Stainless Steel for LNG Cryogenic Service: 304L vs 9% Nickel Steel

Liquefied natural gas (LNG) is stored and transported at –162 °C (–260 °F), one of the most demanding cryogenic conditions in industrial materials engineering. Selecting the wrong structural material for LNG storage tanks, pipelines, and process piping can result in brittle fracture, cryogenic embrittlement, or stress corrosion cracking - failures that carry catastrophic safety, environmental, and reputational consequences.

Stainless Steel for LNG Cryogenic Service

This article provides a structured, evidence-based comparison of two leading LNG-compatible structural materials: austenitic stainless steel 304L (UNS S30403) and 9 % nickel steel (UNS K81340, also referred to as ASTM A353 / A553 Type II / 9Ni steel). Each material is evaluated across chemistry, mechanical and cryogenic toughness properties, weldability, corrosion performance, regulatory compliance, fabrication practicality, and total installed cost.

Liquefied natural gas (LNG) is natural gas (predominantly methane, CH₄) that has been cooled to –162 °C (–260 °F) at atmospheric pressure, reducing its volume by approximately 600× and enabling efficient ocean transport and storage. The LNG value chain spans liquefaction plants, LNG carriers (tankers), regasification terminals, and inland distribution networks - each requiring structural materials that can operate safely at cryogenic temperatures.

At –162 °C, most carbon and low-alloy steels undergo a ductile-to-brittle transition (DBTT): their fracture toughness drops sharply, and a crack that would bend safely at room temperature can propagate as a catastrophic brittle fracture. This phenomenon, known as low-temperature brittleness, is the primary material selection driver for LNG applications. The key material requirements for LNG service are:

Retained ductility and toughness at –196 °C (liquid nitrogen temperature, as a test benchmark) and –162 °C (operational LNG temperature)

Absence of phase transformations (ferrite → martensite) at cryogenic temperatures

Low probability of brittle fracture in the presence of sharp defects or weld discontinuities

Resistance to thermal shock during rapid cool-down or warm-up transients

Adequate yield strength to contain LNG hydrostatic pressure at design temperature

LNG Facility Zones and Their Material Challenges

Different LNG facility zones impose distinct demands on structural materials:

Table. LNG Facility Zones and Their Distinctive Material Challenges


Zone	Temperature Range	Primary Material Challenge
LNG Storage Tank (inner container)	–162 °C (–260 °F)	Cryogenic toughness; thermal contraction; fatigue from fill/drain cycles
LNG Transfer Piping (primary cryogenic)	–162 °C	Thermal contraction stresses; fatigue; pressure containment at cryogenic temperature
Secondary Containment / Outer Tank	–50 °C to +50 °C	Thermal gradient; moisture ingress; fire-rated design
Boil-Off Gas (BOG) piping	–162 °C to 0 °C	Two-phase flow; thermal cycling; potential condensation
Process / Regasification piping	+0 °C to +50 °C (ambient)	CO₂ corrosion; sour species; standard carbon steel may apply
LNG Carrier Cargo Tank (membrane / spherical)	–162 °C	Membrane stress; sloshing fatigue; ullage gas pressure cycling

Source: GIIGNL (Groupe International des Importateurs de GNL) 'Study of the LNG Industry' and API 620 'Design and Construction of Large, Welded, Low-Pressure Storage Tanks' (14th Edition, 2020).

304L is the low-carbon (C ≤ 0.03 %) variant of the 18/8 austenitic stainless steel family, widely used in the food, chemical, and process industries. Its austenitic (face-centred cubic, FCC) crystal structure inherently provides excellent ductility and toughness at cryogenic temperatures - unlike ferritic or martensitic steels, the FCC structure does not undergo a ductile-to-brittle transition. 304L is the standard choice for LNG process piping and has been used for inner tanks of LNG storage facilities for decades. However, its maximum allowable stress is lower than 9Ni steel, and it is susceptible to chloride stress corrosion cracking (Cl-SCC) above approximately 60 °C.

