Distribution Mechanism and Harm of Welded Steel Pipe Defects

The production process of welded steel tube is to use various forming methods to roll steel plates, steel strips, etc. straightly or spirally into the required cross-sectional shape. Then, with the help of heating and pressure, different welding methods are used to weld the welds together to obtain steel pipes. Therefore, the defects of welded steel pipes are divided into two parts: steel plate base metal defects and weld defects.

1. Defects of base metal of steel plate

After rolling and other processes, most of the defects in the plate are planar and parallel to the surface; the main defects include delamination, inclusions, cracks, folds, etc., among which delamination is the most common internal defect. Delamination will produce various cracks. When the plate is subjected to tensile stress perpendicular to the surface, delamination will seriously affect the strength of the steel pipe. It is an unacceptable defect.

2. Weld defects

Weld defects refer to the defects generated in the weld during or after welding, and are divided into weld defects such as cracks, pores, slag inclusions, incomplete penetration, incomplete fusion, and undercut. Dense pores and slag inclusions in the weld are dense three-dimensional defects, and cracks and incomplete fusion are planar defects, which are very harmful. Strip-shaped slag inclusions and incomplete penetration are strip-shaped defects, which are very harmful. Pores, small slag inclusions, etc. are point defects. Defects in the weld are more likely to cause problems such as strength and plasticity of the steel pipe, seriously affecting the quality of the steel pipe. The quality of welded steel pipes directly affects the safe operation and service life of oil and gas pipelines. Therefore, the flaw detection of welds is mainly aimed at the dangerous defects such as cracks, pores, slag inclusions, incomplete penetration, and incomplete fusion in welds.

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