PSI - Issue 52

Long Jin et al. / Procedia Structural Integrity 52 (2024) 12–19

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Author name / Structural Integrity Procedia 00 (2019) 000–000

1. Introduction The low alloy steel is widely used for the structure of reactor pressure vessels (RPV) in nuclear power plants (NPPs). Meanwhile, the reactor pressure vessels are worked at high temperature and harsh cyclic internal pressure environment during long service time according to Mehmanparast and Nikbin (2022). Due to the safe operation and economy of pressure vessels during nuclear power operation, it is extremely important to understand the low cycle fatigue fracture mechanism and life prediction of low alloy steels after a long aging time. Some studies have been conducted on the low cycle fatigue properties of stainless steels after aging. Guo et al. (2022) studied the fatigue failure mechanism of cast duplex steels after aging. They estimated that the fatigue life was enhanced after aging, mainly due to the release of stress at grain boundaries caused by embrittlement of the ferrite phase. In the limited available researches of low alloy steel, Sarkar et al. (2015) investigated the cyclic softening behaviour of 20MnMoNi55 pressure vessel steel. Fekete et al. (2015) reported that 15Ch2MFA steel exhibited cyclic softening while 08Ch18N10T showed cyclic hardening in thermal fatigue test. The cyclic softening of 15Ch2MFA steel was caused by recovery and density reduction of dislocation with cellular structure, while the hardening of 08Ch18N10T was a result of increase in immobile dislocation density with veins or channels structure. However, researches on the aging effect of low alloy steels are still inadequate. A comprehensive consideration of the effect of aging on life model and fracture mechanism is needed. Therefore, in this study, the fatigue tests were performed on as-received and aged specimens in low cycle regime. The effect of aging on mechanical properties was studied, experimental Coffin-Manson life prediction models were proposed and the differences between fatigue initiation and propagation before and after aging were analyzed by fracture. The relative importance of aging on fatigue strength was also clarified. 2. Materials and experimental method 2.1. Material and microstructure observation The material was taken from the nuclear grade forged 16MND5 RPV. Its chemical composition is given in table 1. The 16MND5 steel was water quenched and tempered (referred to as-received specimen). The as-received material was then aged in chamber furnace at 450 ° C for 1000h to obtain aged specimen. The optical microstructures of as-received and aged material are shown in Fig. 1, Bainite micro-structure and dispersed distribution of precipitated phases can be observed in both materials. Fig. 2 shows the SEM morphology of microstructure in magnified views. The white precipitates of manganese-rich carbides (from EDS) are located both at grain boundaries and inside grains. The size of ferrite appears to be wider in the as-received material, while the carbide distribution appears to be more diffusive in the aged material.

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Fig. 1. Optical micrographs of as-received (a) and aged at 450°C for 1000h (b) material 10μm 10μm