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The following
questions reflect the experience of fabricators in the questions
most typically asked during fabrication of duplex stainless
steel. Answers are suggested but in these practical matters,
there is a wide range of possibly "correct" answers. The answer
given may not be applicable to all possible situations.
1. Although it is recommended to use plasma
torches for back gouging of the root and defect removal, can
a conventional carbon arc be used? What is a minimum grinding
that should follow arc gouging in order to remove heat-affected
layer?
Carbon arc back gouging has been successfully
used in the construction of 2205 duplex stainless steel vessels,
but care must be taken to minimise the heating and the potential
for carbon contamination. When care is taken in the back gouging
procedure, the minimum grinding is not burdensome. It would
be appropriate to perform a weld procedure qualification in
which the typically applied back gouging has been included
as it will be used in the practical construction.
2. What is the maximum allowed thickness
reduction resulting from cold forming before solution anneal/
water quenching treatment would be required?
A precise answer to this question has not been
developed. However, it has been common in construction of
2205 duplex stainless steel vessels to apply the same limits
that are applied to carbon steels by the ASME Code. This limitation,
while possibly overly conservative, has not led to any problems
in service. For vessels not being constructed to ASME Code,
significantly more aggressive deformation has been permitted,
with no reports of problems attributed to this cold worked
condition.
3. What is a proper method of repairing
small defects and metal tears on the process side (for example,
caused by knocking off of the pre-cut ladder supports and
lifting lugs - usual method of removal)?
The repair procedure most typically satisfactory
is to open the defect by grinding, if necessary, and then
to repair by GTAW with the typical matching filler. Because
of the size of the weld involved, it is unlikely that small
defects or tears will lead to excessive time at temperature
for these repair welds. However, care must be taken to avoid
too rapid cooling of the weld (with resulting excessive ferrite).
Slight warming of the metal under shielded conditions using
the weld torch before the filler is introduced will typically
prevent too rapid cooling. Autogenous repairs are not recommended
because of the likelihood of forming excessive ferrite in
the weld.
4. Excessive heat input may result from
weld repair of the defect discovered by post-weld NDT. Should
such procedure be pre-qualified, and how?
It is appropriate to demonstrate that the
weld repair has not damaged the material, i.e., to qualify
the repair in much the same way as the procedure was qualified.
So fabricators have qualified reasonably anticipated repair
procedures in advance. Alternatively, the repair practice
can be documented and simulated on a production runout tab,
with the usual production test plate procedures then being
applied to the repaired weld.
5. What is in fact the upper limit for weld
heat input, provided base metal fully passed A-923 criteria?
Because the goal is to limit total time at
temperature, it is generally better to complete a weld in
fewer passes with relatively high heat input than many passes
of lower heat input. The duplex stainless steels can tolerate
relatively high heat inputs. It is not impossible to hot crack
a duplex stainless steel during welding, but it is rare. The
duplex stainless steels have relatively low thermal expansion
and high thermal conductivity. The solidification of the duplex
filler metals is not prone to hot cracking as is a fully austenitic
solidification. Maximum heat input values as high as 65-100
kJ/mm have been found to be satisfactory, depending on the
welding process.
6. Can heat input be allowed below the mentioned
bottom value of 0.5 kJ/mm as long as the ferrite content does
not exceed 70% (for example, due to the over-alloying of the
base and electrodes)?
Exceedingly low heat input is permitted, provided
that the result is demonstrated to meet the usual requirements
for phase balance and corrosion resistance.
7. Does soda lime glass bead blasting
provide an adequate finish for corrosive service, as an alternative
to pickling and what is the recommended surface profile range?
Whether or not a glass blasting will be sufficient
for corrosive service will depend on the degree and nature
of the oxidized surface and the corrosivity of the service,
including the tendency of the medium to adhere to the surface
of the steel. While a pickled surface provides corrosion resistance
to the maximum capability of the grade, a thoroughly blasted
surface may be sufficient and economical. Scale and heat tint
for the duplex stainless steels are especially adherent and
resistant to both mechanical and chemical removal.
8. What is the best way to prepare weld/HAZ
specimens for A 923 Method C testing?
The specimen should be removed by the method least disruptive
of the metal condition. Cold cutting is recommended if possible.
If a hot cutting method is applied, then there should be further
cold cutting or grinding to remove all material that was affected
by the hot cutting. In order to avoid weight loss during the
test that could be associated with heat tint, it is a good
practice to pickle the whole specimen before final grinding
of the specimen surfaces. However, the surfaces that are actually
tested should be as-ground without any subsequent pickling
or other chemical treatment that might clear the surface of
detrimental phases. It is permitted to leave the weld faces
of the specimen in the as-pickled condition as long as the
cross-sectional edges are tested in the ground condition.
