Stainless Steel Coil Tube — Ovality Dimensional Specification
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 114) manufactures precision-engineered stainless steel coil tubes (盘管 / Coil Tube) designed for demanding industrial applications requiring superior dimensional accuracy. This technical specification addresses the critical geometric parameter of ovality (椭圆度) — the deviation of the cross-sectional profile from a perfect circle — which is recognized as the primary geometric cause of ferrule seal failure (卡套密封失效) in instrument-grade tubing systems.
Ovality directly governs the uniformity of contact pressure between the tube end and the ferrule sealing surfaces. Excessive ovality results in non-uniform compression, leading to micro-gaps, stress concentration, and ultimately catastrophic seal integrity loss under pressure cycling or thermal cycling conditions.
Material & Properties
Coil tubes supplied under this specification are typically fabricated from austenitic stainless steel grades including 304 (UNS S30400), 316 (UNS S31600), and 321 (UNS S32100), selected for their excellent corrosion resistance, formability, and dimensional stability during cold working processes. The material's elastic recovery characteristics influence post-draw ovality retention, making material grade a contributing factor to final dimensional accuracy.
- Material Grades: SUS 304, 316, 321, 316L, 904L
- Alloying Philosophy: Chromium (Cr) ≥ 18%, Nickel (Ni) ≥ 8% (austenitic base)
- Formability: High elongation-to-thickness ratio enabling tight coil winding without work hardening
- Dimensional Stability: Low residual stress after cold drawing and annealing
Specifications — Ovality Dimensional Inspection
Ovality Measurement Formula
Ovality (%) = (Dmax − Dmin) / Dnom × 100%
Where:
- Dmax — Maximum cross-sectional diameter (mm)
- Dmin — Minimum cross-sectional diameter (mm)
- Dnom — Nominal (nominal) outer diameter of the tube (mm)
Acceptance Criteria
| Parameter | Grade / Classification | Maximum Ovality | Applicable Standard | Typical Application |
|---|---|---|---|---|
| Ovality (椭圆度) | Precision Grade (精密级) | ≤ 3% | ASTM A213 / GB/T 12771 | Instrumentation, hydraulic systems, medical devices |
| Ovality (椭圆度) | Standard Grade (普通级) | ≤ 5% | ASTM A213 / GB/T 12771 | General process piping, structural applications |
Supplementary Dimensional Parameters
| Dimensional Parameter | Precision Grade | Standard Grade | Measurement Method |
|---|---|---|---|
| Outer Diameter Tolerance (外径公差) | ±0.05 mm | ±0.10 mm | Optical comparator / CMM |
| Wall Thickness Uniformity (壁厚均匀性) | ±10% | ±15% | Ultrasonic thickness gauge |
| Roundness (圆度) | ≤ 2% | ≤ 4% | Profile projector |
| Coil Straightness (直度) | ≤ 1.0 mm/m | ≤ 2.0 mm/m | Surface plate / laser alignment |
Manufacturing Process
The ovality control in Wenzhou Jixing's coil tube production follows a multi-stage cold working process optimized to minimize cross-sectional distortion:
- Hot Rolling (热轧): Initial production of stainless steel strip or seamless tube blank with controlled thickness tolerance.
- Cold Drawing / Cold Forming (冷拔/冷成型): The tube is drawn through precision-die tooling to achieve target dimensions. Die geometry and lubrication are critical to ovality control.
- Intermediate Annealing (中间退火): Stress relief annealing at 1050–1100°C followed by rapid cooling (pickling and passivation) to eliminate residual stresses that cause dimensional drift.
- Final Cold Drawing (最终冷拔): Final dimensional pass through calibrated tungsten carbide (WC) dies to achieve target ovality within specification.
- Coil Winding (盘卷): Precision-controlled winding onto mandrels with uniform tension to prevent post-forming ovality growth.
- Dimensional Inspection (尺寸检测): In-line and end-of-line ovality verification using optical or ultrasonic measurement systems at defined intervals (typically every 3–5 meters).
