Cryogenic Nickel Alloy Selection for LNG and ASU (54 chars)

By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012

Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360

Quick Answer: How to Select Alloys for Cryogenic Service

Select on toughness at the service temperature, not on room-temperature strength. Austenitic grades — 304L, 316L, 625, 718 — stay ductile because their face-centred cubic structure does not embrittle; 9% Ni steel is the economic choice for LNG tanks, and Invar 36 is used where thermal movement must be near zero.

Key Takeaways

• Toughness, not strength, is the design driver. A cryogenic failure is a brittle fracture, so the specification is built around Charpy impact energy, lateral expansion and fracture toughness at the service temperature.
• Face-centred cubic structures stay ductile. Austenitic stainless and nickel-based alloys do not show the ductile-to-brittle transition that ferritic steels do, which is why they dominate cold service.
• 9% nickel steel is the LNG workhorse. Quenched and tempered 9% Ni plate is the standard for large LNG storage tanks because it combines adequate toughness at −162 °C with a cost far below stainless.
• Invar 36 solves a different problem. Its very low coefficient of thermal expansion keeps membrane containment and piping from contracting, rather than providing toughness.
• Welding is the weak link. Filler metal selection, heat input and impact testing of the procedure are where cryogenic equipment most often fails to qualify.
• Test at temperature, not at room temperature. A certificate showing 20 °C impact values says nothing about behaviour at −196 °C.

What Changes When a Metal Goes Cold

When a metal goes cold, its yield strength rises, its ductility falls, and its fracture mode can change from ductile tearing to brittle cleavage. For ferritic steels the change is abrupt and occurs over a narrow temperature range called the ductile-to-brittle transition; below it, the steel absorbs very little energy before breaking. For austenitic and nickel-based alloys the change is gradual, and the material retains substantial toughness down to the lowest temperatures used in industry.

The physical reason is the crystal structure. Ferritic steels have a body-centred cubic lattice, in which dislocation mobility is strongly temperature-dependent and cleavage planes are well defined. Face-centred cubic lattices, which include austenitic stainless steels and nickel-based superalloys, have more slip systems, no well-defined cleavage plane, and a much weaker temperature dependence of yield. The result is that an austenitic alloy at −196 °C is stronger than at room temperature but still ductile, while a carbon steel at the same temperature is brittle.

The second change is thermal contraction. Every metal contracts as it cools, and the amount depends on the coefficient of thermal expansion. Austenitic stainless steels contract about 50% more than carbon steel over the same range, aluminium contracts more again, and Invar 36 contracts very little. In a large tank or a long pipe run, the contraction has to be absorbed by loops, bellows or sliding supports; in a membrane containment system it has to be accommodated by the membrane geometry. Where the contraction cannot be absorbed, the material choice changes to one that moves less, which is why Invar exists as a cryogenic material.

The third change is thermal conductivity, which affects cooldown rate, thermal stress and boil-off. Austenitic stainless has low conductivity, nickel alloys lower still, aluminium high, and 9% Ni steel intermediate. A low-conductivity material reduces heat leak but also produces steeper thermal gradients during cooldown, so the cooldown procedure has to be controlled to keep thermal stress within limits.

Property	Behaviour on cooling	Design consequence	Material implication
Yield strength	Rises	Higher allowable stress at low temperature	Sections can sometimes be thinner
Ductility	Falls, but remains high in FCC alloys	Brittle fracture risk in BCC steels	Austenitic or nickel alloy required below transition
Charpy impact energy	Falls sharply in ferritic steels	Minimum energy specified at service temperature	Impact test at temperature is mandatory
Thermal expansion	Contracts	Piping loops, sliding supports, membrane geometry	Invar where movement must be minimal
Thermal conductivity	Falls for austenitic, rises relatively for aluminium	Cooldown rate, thermal stress, boil-off	Cooldown procedure must match material
Magnetic properties	Austenitic stays non-magnetic	Instrumentation, magnetic interference	Nickel alloys preferred near sensors

Table note: the table describes general physical behaviour. Design values must come from the applicable code and from material data at the service temperature. ASME Section VIII Div 1 UHA-51 governs impact testing of austenitic and nickel alloy materials in pressure service.

Why Austenitic and Nickel-Based Alloys Stay Tough

Austenitic and nickel-based alloys stay tough because their face-centred cubic lattice deforms by slip on many equivalent planes, so a crack tip is blunted by plastic deformation rather than propagating as a cleavage crack. Nickel content stabilises the austenitic structure, and in sufficient quantity it suppresses the formation of martensite on cooling, which is the transformation that would otherwise make the alloy brittle.

