| As a clean,reliable and efficient energy source,nuclear energy plays a significant role in the global energy structure.On the basis of China’s ambitious development of low-carbon energy,advancing nuclear energy is an essential choice for ensuring energy security in the new period.The service performance of nuclear materials has always been an important issue in the safety assessment and life extension of nuclear power plants.Steam generator tubes are the critical barrier between the primary and secondary sides of pressurized water reactor(PWR)nuclear power plant,and hence keeping their service safety is of paramount importance.Stress corrosion cracking(SCC)is one of the main failure modes of steam generator tubes due to their long-term operation in high temperature pressurized water.So far,surface scratches have caused many SCC failures of steam generator tubes worldwide.Surface scratches can affect the SCC resistance of steam generator tubes,which has been of great concern.However,scratches with varying depths are inevitable formed on the surface of steam generator tubes during manufacturing processes,and data on SCC behavior of domestic nuclear alloy 690TT scratched tubes are very rare.To address the above issues,this thesis investigated the effect of scratch depth on both material damage and SCC behavior of alloy 690TT scratched area.Meanwhile,main factors that enhancing SCC sensitivity in scratched area were identified,and the micro-mechanism of SCC crack growth was elucidated.Then,this thesis proposed and evaluated a feasible scratch repairing process for engineering applications.Finally,the influence of various coupling factors on SCC behavior were systemically revealed.The microstructure,residual strain and SCC behavior in scratched area with different depths were studied.Firstly,the characterization results show that the scratched area can be divided into four regions according to the microstructure and residual strain distribution characteristics:fine grain zone(FGZ),severe deformation zone(SDZ),general deformation zone and submatrix.Secondly,the results of corrosion test(performed at 325℃,in deaerated alkaline high-temperature pressurized water,for 1000 h)show that the number and length of cracks increase with scratch depth.The deeper scratches own more higher SCC sensitivity due to their higher residual strain,larger deformation zones and more slip steps.Thirdly,the microstructure changes,the high residual strain and the special geometric structure are the main factors for the enhancement of SCC sensitivity in scratched area.Besides,PAZ is more likely to be the location for SCC initiation.The microscopic mechanism of SCC behavior in scratched area was studied.The microstructure,oxidation characteristics,carbide morphologies and distribution near the typical SCC cracks in scratched area were characterized systemically.Results demonstrate that the structural mismatch at the interface between the FGZ and SDZ is high,so that SCC can easily continuously propagate along this interface.Moreover,the scratching can lead to the fracture and displacement of carbides and change the morphologies and distribution of carbides.The above carbides then lose the hindering effect(compared to the original semi-continuous carbides)on the SCC propagation,thus affecting the SCC behavior in the scratched area.Finally,compared to the FGZ,the SDZ is more susceptible to erosion.Once the crack changes the path at a certain position at the interface between FGZ and SDZ,the crack will bifurcate,and then grows in SDZ.The influence of grinding treatment on the microstructure,residual strain and SCC sensitivity of scratched area was investigated.Two types of sandpaper with different grit sizes were applied for treatment.The results show grinding treatments eliminate the PAZ and reduced the number of crack initiation sites by removing deformation steps and geometric structure.However,grinding treatments increase the activity of the subsurface,leading to preferential oxidation(PO)and short cracks.The grinding treatment using finer abrasive paper performs better than that using abrasive paper with larger grit size.Furthermore,severe SCC may still occur on samples with deep scratches,because the risk factors(such as microstructure changes and residual strain)have not been entirely eliminated.After being ground,deep scratches(≥ 70 μm)still need extra attention in engineering applications.The independent effects of various factors on SCC behavior in scratched areas were investigated.Two different heat treatments,namely stress relief annealing(RA)and solution+TT treatment(SATT),were applied to separate the coupling factors.After heat treatment,the microstructure and residual strain of scratched area have been changed significantly.Firstly,for RA-scratched area,the residual strain is nearly eliminated in FGZ and SDZ,but microstructure stratification in FGZ and SDZ still exists.While for SATT-scratched area,the microstructure and residual strain changes caused by scratching are completely eliminated.Secondly,the SCC behavior in scratched area changes significantly after heat treatment.For RA-scratched area,cracks could still initiate at the interface between FGZ and SDZ,but the crack length significantly decreases,indicating the SCC sensitivity significantly decreases.For the SATT-scratched area,no SCC is observed in all samples,showing excellent SCC resistance.Thirdly,the three coupling factors of microstructure,residual strain and geometric structure can be separated and investigated independently.The special geometric structure caused by scratch is a necessary condition for crack initiation,but it is not enough to cause crack initiation by itself.The effect of high residual strain is mainly reflected in promoting crack propagation.The microstructure change can provide favorable conditions for both initiation and propagation of cracks. |
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