Characteristics and causes of 8 major defects of seamless steel pipes

1. Seamless Steel Pipe Folding

Folding appears as straight or spiral, continuous or discontinuous lines on the inner or outer surfaces of seamless steel pipes. This defect often results from poor material quality, existing folds, inclusions, severe scratches, or cracks in the pipe material. Grinding areas with edges and corners can also lead to folding after drawing and stretching. To prevent folding, it is crucial to improve the quality of pipe materials and ensure thorough inspection and grinding.

2. Seamless Steel Pipe Cracks

Cracks are small, linear, or spiral defects on the inner and outer surfaces of seamless steel pipes, typically deeper than 1 mm and can be continuous or discontinuous. Causes include subcutaneous bubbles and inclusions in hot-rolled tube blanks, pre-existing cracks or deep pits in the steel tubes, and longitudinal scratches from hot rolling or cold drawing. Preventing cracks involves improving pipe blank quality, reinforcing inspection and grinding, and avoiding pitting and scratches during cold drawing.

3. Seamless Steel Pipe Scratches

Scratches manifest as longitudinal linear marks of varying lengths on the inner and outer surfaces of seamless pipes, appearing as grooves or raised stripes. Causes include internal scratches in the pipe material that persist through drawing, remaining oxide iron scale, mold adhesion, insufficient or uneven mold strength and hardness, mold chipping, and hammerhead defects. To prevent scratches, enhance the quality of preparation processes before drawing and use molds with high strength, hardness, and smooth surfaces.

4. Seamless Steel Pipe Pits

Pits are local depressions of varying sizes on the surface of seamless pipes, either periodic or irregular. These defects arise from oxide scale or hard dirt pressing into the pipe surface during drawing or straightening or from existing surface warping peeling off. Prevention measures include thorough inspection and removal of defects like warped skin, maintaining clean work sites, tools, and lubricants, and preventing oxide scale and dirt from contacting the pipe surface. Additionally, check the straightening machine for alignment and roller condition to avoid surface dimples.

5. Seamless Steel Pipe Pitted Surface

Pitted surfaces are characterized by small, point-like pits on the steel pipe surface. Causes include pitting corrosion during pickling, thick oxide scale after annealing, and humid storage conditions leading to pitting corrosion. Preventing pitting involves addressing rust spots promptly and ensuring proper handling and storage to avoid water contact.

6. Seamless Steel Pipe Spots

Spots occur when seamless steel pipes come into contact with water or water vapor during processing or storage without timely treatment, such as drying. Water spots leave marks after evaporation, while prolonged exposure to water or corrosive substances leads to rust spots, which can corrode the pipe surface and leave pitted marks after removal.

7. Seamless Steel Pipe Warping

Warping appears as thin flakes partially separated from the metal matrix on the inner and outer surfaces, which may be lumpy and discontinuous. Causes include poor steel quality with subcutaneous bubbles exposed after cold drawing, warping produced during hot rolling not removed from the tube blank, and deep, angular transverse pits transforming into warped skin during drawing.

8. Seamless Steel Pipe Scarring

Scarring results from defects like folding and warping not removed in time, forming thin flakes during subsequent processing. These flakes may be lumpy, discontinuous, and rooted on the pipe wall but cannot peel off naturally. Preventing scarring requires careful inspection and timely removal of surface defects.