The key metallurgical variable is the martensite start temperature, usually estimated from the composition using an empirical relationship based on nickel, chromium, carbon, manganese, silicon and nitrogen. When the martensite start temperature is well below the service temperature, the austenite remains stable and the toughness remains high. When it is close to or above the service temperature, some austenite transforms to martensite on cooling or on deformation, and toughness falls. This is why the nickel content of a cryogenic stainless steel matters more than its room-temperature strength, and why 304L is preferred over 304 for low-temperature service: the lower carbon reduces the risk of sensitisation and improves toughness.

Nickel-based alloys go further. Inconel 625 and Hastelloy C-276 are fully austenitic with high nickel and molybdenum content, and they retain toughness to liquid helium temperature. Inconel 718 is precipitation hardened and retains high strength at cryogenic temperature, which is why it is used for pump shafts, valve stems and rocket engine hardware where both strength and toughness are needed. Monel K-500 is used for cryogenic pump shafts in LNG service because it combines strength with good low-temperature toughness and resistance to the process fluid.

The practical consequence is that the specification should state the service temperature and require impact testing at that temperature, not a generic "low temperature" requirement. ASME Section VIII Div 1 UHA-51 sets impact test requirements for austenitic and nickel alloy materials and defines the exemptions and the test temperatures; a material that is exempt from impact testing under the code may still need testing under a project specification for a more severe duty.

Grade	Structure	Service temperature capability	Toughness at low temperature	Typical cryogenic use
304L (S30403)	Austenitic	To about −196 °C	Excellent; no transition	Piping, tanks, cold box internals
316L (S31603)	Austenitic	To about −196 °C	Excellent	Piping, instruments, pumps
9% Ni steel (K81340)	Martensitic / ferritic	To −196 °C when qualified	Good if quenched and tempered and correctly welded	LNG storage tanks
Invar 36 (K93600)	Austenitic Fe-Ni	To −180 °C and below	Good	Membrane containment, piping
Inconel 625 (N06625)	Austenitic Ni-Cr-Mo	To −196 °C and below	Excellent	High-pressure cryogenic valves, bellows
Inconel 718 (N07718)	Austenitic, aged	To −253 °C	Good to excellent when aged correctly	Pump shafts, fasteners
Monel K-500 (N05500)	Austenitic Ni-Cu, aged	To −196 °C	Good	LNG pump shafts
Aluminium 5083 (A95083)	Al-Mg	To −196 °C	Excellent; no transition	Secondary barriers, small tanks
Carbon steel	Ferritic	Limited; transition above −46 °C for many grades	Poor below transition	Not for cryogenic product

Table note: temperature capability is indicative and depends on the product form, heat treatment, welding and the governing code. ASME VIII UHA-51 and the applicable material specification set the actual testing and acceptance requirements. Do not use this table as a substitute for code compliance.

LNG Containment: Three Technologies and Three Material Choices

LNG containment at −162 °C is dominated by three technologies, and each one uses a different material for a different reason. The choice is not about which alloy is "best"; it is about which combination of toughness, thermal expansion, cost and construction method fits the project.

The first is the self-supporting 9% nickel steel tank, used for large onshore storage and for Moss-type spherical tanks. The plate is quenched and tempered to a fine-grained martensitic structure and welded with nickel-based filler, and the design relies on the toughness of the base metal and the weld at −162 °C. ASTM A553 Type I covers the quenched and tempered grade, and ASTM A353 covers the double-normalised grade; the two are not interchangeable in a specification because the heat treatment route and the resulting properties differ. The advantage of 9% Ni is cost: it is far cheaper than stainless or nickel alloy for a large plate structure. The disadvantage is that the welding is demanding and the toughness depends critically on the heat treatment and the welding consumable.

The second is the membrane containment system, where a thin stainless steel membrane (typically 304L or a nickel-based alloy) is supported by insulation and a concrete or steel structure. The membrane contains the liquid and transmits the hydrostatic load to the insulation, so it does not need structural strength; it needs ductility, a low expansion coefficient relative to the support, and weldability in thin sections. Invar 36 is used in the membrane systems that rely on very low thermal expansion to control the strain in the membrane, because its expansion coefficient is an order of magnitude lower than austenitic stainless over the cryogenic range. The trade is cost and availability: Invar is an expensive, specialised product with limited suppliers.

