Stainless Steel Coil Tube – TIG Self-Melt Welding with Back-Side Argon Purge
Wenzhou Jixing Metal Materials Co., Ltd. presents a specialized stainless steel coil tube (Coiltube) system engineered for installation and maintenance applications, utilizing advanced TIG (Tungsten Inert Gas) self-melt welding with back-side argon purge protection. This document provides a comprehensive technical specification covering material properties, welding parameters, manufacturing processes, quality control protocols, and applicable industry standards.
1. Product Overview
The stainless steel coil tube is a flexible, pre-formed tubular product designed for routing, conduit, and installation applications where bends, turns, and complex geometries are required. Unlike rigid pipe sections, coil tubes are delivered in continuous spool or coil configurations, enabling on-site forming and cutting to precise dimensions. The integrity of these tubes—particularly their internal corrosion resistance—is maintained through a rigorous welding and fabrication process employing TIG self-melt welding with back-side argon purge protection.
Unique Selling Points:
- Full internal and external argon shielding ensures metallurgical purity at the weld interface
- Self-melt TIG technique eliminates the need for filler metal, reducing contamination risk
- Continuous coil delivery minimizes field joints and potential failure points
- Corrosion-resistant internal surface preserved throughout the welding process
- Compliant with international standards for stainless steel welding and fabrication
2. Material & Properties
Coil tubes are typically fabricated from austenitic stainless steel grades, most commonly AISI 304 (UNS S30400), AISI 304L (UNS S30403), AISI 316 (UNS S31600), and AISI 316L (UNS S31603). These grades exhibit superior corrosion resistance, high strength-to-weight ratios, and excellent weldability due to their austenitic crystal structure.
| Property | AISI 304 / 304L | AISI 316 / 316L |
|---|---|---|
| Chromium (Cr) | 18.0 – 20.0% | 16.0 – 18.0% |
| Nickel (Ni) | 8.0 – 12.0% | 10.0 – 14.0% |
| Molybdenum (Mo) | — | 2.0 – 3.0% |
| Carbon (C, max) | 0.08% (304) / 0.03% (304L) | 0.08% (316) / 0.03% (316L) |
| Tensile Strength (UTS) | ≥ 515 MPa | ≥ 515 MPa |
| Yield Strength (0.2% offset) | ≥ 205 MPa | ≥ 205 MPa |
| Elongation (A50) | ≥ 40% | ≥ 40% |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 16.3 W/(m·K) |
| Maximum Service Temperature | 870°C | 870°C |
The "L" designations (304L, 316L) refer to low-carbon variants specifically intended for welding applications, as reduced carbon content mitigates intergranular corrosion susceptibility during the heat-affected zone (HAZ) sensitization window (450–850°C).
3. Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Diameter (OD) | 6.0 mm – 159.0 mm | ASTM A269 / ASTM A213 |
| Wall Thickness | 0.5 mm – 6.0 mm | ASTM A269 |
| Coil Length (continuous) | Up to 50 m per coil | Manufacturer specification |
| Surface Finish | No. 1 (hot-rolled), 2B (cold-rolled), BA (bright annealed), Electropolished | ASTM A480 |
| Tolerance (OD) | ± 0.05 mm (precision) / ± 0.10 mm (standard) | ASTM A269 |
| Welding Process | TIG Self-Melt (GTAW without filler) | ASME IX, AWS D10.6 |
| Shielding Gas (Front) | Pure Argon (Ar), ≥ 99.99% purity | AWS A5.32 |
| Shielding Gas (Back Purge) | Pure Argon (Ar), ≥ 99.99% purity | AWS A5.32 |
| Front Gas Flow Rate | 8 – 15 L/min | Manufacturer process specification |
| Back Purge Flow Rate | 5 – 12 L/min (tube diameter dependent) | Manufacturer process specification |
| Pre-Flushing Duration | 30 – 60 seconds prior to arc initiation | Welding procedure specification (WPS) |
| Post-Flushing Duration | 15 – 30 seconds after arc termination | Welding procedure specification (WPS) |
| Welding Current | 40 – 250 A (diameter/thickness dependent) | WPS |
| Travel Speed | 30 – 150 mm/min | WPS |
| Internal Surface Condition (post-weld) | Silver-gray to pale straw, no blackening or dark oxide scale | Visual acceptance criterion |
4. Manufacturing Process
4.1 Process Description: TIG Self-Melt with Back-Side Argon Purge
The TIG self-melt welding technique, also referred to as Tungsten Inert Gas Welding without filler metal (GTAW without filler), is a precision welding method specifically suited for thin-wall stainless steel coil tubes. The process relies on the tungsten electrode to generate the arc heat, which melts the base metal at the joint interface without the addition of any external filler material. This eliminates potential contamination vectors associated with filler wire and ensures that the weld metal composition remains consistent with the base material.
4.2 Argon Purge Protocol
The critical differentiator in maintaining the corrosion integrity of stainless steel coil tubes during welding is the implementation of a dual-sided argon shielding system:
- Pre-Flushing: Pure argon gas is introduced to both the front (outside) and back (inside) surfaces of the tube for a minimum of 30 seconds prior to arc initiation. This displaces atmospheric oxygen and nitrogen from the joint area, creating an inert atmosphere essential for preventing oxidation.
