut für Eisenhüttenkunde der RWTH
Kopernikusstraße 16
D-52056 Aachen
J. Vuik, S. C. Martijn
TNO Institute of Industrial Technology
Laan van Westenenk 501
Postbus 541
7300 Apeldoorn
Netherlands
Contract No 7210-KA/121/609
1 December 1992 to 30 November 1995
Final report
Directorate-General
Science, Research and Development
1999 EUR 18777 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
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Luxembourg: Office for Official Publications of the European Communities, 1999
ISBN 92-828-5290-3
© European Communities, 1999
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER SUMMARY
A research programme has been carried out in order to determine the effect of the application of a
very low yield strength electrode in the penetration bead in weldments in S460 and S690 on the
required preheat temperature and the mechanical properties of the welded joint.
These very low yield strength (VLYS) electrodes contain hardly any alloying elements: approx.
0.02 %C, 0.2 - 0.3 %Mn, 0.1 - 0.2 %Si, and give weld metal with (in the undiluted condition) a
yield strength of approx. 380 - 400 N/mm2.
The reduction in required preheat temperature has been determined with a K-bevel Tekken test,
which is more severe (requires a preheat temperature that is approx. 50 °C higher) than the standard
Y-sloped Tekken test. From these K-bevel Tekken tests it appeared that in very high restraint
situations, the use of VLYS electrodes in the penetration bead does allow for a reduction of the
preheat temperature of approx. 50°C compared to matching electrodes. The exact value depends on
the strength level, the chemical composition and the thickness of the base material.
Just like with ordinary matching electrodes, there is some scatter in the required preheat
temperature for the VLYS electrodes, even if strength level, chemical composition and hydrogen
level are the same.
It has been established that for S690 weldments, the application of VLYS electrodes in the
penetration bead can reduce the required preheat level to such an extent that the high strength weld
metal becomes susceptible to the formation of transverse cold cracks. It has been shown that the
risk for weld metal cold cracking can be limited by choosing a filler material with proper yield and
tensile strength. In case the strength properties of the weld metal largely overmatch those of the
S690 base material, there is a strong increase of the chance for transverse cold cracks in the weld
metal.
The proportion of VLYS penetration bead weld metal that is still present after filling a K or
double-V bevel is very limited. In practice, a layer VLYS weld metal will remain with a thickness
less than 1 mm. Only in V or ViV bevels, a layer of approx. 2.5 - 3.5 mm will remain. The yield
strength of this VLYS penetration bead amounts to approx. 450 N/mm2.
In S460, the presence of a VLYS penetration bead does not lead to a lack of strength of the
weldment, not even in thin plates. In S690 however, a single-sided weld in thin plates can lead to
fracture of transverse tensile specimens in the weld metal. For thicker plates (double-sided
weldments) the VLYS penetration bead does not influence the strength level of the weldment as a
whole.
The results of the test programme show a rather frequent appearance of cracks in reverse bend test
on single-sided weldments with a VLYS penetration bead. Generally speaking, the Charpy-V notch toughness properties at the position weld centre are
slightly less in case a VLYS penetration bead is applied instead of a matching penetration bead.
From previous research programmes is became clear that the degradation of Charpy-V properties
increased with the thickness of the remaining layer of the VLYS penetration bead.
The considerable difference in Charpy-V properties between the position Fusion Line root for a
matching and a VLYS penetration bead confirms the previous results: along the fusion line, the
height of VLYS weld metal can be as high as 7 - 8 mm.
Two factors cause the poor Charpy-V notch toughness properties of the VLYS weld metal:
1 due to the low alloying content, a very coarse microstructure is formed,
2 due to the very low carbon content, no pearlite is formed during cooling. During subsequent
heating (following passes) the supersaturated carbon precipitates at grain boundaries as tertiary
cementite.
Both factors contribute to the poor resistance to brittle fracture of the VLYS weld metal.
Extensive CTOD testing revealed that the resistance to initiation of brittle fracture of the VLYS
weld metal is poor. However, this effect can only be established when surface cracked B*B
specimens are used. When the preferred 2B*B specimens are used, the resistance to initiation of
brittle fracture is less than in case a matching penetration bead is applied, but the measured values
are acceptable.
The practical relevance of the poor CTOD properties of the VLYS penetration bead is limited, both
for single-sided and for double-sided weldments.
For double-sided weldments, a thin layer of VLYS weld metal remains. This very thin layer can
only influence the fracture behaviour when it is hit by the fatigue crack. This means that the fatigue
crack must have grown until half plate thickness, which makes it easily detectable. For embedded
defects with a height equal to the remaining part of the penetration bead, calculations according to
PD6493 show that no initiation of unstable fracture can occur.
It is recommended only to use these type of consumables when a surface breaking fatigue crack
that ends in the thin VLYS layer can be detected easily. As it is generally accepted that defects
50*10 mm can easily be detected, the minimum plate thickness in which VLYS electrodes can be
used for welding the penetration bead is set to 30 mm.
For single-sided weldments, the remaining layer of VLYS weld metal has a thickness of approx. 3 -
4 mm. The driving force for initiation of instable crack growth increases with crack depth.
Calculations with the actual CTOD values and strength properties of the VLYS penetration bead
show that (with residual stresses as high as the yield strength of the VLYS penetration bead, and an
applied stress level of 66% of the MSYS of the base material) before the crack has grown out of the VLYS penetration bead, initiation of unstable fracture can take place. The results of the wide
plate tests performed at TEhK RWTH-Aachen indicate that these calculations are very conservative.
From the comparison of the calculated fracture loads (using the average CTOD values) and the
experimental results of the wide plate tests, it appeared that the predictions according to PD 6493
were safe. In fact, even with the assumption that 0 residual stresses were present in the weldment,
the predictions were (over) conservative.
On the other hand, it is true that there is a difference in behaviour between the wide plates from
weldments with a matching penetration bead and with a VLYS penetration bead.
However, this occurrence of pop-ins or even total brittle fracture in case of the presence of VLYS
penetration bead happened after the double edge (DENT) wide plates had started yielding in the net
section. Only the 12 mm S690 broke without plastification before net section yielding occurred.
Even with surface notched wide plates (SSCT), in which the crack has grown in the VLYS weld
metal at half plate thickness, the predictions of PD 6493 were safe. Still, the results of these tests
showed that the VLYS layer was present, and indeed had poor resistance to initiation of instable
fracture.
The final conclusion of the research programmes is that application of very low yield strength
electrodes in the penetration bead of weldments in S460 and S690 does offer considerable benefits
in reduction of prehe