Views: 5 Author: Monica Publish Time: 2026-07-23 Origin: Site
Table of Contents
Inconel 625 is the preferred nickel-chromium-molybdenum alloy for critical components in LNG processing plants. It retains excellent ductility and impact toughness at LNG temperatures of -162°C, resists corrosion from amine-based acid gas removal (AGR) solvents used upstream of liquefaction, and maintains structural integrity under thermal cycling between ambient and cryogenic conditions.
Key applications include cryogenic heat exchanger tubing, acid gas removal column internals, cryogenic piping, and weld overlay cladding on carbon steel pressure vessels.
Key Properties for LNG Service
Property | Value | Why It Matters for LNG |
UNS Designation | N06625 | Globally recognized; listed in ASME Section II Part D |
Service Temperature Range | -196°C to 980°C | Spans from LNG cryogenic to gas turbine exhaust |
Charpy V-Notch at -196°C | > 100 J (74 ft-lb) | No ductile-to-brittle transition at LNG temperature |
Yield Strength (annealed) | 414 MPa min (60 ksi) | 2× the strength of 304L at cryogenic temperature |
Tensile Strength (annealed) | 827 MPa min (120 ksi) | Enables thinner, lighter pressure vessel walls |
PREN Value | ≥ 45 | Immune to chloride pitting in seawater-cooled systems |
Thermal Expansion (20–100°C) | 12.8 × 10⁻⁶ / °C | Lower than austenitic stainless steel → reduced thermal stress |
ASME Code Allowable Stress | 138 MPa at RT | Per ASME Section II Part D for pressure vessel design |
Inconel 625 is suitable for LNG processing because it combines three properties: cryogenic toughness at -162°C without ductile-to-brittle transition, resistance to amine-based acid gas removal solvent corrosion, and high yield strength that enables thinner-walled, lighter pressure-bearing components.
LNG processing involves three demanding stages:
1.Gas treatment (acid gas removal): Raw natural gas contains CO₂ and H₂S that must be removed before liquefaction. Amine-based solvents (MDEA, DEA) used for acid gas removal are corrosive at elevated temperatures (60–110°C) and can cause stress corrosion cracking in standard stainless steels.
2.Liquefaction (cryogenic cooling): Treated gas is cooled to -162°C to condense it into liquid form, reducing volume by ~600×. At this temperature, many steels become brittle and fracture catastrophically under load.
3.Storage and transfer: LNG is stored in cryogenic tanks and loaded onto carriers. Components experience thermal cycling from ambient to -162°C during cool-down and warm-up, generating significant thermal stress.
Inconel 625 addresses all three challenges:
Cryogenic toughness: Its face-centered cubic (austenitic) crystal structure has no ductile-to-brittle transition temperature, so it remains tough at -162°C and below.
AGR solvent resistance: Its 58%+ nickel content provides immunity to chloride stress corrosion cracking, and its 20–23% chromium + 8–10% molybdenum resist the localized corrosion that amine solutions can cause in carbon steel and standard stainless steel.
Thermal cycling endurance: Its coefficient of thermal expansion is lower than that of 304L stainless steel, meaning less dimensional change and lower thermal stress during LNG plant cool-down and warm-up cycles.
Inconel 625 retains excellent mechanical properties at LNG temperatures. At -196°C, it exhibits Charpy V-notch impact values exceeding 100 J, yield strength that actually increases compared to room temperature, and elongation values above 40%. There is no ductile-to-brittle transition — the alloy remains fully ductile and tough throughout the entire LNG temperature range.
The absence of a ductile-to-brittle transition temperature is the single most important metallurgical property for LNG service. Ferritic and martensitic steels undergo a DBTT below which they fracture in a brittle mode with negligible energy absorption. Inconel 625's face-centered cubic (FCC) crystal structure does not undergo this transition, so its toughness at -162°C is comparable to its toughness at room temperature.
