304LN / 316LN Nitrogen-Alloyed Austenitic Stainless Steel Coil Tube for Cryogenic Service
1. Product Overview
The 304LN / 316LN Coil Tube is a nitrogen-stabilized, ultra-low-temperature austenitic stainless steel coil tube engineered by Wenzhou Jixing Metal Materials Co., Ltd. for demanding cryogenic and low-temperature industrial applications. The deliberate addition of nitrogen (N) as an interstitial alloying element significantly enhances the strength and toughness characteristics of the base 304L and 316L austenitic grades without compromising weldability or corrosion resistance. This material maintains outstanding ductility and impact toughness at temperatures as low as -196°C (77 K), making it an ideal selection for Liquefied Natural Gas (LNG), Liquid Nitrogen (LN2), and Liquid Oxygen (LO2) storage and transportation systems.
Unlike conventional austenitic stainless steels that may exhibit a ductile-to-brittle transition at cryogenic temperatures, the nitrogen-alloyed 304LN and 316LN grades retain a fully austenitic microstructure across the entire cryogenic temperature range, eliminating the risk of low-temperature embrittlement and ensuring long-term structural integrity under extreme thermal cycling conditions.
2. Material & Properties
2.1 Chemical Composition
| Element | 304LN (UNS S30453) | 316LN (UNS S31653) |
|---|---|---|
| C (Carbon) | ≤ 0.030% | ≤ 0.030% |
| Si (Silicon) | ≤ 1.00% | ≤ 1.00% |
| Mn (Manganese) | ≤ 2.00% | ≤ 2.00% |
| P (Phosphorus) | ≤ 0.045% | ≤ 0.045% |
| S (Sulfur) | ≤ 0.030% | ≤ 0.030% |
| Ni (Nickel) | 8.00 – 11.00% | 10.00 – 14.00% |
| Cr (Chromium) | 18.00 – 20.00% | 16.00 – 18.00% |
| N (Nitrogen) | 0.030 – 0.10% | 0.030 – 0.10% |
| Mo (Molybdenum) | — | 2.00 – 3.00% |
| Fe (Iron) | Balance | Balance |
2.2 Mechanical Properties (Solution-Annealed Condition)
| Property | 304LN | 316LN |
|---|---|---|
| Yield Strength (Rp0.2) | ≥ 290 MPa (42 ksi) | ≥ 310 MPa (45 ksi) |
| Tensile Strength (Rm) | 520 – 700 MPa (75 – 102 ksi) | 550 – 720 MPa (80 – 104 ksi) |
| Elongation (A50) | ≥ 40% | ≥ 40% |
| Impact Toughness at -196°C (Charpy V-Notch) | ≥ 100 J | ≥ 100 J |
| Density | ~7.93 g/cm³ | ~7.98 g/cm³ |
| Thermal Conductivity at 25°C | ~16.3 W/(m·K) | ~16.3 W/(m·K) |
2.3 Unique Selling Points
- Nitrogen Strengthening: The interstitial nitrogen atoms (N) in solid solution within the austenitic matrix provide a substantial increase in yield strength (up to 30–50% higher than conventional 304L/316L) without the need for cold work or additional alloy additions, resulting in thinner-wall designs and reduced material weight.
- Superior Cryogenic Performance: Maintains full austenitic microstructure and excellent impact toughness down to -196°C, with no ductile-to-brittle transition observed, ensuring reliability in the most extreme cryogenic environments.
- Enhanced Corrosion Resistance: The 316LN grade, with its combined Ni-Cr-Mo-N alloy system, offers superior resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride-containing environments compared to standard 316L.
- Excellent Weldability: The low carbon content (≤ 0.030%) combined with nitrogen stabilization ensures minimal sensitization risk during welding, maintaining corrosion resistance in the Heat-Affected Zone (HAZ).
- Formability & Coil Configuration: Supplied in coil form for maximum flexibility in downstream fabrication, allowing seamless integration into automated tube-forming, bending, and welding processes.
3. Specifications
| Parameter | Specification |
|---|---|
| Product Form | Coil Tube (Coiltube) |
| Material Grades | 304LN (UNS S30453) / 316LN (UNS S31653) |
| Outer Diameter (OD) | 6.00 mm – 50.80 mm (1/4" – 2") |
| Wall Thickness | 0.50 mm – 3.00 mm |
| Coil Inner Diameter (ID) | 200 mm – 600 mm |
| Coil Weight | Up to 2,000 kg per coil |
| Surface Finish | Pickled & Passivated (2B), Bright Annealed (BA), or Custom |
| Hardness | ≤ 220 HV (Solution-Annealed) |
| Operating Temperature Range | -196°C to +450°C |
| Standards | ASTM A270 / A778, EN 10217-7, ISO 15553, GB/T 14976 |
| Heat Treatment | Solution Annealed at 1050 – 1100°C, followed by rapid water quench |
| Packaging | Wooden crate or steel banding with moisture-proof wrapping |
4. Manufacturing Process
- Raw Material Selection: Virgin steelmaking-grade stainless steel billets or slabs are sourced with tightly controlled chemistry, particularly regarding carbon (≤ 0.030%) and nitrogen (0.030 – 0.10%) content. Only certified, traceable feedstock from approved suppliers is used.
- Hot Rolling: The selected billet is reheated to 1200 – 1300°C and hot-rolled to the desired semi-finished thickness. The rolling schedule is carefully optimized to achieve a homogeneous austenitic grain structure with controlled grain size.
- Precision Cold Drawing / Cold Reduction: The hot-rolled strip or tube blank undergoes cold reduction to achieve the target outer diameter and wall thickness with tight dimensional tolerances (typically ±0.1 mm). Multi-pass drawing ensures uniform cross-sectional geometry.
- Intermediate Annealing: Periodic solution annealing is performed during the cold working process to relieve accumulated cold work stresses and restore ductility, preventing cracking and maintaining the required mechanical properties.
- Final Solution Annealing: The finished coil tube is solution-annealed at 1050 – 1100°C followed by rapid water quenching. This step dissolves all carbides and other precipitates, ensuring a fully austenitic microstructure with maximum nitrogen in solid solution.
- Pickling & Passivation: The annealed coil tube is subjected to acid pickling (HNO3/H2SO4 mixture) to remove surface oxide scale, followed by passivation treatment to form a protective chromium oxide layer. Surface roughness is verified to meet Ra ≤ 0.8 μm.
- Coiling & Packaging: The finished product is coiled onto precision mandrels, wrapped with moisture-proof protective film, and packed in export-grade wooden crates or steel-banded bundles for safe transportation.
5. Applications
5.1 Primary Applications
- LNG (Liquefied Natural Gas) Storage Tanks: 316LN coil tubes are used for the construction of cryogenic LNG storage vessels, including inner containment layers, heat exchanger tubes, and vent piping systems operating at -162°C (LNG boiling point). The nitrogen-strengthened alloy provides the required strength at cryogenic temperatures, allowing for thinner wall designs that reduce overall vessel weight and cost.
- Liquid Nitrogen (LN2) Transfer Pipelines: 304LN coil tubes are extensively used in industrial LN2 distribution systems, including vacuum-insulated transfer lines, cryogenic pump manifolds, and process piping in steel mills, food processing plants, and research laboratories.
- Liquid Oxygen (LO2) Transport Lines: 316LN coil tubes serve in LO2 pipeline systems for medical oxygen plants, aerospace fuel systems, and industrial air separation units, where the combination of cryogenic toughness and oxygen-compatible surface finish is critical.
5.2 Secondary Applications
- Cryogenic Heat Exchangers: Double-pipe heat exchangers and spiral-wound heat exchangers for natural gas liquefaction plants, hydrogen liquefaction facilities, and helium cooling systems.
- Cryogenic Valve Bodies & Actuators: Tubular components for cryogenic control valves, ball valves, and gate valves operating in LNG processing facilities.
- Spacecraft & Aerospace Cryogenic Systems: Propellant feed lines, turbopump housings, and cryocooler components for liquid hydrogen (LH2) and liquid oxygen rocket engine systems.
- Medical & Pharmaceutical Cryogenic Equipment: Liquid nitrogen storage dewars, cryopreservation equipment, and medical gas delivery systems requiring ultra-low-temperature service.
- Superconducting Magnet Systems: Conduit tubes and structural components for MRI (Magnetic Resonance Imaging) systems and particle accelerator cryostat assemblies operating at 4.2 K (liquid helium temperature).
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system and adheres to the following international standards and certifications:
- Quality Management: ISO 9001:2015 Certified Quality Management System
- Material Standards: ASTM A270 (Standard Specification for Stainless Steel Tubular Products — Seamless, Austenitic), ASTM A778, EN 10217-7 (Stainless Steel Tubes for Special Purposes), ISO 15553 (Stainless Steel Tubes for Cryogenic Service), GB/T 14976 (Chinese National Standard for Seamless Stainless Steel Tubes)
- Product Certification: PED (Pressure Equipment Directive) 2014/68/EU certification for pressure vessel components; ASME Section VIII Div. 1 stamp for pressure vessel applications
- Testing & Inspection: 100% ultrasonic testing (UT) for internal defects, positive material identification (PMI) via optical emission spectroscopy (OES), Charpy V-Notch (CVN) impact testing at -196°C, intergranular corrosion (IGC) testing per ASTM A262 Practice No. 1E, and hydrostatic testing per ASTM A270
- Mill Certification: Full EN 10204 3.1 material certificates provided with every shipment, including chemical composition analysis, mechanical test results, and traceability documentation
- Environmental & Social Compliance: ISO 14001 Environmental Management System, ISO 45001 Occupational Health & Safety Management System
Technical Note: The nitrogen-alloyed 304LN and 316LN grades represent the current state-of-the-art in cryogenic austenitic stainless steel technology. The synergistic effect of low carbon (preventing sensitization), high nickel (stabilizing austenite), and interstitial nitrogen (providing solution strengthening) creates a material system that is uniquely suited for applications requiring the simultaneous satisfaction of extreme low-temperature toughness, high strength, and excellent corrosion resistance. Wenzhou Jixing Metal Materials Co., Ltd. guarantees that all 304LN/316