ISO 21028-2:2018

Overview - ISO 21028-2:2018 (Cryogenic vessels, −80 °C to −20 °C)

ISO 21028-2:2018 defines toughness requirements for metallic materials intended for cryogenic vessels operating at temperatures between −20 °C and −80 °C. The standard applies to fine‑grain and low‑alloy steels (specified yield strength ≤ 460 N/mm2), aluminium and aluminium alloys, copper and copper alloys, and austenitic stainless steels. Its purpose is to ensure materials, welds and heat‑affected zones have adequate resistance to brittle fracture at cryogenic service temperatures.

Key technical topics and requirements

• Design reference temperature (TR) - TR is calculated by adding a temperature adjustment term (TS) to the minimum metal temperature (TM): TR = TM + TS. TS depends on pressure‑induced membrane stress and construction/weld condition.
• Impact test temperature (TKV) - Materials must meet Charpy V‑notch impact energy at required test temperatures derived from TR. Testing is specified in ISO 148 (Charpy) and specimen location/orientation in ISO 9016.
• Impact energy levels - For 10×10 mm Charpy specimens the standard defines typical pass energies of 27 J (for SMYS ≤ 310 N/mm2) and 40 J (for higher SMYS ranges). These values are used to determine acceptance of base metal, weld metal and HAZ.
• Sub‑sized specimens and thin sections - Specific rules convert full‑size requirements for materials <10 mm thick or when only sub‑sized specimens can be obtained (equivalent energies and prescribed temperature shifts are provided).
• Welds and PWHT - Weld consumables and procedures must produce welds and HAZ meeting the same impact energy requirements; post‑weld heat treatment can affect TS (temperature adjustment) allowances.
• Fracture‑mechanics basis - The method is grounded in fracture mechanics to prevent brittle fracture under specified membrane stress conditions.
• Acceptance rules - Typically three specimens per position/temperature; mean must meet the requirement and at most one specimen may be below requirement but not less than 70% of it.

Practical applications and users

ISO 21028-2 is used by:

• Materials engineers and designers specifying alloys for LNG tanks, cryogenic storage and transport vessels, and other low‑temperature pressure equipment.
• Fabricators and welders selecting consumables and PWHT procedures.
• Testing laboratories performing Charpy tests and issuing conformity reports.
• Certification bodies and inspectors assessing compliance with cryogenic vessel safety requirements.

Related standards

• ISO 148 (Charpy pendulum impact test)
• ISO 9016 (Impact tests on welds)
• EN 13445‑2 and EN 13480‑2 (alternative brittle fracture prevention methods referenced for some steels)

ISO 21028-2:2018 is essential for ensuring material toughness and preventing brittle fracture in cryogenic vessel design, fabrication and qualification.