Characteristics and Causes of the Eight Major Defects in Seamless Steel Pipes

Defects in seamless steel pipes can have a devastating impact on product quality. How can we avoid these defects? Understanding their causes is crucial. Let's explore the specific causes of the eight major defects in seamless steel pipes:

1. Seamless Steel Pipe Folds
These defects occur on the outer or inner surface of seamless steel pipes, appearing linearly or spirally, and can be continuous or discontinuous. Folds are primarily caused by poor steel pipe material quality, inherent folds, surface inclusions, severe scratches and cracks, sharp corners at the grinding site, and post-drawing stretching. To avoid folds, improve steel pipe material quality, and conduct thorough inspection and grinding.

2. Seamless Steel Pipe Cracks
These cracks occur on the inner and outer surfaces of seamless steel pipes, appearing linearly or spirally, and can be 1 mm or more in depth, and can be continuous or discontinuous. Causes of crack formation include: subcutaneous bubbles and inclusions in the hot-rolled steel tube billet; cracks or deep pitting on the steel tube before drawing; and longitudinal scratches or abrasions during hot rolling or the cold drawing process. Preventing crack formation also relies on improving the quality of the steel tube billet and strengthening inspection and grinding of the steel material. Pitting, scratches, and abrasions during the cold drawing process should also be avoided.

3. Scratches on Seamless Steel Tubes
This defect is characterized by longitudinal, linear scratches of varying lengths on the inner and outer surfaces of the seamless steel tube. These scratches are usually groove-like, but may also appear as raised streaks. The main causes of scratches (scratches) include: internal scratches on the steel tube billet that were not removed during drawing; residual iron oxide scale on the steel tube; die adhesion; insufficient or uneven die strength and hardness; die chipping and wear; defective hammer heads; and damage to the die caused by sharp edges and corners in the transition area of the hammer head. To prevent scratches and abrasions, the quality of all pre-drawing preparations should be improved, and molds with high strength, hardness, and a good finish should be used.

4. Seamless Steel Pipe Pits
This is one of the most common surface defects on seamless steel pipes. These are localized depressions of varying sizes distributed across the pipe surface, sometimes periodically or irregularly. Pits are caused by scale or other hard contaminants being pressed into the pipe surface during the drawing or straightening process, or by the flaking of pre-existing surface scale. Preventing pits requires careful inspection of the pipe material and removal of scale and other defects. Maintaining a clean work area, tools, and lubricants prevents scale and contaminants from settling on the pipe surface. If spiral marks, either smooth or sharp, appear on the outer surface of a seamless steel pipe, the straightening machine should be inspected. Pits can occur due to improper straightening roller positioning or angle, the pipe rubbing against the shoulder of the straightening roller edge during straightening, worn grooves on the straightening roller, or excessive bending at either end of the pipe.

5. Seamless Steel Pipe Roughing
This is characterized by small, patchy, and dot-like pits on the surface of the steel pipe. The main causes of pitting are pitting during pickling; excessively thick scale after annealing and straightening, which presses into the steel pipe surface and causes pitting; and pitting caused by humid storage environments, or even water ingress, which can lead to pitting. Removal of these rust spots can also cause pitting.

6. Seamless Steel Pipe Spotting
Water spots are caused by contact with water (or water vapor) during processing or storage, which is not promptly treated (e.g., by drying). Water spots are left behind after evaporation. Rust spots form when contact with water is prolonged or when the water contains corrosive substances. Water spots are generally shallow, while rust spots often corrode the surface of the steel pipe, leaving pitting even after removal.

7. Seamless Steel Pipe Warping
This is characterized by thin flakes on the inner and outer surfaces of the seamless steel pipe that are partially separated from the metal matrix. Some flakes may be discontinuous and blocky, rooted or not rooted to the steel pipe wall, but they do not naturally flake off. Causes include poor steel quality, the presence of subsurface bubbles that become exposed after cold drawing; scale warping caused during hot rolling that is not removed from the tube billet and carried over to cold drawing; and deep, angular transverse pits on the steel pipe that form scale warping after drawing.

8. Scabbing in Seamless Steel Pipes
Failure to promptly remove defects such as folding and scale warping on the inner and outer surfaces of steel pipes leads to the formation of thin flakes that cannot be separated from the metal matrix during subsequent processing. Some flakes may be lumpy and discontinuous, rooted or not rooted in the steel pipe wall, and resist natural peeling. This is called a scab. Surface defects must be carefully inspected and promptly removed to eliminate scabs.