It is well known that solid solution-strengthened alloy 617 and γ’ precipitation-strengthened alloy 263 have excellent mechanical properties and corrosion resistance at high temperatures.Hyper-supercritical power plants work at temperatures above 700 ℃,and these superalloys are considered candidate materials for steam turbines components of these power plants.In this study,gas tungsten arc weldability of these superalloys was evaluated,and the effects of postweld heat treatment(PWHT) on the microstructural characteristics and the mechanical properties of their weld metals were investigated.Scanning transmission electron microscopy,energy-dispersive spectroscopy and electron probe microanalysis were utilized for the investigation.The experimental results confirmed that these weld metals had different characteristics in microstructure and mechanical properties.PWHT resulted in the precipitation of intergranular carbides,γ’ particles and an increase in tensile strength of these superalloy weld metals.Furthermore,fine γ’ particles,which were not detected in the as-welded metal of alloy 263,were precipitated after PWHT and those particles were the reason for the drastic increase in tensile strength. It is well known that solid solution-strengthened alloys 617 and γ ’precipitation-strengthened alloys 263 have excellent mechanical properties and corrosion resistance at high temperatures. Hyper-supercritical power plants work at temperatures above 700 ° C., and these superalloys are considered candidate materials for steam turbines components of these power plants. In this study, gas tungsten arc weldability of these superalloys was evaluated, and the effects of postweld heat treatment (PWHT) on the microstructural characteristics and the mechanical properties of their weld metals were investigated. Scanning transmission electron microscopy, energy-dispersive spectroscopy and electron probe microanalysis were utilized for the investigation. The experimental results confirmed that these weld metals had different characteristics in microstructure and mechanical properties. PWHT resulted in the precipitation of intergranular carbides, γ ’particles and an increase in tensile strength of these supera lloy weld metals. Finemore ’, fine γ’ particles, which were not detected in the as-welded metal of alloy 263, were precipitated after PWHT and those particles were the reason for the drastic increase in tensile strength.