High Cr ferritic steels have a complex lath martensitic structure consisting of several microstructural units, i.e., fine lath, block, packet and prior austenite grain. Additionally, precipitation, solid-solution and dispersion strengthening mechanisms contribute to their excellent strength. However, it is by no means easy to separate the contributions of such strengthening factors and quantitatively understand them because of the extremely fine and complicated microstructure. In this study, the instrumented indentation test was carried out to clarify the change in contribution of each microstructural factor, particularly, “block” during creep. The material used in this study was turbine rotor steel (Fe-10Cr-1Mo-1W-VNbN). The indentation test was applied to the as-tempered and the creep damaged specimens under a wide variety of maximum loads. The test results revealed that the decrease in contribution of block grain was the predominant factor of the decrease in macroscopic hardness at the early stage of creep life. On the other hand, during the second half of the life, the decrease in macroscopic hardness was mainly caused by the decrease in matrix hardness. The decrease in block’s contribution was attributable to the decrease in the resistance of block boundary to deformation, rather than the coarsening of block grain.

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