Section
7 Austenitic and duplex stainless steels
7.1 Scope
7.1.1 Provision is made in this Section for rolled products in austenitic and
duplex stainless steels intended for the construction of marine equipment such as cargo
tanks, storage tanks and process pressure vessels for chemicals and liquefied gases.
7.1.2 Austenitic stainless steels are suitable for applications where the design
temperature is not lower than –165°C.
7.1.3 Austenitic
stainless steels are also suitable for service at elevated temperatures,
and for such applications the proposed specification should contain,
in addition to the requirements of Ch 3, 7.1 Scope 7.1.6,
minimum values for 0,2 and 1,0 per cent proof stresses at the design
temperature.
7.1.4 Duplex stainless steels are suitable for applications where the lowest
design temperature not lower than -20°C.
7.1.5 Duplex stainless steels are also suitable for service at temperatures up to
300°C, and for such applications the proposed specification should include, in addition
to the requirements of Ch 3, 7.1 Scope 7.1.6, a minimum value for 0,2 per cent proof stress at the design
temperature.
7.1.6 Where it is proposed to use alternative steels, particulars of the specified
chemical composition, heat treatment and mechanical properties, including, for the
austenitic grades, both the 0,2 and 1,0 per cent proof stresses, is to be submitted for
consideration and approval.
7.2 Chemical composition
7.2.2 Consideration
will be given to the use of steels whose compositions are outside
the scope of Table 3.7.1 Chemical composition.
Table 3.7.1 Chemical composition
Type and grade of steel
|
Chemical composition % (see Note)
|
C
|
Si
|
Mn
|
P
|
S
|
Cr
|
Ni
|
Mo
|
N
|
Other
|
Austenitic
|
|
|
|
|
|
|
|
|
|
|
304 L
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
17,0-20,0
|
8,0-13,0
|
-
|
0,10
|
-
|
304 LN
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
17,0-20,0
|
8,0-12,0
|
-
|
0,10-0,22
|
-
|
316 L
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
16,0-18,5
|
10,0-15,0
|
2,0-3,0
|
0,10
|
-
|
316 LN
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
16,0-18,5
|
10,0-14,5
|
2,0-3,0
|
0,10-0,22
|
-
|
317 L
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
18,0-20,0
|
11,0-15,0
|
3,0-4,0
|
0,10
|
-
|
317 LN
|
0,03
|
1,0
|
2,0
|
0,045
|
0,03
|
18,0-20,0
|
12,5-15,0
|
3,0-4,0
|
0,10-0,22
|
-
|
321
|
0,08
|
1,0
|
2,0
|
0,045
|
0,03
|
17,0-19,0
|
9,0-12,0
|
-
|
0,10
|
5 x C
≤ Ti ≤ 0,7
|
347
|
0,08
|
1,0
|
2,0
|
0,045
|
0,03
|
17,0-19,0
|
9,0-13,0
|
-
|
0,10
|
10 x C
≤ Nb ≤ 1,0
|
Duplex
|
|
|
|
|
|
|
|
|
|
|
UNS
|
S
31803
|
0,03
|
1,0
|
2,0
|
0,03
|
0,02
|
21,0-23,0
|
4,5-6,5
|
2,5-3,5
|
0,08-0,20
|
-
|
UNS
|
S 32750
|
0,03
|
0,80
|
1,2
|
0,035
|
0,02
|
24,0-26,0
|
6,0-8,0
|
3,0-5,0
|
0,24-0,32
|
Cu 0,50
max.
|
Note All figures are a maximum value except where a range is
shown.
|
7.3 Heat treatment
7.3.1 All materials
are to be supplied in the solution treated condition.
7.4 Mechanical tests
7.4.2 For the duplex grades, one set of three Charpy V-notch impact test specimens
machined from the longitudinal direction for each tensile test is to be tested at –20°C
or 5°C below the design temperature, whichever is lower. The average energy value of the
three specimens is to be not less than 41 Joules.
7.4.3 Unless otherwise
agreed, impact tests are not required from the austenitic grades of
steel given in this Section.
7.5 Metallographic examination for sigma phase
7.5.1 The microstructure of duplex stainless steel grades listed in Table 3.7.1 Chemical composition are to be examined metallographically at x400
magnification to demonstrate that there is no evidence of sigma phase in the observable
area. The frequency of testing shall be one test per heat.
7.6 Corrosion tests
7.6.1 For certain specific applications such as storage tanks for chemicals, it
may be necessary to demonstrate that the material used is not susceptible to corrosion
resulting from precipitation of deleterious phases.
7.6.2 When required, a corrosion test is to be carried out for each tensile test.
The material for the corrosion test is to be taken adjacent to that for the tensile
test.
7.6.3 Unless otherwise agreed or required for a particular chemical cargo, an
intergranular corrosion test for austenitic stainless steels and a pitting corrosion
test for duplex stainless steels are to be performed. The testing procedures are given
in , see
Ch 2, 9 Corrosion tests and, for the pitting corrosion test, unless otherwise
specified, the test temperatures shall be 20°C for S31803 and 30°C for S32750.
7.6.4 Wherever
practical, exposed cut edges should be avoided. However, where any
such edges are to remain after fabrication is completed it is to be
shown by an appropriate test that the corrosion resistance is adequate
for the cargoes expected to be encountered.
7.7 Clad plates
7.7.1 Carbon or
carbon-manganese steel plates, clad on one or both surfaces with a
suitable grade of austenitic or duplex stainless steel, may be used
for the construction of cargo or storage tanks for chemicals.
7.7.3 The process
of manufacture is to be specially approved and may be either by roll
cladding or by explosive bonding.
7.7.4 Where the
use of clad materials is proposed, the material specification is to
be submitted for consideration, together with details of the extent,
and the acceptance standards for non-destructive examination.
7.8 Identification of materials
7.8.1 The particulars
detailed in Ch 3, 1.12 Identification of materials are to be marked
on all materials which have been accepted.
Table 3.7.2 Mechanical properties for
acceptance purposes
Type and grade of
steel
|
0,2% Proof stress
(N/mm2) minimum
|
1% Proof stress
(N/mm2) minimum
|
Tensile strength
(N/mm2) minimum
|
Elongation on 5,65 % minimum
|
Austenitic
|
|
|
|
|
304L
|
170
|
210
|
485
|
40
|
304LN
|
205
|
245
|
515
|
40
|
316L
|
170
|
210
|
485
|
40
|
316LN
|
205
|
245
|
515
|
40
|
317L
|
205
|
245
|
515
|
40
|
317LN
|
240
|
280
|
550
|
40
|
321
|
205
|
245
|
515
|
40
|
347
|
205
|
245
|
515
|
40
|
Duplex
|
|
|
|
|
UNS S
31803
|
450
|
–
|
620
|
25
|
UNS S
32750
|
550
|
–
|
795
|
15
|
7.9 Certification of materials
7.9.1 At least two copies of each test certificate are to be provided. They are
to be of the type and give the information detailed in Ch 3, 1.13 Certification of materials and, where
applicable, the results obtained from corrosion tests, and sigma phase assessment. As a
minimum, the chemical composition is to include the contents of elements used including
the residual elements, as detailed in Table 3.7.1 Chemical composition.
|