304LN/316LN Austenitic Stainless Steel Coil Tube for Cryogenic Service
Product Overview
The 304LN/316LN austenitic stainless steel coil tube is a nitrogen-stabilized, low-temperature-grade seamless and welded coil tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Designed specifically for cryogenic and sub-zero service environments, this product line incorporates deliberate nitrogen (N) alloying into the classic austenitic 304 and 316 stainless steel matrices. The nitrogen addition serves a dual purpose: it provides substantial solid-solution strengthening without compromising ductility, and it maintains the face-centered cubic (FCC) austenitic crystal structure at extremely low temperatures down to -196°C (77 K, the boiling point of liquid nitrogen). This makes 304LN/316LN coil tubes the material of choice for liquefied natural gas (LNG) infrastructure, liquid oxygen (LO2) and liquid nitrogen (LN2) transport systems, and other cryogenic applications where impact toughness retention is critical.
Unlike conventional 304 and 316 austenitic stainless steels, which rely primarily on nickel for austenite stabilization, the LN variants leverage nitrogen as a potent austenite stabilizer. This allows for reduced nickel content while maintaining full austenitic microstructure, resulting in improved mechanical properties and cost efficiency without sacrificing cryogenic performance.
Material Composition and Properties
Chemical Composition
| Element | 304LN (% max) | 316LN (% max) | Notes |
|---|---|---|---|
| Carbon (C) | 0.030 | 0.030 | Low carbon to prevent intergranular corrosion |
| Nitrogen (N) | 0.030–0.100 | 0.030–0.100 | Key differentiator; provides strengthening and austenite stabilization |
| Chromium (Cr) | 18.0–20.0 | 16.5–18.5 | Primary corrosion resistance element |
| Nickel (Ni) | 8.0–10.0 | 10.0–14.0 | Austenite stabilizer; slightly lower than standard grades due to N compensation |
| Molybdenum (Mo) | — | 2.0–3.0 | Enhanced pitting and crevice corrosion resistance in 316LN |
| Manganese (Mn) | 2.0 | 2.0 | Austenite stabilizer |
| Silicon (Si) | 1.0 | 1.0 | Oxide former |
| Sulfur (S) | 0.030 | 0.030 | Limited for ductility |
| Phosphorus (P) | 0.045 | 0.045 | Impurity limit |
Mechanical Properties
| Property | 304LN (Annealed) | 316LN (Annealed) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 550 MPa | ≥ 550 MPa | ASTM A270 / EN 10216-5 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A270 / EN 10216-5 |
| Elongation (A50) | ≥ 35% | ≥ 35% | ASTM A270 / EN 10216-5 |
| Impact Toughness @ -196°C (Charpy V-Notch) | ≥ 47 J (CVN) | ≥ 47 J (CVN) | ASTM E23 / ISO 148 |
| Density | ~ 7.93 g/cm³ | ~ 8.03 g/cm³ | — |
| Thermal Conductivity @ 25°C | ~ 16.3 W/(m·K) | ~ 14.6 W/(m·K) | — |
| Modulus of Elasticity | ~ 193 GPa | ~ 193 GPa | — |
Corrosion Resistance
- General Corrosion: Superior resistance to atmospheric, aqueous, and mild chemical environments. The low-carbon designation ("L") ensures resistance to intergranular corrosion even after welding or heat exposure in the sensitization range (450–850°C).
- Pitting and Crevice Corrosion: 316LN grade offers enhanced resistance due to the addition of 2.0–3.0% molybdenum, making it suitable for chloride-containing environments such as seawater and industrial chemical processing.
- Acid Resistance: Excellent resistance to dilute mineral acids, organic acids, and alkaline solutions. 316LN demonstrates superior performance in sulfuric acid and phosphoric acid environments compared to 304LN.
- Cryogenic Stability: The nitrogen-stabilized austenitic structure does not undergo martensitic transformation at cryogenic temperatures, ensuring consistent mechanical behavior from ambient to -196°C.
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Form | Seamless & Welded Coil Tube (Coiltube) | ASTM A270 / EN 10216-5 / GB/T 12771 |
| Material Grades | 304LN (UNS S30453) / 316LN (UNS S31653) | ASTM A270 / EN 10088-1 | Outer Diameter (OD) | 6.0 mm – 168.3 mm (¼" – 6½") | ASTM A511 / EN 10217 |
| Wall Thickness | 0.5 mm – 6.0 mm | ASTM A511 / EN 10217 |
| Length (Coil) | Continuous coil, typically 20–60 m per coil | Manufacturer standard |
| End Condition | Plain end, beveled end, or threaded end (upon request) | Customer specification |
| Surface Finish | Bright annealed (BA), pickled & passivated, or polished | ASTM A270 / EN 10217-7 |
| Heat Treatment | Solution annealed at 1010–1150°C, followed by controlled cooling | ASTM A270 |
| Temperature Range | -196°C to +450°C (continuous service) | — |
| Maximum Design Pressure | Up to 16 MPa (subject to sizing calculations per ASME B31.3) | ASME B31.3 |
| Coating | Uncoated (passivated surface film) | — |
| Packaging | Wooden crate or steel banding with desiccant; coil wrapped in protective film | ISO 4126 |
Manufacturing Process
The production of 304LN/316LN stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage process ensuring dimensional accuracy, microstructural integrity, and compliance with international standards.
- Raw Material Selection and Ingot Casting: High-purity stainless steel billets or continuous-cast slabs are produced from carefully controlled scrap and alloy additions. Nitrogen is introduced during the Vacuum Arc Remelting (VAR) or Vacuum Induction Melting (VIM) stage to achieve precise N content within the specified range of 0.030–0.100%. Chemical composition is verified via Optical Emission Spectrometry (OES) and Luminance Spectrometry.
- Hot Rolling and Piercing: Billets are reheated to 1150–1200°C and hot-rolled into hollow sections. For seamless tubes, the Mannesmann piercing or plug piercing method is employed to create the initial hollow shell. The hot-rolled coil is then coiled and cooled under controlled conditions.
- Pickle and Scale Removal: The hot-rolled coil is subjected to acid pickling (mixed HNO3/H2SO4 solution) to remove the oxide scale and restore the passive chromium oxide surface layer.
- Cold Drawing / Cold Rolling: The pickled coil is cold-worked to achieve final dimensions. For smaller ODs, multi-pass cold drawing through tungsten carbide (WC) or diamond dies is employed. For larger ODs, cold rolling on a 3-roll or 4-roll mill is used. Cold working provides significant strain hardening, improving yield strength by 30–50% above the annealed condition.
- Solution Annealing: After cold working, the coil tube is solution annealed at 1010–1150°C in a protective atmosphere (high-purity nitrogen or argon) to restore full ductility, eliminate residual stresses, and re-establish the single-phase austenitic microstructure. Controlled cooling rate (typically < 50°C/min) prevents formation of detrimental δ-ferrite or carbide precipitation.
- Precision Sizing and Straightening: Final dimensional accuracy is achieved through precision sizing rolls and straightening operations. Wall thickness is verified using ultrasonic thickness gauging (UT) or eddy current testing (ECT).
- Passivation and Final Surface Treatment: The annealed tube is passivated using a citric acid or nitric acid solution to remove any surface contamination and enhance the passive chromium oxide film (Cr2O3). This ensures optimal corrosion resistance from delivery.
- Non-Destructive Testing (NDT): Every coil lot undergoes 100% volumetric inspection. Methods include:
- Hydrostatic testing (for welded tubes) at 1.5× design pressure for minimum 30 seconds per ASTM A530
- Electromagnetic testing (ECT) for wall thickness and surface defect detection
- Visual inspection (VT) of both inner and outer surfaces
- Positive Material Identification (PMI) via X-Ray Fluorescence (XRF) spectroscopy to verify grade
- Coiling, Packaging, and Dispatch: The finished coil tube is wound onto a coil reel, wrapped in polyethylene protective film, and secured with steel strapping. Each coil is labeled with heat number, grade, dimensions, and mill test certificate (MTC) reference. Packaging complies with ISO 4126 for international shipment.
Applications
The cryogenic toughness and corrosion resistance of 304LN/316LN coil tubes make them indispensable in the following industries and applications:
Liquefied Natural Gas (LNG) Infrastructure
- LNG Storage Tanks: Primary material for internal tank linings, vent lines, and ancillary piping operating at -162°C (LNG boiling point). The nitrogen-stabilized austenitic structure ensures zero risk of ductile-to-brittle transition.
- LNG Loading Arms and Transfer Lines: Flexible coil tube configurations are used in loading arm umbilicals and subsea transfer lines where thermal cycling and low-temperature impact loading are present.
- LNG Vaporizers and Regasification Plants: Piping systems handling LNG at cryogenic temperatures up to ambient regasification temperatures.
Liquid Cryogen Systems
- Liquid Nitrogen (LN2) Transport and Distribution: Cryogenic piping at -196°C for industrial gas supply, food processing, and scientific research applications.
- Liquid Oxygen (LO2) and Liquid Air Systems: Oxygen-compatible cryogenic piping for steelmaking, chemical processing, and aerospace ground support equipment. 316LN is preferred where oxygen compatibility and enhanced corrosion resistance are required.
- Liquid Hydrogen (LH2) Systems: Emerging applications in hydrogen energy infrastructure, where -253°C service requires materials with exceptional low-temperature ductility.
Additional Applications
- Cryogenic Heat Exchangers: Tubular bundles in air separation units (ASUs) and hydrogen liquefaction plants.
- Superconducting Magnet Cables: Cryostat piping and support structures for MRI systems and particle accelerators operating at 4.2 K (liquid helium temperature).
- Oil and Gas Subsea Systems: Cryogenic process piping in subsea separation and gas treatment facilities.
- Food and Beverage Industry: Cryogenic freezing lines and rapid cooling systems.
- Spacecraft and Aerospace: Propellant feed lines and cryogenic fuel system tubing for liquid oxygen and liquid hydrogen rocket engines.
Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains comprehensive quality management systems and product certifications ensuring full traceability and compliance with international standards: