316 Stainless Steel Coil Tube — Austenitic Corrosion-Resistant Grade
Product Code: SS-CT-316 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The 316 Stainless Steel Coil Tube is a premium austenitic corrosion-resistant alloy coil tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product belongs to the austenitic corrosion-resistant category (奥氏体-耐蚀型) within the stainless steel material grade classification. The defining metallurgical feature of the 316 grade is the deliberate addition of 2–3% Molybdenum (Mo) to the base austenitic matrix of Chromium-Nickel (Cr-Ni), which confers significantly enhanced resistance to chloride-induced pitting corrosion (Pitting Corrosion) and crevice corrosion (Crevice Corrosion) in aggressive environments.
Supplied in continuous coil form, this product offers superior material utilization, reduced cutting waste, and streamlined downstream processing efficiency. The coil configuration is particularly advantageous for high-volume production runs in heat exchanger manufacturing, chemical process piping, and marine structural components.
2. Material Composition & Metallurgical Properties
2.1 Chemical Composition (ASTM A240 / GB/T 24511)
| Element | Minimum (%) | Maximum (%) | Role in Alloy System |
|---|
| Chromium (Cr) | 16.0 | 18.0 | Primary corrosion-resistant element; forms passive Cr₂O₃ film |
| Nickel (Ni) | 10.0 | 14.0 | Stabilizes austenitic microstructure; enhances ductility |
| Molybdenum (Mo) | 2.0 | 3.0 | Key differentiator; imparts chloride pitting and organic acid resistance |
| Carbon (C) | — | 0.08 | Limited to prevent sensitization and intergranular corrosion |
| Manganese (Mn) | — | 2.0 | Deoxidizer; secondary austenite stabilizer |
| Silicon (Si) | — | 1.0 | Deoxidizer; enhances high-temperature strength |
| Phosphorus (P) | — | 0.045 | Impurity; controlled to prevent embrittlement |
| Sulfur (S) | — | 0.030 | Impurity; limited to maintain weldability and surface finish |
2.2 Mechanical Properties (Annealed Condition)
| Property | Specification | Test Standard |
|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A376 / GB/T 24511 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) | ASTM A376 / GB/T 24511 |
| Elongation (at break) | ≥ 40% | ASTM A376 |
| Hardness (HB) | ≤ 217 | ASTM A262 |
| Thermal Conductivity (20°C) | ~16.3 W/(m·K) | ASME BPV Code |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /K | ASME BPV Code |
| Density | 8.00 g/cm³ | — |
2.3 Corrosion Resistance Characteristics
- Chloride Pitting Resistance: The 2–3% Molybdenum content raises the Pitting Resistance Equivalent Number (PREN) to approximately 24–26, substantially outperforming 304 grade (PREN ≈ 19). The PREN is calculated as: PREN = %Cr + 3.3 × %Mo + 16 × %N.
- Organic Acid Resistance: Demonstrates excellent resistance to dilute acetic acid, citric acid, formic acid, and other organic acids commonly encountered in food processing and chemical synthesis.
- Crevice Corrosion: Molybdenum addition significantly mitigates crevice corrosion risk in stagnant or poorly ventilated chloride environments.
- Intergranular Corrosion: With controlled carbon content (≤ 0.08%), the alloy maintains resistance to sensitization in the 450–850°C temperature range. For critical applications, the low-carbon variant 316L (C ≤ 0.03%) is recommended.
- Oxidation Resistance: Maintains dimensional stability and surface integrity up to approximately 870°C in cyclic oxidation conditions.
3. Product Specifications
| Dimensional & Geometric Specifications |
|---|
| Outer Diameter (OD) | 6.0 mm – 304.8 mm (¼" – 12") |
| Wall Thickness | 0.5 mm – 12.0 mm (0.020" – 0.472") |
| Coil Inner Diameter (ID) | 300 mm – 1000 mm (standard configurations) |
| Coil Length | Continuous coil; typical cut lengths 3 m – 12 m or as specified |
| OD Tolerance | ±0.05 mm (precision grade) / ±0.5% (standard grade) |
| Wall Thickness Tolerance | ±10% (standard) / ±5% (precision) |
| Roundness | ≤ 5% of wall thickness |
| Coil Straightness | ≤ 1.5 mm/m |
| Surface Finish Options |
|---|
| 2B (Cold Rolled & Annealed) | Smooth, semi-matte finish; standard industrial surface |
| No. 4 Brushed | Uniform unidirectional grain; aesthetic applications |
| No. 8 Mirror | Highly reflective surface; food-grade and architectural use |
| Baelted (Pickled & Passivated) | Acid-cleaned surface; optimal corrosion performance |
| 2D (Cold Rolled) | Flat, matte surface; intermediate finish |
| Available Standards & Norms |
|---|
| ASTM A213 / A269 | American Society for Testing and Materials — Seamless tubes |
| ASTM A376 | Welded austenitic stainless steel tubes |
| EN 10216-5 / EN 10217-7 | European standard — Seamless and welded tubes |
| GB/T 12771 | Chinese standard — Welded stainless steel tubes for fluid transport |
| GB/T 24511 | Chinese standard — Seamless stainless steel tubes for general engineering |
| ASME BPV Code | American Society of Mechanical Engineers — Boiler & Pressure Vessel |
4. Manufacturing Process
4.1 Process Flow Overview
- Raw Material Selection & Melting: Virgin stainless steel scrap and controlled alloy additions (Cr, Ni, Mo, C) are melted in a Vacuum Arc Remelting (VAR) or Argon Oxygen Decarburization (AOD) furnace. The AOD process ensures precise control of carbon and sulfur levels, critical for achieving the specified corrosion resistance.
