A novel strategy to prevent hydrogen charging via spontaneously molten-slag-covering droplet transfer mode in underwater wet FCAW

Underwater wet welding is widely used in offshore platform maintenance, oil pipeline repair and wreck salvage operations. A high diffusible hydrogen content is recognized as one of the critical reasons behind the reduction in the reliability of underwater wet welded joints. In this study, the entry route of hydrogen during underwater wet flux-cored arc welding of 304 stainless steel at a depth of 0.5 m was first con-firmed experimentally using an in-situ X-ray imaging system. The hydrogen charging was deemed to dominantly occur when pendant droplets were formed due to the extremely high temperature of molten metal and the absence of slag coverage. This resulted in 13.51 mL/100 g of diffusible hydrogen remaining in the droplet and 9.42 mL/100 g in the deposited metal. To prevent hydrogen charging, an unprece-dented molten-slag-covering droplet transfer mode was developed by reducing the CaF2 content and maintaining the CaF2/TiO2 to an appropriate ratio in the flux, through which the molten slag sponta-neously covered the droplet, electric arc and molten pool completely during the entire welding process and the entry of hydrogen was successfully blocked and the diffusible hydrogen content was reduced to the same level as that achieved via onshore welding (3.26 mL/100 g).(c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Underwater wet welding is widely used in various applications such as offshore platform maintenance and oil pipeline repair. High diffusible hydrogen content is identified as a critical factor affecting the reliability of underwater wet welded joints. This study confirmed experimentally that hydrogen primarily enters during droplet formation due to the high temperature of molten metal and the absence of slag coverage. To prevent hydrogen charging, a novel molten-slag-covering droplet transfer mode was developed, which successfully blocked the entry of hydrogen and reduced the diffusible hydrogen content to the level achieved in onshore welding (3.26 mL/100 g).