S31803 Duplex Stainless Steel Coil Tube — Technical Product Specification
1. Product Overview
The S31803 Duplex Stainless Steel Coil Tube (also designated as UNS S31803 / EN 1.4539) is a legacy-grade 2205-class duplex austenite-ferrite stainless steel supplied in the form of precision coil tubing (盘管 / Coiltube). Manufactured and supplied by Wenzhou Jixing Metal Materials Co., Ltd., this product represents the original designation of the 2205 duplex family, widely employed in demanding industrial applications where high mechanical strength, excellent corrosion resistance, and superior resistance to stress corrosion cracking (SCC) are required.
S31803 is distinguished from its successor designation, UNS S32205, by a slightly lower minimum nitrogen (N) content requirement. While both grades share the same nominal chromium (Cr) and nickel (Ni) composition, the elevated nitrogen content in S32205 contributes to marginally higher yield strength and improved pitting resistance. Nevertheless, S31803 remains in active use in legacy projects, established piping systems, and specifications governed by older engineering standards.
Classification: Materials & Metallurgy → Material Grade → Duplex Steel — Standard Type (双相钢-标准型)
Industry Position: Legacy 2205 designation; superseded by S32205 for new designs, yet retained for backward compatibility and ongoing project continuity.
2. Material Composition & Metallurgical Properties
2.1 Chemical Composition (Typical)
S31803 is a balanced 2205 duplex stainless steel with an approximately 1:1 austenite-to-ferrite microstructure. The following table presents the nominal chemical composition in accordance with ASTM A790 / EN 10216-5 requirements for the UNS S31803 designation:
| Element | Minimum (%) | Maximum (%) | Notes |
|---|---|---|---|
| Carbon (C) | — | 0.030 | Low carbon to minimize intergranular corrosion |
| Manganese (Mn) | — | 2.00 | — |
| Phosphorus (P) | — | 0.030 | — |
| Sulfur (S) | — | 0.020 | — |
| Chromium (Cr) | 22.0 | 23.0 | Primary corrosion-resistance element |
| Nickel (Ni) | 4.0 | 7.0 | Austenite stabilizer |
| Molybdenum (Mo) | 3.0 | 3.5 | Pitting and crevice corrosion resistance |
| Nitrogen (N) | 0.15 | 0.25 | Key differentiator from S32205 (N ≥ 0.18%) |
| Silicon (Si) | — | 1.00 | — |
| Titanium (Ti) | — | 0.70 | — |
| Copper (Cu) | — | 0.30 | — |
| Iron (Fe) | Balance | — | — |
2.2 Mechanical Properties
The mechanical properties of S31803 duplex coil tube are characterized by significantly higher yield strength compared to conventional austenitic stainless steels (e.g., 304/304L or 316/316L), owing to the dual-phase microstructure:
| Property | Value | Standard / Condition |
|---|---|---|
| Tensile Strength (UTS) | ≥ 550 MPa (80 ksi) | ASTM A790, Annealed |
| Yield Strength (0.2% offset) | ≥ 450 MPa (65 ksi) | ASTM A790, Annealed |
| Elongation (at fracture) | ≥ 25% | ASTM A790, Annealed |
| Hardness (HBW) | ≤ 260 | Annealed Condition |
| Density | 7.81 g/cm³ | — |
2.3 Corrosion Resistance
- Pitting Resistance: PREN (Pitting Resistance Equivalent Number) ≈ 34–35, calculated as Cr + 3.3 × Mo + 16 × N. This confers excellent resistance to localized corrosion in chloride-containing environments.
- Stress Corrosion Cracking (SCC): The ferritic phase in the microstructure provides inherent resistance to chloride-induced SCC, a well-documented vulnerability of austenitic grades such as 304 and 316.
- Crevice Corrosion: Superior performance in stagnant or semi-stagnant chloride solutions compared to 316L, particularly at elevated temperatures.
- General Corrosion: Effective resistance in sulfuric acid (dilute to moderate concentrations), phosphoric acid, and various organic acids.