Key Process Parameters Affecting Ovality
- Die Alignment: Misalignment of drawing die exceeds 0.02 mm, introducing asymmetric deformation
- Pull Tension: Excessive tension during cold drawing causes springback asymmetry
- Material Work Hardening: Excessive strain hardening increases residual stress, promoting ovality growth during coil winding
- Winding Mandrel Diameter: Mandrel diameter relative to tube diameter ratio (Dmandrel/Dtube) must exceed minimum springback threshold
Applications
Coil tubes with controlled ovality are essential in the following application domains:
- Instrumentation & Automation: Pressure transmitters, flow meters, and level gauges where ferrule-type fittings (卡套接头) require uniform sealing contact
- Hydraulic & Pneumatic Systems: High-pressure fluid conveyance where seal integrity is safety-critical
- Medical Device Manufacturing: Endoscopic tubing, implantable device housings, and pharmaceutical fluid transfer lines
- Oil & Gas Industry: Downhole instrumentation, wellhead connections, and subsea control lines
- Food & Beverage Processing: Sanitary process piping requiring CIP/SIP compatibility and leak-free connections
- Automotive & Aerospace: Fuel lines, brake lines, and hydraulic actuator tubing
Why Ovality Control Is Critical for Ferrule Sealing
Ferrule (ferrule-type) fittings achieve leak-tight seals through controlled compression of a hardened ferrule against the tube's outer diameter. The seal relies on uniform circumferential contact pressure. When ovality exceeds specification:
- High points of the oval profile experience over-compression, causing ferrule deformation and material fatigue
- Low points create gaps that permit fluid ingress/egress under pressure
- Asymmetric stress distribution accelerates fretting corrosion at the tube-ferrule interface
- Thermal cycling exacerbates differential expansion, widening gaps at low-stress regions
Therefore, ovality is classified as the primary geometric root cause of ferrule seal failure in field service.
Quality Certifications & Inspection Protocols
Applicable Standards
- ASTM A213 / A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing
- GB/T 12771 — Chinese National Standard for Welded Stainless Steel Tubes
- ISO 15497-1 — Stainless Steel Tubing for Instrumentation
- ASME B31.3 — Process Piping (applicable for pressure vessel connections)
- EN 10217-7 — European Standard for Stainless Steel Tubes
Factory Quality Assurance Program
| Inspection Stage | Test Method | Frequency | Acceptance Criteria |
|---|---|---|---|
| Raw Material Receipt | Mill certificate verification, chemical analysis | Each heat lot | Composition per ASTM/GB grade spec |
| In-Process (Post-Drawing) | Optical dimensional measurement (OD, WT, ovality) | Every 3–5 m along coil | Ovality ≤ grade limit; OD ± tolerance |
| Post-Annealing | Dimensional re-check, surface inspection | Each coil | No dimensional drift > 0.5% |
| Final Inspection | CMM / laser scanning ovality profile | 100% or AQL sampling | Precision ≤3%; Standard ≤5% |
| Hydrostatic Pressure Test | Water pressure test at 1.5× working pressure | Each coil | No leakage, no permanent deformation |
Documentation Provided
- Mill Test Certificate (MTC / 材质证明书) — EN 10204 3.1
- Dimensional Inspection Report with ovality trace data
- Heat treatment and pickling/passivation certificate
- Batch traceability documentation (lot number, coil ID, production date)
Unique Selling Points
- Industry-Leading Ovality Control: Precision grade coils achieve ovality ≤ 3% consistently, exceeding the 5% tolerance common in general-purpose coil tube production
- Ferrule Seal Reliability: Directly addresses the root cause of field failures in ferrule-fitting connections, reducing maintenance costs and system downtime
- Full Traceability: Each coil carries complete dimensional trace data from raw material through final inspection
- Grade Flexibility: Dual-grade offering (Precision / Standard) allows customers to match specification to application criticality and cost requirements
- Comprehensive Material Range: Available in 304, 316, 321, 316L, and 904L grades to suit diverse corrosion environments
- Coil Format Advantage: Continuous coil format eliminates field welding, reduces installation time, and preserves material integrity throughout the tubing run
Technical Summary
| Attribute | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (盘管) |
| Product No. | 114 |
| Critical Inspection Parameter | Ovality: (Dmax − Dmin) / Dnom × 100% |
| Precision Grade Limit | ≤ 3% |
| Standard Grade Limit | ≤ 5% |
| Functional Significance | Primary geometric cause of ferrule (卡套) seal failure |
| Manufacturer | Wenzhou Jixing Metal Materials Co., Ltd. |