| Abstract Scope |
Fe-10Ni is a high-strength, high-toughness steel developed for naval structural applications. U.S. Navy researchers developed a matching solid wire consumable based on the Fe-10Ni alloy system that demonstrated acceptable mechanical properties for both gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW). However, GTA weld metal exhibits significantly higher impact toughness than GMA weld metal. This work aims to understand the mechanisms controlling impact toughness variability in Fe-10Ni groove welds produced with GMAW.
Multipass GMAW of Fe-10Ni steel produces complex thermal histories that generate significant microstructural heterogeneity throughout the weld metal, resulting in local variations in impact toughness. The as-welded condition was first tested and characterized as a baseline against which reheated regions were compared. The Gleeble 3800 thermo-mechanical simulator was used to replicate heat-affected zone (HAZ) conditions observed in multipass welds, and Charpy V-notch (CVN) toughness was evaluated at -51°C and -17.8°C. Advanced characterization techniques, including electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), transmission Kikuchi diffraction (TKD), and X-ray diffraction (XRD), were then employed to link martensitic microstructure evolution and dislocation density to the observed toughness behavior across all reheated conditions.
The as-welded condition exhibited the lowest impact toughness and greatest extent of cleavage fracture, followed by the coarse-grain heat-affected zone (CGHAZ), both attributed to coarse martensitic microstructures, dense dislocation tangles, and transformation twins. Multiple reheating cycles produced the greatest toughness improvements. Intercritical reheating refined the martensitic structure through partial reaustenitization while the remaining microstructure underwent tempering, and subcritical reheats above the minimum tempering temperature promoted dislocation rearrangement into lower-energy substructures, eliminated transformation twins, and facilitated the formation of nanoscale retained-austenite films. These results establish the microstructural mechanisms governing impact toughness variability in Fe-10Ni GMAW welds, providing a scientific foundation for developing improved welding procedures and more robust naval structural components. |