Contributions of Matrix and Block Boundary Strength to Hardness Change of Reduced Activation Ferritic Steel during Creep

Reduced activation ferritic/martensitic steels (RAFs) have a complex and fine lath martensitic structure consisting of several kinds of microstructural units, namely, lath, block, packet and prior austenite grain. However, it is by no means easy to distinguish the contributions of such units to the strengthening mechanism. In this study, the instrumented indentation test was carried out under a wide range of maximum indentation loads using the crept specimens of the RAFs, JLF-1, in order to clarify the contribution of each grain boundary to it's strength and degradation. Experimental results revealed that the block boundary was the most effective boundary and at least seven block grains were necessary for reflecting fully the effect of block boundary in the macroscopic hardness. In addition, it was also revealed that the decrease in matrix strength of block grain was predominant factor of the reduction in macroscopic hardness due to creep. The retardation of the decrease in block boundary strength restrained the significant reduction in macroscopic hardness in JLF-1.