Microstructure characteristic of surfacing layers for underwater shielded metal arc welding

This study delves into the microstructure characteristic of surfacing layers formed through underwater shielded metal arc welding (SMAW), a critical process in marine engineering for ship repair and subsea pipeline maintenance. The research focuses on how varying welding currents influence the grain size, phase structure, and texture evolution of the surfacing layer metal deposited on a Q235 substrate. Key findings reveal that lower welding currents result in columnar dendritic grains due to reduced heat input and accelerated cooling rates. As the welding current increases, the formation of ferrite and pearlite in the fine grain (FG) region is observed, along with the emergence of acicular ferrite (AF), ferrite with second phase arrangement (FSP), and polygonal ferrite (PF) phases within the surfacing layer. The study also highlights the impact of welding currents on the proportion of AF and the formation of aligned AF packets, which contribute to elevated dislocation density and a higher proportion of low-angle grain boundaries (LAGBs). The microhardness of the underwater surfacing layers increases with higher welding currents, reaching a maximum value of 216 HV 0.5. Smaller grain size, equiaxial grain morphology, and higher dislocation density are identified as key factors that enhance both microhardness and corrosive-wear resistance. The research concludes that the utilization of higher welding currents in the underwater SMAW process increases the proportion of AF and promotes the formation of aligned AF packets within the surfacing layer, consequently leading to elevated dislocation density and a higher proportion of LAGBs. This comprehensive analysis provides valuable insights into the formation and performance of the surfacing layer, making it a crucial read for those involved in marine engineering and welding technologies.