Corrosion performance of welds in duplex

A CORROSION MANAGEMENT AND APPLICATIONS ENGINEERING MAGAZINE FROM OUTOKUMPU
1/2014
Corrosion performance of welds
in duplex, superduplex and lean
duplex stainless steels
2
1/2014 |
Corrosion performance of welds
in duplex, superduplex and lean
duplex stainless steels
Rachel Pettersson, Avesta Research Centre, Outokumpu Stainless AB, Avesta, Sweden
Mikael Johansson, Avesta Research Centre, Outokumpu Stainless AB, Avesta, Sweden
Elin M. Westin, Avesta Research Centre, Outokumpu Stainless AB, Avesta, Sweden
Now at: Böhler Schweißtechnik GmbH Austria, Kapfenberg, Austria
Abstract
To achieve good corrosion resistance in a weld is much more
challenging than for plate or sheet material, which has undergone
controlled rolling, heat treatment and pickling processes in the
steel mill. The choice of weld procedure and parameters, filler
metal and shielding gas all play important roles. For duplex and
lean duplex grades it is essential to ensure that an adequate
amount of austenite is formed in the weld metal and heat affected
zone, in order to fulfil requirements in terms of austenite-ferrite
phase balance, but also to avoid the precipitation of detrimental
phases such as intermetallics and nitrides. In addition, the
importance of good post-weld cleaning to ensure good corrosion
properties cannot be over-emphasised.
This paper focuses on a number of different examples of pitting
corrosion of welds in the duplex grades UNS S32101, S32304,
S32205, S82441 and S32750. These illustrate the role of
microstructure control to attain good corrosion performance.
The effect of residual weld oxides or heat tint in degrading the
corrosion performance is also explored, together with evaluation
of the degree to which the corrosion resistance can be restored
by the use of appropriate pickling processes.
modified in terms of composition by addition of filler metal, and
the surrounding heat affected zone in which there is an imposed
heating and cooling cycle on the base material. An indication of
the transformations which can occur is shown in the equilibrium
diagram in Figure 1. On solidification from the melt, ferrite is
formed first, then some of this ferrite transforms to austenite in
the solid state. In practice what this means for welding is that an
almost fully ferritic weld can be produced if the cooling is very fast
so that there is insufficient time for the transformation to austenite
to occur. This has the further drawback that nitrogen, which
partitions preferentially to the austenite phase, may become
trapped in the ferrite where it forms nitrides. This may typically
occur if the heat input is low and is typically encountered for laser
welds in which the melted volume is small and cools rapidly due to
heat loss to the surrounding material. Slower cooling allows time
for an adequate amount of austenite to form but should not be so
slow as to promote the precipitation of intermetallic phases such
as sigma phase or, at lower temperatures, nitrides and carbides.
0.9
0.8
Duplex stainless steels combine a number of attractive features.
The two phase microstructure, with approximately equal amounts
of austenite and ferrite, imparts a higher strength than the
corresponding austenitic grades and a good resistance to stress
corrosion cracking. The lower nickel contents of duplex grades,
typically in the range 1 – 7% compared with 8 – 25 % in the
austenitic grades also gives cost advantages and better price
stability in times of nickel price volatility. In the steel mill, the
duplex stainless steels are produced by a very well controlled
process of rolling, annealing and pickling in order to impart the
optimal properties to the material, whether it be as thick plate,
pipe, tube, precision strip or bar.
However, the vast majority of applications require welding,
which introduces the metallurgical challenges that heating should
not give rise to undesirable phase changes and that a favorable
structure must be reformed in a matter of seconds after melting.
This applies both to the weld metal, which is melted and can be
Phase fraction (NP°)
Key words: Stainless, duplex, weld, corrosion, oxide
Introduction
Melt
1.0
Ferrite
Austenite
0.7
0.6
0.5
0.4
Sigma phase
0.3
0.2
0.1
0.0
500
Nitrides, carbides
1000
1500
Temperature (°C)
Figure 1 Equilibrium phases in a 22Cr duplex stainless steel as a function of
temperature, calculation using ThermoCalc software with the TCFE5 database.
The microstructural issues to be faced when welding duplex
stainless steels are thus multiple and only when a satisfactory
microstructure is attained will adequate corrosion properties be
achieved. Having said that, however, there is a certain degree of
tolerance to the presence of minor amounts of precipitated
phases before the corrosion performance is affected.
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1/2014 |
Materials
Equivalent (PREN) values. This is here defined as PREN = % Cr +
3.3 (% Mo) + 16 (% N) and is a rough predictor of the localized
corrosion resistance for a stainless steel in the perfectly annealed
state with a good surface finish.
