| Nuclear power,standing side by side with hydropower and thermal power as the three pillars of the world's energy,plays an important role in the world's energy structure.As a clean,reliable and efficient energy source,nuclear energy has not only become an important energy source for mankind,but also an important means for mankind to deal with energy crises and global climate change.The long-term,safe and stable operation of nuclear power plants strongly depends on the service behavior of the structural components.Experience has shown that corrosion issue is one of the main reasons for the failure of nuclear power structural materials.Corrosion usually only occurs at the surface of the material.Therefore,the near-surface microstructure plays a key role on the corrosion behavior of nuclear power materials.Processing technology is one of the main reasons that lead to changes in the microstructure of materials,especially the near-surface microstructures.After machining processing,a deformation layer was produced.Once the improper processing was used during processing,inappropriate or even deterious microstructures might be introduced into the material,which would directly affect the performance of the material in high temperature water.On the other hand,the internal components of the reactor and the primary circuit structures are inevitably affected by neutron radiation during the operation.Irradiation mainly leads to displacement of atoms,resulting in the microstructural changes of the material,such as material hardening,element segregation,formation of precipitation phases,or generation of radiolysis,which leads to the increase in the corrosion potential,thereby accelerating the corrosion process.This dissertation will choose nuclear 304 stainless steel as the research object to study the effects of surface machining and irradiation on change of the near-surface microstructure and corrosion behaviors in high-temperature water of the material.Seven samples with different cutting parameters(including three main variables of machining cutting:feed rate,cutting speed&depth of cut)were selected to perform detailed characterization on their surface morphology,deformation structure,work hardening,and residual stress.Results show that the cutting parameters can significantly affect near-surface microstructure of the material.For samples with similar surface roughness,due to the different processing parameters used,their near-surface microstructures can be very different.The order of the influence of cutting parameter variables on the near-surface machining-affected zone(MAZ)is:feed rate>cutting depth>cutting speed.Sample with a larger feed rate has a larger MAZ,and sample with a low speed also has a larger MAZ.When the cutting depth is reduced,the MAZ will be enlarged as well.Surface machining produced a gradient structure with depth of several hundred-microns near the surface.The top surface layer is nanocrystalline within a few microns,followed by the grain distortion zone.A large number of deformation bands and dislocations are formed in the MAZ.The deformation band is composed of mechanical twins and stacking faults.Residual stress exists in the range of several tens of microns in the near-surface region.In the range of a few microns near the surface,the residual stress is tensile while it turns into compressive thereafter and gradually decreases to zero.The corrosion and stress corrosion behavior of the machined samples in high temperature water environment were studied.Results demonstrate that within the range of MAZ,the corrosion level decreases with the increase of the depth.The corrosion rate of the deformation band is higher than the corrosion along the grain boundary,and both are significantly higher than the average corrosion rate at the same depth from the surface.The corrosion rate of different microstructures(nanograins,deformed bands,etc.)near the surface differs by two orders of magnitude.Moreover,after long-term corrosion in high temperature water environment with dissolved oxygen,cracks began to initiate on the machined surface.SCC cracks initiate at the site of surface grooves and expand into the material along the deformed grain boundaries.Further analysis shows the tensile residual stress could promote the initiation and propagation of cracks,while the compressive residual stress near the surface region can inhibit further crack propagation to a certain extent.The domestic nuclear-grade 304 stainless steel was irradiated with 3.5 MeV Fe ions,with the irradiation doses of 3.05×1015 ions/cm2 and 1.55×1016 ions/cm2(the damage level at the peak calculated by the simulation was about 3.2 dpa and 16 dpa).Positron annihilation spectrum and transmission electron microscopy were used to characterize the internal defects of the irradiated materials.Nanoindentation was used to characterize the radiation hardening phenomenon.The results show that Frank dislocation loop is the main defect caused by irradiation,with density of about 1.22 m-3.The size of the loop ranges from a few nanometers to tens of nanometers.The characteristics of radiation-induced defects under the two irradiation doses are different.Irradiation at high doses causes the size of dislocation loops to increase but decrease in number density.Irradiation produces obvious hardening effect,which tends to be saturated within a few dpa dose range.The hardness of the material increased by 55%and 71%at the two irradiation doses,respectively.The sessile Frank dislocation loop is thought to be the main cause of material hardening.In addition,statistical study also found that the characteristics of irradiation defects will affect the evaluation results of theoretical calculations.The corrosion behavior of the irradiated samples in high temperature water environment was studied.Studies show that the formed oxide film of the irradiated sample is more uneven.The defects produced by irradiation accelerate the ion diffusion process,leading the formed oxide film becomes less protective and discontinuous.At the same time,the increased dissolution of solute atoms results in a decrease in the number of oxide particles and an increase in size.Therefore,acceleration of the overall corrosion phenomenon is observed.Moreover,the irradiated samples represent obvious intergranular corrosion behavior,and the degree of intergranular corrosion was further aggravated at higher doses.Further analysis show that the structure of grain boundary changed during the followed corrosion process,which account for the migration of grain boundaries and the aggravation of IGC in high temperature water environment.In general,the influence of the two-surface treatment on the corrosion behavior are achieved by changing the near-surface microstructure of the material.Irradiation mainly affects the intergranular corrosion behavior of the material in high temperature water.In contrast,the near-surface microstructure changes more drastically under the operation of machining cutting.Machined samples not only present intergranular corrosion in high-temperature water,but also preferential corrosion at the deformation bands.Additionally,the machining-induced surface defects and local stress concentrations may lead to the initiation and propagation of the SCC cracks. |
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