Super Duplex Stainless Steel Pipe UNS S31803 S/ UNS S32750 / 2205 /
2507 Duplex Stainless Pipe
Duplex and Super Duplex Stainless Steel (UNS S32205, S31803,
S32750/S32760)
Duplex stainless steels have a microstructure which is 50% ferritic
and 50% austenitic. This composition gives them a better strength
than either ferritic or austenitic steels. Further details about
this topic are given below (see ASTM A790 pipes).
DUPLEX PIPE
A duplex pipe features, at the same time, the qualities of a
ferritic SS pipe (high resistance to corrosion cracking and tensile
strength) and of an austenitic SS pipe (easy manufacturing and good
resistance to corrosion and erosion). Duplex pipes 2205 (UNS
S31803/UNS S32205) are used for applications below 600 degrees (F).
Duplex and super duplex stainless steels are weldable and have
moderate formability. These types of stainless steels are magnetic
but to a lower extent than ferritic, martensitic and precipitation
hardening grades due to the 50% austenitic content.
The common grades under this family are:
- Lean duplex: UNS S32101, S32202 (UR2202), S32304, and S32003.
- Standard duplex: UNS S31803/S32205
- Super duplex: UNS S32760 (Zeron 100), S32750 (2507), and S32550
(Ferralium 255)
- Hyper duplex: UNS S32707 and S33207
The ASTM A790 specification covers seamless duplex and super duplex
pipes for demanding applications, with high corrosion, temperature,
and pressure. Electro-fusion welded duplex and super duplex welded
pipes are covered, instead, by the ASTM A928 specification.
Duplex and super duplex can be considered as ‘higher grade’
stainless steel pipes, and are available on the market in the same
dimensional range of conventional A312 stainless steel pipes (size
range covered by the ASME B36.19 specification).
There are a few differences between duplex/super duplex pipes and
conventional stainless steel pipes, which are explained below.
WHAT IS DUPLEX STEEL?
The term “duplex” refers to a family of stainless steels that have
double structure – i.e. they are neither a fully austenitic SS
structure, like 304 stainless, nor a standard ferritic, such as the
430 series.
Duplex stainless steel (UNS S31803/UNS S32205) has been developed
to overcome the issues of the 300 series stainless steels as 316L
and 317L (chloride stress corrosion cracking, under tensile stress
in cases where the metal is exposed to fluids containing chlorides
or by steel exposure to high temperatures).
The microstructure of duplex steels consists of austenite pools
surrounded by a continuous ferrite phase (approx 40-50% ferrite).
Therefore, duplex pipes combine the qualities of ferritic and
austenitic stainless steel materials.
UNS 2205 duplex steel finds application in marine projects, oil &
gas exploration, closed loops water systems, desalinization plants,
hydrocarbons storage and transportation, downstream refining, pulp,
and paper.
Duplex are resistant to stress corrosion cracking. While standard
duplex steels have corrosion resistance comparable to standard
austenitic steels (but show better strength and resistance to
stress-corrosion-cracking), the so-called super duplex steels
feature enhanced strength and resistance to any type of corrosion
compared to standard austenitic steels.
DUPLEX CORROSION RESISTANCE
Duplex stainless steel shows a superior corrosion resistance to
grade 316 for most applications (excellent resistance to localized
corrosion including intergranular, pitting and crevice corrosion;
the CPT of 2205 is generally at least 35ºC). Duplex is also
resistant to chloride corrosion cracking (SCC) at temperatures up
to about 150ºC.
Grade 2205 duplex will often perform well in environments which
cause premature failure of the standard austenitic grades of series
300 stainless steel. It has also stronger resistance to seawater
corrosion, compared to stainless steel 316.
Note that, despite Duplex stainless steel shows good
high-temperature oxidation resistance, 2205 suffers from
embrittlement if kept at temperatures above 300ºC for some period
of time. (a situation that can be rectified only by a full solution
annealing treatment).
For this reason, Duplex stainless steels are never used for
temperatures in excess of 300ºC. Duplex pipes match the following
fittings materials: ASTM A815 (butt weld fittings) and ASTM A182
F51 / F53 / F55 (flanges / forged socket weld – threaded fittings).
A790 DUPLEX PIPE PROPERTIES
| UNS | Chemical Composition | Min.Tensile (KSI) | Min.Yield
(KSI) | Elongation
% |
|---|
| UNS S31803 | C 0.30 max
Cr 21.0-23.0
Mn 2.00 max
Mo 2.50-3.50
N 0.08-0.20
Ni 4.50-6.50
P 0.030 max
S 0.020 max
Si 1.00 max | 90 | 65 | 25 |
| UNS S32205 | C 0.30 max
Cr 22.0-23.0
Mn 2.00 max
Mo 3.00-3.50
N 0.14-0.20
Ni 4.50-6.50
P 0.030 max
S 0.020 max
Si1.00 max | 95 | 65 | 25 |
Duplex pipes are manufactured according to the ASME B36.19
standard.
SUPER DUPLEX PIPE
WHAT IS SUPER DUPLEX?
Super duplex stainless steel is a variation of the duplex steel
alloy which is used in the oil & gas, chemical, power generation,
mechanical and desalination industries:
- oil & gas production equipment
- Offshore platforms
- heat exchangers
- process and service water systems
- fire-fighting systems
- injection and ballast water systems
- piping
- high-pressure RO plants and seawater piping
- high strength and corrosion-resistant parts in mechanical
applications
- FGD systems (power generation)
- Utility and industrial scrubber systems
- Absorber towers
The higher chromium and molybdenum content of Super Duplex vs.
Duplex makes it extremely resistant to uniform corrosion by organic
acids like formic and acetic acid.
Super Duplex also provides excellent resistance to inorganic acids,
especially those containing chlorides.
The pitting Resistance equivalent of Super Duplex, calculated by
PREN = Cr + 3.3Mo + 16N, will exceed 40 in most material forms.
A790 SUPER DUPLEX PIPES PROPERTIES
