Inner Surface Defects of Stainless Steel Seamless Pipes

Inner surface defects of stainless steel seamless pipes are flaws that form on the inside of the pipe during production. They are generally related to tube billet quality, heating, piercing, rolling and the condition of the related equipment. This article describes the characteristics, causes and inspection requirements of eight stainless steel seamless pipe defects: inward folds, internal scarring, warping, internal straight lines and inner edges (both linear defects in the steel tube), internal bulges, dimples and internal thread.

Inner Surface Defects

Acceptance limits for these defects depend on the applicable product standard and the purchase order. Where a limit is mentioned below, confirm it against the standard for the specific product.

1. Inward Fold

Characteristics: Straight, spiral or semi-spiral serrated defects appear on the inner surface of the stainless steel seamless pipe.

Causes:

1) Tube billet: loose center, segregation, serious residual shrinkage holes, or non-metallic inclusions exceeding the standard.

2) Uneven heating of the tube billet, temperature that is too high or too low, or heating time that is too long.

3) Piercing area: serious plug wear, improper adjustment of the piercer parameters, aging of the piercing rolls, etc.

Inspection: Inward folds are not allowed on the inner surface of the steel pipe. Inward folds at the pipe ends should be ground off or the ends re-cut. After grinding, the actual wall thickness at the ground area shall not be less than the minimum value required by the standard. Inward folds running the full length of the pipe are judged as scrap.

2. Internal Scarring (Scabs)

Characteristics: Scab-like defects appear on the inner surface of the stainless steel seamless pipe. They generally have no root and peel off easily

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: Internal scarring is not allowed on the inner surface of the steel pipe. At the pipe ends it should be ground off or the ends re-cut, and the grinding depth should not exceed the negative deviation required by the standard. The actual wall thickness shall not be less than the minimum value required by the standard. Internal scarring along the whole length is scrapped.

3. Warping (Raised Skin)

Characteristics: Warping on the inner surface of the steel pipe appears as straight or intermittent small raised skins in the shape of nails. It often appears at the head of the rough pipe and peels off easily. Here, warping refers to these raised skins, not to distortion of the pipe itself.

Causes:

1) Improper adjustment of the perforator parameters.

2) Steel sticking to the head.

3) Iron oxide scale accumulation in the rough pipe, etc.

Inspection: The inner surface of the stainless steel seamless pipe is allowed to have warping without roots that are easy to peel off (or can be burned during heat treatment). Warping with roots should be ground or cut off.

4. Internal Straight Lines

Characteristics: Linear scratches of a certain width and depth appear on the inner surface of the steel pipe.

Causes:

1) Low rolling temperature, with hard metal objects stuck to the core rod (mandrel).

2) Impurities in the graphite, etc.

Inspection:

1) For casing pipe and general pipe, internal straight lines within the depth limit of the applicable standard are allowed. Pipe for pressure vessels is subject to a separate depth limit.

2) Internal straight lines with sharp edges should be ground smooth.

Read a full guide: Inner Surface Defects of Stainless Steel Seamless Pipes

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