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Duplex Stainless Steels - Fabrication &
Welding
With the ever-increasing demand
for duplex stainless steel process equipment fabricators have developed
procedures for the welding and fabrication of these grades. A lot of data
on these procedures as well as practical experiences have become
available. When fabricating duplex stainless steels special attention
should be paid to heat treatment and welding. Unsuitable heat treatment
can result in precipitation of intermetallic phase and deterioration of
toughness and corrosion resistance. Although most welding methods can be
used to weld duplex steels, they require special procedures for the
retention of properties after welding. Below you will find some general
guidelines for welding duplex stainless steels
Introduction
It is assumed that the reader already has experience in welding
of austenitic stainless steels such as Type 316L.This section addresses
some to commonly discussed welding characteristics and procedures of the
duplex stainless steels in terms of how they differ from austenitic
stainless steels. Addressing each of these features is essential for the
design of technically and economically effective welding procedures to be
qualified.
Differences between Duplex and
Austenitic Stainless Steels
Duplex stainless
steels are typically twice as strong as common austenitic stainless
steels. The thermal expansion of the duplex grades is intermediate to that
of carbon steel and the austenitic stainless steels. The thermal
conductivity of the duplex stainless steels is also intermediate to that
of carbon steels and the austenitic stainless steels.
When there
are problems with welding of austenitic stainless steels, those problems
are most frequently associated with hot cracking of the weld metal itself.
This hot cracking tendency is aggravated by fully or predominantly
austenitic solidification, and by the combination of high thermal
expansion and low thermal conductivity. For the more common austenitic
stainless steels, hot cracking is minimised by adjusting the composition
of the filler metal to provide a significant ferrite content. For the more
highly alloyed austenitic stainless steels where the use of a nickel-base
filler metal is necessary, austenitic solidification is unavoidable. In
these cases these problems must be managed by minimising joint constraint
and by low heat input, often requiring many passes to build up the
weld.
Duplex stainless steels have good hot cracking resistance.
Hot cracking of the duplex weld metal is seldom a concern. The problems
most typical of duplex stainless steels are associated with the
heat-affected zone (HAZ), not with the weld metal. The HAZ problems are
not hot cracking but rather a loss of corrosion resistance and toughness,
or of post-weld cracking. To avoid these problems, the welding procedure
should focus on minimising total time at temperature in the "red hot"
range for the whole procedure rather than managing the heat input for any
one pass. Experience has shown that this approach can lead to procedures
that are both technically and economically optimal.
The data shown
in the appendix of ASTM A 923 suggest how rapidly intermetallic phases can
precipitate to the extent that corrosion resistance and toughness are
significantly affected.
With this introduction in mind, it is
possible to give some general guidelines for welding of duplex stainless
steels and then to apply this background and those guidelines to specific
welding methods.
The welding characteristics of duplex
stainless steels are much more sensitive to minor within-grade variations
in chemistry or processing than are austenitic stainless steels. For
example, the importance of having sufficient nitrogen in the duplex
stainless steel base metal has been repeatedly emphasised. Air cooling of
a plate, even when rapid, through the 705 to 980°C (1300 to 1800°F) range
will use up some of the "time on the clock" for the welder to complete the
weld before detrimental reactions occur. Similarly, if a plate is allowed
to air cool into this range during transfer to water quenching, that time
is no longer available to the welder. The metallurgical condition of the
material used in actual fabrication should be the same quality with regard
to composition and production practice, as the material used to qualify
the welding procedure.
Cleaning Before Welding.
The need to clean prior to welding applies to all stainless
steels. But the duplex stainless steels are more sensitive to
contamination, and especially to moisture, than the austenitic stainless
steels. The chemistries of the base metal and the filler metal have been
developed assuming no additional sources of contamination. Dirt, grease,
oil, paint, and sources of moisture of any sort will interfere with
welding operations and adversely affect the corrosion resistance and
mechanical properties of the weldment. No amount of procedure
qualification is effective if the material is not thoroughly clean before
welding.
Joint Design.
Duplex stainless steels require good
joint preparation. For duplex stainless steels, a weld joint design must
facilitate full penetration and avoid autogenous regions in the weld
solidification. It is best to machine rather than grind the weld edge
preparation to provide uniformity of the land thickness or gap. When
grinding must be done, special attention should be given to uniformity of
the weld preparation and the fit-up. Any grinding burr should be removed
to maintain complete fusion and penetration. For an austenitic stainless
steel, a skilled welder can overcome some deficiencies in joint
preparation by manipulation of the torch. For a duplex stainless steel,
these techniques can cause a longer than expected exposure in the harmful
temperature range, leading to results outside of those of the qualified
procedure.