Property | Room Temperature (20°C) | LNG Temperature (-162°C) | Liquid Nitrogen (-196°C) |
Yield Strength (MPa) | 414 min (typically 450–655) | ~550–700 (increased) | ~570–720 (increased) |
Tensile Strength (MPa) | 827 min (typically 860–1034) | ~1000–1100 | ~1050–1150 |
Elongation (%) | 30 min (typically 35–60) | ~40–50 | ~40–50 |
Charpy V-Notch (J) | > 100 | > 100 | > 100 |
DBTT | None (FCC structure) | None | None |
Failure Mode | Ductile | Ductile | Ductile |
Key observation: yield and tensile strengths actually increase at cryogenic temperatures — a characteristic of nickel alloys with FCC structures.
By comparison, 304L stainless steel — the most common cryogenic material — also retains toughness at LNG temperatures, but its yield strength at -162°C (~300–400 MPa) is still only 60–70% of Inconel 625's. This means Inconel 625 components can be designed with thinner walls for the same pressure rating, reducing weight and material cost in high-pressure cryogenic service.
Inconel 625 is used in four critical areas of the LNG liquefaction process:
(1) acid gas removal (AGR) column internals and heat exchangers exposed to amine solvents.
(2) main cryogenic heat exchanger (MCHE) tubing in the liquefaction cold box.
(3) cryogenic piping and valves handling LNG at -162°C.
(4) weld overlay cladding on carbon steel pressure vessels and pipe spools for combined cryogenic and corrosive service.
An LNG plant processes natural gas through several stages before it reaches the liquefaction cold box. Inconel 625 is specified selectively at points where its unique combination of cryogenic toughness and corrosion resistance is required.
LNG Process Flow and Inconel 625 Application Points
LNG Process Stage | Operating Conditions | Inconel 625 Application | Why 625? |
Acid gas removal (AGR) | 60–110°C, amine solvents (MDEA/DEA), CO₂ + H₂S | Column internals, heat exchanger tubes, reboiler tubing | Resists amine SCC + localized corrosion |
Dehydration | Ambient to 200°C, glycol contact | Glycol heat exchanger tubes (selective) | Oxidation resistance + chloride immunity |
Mercury removal | Ambient, mercury-laden gas | Guard bed vessel internals | Resists mercury embrittlement |
Liquefaction (MCHE) | -162°C, mixed refrigerant, high pressure | Main cryogenic heat exchanger tubes | Cryogenic toughness + high strength |
LNG storage | -162°C, static liquid | Tank impingement plates, penetrations | Cryogenic toughness + weldability |
LNG loading | -162°C, dynamic loading | Loading arm swivel joints, cryogenic valves | Cryogenic toughness + wear resistance |
Cryogenic flare | -162°C to 1000°C cycling | Flare tip components | Thermal cycling endurance + oxidation resistance |
In most LNG plants, Inconel 625 is not used for the entire facility. Instead, it is specified selectively where its properties justify the cost premium. The predominant use pattern is:
AGR system: Heat exchanger tubes and column internals exposed to hot amine solvents.
MCHE cold box: Brazed aluminum heat exchanger (BAHX) cores are standard, but the connecting pipe spools and tube-to-tubesheet joints often use Inconel 625 for its cryogenic strength and dissimilar-metal weld compatibility with aluminum.
Cryogenic piping: High-pressure LNG transfer piping where 304L wall thickness would be excessive or where additional corrosion margin is required.
Weld overlay cladding: Inconel 625 weld overlay (ERNiCrMo-3) on carbon steel vessels to provide a corrosion-resistant inner surface without the cost of solid nickel alloy construction.
Inconel 625 resists amine-based AGR solvent corrosion through three mechanisms:
(1) its 58%+ nickel content provides immunity to chloride stress corrosion cracking that can affect austenitic stainless steels in amine service.
(2) its 20–23% chromium maintains a stable passive oxide film in the oxidizing zones of the amine circuit (rich amine loading, lean amine regeneration).
