Stainless Steel Cold-Bent Spiral Coil Tube — Springback-Controlled Coiltube (Product No. 1602)

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Category: Stainless Steel Coil Tube / Spiral Coil Pipe
Technical Reference: Cold-Bent Spiral Coil Tube Springback Calculation Methodology

1. Product Overview

The Stainless Steel Spiral Coil Tube (盘管, Coiltube) is a precision-engineered cold-bent tubular product formed from stainless steel seamless or welded pipe stock. Unlike conventional straight-length piping, this product is manufactured in continuous spiral or helical coil configurations, offering significant advantages in space utilization, installation efficiency, and structural performance for applications requiring compact, high-density piping arrangements.

The defining technical characteristic of this product line is the rigorous engineering control of springback (回弹量) — the elastic recovery phenomenon that occurs when cold-bent stainless steel pipe is released from the forming die. Springback is a well-documented challenge in cold-forming operations involving stainless steel due to its high yield strength, pronounced strain hardening behavior, and relatively low elastic modulus compared to its tensile strength. Wenzhou Jixing Metal Materials Co., Ltd. has developed proprietary methodologies for the accurate calculation and compensation of springback in spiral coil tube geometry, ensuring dimensional accuracy and geometric fidelity after forming.

Key Technical Insight: The springback angle in cold-bent spiral coil tubes is governed by the ratio of yield strength to elastic modulus (σy/E), the bend radius-to-wall thickness ratio (R/t), and the strain hardening exponent (n-value) of the specific stainless steel grade. Accurate prediction of these parameters enables pre-compensation in the bending tooling, resulting in coils that meet tight tolerance requirements without post-forming correction.

2. Material & Properties

2.1 Applicable Stainless Steel Grades

2.2 Mechanical Properties (Typical for 304L Grade, Annealed Condition)

Property Symbol Typical Value Relevance to Cold Bending
Tensile Strength (拉伸强度) σb 515–700 MPa Determines forming limit and springback magnitude
Yield Strength (屈服强度) σy 205–310 MPa Primary driver of elastic recovery (springback)
Elastic Modulus (弹性模量) E 193–200 GPa Lower E relative to σy increases springback
Poisson's Ratio (泊松比) ν 0.28–0.30 Affects cross-sectional deformation during bending
Strain Hardening Exponent (应变硬化指数) n 0.50–0.60 Higher n reduces springback through uniform deformation
Formability Index (成形性指数) r 1.6–2.0 Influences thinning behavior at bend apex
Maximum Cold Bend Radius (最小冷弯半径) Rmin 1.5D – 3.0D Depends on grade, wall thickness, and temper condition

2.3 Chemical Composition (AISI 304L, Representative)

Element C Mn P S Si Ni Cr N
Max/Min (%) 0.03 2.00 0.045 0.03 1.00 8.00–10.50 18.00–20.00 0.10

3. Specifications

3.1 Dimensional Specifications

Parameter Range / Specification Tolerance Standard Reference
Outer Diameter (OD) 6 mm – 323.9 mm (¼" – 12") ±0.5 mm or ±0.5% OD ASTM A213, EN 10216-5
Wall Thickness (WT) 0.5 mm – 12.7 mm (0.020" – ½") ±10% of nominal ASTM A269, ASME B36.19M
Coil Pitch (螺旋节距) Custom, typically 150 mm – 600 mm ±3 mm per pitch Per customer drawing
Coil Diameter (盘管直径) 300 mm – 3000 mm ±5 mm Per customer drawing
Bend Radius (弯曲半径) 1.5D – 10D (minimum dependent on grade) ±1% of nominal ISO 13870
Tube Length (per coil) 5 m – 500 m (continuous) ±1% Per customer drawing
End Condition Plain, beveled, threaded, or custom Per specification ASME B1.20.1
Surface Finish 2B, BA (Bright Annealed), Pickled, Polished (Ra ≤ 0.4 μm) Per specification ASTM A473

