Common Surface Defects of Seamless Tubes

Seamless steel pipe & tube, as an indispensable material in the modern industrial field, its quality and performance have a vital impact on the safety and stability of various engineering projects. However, in the production and use of seamless tubes, various defects often occur, which not only affect the performance of the steel pipe, but also may threaten the safety of the engineering project. Therefore, understanding the defect characteristics and causes of seamless tubes is of great significance to improving the quality and use effect of seamless steel pipes.

Common outer surface defects of seamless tubes (smls tube):

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.

Seamless steel pipe, as an indispensable material in the modern industrial field, its quality and performance have a vital impact on the safety and stability of various engineering projects. However, in the production and use of seamless steel pipes, various defects often occur, which not only affect the performance of the steel pipe, but also may threaten the safety of the engineering project. Therefore, understanding the defect characteristics and causes of seamless steel pipes is of great significance to improving the quality and use effect of seamless steel pipes.

Ultrasonic detection technology for seamless steel tubes:

Ultrasonic detection refers to the detection of seamless steel tubes using the principle of electromagnetic induction. When using the ultrasonic detection method for detection, it is first necessary to excite a beam of ultrasonic waves inside the seamless steel tube, which will propagate inside the pipe of the seamless steel tube. If the ultrasonic wave is reflected, it indicates that there are defects on the surface of the seamless steel tube. During the detection process, the ultrasonic wave will produce a series of changes inside the pipe of the seamless steel tube. The quantitative information of the defects of the seamless steel tube can be fully displayed by processing this information with a flaw detector. This detection method can detect multiple steel pipe defects at the same time with high sensitivity. It should be noted that this detection method is not suitable for fast production lines due to its slow detection speed. Therefore, this detection method is mostly used when inspecting high-quality seamless steel tubes in detail.

Causes of seamless tube defects:

1. Raw materials
The poor internal structure and poor surface quality of raw materials are important causes of seamless steel tube defects. Poor internal structure may cause defects such as internal folding, internal cracks in the pipe wall, and delamination; poor surface quality may cause defects such as external folding.

2. Process operation
Improper process operation such as improper heating, excessive deformation pressure, and incomplete cleaning of the pipe surface are also important causes of seamless steel pipe defects. Improper heating may cause defects such as internal folding, external folding, and internal cracks in the pipe wall; excessive deformation pressure may cause defects such as cracks.

3. Equipment
Poor equipment performance, looseness, scratches, etc. may also cause seamless steel tube defects. For example, damage to the piercing machine roll or the tube rolling mill roll may cause defects such as rolling folding.

4. Problems during transportation
Seamless tubes may be subject to adverse effects such as impact or extrusion during transportation, which may cause defects of varying degrees on the surface or inside of the pipe.

Prevention and control of seamless tube defects:

In order to reduce the occurrence of seamless steel tube defects and improve the quality and use effect of seamless steel tubes, the following measures need to be taken:

1. Strengthen raw material quality control
Strictly control the internal organization and surface quality of raw materials, and select high-quality raw materials for production.

2. Optimize process operation
Reasonably set process parameters such as heating temperature and deformation pressure to ensure the stability and accuracy of process operation. At the same time, strengthen the surface cleaning of the tube blank to ensure the smoothness and flatness of the tube blank surface.

3. Strengthen equipment maintenance and management
Regularly inspect and maintain production equipment to ensure the normal operation and stable performance of the equipment. Damaged equipment parts should be replaced and repaired in time to avoid equipment failures affecting the quality of seamless steel pipes.

4. Strengthen quality inspection and monitoring
Establish a complete quality inspection and monitoring system to monitor and inspect the production process of seamless steel pipes throughout the process. The defects found should be handled and rectified in time to ensure that the quality of seamless steel pipes meets the standard requirements.

Conclusion:

The defect characteristics and causes of seamless steel tubes are multifaceted, and comprehensive analysis and control are required from the aspects of raw materials, process operation, equipment, transportation, etc. By strengthening raw material quality control, optimizing process operation, strengthening equipment maintenance and management, and strengthening quality inspection and monitoring, the generation of seamless steel tube defects can be effectively reduced, and the quality and use effect of seamless steel tubes can be improved.

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