This study investigates the effects of five heat treatment processes on the microstructure and mechanical properties of forged 3J40 alloy at room and high temperatures. The results indicate that with the increase in solution treatment temperature, the volume fraction of the α-Cr phase in the 3J40 alloy gradually decreases. Following aging treatment at 800 °C, spherical and lamellar α-Cr phases with distinct morphologies precipitate along the grain boundaries and within the grains, respectively. Furthermore, the differences in the alloy's mechanical properties at room temperature are primarily attributed to the dual influence of the morphology, location, quantity of precipitated α-Cr phases, and grain size. The precipitation of intragranular lamellar α-Cr phases effectively enhances room-temperature tensile strength, while a certain amount of α-Cr phase at the grain boundaries effectively retards grain growth during high-temperature heat treatment, thereby contributing to improved alloy ductility. Considering strength, toughness, and heat processing costs, the optimal heat treatment process for 3J40 alloy is 1100 °C/1h/WQ + 800 °C/24h/AC.