8 Common Seamless Steel Pipe Defects

The 8 most common seamless steel pipe defects are folding, cracks, scratches, pits, pitted surface, water and rust spots, warped skin and scars, which show up as lines, grooves, depressions, marks and raised flakes on the pipe surface. Their main causes are poor tube blank quality, surface defects that were not removed in time, worn or unsuitable dies, scale or dirt pressed into the surface during drawing and straightening, and moisture during processing or storage.

The sections below give each defect’s signs, main causes and prevention measures, so you can identify problems in seamless steel pipes during inspection and trace them to their source. Defects 9–16 are covered as additional defects at the end.

The 8 Common Seamless Steel Pipe Defects

1. Folding

Folding appears as linear or spiral lines, continuous or discontinuous, on the outer or inner surface of seamless steel pipe. The main causes are poor quality of the tube material, folds already present in the material, inclusions, serious scratches and cracks on the surface, sharp edges and corners at ground areas, and folding after drawing. To avoid folding, improve the quality of the tube material and pay attention to inspection and grinding.

2. Crack

Cracks are linear or spiral, continuous or discontinuous, on the inner and outer surfaces of seamless steel pipe, with a depth of 1 mm or more. According to the analysis of Permanent Steel Manufacturing Co., Ltd, the causes are subcutaneous bubbles and subcutaneous inclusions in the hot-rolled tube blank, cracks or deep pits on the pipe before drawing, and longitudinal scratches or scores produced during hot rolling or cold drawing. material, and avoid pits and scratches during cold drawing.

3. Scratches

Scratches are longitudinal linear marks of varying length on the inner and outer surfaces of seamless pipe. Most are groove-shaped, but they may also be raised stripes. The main causes are:

existing marks on the pipe material that cannot be eliminated during drawing;
iron oxide film left on the pipe, or steel sticking to the die;
die strength and hardness that are insufficient or uneven, leading to chipping and wear;
a defective swaged tube end (hammer head), whose sharp edges and corners at the transition section damage the die.

To prevent scratches, improve the quality of every preparation process before drawing and using dies with high strength, high hardness and good smoothness.

4. Pit

Pits are one of the most common surface defects of seamless pipes. Local depressions of different areas are distributed on the pipe surface; some are periodic and some are irregular. They are caused by iron oxide scale or other hard dirt pressed into the surface during drawing or straightening, or by the shedding of warped skin already present on the surface. To prevent pits, inspect the tube material carefully and remove defects such as warped skin, keep the work site, tools and lubricants clean, and prevent oxide scale and from dirt falling on the pipe surface.

Straightening can also leave marks or pits. If smooth or sharp spiral marks appear on the outer surface of the pipe, check the straightening machine. An incorrect position or angle of the straightening roller can make the pipe rub against the roller edge during straightening, and worn grooves on the roller can leave marks or pits on the outer surface.

5. Pitted Surface

A pitted surface is characterized by small pits in patches or dots on the pipe surface. The main causes are pitting corrosion during pickling; oxide scale that is too thick after annealing and is pressed into the surface during straightening; and storage in a humid environment, where water ingress can cause pitting rust.

6. Water Spots and Rust Spots

When seamless steel pipes contact water or water vapor during processing or storage and are not dried in time, the evaporating water leaves marks called water spots. Long contact with water or corrosive substances forms rust spots. Water spots generally have no depth, while rust spots often corrode the pipe surface and leave a pitted surface even after they are removed. Drying pipes in time after contact with water helps prevent these marks.

7. Warped Skin

Warped skin is characterized by partial separation of the inner or outer surface from the metal substrate. The pieces are lumpy and discontinuous, rooted or not rooted in the pipe wall, but cannot peel off naturally. The causes are:

poor steel quality, with subcutaneous bubbles that are exposed after cold drawing;
warped skin produced during hot rolling that was not removed from the tube blank before cold drawing;
deep, angular transverse pits on the pipe that turn into warped skin after drawing.
 

Removing these defects from the tube blank before cold drawing helps prevent warped skin.

8. Scar

Scars form when surface defects such as folding and warped skin on the inner or outer surface are not removed in time and become thin flakes during later processing. They look similar to warped skin but cannot be separated from the metal substrate or peel off naturally. Surface defects must be checked and removed in time to eliminate scars.

Read a full guide: 8 Common Seamless Steel Pipe Defects

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