9% Nickel Steel (UNS K81340 / ASTM A353 / A553 Type II)

9 % nickel steel is a high-strength, quenched-and-tempered (Q&T) alloy containing 8.5–9.5 % nickel by weight. The nickel addition lowers the steel's DBTT below –196 °C, making it fully austenitic and tough at LNG operating temperatures. 9Ni steel achieves significantly higher yield strength (485 MPa min, vs. 170 MPa for 304L) while retaining outstanding notch toughness at cryogenic temperatures. It is the most widely specified material for large-scale LNG above-ground storage tanks (ASTM A553 Type II, ASTM A353) and has been the dominant choice for LNG carrier cargo tanks for over 50 years. Its limitation is susceptibility to temper embrittlement above approximately 350 °C and the need for post-weld heat treatment (PWHT) on thicker sections.

Table. 304L vs 9% Nickel Steel - Specified Chemical Composition (wt %)


Element	304L (UNS S30403)	9Ni (UNS K81340 / A553 Type II)	9Ni (ASTM A353)
Fe	Balance	Balance	Balance
Ni	8.0–12.0	8.5–9.5	8.5–9.5
Cr	18.0–20.0	≤ 0.25	≤ 0.25
C	≤ 0.030	≤ 0.13	≤ 0.15
Mn	≤ 2.00	≤ 0.90	≤ 0.90
Si	≤ 0.75	0.15–0.40	0.15–0.40
P	≤ 0.045	≤ 0.015	≤ 0.020
S	≤ 0.030	≤ 0.015	≤ 0.020
Mo	≤ 0.75	≤ 0.10	≤ 0.10
Cu	≤ 1.00	≤ 0.35	≤ 0.35
N	≤ 0.10	-	-
Al	-	≤ 0.020	≤ 0.020
V	-	≤ 0.05	≤ 0.05
Product Standard	ASTM A240/A276/A480	ASTM A553 Type II (Q&T)	ASTM A353 (DPLT)

Source: ASTM A240/A240M-22 'Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip' (S30403 limits); ASTM A553/A553M-22 'Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched-and-Tempered 9 % Nickel Steel' (Type II, UNS K81340); ASTM A353/A353M-17 'Standard Specification for Pressure Vessel Plates, Alloy Steel, Double-Tempered 9 % Nickel Steel.'

Note: 'Q&T' = Quenched and Tempered; 'DPLT' = Double Normalised and Tempered (less common in modern LNG service). ASTM A553 Type II is the standard specification for LNG storage tanks. UNS K81340 is the unified numbering designation for 9Ni Q&T steel.

Table. 304L vs 9Ni Steel - Room-Temperature Mechanical Properties


Property	304L (UNS S30403)	9Ni (A553 Type II, Q&T)	Test Standard
Min. Tensile Strength (UTS)	485 MPa (70 ksi)	690 MPa (100 ksi)	ASTM A370
Min. Yield Strength (0.2 % offset)	170 MPa (25 ksi)	485 MPa (70 ksi)	ASTM A370
Min. Elongation in 50 mm	40 %	20 %	ASTM A370
Hardness (max)	202 HB	217 HB	ASTM E10
Young's Modulus	193 GPa	200 GPa	ASTM E111
Poisson's Ratio	0.29	0.30	ASTM E132
Charpy V-Notch (–40 °C, min avg)	~200 J (typical)	~80 J (typical)	ASTM E23

Source: ASTM A240/A240M-22 for 304L; ASTM A553/A553M-22 Type II for 9Ni steel; Special Metals Corporation SMC-045 'N08811 (Alloy 800HT) / S30403' data; Linde Steel '9Ni Steel Plate Data Sheet.'

The defining performance criterion for LNG materials is Charpy V-Notch (CVN) impact toughness at –196 °C (liquid nitrogen, test benchmark) and –162 °C (LNG operating temperature). Both 304L and 9Ni steel maintain austenitic (FCC) structure at these temperatures, but their toughness levels differ significantly.

Table. 304L vs 9Ni Steel - Cryogenic Impact Toughness and Tensile Properties


Property	Temperature	304L (UNS S30403)	9Ni (A553 Type II)
Charpy V-Notch (min avg)	–196 °C (LN₂)	≥ 125 J (41 ft·lbf)	≥ 27 J (20 ft·lbf)
Charpy V-Notch (min avg)	–170 °C	≥ 140 J (typical)	≥ 35 J (typical)
Charpy V-Notch (min avg)	–162 °C (LNG)	~150 J (typical)	~40 J (typical)
Tensile Strength (typical)	–162 °C	~650 MPa (typical)	~850 MPa (typical)
Yield Strength (0.2 % offset)	–162 °C	~220 MPa (typical)	~560 MPa (typical)
DBTT (Ductile-Brittle Transition)	-	–< –196 °C (no DBTT)	–< –196 °C (no DBTT)
CTOD (Critical Crack Tip Opening Displacement)	–162 °C	> 0.25 mm (typical)	> 0.20 mm (typical)

