The experience of fabricators.
Kiriman dari Moderator KBK Welding – Bapak Darmayadi.
 
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.