Pre-martensite and bainite reactions: A thermal stability study

The comparative analysis of two ways to achieve ultra-fine bainitic steels considering isothermal heat treatment above and below martensite start (M) temperature was formulated. Although the presence of pre-martensite below the M temperature accelerates the nucleation time of the bainitic transformation, the time to finish the transformation is longer (∼11.5 h) compared to the treatment above M (∼5 h). A qualitative and quantitative analysis of the morphology of the bainitic structure demonstrated that there are no significant differences in the thickness of the bainitic ferrite laths in both cases. However, a clear refinement of the retained austenite blocks of the steel treated below M was observed. Moreover, TEM and STEM/EDS studies revealed precipitation processes associated with the decomposition of the metastable bainitic and bainitic-martensitic structures during the tempering. It was shown that both heat treatment routes lead to secondary hardening during the tempering at 550 °C. At lower tempering temperatures, the bainitic matrix obtained above M was characterized by higher thermal stability, while the presence of pre-martensite in the steel treated below M has greater effectiveness in the secondary hardening effect at higher tempering temperatures and therefore in thermal stability of bainite at the high-temperature tempering range. These results provide a new perspective for designing heat treatment processes for advanced bainitic steels in applications subjected to operation at elevated temperatures.

This work was supported by the National Science Centre Poland (OPUS Project, No. 2023/51/B/ST8/01393, title: “INtermetallics STrengthening in nanocrystalline BAINitic steels considering the additive manufacturing method (INSTaBAIN)”). F.G. Caballero would like to thanks the European Commission for funding her investigations on bainite (RFCS-02–2022-RPJ Project: 101112398 — NanoWinTur).