Avesta Welding’s flux cored wires for welding super duplex stainless steel
Johan Ingemansson, Böhler Welding Group Nordic AB
Contents
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Summary
What are the differences between duplex and super duplex stainless steels?
The most cost-efficient method for manual welding
Weld metal porosity
Why is the highly productive submerged arc welding (SAW) method not more
commonly used when welding super duplex steel?
How does the slag’s chemical composition affect welding results?
Why two variants of flux cored wire for super duplex stainless steel?
General rules for welding with Avesta 2507/P100-PWNOR and Avesta 2507/P100-PW
flux cored wires
Results:
o Welding procedure
o Corrosion testing (ASTM G48 method E)
o Chemical composition
o Mechanical values
o Bend testing
o Corrosion testing of PA, PF welded (normal condition) and pickled samples
(ASTM G48 method A, welded against a ceramic root backing)
o Welding data
References
Abstract
The use of duplex and super duplex steels is increasing worldwide. Thus, naturally enough,
there is great interest in the most productive welding method with the highest possible
quality – even when welding manually. The most productive manual method is flux cored
arc welding (FCAW). This is especially true for position welding.
Avesta Welding has two variants of flux cored wires for welding super duplex stainless steel.
One of these, Avesta 2507/P100-PWNOR, gives a pure weld metal that satisfies the high impact
strength requirements in NORSOK M-601 and similar standards. The impact strength of the
pure weld metal is a minimum of 45 J at -50°C. Our other wire, Avesta 2507/P100-PW, has
slightly better weldability and slag removal. Nonetheless, the pure weld metal still manages
32 J at -40°C.
To ensure that appropriate mechanical properties and corrosion resistance are attained, both
variants are slightly over-alloyed with nickel (to give the optimum ferrite-austenite balance).
With a critical pitting temperature (CPT) above 40°C, the weld metal demonstrates high
resistance to pitting corrosion and stress corrosion in chloride-containing environments
(corrosion testing as per ASTM G48-A).
Both variants have been developed for welding in all positions. They give a stable arc and
full control of the weld pool and slag.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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What are the differences between duplex and super duplex stainless steels?
The distinction between “ordinary” duplex stainless steel and super duplex steels is that the
latter have a pitting resistance equivalent number (PREN) above 40. Now a widely accepted
concept in industry, PREN makes it easy to rank the corrosion resistance of stainless steels.
However, PREN does not, for example, take microstructure into account. Thus, two steels
with the same PREN may still have different resistance to corrosion.
Expressed simply, PREN is a number that states resistance to, primarily, pitting (i.e. pitting
corrosion). PREN is easily calculated from the chemical composition of the stainless steel in
question. Most often, the following formula is used: PREN = % Cr + (3.3 x % Mo) + (16 x % N).
However, there are also other variants of the PREN formula. One of these includes: 0.5 x % W.
As is obvious from the above, a higher number means, in theory, better corrosion resistance.
It also means that chemical composition is the factor distinguishing super duplex from
ordinary duplex. Super duplex steels usually have higher molybdenum (Mo), nickel (Ni),
nitrogen (N) and chromium (Cr) contents. Normally, super duplex steels also usually
demonstrate superior mechanical properties (yield strength and tensile strength).
Furthermore, super duplex steels have very high resistance to crevice corrosion, stress
corrosion, general corrosion in acids, erosion corrosion and corrosion fatigue.
The above-mentioned excellent material properties, together with the almost twice as high
mechanical strength (yield strength and tensile strength) compared to equivalent austenitic
steels (e.g. 254 SMO), mean that various grades of super duplex steels are being increasingly
used in various industries. Oil and gas, offshore, marine, pulp and paper, foodstuffs, water
purification, water desalination, flue gas treatment, desulphurisation systems (power
industry), marine oil and chemical tankers and chemicals production are just some of these.
Examples of common names and designations of the market’s super duplex steels are SAF
2507, Zeron 100, DP-3W, S32760 (1.4501), URANUS 52N+, S32550 (1.4507) and S32750
(1.4410).
ASTM
EN
Outokumpu
Cr
Ni
Mo
N
PRE
Rp0.2
S31254 1.4547
254 SMO
20
18
6.1
0.20
43
300
S32205 1.4462
2205
22
5.7
3.1
0,17
35
460
S32750 1.4410
2507
25
7
4
0.27
43
530
Table 1: Typical minimum values for hot-rolled plate at 20°C (as per EN 10088). Note the difference in strength
(Rp0.2) between the austenitic steel (254 SMO) and the two duplex steels (2205 and 2507). Note also the
difference in chemical composition and pitting resistance equivalent number (PREN) between “ordinary” duplex
2205 and super duplex 2507.
