Microstructural characterization and toughness evaluation of 10 wt% Ni steel weld metal gas tungsten arc and gas metal arc weld fusion zones

Abstract A novel 10 wt% Ni steel weld metal under development for use in high strength/high toughness applications was used to fabricate gas tungsten arc (GTAW) and gas metal arc (GMAW) welds. Both welds exhibit excellent toughness at sub-ambient temperatures, yet the toughness of the GTAW is superior to the GMAW. Using scanning electron microscopy, three differences in microstructure were identified in the GMAW compared to the GTAW, all of which are known to be detrimental to toughness: (1) an overall coarser effective grain size of the martensitic microstructure, (2) a larger presence of a coarse martensite constituent, and (3) a higher number density of oxide inclusions. The difference in the effective grain size of the two welds is rationalized based on inherent differences in the welding processes that produces much larger weld beads and a finger penetration profile in the GMAW thereby limiting the amount of reheating each bead receives. The higher number density of oxide inclusions in the GMAW is from the use of a 98% Ar/2% O2 shielding gas, as compared to 100% Ar used in the GTAW. A GMAW produced using 100% Ar as the shielding gas demonstrated improvements in toughness because of smaller oxide inclusions, yet the toughness was not as high as the GTAW, demonstrating the complexity of the interactions of the three different toughness influences. These results are significant in that they allow modifications to be made to the alloy composition and/or welding process parameters based on a sound scientific basis.