저합금강 배관재의 동적변형시효 특성에 미치는 미세조직의 영향 연구

Abstract

SA508 Gr.1a low-alloy steel (LAS) is used as a reactor coolant piping material of nuclear power plants (NPPs). SA508 Gr.1a LAS is made by forging followed by quenching and tempering, and it has a good mechanical properties. However, it is known that the LAS is typically susceptible to dynamic strain aging (DSA) in the range of operating temperatures of NPPs, and the DSA phenomenon increases strength and decreases ductility and fracture resistance of the materials. Also, it causes abnormal strain rate dependence of LAS. Thus, the mechanical properties of SA508 Gr.1a LAS at operating temperature of NPPs are influenced by its DSA characteristic, and it is important to understand the DSA characteristics for ensuring the reliability of mechanical properties of SA508 Gr.1a LAS tested at operating temperature of NPPs. The DSA phenomenon in LAS is known to be caused by the interaction between diffusing atoms, i.e., free carbon and nitrogen atoms, and mobile dislocations. Therefore, the susceptibility and temperature range of DSA are dependent on the amount of free carbon and nitrogen atoms, which is affected by the chemical composition, heat-treatment, and deformation rate of materials. A number of studies have been carried out to understand the parameters affecting the DSA characteristics and to mitigate the DSA susceptibility of LAS. In this study investigates the effect of microstructure, which is characterized by manufacturing process, on the DSA characteristics of SA508 Gr.1a LAS pipe material via tensile tests.