The outer surface defects of steel pipes mainly have the following 13 defects

1. External folding feature: spiral layered folding on the outer surface of the steel pipe.

Causes:

1) There are folds or cracks on the surface of the tube blank.

2) The subcutaneous pores and subcutaneous inclusions of the tube blank are serious.

3) The surface of the tube blank is not cleaned well or there are ears, misalignment, etc.

4) During the rolling process, the surface of the steel pipe is lifted and scratched and then pressed onto the base of the steel pipe through rolling, forming external folds, etc.

Inspection: Not allowed to exist: Minor ones can be repaired and ground, and the actual values of the wall thickness and outer diameter after grinding shall not be less than the minimum values required by the standard.

2. Delamination feature: spiral or block-shaped stratification and rupture on the surface of the steel pipe.

Causes: serious non-metallic inclusions, residual shrinkage holes, or severe looseness in the tube blank.

Inspection: Not allowed to exist.

3. External scarring feature: scars appear on the outer surface of the steel pipe.

Causes:

1) The rollers are sticky, aged, severely worn, or rubbing.

2) The conveyor rollers are sticky with foreign matter or severely worn.

Inspection:

1) External scars should be ground or removed if they are distributed in pieces.

2) On the pipe section with external scars, the area of external scars exceeding 10% should be removed or ground.

3) External scars with a depth exceeding 5% of the wall thickness should be ground.

4) The actual values of the wall thickness and outer diameter at the grinding point shall not be less than the minimum values required by the standard.

4. Hemp surface characteristics: The surface of the steel pipe presents uneven pits.

Causes:

1) The steel pipe stays in the furnace for too long or the heating time is too high so the surface generates too thick an iron oxide scale, which is not cleaned up and rolled into the surface of the steel pipe.

2) The high-pressure water dephosphorization equipment does not work properly, and the dephosphorization is not clean.

Inspection:

1) Hemp surfaces that do not exceed the negative deviation of the wall thickness are allowed to exist locally.

2) The area of the pitted surface shall not exceed 20% of the area of the pitted pipe section.

3) The pitted surface with excessive tolerance can be ground or removed, and the actual value of the wall thickness and outer diameter at the ground part shall not be less than the minimum value required by the standard.

4) Severe pitting is scrapped.

5. Blue line characteristics: The outer surface of the steel pipe presents symmetrical or asymmetrical straight-line rolling marks.

Causes:

1) The hole type of the sizing machine is misaligned or seriously worn.

2) The hole-type design of the sizing machine roller is unreasonable.

3) Rolling low-temperature steel.

4) The roller is not processed well, and the chamfer of the roller edge is too small.

5) The roller is not assembled well, the gap is too large, etc.

Inspection:

1) The outer surface of the casing is allowed to have a height of no more than 0.2mm blue line, and the excessive tolerance should be ground.

2) High-pressure container pipes are not allowed to have a hand-touch blue line. If there is a hand-touch blue line, it must be removed. The grinding area should be smooth and without edges and corners.

3) For general steel pipes (structures, fluids, hydraulic supports, etc.), the height of the blue line is allowed to be no more than 0.4mm. If the tolerance is exceeded, it should be ground.

4) The blue line with sharp edges should be ground smooth.

5) The actual value of the wall thickness and outer diameter at the grinding point shall not exceed the minimum value required by the standard.

6. Hairline characteristics: On the outer surface of the steel pipe, continuous or discontinuous hair-like fine lines appear.

Causes:

1) The tube blank has subcutaneous pores or inclusions.

2) The surface of the tube blank is not cleaned thoroughly, and there are fine cracks.

3) Excessive wear and aging of the roller.

4) The machining accuracy of the roller is not good, etc.

Inspection: No visible hairline is allowed on the outer surface of the steel pipe. If it exists, it should be completely removed. After removal, the actual value of the wall thickness and outer diameter shall not be less than the minimum value required by the standard.

7. Net crack characteristics: Small fish-scale cracks with bands and large pitches appear on the outer surface of the steel pipe.

Causes:

1) The harmful element content of the tube is too high (such as arsenic).

2) The perforation roller is aging and sticking to steel.

3) The guide plate is sticking to steel, etc.

Inspection: It should be completely removed. The actual wall thickness and outer diameter after removal shall not be less than the minimum value required by the standard.

8. Scratching characteristics: The outer surface of the steel pipe has spiral or linear groove defects, and the bottom of the groove can be seen in most cases.

Causes:

1) Mechanical scratches are mainly caused by rollers, cooling beds, straightening, and transportation.

2) The rollers are not processed well or are severely worn or there are foreign objects in the roller gap.

Inspection:

1) The outer surface of the steel pipe is allowed to have scratches not exceeding 0.5mm locally, and scratches exceeding 0.5mm should be ground. The actual wall thickness and outer diameter of the grinding part shall not be less than the minimum value required by the standard.

2) Scratches with sharp edges should be ground smooth.

9. Characteristics of dents: The outer surface of the steel pipe is concave and convex, but the wall thickness of the steel pipe is not damaged.

Causes:

1) It is dented during lifting.

2) It is dented when the straightening is bitten.

3) It is dented by the roller after the sizing machine, etc.

Inspection: Local dents that do not exceed the negative deviation of the outer diameter and have a smooth surface can exist. Cut off when the deviation exceeds the tolerance.

10. Characteristics of dents: Irregular scars are produced on the outer surface of the steel pipe due to collision.

Causes: It can be caused by various collisions in the cold zone and the hot zone.

Inspection:

1) Local dents with a depth of no more than 0.4mm are allowed on the outer surface.

2) Scratches exceeding 0.4mm should be smoothed and the actual values of the outer diameter and wall thickness at the grinding point shall not be less than the minimum value required by the standard.

11. Characteristics of concave correction: The outer surface of the steel pipe is spirally concave.

Causes:

1) Improper adjustment of the roller angle of the straightening machine and excessive pressure reduction.

2) Severe wear of the straightening rollers, etc.

Inspection: The outer surface of the steel pipe is allowed to have a straightening concave without obvious edges and corners and the inner surface is not protruding, and the outer diameter size meets the tolerance requirements. The excessive straightening concave should be removed.

12. Rolling fold characteristics: The steel pipe wall presents wrinkles with concave and convex outside and inside along the longitudinal direction locally or throughout the length, and the outer surface is strip-shaped.

Causes:

1) The hole width expansion coefficient is too small.

2) Improper adjustment of the rolling mill causes the hole to be misaligned or the rolling center line to be inconsistent.

3) Improper distribution of the pressure reduction of each frame of the continuous rolling mill, etc. Due to the above reasons, the metal enters the gap between the rollers during the rolling process of the steel pipe or the pipe loses stability, causing the pipe wall to wrinkle.

Inspection: It is not allowed. It should be removed or scrapped.

13. Pull-out characteristics: There is a pull-out crack phenomenon on the surface of the steel pipe, which mostly occurs on thin-walled pipes.

Causes:

1) Due to the uneven heating temperature of the tube billet, the deformation part is strong, and the low-temperature part is tensile rolled. When the tensile force is large, the tube will be torn.

2) Improper adjustment of the speed and roll gap of each frame of the continuous rolling mill causes the steel to tear.

3) The influence of the rough tube wall thickness. When the rough tube wall thickness supplied by the piercing machine to the continuous rolling mill is small, the metal deformation of the continuous rolling mill is smaller than the designed deformation, causing the continuous rolling mill to tensile roll, and tearing when the tensile force is large.

4) There are serious inclusions in the tube billet itself.

Inspection: Not allowed. It should be removed or scrapped.