The most cost-efficient method for manual welding
Just as other industries, welding is always seeking to reduce costs. It does this primarily
through making it possible for welders to carry out jobs in the shortest possible time (e.g. by
selecting a welding method that, with the highest possible productivity, gives acceptable
quality for the application in question).
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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TIG welding (GTAW) generally gives higher quality welded joints. However, the method is
rather slow and requires optimal gas shielding. Nonetheless, the high quality of the weld
metal means that the TIG process is normally used for depositing the root bead and a couple
of beads thereafter (i.e. the hot passes). Currently, the rest of the joint is most often welded
using a more efficient/productive method.
Covered electrodes (SMAW, MMA welding) have gradually been replaced by solid wires
(MAG welding, GMAW) or flux cored wires (FCAW). Using flux cored wires is the most
productive of the previously mentioned welding methods. This is particularly noticeable in
position welding (see figure 1).
Figure 1: The productivity difference in the PG (3F) position using three different welding methods (same arc
time).
Weld metal porosity
When using solid wires (MAG welding, GMAW), it can, in many cases, be difficult to get a
satisfactory result as regards porosity. This is because of the relatively high nitrogen (N)
content in the parent metal (2507) – see figure 2. Here, flux cored wires (FCAW) can be a
better option.
Figure 2: Radiograph of porosity in super duplex weld metal, position PA (1G), welded using solid wire (GMAW,
MAG welding). Note that this welding position is considered the best for avoiding porosity.
Even with flux cored wires (FCAW), the weld metal may exhibit small, but acceptable,
porosity. This has primarily been noticed in overhead welding (PE, 4G) – see figure 3.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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Figure 3: Radiograph of porosity in super duplex weld metal, position PE (4G), welded using flux cored wire
Avesta 2507/P100-PW (FCAW). Note that this welding position is considered the worst/most difficult for avoiding
porosity. However, the small number of discernible pores is acceptable here.
Compared with solid wires (MAG welding, GMAW), flux cored wires (FCAW) usually also
give considerably less spatter, improved wetting properties and a more even distribution of
energy in the arc. This gives better side penetration (see figure 4) and reduces the risk of lack
of fusion.
Figure 4: Note the good side penetration of the weld using flux cored wires (FCAW, FCW) compared to that using
solid wires (GMAW, MAG).
Another pure saving, one which usually surprises many people, is how large the difference
in costs for shielding gases actually is. Welding with solid wires (MAG welding, GMAW)
usually requires a three-component gas. The expensive helium makes up a considerable part
of this (normally around 30%). Flux cored wires (FCAW), on the other hand, use a
significantly cheaper two-component gas mixture (Ar +15 – 25% CO2).
Why is the highly productive submerged arc welding (SAW) method not more
commonly used when welding super duplex steel?
In normal cases, the relatively highly productive submerged arc welding (SAW) method is
recommended for welding other stainless steels, e.g. ordinary duplex (2205) and austenitic
steels (316L, etc.).
SAW generally gives a high quality weld metal. However, for super duplex grades, the high
heat input inherent in the method leads to a large decrease in, primarily, corrosion
resistance. Impact strength may also be affected. This is because the high heat input leads to
slow cooling, which can result in an altogether too high content of intermetallic phases.
These are formed (precipitated) in the weld metal’s and the parent metal’s microstructure
(primarily sigma and chi phases) and can drastically impair the above-mentioned properties.
The precipitation problem can be avoided by a well-controlled solution heat treatment,
adapted to the material.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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With the right know-how, these intermetallic phases can be kept to a low level and
satisfactory results achieved using multipass welding. For example, in the previously
mentioned TIG welding of root beads and subsequent passes, the following procedure
should be used: Without exceeding the recommended heat input (max. 1.5 kJ/mm), weld a
relatively large root bead. For the subsequent 1 – 2 beads, weld using 70 – 80% of the heat
input used for the root bead. The interpass temperature should be under 100°C. As a rule of
thumb, it can be said that, apart from the root bead, all beads are to be welded with a heat
input below 1.0 kJ/mm.
If the heat input is too low (normally under 0.5 kJ/mm), the cooling rate will be too high. This
can also be harmful as it may result in the ferrite content in the weld metal and the heataffected zone being too high. Too high a ferrite content (generally over 70%) impairs
corrosion properties and impact strength.
How does the slag’s chemical composition affect welding results?
