The production process of welded steel pipes involves rolling steel plates or strips into the desired cross-sectional shape either directly or in a spiral direction using various forming methods. Subsequently, the seams are welded together through heating and pressing, utilizing different welding techniques to obtain the steel pipe. Consequently, defects in welded steel pipes can be categorized into two parts: weld defects and base material defects.
Weld defects refer to imperfections that occur during or after fusion welding within the weld seam. These include cracks, porosity, slag inclusion, incomplete penetration, lack of fusion, undercutting, among others. Dense porosity and slag inclusion in the weld seam are classified as dense volumetric defects, while cracks and lack of fusion are considered planar defects, both of which pose significant hazards. Linear slag inclusion and incomplete penetration are categorized as linear defects, also posing considerable risks. Porosity and small slag inclusions, on the other hand, are point defects.
Defects within the weld seam are more prone to causing issues with the strength and plasticity of steel pipes, significantly impacting their quality. Furthermore, the quality of welded steel pipes directly affects the safe operation and service life of oil and gas pipelines. Therefore, weld inspection primarily focuses on detecting hazardous defects such as cracks, porosity, slag inclusion, incomplete penetration, and lack of fusion within the weld seam.
Defects in the steel plate, after processes like rolling, mostly appear as planar defects, parallel to the surface. The primary defects include lamination, inclusions, cracks, and folds, with lamination being the most common internal defect. Lamination can lead to various cracks, and when the plate is subjected to tensile stress perpendicular to the surface, it severely affects the strength of the steel pipe, making it an unacceptable defect.
