Ultra-Narrow Gap Welding Process Experiment of Underwater Cast Steel Pipeline

The installation and laying of underwater pipelines are the cornerstone of ocean engineering. They are crucial to the distribution of important fossil energy sources such as oil and natural gas around the world. Welding technology is an indispensable part of pipe connection. A new welding method is proposed to improve the quality of welded joints of underwater cast steel pipelines, which is the flux band-constrained arc ultra-narrow gap welding (UNGW) method. UNGW has unique welding advantages compared to traditional welding methods, including high welding efficiency, excellent welding joint performance, small welding deformation, and low welding heat input. The experimental study on UNGW for the application of underwater cast steel pipeline is conducted by manipulating the welding parameters, such as the welding arc voltage and welding current, and observing the microstructural and mechanical properties of the weld. The optimal range of welding parameters for weld formation are welding voltage of 22 V, welding current of 200-230 A, and welding speed of 8.25-9 mm/s. The microstructure analysis of the welded joints observed a mixture of tempered martensite, bainite, and retained austenite are detected at Heat Affected Zones (HAZ), whereas the results of microhardness showed no softening occurred in the welded joints. The composition of the flux band can be manipulated to improve the fusion of air bubbles and side walls.