Source: ASTM E23-22 'Standard Test Methods for Notched Bar Impact Testing of Metallic Materials'; ASTM A553/A553M-22 Table 2 (Charpy V-Notch requirements at –196 °C); Brostrom et al., 'Cryogenic Properties of Austenitic Stainless Steels' (2nd ed., 2018); Linde Engineering '9Ni Steel for LNG Storage Tanks - Design Basis Report,' 2021.

Note: Although 304L has a higher absolute CVN value than 9Ni steel at cryogenic temperatures, the required minimum CVN for 9Ni steel is lower because the steel's higher yield strength provides a larger plastic zone ahead of the crack tip, effectively resisting crack propagation. Both materials are fully acceptable for LNG service per ASME BPVC Section VIII Division 1 and API 620.

Both materials contract when cooled from ambient to –162 °C. This thermal contraction creates significant residual stresses in multi-span piping and tank-to-piping connections. Design engineers must account for this strain in piping flexibility analyses (ASME B31.3 Chapter II).

Table. 304L vs 9Ni Steel - Thermal Contraction and Physical Properties


Property	304L	9Ni (A553 Type II)	Units
Coefficient of Thermal Expansion (0–100 °C)	17.3 × 10⁻⁶ /°C	13.9 × 10⁻⁶ /°C	mm/mm/°C
Coefficient of Thermal Expansion (–162 °C)	~11.5 × 10⁻⁶ /°C	~9.5 × 10⁻⁶ /°C	mm/mm/°C
Thermal Contraction (20 °C → –162 °C, per 10 m)	~18 mm	~15 mm	mm per 10 m
Thermal Conductivity (20 °C)	14.6 W/m·K	29 W/m·K	W/m·K
Thermal Conductivity (–162 °C)	~9 W/m·K	~18 W/m·K	W/m·K
Specific Heat Capacity (20 °C)	500 J/kg·K	460 J/kg·K	J/kg·K
Density	8,000 kg/m³	7,850 kg/m³	kg/m³

Source: ASME B31.3 'Process Piping' (2022 Edition); ASM Handbook Vol. 4 'Heat Treating' (1991); Special Metals SMC-045 (Incoloy/304L thermal data); US Steel '9% Nickel Steel Plate for Cryogenic Service - Data Sheet US-9Ni-2022.

In the dry, inert LNG environment, both 304L and 9Ni steel exhibit excellent general corrosion resistance. LNG boil-off gas is predominantly methane with trace CO₂ and nitrogen; it is non-corrosive in the absence of free moisture. The primary corrosion concerns arise during the following operational scenarios:

Moisture condensation on external tank surfaces (atmospheric corrosion; mitigated by insulation)

CO₂ / H₂O carryover in BOG piping (carbonic acid formation on 9Ni steel; 304L largely immune)

Seawater cooling tower environments in coastal LNG terminals (Cl⁻ pitting risk on 304L; controlled by coating)

Table. 304L vs 9Ni Steel - Stress Corrosion Cracking Susceptibility


SCC Mechanism	304L (UNS S30403)	9Ni (A553 Type II)	LNG Relevance
Chloride SCC (Cl-SCC)	Susceptible above ~60 °C with Cl⁻ > 50 ppm	Not susceptible (low Cr, low C)	Moderate - coastal terminals, cooling tower drift
Hydrogen Embrittlement (HE)	Not susceptible (FCC structure)	Low susceptibility in Q&T condition	Low - H₂ charging during acid cleaning
Sulfide Stress Cracking (SSC)	Not susceptible (not a ferritic steel)	Not susceptible (Q&T low-hardness HAZ)	Low - only in sour LNG with H₂S
Caustic SCC	Not susceptible	Not susceptible	Not applicable in LNG
Ammonia SCC	Not susceptible	Not susceptible	Not applicable in LNG

Source: NACE International SP0472 'Recommended Practices for Methods and Equipment for Preventing Corrosion of Austenitic Stainless Steels and Nickel-Based Alloys' (2021); NACE SP0103 'Control of Corrosion Under Thermal Insulation' (2021); ISO 15156-3:2015 Annex C (sour service applicability); US Steel '9Ni Steel Corrosion Guide US-9Ni-Corrosion-2022.'

