| The pressure boundary between the primary and secondary circuits of the steam generator(SG)tube is the core component of the pressurized water reactor(PWR)nuclear power plant,and its integrity plays an important role in the safety and economy of the nuclear power plant.With the increase of service life,the problems of high-cycle fatigue and corrosion fatigue due to flow-induced vibration and thermal cycling become more and more prominent,especially the surface defects of SG tubes(such as scratches,dents,fretting wear or microcracks)will significantly reduce the fatigue strength of the SG tubes,rusulting in premature initiation of cracks on the surface or defects of the SG tube,or even rupture of the SG tube,causing radioactive contamination or even scrapping the entire SG.Therefore,it is of great significance to study the fatigue behavior of SG tubes of nuclear power plant for evaluating their structural integrity,reliability,failure analysis and life prediction.In this paper,taking the actual domestic Alloy 690TT SG tube as the research object,a boat-shaped fatigue specimen and its supporting fixture based on actual SG tube were designed,and the high-cycle fatigue behavior of the SG tube and the effect of the surface scratch depth on the microstructure evolution and highcycle fatigue behavior of the SG tube were studied by combining the experiment and numerical simulation;The cyclic deformation behavior and fatigue crack initiation mechanism of the SG tube in room-temperature air and 325℃ hightemperature air were studied;Combined with high-temperature and high-pressure water corrosion fatigue tests and advanced characterization methods,the corrosion fatigue mechanism of SG tube was systematically described,and the model evaluation of the corrosion fatigue life of the SG tube was carried out,and the influence law and mechanism of surface scratch depth on the corrosion fatigue of SG tubes were studied.The crack initiation mode of the tubular specimen was different from that of the bar specimen,which affected the fatigue life evaluation of the actual SG tube.Under the low stress level,the fatigue crack of tubular specimens initiated at the persistent slip bands(PSBs)on the internal surface,while the cyclic deformation of the surface of bar specimen was not enough to form the PSBs,that was,the formation limit of the surface PSBs had not been reached,and fatigue cracks were initiated at subsurface material inhomogeneities(about 30 μm from the surface),resulting in a lower fatigue life for tubular specimens than for bar specimens.Combined with fatigue test and numerical simulation,it was demonstrated for the first time that the boat-shaped specimen could evaluate the fatigue life of the complete SG tube,and the best fitting curve of the high-cycle fatigue of the SG tube was given.The fatigue life simulation flow chart of SG tube based on cumulative damage theory was established,and the predicted lives were consistent with the experimental fatigue life.For the scratched specimen on the external surface of the SG tube,the stress distribution at the bottom of the scratched groove was redistributed and tended to be more uniform during the fatigue test,and the fatigue notch sensitivity of the material became lower.With the increase of surface scratch depth,the deformation band density and the grain boundary curvature at the bottom of scratched groove increased.Fatigue cracks mainly initiated at the bottom of the scratched groove,and the interaction between the deformation strain accumulation and the uneven surface defects formed by scratch would also lead to the crack initiation and propagation of the cracks.Under a given fatigue life,the maximum stress level had an exponential relationship with the depth of surface scratch.When the maximum stress level was high,the effect of surface scratch depth on fatigue life was more obvious.With the decrease of the maximum stress level,the effect of surface scratch depth on fatigue life decreased gradually,and the fatigue limit fluctuated in a small range above and below the yield strength.The SG tube showed serrated secondary cyclic hardening under 325℃ high temperature air and led to fatigue crack initiation,which was the fundamental reason why the fatigue life of Alloy 690TT SG tube under 325℃ high temperature air was lower than that under room temperature air(about 70%).SG tube showed cyclic saturation under room temperature air,the fatigue crack initiated when the peak stress decreased by 5%,and the crack initiated at the intersection step of the PSBs-matrix interface on the internal surface.Howover,under high temperature fatigue at 325℃,a large number of deformation twins burst at the grain boundary,the interaction between stacking faults and dislocations and dislocation entanglement,leading to secondary cyclic hardening,the density of PSBs on the internal surface increased significantly,and the fatigue crack initiated at the PSBs on the internal surface,resulting in the fatigue life under high temperature air being lower than that under room temperature air.The fatigue correction coefficient of the SG tube under high temperature and high pressure water was lower than that of bar specimen in the improved ANL model,which may be related to the PSBs film rupture-slip dissolution caused by the shape effect of SG tube.More dense oxide particles were distributed along the PSBs on the internal surface of the SG tube.The outer oxides of the PSBs were large-size NiFe2O4 spinel and small-size NiO particles,and the inner layer were Ni-rich oxides and Cr-rich oxides interface.The PSBs formed intrusion grooves and extrusion ridges on the surface,resulting in internal stress on the oxide film of the PSBs and rupture of the oxide films.Due to the high dislocation density,the PSBs produced high electrochemical activity,and formed corrosion couples with the surrounding non-slip region,which became the preferred dissolution sites.The crack initiation mechanism of SG tube under high temperature and high pressure water was PSBs film rupture-slip dissolution mechanism.The research showed that the corrosion fatigue data of SG tube under high temperature and high pressure water was below the ASME average curve and ANL room temperature air curve,but above the most conservative ASME design curve,the envelope curve divided by strain coefficient 2 could be selected as the design curve of SG tube.The corrosion fatigue life of the scratched specimen on the external surface of the SG tube under high temperature and high pressure water were significantly lower than that of the non-scratched specimen on the surface,and the corrosion fatigue life decreased gradually with the increase of the scratch depth,which was related to the corrosion fatigue cracking induced by the microstructure defects at the bottom of the scratched groove.The surface fine grain region and deformation bands at the bottom of the scratched groove stored a lot of deformation energy and had high electrochemical activity,so oxidation occurred preferentially.During the process of fatigue,the local weak area at the bottom of the scratched groove produced cyclic slip,and gradually formed slip steps or squeeze gaps,which made the oxide film on the surface crack and preferentially dissolve.The research shows that for the scratched specimen on the external surface of SG tube,the corrosion fatigue data of high temperature and high pressure water were below the most conservative ASME design curve,and there was insufficient safety margin under the test conditions in this paper. |
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