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 (Ms) temperature was formulated. Although the presence of pre-martensite below the Ms 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 Ms ( 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 Ms 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 degrees C. At lower tempering temperatures, the bainitic matrix obtained above Ms was characterized by higher thermal stability, while the presence of pre-martensite in the steel treated below Ms 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.