Cold drawing is the secondary manufacturing process that transforms a hot-finished mother tube into a precision product. The process is deceptively simple in principle — pull a tube through a die and over a mandrel — but the engineering that makes it work reliably involves precise tooling design, controlled lubrication, intermediate heat treatment, and rigorous quality control at every stage.
Starting Material: The Mother Tube
Every cold-drawn tube begins as a hot-finished seamless (HFS) mother tube produced by rotary piercing and elongation. The mother tube’s quality directly determines the quality of the finished CDS product. Its chemistry must be within specification, its wall thickness must be uniform enough for the drawing die to produce consistent results, and its surface must be free of laps, seams, and deep scale pits that would propagate through the cold-drawing process as surface defects on the finished tube.
Mother tubes are typically supplied in an annealed or normalized condition to ensure adequate ductility for the cold-drawing reduction. Hard or partially transformed microstructures will crack during drawing, generating scrap and potentially damaging expensive drawing dies.
The Drawing Process
The tube is pointed (reduced at one end to pass through the die), lubricated internally and externally, then pulled through a precision-ground carbide or tool steel die while riding over a fixed or floating mandrel inside the bore. The die controls the outside diameter; the mandrel controls the inside diameter and wall thickness.
Each drawing pass reduces the cross-sectional area by 15–35%, depending on the steel grade, tube dimensions, and lubrication conditions. Heavier reductions produce more work-hardening in a single pass but increase the risk of surface defects and require higher drawing forces. The total reduction from mother tube to finished tube may require 2–5 drawing passes, with intermediate annealing between passes to restore ductility.
Intermediate Annealing
After each drawing pass, the tube is stress-relief annealed or fully annealed to relieve the work-hardening accumulated during drawing and restore sufficient ductility for the next pass. Without intermediate annealing, cumulative work-hardening would make the tube too hard and brittle to survive further reduction, resulting in cracking, splitting, or catastrophic failure during drawing.
The annealing atmosphere is critical for tubes requiring clean surface finish. Bright annealing in hydrogen or dissociated ammonia prevents oxidation and eliminates the need for subsequent pickling. Conventional annealing in air or endothermic gas produces a thin oxide layer that must be removed by acid pickling before the next drawing pass.
Final Processing
After the final drawing pass, the tube undergoes straightening (typically on a rotary straightener), cutting to length, and final inspection. Eddy current testing (ECT) scans every tube for surface and near-surface defects. Ultrasonic testing (UT) verifies wall thickness and detects internal flaws. Dimensional inspection confirms OD, ID, wall thickness, and straightness within specification tolerances. Hydrostatic testing verifies pressure integrity.
The final delivery condition — as-drawn (BK), stress-relieved (BKW), soft-annealed (BKS), or normalized (NBK) — determines the tube’s mechanical properties and must be specified on the purchase order based on the intended application.
Global Seamless Cold Drawing Capability
Global Seamless operates multiple cold-drawing benches covering OD ranges from 10 mm to 168 mm. In-house annealing furnaces, pickling lines, and NDT equipment provide complete process control from mother tube to finished CDS product, all under one quality system and one MTC.
Need cold-drawn seamless tubes? Send us your specification for a quotation with confirmed delivery.