Metallurgical effects of nitrogen on the microstructure and hot corrosion behavior of Alloy 718 weldment

The effects of nitrogen added by autogenous gas tungsten arc (GTA) welding on weld microstructure and hot corrosion resistance of Alloy 718 exposed to Na2SO4-25%wt NaCI salt compound at 650 degrees C have been studied. The weld metal microstructure was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that nitrogen has low solubility in Alloy 718, leading to precipitation of (Ti,Nb)CN and (Ti,Nb)N precipitates inside the weld. The optimum nitrogen content for hot contusion resistance was 150 ppm due to formation of compact Cr2O3 and adhesive Nb2O5 layers. Higher concentration of nitrogen aggravates hot corrosion resistance owing to severe microsegregation of Nb and Mo solutes as well as increased NbC and Laves phase in interdenderitic regions.