Change in contribution of block boundary to macroscopic strength of 10Cr–1Mo–1W–VNbN steel due to creep

Outstanding strength performance of high Cr ferritic steels is attributable to the combined strengthening mechanisms of matrix and various grain boundaries. However, it is by no means easy to separate the contributions of such strengthening factors and quantitatively understand them because of extremely fine and complicated microstructures. In this study, the instrumented indentation test was carried out to clarify the change in contribution of “block” during creep. The materials used in this study were turbine rotor steel (Fe–10Cr–1Mo–1W–VNbN). The indentation test was applied to the as-tempered and creep damaged specimens under the maximum loads ranging from 1 to 1000 mN. The test results revealed that the decrease in contribution of block was the predominant factor controlling the material deterioration, namely, softening at the early stage of the creep life. This decrease in block's contribution was caused by the decrease in resistance of the block boundary to deformation.