The focus of the present work is on the corrosion performance
of sensitised materials and welds. For details of the welding
processes per se reference is made to the quoted data sources.
The materials investigated in this work were five duplex stainless
steels spanning a wide range of alloying levels and thus corrosion
resistance. Because a number of different heats were used for
the various welds, the typical compositions are given in Table 1.
All materials were in the mill annealed + pickled condition with 2B
or 2E surfaces. Also included in Table 1 are the Pitting Resistance
UNS
EN
Cr
Ni
Mo
Mn
N
PREN
S32101
1.4162
21.5
1.5
0.3
5
0.22
26.0
S32304
1.4362
23
4.8
0.3
1
0.1
25.6
S82441
1.4662
24
3.6
1.6
3
0.27
33.6
S32205
1.4462
22
5.7
3.1
1
0.17
35.0
S32750
1.4410
25
7
4
1
0.27
42.5
Table 1 Typical compositions of the duplex stainless steels alloys investigated
Experimental
Results & Discussion
Critical pitting temperatures were evaluated using either ASTM
G48E or ASTM G150 and critical crevice corrosion temperatures
using ASTM G48F. The G48 methods involve immersion testing in
which separate specimens are exposed to 6 % FeCl3 + 1 % HCl for
a period of 24 hours at different temperatures. The disadvantage
of using these method for welds is that both top and root surfaces
are exposed during the test, so no differentiation can be made
between the two, also that the cut edges of the specimen are
exposed. This may be considered unrealistic since such cross-sections are seldom exposed to a corrosive medium in any actual
application. The G48 immersion testing is also a fairly rough
evaluation method, since steps of 5 °C are used according to the
standard and some scatter is also seen between duplicates.
The electrochemical ASTM G150 method allows only a small
area, delineated by a water-flushed filter paper, to be in contact
with the test solution of 1 M NaCl. An applied potential of +700
mVSCE is imposed then the temperature increased by 1 °C /minute
from 0 °C until an irreversible current increase indicates the onset
of pitting corrosion. The drawback with this method is that it
requires a close tolerance between the specimen holder and the
area to be tested, otherwise leakage will occur, so special seals
were prepared for testing over the raised weld reinforcement. In
addition, it was found that the presence of weld oxides frequently
caused the measurement to terminate shortly after starting. The
reason was that dissolution of the oxide occurred, giving rise to
an increase in measured current without any actual pitting
corrosion. In some cases measurements were made in a more
dilute solution of 0.1 M NaCl, which increased the CPT by around
5 – 10 °C. All measurements were made as at least duplicates,
and the scatter between measurements was typically 1 – 3 °C.
Critical pitting and crevice corrosion temperatures for the duplex
base materials are shown in Figure 2. Good correspondence is
actually seen between the G48E and G150 CPT values, although
the former are a few degrees lower and the difference becomes
more pronounced at higher alloying levels. The CPT for the lean
duplex S32101 is 15 °C in G48E and in the range 15 – 20 °C in
G150, while the corresponding temperatures for the highest
alloyed S32750 were 65 °C for G48E and 80 – 85 °C for G150.
The two can never be used interchangeably.
100
CPT G48E
CCT G48F
Critical temperature (°C)
80
60
40
20
0
S32101
S32304
S82441
S32205
S32750
S32205
S32750
100
Critical pitting temperature (°C)
Normal range
CPT G150
80
60
40
20
0
S32101
S32304
S82441
Figure 2 Typical critical temperatures for pitting and crevice corrosion of base
metal of different duplex base materials
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If duplex stainless steels are subject to slow cooling, as may occur
in conjunction with welding with a very high heat input, or
inappropriate heat treatment in the temperature range
300 – 1000 °C (300 – 800 °C for the leaner duplex grades)
microstructural changes, including precipitation of secondary
phases and 475 °C embrittlement, can occur. This is illustrated in
Figure 3 which includes typical microstructures after longer heat
treatment times. The effect of holding times of 1, 10 or 30
minutes at either 850 °C or 700 °C on pitting corrosion resistance
is shown in Figure 4. This indicates that a slight drop in CPT occurs
after only 1 minute at 850 °C for the standard duplex S32205 and
the superduplex S32750, with an appreciable decrease occurring
after 10 minutes at this temperature. Scatter bands have been
omitted from the figure for clarity, but are detailed in [1]. The
grades S32304 and S32101 are largely insensitive to heat
treatment in this temperature range, because their leaner alloying
concept means that they are remarkably resistant to precipitation
of intermetallic phases. However, they do show some decrease in
CPT as a result of holding times at 700 °C, where precipitation is
dominated by nitrides and carbides which can form in the phase
boundaries.
