The influence of low carbon equivalents and low sulphur levels on the allowable welding and bending procedures to be used in practice

Directorate-General For Research And Innovation European Commission

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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: 3 Mo

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EUROPEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
roperties and in-service performance
1
The influence of low
carbon equivalents
and low sulphur levels
on the allowable
welding and bending
procedures to be used
in practice
h
Report
EUR 18778 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
steel research
Properties and in-service performance
The influence of low carbon equivalents and low
sulphur levels on the allowable welding and
bending procedures to be used in practice
J. Vuik
Nederlands Instituut voor Lastechniek
Laan van Westenenk 501
Postbus 541
7300 Apeldoorn
Netherlands
Contract No 7210-MC/601
1 July 1992 to 31 December 1995
Final report
Directorate-General
Science, Research and Development
1999 EUR 18778 EN LEGAL NOTICE
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Luxembourg: Office for Official Publications of the European Communities, 1999
ISBN 92-828-5296-2
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Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER SUMMARY
A research programme has been performed in order to determine the influence of low carbon
equivalents (CE) and low sulphur contents on the allowable welding and cold bending procedures
to be used in practice.
The project originated from contradictory literature data with respect to the influence of sulphur on
the susceptibility to cold cracking, and the extent to which the metallurgical softening in the fine
grained Heat Affected Zone (HAZ) of low CE steels did influence the strength that is measured
with transverse tensile specimens.
In order to be able to determine the influence of sulphur on the cold cracking susceptibility,
laboratory steel casts have been produced. Three series of steels have been produced, each with a
sulphur content increasing from approx. 0.002% to approx. 0.0011%. Two of the series were
normalized, and had a minimum specified yield strength (MSYS) of 355 N/mm2. One of these
series was micro-alloyed with niobium, the other one contained as less niobium as possible. The
third series was quenched and tempered, and had a MSYS of 550 N/mm2.
Using these steels, weld thermal simulation tests have been performed. Neither the hardness of the
coarse grained HAZ, nor the transformation temperatures did show any correlation with the sulphur
content. Hydrogen determination tests showed that neither the hydrogen content of the weld metal
(HDM) nor the effusion rate was influenced by the sulphur contem.
Implant tests have been used to determine the susceptibility to cold cracking: in the in-plane
direction, the cold cracking susceptibility (= the critical stress level in the implant test) did not
show a correlation with the sulphur content.
In the Z-direction (perpendicular to the plate surface), the critical stress level decreased with
increasing sulphur level, which is in accordance with information published in literature.
In order to determine the relevance of softening for the welding procedures to be applied in
practice, commercially available steels with a MSYS ranging from 355 to 690 N/mm2 have been
procured. Within each group, the steels were selected to give the range of chemical compositions
that are available on the market.
Weld thermal simulation tests, with a cooling time At 8/5 from 15 to 90 s, revealed that both the
hardness and the tensile properties of the fine grained HAZ decreased compared to the values of
the base material. Whereas the results of the simulation tests indicated that for a S355 material
softening occurred up to a carbon content of 0.07% and a corresponding carbon equivalent of 0.35,
in the real weldments no HAZ fracture occurred in the transverse tensile tests for cooling times At
8/5 up to 60 s.
For S460 materials, the softening in the weld thermal simulation tests occurred at least up to a
carbon content of 0.095 %, with a corresponding CE of 0.38.
In real weldments, HAZ fracture in transverse tensile specimens did occur in the 0.061 %C, 0.31 CE HSM material, even for a rather modest cooling time of 30 s. A cooling time of 15 s led
to base material fractures in transverse tensile tests.
The materials with a carbon content of 0.085 resp. 0.095 and a CE of 0.38 did not show HAZ
fractures in the transverse tensile tests of 60 s cooling time weldments.
This means that, metallurgically speaking, softening does occur for S355 and S460 steels, but in
practice only HSM 460 with (in this programme) a CE of 0.31 suffers from a drop in tensile
strength. It is not clear what CE is critical for the drop in tensile strength in high heat input
welding, but the transition from softening to base material strength level will approximately be at
the level of 0.35/0.36.
The results of this investigation indicate that the commercially available thermo-mechanically rolled
S355 and S460 steels can be welded with a cooling time At 8/5 up to 60 s without a loss of
strength in the fine grained HAZ.
For the very lean alloyed hot strip mill steels with MSYS above 550 N/mm2, the allowable cooling
time At 8/5 decreases from 15/20 s for S590 to 15 s (or slightly less) for S690. The S690 grades
with a higher alloying content (> approx. 0.45 CE) tolerate higher cooling times At 8/5 (approx. 30
s). However, the experiments showed that it is very difficult to obtain weld metal that overmatches
the base material at high heat-input weldments (high cooling times
At 8/5), especially when the base material has a tensile strength over 800 N/mm2.
For all high heat input weldments, especially in steels with a yield strength well above 460 N/mm2,
it should be kept in mind that besides to a loss of strength, probably also a loss of notch toughness
appears. For the HSM materials welded at a cooling time At 8/5 of 30 s, in fact there remains no
resistance to initiation of instable fracture.
In order to determine the tolerance to cold deformation/bending of the lean alloyed HSM and
TMCP steels, the materials have been bent over dies with a diameter down to plate thickness.
The results revealed that small diameter bending dies can be applied, although only a few materials
could withstand bending over a die with a diameter less than 3 times the plate thickness. The HSM
materials, that are in practice specially selected for cold forming operations, could withstand
bending over D = t only at a yield strength level of 355 N/mm2. The mechanical properties of such
cold deformed material drastically drop. The Charpy-V transition temperatures shift towards higher
temperatures for max. 50°C.
The shift in transition temperature is most pronounced for the HSM materials with a yield strength
of 590 N/mm2 and above.
The yield and tensile strength and the yield over tensile ratio of the cold bent materials are higher
than of the base material, but this is not catastrophic. The high yield over tensile ratio in principle
prevents the material from deforming, but deformation then will take place in the surrounding non
deformed base material. The one thing that bothers however, is the fact that the cold deformation has consumed all
plasticity that can be supplied by the material. The elongation of the cold deformed materials,
measured with standard Dp5 specimens still lies at a level of approx.
10 - 15 %, but this elongation is almost completely caused by the contraction. The uniform
elongation of the specimens amounts to max. 2.5%, but values down to 0.5% have been measured,
too.
This means that such cold bent material not only can have rather poor Charpy-V notch toughness
properties (depending on the properties of the base material), but also cannot withstand plastic
deformation. SAMENVATTING
Er is een onderzoekprogramma uitgevoerd teneinde de invloed te bepalen van een laag
koolstofequivalent (CE) en een laag zwavelgehalte op de las- en koudbuigprocedures welke in de
praktijk kunnen worden gebruikt.
Het project is geëntameerd door tegenstrijdige gegevens uit de literatuur met betrekking tot de
invloed van zwavel op de gevoeligheid voor koudscheuren, en de mate waarin de metaalkundige
"softening" in de fijnkorrelige warmte beïnvloede zone (WBZ) van laag CE staalsoorten de sterkte
beïnvloede zoals die gemeten wordt met dwarstrekstaven.
Teneinde in staat te zijn de invloed van zwavel op de koudscheurgevoeligheid vast te stellen, zijn
laboratorium staalsmelten gemaakt. Er zijn drie series vervaardigd, elk met een zwavelgehalte dat
opüep