Stainless Steel Coil Tube — Springback Compensation Bending Technology
Product Overview
The Stainless Steel Coil Tube (Coil Tube / Coiltube) produced by Wenzhou Jixing Metal Materials Co., Ltd. represents a precision-engineered cold-formed tubular product designed for downstream bending applications. A critical technical differentiator of this product lies in its springback compensation methodology — the systematic presetting of over-bend angles based on the material's hardness state prior to forming operations. Springback (回弹), the elastic recovery of a material after the removal of forming forces, is a fundamental challenge in tube bending that directly affects dimensional accuracy and part quality. By characterizing the springback behavior across different hardness conditions and compensating through controlled over-bending, Wenzhou Jixing ensures that finished bent components meet tight geometric tolerances without the need for secondary correction.
Material & Properties
The coil tube is manufactured from austenitic stainless steel grades, primarily SUS304 (UNS S30400) and SUS316 (UNS S31600), supplied in a continuous coil configuration for high-volume forming operations. The material's mechanical state — whether annealed (soft/soft-annealed) or cold-worked (hard/hard-drawn) — fundamentally governs the springback magnitude during bending. The following table summarizes the critical material-dependent springback characteristics:
| Property | Soft State (Annealed / Soft-Annealed) | Hard State (Cold-Worked / Hard-Drawn) |
|---|---|---|
| Hardness Condition | Full Annealed (FA) / Solution Annealed | Cold-Worked Hard (CW-H) / Hard-Drawn |
| Springback Angle (回弹角) | 1° – 3° | 5° – 10° |
| Yield Strength (Typical) | 205 – 310 MPa | 310 – 520 MPa |
| Ultimate Tensile Strength (Typical) | 515 – 720 MPa | 620 – 830 MPa |
| Elongation (Typical) | ≥ 40% | 15% – 30% |
| Over-Bend Compensation Required | Low — minimal angle preset | High — significant angle preset required |
| Recommended Bending Method | Standard mandrel bending | Mandrel bending with enhanced die force |
Specifications
| Parameter | Specification |
|---|---|
| Product Category | Installation & Maintenance — Bending Processing — Springback Compensation |
| Product Type | Stainless Steel Coil Tube (Coiltube) |
| Item No. | 217 |
| Material Grades | SUS304, SUS316, SUS321, SUS316L (per customer specification) |
| Supply Form | Continuous coil / Coil-wound lengths |
| Hardness States Available | Soft-Annealed (SA), Half-Hard (1/2H), Hard-Drawn (HD) |
| Springback — Soft State | 1° – 3° (low elastic recovery) |
| Springback — Hard State | 5° – 10° (high elastic recovery) |
| Compensation Method | Hardcoded over-bend angle preset per hardness condition |
| First-Piece Protocol | Mandatory trial bend and springback calibration before batch production |
| Dimensional Tolerance | ±0.5° on final bend angle after springback compensation |
Manufacturing Process
3.1 Springback Characterization and Classification
Upon receipt of coil tube material, the hardness state is verified through Rockwell hardness testing (HRB/HRC) and correlated to the expected springback range. The material is classified into one of two primary categories:
- Soft-Annealed (SA) State: Materials in this condition exhibit minimal elastic recovery upon unloading, with springback angles in the range of 1° to 3°. The over-bend compensation angle is correspondingly small, and standard bending parameters suffice.
- Hard-Drawn (HD) State: Cold-worked materials display significantly greater elastic recovery, with springback angles reaching 5° to 10°. These require substantially larger over-bend compensation angles to achieve the target final geometry.
3.2 First-Piece Trial Bending and Springback Calibration
A defining quality protocol in the manufacturing workflow is the first-piece trial bend procedure (首件试弯标定). Before commencing any batch production run, a single representative tube segment is bent at the nominal target angle without compensation. The actual springback magnitude is then measured precisely using angular gauges or coordinate measuring instruments (CMM). This measured value establishes the empirical springback delta for that specific coil lot, hardness condition, tube diameter, wall thickness, and bend radius combination. The over-bend compensation angle is calculated as:
Over-Bend Angle = Target Final Angle + Measured Springback Angle
This calibrated compensation value is then input into the bending machine's control system for the entire batch, ensuring uniformity and repeatability across all produced parts.