Definitively, 304L's primary SCC vulnerability is chloride SCC (Cl-SCC) at temperatures above ~60 °C - a condition that can occur on the external (ambient) surface of insulated LNG tanks in hot climates, if moisture penetrates the insulation. 9Ni steel has no documented susceptibility to Cl-SCC in its Q&T condition. For insulated cryogenic tanks in humid or coastal environments, 9Ni steel carries a lower SCC risk.

Two cryogenic embrittlement mechanisms are relevant to LNG materials selection:

Temper embrittlement (9Ni only): 9Ni steel tempered in the 350–575 °C range can absorb trace impurity elements (P, Sn, Sb, As) at grain boundaries, reducing notch toughness. This is managed by specifying low residual element heats and avoiding PWHT temperatures in the embrittlement range.

Liquid metal embrittlement (LME): Molten aluminium or zinc (from hot-dip galvanised scaffolding or equipment) can cause LME on both materials; strict exclusion of Zn/Al contamination near weld zones is required per AWS D18.1

304L is immune to temper embrittlement (no tempering step) but is susceptible to sigma phase embrittlement if held in the 540–900 °C range for extended periods (typically not a concern for standard welding if preheat is controlled).

304L is readily weldable by all common arc welding processes (GTAW / TIG, GMAW / MIG, SMAW / stick, SAW). Its low carbon content (≤ 0.03 %) minimises sensitisation risk - the precipitation of chromium carbides at grain boundaries that causes intergranular corrosion (IGC) in the HAZ.

Weldability and Fabrication

However, the following precautions are required:

Use low-carbon filler metal (ER308L or ER316L) to maintain HAZ carbon below 0.03 %

Interpass temperature ≤ 150 °C to prevent carbide coarsening and maintain austenite stability

Argon or argon-based shielding gas for GTAW/GMAW (no nitrogen pickup, which can cause porosity)

Post-weld cleaning: remove oxide scale with pickling paste (HNO₃ + HF) to restore full corrosion resistance

No PWHT required for 304L (annealed base metal self-relieves stress)

9Ni Steel - Welding Characteristics

9Ni steel presents greater welding complexity due to its Q&T microstructure and higher hardenability. Key requirements include:

Preheat: 50–100 °C mandatory for plate thickness > 25 mm to prevent HAZ hydrogen cracking

PWHT: Mandatory for thickness > 35 mm (per ASME BPVC Section VIII Div. 1 UW-39); typical PWHT 560–590 °C for 1–2 hours; must avoid the 350–575 °C temper embrittlement window

Filler metal: ENiCrFe-3 (Inconel 182) or ENiCrMo-3 (Inconel 625) for sour-service or cryogenic service; ERNiCr-3 (Inconel 82) for cryogenic-only service. Do NOT use carbon steel or 309 stainless filler on 9Ni steel.

Post-weld heat treatment temperature must be ≥ 535 °C to avoid the 350–575 °C embrittlement range, or ≤ 300 °C (no practical risk zone)

Welds must meet CVN requirements at –196 °C: separate weld procedure qualification (WPQR) with Charpy testing at –196 °C is required by ASME BPVC Section VIII Div. 1 and API 620 Annex Q

Post-Weld Heat Treatment (PWHT) Requirements

Table. 304L vs 9Ni Steel - Post-Weld Heat Treatment Requirements


Condition	304L (UNS S30403)	9Ni (A553 Type II)	Code Reference
Thin wall (< 20 mm)	No PWHT required	No PWHT required (if preheated)	ASME BPVC VIII-1 UW-39
Medium wall (20–35 mm)	No PWHT required	PWHT recommended (560–590 °C)	ASME BPVC VIII-1
Thick wall (> 35 mm)	No PWHT required	PWHT mandatory (560–590 °C, 1–2 h)	ASME BPVC VIII-1 UW-39
LNG Tank (full-penetration welds)	No PWHT required	PWHT required for tank wall > 16 mm	API 620 Annex Q
PWHT temperature range to avoid	N/A (no PWHT)	350–575 °C (temper embrittlement)	ASME BPVC II-D
Max. interpass temperature	150 °C	150 °C	AWS D1.1 / AWS D1.6

Source: ASME Boiler and Pressure Vessel Code, Section VIII Division 1, UW-39 'Postweld Heat Treatment' (2023 Edition); API Standard 620 'Design and Construction of Large, Welded, Low-Pressure Storage Tanks' (14th Edition, 2020) Annex Q; AWS D1.1/D1.6 Structural Welding Code; US Steel '9% Nickel Steel - Fabrication Handbook,' Rev. 3, 2021.