The third is the aluminium alloy tank, typically 5083, used for smaller tanks, for fuel gas tanks on ships and for secondary barriers. Aluminium retains toughness at cryogenic temperature, is light, and is easy to weld, but its strength is low and its expansion coefficient is high, so it is used where the geometry can accommodate contraction and the load is moderate.

Containment type	Primary material	Why this material	Standard basis	Main fabrication risk
Self-supporting 9% Ni tank	9% Ni steel plate, Ni-based filler	Toughness at −162 °C at low cost	ASTM A553 Type I, ASTM A353	Weld toughness, heat input
Membrane containment	304L or Invar 36 membrane	Ductility, low expansion, thin-section weldability	ASTM A240, ASTM F1684	Weld distortion, membrane fatigue
Aluminium tank	5083 plate	Toughness, light weight, weldability	ASTM B209	Strength limitation, expansion
Spherical Moss tank	9% Ni steel or aluminium	Toughness and structural efficiency	ASTM A553, ASTM B209	Field welding, support design
Secondary barrier	5083 or stainless liner	Containment of leakage	ASTM B209, ASTM A240	Continuity, penetration details

Table note: the table matches containment technology to material and standard. It does not cover the insulation, the concrete or the structural steel, which have their own specifications. Design must follow the applicable LNG code and the project specification.

For a buyer, the material decision follows from the containment type, and the containment type is usually fixed by the project's licensor. The purchasing task is therefore to specify the material correctly within that choice: the exact grade and heat treatment for 9% Ni, the exact expansion grade for Invar, the exact temper for aluminium, and the impact test temperature and acceptance values for each. A purchase order that says "9% Ni steel to A553" without stating the type, the heat treatment and the impact test temperature leaves the most important properties undefined.

Air Separation, Cold Boxes and Cryogenic Process Equipment

Air separation and cold box service operates at −196 °C and below, and the material selection is driven by a different combination of requirements: internal pressure, thermal cycling, insulation space constraints and the need for clean, non-contaminating surfaces. Austenitic stainless dominates, with nickel alloys used where pressure, strength or a specific corrosion duty justifies the cost.

For cold box piping and internals, 304L and 316L are the default. Both remain tough at −196 °C, both are weldable in thin sections, and both are available in the full range of pipe, fittings, plate and bar. The choice between them is usually corrosion-driven rather than cryogenic: 316L is selected where chlorides or a more aggressive process stream are present, and 304L where cost and availability dominate. The low-carbon variants are preferred because they reduce the risk of sensitisation during welding, which matters both for corrosion and for toughness in the heat-affected zone.

For high-pressure cryogenic service — pumps, valves, bellows, heat exchangers in high-pressure cycles — nickel alloys become necessary. Inconel 625 is used for valve bodies, bellows and high-pressure tubing because it combines cryogenic toughness with high strength and corrosion resistance. Inconel 718 is used for pump shafts and fasteners where the strength requirement is high, and Monel K-500 is used for LNG pump shafts where the combination of strength and resistance to the process fluid is required. These alloys are not chosen for toughness alone; they are chosen because the austenitic stainless grades cannot carry the load or resist the fluid at the required section size.

Aluminium 5083 appears in air separation as plate-fin heat exchanger material and in some cold box structures, where its high thermal conductivity and low cost are advantageous. Titanium is used in specific applications where weight and corrosion resistance matter, but it is not a general cryogenic structural material because of cost and availability. Copper alloys appear in brazed aluminium plate-fin exchangers and in some instrument lines, but they are outside the nickel alloy scope of this article.

Equipment	Service temperature	Preferred material	Alternative	Selection driver
Cold box piping	−196 °C	304L	316L, 625	Toughness, weldability, cost
High-pressure cryogenic valve	−196 °C	625	316L for lower pressure	Strength plus toughness
Pump shaft, LNG	−162 °C	K-500	718	Strength, wear, process fluid
Bellows and expansion joints	−196 °C	625	316L	Fatigue, toughness, thin section
Plate-fin heat exchanger	−196 °C	5083 aluminium	3003	Conductivity, brazeability
Instrument tubing	−196 °C	316L	625	Availability, clean surface
Storage tank, small	−196 °C	304L	5083	Cost, shop fabrication
Fasteners	−196 °C	718, A286	316L for low load	Strength with toughness

Table note: the table reflects common practice in air separation and cryogenic process plants. Final selection must consider pressure, cycling, corrosion and the governing design code, and should be confirmed by impact testing at the service temperature.

