Stainless Steel Coil Tube with Post-Bending Local Solution Heat Treatment Control
Product Overview
The Stainless Steel Coil Tube (Coiltube) produced by Wenzhou Jixing Metal Materials Co., Ltd. represents a precision-engineered tubular product designed for high-performance applications requiring both formability and post-forming metallurgical integrity. This product is distinguished by its advanced post-bending local solution heat treatment process, which ensures the restoration and preservation of mechanical properties, corrosion resistance, and microstructural homogeneity in regions subjected to plastic deformation during the bending operation. The coil tube format enables efficient material utilization, continuous processing, and cost-effective delivery in bulk configurations.
Unlike conventional coil tubes that may exhibit localized degradation in mechanical properties and increased susceptibility to stress corrosion cracking (SCC) at bent sections, Wenzhou Jixing's proprietary heat treatment protocol addresses these deficiencies through controlled, localized thermal cycling. This approach minimizes thermal distortion to unaffected regions while fully annealing the work-hardened bend zones, delivering a product with uniform performance characteristics throughout its entire length.
Material & Properties
Base Material Grades
- Austenitic Stainless Steels: Type 304 (UNS S30400/S30403), Type 304L (UNS S30403), Type 316 (UNS S31600/S31603), Type 316L (UNS S31603), Type 321 (UNS S32100/S32101), Type 347 (UNS S34700/S34701)
- Super-Austenitic Grades: Type 904L (UNS N08904), Type 2205 Duplex (UNS S32205), Type 2507 Super Duplex (UNS S32750)
- Custom Alloy Compositions: Available upon request for specialized applications
Mechanical Properties (Post Heat Treatment)
- Tensile Strength (UTS): 515–720 MPa (75,000–104,000 psi), depending on grade and condition
- Yield Strength (0.2% Offset): 205–450 MPa (30,000–65,000 psi)
- Elongation at Break: ≥40% (minimum), typically 50–60%
- Hardness: ≤230 HV (Brinell equivalent ≤95 HB)
- Impact Toughness (Charpy V-Notch, 25°C): ≥275 J for austenitic grades
Corrosion Resistance
- Pitting Resistance (PREN): 19–33 (varies by grade; Type 316L: PREN ≈ 24–26; Type 2205: PREN ≈ 33)
- Stress Corrosion Cracking Resistance: Significantly enhanced through post-bend solution treatment eliminating residual work-hardening
- Intergranular Corrosion Resistance: Verified per ASTM A262 Practice 1E (acid immersion test)
- General Corrosion Rate: <0.05 mm/year in most aqueous environments
Chemical Composition (Typical – Type 316L)
| Element | Specification (wt%) | ASTM A269 / A270 Reference |
|---|---|---|
| Carbon (C) | ≤0.030 | Max 0.030 |
| Chromium (Cr) | 16.0 – 18.0 | 16.0 – 18.0 |
| Nickel (Ni) | 10.0 – 14.0 | 10.0 – 14.0 |
| Molybdenum (Mo) | 2.0 – 3.0 | 2.0 – 3.0 |
| Manganese (Mn) | ≤2.0 | Max 2.0 |
| Silicon (Si) | ≤1.0 | Max 1.0 |
| Phosphorus (P) | ≤0.045 | Max 0.045 |
| Sulfur (S) | ≤0.030 | Max 0.030 |
| Iron (Fe) | Balance | Balance |
Specifications
Dimensional Specifications
| Parameter | Specification | Tolerance | Reference Standard |
|---|---|---|---|
| Outer Diameter (OD) | 3.18 mm – 168.3 mm (⅛" – 6") | ±0.10 mm (±0.004") | ASTM A269 / A270 |
| Wall Thickness | 0.25 mm – 12.70 mm (0.010" – 0.500") | ±10% of nominal | ASTM A269 / A270 |
| Length (Coil Length) | 50 m – 300 m (164 ft – 984 ft) | ±5% | Manufacturers' Specification |
| Coil Inner Diameter | 150 mm – 500 mm (6" – 20") | ±10 mm | Manufacturers' Specification |
| Coil Outer Diameter | 300 mm – 1200 mm (12" – 48") | ±15 mm | Manufacturers' Specification |
| Minimum Bend Radius | 3D (3× Outer Diameter) | — | ASTM A213 / ASME B31.3 |
| Surface Roughness (Ra) | ≤0.8 μm (32 μin) – Pickled & Passivated | — | ASTM A403 / A666 |
| Surface Roughness (Ra) | ≤0.4 μm (16 μin) – Electropolished | — | ASTM A666 |
| Straightness | ≤1.5 mm/m (0.060"/ft) | — | ASTM A269 |
| Roundness | ≤2% of wall thickness | — | ASTM A269 |
Available Surface Finishes
| Finish Type | Description | Typical Application |
|---|---|---|
| Pickled & Passivated (P&P) | Acid pickling followed by passivation treatment | General industrial, chemical processing |
| Mechanically Polished (No. 4) | Linear polish, 100–220 grit equivalent | Food processing, architectural |
| Mirror Polished (8K) | Highly reflective surface, Ra ≤ 0.1 μm | Decorative, pharmaceutical |
| Electropolished (EP) | Electrochemical polishing, Ra ≤ 0.4 μm | Medical, semiconductor, hygienic |
| Baused & Passivated | Bead blasting followed by passivation | Uniform matte appearance |
| Industrial Annealed (2B) | Standard annealed finish | General structural applications |
Manufacturing Process
Process Flow
- Raw Material Receiving & Inspection: Incoming stainless steel coil or strip is verified for chemical composition (spectrometric analysis), mechanical properties, and surface condition per ASTM A269/A270 requirements.
