External Folds

Defect Characteristics:

The surface of the hot-rolled seamless steel pipe exhibits regular folds that may appear triangular, double-line, single-line, or irregular flaky patterns.

Longitudinal folds on the steel pipe’s surface can be continuous or intermittent, resembling sewing machine stitches or offset stitches at 60°, 120°, or 180°.

Spiral folds are present on the pipe’s longitudinal surface.

The surface shows continuous or short oblique lines of point-like folds longitudinally, which can appear as two or three lines offset at 120° when severe.

Causes:

Longitudinal cracks or severe inclusions and shrinkage cavities on the pipe billet surface cause spiral folds.

Poor surface cleaning of the pipe billet, sharp edges, or external raised skin.

Nodules on the piercing machine’s guide disc or scratches on the exit nozzle of the piercing machine.

Cracks or damage on continuous rolling or tension reducing rolls.

Improper chamfering of tension reducing rolls, exceeding service life, or improper matching of old and new frames.

Mechanical scratches or bumps during the hot-rolled seamless steel pipe transport process in the tension reducing machine.

Insufficient or absent cooling water for the tension reducing machine rolls, leading to roll adhesion.

Indentations

Defect Characteristics:

Irregular or regular local indentations and material removal on the longitudinal surface of the hot-rolled seamless steel pipe. Shapes include nuclear-like, strip, transverse indentations, raised skin, fragmentation, and iron filings.

Causes:

Metal objects adhering to the surface of the tension reducing machine’s finished frame rolls.

Scabs or other foreign objects pressing against the hot-rolled seamless steel pipe surface and falling off.

Hot state steel pipe impacts or scratches.

Damage or imprints from transverse transfer devices or sawing devices.

Inspection Criteria:

General Pipes:

For wall thickness ≤ 8mm, indentation depth must not exceed 90% of the negative tolerance. For wall thickness > 8mm, the depth must not exceed 80% of the negative tolerance, with a maximum depth of 1.0mm.

Boiler Pipes:

Indentation depth must not exceed half of the wall thickness’s negative tolerance, with a maximum depth of 0.6mm.

Oil Casing Pipes:

Indentation depth must not exceed half of the wall thickness’s negative tolerance. For wall thickness (S) ≥ 10mm, the maximum depth is 0.6mm. For S < 10mm, the maximum depth is 0.5mm.