페라이트강의 동적변형시효 거동에 미치는 기하학적 불연속부의 영향 평가

Abstract

Ferritic steels are commonly used as a piping material of nuclear power plant (NPP). It is known that dynamic strain aging (DSA) for ferritic steels occurs in the range 150∼350oC, which includes operating temperature of NPPs. DSA causes an abnormal behavior of material properties; increasing tensile strength and decreasing ductility and toughness of materials at a certain range of temperature. Also, the temperature region of DSA strongly depends on strain rate applied to material, and the mechanical properties are sensitively changed by strain rate at a given temperature. It is important to understand the characteristics of DSA because DSA is a cause of uncertainty in the integrity evaluation of ferritic steel piping components. In particular, it is necessary to clarify the effect of geometrical discontinuity on the DSA behavior of material, since the geometrical discontinuity changes stress and strain states and strain rate in the local region of structure. Therefore, the objective of this study is to investigate the effect of different stress state induced by geometrical discontinuity on DSA behavior of ferritic steels. For this purpose, the present study performed a series of tensile tests using notched-bar specimens including standard tensile specimen under three different displacement rates and various temperatures and evaluated the DSA characteristics of material as a function of notch radius of specimen. ASTM A106 Gr.B carbon steel, which is commonly used as a piping material of nuclear power plant, was used in the experiment. The results of tensile test on the standard specimen showed that DSA phenomena of A106 Gr.B piping material appear in the temperature range 125∼230oC under quasi-static loading rate. The DSA region of A106 Gr.B was moved to higher temperature, about 50∼90oC, when the loading rate increased about 100 times compared to quasi-static loading rate. Also, it was indicated that the notch of specimen moves DSA region of material toward higher temperature under the same displacement rate. Finite element analysis showed that the primary notch effect on the DSA behavior of material is an increase in the strain rate by notch.