Fracture toughness of thermal aged 16MND5 bainitic forging steel under varying 3D constraint conditions: An experimental study using SENT specimens

Abstract

This paper presents the experimental investigations of the effects of thermal aging on the embrittlement failure mechanism and fracture toughness of 16MND5 reactor pressure vessel (RPV) steel using clamped single edge notched tension (SENT) specimens with different sizes. In order to study the embrittlement induced by thermal aging, the 16MND5 steel was subjected to accelerated thermal aging for 1000 h, 3000 h and 5000 h at 425 °C, and then the Vickers hardness and tensile properties were determined. To investigate the combined effects of thermal aging and specimen geometric sizes, the fracture toughness of various thermal aged materials was studied using SENT specimens with different specimen thickness ratios, B/W, and crack depths, a/W. A total of 59 SENT specimens were tested covering a wide range of thermal aging time and geometric size combinations. Results showed that as the aging time increases, the material’s micro hardness and tensile properties change slightly, but the R-curves (J-R and CTOD-R) decrease significantly. The ductile fracture properties of thermal aged RPV steels depend markedly on the geometric dimensions (crack depth, a/W and specimen thickness, B/W) of tested specimens. Furthermore, the effects of geometric sizes and thermal aging on fracture behavior are shown to have a strong interaction. Specifically, as the aging time increases, the influence of the geometric sizes on the fracture toughness decreases, especially with the variations of a/W. Also, as the geometric dimensions increase, the effect of thermal aging on fracture toughness decreases. Finally, combined with fractographic observations, the influence of thermal aging on the embrittlement mechanism and ductile fracture properties of 16MND5 steel was discussed.