Common outer surface defects of seamless tubes (smls):

1. Folding defect
Irregular distribution: If mold slag remains locally on the surface of the continuous casting slab, deep folding defects will appear on the outer surface of the rolled tube, and they will be distributed longitudinally, and "blocks" will appear on some parts of the surface. The folding depth of the rolled tube is about 0.5 ~ 1mm, and the distribution folding direction is 40° ~ 60°.

2. Large folding defect
Longitudinal distribution: Crack defects and large folding defects appear on the surface of the continuous casting slab, and they are distributed longitudinally. Most of the folding depths on the surface of seamless steel tubes are about 1 to 10 mm.

3. Small crack defects
When testing seamless steel tubes, there are surface defects on the outer wall of the pipe body that cannot be observed by naked eyes. 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, the depth is about 0.2mm.

4. Linear defects
There are linear defects on the outer surface of the seamless steel tube, and the specific characteristics are shallow depth, wide opening, visible bottom, and constant width. The outer wall of the cross-section of the seamless steel pipe can be seen with scratches with a depth of <1mm, which are in the shape of a groove. After heat treatment, there is oxidation and decarburization at the edge of the groove of the pipe.

5. Scarring defects
There are shallow pit defects on the outer surface of the seamless steel tube, with different sizes and areas. There is no oxidation, decarburization, and aggregation and inclusions around the pit; the tissue around the pit is squeezed under high temperature, and plastic rheological characteristics will appear.

6. Quenching crack
Quenching and tempering heat treatment is carried out on the seamless steel tube, and longitudinal fine cracks appear on the outer surface, which are distributed in strips with a certain width.

Common inner surface defects of seamless tubes:

1. Convex hull defect
Macroscopic features: The inner wall of the seamless steel tube has randomly distributed small longitudinal convex defects, and the height of these small convex defects is about 0.2mm to 1mm.
Microscopic characteristics: There are chain-like black-gray inclusions at the tail, middle and surrounding of the convex hull on both sides of the inner wall of the cross-section of the seamless steel pipe. This type of black-gray chain contains calcium aluminate and a small amount of composite oxides (iron oxide, silicon oxide, magnesium oxide).

2. Straight defect
Macroscopic features: Straight-type defects appear in seamless steel tubes, with a certain depth and width, similar to scratches.

Microscopic characteristics: The scratches on the inner wall of the cross-section of the seamless steel tube are in the shape of a groove with a depth of 1 to 2 cm. Oxidative decarburization does not appear at the edge of the groove. The surrounding tissue of the groove has the characteristics of metal rheology and deformation extrusion. There will be microcracks due to sizing extrusion during the sizing process.