Table. Comprehensive Comparison - 304L vs 9% Nickel Steel for LNG Cryogenic Service


Criterion	304L (UNS S30403)	9Ni (A553 Type II, Q&T)	Winner
Min. Yield Strength (MPa)	170 MPa (25 ksi)	485 MPa (70 ksi)	9Ni ✓
Min. Tensile Strength (MPa)	485 MPa (70 ksi)	690 MPa (100 ksi)	9Ni ✓
Cryogenic CVN toughness (–196 °C)	~125 J min avg; ~150 J typical	~27 J min avg; ~40 J typical	304L ✓ (higher absolute)
DBTT (ductile-brittle transition)	None (FCC retained)	None (FCC retained)	Equal
Corrosion resistance (LNG environment)	Excellent; no SCC in cryogenic service	Excellent; no SCC; low temper embrittlement risk	Equal
Cl-SCC risk (above 60 °C)	Moderate risk in coastal/humid climates	Very low risk	9Ni ✓
Weldability	Good; ER308L filler; no PWHT; interpass < 150 °C	Moderate; ENiCrFe-3 filler; PWHT mandatory > 35 mm; preheat 50–100 °C	304L ✓ (simpler fabrication)
PWHT requirement	Not required	Mandatory for > 35 mm; avoid 350–575 °C range	304L ✓
Temperature limit (max service)	~425 °C (continuous); ~870 °C (intermittent)	~450 °C (short-term); cryogenic is primary service	Equal
Thermal contraction (20 °C → –162 °C)	~18 mm per 10 m	~15 mm per 10 m	9Ni ✓ (less contraction)
Coefficient of thermal expansion	17.3 × 10⁻⁶ /°C (higher)	13.9 × 10⁻⁶ /°C (lower)	9Ni ✓ (better dimensional stability)
Density	8,000 kg/m³ (heavier)	7,850 kg/m³ (lighter)	9Ni ✓ (lower weight)
Max. allowable stress (ASME VIII-1, 250 °F)	~20,000 psi (137 MPa)	~30,000 psi (207 MPa)	9Ni ✓ (thinner wall possible)
Wall thickness for 690 kPa (100 psi) LNG pipe, DN 300	~14 mm WT	~6 mm WT (higher allowable stress)	9Ni ✓ (significant material saving)
Standard availability (plate / pipe)	Full range: DN 50–600; plate up to 50 mm	Plate up to 100 mm; pipe availability more limited	304L ✓ (wider availability)
Approx. material cost index (304L = 1.0)	1.0× baseline	~1.6–2.2× (Q1 2024)	304L ✓ (lower first cost)
Typical fabricated tank cost (per tonne)	~USD 4,000–6,000/t	~USD 7,500–12,000/t	304L ✓ (lower installed cost)
ISO 15156 sour-service applicability	Category 2 (restricted by Cl⁻, temp.)	Not listed (ferrous alloy, not nickel-based); separate testing required	304L ✓ (more defined sour-service path)
LNG standard acceptance	API 620 Annex D; NFPA 59A §6.4	API 620 Annex Q; NFPA 59A §6.4; primary LNG tank material	Equal (both accepted)
Recommended primary application	LNG process piping; BOG lines; secondary containment; small tanks	Large LNG storage tanks; LNG carrier cargo tanks; primary containment	Application-dependent

Source: ASME BPVC Section VIII Div. 1 (2023) Tables UHA-1, UNF-79; ASTM A240/A240M-22; ASTM A553/A553M-22; API 620 Annexes D and Q (14th Ed., 2020); NFPA 59A (2021 Ed.); US Steel '9% Nickel Steel Plate - Technical Data Sheet,' 2022; S&P Global Commodity Insights 'Metals Daily' LNG Materials Price Index Q1 2024; GIIGNL 'LNG Industry Report,' 2023.