What are the 8 major defects of seamless steel pipes and their causes

1. Seamless steel pipe folds: These are folds that exist on the outer or inner surface of a seamless steel pipe, appearing as straight lines or spirals, continuous or discontinuous. The main cause of folds is poor pipe quality, inherent folds, or surface inclusions, severe scratches and cracks, or sharp edges at the grinding point, which extend after drawing and cause folds. To avoid folds, the quality of the pipe material should be improved, and attention should be paid to inspection and grinding.

2. Seamless steel pipe cracks: These are fine cracks distributed in straight lines or spirals on the inner and outer surfaces of the seamless steel pipe, with a depth of 1 mm or more, and can be continuous or discontinuous. The causes are: subcutaneous air bubbles and inclusions in the hot-rolled billet, cracks or deep pits on the seamless steel pipe before drawing, and longitudinal scratches or abrasions during hot rolling or cold drawing processes. Preventing crack formation also depends on improving the quality of the billet and strengthening the inspection and grinding of the seamless steel pipe material. Simultaneously, pitting, scratches, and abrasions should be avoided during the cold drawing process of seamless steel pipes.

3. Scratches on Seamless Steel Pipes: The characteristic of this defect is longitudinal straight scratches of varying lengths on the inner and outer surfaces of the seamless steel pipe, often groove-like, but also possibly raised stripes. The main causes of scratches (scratches) are: internal scratches on the pipe material that were not removed during drawing; residual iron oxide scale on the seamless steel pipe; die adhesion; insufficient or uneven strength and hardness of the die; die breakage and wear; defective hammers; and sharp edges of the hammer transition section damaging the die. To prevent scratches and abrasions, the quality of each preparation process before drawing should be improved, and dies with high strength, hardness, and good surface finish should be used.

4. Pitfalls on Seamless Steel Pipes: This is one of the most common surface defects of seamless steel pipes. It refers to localized depressions of varying areas distributed on the surface of the seamless steel pipe, some periodically and some irregularly. Pits are caused by iron oxide scale or other hard contaminants being pressed into the surface of the seamless steel pipe during drawing or straightening, or by the peeling off of pre-existing surface deposits. Preventing pits involves carefully inspecting the pipe material and removing defects such as peeling scale, maintaining a clean work area, tools, and lubricants, and preventing iron oxide scale and contaminants from falling onto the seamless steel pipe surface. When smooth or sharp-edged spiral marks appear on the outer surface of the seamless steel pipe, the straightening machine should be inspected. Pits can occur due to improper positioning or angle of the straightening rollers, the seamless steel pipe rubbing against the shoulder of the straightening roller during straightening, worn grooves on the straightening rollers, or excessive bending at both ends of the seamless steel pipe.

5. Seamless steel pipe pitting: Characterized by patches of small, dotted pits on the surface of the seamless steel pipe. The main causes of pitting are: pitting corrosion during pickling; excessively thick iron oxide scale after annealing, which is pressed into the surface of the seamless steel pipe after straightening, forming pitting; and pitting corrosion after the seamless steel pipe is stored in a humid environment or even gets wet, which leaves pitted rust after the rust spots are removed.

6. Seamless steel pipe spots: These are marks left after the seamless steel pipe comes into contact with water (or moisture) during processing or storage and is not treated promptly (e.g., dried). If the contact time with water is long, or if the water contains corrosive substances, rust spots will form. Water spots are generally shallow, while rust spots often corrode to the surface of the seamless steel pipe, and pitting will still exist after removal.

7. Seamless steel pipe peeling: This is characterized by thin flakes on the inner and outer surfaces of the seamless steel pipe that are locally separated from the metal matrix. These flakes are sometimes in patches, discontinuous, and may or may not be embedded in the pipe wall, but cannot peel off naturally. The causes include poor steel quality, the presence of subcutaneous air bubbles that are exposed after cold drawing; the presence of scale generated during hot rolling that was not removed from the billet and carried over to cold drawing; and deep, angular transverse pits on the seamless steel pipe that form scale after drawing.

8. Seamless Steel Pipe Scale: Due to defects such as folds and scale that are not promptly removed from the inner and outer surfaces of seamless steel pipes, thin flakes that cannot be separated from the metal matrix are formed during subsequent processing. These flakes may be in discontinuous form, forming patches on the pipe wall, or they may not be, but they cannot peel off naturally; these are called scales. Surface defects must be carefully monitored and removed promptly to eliminate scale.