Seamless Stainless Steel Coil Tube — Extrusion + Cold Drawing Process

Product Category: Manufacturing Process → Forming Process → Seamless Pipe

Product Code: 52

Supplier: Wenzhou Jixing Metal Materials Co., Ltd.

1. Product Overview

The Seamless Stainless Steel Coil Tube produced via the Extrusion + Cold Drawing process represents a high-precision forming solution specifically engineered for small- to medium-diameter seamless coil tube applications. Unlike conventional hot-rolled piercing methods, this dual-stage manufacturing approach—combining warm or hot extrusion with subsequent multi-pass cold drawing—delivers exceptional surface finish quality, tight dimensional tolerances, and superior microstructural uniformity.

Coil tubes (盘管), supplied in continuous helical or coiled configurations, are particularly advantageous for applications requiring long, uninterrupted tube lengths without welded joints. The extrusion + cold drawing process is the preferred manufacturing route for the parent pipe stock from which these high-precision coil tubes are formed, offering a decisive quality advantage over hot-rolled pierced alternatives.

2. Material & Properties

The coil tubes are manufactured from a range of austenitic, ferritic, duplex, and precipitation-hardening stainless steel grades, selected according to the target application's corrosion resistance, mechanical strength, and temperature requirements.

2.1 Commonly Available Grades

2.2 Mechanical Properties (Typical for SUS304 after Cold Drawing)

Property Specification Test Standard
Tensile Strength (抗拉强度) ≥ 520 MPa (75,400 psi) ASTM A269 / GB/T 12771
Yield Strength (屈服强度) ≥ 205 MPa (29,730 psi) ASTM A269 / GB/T 12771
Elongation (延伸率) ≥ 40% ASTM A269 / GB/T 12771
Hardness (硬度) ≤ 92 HRB ASTM A269 / GB/T 12771

2.3 Surface Finish Quality

A defining characteristic of the extrusion + cold drawing process is the superior surface quality achieved relative to hot-rolled piercing. The cold drawing stage refines the surface through controlled die contact, eliminating scale, oxide inclusions, and surface irregularities inherent to hot-rolled products. Surface roughness values as low as Ra ≤ 0.4 μm are achievable in the final cold-drawn condition, with optional finishing to achieve polished (Ra ≤ 0.2 μm) or electropolished surfaces.

3. Specifications

3.1 Dimensional Specifications

Parameter Specification / Range Standard Reference
Outer Diameter (OD) Range Φ3.0 mm – Φ114.3 mm (1/8" – 4-1/2") ASTM A269 / GB/T 12771
Wall Thickness Range 0.3 mm – 12.0 mm ASTM A269 / GB/T 12771
OD Tolerance ±0.1 mm (Precision) / ±0.25 mm (Standard) ASTM A269
Wall Thickness Tolerance ±0.05 mm (Precision) / ±0.15 mm (Standard) ASTM A269
Coil Inner Diameter (ID) Φ500 mm – Φ1500 mm (customizable) Customer Specification
Coil Length (per loop) 2 m – 10 m (adjustable per application) Customer Specification
Coil Total Length Up to 500 m per coil Customer Specification
Surface Condition Mill Finish / Pickled & Passivated / Polished / Electropolished ASTM A269 / GB/T 12771
Delivery Condition Cold Worked (Cold Drawn) / Solution Annealed ASTM A269

3.2 Applicable Standards

Standard Designation Scope
ASTM A269 / A269M American Welded and Seamless Austenitic Stainless Steel Tubing
ASTM A213 American Boiler, Heat Exchanger, and Heat Transfer Equipment Tubing
GB/T 12771 Chinese Stainless Steel Seamless Tubes for General Application
GB/T 14976 Chinese Stainless Steel Seamless Tubes for Fluid Transport
JIS G3459 Japanese Stainless Steel Seamless Tubes
EN 10217 European Non-Alloy and Alloy Steel Tubes for Mechanical and Thermal Purposes
ISO 11431 International Stainless Steel Tubes for General Purposes

4. Manufacturing Process

The Extrusion + Cold Drawing process is a two-stage forming methodology that combines the shape-forming capabilities of extrusion with the precision-finishing advantages of cold drawing. This process is specifically selected for small- to medium-diameter high-precision seamless coil tube production, where surface quality and dimensional accuracy are critical.

