چكيده به لاتين
Inconel 718 is a widely used superalloy in welding of aero-engine and gas-turbine components thanks to its excellent resistance to strain age cracking during post weld heat treatment. In this research, the effect of nitrogen added by Ar-0-6%N2 shielding gases on weld microstructure and hot corrosion resistance and mechanical properties of Alloy 718 made by autogenous gas tungsten arc welding. After welding, the weld metal microstructure was studied by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). The behavior of hot corrosion with Na2SO4–25%wt NaCl salt compound have been studied at 650 °C. Tensile test was carried out on the welded samples and base metals. Vickers microhardness testing was done on different samples with varying nitrogen contents.
The results showed that the increment of nitrogen leads to increase of weld depth and depth/width ratio. Nitrogen was found to have low solubility in Alloy 718, leading to precipitation of denderitic (Ti,Nb)CN and blocky (Ti,Nb)N precipitates inside the weld.
The corrosion scales developed on the weldments consist of multiple layers of mixed oxides, mostly Fe2O3, Cr2O3 and Nb2O5 followed by a depleted zone in substrate. The addition of 1%N2 to Ar was found to improve hot corrosion resistance due to the formation of two protective layers containing compact Cr2O3, and adherent Nb2O5-rich layer. Higher concentration of nitrogen aggravates hot corrosion resistance owing to its negative influence on microsegregation of Nb as well as increased NbC and Laves phase in interdenderitic regions which, in turn, retard the outward diffusion of Cr to the surface and subsequent formation of compact Cr2O3. Regarding tensile test, addition of 1%N2 to Ar considerably increased the UTS from 1210±10 to 1587±55 MPa while elongation was approximately unchanged, whereas addition of 5%N2 to Ar decreased the UTS to 1410±42 MPa due to the increased content of laves phase and increased size of gamma double prime phase. The hardness of the weld metal increased with increasing N2 content in the shielding gas owing to solid solution strengthening and increase of volume fraction of carbonitrides.
