Welded Stainless Steel Coil Tube — GB/T 24592 Heat Exchanger Tube Specification
1. Product Overview
The Stainless Steel Coil Tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered welded tubular product designed specifically for heat exchanger applications. Governed by the Chinese National Standard GB/T 24592 (Welded Tubes for Heat Exchangers), this product represents a rigorous subset of stainless steel tubing engineered to withstand the demanding thermal, mechanical, and chemical environments inherent in industrial heat exchange systems.
Unlike general-purpose fluid transport tubes, heat exchanger tubes are subject to elevated quality requirements, particularly regarding weld integrity. The defining characteristic of this product is its enhanced non-destructive testing (NDT) protocol — the sampling ratio for Radiographic Testing (RT) and Ultrasonic Testing (UT) of weld seams is significantly higher than that required for standard fluid tubes, ensuring superior joint reliability under cyclic thermal loading.
2. Material & Properties
2.1 Material Grades
The coil tube is available in a range of austenitic and duplex stainless steel grades commonly specified for heat exchange service, including but not limited to:
- 304 / 06Cr19Ni10 — General corrosion resistance, moderate temperature capability
- 304L / 022Cr19Ni10 — Low-carbon variant for improved weldability and resistance to intergranular corrosion
- 316L / 022Cr17Ni12Mo2 — Enhanced pitting and crevice corrosion resistance due to molybdenum addition
- 321 / 06Cr18Ni11Ti — Titanium-stabilized grade for elevated temperature service
- 316Ti / 06Cr17Ni12Mo2Ti — Titanium-stabilized 316 variant for high-temperature and corrosion-critical applications
- 310S / 06Cr25Ni20 — High-temperature oxidizing atmosphere resistance
2.2 Mechanical Properties (Typical)
| Property | 304 / 304L | 316 / 316L | 321 | 310S |
|---|---|---|---|---|
| Tensile Strength (Rm, MPa) | ≥ 520 | ≥ 515 | ≥ 520 | ≥ 515 |
| 0.2% Yield Strength (Rp0.2, MPa) | ≥ 205 | ≥ 170 | ≥ 205 | ≥ 190 |
| Elongation (A, %) | ≥ 40 | ≥ 40 | ≥ 40 | ≥ 40 |
| Hardness (HV10) | ≤ 210 | ≤ 210 | ≤ 210 | ≤ 210 |
| Operating Temperature Range (°C) | -196 to 870 | -196 to 870 | -196 to 900 | -196 to 1150 |
2.3 Chemical Composition Requirements
All material grades conform to the chemical composition limits specified in GB/T 24592 and the corresponding material standards (e.g., GB/T 12771, ASTM A213, ASTM A269). Carbon content is tightly controlled in low-carbon (L) grades to a maximum of 0.030% C to prevent chromium carbide precipitation at grain boundaries.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Tolerance (Typical) |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm – 76.2 mm (¼" – 3") | ±0.05 mm (precision grades); ±0.10 mm (standard) |
| Wall Thickness | 0.50 mm – 6.00 mm | ±10% of nominal thickness |
| Tube Length (Coil) | Continuous coil, 10 m – 200 m per coil | As specified |
| Coil Inner Diameter | 300 mm – 1200 mm | As specified |
| Coil Outer Diameter | 500 mm – 2000 mm | As specified |
| Surface Finish (Roughness) | Ra ≤ 0.8 μm (standard); Ra ≤ 0.4 μm (polished) | Per GB/T 24592 requirements |
| Roundness | ≤ 0.5% of nominal OD | Per GB/T 24592 |
| Straightness (per 1000 mm) | ≤ 1.5 mm | Per GB/T 24592 |
3.2 Weld Seam Specifications
| Parameter | Requirement |
|---|---|
| Weld Process | Automatic orbital TIG (Tungsten Inert Gas) welding with internal/external gas shielding |
| Weld Appearance | Smooth, uniform, no undercut, porosity, or weld spatter; bead width ≤ 1.5× wall thickness |
| Weld Penetration | Full penetration with no root defects |
| Weld Heat-Affected Zone (HAZ) | Minimal sensitization; grain structure comparable to base metal |
3.3 Surface Treatment Options
- Pickled & Passivated (P&P) — Standard delivery condition; removes scale and forms a protective chromium oxide film
- Mechanically Polished (MP) — Interior/exterior polishing to Ra ≤ 0.4 μm for pharmaceutical or hygienic applications
- Electrolytic Polished (EP) — Superior surface smoothness (Ra ≤ 0.2 μm) with enhanced corrosion resistance
- Acid Pickled & Passivated (AP) — Enhanced passivation treatment for aggressive chemical environments
4. Manufacturing Process
4.1 Process Flow
- Strip Preparation: Cold-rolled stainless steel strip is inspected for surface quality, dimensional accuracy, and chemical composition. Strip is cleaned and decontaminated to ensure weld quality.
- Coiling & Feeding: Strip is uncoiled and fed through the tube-forming line at controlled tension to maintain dimensional stability.
- Tube Forming: The flat strip is rolled into a tubular shape using precision forming rolls. The forming process ensures uniform wall thickness and roundness.
