Effect of Ni-based filler metal types on microstructure and cryogenic impact toughness of FCAW welded 9 % nickel steel

In this study, the effects of three Ni-based filler metals (Alloy 625, 609, and 709) on the weldability and cryogenic performance of 9% Ni steel fabricated using multipass flux-cored arc welding (FCAW) were evaluated. All welds were produced under identical conditions to isolate the effects of the filler composition. Alloy 625 resulted in the tallest weld bead, whereas Alloy 609 exhibited the greatest penetration owing to differences in thermal conductivity and melt fluidity. All the weld metals displayed dendritic structures with varying degrees of interdendritic segregation. Charpy impact tests conducted at –196 °C revealed that Alloy 625 achieved the highest impact toughness in the weld metal, whereas Alloy 709 exhibited superior toughness in the heat-affected zone (HAZ). Electron backscatter diffraction (EBSD) analysis showed that Alloy 625 maintained high phase stability, characterized by a low high-angle grain boundary (HAGB) fraction of 2.1%. In contrast, Alloy 709 exhibited coarse but uniform grains (29.3 μm), moderate HAGB fraction (38.9%), and the lowest kernel average misorientation (KAM = 0.909), effectively suppressing strain localization and enhancing cryogenic toughness. These findings highlight the critical role of filler metal selection in tailoring the weld microstructure and optimizing the mechanical performance under cryogenic conditions. To our knowledge, this is one of the few studies to systematically investigate the comparative effects of multiple Ni-based filler metals on both the weld metal and HAZ in 9% Ni steel, providing new insight into filler optimization strategies for cryogenic application.