This study investigates the precipitation behavior of γ′, γ″, and δ phases in the fusion zone of IC10/GH4169 dissimilar electron beam welded joints under different heat treatment conditions (650, 750, 850, and 950 °C for 8 h). The effects of these phases on the microhardness and mechanical properties of the joints were systematically analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and microhardness testing. Results indicate that the fusion zone exhibited the highest microhardness (310–330 HV) at 650 and 750 °C due to the precipitation of γ′ and γ″ phases. At 750 °C/8 h, both the fusion zone and GH4169 base metal achieved peak hardness (370–390 HV for GH4169), attributed to the synergistic effects of fine γ′/γ″ precipitates and dislocation pinning by δ phases. However, excessive δ-phase precipitation at higher temperatures (850 and 950 °C) suppressed γ″-phase formation, leading to reduced microhardness (250–270 HV at 850 °C and below 220 HV at 950 °C). The optimal heat treatment process was identified as 750 °C/8 h, balancing high hardness and microstructural stability. These findings provide critical insights into tailoring heat treatment protocols for enhancing the performance of dissimilar high-temperature alloy joints in turbine applications.