Relationship between microstructure and manufacturing condition of 9Cr-ODS ferritic/martensitic steels

Oka, Hiroshi   ; Tanno, Takashi  ; Yano, Yasuhide  ; Otsuka, Satoshi   ; Kaito, Takeji ; Tachi, Yoshiaki  

For the optimization of the fabrication process for 9Cr-ODS steels, it is important to know the influence of the processing steps, i.e. hot isostatic pressing, hot extrusion, and hot forging, on the evolution of nano-structure. In this study, the effect of manufacturing process on the evolution of the nano-structure was investigated. The investigation was performed by multi-scale method, including small angle neutron scattering. The multi-scale microstructure analysis for the sample obtained from each step of processing revealed that the hot extrusion and hot forging reduced the proportion of residual ferrite phase, resulting in the decrease of the total number density of the nano-sized particles. Moreover, the hardness decreased with decreasing the proportion of residual ferrite phase. This is clearly expressing that the residual ferrite phase is the strength controlling factor of the 9Cr-ODS steel and the control of this phase is important towards the process optimization.