The Working Principle of Cold Drawing
Cold drawing reduces the cross section of a tube by pulling it through a die while the metal is at room temperature. The die sets the outside diameter and a mandrel inside the tube sets the inside diameter, so the two dimensions and the wall thickness between them are all controlled by tooling rather than by a rolling pass. Because the deformation happens below the recrystallisation temperature, the metal work hardens as it is reduced, which raises its strength and hardness and improves the surface finish at the same time.
Two consequences follow directly from the principle. The first is accuracy: a die and mandrel combination produces the same dimensions on every piece, so the tolerance band is much narrower than in hot rolling. The second is a limit on how much can be done in one pass, because a heavily work hardened tube loses ductility and will crack before it reaches the next die. Intermediate annealing is inserted whenever the accumulated reduction approaches that limit, which is why a cold drawn product is described by its drawing schedule.
Process Sequence
Mother tube selection and inspection: a hot rolled seamless hollow of the appropriate size, grade and wall is chosen so that the required reduction is achievable.
Heat treatment and descaling: the hollow may be annealed to soften it, and its surface scale is removed by pickling in acid or by shot blasting.
Chemical surface treatment: a phosphate conversion coating and a lubricant or soap film are applied, so that the tube carries lubricant into the die and the metal does not seize against the tooling.
Cold drawing: the pointed tube end is gripped and pulled through the die over a mandrel on a draw bench, or the tube is cold rolled in a pilger mill for heavier reductions.
Intermediate annealing: applied where the total reduction has to be split across several passes, restoring ductility before the next draw.
Final heat treatment: bright annealing in a protective atmosphere is used where an oxide free surface is required, and a stress relief treatment is used where dimensional stability matters.
Straightening, cutting and end finishing: the tube is straightened, cut to fixed length and the ends are faced or chamfered to the drawing.
Final inspection: dimensional checks, eddy current or ultrasonic testing, hydrostatic or air pressure testing and visual inspection of the bore, followed by marking and protection.
Drawing Methods and What They Change
| Method | Effect on the tube |
|---|---|
| Sinking without a mandrel | Reduces the outside diameter while the wall thickens; used for intermediate sizing rather than final accuracy |
| Drawing over a fixed plug or mandrel | Controls the inside diameter as the outside is reduced, which gives the best wall control and is the basis of precision tube production |
| Drawing over a floating plug | Simpler tooling with good inside diameter control, used widely for small and medium sizes |
| Moving mandrel drawing | The mandrel travels with the tube, reducing friction and allowing heavier reductions per pass |
| Cold pilgering | Rolls rather than pulls the tube, giving very large reductions with a good surface and tight wall control |
Property Changes Produced by Cold Work
Cold drawing raises yield and tensile strength, lowers elongation, and increases the internal stress level of the tube. The size of the change depends on the reduction and on the grade, so a drawn tube and its annealed counterpart of the same grade are not interchangeable in a strength calculation. The tube is normally supplied in a defined delivery condition: cold drawn and annealed, cold drawn and stress relieved, or cold drawn as supplied. The delivery condition must therefore be stated in the enquiry, because it fixes the mechanical properties that will be certified.
The drawing operation also improves the bore. The die and mandrel produce a smooth surface with a controlled roughness, reduce wall eccentricity inherited from the mother tube, and improve straightness. These are the properties that make a cold drawn seamless tube suitable for hydraulic barrels, instrument lines and heat exchanger tubes, where the bore is a functional surface.
Quality Control Points and Common Defects
Wall eccentricity and inside diameter variation, which follow from the mother tube quality and from the alignment of the die and mandrel.
Internal scratches, laps and pick-up marks caused by inadequate lubrication or a worn mandrel.
Transverse cracking at the drawn end, which indicates that the reduction per pass exceeded the ductility available.
Residual stress and loss of straightness after cutting, controlled by stress relief treatment and by correct straightening practice.
Surface decarbonisation or oxide, controlled by the atmosphere of the annealing furnace.
Inspection for these defects is carried out with dimensional gauges, go and no go plug gauges, eddy current or ultrasonic testing for wall and defect detection, and hydrostatic or air pressure testing where pressure integrity is part of the requirement. Records are traceable to the mother tube heat number and to the drawing schedule used, so that the mechanical properties certified for the finished tube can be traced back through the process.
FAQ
Q: Does cold drawing make the steel stronger?
Yes, by work hardening. Yield and tensile strength rise with the amount of reduction and elongation falls, which is why the delivery condition has to be specified and certified.
Q: Why is annealing needed between drawing passes?
Each pass consumes ductility. When the accumulated reduction would leave the tube too hard or too brittle for the next pass, an anneal restores the ductility and allows further reduction.
Q: What is the advantage of a cold drawn tube over a hot rolled one?
Tighter diameter and wall tolerances, better concentricity, a smoother bore and a better surface finish, all of which reduce machining and improve the performance of the tube as a machine element.
Q: Is a cold drawn seamless tube always annealed at the end?
No. The final delivery condition depends on the application, and both annealed and as drawn conditions are supplied. A stress relieved condition is also used where dimensional stability is critical.
Q: Which standards apply to cold drawn seamless pipe?
GB/T 3639 for cold drawn or cold rolled precision seamless steel tubes, EN 10305-1 for seamless cold drawn tubes, and ASTM A519 for mechanical tubing, with seamless grades also covering A179, A192 and A213 for heat exchanger and boiler service.
Q: What causes cracks at the end of a drawn tube?
An excessive reduction per pass relative to the ductility of the material, or an incomplete anneal. Reducing the pass reduction or adding an anneal removes the problem.