One disadvantage of all welding methods that involve slag is that they generally give a weld
metal that has comparatively low impact strength. In its turn, impact strength is related to
the basicity of the slag. A higher content of basic substances gives a cleaner weld metal with
better impact strength. However, weldability and slag removal are usually poorer.
Why two variants of flux cored wire for super duplex stainless steel?
Avesta Welding has two variants of flux cored wires for welding super duplex stainless steel.
One of these, Avesta 2507/P100-PWNOR, gives a pure weld metal that satisfies the high impact
strength requirements in NORSOK M-601 and similar standards. The impact strength of the
pure weld metal is a minimum of 45 J at -50°C. Our other wire, Avesta 2507/P100-PW, has
slightly better weldability and slag removal. The pure weld metal manages 32 J at -40°C.
To ensure that appropriate mechanical properties and corrosion resistance are attained, both
variants are slightly over-alloyed with nickel (to give the optimum ferrite-austenite balance).
With a critical pitting temperature (CPT) above 40°C, the weld metal demonstrates high
resistance to pitting corrosion and stress corrosion in chloride-containing environments
(corrosion testing as per ASTM G48-A).
Both variants have been developed for welding in all positions. They give a stable arc and
full control of the weld pool and slag.
General rules for welding with Avesta 2507/P100-PWNOR and
Avesta 2507/P100-PW flux cored wires
•
•
•
•
•
Weld in joints that have a root gap (a few exceptions).
Use a larger groove angle than is normal for joints in austenitic stainless steels.
In automated welding, use a reduced welding speed.
To avoid pores, do not deposit weld beads that are altogether too thick.
For fully satisfactory corrosion properties, pickle and passivate after welding.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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Results
Figure 5. Note the difference in heat input between pass 1 (TIG welding, GTAW) and passes 2 – 4 (flux cored
wire, FCAW).
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18
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Figure 6: Corrosion testing of Avesta 2507/P100-PW weld metal – pickled samples tested as per ASTM G48
method E at 40°C.
Avesta FCW
2507/P100-PW
Typical chemical compositions of pure weld metal, weight %
C
N
Cr
Ni
Mo
PREN
0.03
0.23
25.3
9.8
3.7
> 41
FN
≥ 30
Table 2: Chemical composition of Avesta 2507/P100-PW.
Product
(EN ISO)
E 25 9 4 N L
Tensile test, pure weld metal
Rp0.2
Rm
A5
MPa
MPa
%
650
850
30
Impact strength (ISO-V)
+20°C
-50°C
Z
%
38
60
Table 3: Mechanical values for pure weld metal from Avesta 2507/P100-PW
Tensile test
45
NOR
, welding position PA.
Impact strength (ISO-V)
Rp0.2
MPa
Rm
MPa
A5
%
Fracture
+20°C
J
-20°C
J
+46°C
J
706
844
28
Parent metal
52, 56,
63
51,
51, 52
37, 49,
44
Table 4: Mechanical values for weld metal from Avesta 2507/P100-PW
position PF, V-joint in 15 mm plate.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
NOR
, shielding gas Ar + 18% CO2, welding
2012-10-18
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Type
Side bend test
Mandrel diam., mm
4xt
Table 5: Avesta FCW 2507/P100-PW
Position
Condition
PA
As-welded
Pickled
As-welded
Pickled
PF
Bend angle
180°
Results
No cracks
NOR
. Bend test, V-joint, 15 mm plate welded against a ceramic root backing.
Weight loss, g/m
40°C
4.83*
1.52
5.23
0.95
3
Visual appearance
*Pitting corrosion visible to
naked eye; other corrosion
visible only with macroscope
Table 6: Results of corrosion testing as per ASTM G48 method A. 12 mm plate, V-joint welded against a ceramic
NOR
root backing, Avesta FCW 2507/P100 .
Position
PA
Wire diam.
mm
1.2
Current
A
130 – 220
Voltage
V
23 – 31
Wire feed
m/min
6 – 12.5
Table 7: Recommended welding data for Avesta FCW 2507/P100-PW
NOR
Gas flow
l/min
15 – 18
Stick-out
mm
and 2507/P100-PW.
References
Avesta 2507/P100-PW and Avesta 2507/P100-PWNOR have already been introduced in France,
Germany, the Netherlands, the UK, Denmark, Finland and Spain. Avesta 2507/P100-PWNOR
has proved very successful for welding pipes in the offshore industry and pressure vessels
subject to severe demands in respect of impact strength.
Avesta Welding’s flux cored wire for welding super duplex stainless steels
2012-10-18