304H Austenitic Stainless Steel Coil Tube — Technical Specification
1. Product Overview
The 304H Austenitic Stainless Steel Coil Tube is a high-temperature-resistant, general-purpose austenitic alloy tube supplied in continuous coil form. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product is specifically engineered to deliver superior creep resistance and long-term structural integrity in demanding high-temperature service environments where conventional 304 and 304L grades would exhibit accelerated degradation.
The "H" suffix designates a high-temperature variant of the standard 304 austenitic stainless steel, characterized by a deliberately elevated carbon content (C ≥ 0.04%), which promotes the precipitation of fine carbides at grain boundaries. This microstructural feature fundamentally enhances the material's resistance to creep deformation at elevated operating temperatures, making it the preferred choice for power generation, petrochemical, and process heating applications.
2. Material & Properties
2.1 Chemical Composition (ASTM A213 / ASME SA-213)
| Element | Specification (%) | Notes |
|---|---|---|
| Carbon (C) | ≥ 0.04 – 0.10 | Key differentiator from 304 (≤0.08%) and 304L (≤0.03%) |
| Chromium (Cr) | 18.0 – 20.0 | Primary corrosion resistance element |
| Nickel (Ni) | 8.0 – 11.0 | Stabilizes austenitic microstructure |
| Manganese (Mn) | ≤ 2.0 | — |
| Silicon (Si) | ≤ 1.0 | — |
| Sulfur (S) | ≤ 0.03 | — |
| Phosphorus (P) | ≤ 0.045 | — |
| Iron (Fe) | Balance | — |
2.2 Physical Properties
| Property | Value |
|---|---|
| Density | 8.0 g/cm³ |
| Melting Range | 1400 – 1450 °C |
| Thermal Conductivity (100 °C) | 16.3 W/(m·K) |
| Thermal Conductivity (500 °C) | 21.5 W/(m·K) |
| Modulus of Elasticity (RT) | 193 GPa |
| Linear Thermal Expansion (20–500 °C) | 16.5 × 10⁻⁶ /°C |
2.3 Mechanical Properties (Annealed Condition)
| Property | Specification |
|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (75 ksi) |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (30 ksi) |
| Elongation (50 mm gauge length) | ≥ 40% |
| Hardness (Brinell) | ≤ 217 HB |
| Creep Strength (1000 °C / 10,000 h / 1% strain) | ~ 27 MPa (superior to 304/304L) |
2.4 Key Differentiators vs. 304 and 304L
- Elevated Carbon Content: The minimum carbon specification of 0.04% (compared to 0.08% max for 304 and 0.03% max for 304L) ensures a higher density of carbide precipitates, which act as effective obstacles to dislocation motion at elevated temperatures.
- Superior Creep Resistance: At temperatures above 800 °C, 304H demonstrates significantly higher creep strength and longer rupture life than standard 304 and 304L grades.
- Trade-off Awareness: The higher carbon content reduces resistance to intergranular corrosion (IGC) in the sensitized condition. Therefore, 304H is not recommended for applications involving welding in the presence of reducing acids or chloride-containing environments. Post-weld stabilization heat treatment or the use of 321H as an alternative is advised for such service.
3. Specifications
| Parameter | Specification |
|---|---|
| Product Form | Coiled Tubing (Continuous Coil) |
| Material Grade | 304H (UNS S30409) |
| Classification | Austenitic Stainless Steel — General Purpose, High-Temperature |
| Outer Diameter (OD) | 6.0 mm – 159.0 mm (configurable per order) |
| Wall Thickness | 0.5 mm – 8.0 mm (configurable per order) |
| Coil Length | 10 m – 50 m per coil (customizable) |
| Coil Inner Diameter | 150 mm – 500 mm |
| Surface Finish | Mill Finish / Pickled & Passivated / Polished (upon request) |
| Tolerance (OD) | ±0.05 mm (precision) / ±0.10 mm (standard) |
| Tolerance (Wall Thickness) | ±10% of nominal |
| Applicable Standards | ASTM A213, ASME SA-213, EN 10216-5, GB/T 12771 |
| Delivery Condition | Heat-Treated & Pickled (Solution Annealed) |
| Maximum Operating Temperature | Up to 1150 °C (in oxidizing atmosphere) |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Electrolytic Refining (EBSR — Electro-Slag Remelting) or Vacuum Arc Remelting (VAR) ingots are used to ensure ultra-low impurity levels (S, P, N) and homogeneous alloy distribution.
- Hot Rolling: Billet is reheated to 1150–1200 °C and hot-rolled into seamless tube blanks. For coil tube production, a specialized hot-rolling mill with controlled cooling is employed.
- Piercing & Tube Milling: The heated billet is pierced and rolled through a series of reducing passes to achieve the target outer diameter and wall thickness.
- Pipe Milling (Cold Reduction): Hot-rolled tube blanks undergo cold pipe milling (CPM) or cold-rolled reduction to achieve precise dimensional tolerances and improved surface finish.
- Solution Annealing: The tube is reheated to 1050–1100 °C and rapidly quenched in water or air to dissolve all carbide precipitates and restore full austenitic homogeneity.
- Pickling & Passivation: The tube surface is treated with a mixed acid solution (HNO₃ + H₂SO₄) to remove scale, followed by passivation to establish a protective chromium oxide layer.
- Coiling: The finished tube is wound onto a steel core reel using a controlled-tension coiling system to prevent kinking, ovality, or surface damage.
- Final Inspection & Packaging: Each coil undergoes dimensional verification, hydrostatic testing, and non-destructive testing (NDT) before protective packaging.
4.2 Critical Process Controls
- Carbon Content Monitoring: Spectrometric analysis is performed on every heat to confirm C ≥ 0.04% compliance, which is the defining criterion for the "H" grade designation.
- Annealing Temperature Window: Strict control of the solution annealing temperature between 1050–1100 °C is critical to avoid partial melting or excessive grain growth while ensuring complete carbide dissolution.
- Coiling Tension Management: Real-time tension feedback systems prevent micro-plastic deformation that could compromise dimensional accuracy.
5. Applications
5.1 Primary Applications
- Boiler Superheaters: 304H coil tubes are extensively used in once-through and drum-type boilers for superheater elements, where flue gas temperatures routinely exceed 800 °C. The superior creep strength extends tube life significantly compared to 304 or 304L.
- High-Temperature Steam Pipelines: Suitable for main steam lines, reheat steam lines, and feedwater preheating circuits in power plants operating at temperatures up to 550 °C continuously.
- Heat Exchangers: Used in shell-and-tube heat exchangers handling hot process fluids in petrochemical and refining operations.
- Furnace Tubing: Employed in radiant tubes for industrial furnaces, reformers, and catalytic cracking units.
5.2 Secondary Applications
- Oil and gas processing — high-temperature heat exchanger tubes
- Cement kilns — preheater system tubing
- Power generation — auxiliary steam lines and condensate return systems
- Chemical processing — heat-resistant process piping in oxidizing environments
5.3 Application Limitations
Caution: Due to the elevated carbon content, 304H is susceptible to intergranular corrosion in the sensitized (welded without post-weld annealing) condition. It should not be used in applications involving sustained exposure to reducing acids, chloride-rich environments, or where extensive welding is required without subsequent stabilization treatment. In such cases, 321H (stabilized with Ti) or 310H (higher Cr-Ni) may be more appropriate alternatives.
6. Quality Certifications
6.1 Manufacturing Certifications
- ISO 9001:2015 — Quality Management System certification for consistent production quality
- ISO 3834 — Quality requirements for welding of metallic materials (applicable to welded assemblies)
- TÜV Certified — Third-party audit of manufacturing processes and product conformity
- PED 2014/68/EU — Pressure Equipment Directive compliance for pressure-containing applications
6.2 Product Testing & Inspection
| Test Method | Standard | Purpose |
|---|---|---|
| Spectrographic Analysis | ASTM E1251 | Chemical composition verification (C, Cr, Ni, etc.) |
| Tensile Testing | ASTM A370 | UTS, yield strength, elongation |
| Hardness Testing | ASTM E10 | Brinell hardness verification |
| Hydrostatic Pressure Test | ASTM A213 | Leakage and integrity verification |
| Eddy Current Testing | ASTM E309 | Detection of surface and near-surface defects |
| Dimensional Inspection | ASTM A213 / ASME SA-213 | OD, wall thickness, straightness, ovality |
| Intergranular Corrosion Test | ASTM A262 (Practice E) | IGC susceptibility assessment (as required) |
| Creep Testing | ASTM E139 | Long-term creep strength validation (optional) |
6.3 Traceability & Documentation
- Each coil is supplied with a Mill Test Certificate (MTC) conforming to EN 10204 Type 3.1 or Type 3.2, providing full traceability from raw material heat number through to final product.
- Heat numbers, chemical analysis results, mechanical test data, and dimensional inspection records are documented and archived for minimum 10 years.
- Third-party inspection (TPI) by organizations such as SGS, BV, or DNV is available upon customer request.
7. Unique Selling Points
- Engineered for High-Temperature Service: The controlled elevated carbon content (≥ 0.04%) provides measurably superior creep resistance compared to standard 304 and 304L, translating directly to extended service life in superheater and steam line applications.
- Coil Form Efficiency: Continuous coil supply eliminates the need for tube cutting, welding, and alignment at installation sites, reducing labor costs and minimizing potential failure points from field welds.
- Full Standards Compliance: Manufactured to ASTM A213, ASME SA-213, EN 10216-5, and GB/T 12771, ensuring global project eligibility.
- Comprehensive Quality Assurance: Multi-stage NDT, spectrographic verification of carbon content, and full traceability documentation ensure every coil meets the stringent demands of power generation and process industries.