First, inner surface defects
1. Internal fold
Features: Straight or spiral, semi-spiral serrated defects appear on the inner surface of the steel pipe.
Causes:
1) Tube billet: loose center, segregation; serious shrinkage hole residue; non-metallic inclusions exceed the standard.
2) Uneven heating of the tube billet, too high or too low temperature, too long heating time.
3) Perforation area: serious wear of the head; improper adjustment of the perforator parameters; aging of the perforating roller, etc.
Inspection: Internal folds are not allowed on the inner surface of the steel pipe. The internal folds at the pipe end should be ground or re-cut. The actual wall thickness at the grinding point shall not be less than the minimum value required by the standard; the full-length internal fold is judged to be scrapped.
2. Internal scar
Features: The inner surface of the steel pipe shows scars, which generally do not take root and are easy to peel off.
Causes:
1) Impurities in the graphite lubricant.
2) The iron ear at the rear end of the rough pipe is pressed into the inner wall of the steel pipe, etc.
Inspection: The inner surface of the steel pipe is not allowed to exist. The pipe end should be ground and re-cut. The grinding depth should not exceed the negative deviation required by the standard. The actual wall thickness should not be less than the minimum value required by the standard. The scarring inside the full length is considered scrapped.
3. Warping
Features: The inner surface of the steel pipe presents a straight line or intermittent nail-shaped warped small skin. It often appears at the head of the rough pipe and is easy to peel off.
Causes:
1) Improper adjustment parameters of the perforator.
2) Steel sticking to the head.
3) Iron oxide scale accumulation in the rough pipe, etc.
Inspection: The inner surface of the steel pipe is allowed to be rootless and easy to peel off (or can be burned during heat treatment). Rooted warping should be ground or cut off.
4. Internal straight road
Features: There are linear scratches with a certain width and depth on the inner surface of the steel pipe.
Causes:
1) The rolling temperature is low and the core rod is stuck with metal hard objects.
2) Impurities in the graphite, etc.
Inspection:
1) The casing and general pipe are allowed to have inner straights with a depth not exceeding 5% (maximum depth of 0.4mm for pressure vessels).
The inner straights should be ground and cut.
2) The sharp-edged inner straights should be ground and smooth.
5. Inner edge
Features: There are linear protrusions with a certain width and depth on the inner surface of the steel pipe.
Cause: The core rod is severely worn, and the grinding is not smooth or too deep.
Inspection:
1) The casing and pipeline are allowed to have inner edges with a height not exceeding 8% of the wall thickness and a maximum height not exceeding 0.8mm which does not affect the diameter. Excessive tolerance should be repaired and re-cut.
2) General pipes and pipelines are allowed to have inner edges with a height not exceeding 8% of the wall thickness (maximum height is 0.8mm). Excessive tolerance should be ground and re-cut.
3) For L2 grade (i.e. N5) flaw detection steel pipes, the height of the inner edge shall not exceed 5% (maximum height is 0.5mm). Excessive tolerance should be ground and recut.
4) The inner edge with a sharp edge should be ground smooth.
6. Internal bulge
Features: The inner surface of the steel pipe presents regular convexity and the outer surface is not damaged.
Cause: The grinding amount of the continuous rolling roller is too large or the meat is lost, etc.
Inspection: Inspection according to the requirements of the inner edge.
7. Pull-out
Features: The inner surface of the steel pipe presents regular or irregular pits and the outer surface is not damaged.
Cause:
1) Improper adjustment of continuous rolling, the rolling speed of each stand roller does not match.
2) Uneven heating of the tube billet or too low temperature.
3) Departure of the rolling center line, collision between the steel pipe and the roller after continuous rolling, etc. (Note: This reason was proposed in 2003.1, and the principle is still under discussion).
Inspection: Pull-outs that do not exceed the negative deviation of the wall thickness and the actual wall thickness is greater than the minimum value of the wall thickness requirement is allowed to exist. Pull-outs that exceed the standard should be removed. (Note: Severe development of pull-outs is pull-out cracks, and this kind of damage should be strictly inspected).
8. Internal thread (this defect only occurs in the Assel unit)
Features: There are spiral marks on the inner surface of the steel pipe, which mostly appear on the inner surface of thin-walled pipes, and have an obvious uneven feel. Causes:
1) Inherent defects of the oblique rolling process. This defect is more prominent when the process parameters of the Assel tube rolling mill are not adjusted properly.
2) The deformation distribution is unreasonable, and the Assel wall reduction is too large.
3) The Assel rolling roller is improperly configured.
Inspection: The depth of the internal thread defect of the steel pipe is not more than 0.3mm, and it is within a certain tolerance range.
Second, external surface defects
1. External folding
Features: Spiral layered folds appear 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 more serious.
3) The surface of the tube blank is not well cleaned or has ears, misaligned faces, 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: Slight grinding can be performed, and the actual wall thickness and outer diameter after grinding shall not be less than the minimum value required by the standard.
2. Delamination
Features: Spiral or block-shaped stratification and cracking on the surface of the steel pipe.
Causes: Serious non-metallic inclusions in the tube blank, residual shrinkage holes or severe looseness, etc.
Inspection: Not allowed.
3. External scarring
Features: Scarring on the outer surface of the steel pipe.
Causes:
1) Roller sticking to steel, aging, severe wear, or roller rubbing.
2) Conveyor roller sticking to foreign matter or severe wear.
Inspection:
1) External scarring should be ground or removed if it is distributed in pieces.
2) On the pipe section with external scarring, the area of external scarring exceeds 10% and 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 value required by the standard.
4. Pockmarks
Features: 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 that the surface oxide scale is too thick, which is not cleaned up and rolled onto the surface of the steel pipe.
2) The high-pressure water dephosphorization equipment does not work properly, and the phosphorus removal is not clean.