Testing and Acceptance: The Evidence That Matters

Testing at temperature is the only evidence that matters, and the test programme should be built around the failure mode: brittle fracture. A room-temperature tensile test, however complete, does not demonstrate cryogenic toughness, and a certificate that reports only room-temperature Charpy values is not a cryogenic qualification.

The primary test is the Charpy V-notch impact test to ASTM E23, performed at the service temperature or at a temperature specified by the design code. The acceptance criterion is usually a minimum absorbed energy, sometimes combined with a minimum lateral expansion, and the values are set by the code or the project specification rather than by the material standard. For 9% Ni steel, the weld metal and the heat-affected zone must be tested as well as the base metal, because the toughness of the weld is often the limiting property. For austenitic and nickel alloys, ASME VIII UHA-51 defines when impact testing is required and what test temperature applies.

The second test is fracture toughness, either KIC or JIC, determined at the service temperature. This is used where a fitness-for-service assessment is required, where the section is thick, or where the consequence of failure justifies a more rigorous approach. Fracture toughness testing at cryogenic temperature requires a cryostat and careful temperature control, and it is a specialist service rather than a routine mill test.

The third test is the tensile test at temperature, which gives the yield and tensile strength used in the design and confirms that the material has not been embrittled by heat treatment or welding. For austenitic grades the tensile strength rises at low temperature, which can be beneficial, but the design code may impose limits on the allowable stress increase.

The fourth is nondestructive testing of the welds, which is more important in cryogenic service than in most others because a weld defect is a crack starter in a material with limited toughness reserve. Radiography, ultrasonic testing and penetrant testing are applied according to the code and the joint efficiency; for 9% Ni tanks, the weld inspection regime is particularly stringent.

Test	Purpose	Standard	Temperature	Acceptance basis
Charpy V-notch impact	Toughness, transition behaviour	ASTM E23	Service temperature or code-specified	Code minimum energy and lateral expansion
Fracture toughness	Flaw tolerance	ASTM E399 / E1820	Service temperature	Project-specific KIC or JIC
Tensile at temperature	Design strength	ASTM E8/E8M	Service temperature	Code allowable stress basis
Weld and HAZ impact	Weld toughness	ASTM E23	Service temperature	Code minimum, often governing
Radiography	Weld defects	ASTM E94 / ASME V	Ambient	Code acceptance criteria
Ultrasonic testing	Internal defects, laminations	ASTM A578 / ASME V	Ambient	Code acceptance criteria
Penetrant testing	Surface defects	ASTM E165	Ambient	Code acceptance criteria
PMI	Alloy identity	ASTM E1476	Ambient	Certificate match

Table note: the standards named are test methods; acceptance criteria come from the design code, typically ASME Section VIII Div 1 or the project specification. Impact testing of the procedure qualification is mandatory for cryogenic welds in most codes.

Welding Cryogenic Materials: Where the Qualification Is Won or Lost

Welding is where cryogenic material selection is tested, because the weld and the heat-affected zone are the regions where toughness is most easily lost. A base metal that passes its impact test at −196 °C can still produce a joint that fails, because the thermal cycle coarsens the grain, introduces residual stress and, in 9% Ni steel, can produce martensite or retained austenite in the wrong proportions.

For 9% Ni steel, the standard practice is to weld with a nickel-based filler — typically a 60% Ni or higher consumable — because the filler remains tough at cryogenic temperature and accommodates the difference in thermal expansion between the weld and the base metal. The heat input must be controlled within a qualified range: too high coarsens the heat-affected zone and reduces toughness, too low can produce incomplete fusion and hard zones. The procedure qualification must include impact testing of the weld metal and the heat-affected zone at the service temperature, and the test is usually the governing acceptance criterion for the whole fabrication.

For austenitic stainless and nickel alloys, the risks are different. The weld metal is generally fully austenitic and tough, but the heat-affected zone can suffer sensitisation if the carbon content is not controlled and the heat input is high, and it can suffer hot cracking if the ferrite content is too low or the restraint is high. Low-carbon base metal and filler, controlled heat input and a qualified procedure address most of the risk. For 625 and C-276, the weld metal is over-alloyed relative to the base metal, which preserves corrosion resistance but can produce a different thermal expansion behaviour; the joint design should account for that.

