ISO 21635:2018

Overview

ISO 21635:2018 specifies requirements for high manganese (Mn) austenitic steel plates intended for LNG tanks on board ships. The standard targets plates with thicknesses between 6 mm and 40 mm and is primarily applicable to LNG tanks with volumetric capacity up to approximately 30 000 m3. It provides material purchasers, shipyards, steelmakers and tank manufacturers with requirements covering chemical composition, mechanical properties, manufacture, testing and documentation.

Key Topics

• Chemical composition: The standard defines a high Mn chemistry to retain austenite as the primary phase (Mn typically in the range indicated by the standard). Other alloying and impurity elements are controlled per the specification.
• Mechanical properties: Tensile and yield strength, elongation and Charpy V-notch impact energy requirements are specified. Impact testing is required at cryogenic temperatures (default test temperature -196 °C) with minimum average absorbed energies stated for transverse and longitudinal specimens.
• Manufacture and processing: Plates must be produced by recognized steelmaking routes, be killed, hot-rolled and subject to controlled cooling to avoid grain boundary carbide precipitation. A recommended cooling rate faster than 10 °C/s is given and forming above 500 °C is not permitted.
• Testing and acceptance: Tensile tests (ISO 6892-1) and Charpy V-notch tests (ISO 148-1) are required from each piece. Sub-sized specimens are permitted for thin plates (t < 12 mm) as referenced. Retest procedures and optional supplementary tests (e.g., ultrasonic, fracture toughness) are included by agreement.
• Weldability: Welding procedure qualification and a Welding Procedure Specification (WPS) are required. Weldability considerations must follow applicable guidance (e.g., IACS UR W1) and may require bend tests for qualification.
• Marking & documentation: Finished pieces must be marked with manufacturer identification, grade (e.g., “Mn24Cr3”), year and delivery condition. Test certificates shall document cast/piece identification, chemical composition and mechanical properties.

Applications

ISO 21635:2018 is intended for:

• Primary specification of materials for LNG cargo and fuel tanks on ships.
• Consideration for cargo tanks and pressure vessels in LNG terminals and fuel supply/transport systems where high Mn austenitic steel's cryogenic performance is required.

Practical benefits include improved cryogenic toughness, consistent material quality for welded cryogenic structures, and clear testing and documentation practices that support safe design and manufacturing.

Related Standards

• ISO 148-1 (Charpy test)
• ISO 6892-1 (Tensile testing)
• ISO 5173 (Bend tests)
• ASTM A1106/A1106M (high Mn alloy plate for cryogenic use)
• IACS UR W1, W13, W16 (materials, welding and tolerances for ship structures)

For procurement, fabrication and certification, users should refer to ISO 21635:2018 together with the cited normative and industry codes to ensure full compliance with cryogenic material, weldability and testing requirements.