Microstructural Influences on Retained Austenite Stability in High Strength Steels and Kinetics of Lath Martensite Formation in Fe and Fe Alloys

The stability of retained austenite (RA) is a critical factor for enhancing the mechanical performance of advanced high-strength steels. This study experimentally investigated the influence of microstructural factors on RA stability, isolating them from the influence of chemical composition. This is done through a comparative analysis of two-phase (RA/martensite) and one-phase (austenite) microstructures with nearly identical austenite compositions in medium-Mn steels. This approach enabled a focused examination of microstructural factors influencing austenite stability without the influence of composition. The experimental results were further correlated with a thermodynamic Ms model to determine the significance of different microstructural factors. A major part of this thesis is dedicated to understanding microstructural factors and their influence on austenite stability. A range of characterization techniques were employed. (1) Scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) was used to characterize the apparent microstructure. (2) Dilatometry and X-ray diffraction (XRD) were used to assess austenite stability during cooling, and (3) In-situ high-energy X-ray diffraction tensile test was used to assess austenite stability during deformation. The results showed distinct microstructure effects on the thermal and mechanical stabilities of RA, signifying the importance of the microstructural effects on γ/RA stability. In addition, another part of this thesis explored the kinetics of lath martensite formation in Fe and Fe alloys based on ultra-rapid cooling experiments. These experiments provided rare isothermal information on the transformation, indicating that substitutional alloying elements such as Cr, Ni, and Ru have small effects on the rate of lath martensite formation, in contrast to the behavior observed for interstitial C. A mathematical model based on the Arrhenius equation was developed to predict the rate and temperature dependence of lath martensite formation in Fe-C alloys. Stabiliteten hos restaustenit (RA) är en kritisk faktor för att förbättra den mekaniska prestandan hos avancerade höghållfasta stål. Denna studie undersökte experimentellt påverkan av mikrostrukturella faktorer på RA-stabiliteten, och isolerade dem från effekterna av kemisk sammansättning. Detta görs genom en jämförande analys av tvåfasiga (RA/martensit) och enfasiga (austenit) mikrostrukturer med nästan identiska austenitsammansätt