Stainless Steel Coil Tube — TIG Self-Fusing Weld with Back-Side Argon Shielding
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. (Wenzhou Jixing) presents its premium Stainless Steel Coil Tube (盘管 / Coil Tube), engineered for demanding applications where structural integrity, hygienic performance, and long-term corrosion resistance are non-negotiable. The coil tube is delivered in a coiled configuration to facilitate continuous-length deployment in automated and semi-automated piping systems. The product's defining engineering feature lies in its Tungsten Inert Gas (TIG) self-fusing welding process combined with back-side argon gas shielding — a technique that ensures metallurgical purity and seamless internal surfaces free of oxidation, pitting, or weld-induced sensitization.
This product falls under the classification of Installation & Maintenance > Connection Processes > Welding Connections, representing a critical junction technology for stainless steel piping networks in pharmaceutical, food-grade, chemical, and high-purity industrial environments.
Material & Properties
The coil tube is manufactured from austenitic stainless steel grades, most commonly ASTM A270 Type 304 (UNS S30400) and Type 316L (UNS S31603), selected for their superior corrosion resistance, formability, and weldability. Key material properties include:
- Chemical Composition: Carbon (C) ≤ 0.08% (304) / ≤ 0.03% (316L), Chromium (Cr) 18–20%, Nickel (Ni) 8–12% (304) / 10–14% (316L), Molybdenum (Mo) 2–3% (316L)
- Mechanical Properties: Tensile Strength (UTS) ≥ 515 MPa (304) / ≥ 485 MPa (316L); Yield Strength (0.2% offset) ≥ 205 MPa; Elongation ≥ 40%
- Surface Finish: Internal surface Ra ≤ 0.5 μm (electropolished or mechanically polished variants available upon request)
- Corrosion Resistance: Equivalent to or exceeding ASTM A967 passivation requirements; resistance to intergranular corrosion verified per ASTM A262 Practice E (acid solution test)
- Temperature Range: Operational service from cryogenic (−196 °C) to +400 °C without loss of mechanical or corrosion properties
Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Nominal Diameter (OD) | Ø 6 mm – Ø 108 mm | ASTM A270 / ASME B36.19M |
| Wall Thickness | 0.5 mm – 4.0 mm | ASTM A270 |
| Coil Length per Spool | 10 m – 50 m (customizable) | — |
| Material Grade | 304 / 304L / 316 / 316L / 321 / 904L | ASTM A270 / EN 10217-7 |
| Welding Process | TIG (GTAW) Self-Fusing + Back-Side Argon Shielding | ASME Section IX / ISO 6947 |
| Shielding Gas | Pure Argon (Ar 99.995%) | — |
| Gas Flow Rate | 8 – 15 L/min | Manufacturer's Procedure (WPS) |
| Pre-Flushing Duration | Minimum 30 seconds before arc initiation | WPS Requirement |
| Post-Flushing Duration | Minimum 15 seconds after arc termination | WPS Requirement |
| Weld Bead Appearance | Uniform silver-gray; no oxidation, no blackening | Visual Acceptance Criteria |
| Internal Surface Quality | No oxide scale, no discoloration, Ra ≤ 0.5 μm | ASTM A967 / ISO 14283 |
Manufacturing & Welding Process
3.1 Coil Tube Fabrication
The coil tube is produced through a precision cold-rolling and forming process. Starting from stainless steel strip (cold-rolled coil), the strip is slit to the required width, cold-formed into a tubular configuration via a roll-forming mill, and then helically welded along the longitudinal seam using an automated TIG welding system. The helical coil configuration is achieved by winding the continuous welded tube onto a coil spool at a controlled diameter, ensuring minimal residual stress and uniform geometry throughout the length.
3.2 TIG Self-Fusing Weld with Back-Side Argon Shielding (GTAW + Back Purge)
The critical connection process for this product is Tungsten Inert Gas welding (GTAW — Gas Tungsten Arc Welding) in a self-fusing configuration (no filler metal added) with back-side argon gas shielding. This process is the industry benchmark for achieving oxidation-free welds on austenitic stainless steel, particularly where the internal surface must remain pristine for hygiene or corrosion-resistance reasons.
3.3 Process Parameters and Execution
- Pre-Flushing (Pre-Purge): Pure argon gas (purity ≥ 99.995%) is introduced into the interior of the tube at a flow rate of 8–15 L/min for a minimum of 30 seconds prior to arc initiation. This displaces atmospheric oxygen (O₂) and moisture (H₂O) from the weld zone, creating an inert environment essential for preventing oxidation of the molten pool and the heat-affected zone (HAZ).
- Front-Side Shielding: Argon gas is simultaneously supplied from the welding torch nozzle to shield the front face of the weld pool, ensuring a smooth, uniform weld bead with minimal spatter.
- Back-Side Shielding (Internal Purge): A dedicated back-purge gas delivery system — typically via a trailing gas lance, internal purge ring, or through-flow fixture — maintains continuous argon flow inside the tube diameter during and after welding. This is the defining feature of this process: it prevents the formation of iron oxide scale (Fe₂O₃, Fe₃O₄, Cr₂O₃) on the internal weld surface.
- Welding Parameters: DC (Direct Current) with tungsten electrode polarity; amperage typically 40–120 A depending on wall thickness; travel speed 100–300 mm/min; tungsten electrode diameter 1.6–3.2 mm (pure tungsten or lanthanated tungsten).
