Differences Between Cold Drawn and Cold Rolled Seamless Steel Tubes in Manufacturing Process

1. Core Deformation Principle (The Fundamental Difference)

Cold Drawn (CD)

Force condition: Uniaxial axial tension. The steel tube is pulled through a fixed outer die with an internal mandrel to reduce outer diameter and wall thickness via stretching.
Deformation stress: Dominated by tensile stress. Metal flows unidirectionally along the axial direction, similar to hand-pulled noodles.
Single-pass deformation capacity: Section shrinkage rate only 10%~20%, limited forming capacity, requiring multiple successive passes.

Cold Rolled (CR, Pilger Mill LG/LD Type)

Force condition: Bidirectional radial compression. Circumferential rolling by rolls combined with internal mandrel support achieves synchronous compression in radial and circumferential directions for uniform metal elongation, similar to rolling dough.
Deformation stress: Predominantly triaxial compressive stress with even metal flow, free of defects from uniaxial stretching.
Single-pass deformation capacity: Section shrinkage rate reaches 40%~70%, large thickness reduction per pass with fewer production passes.

2. Full Manufacturing Process Differences

Cold Drawing Process (Multiple Passes & Recurring Annealing)

Hot-rolled mother tube → Pickling to remove oxide scale → Phosphating & lubrication → Tube end pointing (tapering) → Drawing on chain draw bench → Stress-relief annealing → Repeated pickling, lubrication & drawing (multiple passes) → Straightening & finishing → Inspection

Drawbacks: Tube end pointing is mandatory before each drawing pass; intermediate annealing is required frequently to eliminate work hardening, leading to complex procedures and high material loss.

Cold Rolling Process (Simplified Procedures, Less Annealing)

Hot-rolled mother tube → Pickling & lubrication → Feeding onto long mandrel for reversible periodic pilger rolling (significant OD & wall reduction in one pass) → Bright annealing (as required) → Straightening & finishing → Inspection

Advantages: No repeated pointing operations; large single-pass deformation minimizes intermediate annealing times, low cutting loss and high yield rate (≥98%).

3. Equipment & Production Mode

Cold Drawn

Equipment: Chain draw benches, fixed carbide dies; low capital investment, simple maintenance.
Production: Intermittent single-tube processing, quick specification changeover; ideal for small-batch, multi-spec custom orders.
Applicable size range: Common OD Φ6~130mm, better performance for heavy-wall tubes; thin-wall tubes prone to fracture during drawing.

Cold Rolled

Equipment: 2-high / 3-high / 20-high precision Pilger cold rolling mills; high equipment cost, strict precision requirements for rolls and mandrels.
Production: Semi-continuous mass rolling; cost-effective for large standardized batches, high cost for frequent specification switching.
Applicable size range: OD Φ4~100mm, dedicated process for ultra-thin-wall tubes (minimum wall thickness 0.1~0.2mm capillary tubes).

4. Dimensional Tolerance & Wall Thickness Uniformity

Index	Cold Drawn	Cold Rolled
Outer Diameter Tolerance	±0.05~±0.10mm	±0.02~±0.05mm with superior stability
Wall Thickness Deviation	±5%~8%; prone to unilateral thinning and eccentricity under tension	±2%~3%; rolls & mandrel correct eccentricity synchronously for highly consistent wall thickness
Ovality & Roundness	Average; thin-wall tubes easily out-of-round	Excellent; multi-roll rolling calibrates circularity
Ultra-Thin Wall Limit	Wall thickness below 0.8mm easily cracks or collapses, unsuitable for ultra-thin applications	Stable mass production of 0.2mm ultra-thin capillaries, standard for medical & heat exchange micro-tubes

5. Internal & External Surface Finish

Cold Rolled

Progressive rolling without dragging friction between tube and tool; no longitudinal drawing marks on inner surface, surface roughness Ra ≤0.4μm. Can be supplied directly after bright annealing with high cleanliness, suitable for food, medical and high-purity fluid piping.
Cold Drawn
Severe dragging friction between tube and carbide fixed die, easily generating fine longitudinal drawing streaks on inner bore; typical roughness Ra 0.4~1.6μm. Extra polishing is required for ultra-high surface finish requirements.

6. Mechanical Property Differences

Cold Drawn (CD)

Severe work hardening from uniaxial stretching: higher axial tensile strength and hardness yet obvious anisotropy; low elongation (8%~15%) with high residual tensile stress. Prone to cracking during subsequent flaring or bending, ideal for high-pressure hydraulic cylinder structural tubes.
Cold Rolled (CR)
Uniform deformation under triaxial compressive stress delivers homogeneous grain structure, superior ductility & elongation (15%~25%) and low residual stress. Resists cracking during secondary forming (bending, flaring, welding) and maintains stable dimensional accuracy long-term.