- Continuous Casting: Molten alloy is cast into rectangular or square billets using a continuous casting machine (CCM). Strand dimensions are optimized for subsequent hot rolling.
- Hot Rolling: Cast billets are reheated to 1100–1200°C and hot-rolled through a tube mill to produce seamless hollow bars or hot-rolled coil strips. This step establishes the initial austenitic grain structure.
- Pickling & Descaling: Hot-rolled products undergo acid pickling (typically HNO₃/HF mixed acid bath) to remove the oxidized scale layer and reveal the clean metallic surface.
- Cold Drawing / Cold Rolling: For coil tube production, hot-rolled strip is cold-rolled to the target wall thickness, then formed into tube shape via cold drawing through a die or cold rolling through a plug mill. This step achieves the required dimensional tolerances and enhances mechanical properties through work hardening.
- Solution Annealing (Heat Treatment): The cold-worked tube is solution-annealed at 1010–1150°C followed by rapid water quenching. This process dissolves carbide precipitates, restores full austenitic structure, eliminates residual stresses, and maximizes corrosion resistance and ductility.
- Pickling & Passivation (Surface Treatment): Post-annealing pickling removes any oxide scale formed during heat treatment. Passivation using a citric acid or nitric acid solution forms a thin, adherent chromium oxide (Cr₂O₃) passive film (~2–5 nm) that provides the initial corrosion barrier.
- Coiling & Packaging: The finished coil tube is wound onto a steel core, wrapped with protective film and moisture-barrier packaging, and secured with steel strapping for transport.
4.2 Key Process Parameters
| Process Step | Temperature (°C) | Duration / Cycle | Critical Control Point |
|---|
| Solution Annealing | 1010 – 1150 | 15–30 min (soak) | Complete carbide dissolution; uniform austenite |
| Water Quenching | — | Immediate | Prevent sensitization during cooling |
| Pickling (HNO₃/HF) | 50 – 60 | 5–15 min | Complete scale removal; no acid attack on base metal |
| Passivation | 20 – 60 | 10–20 min | Uniform passive film formation |
5. Applications
The 316 Stainless Steel Coil Tube is the industry-standard material for applications where chloride environments and organic acid exposure demand superior corrosion performance beyond what 304 grade can provide.
5.1 Primary Application Sectors
- Chemical Processing Industry (化工): Reactor piping, heat exchanger tubes, distillation column internals, and process vessels handling chlorinated solvents, hydrochloric acid (HCl) solutions, and mixed organic acids. The Molybdenum content provides critical protection against chloride stress corrosion cracking (Cl-SCC).
- Paper & Pulp Industry (造纸): Heat recovery systems, effluent treatment piping, and paper-making machinery components exposed to acidic pulping liquors (kraft process) and chlorinated bleaching agents.
- Marine & Offshore Environments (海洋大气环境): Ship superstructure piping, offshore platform plumbing, desalination plant components, and coastal architectural fixtures exposed to salt spray and marine atmospheric corrosion (MAC).
- Food & Pharmaceutical Processing: Sanitary-grade piping for dairy, beverage, and pharmaceutical manufacturing where hygienic surface finish and resistance to cleaning agent corrosion are required.
- Oil & Gas Industry: Subsea production systems, flowlines, and wellhead equipment operating in high-chloride formation water environments.
- Power Generation: Flue gas desulfurization (FGD) systems and boiler economizer tubes where acidic condensate and sulfur compounds are present.
5.2 Comparative Advantage Over 304 Grade
| Environment | 304 Grade Performance | 316 Grade Performance | Recommendation |
|---|
| Seawater (immersed) | Poor; rapid pitting | Moderate; limited service life | Consider 316L or duplex 2205 |
| Marine Atmosphere | Marginal; staining within years | Excellent; long-term protection | 316 recommended |
| Dilute HCl (1–5%) | Poor; rapid attack | Moderate; limited resistance | Consider higher Mo grades |
| Acetic Acid (dilute) | Good | Excellent | 316 recommended |
| Chlorinated Bleach (5% NaClO) | Poor; Cl-SCC risk | Good; significantly improved | 316 recommended |
| Atmospheric (industrial) | Good | Excellent | Either acceptable |
6. Quality Assurance & Certifications
6.1 Testing & Inspection Protocol
- Chemical Analysis: Spectrometric verification (OES or XRF) of every heat lot to confirm compliance with ASTM A240 / GB/T 24511 chemical specifications.
- Mechanical Testing: Tensile testing (ASTM A370) and hardness testing (ASTM E10/E18) performed per heat lot and production batch.
- Hydrostatic Testing: 100% of coils subjected to hydrostatic pressure testing at 1.5× maximum working pressure to verify weld integrity and dimensional conformance.
- Corrosion Testing: ASTM A262 Practice No. 1A (Intergranular Corrosion — Acid Solution Test) and ASTM G48 (Pitting & Crevice Corrosion) performed on representative samples.
- Dimensional Inspection: Optical or ultrasonic measurement of OD, wall thickness, and coil geometry per ASTM A576.
- Surface Quality Inspection: Visual and magnetic particle inspection (MPI) for surface defects, cracks, and inclusions.