Cryogenic material selection starts with one question: will the metal stay tough at the lowest temperature it can reach? Many steels lose toughness sharply as they get cold and can fail by brittle fracture with almost no warning (our guide to toughness versus hardness in weld metal explains the difference), while other metals stay ductile all the way to liquid-helium temperature. This guide explains which metals suit cryogenic service, how the codes treat them, and what changes in welding and design.
Cryogenic material selection means choosing a metal that stays tough at the lowest metal temperature the equipment can reach. Austenitic stainless steels (304L, 316L) and aluminium alloys stay ductile to -196 °C and far below, 9% nickel steel is rated to -196 °C, and impact-tested carbon steel is limited to about -45 °C.
It is written for piping and materials engineers, welding engineers, inspectors and students. Related guides on mechanical behaviour and material choice sit in the Material Properties & Selection section of this site.
Temperatures and grade limits below come from material specifications and published industry guidance. Always confirm them against the code edition, the material specification and the project specification that govern your equipment.
- Body-centred cubic (BCC) steels have a ductile-to-brittle transition, while face-centred cubic (FCC) metals such as austenitic stainless steel and aluminium do not.
- Impact-tested carbon steel such as ASTM A333 Grade 6 is typically tested at -45 °C, 3.5% nickel steel at about -100 °C and 9% nickel steel at about -195 °C.
- Austenitic stainless steel contracts about 0.28% from room temperature to -196 °C, which is roughly 2.8 mm per metre of pipe.
- 9% nickel steel is normally welded with nickel-based filler metal and needs control of heat input and residual magnetism.
- The design minimum metal temperature must include upset conditions such as depressurisation, not only the normal operating temperature.
- A fitting marked WPL6 is impact tested and a fitting marked WPB is not, so markings and certificates must be checked against the order.
What Is Cryogenic Service?
Cryogenic service is the use of equipment and piping at very low temperatures, where materials must keep adequate toughness to resist brittle fracture. Definitions of the temperature threshold vary: many specifications call service below about -50 °C cryogenic, while some definitions start near -150 °C.
In pressure-equipment practice the first checkpoint is much warmer. Carbon and low-alloy steels often need toughness evaluation once the design minimum metal temperature falls below -29 °C (-20 °F), so low-temperature material selection begins well before true cryogenic temperatures.
The coldest condition governs the material. That condition may be the normal operating temperature, a start-up or cool-down, or an upset such as rapid depressurisation. When a liquefied gas is let down quickly, auto-refrigeration can push metal temperatures far below the normal boiling point.
| Fluid | Boiling point (°C) | Boiling point (°F) |
|---|---|---|
| Ammonia | -33.3 | -27.9 |
| Propane | -42.1 | -43.8 |
| Carbon dioxide (sublimation point) | -78.5 | -109.3 |
| Ethane | -88.6 | -127.5 |
| Ethylene | -103.7 | -154.7 |
| Methane (LNG) | -161.5 | -258.7 |
| Oxygen | -183.0 | -297.4 |
| Argon | -185.8 | -302.4 |
| Nitrogen | -195.8 | -320.4 |
| Hydrogen | -252.9 | -423.2 |
| Helium | -268.9 | -452.0 |
Why Do Metals Become Brittle at Low Temperature?
Ferritic steels become brittle at low temperature because their body-centred cubic (BCC) crystal structure has a ductile-to-brittle transition. Above the transition range the steel absorbs large energy before it breaks, and below it a crack can run through the material with little plastic deformation.
The transition is measured with the Charpy V-notch (CVN) impact test, which records the energy absorbed (and often the lateral expansion at the notch) at a set of test temperatures. A steel suited to cryogenic service must show adequate energy or lateral expansion at the design minimum temperature, in the base metal, the weld metal and the heat-affected zone (HAZ).
Face-centred cubic (FCC) metals behave differently. Austenitic stainless steels, aluminium, copper and nickel alloys often keep or gain ductility as temperature falls, which is why they are the default choice for the coldest services. The test method and acceptance criteria are set by the material specification and the governing code, for example UG-84 in ASME Section VIII Division 1.
Which Metals Are Suitable for Cryogenic Service?
Metals suited to cryogenic service are either FCC metals with no transition or nickel steels engineered for a very low transition temperature. The table lists typical lowest service temperatures from TWI Job Knowledge 100 and from material specification test temperatures.
| Material | Crystal structure | Typical lowest service temperature | Example specifications |
|---|---|---|---|
| Fine-grain, impact-tested C-Mn steel | BCC | About -45 to -50 °C | ASTM A333 Gr 6 pipe, A350 LF2 forgings, A420 WPL6 fittings |
| 1.5% to 2.5% Ni steel | BCC | About -60 °C | ASTM A203 Gr B plate, EN 10028-4 15NiMn6 |
| 3.5% Ni steel | BCC | About -100 °C | ASTM A203 Gr E plate, A333 Gr 3 pipe, A350 LF3 forgings, A420 WPL3 fittings |
| 5% Ni steel | BCC | About -130 °C | EN 10028-4 X12Ni5 |
| 9% Ni steel | BCC (tempered martensite with retained austenite) | About -196 °C | ASTM A353 and A553 Type I plate, A333 Gr 8 pipe, A420 WPL8 fittings, EN 10028-4 X8Ni9 |
| Austenitic stainless steel (304L, 316L) | FCC | -196 °C and far below, with no transition | ASTM A240 plate, A312 pipe, A403 fittings |
| Aluminium alloys (5083-O, 6061) | FCC | -196 °C and colder | ASTM B209 plate, B221 extrusions |
| Copper and nickel alloys | FCC | -196 °C and colder | Per product specification |
9% nickel steel (ASTM A353 and A553 Type I) is the usual choice for large LNG and liquid-oxygen tanks. Both grades have excellent notch toughness at -196 °C, and A553 has the higher minimum yield strength (585 MPa against 515 MPa for A353), with a tensile strength range of 690 to 825 MPa. Austenitic stainless steel is preferred for piping and smaller vessels and has the lowest yield strength at room temperature of the main options.
Duplex stainless steels contain ferrite and so show a toughness transition, and projects commonly limit them to about -50 °C unless lower temperatures are qualified by test. See our guide to duplex stainless steels for the metallurgy. Ferritic and martensitic stainless steels also have a transition and are not cryogenic materials.
Non-metallic parts need the same scrutiny. Many elastomers and plastics become hard and lose sealing ability when cold, so gaskets, seals and packing must be rated for the minimum temperature.
How Do You Select a Cryogenic Material Step by Step?
Select a cryogenic material by working from the coldest metal temperature to the code toughness rule, then to fabrication and compatibility. The numbered steps below give a repeatable order.
- Define the fluid and conditions. Record the fluid, operating pressure and temperature, and every upset case such as depressurisation, cool-down and purging.
- Set the design minimum metal temperature. Use the coldest metal temperature the equipment can see, not the normal operating temperature, with a margin that the project specification requires.
- Choose the governing code. Identify whether ASME B31.3, ASME Section VIII, API 620, EN 14620 or another code controls the item.
- Pick candidate materials by temperature. Compare the design minimum metal temperature with the lowest allowed temperature of each material family in Table 2.
- Check the toughness requirement. Confirm whether the code exempts the material from impact testing or requires Charpy tests on base metal, weld metal and the HAZ.
- Check fabrication. Confirm weldability, qualified filler metals, heat treatment and contraction effects for each candidate.
- Check compatibility and cleanliness. Confirm fluid compatibility, such as oxygen cleaning, and the suitability of seals, gaskets and bolting.
What Is a Worked Example of Material Selection for LNG?
The final choice then depends on size, availability and fabrication capability. Stainless steel is usual for process piping, and 9% nickel steel is common where strength and cost favour a ferritic material.
How Do ASME, API and EN Codes Treat Low-Temperature Materials?
Each code sets the design minimum metal temperature, the materials allowed and the impact test rules in its own way. The table summarises the main codes, and the edition and project specification control the actual requirements.
| Code or standard | Scope | How it treats low-temperature materials |
|---|---|---|
| ASME B31.3, Process Piping | Process piping | Chapter III (Materials) and Table 323.2.2 set low-temperature toughness rules; Table 323.3.1 sets impact testing; Table A-1 lists material temperature limits |
| ASME BPVC Section VIII Div. 1 | Pressure vessels | UG-20 sets design temperature; Part UCS (UCS-66) covers carbon and low-alloy steel; Part UHA (UHA-51) covers austenitic stainless steel; Part UHT covers heat-treated ferritic steels such as 9% Ni |
| API 620 Annex R | Refrigerated tanks, about +4 to -51 °C | Low-pressure tanks for refrigerated products |
| API 620 Annex Q | Liquefied gas tanks to about -198 °C | Tanks for liquefied gases not lower than -325 °F |
| EN 14620 | Site-built flat-bottomed tanks, 0 to -165 °C | Design and manufacture of refrigerated liquefied gas tanks |
| EN 1252-1 | Cryogenic vessels | Toughness requirements for materials below -80 °C |
| EN 10028-4 | Nickel alloy steel plate | Flat products with specified low-temperature properties, including 9% Ni |
| IGC Code (IMO) | Ships carrying liquefied gases | Lists suitable tank materials such as 304L, 316L, 5083-O aluminium, 36% Ni steel and 9% Ni steel |
How Does Impact Testing Apply to Stainless Steel and Nickel Steel?