4.1 Stage 1 — Extrusion (挤压)

  1. Raw Material Preparation: Stainless steel billet or rod (typically Φ40 mm – Φ150 mm) is inspected, cut to length, and surface-degreased.
  2. Heating: The billet is heated to a controlled temperature range (typically 900°C – 1200°C depending on grade) in a controlled-atmosphere furnace to minimize surface oxidation and scaling.
  3. Extrusion Forming: The heated billet is forced through a calibrated die system using a hydraulic or mechanical extrusion press. The extrusion ratio is carefully controlled to achieve the desired intermediate tube dimensions while maintaining favorable grain flow patterns.
  4. Intermediate Annealing: The extruded tube is annealed to relieve residual stresses and restore ductility prior to the cold drawing stage.
  5. Pickling & Passivation: Any residual oxide scale is removed via acid pickling, followed by passivation to establish a protective chromium oxide layer.

4.2 Stage 2 — Cold Drawing (冷拔)

  1. Die Selection & Preparation: Precision tungsten carbide or high-speed steel dies are selected and honed to achieve the target final dimensions and surface finish.
  2. Multi-Pass Drawing: The tube is pulled through successive die stations in multiple passes (typically 2–6 passes depending on the reduction ratio). Each pass reduces the OD and/or wall thickness incrementally, with controlled reduction rates to prevent deformation and maintain dimensional accuracy.
  3. Lubrication: Appropriate drawing lubricants are applied to minimize friction, reduce die wear, and enhance surface finish quality.
  4. Inter-Pass Inspection: Dimensional measurements and surface quality assessments are performed between passes to ensure process control.
  5. Final Cold Drawing: The last drawing pass achieves the final dimensions with the tightest tolerances, leveraging the work-hardening effect of cold working to enhance surface smoothness and mechanical properties.

4.3 Stage 3 — Post-Processing & Coil Forming

  1. Final Annealing (if required): Solution annealing may be performed to restore full ductility for applications requiring formability, or the tube may be delivered in the cold-worked condition for enhanced strength.
  2. Final Pickling & Passivation: Ensures a clean, corrosion-resistant surface free of drawing lubricant residues.
  3. Coil Forming: The straight seamless tube is carefully coiled into the desired coil configuration using precision coiling equipment to avoid deformation, kinking, or surface damage.
  4. Final Inspection: Comprehensive dimensional, surface, and mechanical testing is performed on the finished coil tube.
  5. Packaging: Coils are wrapped with protective material and secured for transport.

4.4 Process Advantage Over Hot-Rolled Piercing (热轧穿孔)

Characteristic Extrusion + Cold Drawing Hot-Rolled Piercing
Surface Roughness (Ra) ≤ 0.4 μm (as-drawn); ≤ 0.2 μm (polished) 1.6 μm – 6.3 μm (typical)
OD Tolerance ±0.1 mm (Precision) ±0.5 mm – ±1.0 mm
Wall Thickness Tolerance ±0.05 mm (Precision) ±0.25 mm – ±0.5 mm
Surface Defects Minimal; no scale or oxide inclusions Scale, surface cracks, and oxide inclusions common
Microstructure Uniformity Excellent; controlled grain flow Adequate; potential for segregation
Applicable Diameter Range Small to Medium (Φ3.0 mm – Φ114.3 mm) Medium to Large (Φ20 mm – Φ3000 mm)
Cost Efficiency Higher unit cost; justified by quality Lower unit cost for large diameters

5. Applications

The high surface quality and dimensional precision of extrusion + cold drawing seamless coil tubes make them suitable for demanding applications across multiple industries: