Glossary of Tube Terms

The manufacture of precision tube products is a complex, high-tech operation. If you’ve come across a term that needs clarification, please use our metal tube engineering glossary below.

Annealing: Heat treatment operation intended to increase the material ductility for further working, or to meet end use properties. An effective operation results in reduced hardness and tensile properties from the original temper condition.

Bright Annealing: To produce a colourless oxide without formation of scale on tube surfaces.

Open Annealing: No protective atmosphere, resulting in oxide formation on tube surfaces.

Bar: Metal product with a solid cross section

Blow: Process to remove dust and debris from the bore of the tube by using foam wads.

Bore Blast: Process for blasting abrasive media through the bore of the tube to condition the inside surface.

Bored Bar: Bar which has been bored to produce raw material suitable for tube manufacture.

Bow: See ‘Straightness’.

Burr: Rough ridge left from a cutting or machine operation. Burrs – loose metal particles produced by a break up of a burr. The term may also be applied to the build up at the end of a score line on a tube.

Calibrated Tube: Tube that is slightly cold worked to remove ovality or to improve dimensions and tolerances.

Camber: See ‘Edge Camber’.

Carburisation: An increase in carbon content from the original amount in the material. The increase is established by metallographic or chemical analysis.

Centre Line Shrinkage: In a welded tube, the reduced thickness of the weld bead approximate at the centre of the seam weld.

Chatter: One or multiple circumferential rings along the tube length, caused by vibration during the draw bench operation. This is noticeable by visible and audible symptoms.

Chemical Milling: Process where desired shapes, dimensions and surfaces are achieved through selective or overall removal of metal by controlled chemical dissolution, such as hydrofluoric acid. At Fine Tubes we chemical mill to both improve the surface, to reduce any surface imperfections and to mill into size/tube tolerance, for instance taking 0.05mm from the surfaces of aerospace grade titanium tubes.

Contamination: The reduction of surface or sub-surface quality by adherence or absorption of foreign substances.

Corrosion: Surface attack by chemical and/or electrolytic action

Crack: Narrow surface fissure extending into the material thickness.

Cross sectional Area: The sectional area of a tube, usually in mm2.

Degrease:  Cleaning process using a degreasing agent to remove unwanted grease and dirt.

Dent: : Local depression affecting the tube dimensions, caused by mechanical impact or constriction.

Discoloration: Changes in surface appearance due to oxides or other chemical reaction.

Dog Leg: Double bending at the front end of a tube caused by sudden relaxation of tension following a sink or plug draw operation.

Eddy Current Testing: Non destructive test procedure that examines for outside diameter cracks and burrs.

Edge Camber: Deviation of a strip length from the straight edge over a measured length.

Electro-Chemical Cutting:  Cutting process that utilises electrical spark erosion action as well as abrasive cutting.

Flats: Small changes in the surface contour of the tube circumference giving a multi faceted appearance.

Flatness: Strip width deviation of flatness from a straight edge across the strip width.

Heat treatment: General term for the process involving a furnace operation - annealing, hardening, tempering, etc.

Hollow: Seamless raw material supplied for tube processing. This may be in the form of an extrusion or tube reduced product.

Lap: Fold or overlapping layer of material formed during extrusion, tube reducing or the drawing process.

Orange peel: Surface appearance arising from the stretching of coarse grain metal in forming (drawing, bending etc.). This has been observed in 304 subjected to heavy sink pinch ratio reductions.

Oxide: Chemical combination of oxygen with the parent metal. Always present on the tube surfaces in normal atmospheric conditions. May be invisible to opaque with colour variations, sometimes referred to as tarnish.

Passivate: Process whereby material is dipped in a solution of nitric acid in order to produce a ‘passive’ surface which will reduce the risk of oxidisation or rusting.

Pickup: Single or multiple scoring or pitting during tube processing due to friction or impact between the tube and tooling. The formed mark may be continuous, intermittent straight or helical depending on the process.

  • Die and plug Straight continuous pickup
  • Multiroll Straight continuous pickup
  • Weld mill rolls Straight and intermittent
  • Tube reducer tooling Helical
  • Cross roll stripper Helical and intermittent
  • Straightener Helical and intermittent(but opposite to cross roll strippers)

Pit:  Local surface defect, single or in clusters, approximately hemispherical in shape or cross-section; may be caused by chemical attack or by impression from rolls or other sources.

Pilger: Pilgering is a longitudinal cold-rolling process that reduces the diameter and wall thickness of metal tube in one process step. Depending on the material, the cold pilger process achieves cross-section reductions of more than 90 percent in a single working cycle.

Plastic Impression: Random size, shape and spacing or marks caused by thick spots or runs of hard plastic being impressed into the tube surface during drawing. Frequently the impression has no measurable depth but is detrimental to the tube’s cosmetic appearance.

Plug Draw: Drawing process which uses a hardened tool, called a ‘plug’, to control the inside diameter of a tube.

Polishing  Conditioning process which improves the outside surface of a tube by passing it against belts of abrasive material.

Process Specification: Specification detailing the rules applicable to a particular process.

Ripple: Undulations along the length of the tube, generally on a helical path. These can be visual only, or associated with measurable differences in dimensions and hardness properties. Caused by cross roll stripping of machine straightening where undue pressure has been applied.

Scale: Oxide film that is thicker than the superficial film referred to as tarnish. Although adherent to the tube surface when originally formed, scale may break up during subsequent processing, leading to surface defects.

Score: Notch or deep scratch of length greater than width, caused by cutting action from another component.

Seam: Weld zone in a tube.

Seamless tube: Produced by piercing, boring or extruding a solid metal piece so that no part of the cross section contains a seam of any description.

Softening: General reduction in hardness and tensile strength by heat treatment at a selected temperature and conditions. The term can be more broadly applied than annealing.

Split: Defect similar to a crack. Normally mechanically induced for example by flattening, bonding or pressure testing beyond the material strength.

Straightness: This is the relationship between the tube’s longituditional axis and the permitted deviation from a straight edge over a measured length. Also called ‘bow’.

Surface texture: Small variations in the surface topography resulting from drawing, polishing or other operations. The surface is measured by electronic instruments or by visual comparator. Electronic measurements are registered in micro metres or micro inches (Ra or CLA). Also called ‘surface roughness’.

Temper: Used to describe metal conditioning such as annealed or cold worked. This is also a specific form of heat treatment following hardening.

Tube: Hollow product of circular or other shaped cross-section having a continuous periphery.

Tramlines: Form of pick-up consisting of two or more parallel lines on the tube surface.

Ultrasonic testing: The use of high-frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection and evaluation, dimensional measurements, material characterisation, and more.

Undercutting: Welded tube, where the weld bead has less thickness than the parent metal. Often traceable to misalignment of the formed strip edges during seam welding.

Witness Mark: Surface mark, apparent in certain positions of light reflection but without measurable depth.

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