| || |
Written by Ed Craig
Welding Duplex Alloys.
Avesta: Stainless steel evaporators
at Smurfit Kappa Kraftliner, Piteå.
Once you have read all the typical weld problems that will be generated on duplex alloys with either the pulsed MIG - TIG - FCAW or SMAW processes, the good news is none of those dupplex weld issues should be generated if you select the process thats most suited to all heat sensitive alloys, the TiP TiG process. TIP TIG.
Avesta: Pulp Paper Industry Stainless Steels and Duplex:
In modern pulp and paper mills, nearly all key
equipment (digesters, bleaching systems, evaporators, storage tanks and paper
machines) are made from various types of stainless steels. Approximately 5,000
to 6,000 tonnes of stainless steel go into a large new pulp and paper mill. On
top of that, there is piping, welding consumables and auxiliary equipment. The
materials used are standard austenitic, duplex and high-alloy super-austenitic
Currently, the high price of the alloying elements in
the raw materials is one of the reasons why manufacturers serving the pulp and
paper industry are showing increased interest in duplex steels. However, the most
important factor is that, even though the nickel and molybdenum content is generally
low, duplex steels have very good properties. Previously, so-called acid-resistant
steels were used when the sulphite process (an acid process) was used in cooking
paper pulp. Now that the industry is increasingly switching to sulphate or other
alkaline solutions, such steels are no longer optimal, says Lena Wegrelius.
corrosion presents the greatest risk in alkaline
cooking. Stainless steels with a high
chromium content, low molybdenum content and duplex microstructure are
optimal for these environments. Furthermore, the strength of duplex steels is
"twice" that of standard steels. Consequently this allows pressure vessels
to be built with thinner walls. Duplex steels are more cost-efficient austenitic
Duplex stainless steel are becoming ever more popular with
equipment manufacturers. Avesta has a new duplex grade, LDX 2101®, which is
excellent for the paper industry. However, its a fact that the choice of materials
when building new plants or machinery is not determined solely by what is best
for the application. For example, in Latin America, mills are favoring their own
local producers and these producers do not always manufacture duplex steel. In
those cases, standard materials are being used. Its believed that Chinese manufacturers
will soon be producing duplex steels themselves.
cut through the unique trade names for some duplex alloys you may wish to reference
the UNS number when ordering to the standard specifications in order to get the
desired grade. Many of the duplex grades have become known by a number that reflects
their typical chromium and nickel contents. For example the 2205 grade will have
22% Cr and 5% Ni. These composition-based names, used by many producers, have
recently been added to A 240 and are being added to the ASTM specifications for
other product forms.
[a] Lean duplex: Grade 2304. UNS (S32304) contains little or no deliberate
Mo addition. Corrosion resistance equiverlent of 316L
duplex: Grade 2205. UNS (S32205) accounts for vast majority of duplex use;
[c] 25 Cr duplex: Grade 255. UNS (S32550) and S31260;
[d] Superduplex: Grade 2507. UNS (S32750). Has 25-26% Cr and increased Mo and N.
[e] Hyper Duplex: Grade 2609.
Smurfit Kappa Kraftliner, Piteå, both the peroxide reactor and the oxygen
reactor, behind it, are manufactured from 2205 duplex stainless steel.
Avesta: The commercial development of stainless steel occurred side by side with
that of the modern pulp and paper industry. This was a phenomenon of 1940s
northern Europe. The unique collaborations between engineering companies (developers
of new and more efficient pulp and paper processes) and steel mills have led to
unsurpassed expertise in stainless steels for various pulp and paper applications.
They have also led to the development of new steel grades such as 254 SMO®
and LDX 2101®. To test how its steel grades stand up to various environments,
Outokumpu has a number of research and development collaborations with mills,
machine builders, research institutes and engineering companies. Amongst other
projects, Lena Wegrelius mentions that with an inspection company, Inspecta. In
this, field tests are being run in digester and evaporator environments at the
Smurfit Kappa Kraftliner pulp and paper mill in Piteå. We are also
working with Chempolis Oy in Finland. The latter company is looking at processes
for cooking pulp from raw materials other than wood. This is of future interest
for the Asian market, she declares. In China, for example, trials are being
conducted with the common reed and residual products from arable and rice farming.
New processes and steel grades may be necessary here. What else can be said about
developments in pulp and paper? For environmental reasons, the industry
is using more and more recycled paper in its raw materials. It is often said that
fibres can be used seven times before they are finished. Recycling processes are
thus becoming increasingly more important in pulp production, concludes
NEW DUPLEX FLUX CORED WIRE:
Avesta FCW-2D LDX 2101There is now a new and
improved version of Avesta Weldings flux cored wire for welding Outokumpus
new duplex steel, the LDX 2101®.Avesta FCW-2D LDX 2101 gives a duplex weld
metal that combines excellent mechanical properties with good corrosion resistance.