A slight chamfering of the specimen is helpful, but the should
not be substantial rounding off of the edges. The presence
of burrs on the edges will cause weight losses not related
to the presence of intermetallic phases. Corrosion attack
on the edges must be included in the limiting acceptance criterion.
"Modified G 48" procedures that permit disregarding of edge
corrosion are not correctly testing for the presence of detrimental
intermetallic phases. If intermetallic phases are present,
they are much more likely to occur within the metal, and therefore
be exposed on the specimen edges, than on the faces of the
product.
9. Is "modified G 48" testing the same thing
as A 923 Method C?
ASTM G 48 Practice A and A 923 Method C are
similar to the extent that they use similar equipment and
laboratory procedures. However, they are substantially different
in their application. ASTM G 48 is a description of laboratory
procedure, but it does not specify the temperature of testing,
the time of exposure, the technique of assessing corrosion,
and an acceptance criterion. The "modified G 48" test indicated
that the individual ordering specification was attempting
to address these deficiencies, but few specifications addressed
all of them. ASTM A 923 Method C specifically addresses each
of these issues, and provides a basis for acceptance of the
duplex stainless steels with regard to the absence of detrimental
intermetallic phases.
One important difference is that G 48 permits the tester to
disregard corrosion on the edges of the specimen. This permission
is totally inappropriate for use of the test to demonstrate
the absence of intermetallic phases in duplex stainless steels.
It is unlikely that the intermetallic phases will occur in
the faces of the plate or the faces of the weld, but rather
will occur in the interior of the metal. Therefore, incidents
of pitting on the edges of the sample should be considered
indicative of a problem, and not ignored.
G 48 is usually a procedure performed at a series of temperatures,
with the goal of identifying the critical pitting temperature.
Accordingly, the time of exposure and the inspection for pitting
on the surface are designed to detect subtle pitting initiation.
The single test temperature for each grade in A 923 is chosen
to be below the critical pitting temperature for material
without intermetallic phases, and above the critical pitting
temperature for material with intermetallic phases. The pitting,
when it does occur, is readily visible. However, the weight
loss is what is measured in order to remove the potential
for debate over visual interpretation. That weight loss is
converted to a corrosion rate in order to permit different
sizes and geometries of specimens to respond to a single acceptance
criterion.
An important issue is the surface preparation of the sample.
The goal of the test is to detect intermetallic phases if
present. Chemical treatment of the specimen surface (passivation
or pickling) may reduce the exposure of intermetallic phases
in the surface and thereby cause the test not to detect the
presence of intermetallic phases. The specimen edges should
be fine ground but not chemically treated for most effective
use of the A 923 test. If there is concern that the faces
of the specimen may contribute to the weight loss, the appropriate
specimen preparation is to pickle the specimen before final
grinding of the edges.
10. When you encounter a need to weld
repair a structure of duplex stainless steel and you do not
have a detailed history of the welding during construction,
how do you decide how much welding is safe? What filler metal
do you use?
The correct answer will depend on the nature
of the weld, the conditions of application, and on the application
itself, particularly whether or not the structure was built
to ASME Code, or is being used in a situation of significant
safety risk. The safest approach is to sample the fabrication
weld and perform a qualification of the proposed repair. However,
this approach imposes extra costs and opens the necessity
to repair also the position of sampling. The value of good
records in welding fabrication is amply demonstrated by this
situation. It is appropriate to consult metallurgical engineers
before making the weld repair.
The problem, it there will be one, will most likely occur
in the HAZ of the original fabrication welds. The selection
of the filler metal is unlikely to have any favorable effect
on dealing with this part of the problem. The is no reason
that the filler metal should not be the same filler metal
that would be used with the duplex stainless steel in the
original fabrication welds.
11. Are there any special problems in cleaning
the heat tint of a duplex stainless steel?
Because of the relatively high chromium content
and the relatively low thermal expansion of a duplex stainless
steel, the oxide scale is typically thin and highly resistant
to removal. It is desirable to remove any heat tint in order
to get maximum corrosion resistance, but there are some applications
where the process itself will remove the heat tint. Grinding
to clean bright metal is effective. Blasting can also be effective
but, depending on the scale and the blasting medium, may not
be as effective as grinding for removing the oxide. Pickling,
by solution or by paste, is effective, but longer times or
more aggressive pickling chemistries are required for duplex
grades than are typically required for austenitic grades.
