“Duplex”
means two-phase microstructure consisting of grains
of ferritic and austenitic stainless
steel thats why it is called Duplex stainless steels. The picture shows the
yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When
duplex stainless steel is melted it solidifies from the liquid phase to a
completely ferritic structure. As the material cools to room temperature, about
half of the ferritic grains transform to austenitic grains (“islands”). The
result is a microstructure of roughly 50% austenite and 50% ferrite. Duplex stainless
steels have a two-phase microstructure of austenite and ferrite grains.
The duplex structure
gives this family of
stainless steels a
combination of
attractive
properties:
Strength: Duplex
stainless steels are
about twice as
strong as regular
austenitic or
ferritic stainless
steels.
Corrosion
Resistance:
Duplex stainless
steel grades have a
range of corrosion
resistance, similar
to the range for
austenitic stainless
steels, i.e from
Type 304 or 316
(e.g. LDX 2101©) to
6% molybdenum (e.g.
SAF 2507©) stainless
steels. As with all
stainless steels,
corrosion resistance
depends mostly on
the composition of
the stainless steel.
For chloride pitting
and crevice
corrosion
resistance, their
chromium, molybdenum
and nitrogen content
are most important.
Toughness and
ductility:
Duplex stainless
steels have
significantly better
toughness and
ductility than
ferritic grades;
however, they do not
reach the excellent
values of austenitic
grades.
Duplex stainless
steels show very
good stress
corrosion cracking (SCC)
resistance, a
property they have
“inherited” from the
ferritic side. SCC
can be a problem
under certain
circumstances
(chlorides,
humidity, elevated
temperature) for
standard austenitics
such as Types 304
and 316.
Low Cost:
Duplex stainless
steels have lower
nickel and
molybdenum contents
than their
austenitic
counterparts of
similar corrosion
resistance. Due to
the lower alloying
content, duplex
stainless steels can
be lower in cost,
especially in times
of high alloy
surcharges.
Additionally, it may
often be possible to
reduce the section
thickness of duplex
stainless steel, due
to its increased
yield strength
compared to
austenitic stainless
steel. The
combination can lead
to significant cost
and weight savings
compared to a
solution in
austenitic stainless
steels.
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AS PER THE NATIONAL
& INTERNATIONAL
STANDARD &
SPECIFICATIONS
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