Common internal and external surface defects of seamless steel tubes

 Common defects of seamless steel pipes can be divided into two categories: external surface defects and internal surface defects.

1. External surface defects

(1) Folding defects are irregularly distributed.

Macroscopic features: If there is mold residue left on the surface of the continuous casting billet, the outer surface of the rolled tube will have deep folds.There are stacking defects, and they are distributed longitudinally, and there will be a blockage phenomenon in some parts of the surface, and dark gray blocks will be exposed at the part where the blockage occurs.

Microscopic features: The folding depth of the rolled tube is about 0.5 ~ 1mm, and the folding direction of the distribution is 40°~60°.

(2) Longitudinal distribution of large folding defects.

Macroscopic features: Crack defects and large folding defects appear on the surface of the continuous casting billet, and they are distributed longitudinally.

Microscopic features: Most of the folds on the surface of the seamless steel pipe are about 1 ~ 10mm deep, and the folds contain iron oxide. There are no other defects around and in the middle of the folds, and there is serious oxidation and decarburization around the folds.

(3) Small crack defects.

Macroscopic characteristics: There are surface defects on the outer wall of the pipe body that cannot be observed with the naked eye when the seamless steel pipe is inspected.

Microscopic features: There are many small folding defects on the surface of the seamless steel pipe, the deepest depth is about 0.15mm, the surface of the seamless steel pipe is covered with a layer of iron oxide, and there is a decarburization layer under the iron oxide, with a depth of about 0.2mm.

(4) Linear defects.

Macroscopic features: There are linear defects on the outer surface of the seamless steel pipe, and the specific features are shallow depth, wide opening, visible bottom, and certain width.

Microscopic Features: The visible depth of the outer wall of the cross-section of the seamless steel pipe.

2. Internal surface defects

(1) Convex hull defects.

Macroscopic features: There are randomly distributed longitudinal small convex hull defects on the inner wall of the seamless steel pipe, and the height of these small convex hull defects is about 0.2mm~ 1mm.

Microscopic features: There are chain-like black-gray inclusions at the tail, middle and surrounding sides of the inner wall convex hull of the cross-section of the seamless steel pipe. This type of black-gray chain contains calcium aluminate and few complex oxides (iron oxide, silicon oxide, magnesium oxide).

(2) Straight defects.

Macroscopic features: The seamless steel pipe has straight-shaped defects with a certain depth and width, similar to scratches.

Microscopic features: The scratches on the inner wall of the cross-section of the seamless steel pipe are in the shape of grooves with a depth of 1~2cm. The oxidation and decarburization of the groove edges does not appear. The structure around the grooves has metal rheology and deformation extrusion characteristics. The bottom of the grooves is usually There will be micro-cracks due to sizing extrusion during the sizing process.

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