In both the flat and the horizontal-vertical (PC) positions, weldability is very
good Positive direct current (DC+) is used for welding flux cored wires. Compared
with MIG welding, the flux cored wire parameter range is considerably wider. Avesta
FCW-2D is an 0.045 1.2 mm wire supplied in a layer wound on 15-kilo wire basket
spools. Avesta Weldings range of filler metals for welding LDX 2101®
also includes covered electrodes and solid MIG, TIG and SAW wires.
For welding storage tanks, recovery boiler
pipes, oxygen reactors, etc. in duplex stainless steel 2304 (EN 1.4362, Outokumpu
2304). Avesta Welding is the first supplier in the
world able to offer a complete
range of matching filler metals for this duplex grade. The range comprises covered
electrodes, flux cored wires and solid wires (TiP TiG/MIG/TIG/SAW).
For welding batch and continuous digesters,
oxygen delignification and pressurized peroxide reactors, press washers, pulp
towers and filters, etc. made of 2205 duplex stainless
steel (EN 1.4462, Outokumpu
2205). Giving excellent mechanical properties and corrosion resistance, Avesta
2205 has a long record of success in these applications.
For welding super duplex 2507 (EN 1.4410,
Outokumpu SAF 2507) and similar grades. Avestas 2507/P100 range of highly
alloyed super duplex consumables provides better
strength and corrosion resistance
than ordinary duplex fillers.
Specially designed for welding Outokumpu
LDX 2101 (EN 1.4162), the new high-strength low-alloy duplex stainless steel.
Thanks to its superior strength and good corrosion resistance in alkaline environments,
this cost-efficient material is a maintenance-free option for railings, walkways,
storage tanks and even hydrogen peroxide reactors in pulp bleaching.
LDX 2101, it is easy to achieve a sound weld that, as regards corrosion resistance
and mechanical properties, matches the parent metal.
Note from Ed. None of the world's gas shielded flux cored wires or MIG welds can produce the duplex weld - mettalurgical qualities than can be attained from the TiP TiG process.
ton Brazilian urea reactor
From Avesta. Jaraguá of Brazil manufactured
and delivered a complete urea reactor to Petrobras/FAFEN in Camaçari (Brazil),
all the stainless steel plates and welding consumables came from, respectively,
Outokumpu and Avesta Welding in Sweden. The reactor, which was the first to be
produced in South America, weighs 130 tonnes. Besides conventional non-destructive
testing, the reactor was subjected to helium leakage tests and positive material
identification (PMI). Furthermore, all stainless steel (welded joints included
therein) were corrosion tested as per ASTM A262s practice C
(Huey test). The special technique developed to measure the gap between shell
and lining (maximum permitted limit of 3.0 mm) is worthy of individual highlighting.
Compared with other assignments, one of the greatest differences for Jaraguá
was that, to guarantee perfect handling and cleaning of the stainless steel, manufacture
of the reactor was kept entirely separate from other production.
Note from Ed Avesta and Sanvik are an excellent source for duplex info, its unfortunate that a decade has passed and there engineers still have not figured out the benefits from TiP TiG process for their alloys...
productivity welding of the new lean duplex stainless steel,
LDX 2101® (S32101)
With their strengthening in the
mid 1980's, duplex steels increasingly provided an alternative to conventional
austenitic steels. They were then predominantly used for chemical tankers as well
as process vessels and storage tanks in the pulp and paper industry. It is the
good combination of strength and corrosion resistance that makes duplex stainless
steels so attractive. However, in the beginning, the duplex grades were primarily
seen as alternatives to high-performance austenitic grades (e.g. 904L and 254
SMO®) rather than as alternatives to conventional grades (e.g. 304 and 316).