Impact testing rules differ by material and code. In ASME B31.3, austenitic stainless steel base metal is exempt only if conditions on carbon content and heat treatment are met, and austenitic weld metal is exempt only to a limited temperature (-101 °C, or -150 °F, in recent editions) before toughness tests are needed. In Section VIII Division 1, UHA-51 exempts qualifying austenitic grades down to -196 °C (-320 °F) and requires testing below that. When tests are needed, a lateral expansion of at least 0.38 mm (0.015 in) is the common acceptance criterion.
Ferritic steels such as 9% nickel steel are normally tested at or below the minimum design temperature. For the exemption logic for carbon and low-alloy steels, see our guides to impact test exemption conditions and the ASME Section VIII Division 1 overview. For piping, the ASME B31.3 process piping guide covers the code structure.
Exemption temperatures and clause wording change between code editions. Always confirm limits against the edition named in your contract before you waive any impact test.
How Do You Weld Cryogenic Materials?
Welding cryogenic materials means matching the toughness of the weld metal and the HAZ to the base metal at the design minimum temperature. Filler metal, heat input and procedure qualification are chosen with that aim.
How Is 9% Nickel Steel Welded?
5% and 9% nickel steels are conventionally welded with nickel-based filler metals, because matching-composition filler does not give consistent toughness in the as-welded state. Early nickel-based fillers had much lower strength than the base metal, and higher-strength fillers of the ENiCrMo-3 (Alloy 625) type are now widely used. Our article on nickel alloy consumables under SFA-5.11 explains these filler families.
- Heat input and interpass: TWI recommends a maximum interpass temperature of 250 °C and ideally below 150 °C, with heat input limited to about 3.5 kJ/mm for submerged arc welding and 2.5 kJ/mm for manual metal arc welding.
- Preheat: little or none, because the high austenite content tolerates hydrogen; B31.3 recommends only 10 °C minimum for 5% and 9% Ni steels.
- Post-weld heat treatment (PWHT): not generally required for 9% Ni steel, and EN 13445-4 recommends avoiding it, while ASME codes specify 552 to 585 °C when thickness exceeds 51 mm (2 in).
- Residual magnetism: 9% Ni steel can become strongly magnetised and cause arc blow, so handling, alternating current welding and degaussing may be needed.
For the principles behind this calculation, see our guide to heat input in welding.
How Are Austenitic Stainless Steel and Aluminium Welded?
Austenitic stainless steel for cryogenic service is normally low-carbon (L-grade) or stabilised, and the weld metal composition is controlled so that ferrite content supports hot-crack resistance without hurting toughness. High ferrite lowers toughness at very low temperature, so projects often set limits and require impact tests on weld metal and the HAZ. Aluminium 5083 is commonly joined with 5xxx filler metals that match its strength.
Transition joints between dissimilar materials, such as stainless steel to aluminium, need special joint designs, which our guide to welding dissimilar metals covers in general terms.
What Design Issues Come with Cryogenic Piping?
Cryogenic piping must accommodate large thermal contraction, cold insulation and cold-service fittings. A line that shrinks several millimetres per metre needs flexibility, supports that allow movement, and joints and bolting that stay tight.
| Material | Contraction (% of length) | Per metre (mm) | Per 50 m of pipe (mm) |
|---|---|---|---|
| Austenitic stainless steel (304, 304L) | About 0.28 | About 2.8 | About 140 |
| Aluminium alloys | About 0.4 | About 4 | About 200 |
| Carbon and 9% Ni steels | About 0.2 | About 2 | About 100 |
The stainless steel value is from the NIST cryogenic material properties database for 304, which gives the integrated linear expansion from 293 K. Values for other materials are approximate and vary with alloy, so use the supplier or NIST data for design.
For quick estimates of expansion and contraction at other temperatures, use our pipe thermal expansion calculator. Insulation is also critical. Cold lines can suffer moisture ingress and corrosion beneath insulation, as described in our guide to corrosion under insulation.
- Bolting: low-temperature bolting such as ASTM A320 Grade L7 is used with ferritic flanges.