3.3 Batch Bending with Preset Compensation
Once the springback compensation value is established, the bending machine is configured to bend each tube segment to the pre-calculated over-bend angle. Upon release from the die, the elastic recovery naturally brings the final angle into conformity with the target specification. This method eliminates the need for post-bend correction operations such as press-brake rework or manual adjustment, thereby improving throughput and reducing scrap rates.
3.4 Process Flow Summary
- Material Receipt & Hardness Verification — Rockwell hardness testing to confirm SA or HD classification
- Springback Range Assignment — Map hardness to expected springback range (1–3° for SA; 5–10° for HD)
- First-Piece Trial Bend — Bend one specimen at nominal angle; measure actual springback
- Compensation Calibration — Calculate and input over-bend angle into machine controller
- Batch Production — Execute high-volume bending with preset compensation
- In-Process Inspection — Periodic angular verification to confirm compensation accuracy
- Final Dimensional Verification — Statistical sampling against tolerance specifications
Applications
The springback-compensated coil tube bending technology is applicable across a broad range of industrial and commercial sectors where precision bent stainless steel tubing is required:
- Automotive Industry: Fuel lines, brake lines, exhaust system components, and structural brackets where precise bend angles are critical for fitment and function.
- Aerospace: Hydraulic lines, pneumatic tubing, and fuel system components requiring strict dimensional control and corrosion resistance.
- Medical Equipment: Tubing for surgical instruments, dialysis machines, and medical gas delivery systems where surface integrity and geometric accuracy are paramount.
- Food Processing & Pharmaceutical: Hygienic piping systems, sanitary fittings, and heat exchanger coils requiring corrosion-resistant materials and precise geometry.
- Furniture & Architectural: Handrails, decorative trim, structural supports, and custom fabrication where both aesthetics and dimensional accuracy are important.
- Electronics & Energy: Heat sink tubing, connector housings, and battery pack structural components in electric vehicles and energy storage systems.
Quality Certifications & Standards
Wenzhou Jixing Metal Materials Co., Ltd. adheres to internationally recognized quality management and product standards to ensure consistent performance and reliability:
- ISO 9001:2015 — Quality Management System certification covering the full production lifecycle from raw material procurement through finished product delivery.
- ISO 14001:2015 — Environmental Management System ensuring sustainable manufacturing practices.
- ASTM A213 / ASTM A269 — Compliance with standard specifications for seamless and welded austenitic stainless steel tubing.
- GB/T 12771 — Chinese national standard for welded austenitic stainless steel tubes for general applications.
- EN 10217 — European standard for stainless steel tubes for pressure purposes.
- JIS G3459 — Japanese Industrial Standard for austenitic stainless steel tubing.
Each production batch undergoes rigorous quality control including hardness verification, dimensional inspection (OD, wall thickness, flatness), bend angle verification against compensated targets, and surface finish evaluation. Full traceability is maintained through heat number documentation and batch tracking systems.
Unique Selling Points
- Empirical Springback Calibration: Unlike generic compensation approaches, Wenzhou Jixing employs a lot-specific first-piece trial bend protocol that accounts for actual material behavior rather than relying solely on theoretical springback models. This ensures accuracy even when material properties vary between coil lots.
- Dual-State Springback Database: The company maintains a documented springback database spanning soft (1–3°) and hard (5–10°) conditions across multiple tube diameters, wall thicknesses, and bend radii, enabling rapid initial compensation estimation and reduced setup time.
- Elimination of Secondary Correction: By integrating springback compensation directly into the bending process, the need for post-bend angular correction is eliminated, resulting in faster cycle times, lower labor costs, and reduced risk of surface damage from rework.
- Consistent Batch Uniformity: The calibrated over-bend angle is applied uniformly across the entire batch, ensuring that every tube segment produced within a run meets the same tight angular tolerance (±0.5°).
- Scalability: The coil tube supply format combined with automated bending with preset compensation enables high-volume production suitable for OEM and contract manufacturing programs.