What are the common quality problems of seamless steel pipes?

The common quality problems of seamless steel pipes (SMLS) mainly include the following five types:

1. Surface quality problems

- Cracks: Straight or spiral cracks appear on the inner and outer surfaces of the steel pipe, with sharp corners at both ends and the bottom, and may be distributed in a network. The reasons include poor steel quality, improper heating of the tube billet, excessive deformation pressure, etc. - Hairline: Continuous or discontinuous hair-like fine lines on the outer surface, mostly spiral, opposite to the rotation direction of the piercing machine, caused by poor steel quality, incomplete surface cleaning of the tube billet, etc.

2. Folding

- External folding: It is manifested as a spiral fold on the outer surface, the direction is opposite to the spiral direction of the rough pipe piercing, caused by residual cracks or ears on the surface of the tube billet, poor steel quality, etc.

- Rolling folding: The outer surface shows regular folding, caused by damage to the piercing machine or the rolling mill roll. - Scarring: Shallow pit defects of varying sizes and areas appear in some positions on the outer surface, and there is no oxidation decarburization or aggregation inclusions around the pits.

- Scratches: Scratches on the surface of the steel pipe may be caused by contact and friction between equipment components and the surface of the steel pipe during production, or collision with other objects during transportation and storage.

- Rust: Rust spots appear on the surface, affecting the appearance and corrosion resistance, which is related to the corrosion resistance of the material itself, high humidity in the storage environment, and corrosive media.

3. Dimensional accuracy issues

- Outer diameter deviation: The outer diameter exceeds the standard range, affecting the installation with other components, which is caused by factors such as sizing (reducing) diameter method, equipment operation, and process system.

- Wall thickness deviation: The wall thickness is uneven or does not meet the nominal wall thickness requirements, which is related to the heating quality of the tube blank, the parameters of each deformation process, the quality of the tool and the quality of lubrication.

- Length deviation: The actual length does not match the specified length, which may cause insufficient length or waste during installation. Inaccurate cutting control and measurement errors during production will cause this problem.

- Excessive ovality: The cross section of the steel pipe is not round and exceeds the specified ovality range, which affects the uniformity of force and sealing of the steel pipe. It can be caused by improper equipment adjustment and uneven force on the tube billet during perforation and rolling processes.

4. Internal quality problems

- Internal folding: Straight or spiral serrated defects appear on the inner surface. High-alloy steel pipes may be irregular blocks distributed at the inlet. This is caused by excessive pressure reduction of the plug and severe wear of the plug.

- Inclusions: Non-metallic inclusions such as calcium aluminate and composite oxides exist inside, which reduce strength and toughness. They are caused by impure raw materials and slag mixing during the smelting process.

- Bubbles: The cavities formed by gas inside will become a source of stress concentration and increase the risk of rupture. The main reasons are incomplete deoxidation during smelting and gas entrainment during pouring.

- Layering: Layered separation occurs inside the steel pipe, which is mostly caused by uneven internal structure of the raw materials, segregation, or uneven metal flow during rolling.

5. Performance issues:

- Mechanical properties do not meet the standards: tensile strength, yield strength, toughness, hardness, etc. do not meet the standard requirements, which is related to unqualified chemical composition and improper heat treatment process, which will cause the steel pipe to be easily deformed and broken during use.

- Poor corrosion resistance: Insufficient corrosion resistance in specific environments, such as insufficient content of elements such as chromium and nickel in the chemical composition, or inadequate surface treatment, which causes premature corrosion of the steel pipe and shortens its service life.

- Poor welding performance: cracking, leakage and other problems occur during welding, affecting construction and use, which is related to improper selection of chemical composition of steel pipes and welding process parameters.

Read more: ASTM Standard Seamless Steel Pipe Quality Inspection or Carbon steel tube material and use

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