- Valves: extended bonnets keep the stem packing above the cold zone.
- Cleanliness: oxygen service requires cleaning to the project specification and compatible materials and gaskets.
How Do You Verify Cryogenic Materials and Documents?
Verify cryogenic materials by matching the markings, the material test certificate and the impact test results to the order. A material that looks right but lacks the required impact test is a common non-conformance.
- Check the test temperature: the certificate must show Charpy results at or below the specified temperature and meet the energy or lateral expansion criterion.
- Check the grade: a WPL6 fitting is impact tested while a WPB fitting is not, even though both are B16.9 fittings. See our guide to ASME B16.9 fittings for how the WP prefix and grade markings work.
- Confirm the alloy: positive material identification catches a carbon steel part in a stainless line; see our guide to positive material identification testing.
Mixed-up fittings, bolting and consumables are most likely at receiving and storage, which makes a documented incoming material inspection a practical safeguard.
Quick Reference: Material Choice by Minimum Design Temperature
This table maps the minimum design metal temperature to typical material choices. Confirm each choice against the governing code and project specification.
| Design minimum metal temperature | Typical choices | Notes |
|---|---|---|
| Warmer than -29 °C | Carbon steel within code exemption curves | Check UCS-66 or B31.3 exemption for thickness and grade |
| -29 to -46 °C | Impact-tested carbon steel: A333 Gr 6, A350 LF2, A420 WPL6 | Impact tested at about -45 to -46 °C |
| -46 to -101 °C | 3.5% Ni steel (A333 Gr 3, A350 LF3, A420 WPL3), or austenitic stainless steel | 3.5% Ni test temperature about -100 °C |
| -101 to -130 °C | 5% Ni steel, 9% Ni steel or austenitic stainless steel | Nickel-based filler for Ni steels |
| -130 to -196 °C | 9% Ni steel, austenitic stainless steel or 5083 aluminium | 9% Ni limit about -196 °C |
| Colder than -196 °C | Austenitic stainless steel, aluminium, copper and nickel alloys | 9% Ni steel is not used below its limit |
What Are the Common Mistakes and Limits?
Most cryogenic failures and rejections come from a wrong assumption about temperature, toughness or documents. The mistakes below are the ones that recur.
- Using the normal operating temperature. The design minimum metal temperature must include depressurisation and other upsets.
- Treating a test temperature as a service limit. The -45 °C of A333 Grade 6 is a material test temperature, not an automatic limit for every system.
- Ignoring the weld. Base metal toughness means little if the weld metal or HAZ was not tested at the design temperature.
- Forgetting contraction. A rigid cold line can overload nozzles and supports when it shrinks.
- Mixing grades. A fitting or bolt without the required low-temperature grade can sit unnoticed in a cold line.
- Overlooking non-metals. Gaskets, seals and insulation must also be rated for the temperature.
This guide gives general selection guidance and typical values. Requirements depend on the code edition and contract specification applicable to your project, and the responsible engineer must confirm the material, tests and design for any specific service.
Frequently Asked Questions
What is the best material for cryogenic service?
No single material is best for all cryogenic service. Austenitic stainless steels such as 304L and 316L are the usual choice for piping and small vessels because they stay ductile at -196 °C and below. 9% nickel steel is widely used for large LNG and liquid-oxygen storage tanks, and aluminium alloys such as 5083 are used where weight matters. The choice depends on the minimum design temperature, the code, the size, the cost and the fabrication capability.
Can carbon steel be used for cryogenic service?
Ordinary carbon steel is not suitable for cryogenic service, but impact-tested low-temperature grades can be used down to a limited temperature. ASTM A333 Grade 6, for example, is typically impact tested at -45 °C. Below that, nickel steels or austenitic stainless steel are needed, because body-centred cubic steels lose toughness as they get colder. At -196 °C carbon steel is not acceptable.
Why is austenitic stainless steel used for cryogenic service?
Austenitic stainless steel has a face-centred cubic crystal structure, which does not show a ductile-to-brittle transition. It keeps high impact toughness down to very low temperatures, and type 304L is reported to keep it to about -270 °C. Its main drawback is a low room-temperature yield strength, which can require thicker sections. Low-carbon L grades are preferred to limit sensitisation during welding.
Why is 9% nickel steel used for LNG tanks?
