Quenching and partitioning (Q&P) heat treatments have recently gained attention as promising methods for the third generation of advanced high- strength steels, particularly in industrial applications like automotive. This study investigates the microstructural evolution during Q&P in two medium-carbon high-silicon and aluminum-alloyed steels, exploring potential additional phase transformations controlling the final structure. The choice to focus on Si and Al- medium-carbon steel is linked to the lower cost of these elements compared to commonly alloying elements like Ti, Cr, Mo, and V, while still achieving high mechanical properties through Q&P. The Q&P process is analyzed by varying the volume fraction of primary martensite (M1) at 0.25, 0.50, and 0.75, with parti- tioning temperatures ranging from 350 to 550 °C for 30 min. At 350 °C, a significant volume fraction of stabilized austenite (up to 0.3) is observed. However, concurrent reactions such as nanostructured bainite and martensite formation lead to devi- ations from the theoretical constrained carbon equilibrium (CCE). At higher temperatures (450–550 °C), tempering reactions, including cementite precipitation and pearlite formation, reduced the austenite final fraction. The study highlights that heat treatment design, particularly partitioning temperature, must be tailored to the specific steel composition due to the varying effects of Si and Al.