Investigation on Thermal Aging Embrittlement Behavior of Primary Pipes in CF-8M Cast Stainless Steel

Cast austenitic stainless steel (CASS) materials have a duplex structure consisting of austenite and ferrite phases and are widely used in the manufacture of pressure boundary components for reactor coolant system. A large number of studies have shown that these material will undergo thermal aging embrittlement after long-term service at 290-320 °C, resulting in a decrease in the fracture toughness of the material. However, as a key component in the reactor coolant system, the primary pipe is difficult to replace and has high replacement cost during its service period, which seriously restricts the operation safety and service life of nuclear power plants. Therefore, it is very important for the integrity of the reactor primary pressure boundary to evaluate and predict the thermal aging embrittlement of the primary pipe materials. In order to evaluate and predict the thermal aging embrittlement degree of CASS materials during their service life, through the research and analysis of the ANL prediction model and the accelerated thermal aging experiments of nuclear grade CF-8M static cast stainless steel primary pipe materials were carried out at 400 °C for 10 000 hours. The variations of tensile properties, impact energy and microstructure of stainless steel under different thermal aging time were studied. The impact energy was used as the characterization parameter of thermal aging, and the prediction formula of thermal aging embrittlement of stainless steel was obtained, which was compared with ANL model. The experimental results show that with the increase of thermal aging time, the 0. 2% plastic elongation strength of stainless steel at room temperature and high temperature (350 °C) varies slowly, and the tensile strength increases slowly during the accelerated thermal aging test cycle. In addition, it can be seen from the morphology of the expansion zone of the impact fracture that there are a large number of dimples in the expansion zone of the unaged sample. Although the dimples had different sizes, the overall dimples show the characteristics of ductile fracture, indicating that the original sample had excellent toughness. But the number and size of dimples on the expansion zone change significantly during different thermal aging time, and the morphology of river patterns can be clearly observed, indicating that the toughness of the material gradually decrease with the increase of accelerated thermal aging time. However, the impact energy at room temperature decreases rapidly, and it decreases nearly to saturation after 8 000 h. And the impact energy prediction results of ANL model for this stainless steel in experiment period arc not conservative. 10 000 hours of the accelerated thermal aging experiments of nuclear grade CF-8M material is equivalent to 30. 49 years of service, and the degree of thermal aging is close to the thermal aging saturation state. © 2023 Atomic Energy Press. All rights reserved.