<p>Due to the combined effects of coarse grains and quenching stress, SA508-3 steel heavy forgings appear abnormal blocky microstructure (Bc) composed of blocky ferrite and needle-shaped Mo₂C or cementite particles, thus resulting in the poor impact toughness. Herein, the formation of Bc in SA508-3 steel was suppressed by partitioning undercooled austenite and reducing phase transformation stress during the subsequent phase transformation through the construction of a small amount of a dispersedly distributed ferrite film by intercritical heat treatment on the high-temperature side. The results of a thermomechanical coupling simulation indicated that the quenching and tempering (QT) samples subjected to intercritical heat treatment at 800&#xa0;°C had a residual ferrite film content of 3.4&#xa0;vol&#xa0;pct. The dispersedly distributed ferrite film effectively split the prior austenite grains and alleviated intragranular phase transformation stress, thereby inhibiting the formation of coarse Bc during quenching, and increasing the magnitude and stability of the impact toughness of SA508-3 steel without reducing its strength.</p> Graphical Abstract