On the evolution of mechanical properties and microstructure of ferritic-bainitic (FB) 2.25Cr-1Mo (Grade 22) steel during high-temperature creep

The development of creep damage in ferritic-bainitic 2.25Cr-1Mo (Grade 22) steel was studied by following the evolution of mechanical properties and microstructure by characterisation of a series of interrupted creep specimens (605 °C/80 MPa). The change in mechanical properties of the partially crept specimens was evaluated via hardness measurements, while the microstructural changes were characterized by a range of microscopy techniques. The nano-hardness measurements combined with a statistical analysis allowed for analysis of the strength of the present constitutive phases as a function of creep time (tc), and imparted creep strain (ɛc). In addition, a machine learning methodology was used to identify ferrite and bainite regions in Electron Back-Scatter Diffraction (EBSD) orientation maps, which then allowed for a study of the evolution of dislocation densities (Geometrically-Necessary Dislocations, GNDs) in each phase separately. It is shown that the amount of GNDs in the bainite is reduced significantly during the high-temperature creep, while there is a moderate increase in the amount of GNDs in ferrite grains. These trends are consistent with the observed changes in the strength (hardness) of each phase – bainite is significantly softening, while ferrite slightly hardening. The reduction of the amount of dislocations in the bainite and its consequent softening is caused by the progressive decomposition of the needle-like bainite present in the initial microstructure into polygonal fine-grained ferrite-like bainite. Transmission electron microscopy (TEM) confirmed the changes in dislocation density observations and was also used to identify changes in carbides during the high-temperature creep. [Display omitted]