This limited their use in general construction. Here, high strength is needed
rather than high corrosion resistance. Developed by Outokumpu Stainless, LDX 2101®,
the new "lean" duplex stainless steel, is now used in a very wide range
purpose and structural applications are just two examples. The high strength
and good corrosion resistance of LDX 2101® also make it extremely suitable
for a variety of storage tanks. This article focuses on examining how high productivity
welding methods using Avesta LDX 2101, can be optimised to maximise the good corrosion
resistance and mechanical properties of duplex LDX 2101 stainless steel. Strength
and corrosion resistance Of the duplex grades currently on the market, LDX 2101®
has the "least alloyed" chemical composition with a nickel content of
just 1.5% (balanced with 5% manganese and some 0.22% nitrogen), LDX 2101 offers
high mechanical strength and corrosion resistance at an advantageous price. Nickel
is an expensive metal. Furthermore, its price fluctuates widely over time. This
can present major difficulties when estimating the total cost of a construction.The
chemical composition of LDX 2101 is balanced to give a microstructure with approximately
equal amounts of ferrite and austenite. Thanks to the high nitrogen content, austenite
re-formation after welding is rapid. In general, corrosion properties are as good
as, or better than, those of the Cr-Ni grade EN 1.4301/ AISI 304. Furthermore,
the steel's duplex structure and high nitrogen content result in a mechanical
strength almost twice that of ordinary, austenitic steels.
high strength of LDX 2101 steel can be used to optimise the design of various
storage tanks. On top of this, the steel's cost efficiency can be fully exploited
by selecting the correct filler (Avesta LDX 2101) and welding method.Better properties
with matching filler metals Because it is optimised to ensure the best welding
characteristics and mechanical properties, the matching Avesta LDX 2101 filler
should be used to weld LDX 2101® duplex stainless steel. To obtain a suitable
ferrite/austenite balance in the weld metal, Avesta LDX 2101 is over-alloyed with
The microstructure of metal arc welded LDX 2101 has a typical
duplex appearance with 30 - 65% ferrite.Testing of LDX 2101 welds has shown that
tensile strength is higher than 680 MPa / 99 ksi and that fracture most commonly
occurs in the parent metal. Bending tests to 180°
with a 3 x t mandrel
have shown no signs of cracking.
Methods and Weldability. Excellent results are obtained when welding LDX
2101 using all conventional methods - TiP TiG - SMAW, FCAW, MIG, TIG and SAW. However the TiP TiG process should be the first choice.
Duplex welds and the TiP TiG process.
This extraordinary semi-automatic and automated weld process provides the widest weld joules range, the lowest possible weld joules and the highest possible weld quality. With all position welds TiP TiG is easy to use and produces the lowest possible weld fumes. TiP TiG should be the first process considered for duplex alloys.
2008. A new lean duplex stainless steel grade EN 1.4062 (UNS S32202)
has been developed by Industeel and Ugitech by combining
low nickel content with a nitrogen addition but without an increase in manganese.
Nickel is reduced by a nitrogen addition to develop a microstructure containing
approximately 50% ferrite and 50% austenite. After a preliminary study with laboratory
heats, several industrial heats were produced with composition 22Cr, 2Ni, and
good source of Duplex Weld Data / Electrodes is
Sanvik / ESAB.
July 2008. Sandvik Materials Technology has launched
a hyper duplex stainless steel tube material which is designed to improve umbilical
performance in increasingly demanding subsea environments. Sandvik SAF
3207 HD is designed specifically for deepwater, high temperature, and extreme
Sandvik SAF 3207 HD provides
a tensile strength of 980 to 1180MPa and a maximum operating temperature of up
to 90C. This compares with a tensile strength of 800-1100MPa and maximum water
temperature rating of 65C for the companys standard super-duplex grade SAF
2507. SAF 2507 can be deployed in water depths greater than 2000m. However, as
the pressure at the seabed increases with depth, the wall thickness of the tube
eventually becomes so great that it can become uneconomical to purchase all the
steels offer great mechanical properties and corrosion resistance. The primary
difference between austenitic stainless and duplex steels is the duplex offers
a balanced microstructure of approx. equivalent volumes of both ferrite and austenite.
Typically the duplex steel will have a weld ferrite content of 40 to 60% and the
weld 25 to 40 % ferrite. A 25% to 50% ferrite range is desirable.
comparison to traditional austenitic stainless, the duplex, two-phase microstructure
and high chrome / Mo can result in duplex applications that have;
higher resistance to stresses,
[b] higher resistance to pitting corrosion
[c] improved intergranular corrosion resistance.
Be aware of too much or too little ferrite. Rapid quenching of the duplex weld
as found in arc strikes or small welds on large parts should be avoided as this
can result in very high ferrite content > 60%. The extra high ferrite welds
can result in low toughness or low corrosion resistance.
Ferrite in duplex
results in lower coefficient of thermal expansion in contrast to austenitic stainless
resulting in less weld distortion potential.
Potential Information from Sanvik..
data is from Sanvik: Materials used in oil and gas extraction are affected to
several different types of corrosion, often caused by seawater and spray. The
types of corrosion, which can occur on stainless steels in marine environment,
are pitting and crevice corrosion, and for standard austenitic grades also stress
corrosion cracking (SCC), if the material temperature is above 60°C (140°F).
