It is important to control the microstructure and properties of a weld for the continuous production of silicon steel sheets. Post-weld heat treatment (PWHT) can be applied to adjust the weld properties; however, research on its application to silicon steel weldments remains limited. This study investigated the microstructure and hardness evolution of the weld after PWHT for high-silicon steel, with Inconel 82 used as the filler material. The weldment contained FCC phase and BCC phase regions, and PWHT were conducted at 520, 620, 720, and 920 °C for 8 h. Experimental observations indicate that G-phase precipitations in FCC phase could increase its hardness, and it peaked at 620 °C with an average hardness of 259 HV. By contrast, the BCC phase region was subjected to martensitic transformation and its hardness increased from 305 to 335 HV after PWHT at 920 °C. To elucidate microstructure evolutions, CALPHAD-based simulations successfully predicted BCC to FCC phase transformation at 920 °C, peak G-phase precipitation at 620 °C, and elemental diffusion at the BCC and FCC interface. The findings indicate that CALPHAD-based simulations offer a robust approach that can be extended to understand the effect of PWHT.