Defects of cold drawn seamless steel pipes

1. Fold

Folds appear on the outer or inner surface of cold-drawn seamless pipes in linear or spiral shapes, either continuous or discontinuous. The primary causes are poor material quality, pre-existing folds in the pipe, surface inclusions, severe scratches, cracks, and edges and corners in the grinding area. These defects result in folding during the drawing process. To avoid folds, the quality of the pipe material should be improved, and inspection and grinding should be carefully performed.

2. Cracks (including cracks and hairlines)

Cracks manifest as small straight or spiral fissures on the inner and outer surfaces of cold-drawn seamless pipes, with depths of 1 mm or more, and can be continuous or discontinuous. Causes include subcutaneous bubbles and inclusions in the hot-rolled tube blank, pre-existing cracks or deep pitting, and longitudinal scratches or marks during hot rolling or cold drawing. To prevent cracks, improve the quality of pipe materials, and focus on inspection and grinding. Avoid pitting, scratches, and marks during cold drawing.

3. Dimples

Dimples are local depressions of varying areas on the surface of cold-drawn seamless pipes, which can be periodic or irregular. They are caused by oxide scale or hard dirt pressing into the pipe surface during drawing or straightening, or peeling of pre-existing surface warping. Prevent dimples by inspecting pipe materials thoroughly, removing defects like warped skin, and keeping the worksite, tools, and lubricants clean.

4. Diameter Out of Tolerance

Diameter deviations result from improper mold matching or neglect of mold dimension changes due to wear. Excessive straightening roller reduction during straightening can also cause this issue.

5. Wall Thickness Out of Tolerance

Wall thickness deviations occur due to improper mold matching, neglecting mold dimension changes due to wear, incorrect wall thickness estimation in air-drawing passes, and improper mandrel position adjustment. To prevent this, match the mold correctly and adjust the mandrel position accurately.

6. Ellipse

An elliptical cross-section can result from using a drawing die with an oval die hole, excessive bending of the steel pipe during straightening, or uneven reduction distribution between straightening rollers. Replacing the pipe mold and addressing causes during straightening can eliminate this defect.

7. Uneven Wall Thickness

Uneven wall thickness arises from excessive wall thickness variations in the pipe material, inconsistent axis alignment during drawing, and elliptical die holes. To reduce this, minimize pipe material wall thickness variations and carefully check the mold.

8. Pullout

Pullout results from excessive deformation, poor quality heat treatment, pickling and lubrication, inappropriate hammer head, overheating, and misaligned drawing axes. To prevent pullout, follow drawing table regulations for size and mold matching, ensure mold alignment, and maintain quality in preparatory processes.

9. Excessively Long Drawing Head

When drawing short mandrel rods, a short drawing head is normal. However, an excessively long head increases head cutting and metal consumption. This occurs when the mandrel is not pushed in time or cannot enter the deformation zone. Ensure proper mandrel operation and adjust the mandrel for easier entry into the deformation zone.

10. Shake Lines

Shake lines, occurring during short mandrel drawing, are wavy ring marks on the inner or outer surfaces. They result from vibration during drawing due to poor pickling and lubrication, friction changes, inconsistent mechanical properties, thin mandrels, and excessive deformation. Improve pickling, lubrication, and heat treatment quality, and adjust mandrel position correctly to prevent shake lines.

11. Longitudinal Cracking

Longitudinal cracks penetrate the pipe wall and occur suddenly, often along the entire length or near the hammer head. They are caused by excessive diameter reduction, uneven deformation, high residual stress, improper annealing, and impact during handling. Control diameter reduction, ensure quality heat treatment, and anneal pipes in time to prevent longitudinal cracking.

12. Longitudinal Concave Folds

These occur in thin-walled pipes due to cross-section instability during drawing. Excessive diameter reduction and deep recesses at the hammer head transition cause this defect. Reduce diameter reduction and ensure good hammer head quality to prevent it.

13. Warped Skin

Warped skin appears as thin flakes partially separated from the metal matrix, caused by subcutaneous bubbles, hot rolling warping, or deep transverse pits. Avoid this by ensuring high-quality steel and addressing surface defects before drawing.

14. Inclusions

Non-metallic inclusions embedded in the surface or surface cracks result from poor quality steel or impurities pressed into the surface during heating or annealing. Use high-quality materials and maintain cleanliness during processing.

15. Pitting (Ma Mian)

Pitting appears as small point-like pits due to pitting corrosion during pickling, thick oxide scale after annealing, or rust. Ensure proper pickling, prevent oxide scale buildup, and protect steel pipes from rust.

16. Dents (Hit/Press)

Dents occur mainly in thin-walled tubes due to improper handling, excessive stacking, swinging during straightening, or tight clamping during cutting. Handle pipes carefully and avoid excessive stacking to prevent dents.

17. Correction Dents

These manifest as spiral marks with smooth or sharp edges caused by incorrect straightening roller angles, steel pipe rubbing against the roller edge, worn grooves, or excessive bending. Adjust straightening roller angles and replace worn rollers to prevent this defect.