Dissimilar joints deserve specific attention. A 9% Ni nozzle welded to a stainless pipe, an aluminium heat exchanger connected to a stainless line, or a nickel alloy valve welded into a carbon steel spool all create a joint where the two materials contract differently on cooling. The weld sees a complex stress state, and the interface can be the weakest point in the cryogenic boundary. The solution is usually a transition piece or a bimetallic joint qualified as a procedure, not a direct weld between the two materials.

Material	Filler	Heat input control	Key risk	Qualification test
9% Ni steel	Ni-based (about 60% Ni or higher)	Narrow qualified range	HAZ toughness, hot cracking	Impact test of weld and HAZ at −196 °C
304L / 316L	Matching or over-alloyed	Moderate, interpass control	Sensitisation, ferrite balance	Impact test where code requires
Invar 36	Matching Fe-Ni filler	Low	Cracking, expansion mismatch	Tensile and expansion check
Inconel 625	Matching Ni-Cr-Mo	Moderate	Hot cracking, dilution	Impact and corrosion test
Aluminium 5083	5183 or 5356	Controlled	Porosity, loss of strength in HAZ	Tensile and bend test
Dissimilar joint	Transition or bimetallic	Low	Thermal stress, interface cracking	Procedure qualification and NDT

Table note: filler and heat input guidance is typical practice, not a substitute for a qualified welding procedure. Cryogenic procedures must be qualified with impact testing at the service temperature under the governing code.

National Standards and 2026 Price Reference

The national systems describe cryogenic materials in different ways, and a buyer sourcing across borders should expect the grade designation, the heat treatment and the impact test requirement to be expressed differently in each. The US system centres on ASTM A553 and A353 for 9% Ni steel, ASTM A240 for stainless plate, ASTM F1684 for Invar, and ASME Section VIII UHA-51 for impact testing. Europe uses EN 10028-4 for nickel alloy steel plates for low temperature, EN 10088 for stainless, and the Pressure Equipment Directive for conformity. China uses GB/T 24510 for 9% Ni steel plate and GB/T 4237 for stainless plate, with GB/T 150 governing pressure vessel design. Japan uses JIS G 3127 for nickel steel plates for low temperature service and JIS G 4304/G 4305 for stainless plate. Russia and the CIS use GOST 5632 for alloy grades and their own low-temperature plate specifications.

System	9% Ni plate	Austenitic stainless plate	Low-expansion alloy	Impact test basis
US	ASTM A553 Type I, ASTM A353	ASTM A240	ASTM F1684	ASME VIII UHA-51
Europe	EN 10028-4	EN 10028-7 / EN 10088	EN 10331 (Fe-Ni)	EN 13445 and PED
China	GB/T 24510	GB/T 4237	GB/T 14986 (precision alloys)	GB/T 150
Japan	JIS G 3127	JIS G 4304 / G 4305	JIS C 2534 (Fe-Ni)	JIS B 8265
Russia / CIS	GOST low-temperature plate specs	GOST 5632 grades	GOST 10994 (precision alloys)	GOST and technical supervision

Table note: designations and specifications are listed to show how each system frames the requirement. They are not claimed to be equivalent, and the numerical requirements differ. Confirm the exact grade, heat treatment and impact test temperature with the mill before ordering.

Price for cryogenic material reflects the alloy content, the heat treatment and the test package. 9% Ni steel is the cheapest option per kilogram for a large plate structure but requires a demanding welding procedure; austenitic stainless costs more but is easier to weld; Invar is a specialty product with a limited supplier base and a price to match; nickel alloys sit at the top. The table gives indicative 2026 EXW Shanghai prices for budgeting, not for quotation.

Material and form	Indicative price, 2026, EXW Shanghai, USD/kg	Relative to 304L plate	Cryogenic role	Cost comment
304L plate	4-7	1.0	Cold box piping, tanks	Lowest cost austenitic
316L plate	5-9	1.3	Chloride-bearing cryogenic service	Slightly higher alloy content
9% Ni steel plate	3-6	0.8	LNG tank shells	Cheapest per kg, welding cost high
Invar 36 plate	25-40	6.0	Membrane containment	Specialty product, limited supply
Aluminium 5083 plate	4-7	1.0	Secondary barriers, small tanks	Light, low strength
Inconel 625 plate	40-55	8.5	High-pressure valves, bellows	Premium for strength and toughness
Inconel 625 bar	38-58	8.5	Valve stems, fittings	Size and test package vary
Inconel 718 bar	45-70	10.0	Pump shafts, fasteners	Aging and test package add cost
Monel K-500 bar	40-60	9.0	LNG pump shafts	Hardness control required

Table note: 2026, EXW Shanghai, USD/kg, indicative ranges only, subject to nickel, molybdenum and alloy surcharge movement, quantity and test package. Relative cost uses the midpoint of the 304L plate range. Not an offer.

Grade Reference Data for Cryogenic Design

The tables below give the composition, room-temperature properties and heat treatment of the grades used in cryogenic service. Room-temperature values are the starting point for design, but the acceptance evidence for cryogenic duty is the impact test at the service temperature described earlier.

Grade	UNS	Ni %	Cr %	Other (typical)	Standard
Type 304L	S30403	8.0-12.0	18.0-20.0	C 0.030 max	ASTM A240
Type 316L	S31603	10.0-14.0	16.0-18.0	Mo 2.0-3.0, C 0.030 max	ASTM A240
9% Ni steel	K81340	8.5-9.5	-	C 0.13 max, Mn 0.90 max	ASTM A553 / A353
Invar 36	K93600	35.0-37.0	-	Fe balance, C 0.05 max	ASTM F1684
Inconel 625	N06625	58.0 min	20.0-23.0	Mo 8.0-10.0, Nb+Ta 3.15-4.15	ASTM B443
Inconel 718	N07718	50.0-55.0	17.0-21.0	Nb+Ta 4.75-5.5, Mo 2.8-3.3	ASTM B637
Aluminium 5083	A95083	-	-	Mg 4.0-4.9, Mn 0.4-1.0	ASTM B209

Table note: composition ranges are from the product specification named. Aluminium 5083 contains no nickel and is included because it is a direct alternative in LNG and cold box construction.

Grade	Condition	UTS (typical, MPa)	0.2% YS (typical, MPa)	Elongation (typical, %)	Hardness (typical)	Standard
Type 304L	Annealed	485-690	170-310	40-50	130-180 HB	ASTM A240 / E8
Type 316L	Annealed	485-690	170-310	35-45	130-180 HB	ASTM A240 / E8
9% Ni steel	Quenched and tempered	690-830	585-690	18-25	190-240 HB	ASTM A553 / E8
Invar 36	Annealed	450-550	240-310	30-40	130-170 HB	ASTM F1684 / E8
Inconel 625	Solution annealed	830-930	415-500	30-45	150-220 HB	ASTM B443 / E8
Inconel 718	Solution treated and aged	1,240-1,400	1,030-1,200	12-20	36-44 HRC	ASTM B637 / E8

Table note: room-temperature typical values. Yield and tensile strength rise at cryogenic temperature while elongation falls; the design code allowable stresses and the impact test requirements govern acceptance, not these values.

Grade	Heat treatment (typical)	Cooling	Condition of supply	Standard
Type 304L / 316L	1,040-1,120 °C anneal	Water quench or rapid	Annealed, pickled	ASTM A240 / A480
9% Ni steel	Quench and temper, or double normalise	Water quench	Q&T Type I or double normalised	ASTM A553 / A353
Invar 36	830-900 °C anneal	Rapid cool	Annealed	ASTM F1684
Inconel 625	1,090-1,205 °C solution anneal	Water quench or rapid gas	Solution annealed	ASTM B443 / AMS 5599
Inconel 718	Solution 950-1,065 °C, age 720 °C then 620 °C	Controlled	Solution treated and aged	ASTM B637 / AMS 5662
Aluminium 5083	O temper, no thermal treatment	Air	O or H temper as specified	ASTM B209

Table note: heat treatment ranges are typical practice and the controlling values are those of the product specification and the qualified procedure. The heat treatment of 9% Ni steel is critical to its toughness and must be confirmed on the certificate.

