14
Practical guide to using duplex stainless steels
proper chemistry (i.e. nitrogen in the higher end of the range)
and in optimal heat treatment condition as shown by passing
ASTM A923 or A1084 testing requirements. (See section
Specications and Quality Control.) If the material received has
low nitrogen (but within the specication) or has small amounts
of secondary phases present, it may not be possible to weld
it and obtain satisfactory corrosion or toughness properties.
It is necessary to qualify each welding procedure to ensure
heat input is neither too low nor too high. Low heat inputs
can result in rapid cooling rates, which have the potential
of producing a phase balance in the HAZ that is too high
in ferrite. Too high of a heat input could expose the HAZ
to the critical temperature range for the specic alloy for
too long a time resulting in the precipitation of undesirable
secondary phases. Because problems with duplex stainless
steel welds reveal themselves by a loss of toughness or
corrosion resistance, it is prudent that welding procedure
qualications include a toughness test (e.g. Charpy V-notch)
at an appropriate temperature with acceptance criteria
appropriate to the application, such as described in ISO 17781.
Alternatively, because loss of corrosion resistance can be
associated with precipitation of secondary phases, a corrosion
test can also be used to evaluate weld procedures and it is not
uncommon to have welds evaluated with both a toughness
test and a corrosion test. Test methods for evaluating some
common duplex stainless steel grades are:
Standard Type of duplex
ASTM A923 / ISO 17781 Standard and superduplex
ASTM A1084 / ISO 17781 Lean
Higher nitrogen contents are extremely helpful in avoiding
excessive ferrite content, especially with lower-alloyed grades.
It is still necessary to be concerned about certain geometries,
which can result in very rapid cooling rates, such as liner
sheets installed on heavy plates or small welds on large
plates. In extreme cases a modest preheat or a controlled
interpass temperature for a multiple-pass weld will slow
the cooling rate enough to allow sufcient reformation of
austenite to produce good toughness and corrosion resistance.
The concern of too high heat input relates to the need to
minimise the accumulated total exposure time of the HAZ
in the critical temperature range. Even with a high nitrogen
level, the total time at approximately 850 °C (1550 °F)
before detrimental formation of intermetallic compounds
is as short as ve minutes for Type 2205 and only about
one minute for the higher alloyed super duplex grades.
This time must include both cooling after the nal anneal
and all subsequent fabrication. The ASTM A923, A1084
and ISO 17781 specications were developed to provide
test methods for detecting detrimental phases in duplex
stainless steels. It is recommended that the appropriate
A923, A1084 or ISO 17781 test methods be required to
qualify duplex stainless steel weld procedures.
If intermetallics form in a HAZ of a large fabrication where
post weld heat treatment is not a viable option, the only
remedy may be to cut the weld and HAZ out and start over.
Consequently, welding procedures must be qualied with
respect to maximum thermal exposure, including any repair
or rework practice. When later modications of equipment
are contemplated, it is important that total thermal history
be considered before performing additional welding.
It is common for ller metals to contain increased nickel
content to ensure that the rapidly quenched cast structure
of the weld is comparable to the base metal in toughness
and corrosion resistance. For example, the 2209 weld ller,
most commonly used with Type 2205, has about 9% nickel,
3-4% more than the base metal. Matching welding llers,
over-alloyed with nickel, are available for many duplex grades
and users are encouraged to contact the alloy producers for
recommendations for specic grades. Only a few duplex ller
metals are included in the welding standards such as AWS.
Weld metal toughness is strongly related to the welding
process. Non-ux processes providing greater toughness.
Typically, weld metal toughness as related to welding process
is as follows:
GTAW> GMAW>FCAW>SMAW>SAW
Highly basic uxes have been reported to be benecial to the
as-welded impact toughness of the duplex stainless steels.
When welding a duplex grade to carbon steel a Type 309L
ller is usually a good choice for achieving a sound weld. An
appropriate ller for joining 2205 or a super duplex grade to
an austenitic stainless steel of lower molybdenum content
would be 309LMo.