(3) its 8–10% molybdenum prevents pitting and crevice corrosion in the reducing zones where CO₂ and H₂S are concentrated.
Acid gas removal is the most corrosive stage in an LNG plant. Raw natural gas typically contains 2–10% CO₂ and 1–50 ppm H₂S. The most common AGR technology uses aqueous amine solvents — typically MDEA (methyldiethanolamine) at 40–50% concentration — circulated at 60–110°C.
The amine absorbs CO₂ and H₂S in the absorber column (forming 'rich amine'), then releases them in the regenerator/stripper at 110–125°C (forming 'lean amine' that is recirculated).
This cycle creates multiple corrosion challenges:
Rich amine loading: CO₂-saturated amine is acidic and can cause pitting in carbon steel and stress corrosion cracking in austenitic stainless steels at temperatures above 80°C.
Amine regeneration: The stripper operates at 110–125°C with concentrated CO₂ and H₂S. This is the most aggressive zone. Carbon steel may suffer from uniform corrosion rates of 0.1–0.5 mm/year.
Amine degradation products: Heat-stable amine salts (HSAS) formed from reaction with oxygen or acidic contaminants are corrosive and can accumulate in the system over time.
Erosion-corrosion: High-velocity amine flow at pipe bends and column internals can mechanically damage passive films, accelerating corrosion.
Material | Amine SCC Resistance | Pitting in Rich Amine | Uniform Corrosion Rate (mm/yr) | Suitability for AGR |
Carbon Steel (CS) | Good (if stress-relieved) | Poor above 80°C | 0.1–0.5 | Marginal (needs cladding) |
304L Stainless Steel | Poor above 60°C | Moderate | 0.02–0.05 | Marginal |
316L Stainless Steel | Good | Moderate above 80°C | 0.01–0.03 | Adequate for lean amine |
Inconel 625 | Excellent (Ni > 45%) | Excellent (PREN ≥ 45) | < 0.005 | Excellent (all zones) |
Hastelloy C276 | Excellent | Excellent | < 0.005 | Excellent (but costly) |
304L is the default cryogenic material for non-corrosive LNG components (storage tanks, low-pressure piping). Inconel 625 is specified when additional corrosion resistance (AGR solvent exposure), higher strength (high-pressure piping, thin-wall designs), or resistance to thermal cycling fatigue is required.
For most LNG plants, both materials are used — 304L for the bulk cryogenic system and Inconel 625 for critical components and AGR service.
Inconel 625 vs 304L: LNG Service Comparison
Dimension | Inconel 625 | 304L Stainless Steel | Winner for LNG |
Yield Strength at -162°C (MPa) | ~550–700 | ~300–400 | 625 (1.7× stronger) |
Charpy at -196°C (J) | > 100 | > 100 | Tie (both excellent) |
DBTT | None | None | Tie (both FCC) |
AGR amine SCC resistance | Excellent | Poor above 60°C | 625 |
PREN | ≥ 45 | ~24 | 625 |
Thermal expansion (×10⁻⁶/°C) | 12.8 | 16.0 | 625 (lower stress) |
Weldability | Good (GTAW/GMAW/SMAW) | Excellent | 304L (easier) |
Relative material cost | 1.0 (baseline) | ~0.25 | 304L (4× cheaper) |
Typical LNG use | AGR tubes, MCHE, HP piping, cladding | LNG storage tanks, LP piping | — |
ASME code allowable stress (RT) | 138 MPa | ~115 MPa | 625 |
The practical decision rule for LNG plant material selection:
Use 304L for: LNG storage tank inner vessel, low-pressure cryogenic piping (< 10 bar), structural supports in cold boxes.
Use Inconel 625 for: AGR system heat exchanger tubes and column internals, high-pressure cryogenic piping (> 50 bar), MCHE tube-to-tubesheet joints, cryogenic valve internals, and weld overlay on carbon steel AGR vessels.