These are all localised attacks general corrosion need not be considered
for stainless steels in seawater. High temperatures, high chloride contents and
low pH values increase the risk of localised attacks in any chloride-containing
environment. Of these, temperature is usually the most influential factor.
However, there is a fourth important consideration: the electro-chemical corrosion
potential of the environment. In seawater, this potential is affected by biological
activities on the steel surface. Since seawater is, in a sense, a living corrosive
environment, it is sometimes difficult to define exactly what the service conditions
will be. At normal seawater temperatures, a biofilm will form on the steel surface
and result in a corrosion potential of +300 to +500 mV/SCE.
temperatures above ~40°C (100°F) the biological activity will cease and
the corrosion potential will drop. The use of continuous chlorination, to stop
marine growth, may increase the corrosion potential to values as high as +600
to +800 mV/SCE. This, however, can be avoided through the use of intermittent
rather than continuous chlorination.
over Cu and CuNi-based alloys:
steels are very resistant to erosion corrosion compared with Cu and CuNi-based
alloys, which are very sensitive to this form of attack. Water in harbours, around
offshore platforms, and near chemical plant sites is often contaminated e.g. with
ammonia (NH 3 ) and sulphides (S 2- ). These compounds, even in very small quantities,
cause localised attacks on copper-base alloys, while stainless steels are not
affected at the impurity levels involved.
corrosion: Wet and sour service: The corrosivity of an oil and
gas well is increased by the presence of chlorides in water solutions, carbon
dioxide, and hydrogen sulphide.
environment is considered sweet as long as no hydrogen sulphide is present. Carbon
dioxide alone can however cause high corrosion rates on carbon steel, since it
is acidifying the solution. This is further accelerated if chlorides are present.
environments are defined when the partial pressure of hydrogen sulphide is above
0.05 psi. At higher partial pressures, the corrosion rate on carbon steel is substantially
increased by means of making the water phase more acidic and by forming iron sulphide
scale. Sulphide Stress Cracking (SSC) is common in high strength steels containing
martensite. It can also occur in ferritic steels.
steels are different. Sandvik Sanicro 28, Sanicro 29, SAF 2205 and SAF 2507 grades
are completely resistant to corrosion in wells rich in carbon dioxide with a high
amount of chlorides in the water phase. If hydrogen sulphide is present, there
is still no general corrosion, but the risk of localised corrosion increases,
especially with regard to SSC.
NACE TM-0177 test.
Experiments have been carried out at room temperature in
accordance with the NACE TM-0177 test (5% sodium chloride, 0.5% acetic acid, saturated
with hydrogen sulphide).
threshold stress for cold-worked Sandvik SAF 2205/22Cr is about 90% of the yield
strength, which is very good when compared to results for high strength, ferritic
Sanicro 28, in the cold-worked condition, results in no failures up to very high
stress levels. The high alloy duplex stainless steel Sandvik SAF 2507 is also
resistant to cracking in the solution-annealed condition.
general terms, this test shows that Sandvik Sanicro 28 has a higher resistance
to sulphide stress cracking compared to SAF 2205/22Cr, which is much more resistant
than 13Cr. Sandvik Sanicro 29 has an ever higher resistance to localised corrosion
and sulphide stress cracking than Sandvik Sanicro 28.
should be remembered that the chemistry of the NACE solution is not
representative of the conditions in most sour oil and gas wells. This is especially
true for acidity, where the pH value is lower in the NACE test. Results from the
NACE TM-0177 test, therefore, should not be used for determining the suitability
of different grades, but more as a ranking test. Other tests, more representative
of actual service conditions, must be used to determine the suitability of different
grades. Practical experience of specific grades is, of course, extremely useful.
Visit Sanvik's web site for more excellent data on stainless and duplex
products, however if you want the best stainless MIG process control data visit
Metallurgy of Duplex Stainless Steels.
duplex have an alloy range of,
Cr - 22 to 26%.
Ni - 4 to 7%.
- 0 to 3%.
With 0.1 to 0.3 Nitrogen, (no nitrogen will be in filler metals)
plays an important role of present day duplex alloys. Nitrogen is a potent austenite
stabilizer. The addition of nitrogen to the duplex steel will promote structural
hardening by a solid solution mechanism. The nitrogen therefore will raise both
the yield strength and ultimate strengths of the duplex without impairing toughness.
Nitrogen cannot be added to the filler metal as
it can not transfer across the arc. When welding nitrogen can be added to the
weld through the shielding or purge gas.