Austenitic-Ferritic (Duplex) Stainless Steel
Type)
Austenitic-ferritic steels are
divided into two sub-families, DSS (22%Cr) and SDSS (25%Cr). SDSS has better
corrosion resistance and may also be used for seawater service. SDSS may be
preferred to DSS also for its higher strength, resulting in reduced wall
thickness, which may be of interest and even necessary for HPHT issues
(typically inlet and injection systems).
DSS (22%
Cr)
DSS has significantly better
corrosion resistance than SS316, with typically PRE value of 35. It has a
relatively lean chemistry, particularly with regards to nickel, and is
therefore less sensitive to fluctuations in the price of alloying elements.
Since duplex materials have high strength,i.e. the yield strength is more than
twice of that of SS316, significant weight and cost saving may be obtained,
particularly at high pressure where considerable wall thickness reductions may
be obtained.
DSS has significantly better
resistance to CSCC than SS316. Due to the generally better corrosion
resistance, it is also acceptable to mitigate CSCC by coating, which makes it
possible to use DSS at high temperatures.
SDSS (25%
Cr)
SDSS is mainly used when additional corrosion
resistance is required compared to DSS. Also when the higher strength enables
reduced wall thickness it may be cost and weight effective to SDSS in lieu of
DSS. SDDS can be used up to maximum operating temperature 20˚C in sweater
systems with crevices and max 1.5 ppm free chlorine. It is a relatively cost
efficient material for seawater purposes due to the relatively low content of
expensive alloying elements compared to other seawater resistant materials like
SS6Mo, alloy 625, alloy C276/C22, and titanium. Its PREN value of above 40 is
required for seawater service
Duplex stainless steels are
divided into two sub-families, 22 Cr DS and 25 Cr DS. 22 Cr DS has become the
workhorse among the CRAs used in process systems. 25 Cr DS has better corrosion
resistance and may also be used for seawater service. 25 Cr DS may be preferred
to 22 Cr DS also for its higher strength, resulting in reduced wall
thicknesses. Duplex SS (all types) have to be treated as sensitive materials
wrt. possible formation of unwanted brittle inter metallic phases during
manufacturing (heat treatment) and fabrication (welding).
22 Cr DS has significantly better
corrosion resistance than SS 316L, with typical PRE value of 35. Since duplex
materials have high strength, i.e. the yield strength is more than twice to
that of 316L, significant weight saving may be obtained, particularly at high
pressure where considerable wall thickness reductions may be obtained.
22 Cr DS has significantly better
resistance to CSCC than SS 316L. Due to the generally better corrosion
resistance, it is also acceptable to mitigate CSCC by coating, which makes it
possible to use 22 Cr DS at high temperatures.
25 Cr DS is mainly used when
additional corrosion resistance is required compared to 22 Cr DS. Also when the
higher strength enables reduced wall thickness it may be cost and weight
effective to use 25 Cr DS in lieu of 22 Cr DS.
25 Cr DS can be used up to
maximum operating temperature 30 °C in seawater systems with max 1.5 ppm free
chlorine. 25 Cr DS is a relatively cost efficient material for seawater
purposes due to the relatively low content of expensive alloying elements
compared to other seawater resistant materials like 6Mo, alloy 625, alloy C276, and titanium.
Two grades are indicated in this MOC. However, MOC shall be
as per the class and grade indicated in the P&ID.
PIPES : a) ASTM A790 UNS S 32205
b) ASTM A790 UNS S 31803
note:
On case to case basis for higher thickness if manufacturing
of seam less pipe is not possible, ASTM A 928 Class1 & 3 may be used.
FITTINGS: a) ASTM A790 UNS S 32205,
ASTM A182 Gr. F60 UNS S 32205,
ASTM A815 Gr. WP-S/WX UNS S 32205
b) ASTM A790 UNS S 31803,
ASTM A182 Gr. F51 UNS S 31803,
ASTM A815 Gr. WP-S/WX UNS S 31803
FLANGES: a) ASTM
A182 Gr. F60 UNS S 32205
b) ASTM A182 F51 UNS S 31803
BOLTING: ASTM
A 453 GR 660 Class A
GASKETS: Duplex stainless steel spiral wound
with CNAF, Octagonal ring duplex stainless steel UNS S 32205/ UNS S 31803;
Maximum hardness 22HRC.
VALVES: BODY-DSS, TRIM –DSS
Notes :
1. Maximum hardness shall be limited to 25 HRC. Material
shall meet the requirement of NACE MR 01-75 along with additional requirements indicated
in clause no. 5.2.3 of this document. This specification is in addition to
relevant codes & standards.
2.Maximum temp. Limit shall be limited to 1210C. For max
temp greater than 1210C, company’s approval shall be obtained.
3.Stub end connections are not permitted for branch
connection.
4. For butt weld fittings thickness shall be same as pipe
5. DSS shall have approximately equal proportion of
Austenitic & ferritic matrix phase (ferritic content: 45-55%) for DSS.