Examples of joint designs used with duplex stainless
steels are shown in Figure 1.4 Other designs are possible provided that
they assure full penetration welds and minimise the risk of
burn-through.
Examples of joint designs applied to 2205 duplex
stainless steel:
Fig. 1a)
2 mm (0.08 in) < t < 4 mm (0.16 in)
A = 1-2 mm
(0.04-0.08 in)
A. Square Butt Joint - Suitable for single-sided SMAW or
double-sided SMAW or GMAW.
Fig. 1b)
t < 2.5 mm (0.1 in)
A = 1-2 mm (0.04-0.08 in)
B. Square
Butt Joint - Suitable for GTAW from one side. Backing gas
required.
Fig. 1c)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2 mm
(0.08 in)
B = 2 mm (0.08 in)
C. Suitable for heavier sections with
SMAW or GMAW. Increase A to 3 mm (0.12 in) for vertical-up
SMAW.
Fig. 1d)
12 mm (0.5 in) < t < 60 mm (2.5 in)
A = 3 mm
(0.06 in)
B = 2 mm (0.08 in)
Radius = 6 mm (0.25 in)
D. Suitable
for very thick base metal with SMAW or GMAW.
Fig. 1e)
9 mm (0.36 in) < t < 12 mm (0.5 in)
B = 5 mm
(0.2 in)
E. Suitable for SAW. Grinding after first pass facilitates
full penetration.
Fig. 1f)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2.5 mm (0.1
in)
B = 5 mm (0.2 in)
F. Full penetration Fillet. Suitable for SMAW.
Tack weld with SMAW or GMAW.
Fig. 1g)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2.5 mm (0.1
in)
B = 2.5 mm (0.1 in)
G. Single V Joint. Pipe welding. Suitable
with SMAW
Fig. 1h)
3 mm (0.12 in) < t < 12 mm (0.5 in)
A = 1-2 mm (0.04-0.08
in)
B = 2 mm (0.08 in)
H. Single U Joint. Pipe Welding. Suitable
with GTAW.
Preheating
As a general rule,
preheating of duplex stainless steel is not recommended because it slows
the cooling of the heat-affected zone. Preheating should not be a part of
a procedure unless there is a specific justification.
Preheating
may be beneficial when used to eliminate moisture from the steel as may
occur in cold ambient conditions or from overnight condensation. When
preheating to remove moisture, the steel should be heated to about 95°C
(200°F) uniformly and only after the weld preparation has been
cleaned.
Preheating may also be beneficial in those exceptional
cases where there is a risk for forming a highly ferritic HAZ because of
very rapid quenching. Examples include welding a thin sheet to a plate, as
with a liner to a vessel or a tube to a tubesheet, or any very low heat
input weld where there is exceedingly rapid cooling.
Heat Input and Interpass Temperature.
Compared to austenitic stainless
steels, duplex stainless steels can tolerate relatively high heat inputs.
The duplex solidification structure of the weld metal is resistant to hot
cracking, much more so than that of highly austenitic weld metals. Duplex
stainless steels, with higher thermal conductivity and lower coefficient
of thermal expansion, do not create the same high intensity of local
thermal stresses at the welds of austenitic stainless steels. While it is
prudent to avoid severe restraint, hot cracking is seldom a common
problem.
To avoid problems in the HAZ, the weld procedure should
allow rapid (but not extreme) cooling of this region. The temperature of
the work piece is important because the plate itself provides the most
effective cooling of the HAZ. Typically, the maximum interpass temperature
is limited to 150°C (300°F). That limitation should be imposed when
qualifying a weld procedure, and production welding should be monitored to
assure that the interpass temperature is no higher than that used in the
qualification. Electronic temperature probes and thermocouples are the
preferred instruments for monitoring the interpass temperature. When a
large amount of welding is to be performed, planning the welding to
provide enough time for cooling between passes is good, economical
practice.