Use carbon steel + 625 cladding for: AGR absorber and regenerator columns, rich/lean amine flash drums, and reboiler shells. This approach provides 90% of the performance of solid Inconel 625 at 30–40% of the cost.
The five most common Inconel 625 components in LNG plants are:
(1) AGR system heat exchanger tubes (ASTM B444).
(2) main cryogenic heat exchanger connecting piping and tube-to-tubesheet joints.
(3) weld overlay cladding on AGR column and vessel internals.
(4) cryogenic valve stems and seats.
(5) subsea LNG loading arm swivel joint components.
Each is specified for a combination of cryogenic toughness, corrosion resistance, or both.
Inconel 625 LNG Component Summary
Component | Product Form / Standard | Service Conditions | Key Property |
AGR heat exchanger tubes | Seamless tube, ASTM B444/B704 | 60–125°C, MDEA amine, CO₂ + H₂S | Amine SCC resistance |
AGR column internals (trays, distributors) | Sheet/plate, ASTM B443 | 60–125°C, rich/lean amine | Uniform + pitting corrosion resistance |
AGR reboiler tubes | Seamless tube, ASTM B444 | 110–125°C, steam + amine | Thermal stability + corrosion resistance |
MCHE connecting pipe spools | Seamless pipe, ASTM B444 | -162°C, mixed refrigerant, HP | Cryogenic toughness + strength |
MCHE tube-to-tubesheet joints | Welding wire, ERNiCrMo-3 | -162°C, dissimilar metal joint | Dissimilar weld compatibility |
Cryogenic valve stems/seats | Bar, ASTM B446 | -162°C, cyclic loading | Wear resistance + cryogenic toughness |
LNG loading arm swivel joints | Forgings, ASTM B564 | -162°C, dynamic + wear | Cryogenic toughness + galling resistance |
Cryogenic flare tip components | Sheet/plate, ASTM B443 | -162°C to 1000°C cycling | Thermal cycling + oxidation resistance |
AGR vessel cladding (overlay) | Weld overlay, ERNiCrMo-3 | 60–125°C, amine + acid gas | Corrosion barrier on carbon steel |
Inconel 625 is welded using GTAW (TIG) with ERNiCrMo-3 filler metal for cryogenic service. Key parameters: interpass temperature ≤ 150°C, heat input 0.5–1.5 kJ/mm, 100% argon shielding and backing gas. Post-weld heat treatment is generally not required.
Inconel 625 is one of the most weldable nickel alloys, but LNG service imposes additional requirements:
•Welding process: GTAW (TIG) is preferred for cryogenic service because it produces the cleanest weld metal with the lowest inclusion content, maximizing Charpy toughness at -162°C. GMAW (MIG) with pulsed-spray transfer is acceptable for thicker sections. SMAW (stick) is used for field repairs but may require additional NDE due to potential for slag inclusions.
•Filler metal: ERNiCrMo-3 (AWS A5.14) wire or ENiCrMo-3 (AWS A5.11) electrodes. The filler metal chemistry is over-alloyed compared to the base metal to compensate for dilution and ensure the weld deposit retains corrosion resistance.
•Interpass temperature: Maximum 150°C (300°F). Excessive interpass temperature promotes grain growth and carbide precipitation, which can reduce toughness.
•Heat input: 0.5–1.5 kJ/mm. Low heat input minimizes grain growth in the heat-affected zone (HAZ) and preserves the solid-solution microstructure.
•Shielding gas: 100% argon for GTAW. Argon-helium mixtures can improve penetration on thicker sections. Nitrogen additions should be avoided as they can cause porosity.
•Backing gas: 100% argon for root passes, maintained until the root has cooled below 250°C to prevent oxidation ("sugaring") on the inside surface. Oxidized root surfaces can act as crack initiation sites under cyclic cryogenic loading.
•PWHT: Generally not required. For LNG service, as-welded joints meet ASME B31.3 requirements for cryogenic applications. Stress relieving at 870–980°C may be applied for dimensional stability before final machining.
