Study On Plastic And Fatigue Behavior Of Reactor Materials And Sealing Element

Since 1960's, the hollow metal O-ring of Inconel718 alloy has been widely used as sealing element in nuclear reactor because which has better springback sealing performance but with bigger compressing displacement and smaller preload. During serving, O-ring will suffer atrocious working state such as elevated temperature, high pressure, small amplitude cyclic loading. In order to ensure the sealing reliability of nuclear reactor, it is essential to study systematically compress-springback character, fatigue performance, ratcheting behavior of O-ring.Based on experiments, the researches were completed as following:In this paper, the influence on springback capability of Inconel718 alloy O-ring from several factors such as loading history, compressing ratio A,, temperature, heat-method were investigated. It is shown that, loading history with lower compressing ratio has no effect on springback displacement of O-ring specimens; the relationship between springback displacement and compressing ratio meets logarithmic rule in the compressing ratio range from 2% to 30%, however the increase of compressing ratio will result in the decrease of springback displacement once A, is over 30%; there exists a little temperature effect on springback displacement but it will die down when temperature is over 200℃; the different heat-methods have obvious influence on springback displacement of O-ring specimens, so it is necessary to notice that compressing ratio will exceed design-standard due to the thermal expansion of surrounding components during serving and affects finally the actual springback displacement of O-ring specimens.A series of fatigue tests of Inconel718 alloy O-ring specimens were carried out at room and elevated temperatures, meanwhile the fracture surface of some fatigue specimens suffering from displacement-load alternate cycling were examined by metallographic microscope and scanning electron microscope (SEM). The fatigue results show that: under cyclic 1 oading c ondition, t he fatigue s pecimens w ill be d estroyed o wing to cyclic creep and distributed load amplitude is the independentparameter affecting fatigue life; under another condition, the load response has the character of stress relaxation, the fatigue specimens are disabled by reason of appearing macroscopical cracks but geometrical shape keep as the initial compressing status, compressing ratio amplitude is the independent factor influencing on fatigue life; fatigue performance of Inconel718 alloy at elevated temperature is better than that at room temperature. Micro cosmic analysis indicates that: during fatigue cycling, there appears many twinned grains, some nubby a phase are separated out on the grain boundaries accompanied with coarsening of y' phase; the fatigue cracks initiate from inner surface of O-ring specimens and develop by means of cross-grain manner; the fatigue striations that have been found in the fracture surface of specimens distribute discontinuously as a whole, but the arrangement of fatigue striations in some local grain surface are continuous and regular and accompanied with many micro-cracks; there exists jointly cleavage and fatigue striations even in extension region; some globular inclusions in the frature surface are found because of linear cutting, oxidation, and elemental release.Ratcheting behavior of Inconel718 alloy O-ring specimens were investigated in this paper. Experimental results reveal that: this alloy exhibits cyclic stability, and whose O-ring specimens will produce progressive ratcheting deformation under dissymmetrical cyclic loading, and peak distributed load is the main factor which affects saturated ratcheting deformation.In the case of finite element analysis(FEA), based on contact analyzing principle and Nonlinear Isotropic Hardening (NLISO) constitutive model, numerical simulations of the compressing-springback process about Inconel718 alloy O-ring specimen were performed by ANSYS software, and the simulation results have excellent agreement with experimental results which indicates FEA is feasible and effective. Further, several factors influencing on springback performance of O-ring specimens were analyzed, the results include: the relationship between parameters of NLISO model such as k, R0, R and springback displacement H are respectively increasing linear character, while the relationship of b~H is just contraty,...