Standard Index

Standard	Title or scope	Covers	Product form
ASTM A553	9% nickel alloy steel plate, quenched and tempered	Chemistry, mechanicals, toughness	Plate
ASTM A353	9% nickel alloy steel plate, double normalized	Chemistry, mechanicals, toughness	Plate
ASTM A240	Chromium and chromium-nickel stainless steel plate, sheet and strip	Chemistry, mechanicals	Plate, sheet
ASTM F1684	Iron-nickel alloy for low thermal expansion	Chemistry, expansion, properties	Plate, bar, strip
ASTM B209	Aluminium and aluminium alloy sheet and plate	Chemistry, mechanicals	Plate, sheet
ASTM B443 / B446	625 plate and bar	Chemistry, mechanicals	Plate, bar
ASTM B637	718 bar, forging and ring	Chemistry, mechanicals	Bar, forging
ASTM E23	Notched bar impact testing	Charpy energy and lateral expansion	All forms
ASTM E399 / E1820	Fracture toughness	KIC, JIC	All forms
ASTM E8/E8M	Tension testing	Tensile properties	All forms
ASTM E165	Liquid penetrant examination	Surface defects	All forms
ASTM A578	Ultrasonic examination of plain and clad steel plate	Laminations	Plate
ASME VIII Div 1 UHA-51	Impact test requirements for austenitic and nickel alloys	Test temperature, exemption	Pressure parts
EN 10028-4	Nickel alloy steel plates for low temperature	Properties, toughness	Plate
GB/T 24510	9% Ni steel plate for low temperature	Properties, toughness	Plate
JIS G 3127	Nickel steel plates for low temperature service	Properties, toughness	Plate
GOST 5632	Corrosion- and heat-resistant alloy grades	Grade designation	All forms

Table note: the table separates material specifications from test methods and design codes. Cryogenic supply normally requires all three, and the impact test temperature must be stated explicitly in the purchase order.

FAQ: Cryogenic Nickel Alloy Selection

Q1: Why do austenitic stainless steels stay tough at cryogenic temperature?

Austenitic stainless steels stay tough because their face-centred cubic crystal structure has many equivalent slip systems and no well-defined cleavage plane, so a crack tip is blunted by plastic deformation instead of propagating as a brittle crack. Ferritic steels have a body-centred cubic structure in which dislocation mobility falls sharply with temperature and cleavage planes are well defined, producing the ductile-to-brittle transition. The practical consequence is that 304L and 316L remain ductile at −196 °C while a carbon steel at the same temperature is brittle. The stability of the austenite also matters: if the composition allows martensite to form on cooling or deformation, toughness falls, which is why nickel content and the martensite start temperature are part of the material specification.

Q2: Is 9% nickel steel better than stainless for LNG tanks?

Neither is better in absolute terms; 9% nickel steel is more economical for large plate structures and stainless is easier to weld and more forgiving in thin sections. A large LNG storage tank contains thousands of tonnes of plate, and at roughly a third of the stainless price per kilogram, 9% Ni offers a decisive cost advantage, provided the welding procedure is qualified with impact testing at −162 °C. Stainless becomes competitive for smaller tanks, for membrane systems, for piping and for components where the fabrication is complex. The decision is therefore driven by scale and fabrication method rather than by a material property comparison. A project that chooses 9% Ni for the tank shell may still choose stainless for the internal piping and the pump columns.

Q3: What is Invar 36 used for in LNG service?

Invar 36 is used where thermal contraction must be minimised, most notably in membrane containment systems where a thin stainless or nickel alloy membrane is bonded to an insulated support. The alloy contains about 36% nickel and has a coefficient of thermal expansion an order of magnitude lower than austenitic stainless over the cryogenic range, so the membrane does not contract enough to over-stress its anchors or to develop fatigue cracks at the corrugations. It is also used in cryogenic piping and in instrument components where dimensional stability matters. Invar is not chosen for toughness or strength; it is chosen for expansion, and its high cost and limited supplier base mean it is specified only where the expansion requirement justifies it.

Q4: Can carbon steel be used at −46 °C?

Some carbon steels are qualified for service down to about −46 °C, but only after impact testing at that temperature and only for the specific grade and thickness covered by the code. ASME Section VIII provides impact test exemptions and rules based on the material specification, the thickness and the minimum design metal temperature, and a steel that is exempt at one thickness may require testing at another. Below −46 °C, ferritic steels generally require a nickel addition or a different material family, which is why 3.5% Ni, 5% Ni and 9% Ni steels exist as a ladder of low-temperature options. A buyer should never assume that a carbon steel is suitable for cold service without checking the code curve and the impact test requirement.

Q5: How is cryogenic toughness specified and tested?

Cryogenic toughness is specified as a minimum Charpy V-notch absorbed energy, often with a minimum lateral expansion, measured at the service temperature or at the temperature required by the design code. The test is performed to ASTM E23, and the acceptance values come from the code or the project specification rather than from the material standard. For welds, the test is performed on the weld metal and on the heat-affected zone, and those values often govern the procedure qualification. Where a fitness-for-service assessment is needed, fracture toughness testing to ASTM E399 or E1820 at the service temperature provides the KIC or JIC value used in the assessment. Room-temperature impact values are not a substitute.

Q6: Which nickel alloy is best for a cryogenic pump shaft?

