Inner surface defects of hot rolled seamless carbon steel pipe

Fold inward

Features: There are straight line or helical, semi-spiral zigzag defects on the inner surface of the seamless pipe.
cause:
1) Tube blank: loose center and segregation; serious residual shrinkage; non-metallic inclusions exceeding the standard.
2) The tube blank is heated unevenly, the temperature is too high or too low, and the heating time is too long.
3) Perforation area: serious wear of the plug; improper adjustment of the parameters of the perforator; aging of the perforation roller, etc.
Judgment: Inward folds are not allowed on the inner surface of the steel pipe. The inner folds of the pipe ends should be ground or re-cut. The actual value of the wall thickness at the grinding place shall not be less than the minimum value required by the standard; usually, the inner folds are considered waste.

Inner scarring

Features: The inner surface of the steel pipe presents scars, which generally do not take root and are easy to peel off.
cause:
1) Graphite lubricant contains impurities.
2) The iron lug at the rear end of the waste pipe is pressed into the inner wall of the steel pipe, etc.
Judgment: The inner surface of the steel pipe is not allowed to exist, the pipe end should be ground and re-cut, and the grinding depth should not exceed the negative deviation of the standard requirement; the actual wall thickness should not be less than the minimum value required by the standard;

Warped skin

Features: The inner surface of the steel tube presents straight or intermittent fingernail-like raised skins. It usually appears on the head of the capillary and is easy to peel off.
cause:
1) Improper adjustment parameters of the punching machine.
2) The head is glued to steel.
3) The accumulation of iron oxide scale in the waste pipe, etc.
Judgment: The inner surface of the steel pipe is allowed to have warped skins that are easy to peel off without roots (or burn off during heat treatment). Rooted warps should be ground or excised.

Inside straight

Features: There are linear scratches with a certain width and depth on the inner surface of the steel pipe.
cause:
1) The rolling temperature is low, and the mandrel sticks with metal hard objects.
2) Graphite contains impurities, etc.
Judgment:
1) The casing and ordinary pipes are allowed to have inner straights with a depth not exceeding 5% (the maximum depth of pressure vessels is 0.4mm).
2) The inner straights with sharp edges should be smoothed.

Inner edge

Features: There are linear protrusions with a certain width and depth on the inner surface of the steel pipe.
Cause: The mandrel is severely worn, and the grinding is not smooth or too deep.
Judgment:
1) The casing and line pipes are allowed to have inner edges whose height does not exceed 8% of the wall thickness, and the maximum height does not exceed 0.8 mm without affecting the diameter. If it is out of tolerance, it should be repaired, ground and cut again.
2) Common pipes and line pipes are allowed to have inner edges whose height does not exceed 8% of the wall thickness (maximum height is 0.8mm). Out of tolerance should be ground and re-cut.
3) For the L2 level (ie N5) flaw detection, the inner edge height of the steel pipe shall not exceed 5% (the maximum height is 0.5mm). Out of tolerance should be ground and re-cut.
4) The inner edges with sharp edges should be smoothed.

Inner drum bag

Features: The inner surface of the steel pipe exhibits regular convexity and the outer surface is not damaged.
Cause: Excessive grinding of continuous rollers or loss of meat, etc.
Judgment: Judgment according to the requirements of the inner edge.

Pull concave

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 and mismatch of rolling speed of each stand.
2) The tube blank is heated unevenly or the temperature is too low.
3) The rolling center line deviates, the steel pipe collides with the roller table after continuous rolling, etc. (Note: This reason was proposed in 2003.1, and the principle is still under discussion).
Judgment: The negative deviation of the wall thickness is not exceeded, and the actual wall thickness is greater than the minimum required wall thickness. Excessive pull concave should be removed. (Note: The serious development of the pull concave is the pull crack, and this kind of injury should be strictly inspected).

Internal thread (this defect only occurs in Axel units)

Features: There are spiral traces on the inner surface of the steel pipe, which mostly appear on the inner surface of the thin-walled pipe, and have an obvious bumpy feel.
cause:
1) The inherent defects of the skew rolling process. This defect is more prominent when the process parameters of the Axel tube mill are not adjusted properly.
2) The distribution of deformation is unreasonable, and the Axel wall reduction is too large.
3) Improper configuration of Axel rolling rolls.
Judgment: The defect depth of the inner thread of the steel pipe is not more than 0.3mm, and is within a certain tolerance range.