Stainless Steel Coil Tube — Springback Compensation & Over-Bend Angle Presetting by Hardness Condition

1. Product Overview

Wenzhou Jixing Metal Materials Co., Ltd. (Jixing Metals) offers a precision-engineered stainless steel coil tube (Coil Tube) product line designed specifically for bending applications in installation and maintenance workflows. The core differentiator of this product lies in its hardness-condition-based over-bend angle presetting system — a manufacturing methodology that accounts for the elastic recovery (springback) behavior of the tube material to ensure dimensional accuracy after forming.

Springback (Elastic Recovery) is the tendency of a bent metal tube to partially revert to its original geometry upon release of the bending force. The magnitude of springback is directly governed by the material's hardness state. Jixing Metals classifies its coil tube products into soft condition (Annealed / Soft Temper) and hard condition (Cold-worked / Hard Temper) grades, each exhibiting distinct springback characteristics that are systematically compensated during the bending process.

Key Principle: By pre-setting an over-bend angle during forming — exceeding the target angle by a calculated margin — the final geometry after springback converges precisely to the required specification, eliminating the need for costly post-bending corrections.

2. Material & Properties

2.1 Material Grades

Jixing Metals coil tubes are manufactured from high-quality austenitic stainless steel grades, most commonly:

2.2 Hardness State Classification

Hardness State Designation Typical Hardness (HV) Springback Angle Over-Bend Compensation Formability
Soft / Annealed 1/4H – Soft Temper 80 – 120 HV 1° – 3° Low (minimal pre-set required) Excellent
Half-Hard 1/2H 120 – 160 HV 3° – 6° Moderate Good
Full-Hard 3/4H – Hard Temper 160 – 210 HV 5° – 10° High (significant pre-set required) Fair
Extra-Hard H – Extra Hard 210 – 260 HV 8° – 12° Very High Limited

2.3 Mechanical Properties (Typical Values for Grade 304)

Property Soft Condition (1/4H) Hard Condition (3/4H) Standard Reference
Tensile Strength (Rm) 520 – 700 MPa 700 – 900 MPa ASTM A213 / EN 10216-5
Yield Strength (Rp0.2) 205 – 310 MPa 450 – 650 MPa ASTM A213 / EN 10216-5
Elongation (A5) ≥ 40% ≥ 15% ASTM A213
Elastic Modulus (E) ~193 GPa ~193 GPa Material constant
Hardness (HV 10) 80 – 120 160 – 210 ASTM E92

3. Technical Specifications

3.1 Dimensional Specifications

Parameter Specification Range Tolerance
Outer Diameter (OD) 3.0 mm – 50.8 mm ±0.15 mm
Wall Thickness (WT) 0.3 mm – 3.0 mm ±10% of nominal
Coil Length 50 m – 1000 m (custom available) ±2%
Coil Inner Diameter 150 mm – 300 mm ±10 mm
Minimum Bend Radius 2D – 5D (D = outer diameter) —
Surface Roughness (Ra) ≤ 0.8 μm (polished); ≤ 1.6 μm (pickled) —
Straightness ≤ 1.5 mm/m —

3.2 Springback Compensation Parameters

Target Bend Angle Soft State — Over-Bend Pre-set (°) Hard State — Over-Bend Pre-set (°) Expected Springback — Soft (°) Expected Springback — Hard (°)
90° 91° – 93° 95° – 100° 1 – 3 5 – 10
45° 46° – 48° 50° – 55° 1 – 3 5 – 10
180° 181° – 183° 185° – 190° 1 – 3 5 – 10
Important Note: The springback values listed above are empirical reference data. Actual springback varies with tube diameter, wall thickness ratio, bend radius, and bending speed. First-article trial bending (First-Piece Calibration) is mandatory prior to any batch production run to empirically determine the exact over-bend angle for the specific configuration.

4. Manufacturing Process

4.1 Production Workflow

  1. Raw Material Selection: Selection of stainless steel strip (band) of appropriate grade (e.g., 304, 316L) from certified mills. Incoming material undergoes spectroscopic verification (PMI — Positive Material Identification) for chemical composition confirmation.
  2. Cold Rolling & Hardening: The strip is cold-rolled to the desired thickness. The degree of cold work determines the final hardness state — minimal reduction yields soft temper; high reduction yields hard temper. Controlled rolling schedules ensure uniform hardness across the coil.
  3. Coil Forming (Spiral Winding): The flat strip is spirally wound onto a mandrel to form the coil tube geometry. Precision mandrel sizing ensures accurate outer diameter and wall thickness consistency.
  4. Welding (if applicable): For welded coil tubes, the spiral seam is welded using Automatic Underwater TIG (A-UTIG) or Orbital TIG welding to ensure a high-quality, corrosion-resistant weld seam.
  5. Pickling & Passivation: The coil tube undergoes pickling to remove scale and discoloration, followed by passivation to restore the chromium oxide protective layer.
  6. Hardness Verification: Each coil is hardness-tested using a portable Vickers or Rockwell hardness tester. The measured hardness value determines which springback compensation profile will be applied during downstream bending operations.
  7. First-Article Trial Bending (First-Piece Calibration):
    • A sample segment is cut from each coil lot.
    • The sample is bent to the target angle using standard bending equipment.
    • The actual springback angle is measured using a protractor gauge or coordinate measuring machine (CMM).
    • The over-bend compensation value is calculated: Over-bend angle = Target angle + Measured springback angle.
    • The calibrated compensation value is recorded and programmed into the bending equipment for batch production.
  8. Batch Bending Production: With the calibrated over-bend angle programmed, the coil tube is fed through the bending machine. The machine applies the pre-set over-bend angle, and upon release, the springback naturally returns the tube to the exact target geometry.
  9. Post-Bend Inspection: Finished bent tubes are inspected for dimensional accuracy (angle, radius, straightness), surface integrity, and absence of defects (wrinkling, cracking, ovality).

4.2 Springback Compensation Methodology

The springback compensation methodology employed by Jixing Metals is based on the fundamental relationship between material hardness and elastic recovery. The governing equation for springback in tube bending can be expressed as:

θspringback = f(HV, σy, E, R/D, t/D)

Where:

As hardness increases, the yield strength rises proportionally, resulting in greater elastic strain energy stored during bending and consequently larger springback upon release. This is why hard-state tubes (5–10° springback) require significantly more over-bend compensation than soft-state tubes (1–3° springback).

5. Applications

The coil tube products with hardness-condition-based over-bend compensation are extensively utilized across multiple industries:

5.1 Installation & Maintenance Applications

5.2 Additional Industrial Applications

6. Quality Certifications & Compliance

6.1 Standards Compliance

Standard Scope Applicability
ASTM A213 Welded Austenitic Stainless Steel Tubes for High-Temperature Service Material specification
ASTM A778 Welded Austenitic Stainless Steel Tubes for General Use Material specification
EN 10216-5 Non-alloy and Alloy Steel Tubes — Technical Delivery Conditions — Part 5: Austenitic Stainless Steel Tubes European material specification
ISO 3506 Fasteners — Mechanical Properties — Stainless Steel Reference for mechanical properties
ASME B31.3 Process Piping Installation code compliance
3-A Sanitary Standards Hygienic Design for Food and Dairy Equipment Food-grade applications

6.2 Quality Management & Certifications

6.3 Quality Assurance Protocol for Bending Operations