- Orbital TIG Welding: The formed tube is welded longitudinally using an automatic orbital TIG welding system. Argon gas shielding is applied both internally (through a consumable or permanent internal nozzle) and externally to prevent oxidation of the weld zone.
- Post-Weld Heat Treatment (if required): Solution annealing at 1010–1150°C followed by rapid water quenching to restore full austenitic structure and maximize corrosion resistance. This step is critical for preventing sensitization in the HAZ.
- Surface Treatment: Pickling and passivation (or polishing) to remove weld discoloration and oxide scale, restoring the full corrosion resistance of the stainless steel surface.
- Dimensional Inspection: OD, wall thickness, roundness, and straightness are verified using laser and mechanical gauging systems.
- Non-Destructive Testing (NDT): See Section 4.2 for detailed NDT protocol.
- Final Coiling & Packaging: Tubes are recoiled to the specified coil dimensions, wrapped in protective material, and packaged for shipment.
4.2 Non-Destructive Testing Protocol (Key Differentiator)
Per GB/T 24592, welded heat exchanger tubes are subject to significantly more stringent NDT requirements compared to standard fluid transport tubes. This is the defining quality differentiator of this product.
| Test Method | Heat Exchanger Tube (GB/T 24592) | Standard Fluid Tube (for comparison) | Purpose |
|---|---|---|---|
| Radiographic Testing (RT) | Sampling ratio: 10% – 100% (often 100% for critical applications) | Typically 1% – 5% or visual inspection only | Detects internal weld defects: lack of fusion, porosity, inclusions, cracks |
| Ultrasonic Testing (UT) | Sampling ratio: 10% – 100% (continuous UT available) | Typically 1% – 5% or not required | Detects volumetric and planar defects in weld and base metal |
| Visual Inspection (VT) | 100% of all welds | 100% of all welds | Surface defects: undercut, overlap, burn-through, discoloration |
| Eddy Current Testing (ET) | Available as option; continuous inline inspection | Rarely applied | Surface and near-surface defect detection at high speed |
| Hydrostatic / Pneumatic Testing | Per batch or lot; test pressure ≥ 1.5× maximum operating pressure | Per batch or lot | Leak tightness verification |
Critical Note: The elevated RT/UT sampling ratio for heat exchanger tubes reflects the consequence of failure in heat exchange service. A weld defect in a heat exchanger tube can lead to cross-contamination between process fluids, catastrophic equipment failure, or safety incidents. The enhanced NDT protocol is therefore not merely a quality preference but a safety-critical requirement.
4.3 Weld Quality Assurance Measures
- Weld parameters (current, voltage, travel speed, gas flow rate) are tightly controlled and monitored in real-time via automated process control systems
- Each welding station is qualified through Weld Procedure Qualification (WPQ) per applicable codes
- Weld operators are certified per NB/ISO 9606-1 standards
- In-line weld inspection systems (optical or acoustic emission) provide continuous monitoring of weld quality during production
- Any lot failing NDT is subject to 100% re-testing or rejection — no partial acceptance of defective welds
5. Applications
5.1 Primary Applications
- Shell & Tube Heat Exchangers: Primary application; coil tubes are straightened and cut to length for insertion into heat exchanger tube sheets
- Air Coolers / Finned Tube Heat Exchangers: Coil tubes are finned and used in gas-to-liquid or gas-to-gas heat exchange
- Reboilers & Condensers: Process heat exchange in distillation and refining operations
- Evaporators: Multi-effect and falling-film evaporators in food, chemical, and pharmaceutical industries
5.2 Industry Sectors
- Petrochemical & Oil Refining: Crude oil distillation, cracking, reforming, and hydrotreating heat exchangers
- Power Generation: Boiler economizers, air preheaters, condenser tubes, and waste heat recovery systems
- Chemical Processing: Reactor cooling, product heating, and solvent recovery
- Food & Beverage Processing: Pasteurization, sterilization, and heat recovery systems (316L / 304L with polished finish)
- Pharmaceutical Manufacturing: GMP-compliant heat exchange with electro-polished or mechanically polished tube surfaces
- Marine & Offshore: Seawater coolers, condensers, and heat exchangers for offshore platforms
- HVAC & Refrigeration: Chilled water systems, absorption chillers, and heat pump systems
- Environmental Protection: Flue gas desulfurization (FGD) heat exchangers, scrubber systems
6. Quality Certifications & Compliance
6.1 Applicable Standards
| Standard | Description | Relevance |
|---|---|---|
| GB/T 24592 | Welded tubes for heat exchangers — Chinese National Standard | Primary governing standard; defines dimensional, mechanical, and NDT requirements |
| GB/T 12771 | Stainless steel welded tubes for general use | Base material standard for general stainless steel tubing |
| GB/T 14976 | Stainless steel seamless tubes for fluid transport | Referenced for comparison of NDT requirements |
| ASTM A213 | Standard specification for austenitic stainless steel welded and seamless tubes for heat exchanger service | International equivalent; often specified alongside GB/T 24592 for export orders |
| ASTM A269 | Standard |