Passivation, in the sense of removing free iron (from tooling
contact, etc.), is no different than for austenitic stainless
steels. It is appropriate to confirm the effectiveness of
a passivation treatment by testing such as that listed in
ASTM A 967.
It should be noted that the complete removal of heat tint
may not always be necessary, depending on the application.
For example, removal of all heat tint is not required for
exposure to kraft liquor, but is desirable for service in
acid sulphite liquors.
12. When is post weld heat treatment beneficial,
and what treatments should be used?
There are no heat treatments in the 315-980°
C (600-1800° F) range that are beneficial to duplex stainless
steels. Postweld stress relief heat treatments are used with
steels that are capable of forming martensite, but duplex
stainless steels do not form martensite. The metallurgical
condition of a duplex stainless steel will be severely damaged
if it is exposed to the stress relief treatment applied to
a carbon or alloy steel (a consideration in dissimilar welds).
If the duplex stainless steel for whatever reason is exposed
to conditions that lead to the formation of intermetallic
phases, then the appropriate remedy is to heat treat the whole
structure. The only heat treatment that works for duplex stainless
steel is a full anneal above the minimum temperature listed
in ASTM A 240, (1040°C (1900° F) in the case of 2205) and
quench. When the construction cannot be annealed and quenched,
the only remaining alternatives are to scrap the whole construction,
or to cut out and replace the affected parts of the metal.
13. When is preheating useful or required?
Preheating the duplex stainless steel before
welding is useful in two situations. If the part is damp,
as from condensation, heating uniformly to a maximum of about
95° C (200° F) will avoid the problems associated with moisture
in the weld. Preheating is one alternative for avoiding welds
that are excessively ferritic as a result of too rapid quenching.
Examples include spot resistance welds, superficial surface
repair, and welding of thin material to heavy sections (sheet
liners, tube-to-tubesheet welds). As with the suggested interpass
temperature, 150° C (300° F) is an appropriate maximum temperature
for preheating.
14. What is the correct design for a runout
tab?
The fact that the purpose of the runout tab
is produce a sample of weld that is identical to the production
weld dictates the design of the tab. Ideally, the plate of
the tab is from the same heat and thickness as the workpiece.
It should be of a size that will produce neither unusual heating
or unusual cooling. It should be large enough to readily supply
the samples necessary for the qualification tests selected.
Experience indicates that tabs from 6x6xt to 12x12xt inches
finished size have been satisfactory.
Sample material can be obtained from the plate itself when
there are manways or nozzles to be cut, but this source of
samples may not always be available. When a bill-of-materials
order is made for a large project construction, with special
sizes of plate being rolled, there may not always be off-cuts
from the plates for the sample material. It is a good idea
to obtain the sample material with the purchase of the plate
in order to assure the availability of matching sample material.
15. How significant is the selection of
the temperature for Charpy tests, comparing the -40° C (-40°
F) of A 923 and the ASME minimum design metal temperature?
ASTM A 923 and ASME UHA 51 have in common
only that they both use Charpy tests. However, the purpose
on the tests for the two procedures are quite different. The
purpose of A 923 was to demonstrate that the heat treatment
applied to a duplex stainless steel mill product had eliminated
the intermetallic phases. The Charpy test was chosen because
it was familiar to producer and user. As shown in the appendix
of ASTM A 923, an acceptance criterion of 40 ft-lb at -40°
C (-40° F) was found to correlate with the appearance of the
intermetallic phase in a metallographic examination and a
loss of corrosion resistance. Impact energy was selected as
the acceptance criterion because of its intuitive meaning
and the fact that it is so readily measured in an impact test.
A 923 was not intended to demonstrate suitability for use
at this temperature. The test was chosen to demonstrate the
absence of the intermetallic phase. The high impact energy
and low test temperature were necessary in order to get a
meaningful indicator for the extremely tough annealed mill
product. A 923 states that it is not applicable to a welded
structure.
In comparison, ASME UHA 51 is designed to demonstrate suitability
for use. The temperature is minimum design metal temperature,
a factor of design specific to each installation. The standard
test of three specimens is performed using the lateral expansion
measurement to confirm results. Impact strength well below
40 ft-lb is accepted as suitable for use. It is applicable
to the whole construction, whether base metal, weld metal,
or HAZ. Where appropriate, it is permitted to use the more
demanding test conditions of ASTM A 923, but with the number
of specimens and measurements of both impact energy and lateral
expansion, to qualify for ASME UHA 51, and so reduce testing
costs.
