Alloy 690 합금의 Si, Mo 및 Al 첨가에 따른 탄화물 성장 거동 및 SCC와 마모 거동에 미치는 영향

Abstract

Alloy 690 is widely used for steam generator tubes in nuclear power plants. Intergranular carbide in Alloy 690 is reported to affect SCC and fretting wear resistance. In case that carbide coarsening occurs for long-term operation of nuclear power plants, SCC and fretting wear behavior may change. In the present study, alloying elements such as Si, Mo and Al are selected as candidate to decrease the carbide coarsening rete by decreasing Cr diffusivity. And the effect of Si, Mo and Al addition on the carbide coarsening behavior and its expected effect on the SCC and wear behavior of Alloy 690 was studied. This study was classified as three parts. The first part is about the carbide precipitation behavior of Alloy 690 + Si/Mo/Al alloys. As Si concentration increased, carbide volume fraction and size increased due to formation of Cr3Si which acted as nucleation site of carbide As Mo concentration increased, carbide volume fraction and size also increased due to increase in carbide lattice constant. In contrast, as Al concentration increased carbide volume fraction decreased due to increase in C solubility. The second part is about the carbide coarsening behavior and its expected effect on the SCC behavior of Alloy 690 + Si/Mo/Al alloys. Si and Al did not affect the carbide coarsening activation energy and SCC behavior for long-term operation. As Mo concentration increased to 1 wt%, carbide coarsening activation energy increased about 30%. It is considered that Mo in matrix decreased the Cr diffusivity and increased the carbide coarsening activation energy in Alloy 690. Due to this effect, SCC Mo is considered as a promising element for SCC behavior in Alloy 690. The third part is about the fretting wear behavior of Alloy 690 + Si/Mo/Al alloys. As carbide size and volume fraction increased, fretting wear resistance increased due to increase in load bearing effect. Therefore, Si and Mo increased the wear resistance and Al decreased the wear resistance due to changes in carbide volume fraction and size. It shows that Mo added alloy has equivalent or higher wear resistance than that of Alloy 690 due to increase in carbide size and volume fraction.