| Corrosion fatigue(CF)is one of the potential failure modes for pressure boundary materials in pressurized water reactors(PWRs).Operation experience has verified that except for the normal CF,CF of special status may also happen due to the special structures,such as the fretting corrosion fatigue and the notch corrosion fatigue.The former mainly takes place between the steam generator(SG)tube and the supporting plate/anti-vibration bar,while the latter mainly occurs at the geometrical discontinuities of the reactor components.However,due to lacking of simulation testing equipment and experimental technique,there are limited dataconcerning fretting corrosion fatigue and notch corrosion fatigue of the structural materials in PWRs,as well as the related fatigue failure mechnisms.In the present work,based on the existing CF testing machine,two CF research methods for nuclear-grade materials under special status in high-temperature water were developled,namely,the methods for fretting corrosion fatigue and notch corrosion fatigue investigation in high-temperature pressurized water.Under these circumstances,the fretting corrosion fatigue of the domestic nuclear-grade Alloy 690 in high-temperature pure water and the notch corrosion fatigue of the domestic nuclear-grade 304 SS in high-temperature B/Li water were investigated.Main research progress is as follows.(1)Fretting corrosion fatigue testing apparatus with high-temperature pressurized water loop was designed and constructed.A fretting fatigue specimen and a matched fixture were designed.The fixture can clamp the specimen stably.A normal load applied device with compression springs was designed.The device is suitable to apply a normal load within 0-300 N,and the accuracy can be maintained during the long-term test.The theoretical error is less than 0.5%.(2)Fretting corrosion fatigue of Alloy 690 in high-temperature pure water was investigated.Under the condition of 100 N normal load,the fretting corrosion fatigue life of Alloy 690 in 285 ? water is reduced by about 30%in comparison with that in RT air,indicating a significant environmentally assisted fretting fatigue.Due to the combination of high-temperature pressurized water and fretting damage,the fretting affected zone can be divided into two regions:the contact region and the crevice region.The longitudinal section of the contact region consists of three layers,the oxide layer at the outmost surface,the deformed layer in the middle and the matrix.The oxide layer has a duplex structure,including the outer tribolayer and the inner Cr-rich oxide layer.(3)In RT air,fretting fatigue crack of Alloy 690 is inclined to initiate in the edgearea of contact region due to the localized tangential stress;while in high-temperature pure water,the crack is intended to initiate in the crevice region due to the combination of high-temperatuer pressurized water environment and localized tangential stress.In addition,the crack propagation in high-temperature pure water has an extra perpendicular propagation stage.Based on the surface damage and the cracking behaviour,a fretting corrosion fatigue damage model of Alloy 690 in high-temperature pure water was proposed.(4)Fretting corrosion fatigue of Alloy 690 in 285 ? pure water was investigated under the conditions of different normal loads.With increasing the normal load,the fretting fatigue life decreases,areas of the contact region and the fretting affected zone increase,suggesting a more severe combination of environmental degradation and fretting damage.(5)In 285 ? pure water,fretting corrosion fatigue crack initiates in the crevice region where has the maximum mutual effect.The crack shows an oblique angle to the fretting direction due to the slip oxidation/dissolution cracking mechanism.With increasing the normal load,the slip phenomenon of crack becomes more obvious.Crack propagation depends on the competition between the environment and the mechanics.The crack grows perpendicular to the fretting direction at the early stage,then turns to a certain oblique angle,and finally turns back to grow in a manner perpendicular to the fretting direction.Under the condition of a larger normal load,the subsurface near the crack tip bears more severe deformation,resulting in a more severe environmental degradation,thus the perpendicular propagation depth at the early stage is larger.(6)A method for the notch corrosion fatigue test in high-temperature pressurized water was designed.According to the theoretical calculation,charts analysis and finite element analysis(FEA),a notched specimen with Kt=2.59 was designed.And the relationships between the nominal cyclic stress and the notch root strain amplitude for the notched specimen in RT(25 ?)and high-temperature(280??325 ?)air were obtained by FEA.This method is suitable for the notch fatigue test in RT air and the notch corrosion fatigue test in high-temperature pressurized water.(7)Notch fatigue behaviour of 304 SS notched specimen in RT air was investigated.When ?roitis equal to ?smooth,fatigue life of the notched specimen is longer than that of the smooth specimen,which is attributed to the stress gradient on the section of the notch root.(8)Notch corrosion fatigue of 304 SS notched specimen in 280 ? and 325 ? B/Li water at different average strain rates was investigated.The corrosion rate of 304 SS in 280 ? water is higher than that in 325 ? water,indicating a larger CF life reduction.Under the condition of stress-controlled notch fatigue test,the DSA in 325 ? water is more severe than that in 280 ? water,indicating a larger CF life increasement.Due to the combination of corrosion and DSA,CF life of 304 SS notched specimen in 325 ? water is longer than that in 280 ? water.With decreasing the average strain rate,both the degrees of DSA and corrosion in high-temperature pressurized water become more severe.But in comparison with the more severe corrosion at a lower average strain rate,the enhanced effects of DSA seems less significant.Therefore,with decreasing the average strain rate,the CF life of 304 SS notched specimen decreases. |
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