3.2 Springback Compensation Parameters

Parameter Description Typical Value (304L)
Springback Angle (回弹角, θsb) Elastic angular recovery after die release 2° – 8° (function of R/t ratio)
Springback Compensation Factor Ratio of forming bend angle to target bend angle 1.03 – 1.15
Residual Curvature Deviation Post-forming deviation from target geometry ≤ ±0.5° per bend segment
Wall Thinning at Apex Maximum thickness reduction at outer bend radius ≤ 12% of nominal WT
Ovality after Bending Cross-sectional distortion (ovality ratio) ≤ 3% of OD

4. Manufacturing Process

4.1 Process Flow

  1. Raw Material Procurement & Inspection: Seamless stainless steel tubes conforming to ASTM A213, ASTM A269, or EN 10216-5 specifications are procured from certified mills. Incoming material undergoes full chemical analysis (spectrometric), mechanical testing (tensile, hardness), and dimensional verification.
  2. Pre-Treatment & Surface Preparation: Tubes are cleaned, degreased, and optionally annealed to ensure uniform mechanical properties and optimal formability. For high-precision applications, a controlled solution anneal at 1040–1120°C followed by rapid quench is performed to maximize ductility and minimize prior work-hardening effects.
  3. Springback Calculation & Compensation Modeling: Prior to production, the springback behavior is calculated using the following methodology:
    • The neutral axis shift is determined based on the R/t ratio and material n-value, as the neutral axis migrates toward the outer radius under large plastic deformation.
    • The elastic recovery angle is computed using the formula: θsb = (σy × t) / (E × R × (1 + (t/2R)²)) where t is wall thickness, R is bend radius, E is elastic modulus, and σy is yield strength.
    • The over-bending angle is determined to compensate for predicted springback, incorporating a safety factor of 1.05–1.10 to account for material lot-to-lot variability.
    • Finite Element Analysis (FEA) using Abaqus or AutoForm is employed for complex coil geometries to simulate the full forming sequence and predict post-forming geometry with ±1° accuracy.
  4. Cold Bending & Coil Forming: Tubes are bent using a hydraulic or servo-controlled bending machine equipped with:
    • Programmable die sets for precise control of bend radius and angle
    • Internal mandrel or mandrel-free forming (depending on R/t ratio and wall thickness)
    • Real-time force monitoring to detect material anomalies
    • Compensated die angles incorporating pre-calculated springback offsets
  5. Post-Forming Inspection & Dimensional Verification: Each coil is inspected using:
    • Laser scanning for profile accuracy and springback verification
    • Caliper and micrometer measurements at critical sections
    • Geometric coordinate measurement (CMM) for pitch and coil diameter verification
    • Visual inspection for surface defects, wrinkles, or cracks at bend apex
  6. Optional Post-Forming Heat Treatment: For applications requiring stress relief or property restoration, coils may be subjected to solution annealing (1050°C, air or water quench) or stress relief annealing (400–500°C) to eliminate residual stresses introduced during cold bending.
  7. Final Surface Treatment & Packaging: Tubes receive final surface finishing (polishing, pickling, passivation per ASTM A967) and are packaged in moisture-proof wrapping with desiccant on wooden pallets or in export-grade containers.

4.2 Springback Calculation Methodology — Technical Detail

The springback phenomenon in cold-bent spiral coil tubes is governed by the elastic-plastic deformation behavior of stainless steel. The following analytical framework is applied:

4.2.1 Basic Springback Model

For a tube bent to radius R with wall thickness t, the springback angle Δθ is approximated as:

Δθ = (σy / E) × (t / R) × f(n, R/t)

Where:

4.2.2 Neutral Axis Correction

Under large plastic deformation (R/t < 10), the neutral axis shifts inward from the geometric center. The corrected neutral axis position is:

zn = z0 × (1 + n/(n+1)) × (t/2R)

This shift increases the effective moment arm and must be incorporated into the springback compensation die design.

4.2.3 Spiral Geometry Considerations

Unlike planar bending, spiral coil forming introduces additional geometric complexities: