Section 6 Carbon–manganese and nickel alloy steels for low temperature service

6.1.1 This Section gives specific requirements for carbon-manganese and nickel alloy steels intended for use in the construction of cargo tanks, storage tanks and process pressure vessels for liquefied gases.

6.1.2 The requirements of this Section are also applicable for other types of pressure vessels where the use of steels with guaranteed impact properties at minimum design temperature lower than 0°C is required.

6.1.3 Provision is made for plates and sections up to 50 mm thick.

6.1.4 Alternative steels which comply with National Standard specifications may be accepted under special consideration provided that these specifications give equivalence to the requirements of this Chapter or alternatively are approved for a specific application. For plates of thickness greater than 40 mm and up to 50 mm that are accepted in accordance with a National Standard specification, in addition to each set of impact tests required by the standard, a set of impact tests is required at mid-thickness position; the orientation and acceptance criteria are to be the same as those required by Ch 3, 6.4 Mechanical tests.

6.1.5 The requirements for selection of grades and thickness for plates for appropriate minimum design temperatures are given in the Rules and Regulations for the Construction and Classification of Ships for the Carriage of Liquefied Gases in Bulk, July 2022 and the Rules and Regulations for the Classification of Ships using Gases or other Low-flashpoint Fuels, July 2022 for applications defined in Ch 3, 6.1 Scope 6.1.1 and Ch 3, 6.1 Scope 6.1.2.

6.1.6 For high manganese austenitic steels with thickness up to 40 mm, the specific requirements for approval, manufacturing and inspection are specified in LR’s Guidance Notes for Approval, Manufacture, Testing and Certification of High Manganese Austenitic Steel for Low Temperature Service.

6.2.1 All steels are to be in the killed and fine grain treated condition.

6.2.2 The chemical compositions of carbon-manganese steels are to comply with the appropriate requirements for grades AH, DH, EH and FH strength levels 27S, 32, 36 and 40,see Table 3.3.2 Chemical composition for higher strength steels. For the uses defined in Ch 3, 6.1 Scope 6.1.1 and Ch 3, 6.1 Scope 6.1.2, however, these grades are to be designated LT−AH, LT−DH, LT−EH and LT−FH respectively.

6.2.3 The chemical compositions of nickel alloy steels are to comply with the appropriate requirements of Table 3.6.1 Chemical compositions of nickel alloy steels.

6.2.4 For plate supplied from coil, the chemical analysis may be transposed from the certificate of the coil manufacture onto the re-processor’s certificate.

6.3.1 All materials are to be supplied in a condition complying with the requirements given in Table 3.6.2 Supply conditions.

6.4.1 For plates, one tensile test specimen is to be taken from one end of each piece. A piece is to be regarded as the rolled product from a single slab or from a single ingot if this is rolled directly into plates.

6.4.2 For strips, tensile test specimens are to be taken from both ends of each coil.

6.4.3 Sections and bars are to be presented for acceptance test in batches containing not more than 50 lengths, as supplied. The material in each batch is to be of the same section size, from the same cast and in the same condition of supply. One tensile test specimen is to be taken from material representative of each batch, except that additional tests are to be taken when the mass of a batch exceeds 10 tonnes.

6.4.4 One set of three Charpy V-notch impact test specimens is to be taken for each tensile test specimen required.

6.4.5 For plates, these impact test specimens are to be cut with the principal axis perpendicular to the final direction of rolling. For sections, the impact test specimens are to be taken longitudinally.

6.4.6 The results of all tensile tests are to comply with the appropriate requirements given in Table 3.6.3 Mechanical properties for acceptance purposes (see Note 1) . For carbon–manganese steel grades, the ratio between the yield stress and the tensile strength is not to exceed 0,9 for normalised and TM steels and 0,94 for QT steels.

Table 3.6.3 Mechanical properties for acceptance purposes (see Note 1)

Grade of steel		Yield stress N/mm2 min.	Tensile strength N/mm2	Elongation on 5,65 % min. (see Note 3)		Charpy V-notch impact tests (see Note 2)
						Test temp. °C	Impact energy
LT – AH	27S	265	400 – 530	22		0
	32	315	440 – 590	22
	36	355	490 – 620	21
	40	390	510 – 650	20
LT – DH	27S	265	400 – 530	22		–20	Plates – transverse tests
	32	315	440 – 590	22
	36	355	490 – 620	21
	40	390	510 – 650	20
LT – EH	27S	265	400 – 530	22		–40	Average energy 27 J min
	32	315	440 – 590	22
	36	355	490 – 620	21
	40	390	510 – 650	20
LT – FH	27S	265	400 – 530	22		–60	Sections and bars – longitudinal tests
	32	315	440 – 590	22
	36	355	490 – 620	21
	40	390	510 – 650	20			Average energy 41 J min
11/2 Ni		275	490 – 640	22		–65
2 1/4 Ni		275	490 - 640	21		-70
31/2 Ni		285	450 – 610	21		–95
5Ni		390	540 – 740	21		–110
9Ni		585	680 – 820	18		–196
Note 1. These requirements are applicable to products not exceeding 50 mm in thickness. The requirements for thicker products for approved applications are subject to agreement. Note 2. See Ch 3, 1.9 Test material and mechanical tests 1.9.11 Note 3.For full thickness tensile test specimens with a width of 25 mm and a gauge length of 200 mm (see Figure 2.2.3 Test specimen dimensions for plates, strip and sections - I and aluminum alloys in Ch 2 Testing Procedures for Metallic Materials) the minimum elongation is to be:

6.4.7 The average value for the three impact tests is to comply with the appropriate requirements given in Table 3.6.3 Mechanical properties for acceptance purposes (see Note 1) . One individual value may be less than the required value provided that it is not less than 70 per cent of this average value. See Ch 2, 1.4 Re-testing procedures for re-test procedures.

6.4.8 Where standard subsidiary impact specimens are necessary, see Ch 2, 2.3 Procedure for testing at ambient temperature 2.3.4.

6.4.9 Where plate is supplied from coil, both the tensile tests and the Charpy V-notch tests are to be taken from the de-coiled plate in accordance with the frequency specified for the Grade as required by this Section.

6.5.1 The particulars detailed in Ch 3, 1.12 Identification of materials are to be marked on all materials which have been accepted.

6.6.1 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 together with general details of the heat treatment. As a minimum, chemical composition is to include the contents of any grain refining elements used and the residual elements as detailed in Table 3.3.2 Chemical composition for higher strength steels or Table 3.6.1 Chemical compositions of nickel alloy steels.