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概要

In this study, thermal-aging evaluation has been performed using service aged elbow pipe in PWR plant, aged at 320℃ for 196,500h. As a result, micro Vickers hardness of ferrite in service material (SCS14A), HV(0.025) was 616~630. Since micro Vickers hardness of un-aged ferrite phase is about HV(0.025)=300 in commercial SCS14A, the increasing of ferrite hardness during aging was 300. Cr-rich and Fe-rich regions were observed in the ferrite phase using Atom-probe analysis. In addition, Ni, Si and Mo clustering were also observed in the ferrite phase. So the ferrite phase was hardened caused by these micro-structural changes. Micro Vickers hardness of austenite phase, HV(0.025) was 155~180. Since micro Vickers hardness of un-aged austenite phase is about HV(0.025)=180~200, and no micro-structural change was observed in the austenite phase, so no change was observed in the austenite phase during aging. To compare the micro Vickers hardness of ferrite in service and accelerated materials using activation energy, Q=100kJ/mol, the ferrite hardness of in service material was very low rather than predictive line. This seems the activation energy was too conservative.

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