Primary Degradation Assessment of 12Cr1MoVG Steel by Flat Indentation and Electron Backscattered Diffraction Methods

Most of the allowable design strain in high-temperature component occurs in the primary creep stage, so the accumulation of creep strain in this stage must be considered. Performance degradation of 12Cr1MoVG steel during interrupted creep test at 580 °C and 110 MPa was assessed by the flat micro-indentation method, whereas the creep deformation and microstructural evolution due to primary creep were also evaluated by combination of scanning electron microscopy (SEM) and electron backscattered diffraction. The energy density equivalent model established for flat indentation was introduced to evaluate the macromechanical properties of the 12Cr1MoVG at different creep times, with the overall strain hardening behavior attributed to microstructural degradation associated with precipitates, dislocation substructure and cavities nucleation. Based on the Larson–Miller parameter (LMP), an unusual softening behavior was also found during creep, significantly counteracting the strengthening and work hardening of the steel. Through examining the low-angle grain boundaries and dislocation density distributions, the sudden reduction in creep strength is the result of the dynamic recovery of substructure, which is linked to the rearrangement of dislocation structure into cells by glide and climb processes. It suggested that the flat indentation method is advantageous for microdestructive assessment of early degradation of 12Cr1MoVG steel during the primary stage.