Abstract
The precipitation hardenable nickel-based superalloys are widely used in aerospace, power plants, oil, and gas industries due to their excellent high-temperature strength and remarkable hot corrosion resistance. Though it is weldable, the required properties such as hardness, resistance to fracture, corrosion, etc., depends upon the weld microstructure which is to be obtained by various heat treatment methods. In the present investigation, the influence of aging heat treatment on gas tungsten arc-welded microstructure of alloy 718 was studied. The aging heat treatment was carried out at 780 °C for 1 h, 4.5 h and 8 h and air cooled to enhance the properties of the weld metal. After heat treatment, the welded samples were characterized using an optical microscope (OM), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and microhardness tester. The effect of heat treatment time on the microstructure was evaluated. Gamma prime (γʹ), gamma double prime (γʺ) and the primary carbides of MC, M23C6 were observed in the HAZ and grain boundaries. It was observed that with the increase in aging time microhardness was significantly increased. The mechanical bend test was carried out to evaluate the ductility of the weld metal in the heat-treated conditions. On aging at 780 °C for 8 h, the ductility of the weld metal was found to be good and the microstructure depicted the reduction of laves phases and NbC in weld fusion zone and higher amount of γʺ resulting is higher strength and hardness.
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