- Front Shielding: During welding, argon flows at 8–15 L/min through the torch nozzle, providing a protective blanket over the molten weld pool on the exterior surface.
- Back-Side Purge: Simultaneously, pure argon is introduced into the tube's internal cavity at controlled flow rates (5–12 L/min), protecting the interior weld face from atmospheric contamination. This is typically achieved using purge fittings, inflatable dams, or continuous purge methods depending on the coil tube geometry.
- Post-Flushing: After arc termination, argon flow is maintained for 15–30 seconds to ensure complete cooling of the weld zone under inert atmosphere, preventing post-weld oxidation during the critical temperature descent through the sensitization range.
4.3 Process Flow Diagram
| Step | Operation | Key Control Parameters |
|---|---|---|
| 1 | Coil tube uncoiling and straightening | Springback compensation, straightness ≤ 1.5 mm/m |
| 2 | Dimensional cutting and bevel preparation | Cut perpendicularity ± 1°, V-groove 60°±5° for wall > 2 mm |
| 3 | Joint assembly and alignment | Root gap 0.5 – 1.0 mm, misalignment ≤ 0.5 mm |
| 4 | Back-purge fitting installation | Airtight seal, purge inlet/outlet placement |
| 5 | Pre-flushing (front + back) | 30–60 sec, ≥ 99.99% Ar, flow verification |
| 6 | TIG self-melt welding | 8–15 L/min front, 5–12 L/min back, 40–250 A |
| 7 | Post-flushing | 15–30 sec continuous flow |
| 8 | Visual inspection of weld bead | Silver-gray appearance, no blackening |
| 9 | Non-destructive testing (NDT) | Visual (VT), penetrant (PT), ultrasonic (UT) as applicable |
| 10 | Purge fitting removal and final inspection | Surface cleanliness, dimensional verification |
4.4 Critical Quality Indicator: Internal Surface Color
Acceptance Criterion: The internal weld surface must exhibit a uniform silver-gray to pale straw coloration, indicative of complete argon protection throughout the welding and cooling cycle.
Rejection Criterion: Any evidence of dark gray, brown, or black discoloration on the internal weld face constitutes a protection failure. This indicates that atmospheric oxygen penetrated the purge envelope, forming chromium oxide (Cr₂O₃) and iron oxide (Fe₂O₃/Fe₃O₄) scales on the weld surface. Such oxidation results in:
- Localized depletion of chromium and nickel in the HAZ (intergranular sensitization)
- Potential for intergranular corrosion (IGC) in the sensitized zone
- Compromised long-term corrosion resistance of the internal tube surface
- Violation of process specification and rejection of the weld joint
5. Applications
Stainless steel coil tubes fabricated using the TIG self-melt with back-side argon purge process are deployed across multiple industries where internal corrosion resistance and structural integrity are paramount:
- Food and Beverage Industry: CIP (Clean-In-Place) compatible piping for dairy, brewing, and pharmaceutical processing; internal surface purity is critical for hygiene compliance (EHEDG, 3-A Standards).
- Chemical Processing: Conveyance of aggressive chemicals, acids, and solvents where internal oxidation would catalyze corrosion.
- Petrochemical and Oil & Gas: Instrument tubing, control lines, and cryogenic service applications requiring leak-tight, corrosion-resistant joints.
- Power Generation: Condensate lines, steam sampling systems, and boiler instrumentation requiring high-temperature resistance.
- Medical Devices: Biocompatible tubing for medical gas systems and equipment manifolds.
- Semiconductor Manufacturing: Ultra-high purity (UHP) gas delivery systems where particulate and oxide contamination must be eliminated.
- Construction and Architecture: Decorative and structural installations where both aesthetic and functional performance are required.
6. Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains compliance with the following international standards and certifications:
| Standard / Certification | Scope | Relevance |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Systematic quality assurance throughout production |
| ISO 3834-2 | Welding Requirements – General | Comprehensive welding quality management |
| EN ISO 14732 | Stainless Steel Welding | Specific welding procedures for austenitic stainless steels |
| ASME IX | Welding and Brazing Qualifications | Welder and WPS qualification for pressure-containing equipment |
| AWS D10.6 | Stainless Steel Welding Code | Welding procedures, inspection, and acceptance criteria |
| ASTM A269 | Seamless and Welded Austenitic Stainless Steel Tubing | Material specification and mechanical property requirements |
| ASTM A480 | Hot-Rolled and Cold-Rolled Stainless Steel Plate | Surface finish and dimensional tolerances |
| ASME BPE | BioPharma Equipment and Components | Pharmaceutical-grade fabrication and surface finish requirements |
7. Technical Recommendations
- Ensure argon gas purity of ≥ 99.99% (4-nines) for all shielding and purge operations; impurities such as oxygen and water vapor will compromise weld integrity.
- Implement flow metering and monitoring devices to maintain purge rates within specified parameters throughout the welding cycle.
- Perform a pre-weld gas purity test using an oxygen analyzer at the purge outlet;