"Design strategies for optimized mechanical performance of bainitic steels”

Developing steel products with enhanced properties is crucial for extending the lifespan and reliability of engineering components. Beyond engineering performance, this additionally contributes to environmental sustainability by improving the circularity of the steel industry. To fulfill these requirements, bainitic steels have attracted considerable research attention due to their promising mechanical properties achieved with cost-effective alloying strategies. Modifications in chemical composition and processing parameters result in significant changes to the resulting bainitic microstructure, which in turn impacts the mechanical performance. Although quasi-static tensile properties are commonly used to evaluate the bainitic steels, they do not fully reflect the mechanical performance under service-relevant conditions. Therefore, it is essential to assess the mechanical behavior under different loading conditions. However, due to the complex and multiphase nature of the bainitic microstructure, which may include carbides, retained austenite (γR), and potential martensitic regions as secondary phases, establishing clear correlations remains a major challenge. To address these challenges, the bainitic phase transformation and microstructure evolution were investigated in a Fe–0.2C–2.5Mn base-alloy as a function of heat treatment parameters and Si and Al alloying, which are key alloying elements in suppressing carbide formation to generate carbide-free bainite (CFB). The overall phase transformation behavior was characterized through continuous-cooling-transformation (CCT) and time-temperature-transformation (TTT) diagrams, determined using dilatometry. The effect of isothermal transformation temperature on the quasi-static tensile properties and high-cycle fatigue (HCF) performance of CFB and carbide-bearing bainite (CBB) was comparatively evaluated. Furthermore, the mechanical response under different loading scenarios was investigated. The complex role of microstructural constituents, particularly γR and martensite-austenite (MA) islands, in governing fatigue crack resistance, impact toughness, and fracture mechanisms was analyzed. Throughout the study, the relationship between the resulting microstructures and mechanical performance was established using a wide range of characterization methods, including scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Synchrotron X-Ray diffraction (SYXRD). The results show that Al substantially increases the martensite start temperature (Ms) and strongly promotes the ferritic constituents compared to Si. With Al alloying, Widmanstätten ferrite forms during continuous cooling induced by high cooling rates and large prior austenite grain (PAG) size. Si addition slows bainite transformation by stabilizing austenite through carbon enrichment and solid solution strengthening. While Al exhibits similar effects, it shortens the incubation time during isothermal heat treatment due to increased driving force for bainite transformation, overcompensating for the retardation effects. Al alloying results in achieving a finer CFB microstructure with pronounced film-like γR with enhanced carbon enrichment, leading to improved γR stability and suppressed MA island formation. CFB demonstrates superior quasi-static tensile properties compared to CBB, mainly due to the gradual transformation of γR to martensite under uniaxial tension. Lowering the transformation temperature leads to a finer distribution of carbides in CBB and a more refined CFB microstructure, characterized by pronounced film-like γR morphology and reduced MA island fraction. These microstructural modifications obtained at lower temperatures contribute to improved mechanical performance in both bainite groups. On the other hand, the fracture behavior of CFB is significantly affected by the presence of MA islands and unstable γR at PAG boundaries, particularly under localized deformation, as observed in Charpy impact and bending fatigue tests. While CBB exhibits substantially higher impact toughness with clear ductile characteristics in the fracture surface, CFB shows a tendency for brittle failure. This is primarily due to the abrupt transformation of unstable γR and the brittle nature of MA islands, which either lead to intergranular fracture through the formation of a brittle network along the PAG boundaries or to cleavage fracture initiated by rapid debonding at high deformation rates. Moreover, despite a significantly higher fatigue limit of CFB, which is consistent with its enhanced quasi-static tensile properties, it shows a higher fatigue crack propagation rate and lower threshold of stress intensity range (ΔKth) than CBB. While PAGBs in CBB successfully deflect the fatigue cracks, MA islands, and unstable austenitic constituents in CFB weaken these boundaries, resulting in intergranular fractures through the brittle network or cracks that cut PAGBs without deflection. However, ductile fracture behavior is observed when the crack goes through a PAG instead of cleavage fracture, highlighting the influence of a lower deformation rate compared to Charpy impact tests.