Isothermal holding processes of a reduced activation ferritic/martensitic steel to form a bainitic/martensitic multiphase microstructure and its mechanical properties

Abstract In this study, we developed two novel multi-step phase transformation processes by adjusting the order of bainitic isothermal transformation and martensitic transformation, (i.e., B-M and M-B-M) to successfully improve the ductility and toughness of a modified 9Cr–2WVTa steel by employing the finer bainitic/martensitic (B/M) multiphase microstructure. In particular, compared with the traditional normalizing-tempering (N-T) treatment, the M-B-M process significantly improved the total elongation/impact toughness of the steel from 25.1%/72.0 J/cm−2 to 32.1%/104.6 J/cm−2 at room temperature, but reduced the ultimate tensile strength/yield strength of the steel from 863 MPa/711 MPa to 706 MPa/567 MPa. The increase in ductility is attributed to the lower dislocation density in the matrix and more subgrains within the bainite laths, and the increase in toughness is due to the presence of the secondary cracks and the lower residual stress. The microstructure evolutions and the relationship between the mechanical properties and microstructure features for the different processes have been comparatively discussed.