Degradation of the Grain Boundary Strength Due to the Accumulation of Defects

TEI Kunio, SUZUKI Wataru, SUZUKI Ken, MIURA Hideo

doi:10.1299/jsmemecj.2018.j0610205

<p>A modified 9Cr-1Mo steel is a ferritic heat resistant steel expected to be used for piping materials for FBR (Fast Breeder Reactor). According to the S-N curve obtained at elevated temperatures during high-cycle fatigue test on this steel, the existence of the fatigue limit was not confirmed in the number of cycles up to 1×108 cycles in the test at 500℃ or higher. Furthermore, the lath martensitic texture, which is the reinforced structure of this steel, was confirmed to disappear during the high-temperature tests. The reason why the fatigue limit did not appear can be attributed to the degradation of the strength due to this change of micro texture, the disappearance of the initially dispersed lath martensitic texture, during the fatigue tests at elevated temperatures. In this research, we attempted to elucidate the degradation of the strength of a grain of this alloy accompanying the microstructural change, to guarantee the lifetime and strength reliability of the modified 9Cr-1Mo steel. The microstructure change was observed by the EBSD (Electron Back Scatter Diffraction) method and SIM (Scanning Ion Microscope), and the change of the strength of a grain during the creep test was measured by a micro tensile test method. As a result, the tensile strength and the yield stress of the single crystal grain decreased as the average KAM (Kernel Average Misorientation) value of a grain decreased. By using these methods, it was possible to clarify the degradation process of this alloy from the viewpoint of the change of its micro texture.</p>

Degradation of the Grain Boundary Strength Due to the Accumulation of Defects

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