Common Defects and Causes of Cold Drawn Seamless Steel Tubes

What are the Common Defects in Cold Drawn Seamless Steel Tubes?

For procurement purposes, it helps to think of tube defects in three buckets:

Surface defects — anything visible on the outside or inside of the tube: scratches, folds, scale, corrosion, and surface cracks.
Internal defects — problems hidden inside the tube wall, usually inherited from the original steel billet (the solid round bar the tube is made from) or the casting process: porosity, segregation, non-metallic inclusions, shrinkage cavities, and white spots.
Dimensional defects — the tube doesn't match the size or shape called out on the order: ovality, wall thickness variation, and straightness deviation.

Cracking is the one defect that can land in more than one bucket — it can start at casting (internal), during cold drawing, or later in service (surface). The sections below walk through each category.

Surface Defects of Cold Drawn Seamless Steel Tubes

A buyer can catch most surface defects with nothing more than a good look down the length of the tube.

Scratches and scoring are usually a handling problem — tubes dragged across each other or across equipment during transport or drawing — rather than a sign of a bad batch.

Folds happen when a scar, an uneven patch, or a sharp edge on the original billet gets pressed into the tube surface during forging, rolling, or drawing, or when the rolling equipment isn't set up correctly for the pass. On inspection, a fold looks like a diagonal line crossing the tube surface, usually with a slightly decarburized (softened) patch next to it and a sliver of oxide scale trapped inside.

Scale and oxidation marks come from incomplete pickling (the acid bath used to strip mill scale) or poor atmosphere control during heat treatment. A light dusting of scale is usually within tolerance; heavy, flaking scale generally isn't.

Internal Defects in Cold Drawn Seamless Steel Tubes

Internal defects don't show up on a walk-around inspection — they're baked into the tube wall before drawing ever starts, back at the casting or hot-working stage.

Porosity is essentially trapped gas: as the last bit of molten steel solidifies, it shrinks and releases gas that doesn't get squeezed shut during hot working, leaving small voids behind. Under acid-leaching inspection (etching a cut sample to reveal what's inside), porosity looks like a cluster of irregular pits, sometimes merged into a sponge-like patch in bad cases.

Segregation means the chemical composition isn't even throughout the steel — certain elements crystallize and settle unevenly as the metal cools. It's labeled by where it shows up: ingot-type (spread through the whole ingot), center, or spot segregation.

Non-metallic inclusions are bits of material that shouldn't be there — foreign metal that fell into the mold during pouring, slag or furnace-lining material that didn't separate out of the molten steel, or a skin of partially-solidified metal that got rolled into the ingot during casting.

Shrinkage cavities form when the core of a cooling ingot can't pull in more liquid metal to fill the gap left by solidification, leaving a genuine void at the head of the casting.

White spots are a specific type of internal crack, generally blamed on hydrogen trapped in the steel combined with internal stress. On a cut cross-section they look like short cracks; on a broken fracture surface, they show up as bright, coarse crystal dots.

Because all five of these originate before the tube is ever drawn, no amount of care during drawing can fix them after the fact — they have to be caught at the billet stage. 

Dimensional Defects of Cold Drawn Seamless Steel Tubes

Even a tube with flawless material can still cause problems on the shop floor if it's the wrong shape.

Ovality (out-of-round) comes from uneven wear on the drawing die, or the die being slightly misaligned — and it needs to stay within tolerance for the tube to seal properly or fit into a matching part.

Wall thickness variation happens when the mandrel (the internal rod that shapes the bore during drawing) isn't centered consistently through the pass. It's checked against the minimum wall thickness the standard allows.

Straightness deviation usually comes from uneven cooling or rough handling after drawing. A slight bow can typically be corrected by re-straightening; excessive bow is normally rejected outright.

Read full guide: Common Defects and Causes of Cold Drawn Seamless Steel Tubes

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