Section 7 Austenitic and duplex stainless steels

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.

7.2.1 The chemical composition of ladle samples is to comply with the requirements given in Table 3.7.1 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.

7.3.1 All materials are to be supplied in the solution treated condition.

7.4.1 Tensile test specimens are to be taken in accordance with the appropriate requirements of Ch 3, 4.4 Mechanical tests and Ch 3, 6.4 Mechanical tests 6.4.1.

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.

7.4.4 Where standard subsidiary Charpy V-notch test specimens are necessary, see Ch 2, 2.3 Procedure for testing at ambient temperature 2.3.4.

7.4.5 The results of all tensile tests are to comply with the requirements of Table 3.7.2 Mechanical properties for acceptance purposes or the approved specification.

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.

7.6.1 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.

7.7.1 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.

7.7.2 The carbon or carbon-manganese steel base plates are to comply with the requirements of Ch 3, 4 Steels for boilers and pressure vessels, and the austenitic cladding material generally with the requirements of this Section.

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.

7.8.1 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

7.9.1 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.