Steel pipes are widely used in many industries,if you know the cause of common external surface defects in steel pipes,you can avoid them.
1.Outward Fold
Features: A spiral layered fold on the outer surface of the steel pipe.
cause:
1) There are folds or cracks on the surface of the tube blank.
2) The pores under the skin of the tube blank are more serious.
3) The surface of the tube blank is poorly cleaned or has ears, wrong faces, etc.
4) During the rolling process, the surface of the steel pipe is scratched, and then pressed onto the base of the steel pipe by rolling to form an outer fold.
Inspection: not allowed to exist: slight grinding can be carried out, the actual value of the wall thickness and outer diameter after grinding should not be less than the minimum required by the standard.
3PE anti corrosion coating steel pipes
2.Detachment
Features: Spiral or massive layering and cracking on the surface of the steel pipe.
Causes: Serious non-metallic inclusions in the tube blank, residual shrinkage or severe looseness, etc.
Prosecution: Not allowed to exist.
3.External scarring
Features: Scars appear on the outer surface of the steel pipe.
cause:
1) Roller sticks to steel, aging, serious wear or roller.
2) Foreign matter sticks to the conveyor roller or wears out seriously.
Prosecution:
1) The scars on the outside should be sharpened or removed.
2) On the tube section with external scarring, the external scarring area should be removed or repaired if it exceeds 10%.
3) External scars with a depth exceeding 5% of the wall thickness should be repaired.
4) The actual value of wall thickness and outer diameter of the repaired place shall not be less than the minimum value required by the standard.
Carbon Steel Pipes
4.Ma Noodles
Features: The surface of the steel pipe shows uneven pits.
cause:
1) The steel tube stays in the furnace for too long or the heating time is too high, making the surface of the oxide scale too thick, which is not cleaned and rolled into the surface of the steel tube.
2) The high-pressure water dephosphorization equipment is not working properly, and the dephosphorization is not clean.
Prosecution:
1) A pitted surface that does not exceed the negative deviation of the wall thickness is allowed to exist.
2) The area of the hemp surface must not exceed 20% of the area of the pipe section with hemp surface.
3) The super-differential pitted surface can be repaired or cut off. The actual value of the wall thickness and outer diameter of the repaired place should not be less than the minimum value required by the standard. 4) Severe numbness verdict.
5.Green Line
Features: The outer surface of the steel pipe presents symmetrical or asymmetrical linear marks.
cause:
1) The hole pattern of the sizing machine is misaligned or badly worn.
2) The roll pass design of the sizing machine is unreasonable.
3) Rolling low temperature steel.
4) The processing of the roll is not good, the edge chamfering of the roll is too small.
5) Roller assembly is not good, the gap is too large, etc.
Prosecution:
1) The outer surface of the casing is allowed to have a green line with a height not exceeding 0.2mm, and excessive tolerances should be ground.
2) The high-pressure container pipes are not allowed to have the feel of blue lines. Feel blue lines must be removed. The grinding place should be smooth and without edges.
3) General pipe steel pipes (structures, fluids, hydraulic supports, etc.) are allowed to have a height of no more than 0.4mm green line.
4) Sharp green lines on the edges should be smoothed.
5) The actual value of the wall thickness and outer diameter of the grinding place shall not exceed the minimum value required by the standard.
Pipe Spools Flanged Fittings
6.Hairline
Features: On the outer surface of the steel pipe, continuous or discontinuous hair-like fine lines appear.
cause:
1) The tube blank has pores or inclusions under the skin.
2) The surface of the tube blank is not cleaned completely, and there are small cracks.
3) Excessive wear and aging of rollers.
4) The processing accuracy of the roll is not good.
Inspection: No visible hairline is allowed on the outer surface of the steel pipe. If there is any hairline, it should be completely removed. The actual value of the wall thickness and outer diameter should not be less than the minimum value required by the standard.
7.Mesh crack
Features: On the outer surface of the steel pipe, there are band-shaped and large pitch fish-like small cracks.
cause:
1) The harmful element content of the tube blank is too high (such as arsenic element).
2) Aging perforated rollers and sticky steel.
3) The guide plate is glued with steel, etc.
Judgment: It should be completely cleared. The actual values of wall thickness and outer diameter after removal shall not be less than the minimum value required by the standard.
8.Scratches
Features: The outer surface of the steel pipe is spiral or linear groove-shaped defects, most of which can be seen at the bottom of the groove.
cause:
1) Mechanical scratches are mainly caused by roller table, cooling bed, straightening and transportation.
2) The rollers are not processed well or are worn seriously or there are foreign objects between the rollers.
Prosecution:
1) The external surface of the steel pipe is allowed to have scratches not exceeding 0.5mm, and scratches exceeding 0.5mm should be repaired. The actual value of the wall thickness and outer diameter of the grinding place shall not be less than the minimum value required by the standard.
2) Sharp scratches on the edges should be smoothed.
ASTM A312 TP316L seamless steel pipes
9.Bump
Features: The outer surface of the steel pipe is concave and convex, and the wall thickness of the steel pipe is not damaged.
cause:
1) Struck until deflated during lifting.
2) Collapse when straightening and biting.
3) The roller table after the sizing machine collided.
Judgment: local deflation that does not exceed the negative deviation of the outer diameter and the surface is smooth can exist. Cut off when out of tolerance.
10.Bruises
Features: Irregular scars on the outer surface of the steel pipe due to collision.
Cause: It can be caused by various collisions between the cold zone and the hot zone.
Prosecution:
1) The external surface allows local scratches with a depth not exceeding 0.4mm.
2) Bumps over 0.4mm should be smoothed and the actual value of the outer diameter and wall thickness of the sharpened place should not be less than the minimum value required by the standard.
11.Concave
Features: The outer surface of the steel pipe is spirally concave.
cause:
1) Improper adjustment of the roller angle of the straightener and excessive reduction.
2) Straightening roller wear is serious.
Inspection: The outer surface of the steel pipe is allowed to have no obvious corners and the inner surface is not protruding, and the outer diameter size meets the tolerance requirements of the concave. Excessive correction should be removed.
12.Folded
Features: The wall of the steel pipe is partially wrinkled or convex in the longitudinal direction, and the outer surface is concave and convex.
cause:
1) The selection of hole spreading coefficient is too small.
2) Improper adjustment of the rolling mill causes misalignment of the pass pattern or inconsistent rolling centerline.
3) Improper distribution of the reduction of each stand of the continuous rolling mill.
For the above reasons, during the rolling process, the metal enters the roll gap or the tube loses its stability, causing the tube wall to wrinkle.
Prosecution: Not allowed to exist. Should be removed or rejected.
13.Crack
Features: The surface of the steel pipe is pulled apart and cracked, which mostly occurs on thin-walled pipes.
cause:
1) Due to the uneven heating temperature of the tube blank, the deformed part is handsome, and the part with low temperature is rolled by tension. When the tension is large, the tube is pulled apart.
2) The speed and roll gap of each stand of the continuous rolling mill are not adjusted properly, causing the steel to tear.
3) The influence of the thickness of the capillary wall. When the thickness of the capillary wall supplied by the piercer to the continuous rolling mill is small, the metal deformation in the continuous rolling mill is reduced compared with the design deformation, which causes the continuous rolling mill to be rolled under tension and tear when the tension is large.
4) There are some serious inclusions in the tube blank itself.
Prosecution: Not allowed to exist. Should be removed or rejected.