- Post-Flushing (Post-Purge): Argon flow is maintained for a minimum of 15 seconds after arc termination to allow the weld pool and HAZ to solidify and cool under inert atmosphere, preventing late-stage oxidation during the critical cooling range (1,100 °C to 500 °C).
3.4 Critical Quality Indicator
WARNING: Internal wall oxidation or blackening (内壁氧化发黑) of the weld zone is a definitive indicator of protection failure. If the internal surface exhibits dark discoloration (ranging from straw-yellow through blue to black), it signifies that atmospheric oxygen penetrated the weld zone during solidification. This results in the formation of a thick, non-protective iron oxide layer that compromises the passive chromium oxide film (Cr₂O₃), leading to a permanent and irreversible loss of corrosion resistance (耐蚀性丧失) at the weld. Such welds must be rejected and the tube section scrapped or re-processed.
3.5 Process Flow Diagram (Schematic)
The overall manufacturing sequence follows:
- Strip inspection & surface preparation (degreasing, acid pickling)
- Cold roll-forming into tube blank
- Helical seam welding (TIG, self-fusing, with back-side argon purge)
- Coil winding onto spool
- Internal surface inspection (visual, borescope, or profilometric)
- Passivation treatment (ASTM A967) if required
- Final dimensional and surface quality verification
- Packaging and shipment
Quality Certifications & Compliance
- Material Certification: Mill Test Certificate (MTC) per EN 10204 Type 3.1 or Type 3.2; chemical analysis and mechanical test reports for each heat number
- Welding Procedure: Welding Procedure Specification (WPS) and Welding Procedure Qualification Record (WPQR) in accordance with ASME Section IX or ISO 15614-1
- Welder Qualification: All operators certified per ASME Section IX or ISO 9606-1
- Surface Quality: Internal surface finish verified per ISO 14283 (Surface roughness of tubes for food processing) or 3-A Sanitary Standards
- Passivation: Verified per ASTM A967 (Standard Practice for Chemical Passivation of Stainless Steel Parts) and/or ASTM A380 (Hydrogen-Peroxide-Nitric Acid Passivation)
- Intergranular Corrosion Resistance: Tested per ASTM A262 Practice E (65% Boiling Oxalic Acid Test) or ASTM G58 (Modified ASTM A262 Practice E)
- System Certifications: ISO 9001:2015 Quality Management; ISO 14001 Environmental Management; CE marking per PED 2014/68/EU where applicable
Applications
The stainless steel coil tube with TIG self-fusing back-side argon-shielded welds is deployed in applications where the integrity of the internal surface is critical:
- Pharmaceutical & Biotechnology: Clean-in-Place (CIP) piping systems, reactor internals, hygienic transfer lines, and medical device manufacturing — where bioburden control and absence of surface contamination are paramount
- Food & Beverage Processing: Dairy, brewing, juice, and beverage processing lines; sanitary-grade transfer coils for pasteurization and sterilization circuits
- Chemical & Petrochemical Industry: Transfer lines for aggressive media (acids, halides, chlorides); heat exchanger tubing; reactor coil inserts
- Semiconductor & High-Purity Systems: Ultra-pure water (UPW) distribution, chemical vapor deposition (CVD) gas lines, and vacuum system feedthroughs
- Medical Equipment: Endoscope channels, dialysis tubing, and implantable device manufacturing lines
- Automotive & Aerospace: Fuel lines, hydraulic circuits, and exhaust system components requiring corrosion-free internal surfaces
Unique Selling Points
- Oxidation-Free Internal Welds: The back-side argon shielding process guarantees a clean, silver-gray internal weld surface with no oxide scale formation — a prerequisite for maintaining the full corrosion resistance of the base metal
- Self-Fusing (No-Filler) Weld: Eliminates the risk of filler metal contamination, ensuring the weld metal chemistry matches the base metal exactly, preserving uniform corrosion and mechanical properties across the joint
- Precisely Controlled Gas Flow (8–15 L/min): Optimized flow rate range ensures complete oxygen displacement without turbulence-induced contamination or excessive gas consumption
- Pre- and Post-Flushing Protocol: Mandatory purging before and after welding eliminates the most common cause of weld oxidation — insufficient inert atmosphere during pool initiation and solidification
- Continuous Coil Configuration: Delivered as a coil for maximum deployment flexibility, minimizing field welds and reducing total system failure points
- Traceable Quality Assurance: Full heat-number traceability from mill to finished coil; documented WPS/WPQR for every weld configuration
- Scalable Manufacturing: Processes validated for both automated orbital welding and manual TIG, accommodating batch sizes from prototype to high-volume production
Technical Notes & Recommendations
- Gas purity below 99.99% introduces oxygen and nitrogen contamination that can cause weld porosity and reduced toughness. Only high-purity argon (≥ 99.995%) should be used.
- Back-purge gas delivery must be sealed at both ends of the tube section (using purge plugs, tail-pipe, or through-flow fixtures) to prevent gas short-circuiting and ensure full internal coverage.
- Visual inspection of the internal weld surface is mandatory. Any discoloration — even slight straw-yellow — indicates marginal protection and warrants further evaluation or rejection.
- Post-weld passivation (ASTM A967) is recommended to restore and enhance the passive chromium oxide film, particularly for 316L and higher-alloy grades.
- For cryogenic applications, ensure the weld process avoids excessive heat input to prevent the formation of brittle delta-ferrite or martensite phases in the HAZ.