16. Why is 20 ft-lb impact energy sufficient
for a weld when the specification for the plate requires 40
ft-lb at -40° C (-40° F)?
The ASME has determined that 20 ft-lb is an
adequate toughness for service in a particular class of applications.
This level of toughness is not high enough to correlate well
with the observance of intermetallic phase in the microstructure
and the associated loss of corrosion resistance in a duplex
stainless steel mill plate. The duplex stainless steel plate
structure is tough enough that it may still show significant
impact energy even after significant intermetallic phase formation.
On the other hand, a weld metal may occasionally have toughness
less than 40 ft-lb even when no intermetallic phase is present.
For example, weld toughness is particularly affected by the
presence of oxygen in the weld, as may occur with flux-shielded
welds.
17. Why is 25% ferrite enough for a weld,
when higher ferrite content is specified for the base metal?
The base metal is specified with a composition
that, for a fully annealed and quenched structure, will lead
to about 40 to 50% ferrite, essentially the equilibrium structure.
This chemistry is found to return rapidly to almost that balance
after the thermal cycle that occurs in the HAZ during welding,
retaining corrosion resistance and toughness. It is known
that the oxygen associated with flux shielding reduces the
toughness of the weld metal. Therefore, the compositions of
the filler metal for flux-shielded welds have been adjusted
to produce the highest austenite that can be accepted while
still retaining the benefits of the duplex structure. If there
is significant dilution from the base metal, then the weldment
will have slightly more ferrite. The 25% ferrite represents
the minimum that will be achieved in there is essentially
no dilution, as in a capping pass.
18. Is it necessary to water quench after
every heat treatment of a duplex stainless steel?
It is necessary to water quench after the
final anneal of a mill product or of a constructed and heat-treated
component such as a head, fitting, or forging. However, it
may be convenient to air cool the piece during intermediate
processing and then perform the final anneal and quench as
a separate operation. The air-cooled piece will not have optimal
toughness and corrosion resistance in that condition, but
it is sufficient for further processing. The part will be
brought to maximum toughness and corrosion resistance by the
final heat treatment with its water quench.
19. Are there temperature limits, low and
high, on the use of duplex stainless steels?
The toughness of the duplex stainless steel
mill plate does not undergo an abrupt ductile-brittle transition.
Rather it decreases gradually from its high shelf energy to
a very low impact energy as temperature decreases from about
ambient to temperatures in the range of -45 to -75° C (-50
to -100° F). So the minimum application temperature is determined
in accordance with the tough of the duplex stainless steel.
To date, there have been few applications with minimum design
metal temperature below -40° C (-40° F).
The maximum temperature for ASME Code applications is 315°
C (600° F). The temperature was chosen because it represents
the lowest temperature for the transformation curve for 475°
C (885° F) embrittlement. Below that temperature, the steel
will not be embrittled by this reaction in many years of exposure.
In non-Code applications, it would be possible to consider
use of 2205 in applications where there are limited excursions
in the range just slightly above the limiting temperature.
However, the embrittling reaction is real and exceptions to
the 315° C (600° F) limit should not be undertaken without
full knowledge and evaluation.
20. How do the properties of duplex stainless
steels affect wall thickness, thermal expansion, and heat
transfer in comparison to austenitic stainless steels?
Although it is generally correct to say that
the yield strengths of the duplex stainless steels are twice
that of the common austenitic stainless steels, that relationship
does not imply that the thickness of the duplex stainless
steel will be simply half that of the austenitic stainless
steel in the same design. The higher strength of the duplex
grades is reflected in higher allowable design stresses in
the ASME Code. Depending on the shape of the construction,
it is possible to reduce significantly the thickness of the
material required when using duplex stainless steel, an opportunity
for cost savings.
The thermal expansion of a duplex stainless steel is intermediate
to that of carbon steel and austenitic stainless steels. This
difference can be an advantage in structure with cyclic heating
because there is less necessity to accommodate the large expansions
associated with the austenitic materials. On the other hand,
using duplex stainless steel within a construction of austenitic
stainless steel may create problems when the duplex steel
does not expand to the same extent. The combination of high
strength and lower expansion may mean that the duplex stainless
steel will impose high stresses at the point where it is joined
to the austenitic structure.
Because the duplex stainless steel has a ferritic matrix,
it is more efficient in heat transfer than the austenitic
stainless steels. This property, combined with the thinner
material that results from economical use of the higher strength
of the duplex grades, can be used to significant advantage
in heat transfer applications.
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