10Cr-1Mo-1W-VNbN steel forging for steam turbine rotors was investigated using TEM (Transmission Electron Microscope), SEM (Scanning Electron Microscope) with EBSD (Electron BackScattering Diffraction pattern) method and nano-indentation tester for the assessment of microstructural change during creep damage process. Long term creep rupture tests and interrupted creep tests were conducted for the subject material and then effective damage parameters were assessed and compared with each other. Dislocation substructure which was observed through TEM thin foil method showed increasing lath/block width according to creep damage accumulation and the same feature was observed through EBSD IPF mapping more clearly and easily. EBSD KAM mapping was also conducted and averaged KAM was shown as an effective index for measuring dislocation microstructural changes during creep. Nano-indentation tests were conducted at the same position of EBSD measurement, which revealed that there was a good correlation between hardness value and the square root of averaged KAM. The differential equation of dislocation density with creep time was arranged to estimate the relationship between averaged KAM and time through the relationship between hardness and dislocation density. The creep damage estimation curves were obtained successfully by the equation expressed with stress and temperature term. The dislocation density based creep damage assessment curves could be effective for creep life assessment of high temperature components.

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