Inspection:
1) Pockmarks that do not exceed the negative deviation of the wall thickness are allowed to exist locally.
2) The area of the pits shall not exceed 20% of the area of the pitted pipe section.
3) The out-of-tolerance pits can be ground or removed, and the actual values of the wall thickness and outer diameter at the grinding point shall not be less than the minimum value required by the standard. 4) Severe pits are scrapped.
5. Blue lines
Features: 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 severely worn.
2) The hole-type design of the sizing machine roller is unreasonable.
3) Rolling low-temperature steel.
4) The roller processing is not good, and the chamfer of the roller edge is too small.
5) The roller assembly is not good, 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 excess should be ground.
2) High-pressure container pipes are not allowed to have a hand-felt blue line. If there is a hand-felt blue line, it must be removed. The grinding area should be smooth and without edges.
3) General steel pipes (structures, fluids, hydraulic supports, etc.) are allowed to have a height of no more than 0.4mm blue line, and the excess should be ground.
4) The sharp edge of the blue line should be ground smooth.
5) The actual value of the wall thickness and outer diameter value at the grinding area shall not exceed the minimum value required by the standard.
6. Hairline
Features: On the outer surface of the steel pipe, there are continuous or discontinuous hair-like fine lines.
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) Poor processing accuracy of the roller, 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-like cracks
Features: Small fish-scale cracks with bands and large pitch appear on the outer surface of the steel pipe.
Causes:
1) The harmful element content of the tube blank is too high (such as arsenic).
2) The perforating roller is aged and sticks to the steel.
3) The guide plate sticks to the steel, etc.
Inspection: It should be completely removed. The actual value of the 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 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 values of the wall thickness and outer diameter at the grinding point shall not be less than the minimum value required by the standard.
2) Scratches with sharp edges should be ground smooth.
9. Bumping
Features: The outer surface of the steel pipe is concave and convex, and the wall thickness of the steel pipe is not damaged.
Causes:
1) Bumping during lifting.
2) Bumping during straightening.
3) Bumping of the roller after the sizing machine, etc.
Inspection: Bumping that does not exceed the negative deviation of the outer diameter and has a smooth surface can exist. Cut off when out of tolerance.
10. Bumps
Features: Irregular scratches 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) The outer surface is allowed to have local bruises with a depth not exceeding 0.4mm.
2) Bumps 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. Concave correction
Features: The outer surface of the steel pipe is spirally concave.
Causes:
1) Improper adjustment of the straightening machine roller angle and excessive pressure reduction.
2) Severe wear of the straightening roller, etc.
Inspection: The outer surface of the steel pipe is allowed to have concave corrections without obvious edges and corners the inner surface is not protruding, and the outer diameter size meets the tolerance requirements. Concave corrections that exceed the standard should be removed.
12. Rolling folds
Features: The steel pipe wall presents wrinkles with concave and convex outside and inside along the longitudinal direction locally or throughout the entire length, and the outer surface is concave in strips.
Causes:
1) The hole width 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: Not allowed. It should be cut off or scrapped.
13. Pull-out cracking
Characteristics: There is a phenomenon of pulling and cracking 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 tensile rolled in the low-temperature part. When the tensile force is large, the pipe is pulled and cracked.
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 rough tube wall thickness. When the rough tube wall thickness supplied by the punching 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 roll with tensile force, and sometimes tearing when the tensile force is large.
4) There are serious inclusions in the tube itself.
Inspection: Not allowed. It should be cut off or scrapped.
Third, size out of tolerance
1. Uneven wall thickness
Features: The wall thickness of the steel pipe is uneven on the same section, and the maximum wall thickness and the minimum wall thickness are very different.
Causes:
1) Uneven heating of the tube.
2) The rolling line of the punching machine is not adjusted, and the centering roller is unstable.
3) The head is worn or the back hole of the head is eccentric.
4) Correction of the centering hole of the tube.
5) The curvature and cutting slope of the tube are too large.
Inspection: Measure each tube, and the end with uneven wall thickness should be cut off.
2. Wall thickness tolerance
Features: The wall thickness of the steel pipe exceeds tolerance in one direction. The one with positive deviation is called excessive wall thickness; the one with negative deviation is called excessive wall thickness.
Causes:
1) Uneven heating of the tube blank.
2) Improper adjustment of the piercing machine.
Inspection: Measure each pipe, the end deviation should be cut off, and the full-length deviation should be revised or scrapped.
3. Outer diameter deviation
Features: The outer diameter of the steel pipe exceeds the standard. The one with a positive deviation is called a large outer diameter, and the one with a negative deviation is called a small outer diameter.
Causes:
1) The hole type of the sizing machine is worn too much, or the new hole type design is not reasonable.
2) The final rolling temperature is unstable.
Inspection: Measure each pipe, and the one with excess should be revised or scrapped.
4. Bending
Features: The steel pipe is not straight along the length direction or the bend at the end of the steel pipe is called “goose head bend”.
Causes:
1) Local water cooling during artificial heat inspection.
2) Improper adjustment during straightening, and serious wear of the straightening roller.
3) Improper processing, assembly, and adjustment of the sizing machine.
4) Bending caused during lifting and transportation.
Inspection: When the bending exceeds the standard, it can be re-straightened for a second time, otherwise it will be scrapped. The “goose head bend” that cannot be straightened should be removed.
5. Length deviation
Features: The length of the steel pipe exceeds the requirement, the excess positive deviation is called long length, and the excess negative deviation is called short length.
Causes:
1) The length of the tube billet exceeds the standard.
2) Unstable rolling.
3) Poor control during slitting, etc.
Inspection: Long-length tubes are cut or re-judged, short length tubes are re-judged or scrapped
Post time: Dec-17-2024