Cold pilgering’s compressive deformation mode enables the production of seamless tubes with wall thicknesses that would be impossible to achieve by conventional cold drawing. When the ratio of outside diameter to wall thickness (OD/WT) exceeds 20:1, drawing becomes increasingly risky — the tensile stresses in the drawing direction can exceed the material’s ductility, causing longitudinal cracks, spiral fractures, or wall thinning that renders the tube unusable. Pilgering pushes this boundary to 50:1 and beyond.
The Physics of Thin-Wall Production
During cold drawing, the tube experiences tensile stress in the drawing direction, radial compressive stress at the die, and circumferential stress that varies from compressive at the die contact to tensile between the die and mandrel. For thick-walled tubes (OD/WT below 15:1), the tensile stresses are safely within the material’s ductility limits. As the wall thins, the ratio of tensile to compressive stress increases, and the tube becomes progressively more vulnerable to tensile failure modes.
In pilgering, the stress state is predominantly compressive in all directions. The dies compress the tube radially while the tapered die groove compresses it axially. This triaxial compression suppresses crack nucleation and propagation, allowing much higher deformation without fracture. The result is the ability to produce walls as thin as 0.3–0.5 mm on tubes of 15–25 mm OD — OD/WT ratios that no drawing process can reliably achieve.
Applications for Thin-Wall Tubes
Nuclear fuel cladding: Zirconium alloy tubes (Zircaloy-2 and Zircaloy-4) for nuclear fuel rods are among the most demanding thin-wall tube applications. Typical dimensions are 9.5–11.2 mm OD with 0.57–0.72 mm wall thickness — OD/WT ratios of 14–18:1. While these ratios are within drawing capability, the nuclear industry demands concentricity below 3%, OD tolerance within ±0.03 mm, and WT tolerance within ±5% — precision levels that only pilgering consistently achieves in production.
Aerospace hydraulic tubing: Aircraft hydraulic systems operate at 3000–5000 PSI using thin-wall stainless or nickel alloy tubes where every gram of weight matters. Typical aerospace hydraulic tubes are 6–12 mm OD with 0.5–1.0 mm wall, produced to AMS specifications with OD tolerances of ±0.05 mm and 100% eddy current and ultrasonic inspection.
Medical devices: Surgical instruments, endoscope shafts, catheter reinforcement, and implantable device housings use ultra-thin stainless steel and nitinol tubes with OD as small as 1–3 mm and wall thickness as thin as 0.05–0.15 mm. These dimensions are at the extreme end of pilger capability, often requiring micro-pilger mills specifically designed for sub-millimeter wall production.
Instrumentation and sampling: High-pressure instrumentation tubing for chemical analysis, process sampling, and chromatography uses small-bore, thin-wall tubes where the pressure rating per unit weight must be maximised. Pilgered tubes with precisely controlled wall thickness provide the highest burst pressure for a given tube weight, optimising both safety margin and system weight.
Quality Assurance for Thin-Wall Tubes
Thin-wall tubes demand inspection intensity proportional to the consequences of a wall-thickness deficiency. A 10% wall variation on a 10 mm wall tube costs 1 mm of pressure-carrying capacity; the same 10% on a 0.5 mm wall costs only 0.05 mm, potentially reducing the tube below the minimum required for the design pressure. For this reason, thin-wall pilgered tubes are inspected with 100% eddy current testing (for surface and near-surface defects), 100% ultrasonic wall thickness measurement (for through-wall variations), and statistical OD and concentricity measurement (typically every tube at multiple positions).
Need thin-wall precision tubes? Send us your OD, wall thickness, and material requirements for a quotation from our pilger program.