The size of the test piece used in qualifying a weld
procedure may affect the cooling rate and the interpass temperature. There
is a risk that the test piece for qualification of a multipass weld
procedure may come to a lower interpass temperature than can be reasonably
or economically achieved during actual fabrication. Therefore, the
qualification might not detect the loss of properties that can occur the
higher interpass temperature slows the cooling and increases the time at
temperature for the HAZ in actual practice.
Postweld Heat Treatment.
Postweld stress relief is not
necessary or useful for duplex stainless steels. Unlike the L-grade
austenitic stainless steels, the duplex stainless steels are sensitive to
even relatively short exposures to temperatures in the 300 to 1000 ° C
(600 to 1800° F) range. Thermal stress relief in the 300 to 700° C
(600-1300° F) range may cause precipitation of alpha prime phase ("475°C
(885°F) embrittlement"), causing a loss of toughness and corrosion
resistance. Stress relief in the range of 700 to 1000° C (1300 to 1800° F)
leads to rapid precipitation of intermetallic phases with moderate to
severe loss of toughness and corrosion resistance. Any heat treatment of a
duplex stainless steel for whatever reason, should be a full solution
anneal, meeting the minimum temperatures specified for the mill product in
the ASTM specifications, followed by water quenching. For 2205 that
minimum temperature is 1040° C (1900°F) in most cases.
Some types
of equipment manufactured from duplex stainless steel require a full
anneal. For example, the forming of large heads or the fabrication of some
valve and pipe assemblies may require annealing. When there is a full
solution anneal and quench subsequent to welding, that heat treatment is a
part of the welding procedure. Annealing can restore the equilibrium phase
balance and eliminate the problems associated with excessive ferrite and
intermetallic phases. If the common duplex filler metals are used,
typically overalloyed with nickel, phase balance in the fully annealed
weld may shift toward austenite. Water quenching is essential after the
final anneal, but air cooling from intermediate thermal exposures, such as
in hot forming, have been found to be practical and economical.
Phase Balance in the Weld.
Modern
duplex stainless steel mill products are balanced to have about 40-50%
ferrite with the balance being austenite. It is generally agreed that the
characteristic benefits of duplex stainless steels (strength, toughness,
corrosion resistance, resistance to stress corrosion cracking) are
achieved when there is at least 25% ferrite with the balance
austenite.
The ferrite in the weld metal is typically in the range
of 25 to 60%. In some welding methods, particularly those relying upon
flux shielding, the phase balance of the filler has been adjusted toward
more austenite to provide improved toughness, offsetting the loss of
toughness associated with oxygen pickup from the flux. There have been no
reports of problems associated with the ferrite contents at the lower end
of this range, typically seen in SMAW (shielded metal arc, or stick) or
SAW (submerged arc) welds.
Rapidly
quenched autogenous welds, e.g., arc strikes, repair of arc strikes, small
GTA repair welds, etc., tend to have high ferrite, greater than 60%. Such
welds can have low toughness and reduced corrosion resistance.
Metallographic
evaluation of the phase balance in the HAZ is an appropriate test for
welding procedure qualification. However, metallographic evaluation is not
technically or economically effective for evaluation of annealed mill
products or production welds. Magnetic evaluation of the phase balance is
widely used but has serious accuracy limitations when used on welds or
HAZ.
Dissimilar Metal Welds
Duplex stainless steels can be welded
to other duplex stainless steels, to austenitic stainless steels, and to
carbon and low alloy steels.
Duplex stainless steel filler metals with
increased nickel content relative to the base metal are most frequently
used to weld duplex stainless steels to other duplex grades.
When
welding duplex stainless steels to austenitic grades, the austenitic
filler metals with low carbon and a molybdenum content intermediate
between the two steels are typically used. AWS E309LMo/ER309LMo is
frequently used for these joints. The same filler metal or AWS
E309L/ER309L is commonly used to join duplex stainless steels to carbon
and low alloy steels. Because austenitic stainless steels have lower
strength than duplex grades, welded joints made with austenitic filler
metals may not be as strong as the duplex base metal.
When welding the
highly alloyed austenitic stainless steels, nickel-base fillers are used.
The nickel-base filler metals are not normally used for duplex stainless
steels, but if they are, they should be free of niobium (columbium).
Although not thoroughly documented, there have been suggestions that the
ENiCrMo-3 filler (625) has been less than satisfactory, possibly because
of interaction of the niobium from the filler with the nitrogen from the
duplex base metal.
Table I summarises filler metals frequently used
to weld duplex stainless steels to dissimilar metals. These examples show
the AWS bare wire designation (ER), but depending on the process, joint
geometry and other considerations, electrodes (AWS designation E), and
flux-cored wire may be considered.
Table I: Welding Consumables
Used for Dissimilar Metal Welding.
| |
2304 |
2205 |
25 Cr |
Superduplex |
| 2304 |
2304 / ER2209 |
ER2209 |
ER2209 |
ER2209 |
| 2205 |
ER2209 |
ER2209 |
25Cr-10Ni-4Mo-N |
25Cr-10Ni-4Mo-N |
| 25 Cr |
ER2209 |
25Cr-10Ni-4Mo-N |
25Cr-10Ni-4Mo-N |
25Cr-10Ni-4Mo-N |
| Superduplex |
ER2209 |
25Cr-10Ni-4Mo-N |
25Cr-10Ni-4Mo-N |
25Cr-10Ni-4Mo-N |
| 304 |
ER309LMo
ER2209 |
ER309LMo
ER2209 |
ER309LMo
ER2209 |
ER309LMo |
| 316 |
ER309LMo
ER2209 |
ER309LMo
ER2209 |
ER309LMo
ER2209 |
ER309LMo
ER2209 |
Carbon steel
Low alloy steel |
ER309L |
ER309L |
ER309L |
ER309L |
Applicable Welding Methods
Second-generation (nitrogen-alloyed) duplex stainless steels saw
rapid development in the early 1980s. With only limited understanding of
the formation of intermetallic phases, early views of welding duplex
grades focused on limiting heat input, possibly because this approach is
what is typically applied to special austenitic grades. With such severe
limitations on heat input, many of the more economical welding methods
with high deposition rates, such as submerged arc welding, were thought to
be inappropriate for the duplex stainless steels. However, the final
properties of the duplex stainless steels are of such interest that much
effort was directed to learning how to use the more economical processes.
Now virtually all welding processes, except for oxyacetylene with its
associated carbon contamination of the weld, are applied to duplex
stainless steels. Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding
(GMAW), Shielded Metal Arc Welding (SMAW), Flux Core Arc Welding (FCAW),
Submerged Arc Welding (SAW), and Plasma Arc Welding (PAW) have all seen
practical application. Electric Resistance Welding and Electron Beam
Welding, although much less common, have also been qualified and used in
particular fabrications. There are important differences among the welding
procedures. For example, the decision to use a flux-shielded weld and
selection of flux for that weld effect toughness.
Welding Procedure Qualification
"Qualification" of welding procedures for duplex stainless
steels must be considered in a broad sense, i.e., demonstration that the
welding procedure that will be used in fabrication will have acceptable
engineering properties, especially toughness and corrosion resistance. For
other types of stainless steels, qualification testing for weld procedures
is fairly simple, with only a limited amount of testing to qualify a
material, filler metal, and weld method. With hardness tests and bend
tests, looking for martensite and hot cracking, respectively, these
qualification tests reflect long experience for what can go wrong when
welding ferritic, martensitic or austenitic steels. Duplex stainless
steels are unlikely to have difficulty meeting these requirements, because
these standard tests are unlikely to find intermetallic phases or
excessive ferrite, the most likely problems for duplex stainless steels. A
bend test may still be useful and economical, but it is not conservative
in the sense of always detecting problems if present. Because of the
limitation on total time at temperature for the HAZ, the properties of
duplex grades will be sensitive to section thickness and details of actual
welding practice. So the qualification of procedures for duplex stainless
steels is specific to particular geometries of welding, much more so than
for austenitic stainless steels.
It would be desirable to qualify a
weld procedure for every thickness, geometry, and method of welding
because minor differences in setup may be significant in the results
achieved in production. However, the complex nature of actual
constructions makes such testing costly. Savings are achieved by
qualifying the procedures (defined by section, filler, and method)
determined to be the most demanding on the duplex stainless steel. It is
also prudent to test welds in the most critical joints in a construction,
even when those joints might be exempt from testing of production welds
under a strict reading of the ASME requirements. For example, ASME UHA 51
does not require testing for thickness of 3/8-inch or less, or for minimum
design metal temperature above -29° C (-20° F).5 The temperature for
toughness tests will depend on whether the purpose of the test is to check
the metallurgical condition of a mill product or to demonstrate the
suitability for use of a construction.
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