Effect of Partitioning Time and Silicon Content on Retained Austenite Stability in Medium-Carbon Advanced High-Strength Steels

This study investigates the effects of silicon content and partitioning time on the phase transformations and microstructural evolution of medium-carbon steels. Two compositions—a high-silicon (H-Si) alloy and a low-silicon (L-Si) alloy—were subjected to varying holding times during deformation quenching and partitioning. Their retained austenite (RA) fraction, secondary martensite and bainite content were quantified using X-ray diffraction. Microstructural observations were carried out via electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The results revealed that the H-Si steel achieves a stable RA fraction at shorter partitioning times (around 19% at 1000 s), with secondary martensite formation effectively prevented. In contrast, the L-Si steel exhibits a slower RA stabilization process; after 10000 seconds in the final quench, around 17% RA was stabilized, allowing some secondary martensite to persist. Similarly, while the L-Si steel shows a progressive and sustained increase in the bainite fraction, the H-Si steel remains largely resistant to bainitic transformation under the investigated conditions. Lattice-parameter-based calculations confirm that both steels experience progressive carbon enrichment in RA, although the L-Si steel undergoes a faster initial rise in carbon content. EBSD and SEM analyses illustrate how these phase evolutions are reflected at the microstructural level. H-Si micrographs show larger, more continuous RA regions forming over time, whereas L-Si steel displays finer RA grains and a higher tendency toward martensite tempering. These findings emphasize the critical role of silicon in modulating phase stability, carbon partitioning, and, ultimately, the balance of mechanical properties.