11000
S32750
1000
Temperature (°C)
900
S32205
Intermetallics
S32101
800
700
Carbides
& nitrides
S32304
600
500
Spinodal
”475 C”
embrittlement
400
300
0.01
(36s)
0.1
(6 min)
1
10
100
1000
Time (h)
Figure 3 Precipitation kinetics for duplex grades evaluated as the conditions required to give a 50 % reduction in impact toughness, and microstructures illustrating
intermetallic phases, predominantly sigma phase, and carbides/nitrides
100
100
700 °C
90
S32750
80
70
S32205
60
SensitisedCPT (°C)
SensitisedCPT (°C)
S32750
80
70
0.1 M NaCl
50
S32304
40
S32101
30
20
S32205
60
0.1 M NaCl
50
S32304
40
S32101
30
20
10
10
1 min
0
850 °C
90
0
20
40
10 min
60
Base metal CPT (°C)
30 min
80
1 min
100
0
0
20
40
10 min
60
30 min
80
Base metal CPT (°C)
Figure 4 Effect of holding time at 700 °C or 850 °C in decreasing the CPT compared to that of the base material. 700 °C is the more detrimental temperature
for the lean grades S32101 and S32304 while 850 °C has a larger influence for the standards S32205 or the superduplex S32750
100
5
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Figure 5 Typical pitting in a 22Cr duplex weld, showing how ferrite phase is
attacked while austenite remains more resistant [2].
100
S32705
N2 (Pickled)
N2 (as-welded)
Ar (Pickled)
Ar (as-welded)
90
80
70
Weld CPT (°C)
In a welded structure, as opposed to a base material which has
been sensitized to provoke the type of precipitation which can
occur in association with welding, the basic phase morphology is
also altered. Since the weld is fully ferritic on solidification from the
melt, austenite is reformed at the ferrite grain boundaries and as
Widmanstätten austenite in crystallographic orientations within the
ferrite grains, as illustrated in Figure 5. There is a difference in
composition between the austenite and ferrite phases: the former
is enriched in nickel and nitrogen and the latter in chromium and
molybdenum, and this means that there is usually some difference
in corrosion resistance between them. This is also illustrated in
Figure 5, in which it is seen that pitting has preferentially occurred
within the ferrite phase.
Results from a series of CPT measurements on GTA welds in
the five duplex grades are shown in Figure 6, based on data taken
from [3, 4]. This shows a number of interesting trends. Firstly,
the as-welded CPT is between 20 and 40 °C below that of the
corresponding base material, with the result that no as-welded
CPT can be determined for the lean duplex grades S32101 and
S32304. The larger drop is seen when argon is used as a shielding
and backing gas, but this can be mitigated by using nitrogencontaining shielding and backing gases. The primary reason is
that nitrogen per se improves pitting corrosion (as seen in the
PREN formula) and promotes reformation of austenite to give a
more advantageous phase balance. However, pickling the welds
to remove the very slight amount of weld oxide can almost restore
the CPT to the same level as that of the unwelded base material,
if nitrogen shielding is employed.
An even more demanding welding process is spot welding,
because the low heat input and thus rapid cooling can give a very
ferritic weld. Figure 7 using data from [5] shows how a drop of
around 15 °C in CPT occurs as a result of resistance spot welding
thin sheets of duplex steels. In the case of the lean duplex
S32101, however, this could be restored to a similar level as the
base material by the use of a protective argon atmosphere to
minimize the oxidation of the spot weld. This was particularly
notable since the weld microstructure was far from optimal, with a
high ferrite content of 75 – 85 % and some nitrides present in the
microstructure. The austenite formation was somewhat lower for
the other two duplex grades, S32304 and S2205, due to the lower
nitrogen contents in these grades.
S32205
60
S82441
50
S32304
40
S32101
30
20
10
0
0
20
40
60
80
100
Base metal CPT (°C)
Figure 6 Critical pitting corrosion temperatures according to ASTM G150 for
the root side of single-side GTA welds on 1 mm material with argon as both
shielding and backing gas or with Ar + 2 % N2 as shielding gas and 90 % N2 +
10 % H2 as backing gas, based on data from [3,4].
100
Shielding gas
Visible oxide
90
No shielding
Shielding oxide
80
S32205
Spot weld CPT (°C)
70
60
S32304
50
40
30
S32101
20
10
0
0
20
40
60
80
100
Base metal CPT (°C)
Figure 7 ASTM G150 critical pitting temperature for spot welds in three duplex grades. The beneficial effect of using a shielding gas is apparent for S32101 and
gives a CPT on a par with the base material, in spite of the presence of nitrides in the microstructure, based on data from [5].
