Influence of Pre-Existing Martensite on the Acceleration of Carbide-Free Bainite Transformation Studied by In-Situ Synchrotron XRD

Quenching and partitioning (Q&P) and carbide-free bainitic austempering are widely used to stabilize retained austenite through carbon enrichment, enabling improved ductility and toughness in advanced high-strength steels. Although the sequence of martensite formation, bainitic transformation, and carbon redistribution are largely discussed in literature, Q&P and austempering have been studied in isolation, and no direct, real-time comparison exists between Q&P, single-step austempering, and two-step austempering performed under identical thermal histories. In this work, in-situ high-energy X-ray diffraction was used to monitor the phase evolution during quenching, reheating, partitioning, and austempering of a low-carbon high-silicon (0.2%C-1.5%Si) steel. By applying quench temperatures both below and above the M s temperature, the transformation pathway was shown to depend critically on the initial microstructure. Quenching below M s temperature produced supersaturated martensite that accelerated the bainitic transformation and enabled rapid carbon enrichment of austenite, leading to early stabilization within tens of seconds. In contrast, austempering routes exhibited delayed carbon enrichment of austenite, resulting in heterogeneous austenite and, at higher transformation temperatures, insufficient stabilization of austenite even for holding times over 900 s. These findings demonstrate that the transformation pathway established during quenching governs the efficiency of carbon redistribution and the resulting retained austenite stability. This work guides the selection of Q&P and austempering treatments using in-situ synchrotron analysis.