<p>This research took a novel approach by examining the only fusion zone (FZ) mechanical properties and its corresponding microstructure under various post-weld heat treatment (PWHT) conditions of Inconel 625 (IN625) thick plates. The FZ of as welded sample was mainly comprised of dendritic structures with irregular shapes of brittle phases in the interdendritic regions. The plasma arc-welded specimens were initially solution heat-treated at 1100&#xa0;°C for 1&#xa0;h to dissolved the deteriorate phases. Homogeneous equiaxed grains were achieved by the solution heat treatment (ST) process, along with a few fine residual phases, resulting in an increase in hardness compared to the FZ-welded condition. Following the ST process, aging heat treatments were conducted at 650 and 780&#xa0;°C for 100&#xa0;h each for precipitation of strengthening phases. The specimen of ST + aged at 650&#xa0;°C (SAT1) was found with γ′′ (Ni₃Nb) phase and fine blocky secondary metal carbides (SMCs), showed improved mechanical properties. Conversely, the specimen of ST + aged at 780&#xa0;°C (SAT2) depicted needle-shaped coarse δ-phases in the γ-matrix and coarse SMCs at the boundaries, resulting in negligible enhancement in tensile strength. The ST specimen exhibited UTS of about 754.59 ± 2.87&#xa0;MPa and elongation of 45.35 ± 1.51% due to dissolution of coarse brittle phases. Whereas, the SAT1 specimen depicted tensile strength of about 969.14 ± 9.83&#xa0;MPa, elongation of 12.55 ± 4.63 at a strain rate of 0.0015&#xa0;s⁻¹ with hardness of approximately 270.70 ± 5.27&#xa0;HV. The tensile test at different strain rates indicated that all material conditions significantly showed strain rate sensitivity. The strain hardening exponent of the SAT1 specimens was the lowest, attributed to the precipitation of γ″-phases in the matrix, which limits their work hardening capacity during deformation. The dissolution of coarse, brittle phases in ST specimens exhibited a greater degree of ductile failure compared to the other conditions.</p>