Monel K-500 and Inconel 718 are the two common choices, and the decision depends on the process fluid and the strength requirement. K-500 is used extensively in LNG pumps because it combines good cryogenic toughness with resistance to the process fluid and reasonable wear behaviour, and it can be machined and hardened to the required strength. 718 is used where higher strength is needed and where the corrosion duty allows it; it retains high strength at cryogenic temperature but requires careful control of ageing and hardness. In both cases the hardness and the surface finish matter, because a shaft is a stress concentration and a hydrogen or corrosion initiation site.

Q7: Why must welding procedures be impact tested for cryogenic service?

Welding procedures must be impact tested because the weld and heat-affected zone are where toughness is most easily lost, and a base metal certificate says nothing about the joint. The thermal cycle coarsens grains, can produce untempered martensite in 9% Ni steel, and can sensitise austenitic grades, all of which reduce impact energy. The procedure qualification therefore includes impact tests on specimens taken across the weld, with the test temperature at or below the service temperature and the acceptance values from the code. For 9% Ni steel, the weld metal toughness is often the limiting property, and the filler selection and heat input range are qualified together. A procedure qualified only by tensile testing is not a cryogenic qualification.

Q8: Does thermal expansion matter for cryogenic piping design?

It matters a great deal, because a pipe run that contracts on cooldown generates large forces if the movement is restrained. Austenitic stainless contracts about 50% more than carbon steel over the same temperature range, aluminium more again, and Invar very little. The design must provide expansion loops, bellows, sliding supports or flexible connections to absorb the contraction, and the support spacing and anchor locations must be calculated for the cold condition, not the ambient one. A line that is installed at ambient temperature and then cooled will move, and any restraint that was not designed for that movement becomes a stress concentration at the first cooldown. Flexibility analysis at the cold condition is therefore part of the material selection, not an afterthought.

Q9: Is Inconel 625 suitable for liquid hydrogen or helium service?

Inconel 625 is used in very low temperature service, including liquid hydrogen and helium applications, because it remains austenitic and tough and has a good strength-to-toughness balance. Its face-centred cubic structure does not undergo a ductile-to-brittle transition, and its high nickel content stabilises the austenite. The qualification still depends on the specific product form, heat treatment and weld, and the design code must be consulted because the allowable stresses at these temperatures differ from those at ambient. For the most severe duties, fracture toughness testing at the service temperature is advisable, as the consequence of failure in a hydrogen or helium system is high. Product form also matters: thin-wall tube and bellows behave differently from heavy bar.

Q10: How do I specify cryogenic material in a purchase order?

Specify the grade and product specification with edition, the heat treatment condition, the service temperature, the impact test temperature and acceptance values, the test location and orientation (including weld and heat-affected zone for fabricated items), the nondestructive testing requirements, and the documentation package. Add the design code, because the impact test exemption and the allowable stress depend on it. For 9% Ni steel, state Type I or the double-normalised route explicitly. For Invar, state the expansion requirement and the test method. For nickel alloys, state the condition of supply and the hardness where relevant.

Conclusion: Design for Toughness, Then Prove It at Temperature

Cryogenic material selection is a toughness problem before it is a strength problem. Austenitic stainless steels and nickel alloys stay ductile because their crystal structure does not embrittle, 9% nickel steel provides an economical route for large LNG plate structures, and Invar 36 solves the separate problem of thermal movement in membrane containment. The selection follows from the containment technology, the pressure and the process fluid, and it is confirmed by impact testing at the service temperature rather than by a room-temperature certificate.

The purchasing programme follows the same logic. State the service temperature and the impact test temperature. Name the grade, the heat treatment and the product specification with edition. Require impact testing of the weld and heat-affected zone for fabricated items, and fracture toughness where a flaw assessment is needed. Specify the nondestructive testing and the documentation. Then check that the certificate values are at temperature and at the specified location, because a certificate that reports room-temperature Charpy energy for a −196 °C component is not evidence of anything.

Buyers who need cryogenic material can review the Inconel product line and the stainless and duplex product range, check the low-temperature testing notes and the technical knowledge centre, or send the service conditions to the sales engineering desk. Shanghai Hangbo Alloy Group supplies 304L, 316L, 625, 718, K-500 and Invar 36 in plate, bar, tube and forging, with impact testing at the specified temperature and full documentation.

Related Reading

1. duplex 2205 versus 904L for cold and corrosive service
2. Hastelloy C-276 versus Incoloy 825

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Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier

Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com

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Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA

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