Deformation-induced Martensitic Transformation at Tensile and Compressive Deformations of Bainitic Steels with Different Carbon Contents

Deformation-induced martensitic transformation (DIMT) during tensile or compressive deformations of the bainitic steels with various carbon content (0.15%C, 0.25%C, 0.62%C) was studied. The initial microstructure before the deformation tests was prepared by the austempering at 400 degrees C to obtain bainitic structure consisting of bainitic ferrite and retained austenite. The volume fraction of the retained austenite was larger in the bainitic steel with the larger carbon content. In all of the bainitic steels, the tensile deformation exhibited larger work hardening than the compression. This difference indicates the suppression of the DIMT at the compression, and actually the measurements of electron back scattering diffraction (EBSD) confirmed the less reduction of retained austenite at the compression of all the bainitic steels. Additionally, the steel with the highest carbon content was examined by in situ neutron diffraction and clarified the difference similar to that obtained by the EBSD measurement. The regression of the relation between the fraction of austenite and applied strain with the conventional empirical equation revealed that the kinetic of DIMT is strongly dependent with the stress polarity, but not significantly changed by the carbon content. The mechanism of the DIMT dependence of the stress polarity was discussed with the deformation texture and the crystallographic orientation dependence of DIMT.