Main Influencing Factors of the Inner Surface Finish of Cold Drawn Seamless Steel Tube

1.1 Piercing

Inconsistent piercing temperature and outdated equipment are common causes of uneven heating in the round steel billet during the cold drawn tube piercing process. This creates a temperature gap between the inside and outside of the tube, which shows up later as pitting, pockmarks, or surface peeling — all of which affect inner surface finish. An extended perforation furnace with even heat distribution across the billet helps avoid this problem.

1.2 Cold Drawn Mold

Defects in the drawing die itself carry straight through to the finished tube's internal surface. Cold drawing die precision is one of the most direct levers over the result: dies that are worn, poorly machined, or not matched to the specific tube dimensions will produce a rougher inner bore regardless of how well the rest of the process is controlled.

1.3 Pickling

Because of tightening environmental regulations, many manufacturers have moved pickling to third-party processors without the accumulated process data to get it right — pickling time varies by steel grade, and without that data, the result is either incomplete removal of internal scale and impurities or over-pickling that creates pitting. An in-house, environmentally enclosed pickling process for steel tube, backed by grade-specific time data, is what keeps this step consistent.

1.4 Number of Drawing Passes

The number of drawing passes has a direct effect on tube finish: smaller deformation per pass generally produces a smoother inner surface finish, but it also adds cost, since more passes are needed to reach final dimensions. Reducing the number of passes to cut cost is a common shortcut, but it typically comes at the expense of finish quality.

Read full guide: Cold Drawn Seamless Steel Tubes: What Affects Inner Surface Finish?

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