2.4 Comparison: S31803 vs. S32205
| Parameter | S31803 (Legacy) | S32205 (Current) | Implication |
|---|---|---|---|
| Minimum Nitrogen (N) | 0.15% | 0.18% | S32205 offers marginally higher strength |
| Yield Strength | ≥ 450 MPa | ≥ 450 MPa (often higher in practice) | Higher N in S32205 enhances solid-solution strengthening |
| PREN | ≈ 34 | ≈ 35 | Incremental improvement in pitting resistance |
| Design Status | Legacy / Existing projects | Recommended for new designs | Industry consensus favors S32205 |
3. Product Specifications — Coil Tube Dimensions
The S31803 duplex coil tube is supplied in continuous coil form, suitable for automated tube-forming, bending, and fabrication processes. The following table outlines the standard dimensional range:
| Specification Parameter | Range / Value | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 50.8 mm (⅛" – 2") | ASTM A778 / EN 10217-7 |
| Wall Thickness (WT) | 0.3 mm – 3.0 mm | ASTM A778 / EN 10217-7 |
| Coil Inner Diameter (ID) | 150 mm – 600 mm (typical) | Manufacturer specification |
| Coil Weight | 500 kg – 2,000 kg (typical) | Customizable per order |
| Surface Finish | BA (Bright Annealed), No. 4, No. 8, Mirror, Pickled & Passivated | ASTM A478 |
| Dimensional Tolerance (OD) | ±0.05 mm to ±0.15 mm (depending on OD) | ASTM A778 |
| Dimensional Tolerance (WT) | ±10% to ±12.5% | ASTM A778 |
| Condition | Annealed (soft), Cold Worked (hardened) | ASTM A790 |
| Length per Coil | Continuous (typically 20 m – 100 m+) | — |
4. Manufacturing Process
The production of S31803 duplex stainless steel coil tube follows a rigorous multi-stage metallurgical and mechanical processing sequence to ensure dimensional accuracy, microstructural integrity, and surface quality:
- Vacuum Arc Remelting (VAR) / Electro-Slag Remelting (ESR): The raw ingot is produced via secondary melting processes to achieve ultra-low sulfur and carbon content, eliminate macro-segregation, and ensure compositional homogeneity critical for duplex steel performance.
- Hot Rolling / Forging: The remelted ingot is hot rolled or forged into a flat strip or hollow blank. For coil tube production, hot-rolled strip is the typical starting form.
- Cold Rolling: The strip is cold-rolled to the target thickness with controlled reduction ratios to achieve the desired dimensional tolerance and surface finish. Interpass annealing may be applied to prevent excessive work hardening.
- Tube Forming: The cold-rolled strip is fed through a roll-forming or seamless tube-forming process to produce the continuous tube profile with precise OD and wall thickness control.
- Solution Annealing (Stabilization Heat Treatment): The formed tube is solution-annealed at 1050 °C – 1100 °C followed by rapid water quenching. This critical step dissolves precipitates (including intermetallic phases such as σ-phase), homogenizes the austenite-ferrite distribution, and achieves the target ~50:50 phase balance essential for optimal mechanical and corrosion properties.
- Pickling and Passivation: Post-annealing pickling removes the oxide scale, followed by acid passivation to regenerate the chromium-rich passive film on the surface. This ensures optimal corrosion resistance upon delivery.
- Coiling: The finished tube is wound onto a standard coil core with controlled tension to prevent deformation, wrinkling, or damage during storage and transport.
- Quality Inspection: Each coil undergoes comprehensive non-destructive testing (NDT), including eddy current testing (ECT), hydrostatic pressure testing, and visual surface inspection.
5. Applications
S31803 duplex coil tube finds extensive application across industries where the combination of high strength, corrosion resistance, and cost-effectiveness is critical. Key application domains include:
- Oil & Gas Industry: Subsea piping systems, downhole tubing, flow lines, risers, and wellhead components operating in high-chloride, high-pressure environments. The SCC resistance of the duplex microstructure is particularly valuable in subsea production systems.
- Marine & Offshore Engineering: Seawater cooling systems, ballast water systems, anchor chains, and structural components exposed to marine atmospheric and immersion conditions.
- Chemical Processing: Heat exchangers, reactors, distillation columns, piping systems, and storage vessels handling aggressive chemical media including chlorides, sulfuric acid, and various organic compounds.
- Power Generation: Flue gas desulfurization (FGD) systems, condensate polishing systems, and components in supercritical and ultra-supercritical (USC) boilers where elevated temperatures and corrosive environments prevail.
- Food & Beverage Processing: Hygienic-grade tubing for dairy, brewing, and pharmaceutical processing where resistance to chloride-based cleaning agents (e.g., sodium hypochlorite) is essential.
- Desalination (RO/MSF): Piping and heat exchanger tubes in reverse osmosis (RO) and multi-stage flash (MSF) desalination plants operating in high-salinity seawater.
- Legacy and Retrofit Projects: Existing installations originally specified with S31803 continue to require matching material for maintenance, replacement, and system integrity.
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. adheres to internationally recognized quality management and product certification standards to ensure traceability, consistency, and compliance:
| Standard / Certification | Scope of Applicability |
|---|---|
| ASTM A790 | Standard Specification for Duplex Stainless Steel Bar and Shapes (mechanical property reference) |
| ASTM A778 | Standard Specification for Welded Austenitic and Duplex Stainless Steel Tube (dimensional and material requirements) |
| ASTM A240 / A270 | Sheet and Tube specifications (reference for surface finish and general requirements) |
| EN 10216-5 | European standard for seamless and welded stainless steel tubes for pressure purposes |
| EN 10217-7 | European standard for welded austenitic stainless steel tubes for pressure purposes |
| ISO 3506 | Fastener materials ( |