Creep damage assessment of 10Cr-1Mo-1W-VNbN steel forging through EBSD observation

10Cr-1Mo-1W-VNbN steel forging was observed through TEM (Transmission Electron Microscope), SEM (Scanning Electron Microscope) with EBSD (Electron BackScattering Diffraction) pattern method and tested through nano-indentation tester to investigate the microstructural change during creep damage process. Long-term creep rupture and interrupted creep test samples were investigated and effective damage parameters were selected. Dislocation substructure through TEM thin foil method showed increasing block/lath width especially near grain boundary according to creep damage accumulation and the same feature was observed through EBSD IPF mapping more clearly and easily. Area averaged KAM (Kernal Average Misorientation) KAMave was shown to be effective for evaluating dislocation microstructural changes during creep. Nano-indentation tests were conducted at the same position in EBSD measurement, which showed a good correlation between hardness value and the square root of KAMave. The differential equation of dislocation density with creep time was applied to estimate the relationship between averaged KAMave and time through the relationship between hardness and dislocation density. The creep damage estimation curves were obtained by the integrated form of the equation. As the KAMave showed an apparent drop against time fraction in the primary creep stage near grain boundary followed by almost constant trend for later stage. The statistical distribution of KAMave during creep damage process suggested the localized recovery of dislocation substructure near grain boundary.