- Tube Forming (Cold Drawing or Cold Rolling): The coil strip is formed into seamless tubing via cold drawing (plugging method or cross-roll) or cold rolling (seamless or welded depending on configuration). This establishes the initial tube geometry with controlled dimensional tolerances.
- Intermediate Annealing: Full solution annealing at 1010–1150°C (1850–2100°F) with rapid water quench to restore ductility for subsequent forming operations. This dissolves carbides and recrystallizes the deformed microstructure.
- Pickling & Passivation: Removal of scale and oxide films formed during annealing through controlled immersion in nitric/hydrofluoric acid mixtures, followed by passivation to establish a protective chromium oxide layer (Cr₂O₃).
- Coil Winding: The processed tubing is wound onto precision-machined coil cores with controlled tension to prevent deformation, scratching, or kinking.
- Bending Operations: The coil tube is bent to specified geometries (U-bends, hairpins, custom profiles) using CNC-controlled bending machines. Cold bending introduces plastic deformation, work hardening, and residual stresses in the bend regions.
- Post-Bending Local Solution Heat Treatment (Proprietary Process): This is the critical differentiating step of Wenzhou Jixing's process. The bent sections are subjected to localized thermal treatment as described below.
- Final Inspection & Testing: Comprehensive quality verification including dimensional measurement, hydrostatic testing, surface inspection, and metallurgical evaluation.
- Coil Re-winding & Packaging: Treated tubes are re-coiled with protective wrapping, moisture barriers, and shipping documentation.
Post-Bending Local Solution Heat Treatment – Detailed Process Description
This proprietary process is the cornerstone of Wenzhou Jixing's technical expertise and represents a significant advancement in coil tube manufacturing. The following subsections detail the methodology:
1. Rationale & Metallurgical Basis
During cold bending of austenitic stainless steel, the material undergoes significant plastic deformation (typically 20–60% strain at the outer fiber of the bend). This results in:
- Work Hardening: Dislocation density increases dramatically, raising local hardness by 40–80% relative to the parent material. This creates a mechanically weak point under cyclic loading.
- Transformation-Induced Martensite (TRIP): In metastable austenitic grades (e.g., Type 304), severe deformation can induce α'-martensite formation, which is less corrosion resistant and more brittle than austenite.
- Residual Stress: Tensile residual stresses develop at the outer bend radius and compressive stresses at the inner radius. These can exceed 200 MPa, significantly reducing resistance to stress corrosion cracking.
- Grain Distortion: Austenite grains become elongated and distorted along the deformation direction, creating anisotropic mechanical behavior.
2. Heat Treatment Parameters
| Parameter | Specification | Control Method |
|---|---|---|
| Solution Treatment Temperature | 1020°C – 1120°C (1868°F – 2048°F), optimized per grade | Pyrometric monitoring with ±5°C accuracy |
| Soak Time | 10 – 30 minutes (depending on wall thickness and section geometry) | Timed controller with automated logging |
| Heating Rate | 50°C/min – 100°C/min (controlled to minimize thermal gradient) | Induction or gas-fired localized heating |
| Quench Method | Water quench or air cool (air cool for thick sections >6 mm wall) | Automated quench system with temperature monitoring |
| Heating Zone Width | 1.5× – 2.0× tube OD beyond bend region | Shielded heating apparatus with heat sink boundaries |
| Post-Treatment Surface Condition | Re-passivation required if discoloration exceeds 0.5 μm oxide | Visual inspection + eddy current verification |
3. Heat Treatment Methodology
The local heat treatment employs one or more of the following techniques, selected based on coil configuration, bend geometry, and production volume:
- Induction Heating: High-frequency induction coils are positioned around the bend region, providing rapid, localized heating with excellent thermal confinement. Typical power: 50–200 kW; frequency: 20–300 kHz. This method offers precise control and minimal thermal spread to unaffected tube sections.
- Gas-Fired Localized Heating: Propane or natural gas burners with reflective shields direct heat to the bend zone. Suitable for thicker walls and larger diameters where induction skin depth is insufficient.
- Resistance Heating (for welded tubes): Direct current passing through the tube at the bend region generates uniform Joule heating. Requires electrical contact fixtures.
- Flame Heating with Infrared Monitoring: Oxygen-fuel flame (acetylene/ethylene) with real-time pyrometer feedback for temperature control. Used for field applications or large-diameter tubes.
4. Quality Verification of Heat Treatment
- Hardness Mapping: V