For dissimilar-metal welds between Inconel 625 and carbon steel, ERNiCrMo-3 filler metal provides an intermediate coefficient of thermal expansion, reducing thermal fatigue stress during LNG plant cool-down and warm-up cycles.
Answer: Inconel 625 is used in LNG plants for two reasons that stainless steel cannot satisfy: (1) the AGR (acid gas removal) system uses amine solvents at 60–125°C that cause stress corrosion cracking in 304L/316L stainless steel, and (2) high-pressure cryogenic piping at -162°C benefits from Inconel 625's 550–700 MPa yield strength, which is 1.7× higher than 304L's 300–400 MPa at LNG temperature. For non-corrosive, low-pressure LNG service (storage tanks, low-pressure piping), 304L remains the standard and more cost-effective choice.
Answer: LNG is stored at -162°C (-260°F) at atmospheric pressure. Inconel 625 has no ductile-to-brittle transition temperature (DBTT) due to its face-centered cubic crystal structure, so it retains full toughness at this temperature. Charpy V-notch impact values at -196°C (even colder than LNG) exceed 100 J (74 ft-lb). Inconel 625 is fully qualified for LNG temperature service per ASME Section VIII Division 1 and ASME B31.3.
Answer: Weld overlay cladding deposits a 3–5 mm layer of Inconel 625 (using ERNiCrMo-3 filler wire via FCAW or GTAW) onto the internal surface of a carbon steel pressure vessel. This provides the corrosion resistance of solid Inconel 625 at 30–40% of the cost. In LNG plants, it is the standard approach for AGR absorber columns, regenerator shells, and rich/lean amine flash drums where the process stream is corrosive but the pressure shell can be carbon steel.
Answer: Both Inconel 625 and 304L have face-centered cubic structures with no DBTT, so both retain toughness at -162°C. The difference is strength: Inconel 625's yield strength at -162°C (~550–700 MPa) is approximately 1.7× that of 304L (~300–400 MPa). Inconel 625 also has far superior corrosion resistance (PREN ≥ 45 vs ~24 for 304L). The trade-off is cost: Inconel 625 is 3–5× more expensive than 304L. Use 304L for clean cryogenic service; use 625 where corrosion or high strength is required.
Answer: No. Inconel 625 is a solid-solution strengthened alloy, meaning it does not require precipitation hardening or PWHT to achieve its mechanical properties. Welded joints in the as-welded condition meet ASME B31.3 requirements for cryogenic service. Stress relieving at 870–980°C may be applied for dimensional stability before final machining, but it is not required for mechanical integrity or corrosion resistance at LNG temperatures.
Answer: The standard filler metal is ERNiCrMo-3 (for GTAW/GMAW wire, per AWS A5.14) or ENiCrMo-3 (for SMAW electrodes, per AWS A5.11). This filler metal is over-alloyed in chromium, molybdenum, and niobium compared to the base metal, compensating for dilution from the base metal and ensuring the weld deposit retains the required corrosion resistance and cryogenic toughness. The same filler metal is used for both joining Inconel 625 to itself and for weld overlay cladding on carbon steel.
Answer: Inconel 625 is not typically used for the main LNG subsea pipeline (which operates at -162°C and is usually 9% nickel steel or Invar for thermal contraction management). However, Inconel 625 is used for subsea LNG loading system components, including loading arm swivel joints, cryogenic valve internals, and subsea connector components, where its combination of cryogenic toughness, galling resistance, and seawater corrosion resistance (PREN ≥ 45) is unmatched.
Answer: Yes. Inconel 625 is listed in NACE MR0175 / ISO 15156 as an acceptable material for all sour service conditions (environments containing H₂S) at any temperature and any H₂S partial pressure. This makes it suitable for LNG plant inlet gas treatment where the feed gas may contain H₂S before it is removed by the AGR system.