1/2014 |
The corrosion performance of a number of welds in the lean duplex
grade S32101 are shown in Figure 8, based on data from [6,7]. In
each case the base material CPT is given as a reference point, this
shows some variation because of the different surfaces and
product forms tested. All welds were pickled prior to corrosion
testing, attempts to test the GTA welds with residual weld oxides
caused the CPT measurement to terminate just after the starting
temperature of 0 °C due to oxide dissolution but no pitting. The
figure shows that in the majority of cases the weld CPT was close
to that of the base material. This even applies to autogenous
welds, where there is no filler metal added to adjust the weld
GTA-2.5 mm (A)
GTA-2.5 mm (B)
GTA-2.5 mm (C)
GTA-5 mm (A)
GTA-5 mm (B)
GTA-5 mm (C)
Nd:YAG-1 mm (A)
Nd:YAG-1 mm (B)
Nd:YAG/GTA-1 mm (B)
6
composition. Adding nitrogen via the shielding gas could have a
beneficial effect on the weld microstructure, as is illustrated in the
microstructures, but for the autogenous GTA welds was found to
have only a minor influence on the CPT, increasing it from 15 to
17 °C. Even most laser welds showed acceptable CPT values,
above the normally specified minimum of 15 °C for base material.
The exceptions were the fiber laser weld in 2 mm material and the
Nd:YAG laser weld in 1mm material, but in both cases a switch to
hybrid welding with leading GTA or GMA gave a significant
improvement.
(A)
Base material
Weld
Top
Weld with nitrogen
CO2-1 mm (A)
CO2-2 mm (A)
Fiber-1 mm (A)
Fiber/GMA-1 mm (C)
Fiber-2 mm (A)
Fiber/GMA-2 mm (C)
Nd:YAG-1 mm (A)
Nd:YAG-1 mm (B)
Nd:YAG/GTA-1 mm (B)
Root
(B)
0
10
20
30
40
50
CPT (°C)
Figure 8 Critical pitting temperatures for various welds in the lean duplex grade S32101, based on data from [6,7]. These include GTA and Nd:YAG, CO2 or fiber laser
welds which were either autogenous (A) or with a filler (B = W 23 7 N L, C = W 22 9 3 NL or G 22 9 3 NL). Micrographs show a reduction in the amount of surface
nitride for GTA welds with Ar + 2% N2 (lower photograph) compared to pure argon (top) but little difference in CPT. The lower CPT values (circled) were improved by
hybrid welding.
1/2014 |
7
Conclusions
References
Results presented in this paper have demonstrated that excellent
pitting corrosion resistance, on a par with that of the base material,
can be achieved for welds in duplex stainless steels. A critical
factor is the removal of weld oxide by pickling, or minimization of
oxidation by efficient use of shielding and backing gas. Nitrogen
additions to the shielding gas can also have a beneficial effect on
weld metal pitting resistance by increasing the weld metal nitrogen
content and promoting austenite reformation. The high nitrogen
content of the lean duplex steel UNS S32101 means that good
corrosion resistance can be achieved even in autogenous GTA
welds, laser welds and resistance spot welds.
[1] H. Liu, P. Johansson and M. Liljas: Structural evolution of LDX
2101 (EN1.4162) during isothermal ageing at 600 – 850 °C.
Proc. 6th European Stainless Steel conference, Helsinki
2008).
[2] E. M. Westin: Pitting corrosion resistance of GTA welded lean
duplex stainless steel. Welding in the World 54 (2010) 11/12
R308-321.
[3] E. M. Westin & D. Serrander: Experience in welding stainless
steels for water heater applications. Welding in the World I56
(2012) 5/6 14 – 28. IIW Doc.-No. IX-2357-11
[4] E. M. Westin, M. M. Johansson and R. F. A. Pettersson: Effect
of nitrogen-containing shielding and backing gas on the pitting
corrosion resistance of welded lean duplex stainless steel
EN 1.4162. IIW Doc. II-C-437-11. Accepted for publication in
Welding in the World.
[5] A. Thulin, M. Johansson, S. Mameng: Properties of resistance
spot welded duplex stainless steel. Proc. Duplex Stainless
Steel World 2010
[6] E. M. Westin: Corrosion resistance of welded lean duplex
stainless steel. Proc. Stainless Steel World 2008.
[7] M. M. Johansson, E. M. Westin, J. Oliver and R.F.A. Pettersson:
Localised corrosion resistance of welded austenitic and lean
duplex stainless steels. Welding in the World 55 (2011) 9/10
19 – 27.
Reproduced with permission from NACE International, Houston, TX.
All rights reserved. Paper C2013-002697 presented at CORROSION/2013, Orlando, FL.
© NACE International 2013.
1357.EN-GB, Art 58, 04, 14.
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