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March 2015
AFRICAN FUSION
35
S
uper duplex stainless steel (SDSS)
has been increasingly applied in
the upstreamoil and gas industry
segment in recent years. This steel grade
is normally used in the manufacture of
subsea components and in the construc-
tion of topside equipment and pipes on
oil and gas platforms. The extraordinary
growth on SDSS use for manufacturing
such components and equipment can
be mainly explained due to two factors.
•
First, SDSS has excellent mechani-
cal properties when compared
to austenitic stainless steels of
the 3xx series. The yield strength
of UNS S 32750 super duplex, for
example, is close to twice that of
austenitic stainless steels types 304L
and 316L This strength advantage
enables significant weight, thick-
ness and lifecycle cost reductions. In
addition to higher tensile strength,
excellent toughness at low tempera-
tures (down to -46 ºC) is available
from SDSS.
•
Second, super duplex offers high
resistance to pitting corrosion and
stress corrosion cracking in media
containing hydrochloric acid, sul-
phuric acid, acetic acid and chloride
ions, among others.
Despite the noted advantages described
above, inadequate welding procedures
of SDSS can result in metallurgical
changes in its microstructure, resulting
in loss of the desired properties of this
grade of stainless steel. Figures 1 and 2
show one of the typical problems that
can occur in the heat affected zone (HAZ)
of SDSS when welders and or welding
procedures are not properly qualified,
consumables and welding processes
are wrongly selected, and the specific
requirements, suchas heat input control,
are not properly followed.
To avoid such problems, technical
welding expertise when using these
material grades is necessary. Voestalpine
BöhlerWelding develops, manufactures,
and supplies SDSSwelding consumables
for all of the major welding processes.
They meet all main global requirements
for this alloy type and are approved by
oil and gas companies such as Petrobras,
Norsok and NACE, for example. Welding
consumables for each of the recom-
mended welding process are listed in
Table 1.
Welding of super duplex
stainless steels
Omar Abou Samra Filho, voestalpine Böhler Welding
Process voestalpine Böhler
Welding consumable
GTAW
THERMANIT 25/09 CuT
SMAW
THERMANIT 25/09 CuT
GMAW
THERMANIT 25/09 CuT
FCAW
Avesta2507/P100-PW NOR
SAW wire
THERMANIT 25/09 CuT
SAW flux
Marathon 431
Table 1: Böhler Welding consumables
suitable for welding of super duplex
stainless steel UNS S 32750 and UNS N
32760.
Figure 1: The microstructure of welded
UNS S 32750 super duplex stainless steel.
Region A: base metal containing an
optimum balance of ferrite and austenite
phases (approximately 50% each). Region
B: the HAZ with unbalanced ferrite and
austenite phases due to unsuitable
welding procedure (approximately 75%
and 25% respectively). Region C: The weld
metal region that contains the correct
phase balance due to the use of a welding
consumable with a well-balanced chemical
composition.
Welding super duplex stainless steels
Figure 2: A super duplex UNS S 32750 pipe removed
from an oil platformmodule. Corrosion of HAZ can
be observed due to a HAZ microstructure imbalance
presented in Figure 1.
Figure 3: Welding procedure qualification of an SDSS
pipe in the 6G position.
Figure 4: Welding of an SDSS pipe for a
desulphurisation system on an oil platform.
Voestalpine Böhler Welding has
regional teams of highly trained and
qualified professional engineers – sup-
ported by central R&D and Applications
in Europe – to assist when selecting the
best welding processes and welding
consumables; trainingwelders; develop-
ing welding procedure qualifications;
interpreting technical standards; and
for all the choices necessary to ensure
anoptimumsolution fromSDSSwelding.
Voestalpine Bohler Welding’s SDSS
solutions and consumables for the oil
and gas industry comply fully with all
national and international industry seg-
ment standards.