9% nickel steel combines high strength with excellent toughness at -196 °C, which is well below the -162 °C boiling point of LNG. Its microstructure of tempered martensite with some retained austenite resists brittle fracture, and it is moderately priced for large tanks. It is standardised as ASTM A353 and A553 Type I and in EN 10028-4. It is welded with nickel-based filler metals and needs control of heat input and magnetism.
What is the minimum temperature for ASTM A333 Grade 6?
ASTM A333 Grade 6 is impact tested at -45 °C (-50 °F), and that is a material test temperature rather than a universal service limit. Whether it can be used at a given temperature depends on the code, the thickness and the impact test requirements of the project. For colder service, A333 Grade 3 (3.5% Ni) is tested at about -100 °C and Grade 8 (9% Ni) at about -195 °C.
Is impact testing required for stainless steel at -196 °C?
It depends on the code and conditions. Under ASME Section VIII Division 1, UHA-51 exempts qualifying austenitic grades down to -196 °C, and testing is needed below that. ASME B31.3 sets conditions on carbon content and heat treatment for base metal, and limits the exemption for austenitic weld metal to about -101 °C and warmer in recent editions. Many project specifications require Charpy tests on procedure qualification regardless, so check both the code and the specification.
How much does pipe shrink when cooled to -196 °C?
Austenitic stainless steel contracts about 0.28% from room temperature to -196 °C, which is about 2.8 mm per metre. A 50 m line therefore shortens by roughly 140 mm. Aluminium alloys contract about 0.4%, and carbon and 9% nickel steels about 0.2%. These are approximate values, so use supplier or NIST data for design.
Key Terms
- Cryogenic service
- Operation at very low temperatures where materials must resist brittle fracture, commonly below about -50 °C in material practice.
- Ductile-to-brittle transition
- The temperature range over which a ferritic steel changes from absorbing large impact energy to fracturing in a brittle manner.
- BCC and FCC
- Body-centred and face-centred cubic crystal structures; FCC metals such as austenitic stainless steel and aluminium do not show a transition.
- Charpy V-notch (CVN) test
- An impact test that measures the energy absorbed or lateral expansion of a notched specimen at a set temperature.
- Design minimum metal temperature (MDMT)
- The lowest metal temperature at which equipment is permitted to carry full design pressure.
- Lateral expansion
- The increase in specimen width opposite the notch after a Charpy test, used as an acceptance criterion for austenitic and some other steels.
- Auto-refrigeration
- The cooling of a fluid and the metal around it that occurs when a liquefied gas is depressurised rapidly.
Conclusion
Cryogenic material selection follows the lowest metal temperature: impact-tested carbon steel to about -45 °C, nickel steels down the scale to 9% nickel steel at -196 °C, and austenitic stainless steel, aluminium and other FCC metals for the coldest service. Check the code toughness rule, test the weld metal and HAZ, allow for contraction, and verify markings and certificates on receipt. Treat the limits here as typical, and confirm them against the code edition and project specification before you finalise any design. As a next step, test your knowledge of impact testing and design temperature with the ASME Section VIII Division 1 quiz.
This guide was prepared by the WeldFabWorld technical team from the standards and sources listed in Standards and References. Verify every requirement against the code edition and specification applicable to your project.
Standards and References
- American Society of Mechanical Engineers (ASME): ASME B31.3, Process Piping; ASME BPVC Section VIII Division 1, Rules for Construction of Pressure Vessels; ASME BPVC Section II and Section IX
- American Petroleum Institute (API): API 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks, including Annexes Q and R
- ASTM A333/A333M, Seamless and Welded Steel Pipe for Low-Temperature Service; ASTM A350/A350M; ASTM A420/A420M; ASTM A353/A353M; ASTM A553/A553M; ASTM A203/A203M; ASTM A240/A240M; ASTM A312/A312M; ASTM A403/A403M; ASTM A320/A320M; ASTM B209, ASTM International
- EN 14620, Design and manufacture of site built, vertical, cylindrical, flat-bottomed steel tanks for the storage of refrigerated, liquefied gases with operating temperatures between 0 °C and -165 °C, CEN
- EN 1252-1, Cryogenic vessels, Materials, Toughness requirements for temperatures below -80 °C, CEN
- EN 10028-4, Flat products made of steels for pressure purposes, Nickel alloy steels with specified low temperature properties, CEN
- EN 1011-1, Welding, Recommendations for welding of metallic materials, General guidance for arc welding, CEN
- TWI Job Knowledge 100: Welding of ferritic cryogenic steels, TWI Ltd
- NIST cryogenic material properties: 304 stainless steel, National Institute of Standards and Technology
- IGC Code, International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, International Maritime Organization (IMO)