Section 8 Ferritic steel forgings for low temperature service

8.1 Scope

8.1.1 The requirements for carbon-manganese and nickel steels suitable for low temperature service are detailed in this Section. They are applicable to all forgings used for the construction of cargo tanks, storage tanks and process pressure vessels for liquefied gases and, where the design temperature is less than 0°C, to forgings for the piping systems.

8.1.2 The requirements are also applicable to forgings for other pressure vessels and pressure piping systems where the use of steels with guaranteed impact properties at low temperatures is required.

8.1.3 In all cases, details of the proposed chemical composition, heat treatment and mechanical properties are to be submitted for approval.

8.1.4 In addition to the steels in this Section, the austenitic stainless steels detailed in Ch 5, 9 Stainless steel forgings may also be used for low temperature applications.

8.2 Chemical composition

8.2.1 The chemical composition of ladle samples is, in general, to comply with the requirements given in Table 5.8.1 Chemical composition of ferritic steel forgings.

8.3 Heat treatment

8.3.1 Forgings are to be normalised, normalised and tempered or quenched and tempered in accordance with the approved specification.

8.4 Mechanical tests

8.4.1 At least one tensile and three Charpy V-notch impact test specimens are to be taken from each forging or each batch of forgings. Where the dimensions and shape allow, the test specimens are to be cut in a longitudinal direction.

8.4.2 The impact tests are to be carried out at a temperature appropriate to the type of steel and for the proposed application. Where forgings are intended for ships for liquefied gases, the test temperature is to be in accordance with the requirements given in Ch 3, 6.4 Mechanical tests 6.4.6 in Ch 3 Rolled Steel Plates, Strip, Sections and Bars.

8.4.3 The results of all tensile tests are to comply with the approved specification.

8.4.4 The average energy values for impact tests are also to comply with the approved specification and generally with the requirements of Ch 3, 6 Carbon–manganese and nickel alloy steels for low temperature service. One individual value may be less than the required average value provided that it is not less than 70 per cent of this value. See Ch 2, 1.4 Re-testing procedures for re-test procedures.

8.5 Non-destructive examination

8.5.1 Non-destructive testing is to be carried out in accordance with the requirements of the approved forging drawing and specification, or as otherwise agreed between the manufacturer, purchaser and Surveyor.

8.6 Pressure tests

8.6.1 When applicable, pressure tests are to be carried out in accordance with the requirements of the relevant Rules.

Table 5.8.1 Chemical composition of ferritic steel forgings

Grade of steel	C %	Si %	Mn %	Ni %	P %	S %	Residual elements %	Grain refiners %
								Al	Other
LT–AH (AH40)	0,18 max.	0,50 max.	0,90 – 1,60	0,40 max.	0,035 max.	0,030 max.	Cu 0,35 max. Cr 0,25 max. Mo 0,08 max. Total 0,60 max.	Total 0,020 min. Acid soluble 0,015 min.	(See Note) Nb 0,02 – 0,05 V 0,03 – 0,10 Ti 0,02 max.
LT–DH (DH40)
LT–EH (EH40)
LT–FH (FH40)	0,16 max.			0,80 max.	0,025 max.	0,025 max.
1 ¹/₂Ni	0,18 max.	0,10–0,35	0,30 – 1,50	1,30 – 1,70		0,020 max.	Cu 0,35 max. Cr 0,25 max. Mo 0,08 max. Total 0,60 max.
3 ¹/₂Ni	0,15 max.		0,30 – 0,90	3,20 – 3,80
5Ni	0,12 max.			4,70 – 5,30
9 Ni	0,10 max.			8,50 – 10,0
Note The steel is to contain aluminium, niobium, vanadium or other suitable grain refining elements, either singly or in any combination. When used singly, the steel is to contain the specified minimum content of the grain refining element. When used in combination, the specified minimum content of each element is not applicable.