| The future development of ultra-supercritical units aims to be large-scale,integrated and long-life.The realization of this goal depends to a large extent on the fatigue strength and corrosion resistance of key large components of the steam turbine.Turbine transition zone blades are subjected to the combined destruction of corrosive media,cyclic loads,and high-temperature water vapor in the working environment.Pitting corrosion,corrosion fatigue and cavitation damage on the surface will lead to the fracture failure of the blade.Surface plastic deformation technology can significantly improve the corrosion resistance,mechanical properties and cavitation resistance of metal materials,thereby increasing the service life of steam turbine blades.Laser shock processing(LSP)technology has remarkable characteristics such as ultra-high strain rate,high pressure,ultra-fast,etc.,and it has advantages that conventional surface processing methods cannot match.However,there is still a lack of systematic research on the evolution of microstructure and the influence of residual stress and microstructure on the corrosion/cavitation behavior of laser shocked dual phase materials.This paper takes 2Cr13 martensitic stainless steel as the research object,and four aspects including laser shock-induced gradient nanocrystallization mechanism,electrochemical corrosion resistance,corrosion fatigue crack growth resistance,and cavitation resistance performance have been systematically studied.The main conclusions and innovative results are as follows:(1)Gradient nanocrystallization mechanism of the matrix and carbide in2Cr13 steel subjected to LSP with different coverage layers: Based on transmission electron microscopy observation,energy dispersive spectrum element mapping and X-ray diffraction phase analysis,the matrix and carbide at different depths in the plastic-deformed layer of 2Cr13 steel subjected to LSP were characterized,and the mechanisms of dual phase gradient nanocrystallization for martensitic lath and carbide were revealed for the first time.Results show that the formation mechanism of the gradient nanostructure is mainly through the dislocation movement,and the presence of carbides promotes the dislocation movement.When the martensitic laths are refined to a size smaller than that of carbides,the carbides are also refined or even nanocrystallized,and the multi-directional dislocation slip subdivides the carbides into nanocrystalline carbides.Under the mechanical action of laser shock wave with ultra-high strain rate and ultra-high pressure,the carbides are also fragmented and separated,and the reason is that the microcracks propagate along the amorphous shear band and run through the carbides.Besides the refinement and fragmentation of carbides,the precipitation of nano-carbides can also be induced by LSP.The temperature rise induced by ultra-high strain rate can promote the nucleation and growth of nano-carbides at the high-density dislocation.In addition,during the plastic deformation process,the carbide can decompose obviously,which is mainly manifested as the decrease of Cr content and the increase of Fe content in the carbide.The carbide decomposition is the synergistic result of the interface wear of carbide caused by the matrix flow and the plastic deformation of carbide.Finally,compared with the LSP treatment with one coverage layer,the refinement degree of martensitic lath and carbide induced by the LSP treatment with two coverage layers is higher.(2)Electrochemical corrosion initiation and propagation behavior of 2Cr13 steel subjected to multi-layer LSP: The inhibition mechanism of LSP on electrochemical corrosion initiation and propagation was analyzed from the aspects of compressive residual stress and microstructure by measurement of impedance spectrum and potentiodynamic polarization curve as well as scanning electron microscope observation of corrosion morphology.The corresponding relationship between corrosion morphology and microstructure was established.The results show that the charge transfer resistance and pitting potential of LSPed specimens are significantly increased,indicating that LSP has a significant inhibition effect on the corrosion initiation.The corrosion current density of LSPed specimens is significantly reduced,which indicates that LSP significantly reduces the corrosion propagation rate.The main reasons for the improvement of electrochemical corrosion performance are due to the martensitic lath nanocrystallization,carbide decomposition,Cr segregation and compressive residual stress.The electrochemical corrosion performance is gradually improved with the increase of coverage layer.The corrosion initiation of the as-received specimen occurred in the Cr-depleted zone around the Cr-rich carbides,and the corrosion propagation was mainly along the martensitic lath boundary.The corrosion initiation near the carbide in LSPed specimen is inhibited,and the corrosion propagation is mainly along the ultrafine grain/nanocrystalline boundary.(3)Cavitation erosion initiation and propagation behavior of 2Cr13 steel subjected to multi-layer LSP: The effect of LSP with different coverage layers on cavitation erosion behavior in distilled water and sand suspension was investigated by cavitation erosion test as well as scanning electron microscope observation and energy dispersive spectrum element mapping of surface morphologies.The inhibition mechanism of LSP on cavitation erosion initiation and propagation was discussed from two aspects of compressive residual stress and microstructure,and the corresponding relationship between cavitation erosion morphology and microstructure was established.The results show that the cavitation erosion resistance of 2Cr13 steel in distilled water and sand suspension is significantly improved by LSP due to the synergistic effect of compressive residual stress and microstructural evolution,and the cavitation erosion resistance steel increases with the increase of coverage layer.The cavitation erosion crack of the as-received specimen first initiates around the coarse carbide near the wear mark and then propagates along the wear mark and martensitic lath boundary.However,due to the carbide fragmentation induced by LSP,the number of initiation points of cavitation erosion fatigue crack near carbide decreases,and the initiation of cracks is inhibited.Moreover,the crack propagation occurs along the wear mark and the grain boundary of refined grains,and the crack is hindered by a large number of grain boundaries during the propagation process.(4)Corrosion fatigue crack propagation behavior of 2Cr13 steel subjected to LSP with surface gradient pulse energy: The effect of LSP with gradient pulse energy on corrosion fatigue crack growth performance of 2Cr13 steel in Na Cl solution were studied by microstructure characterization,residual stress measurement,corrosion fatigue test and fracture morphology observation.Based on the surface gradient compressive residual stress and solution p H value,a mathematical model of corrosion fatigue crack growth rate was developed.The corresponding relationship between fracture morphology and microstructure was established.The experimental results show that LSP can effectively improve the corrosion fatigue property of 2Cr13 steel in acidic(p H 3),neutral(p H 7)and alkaline(p H 11)Na Cl solutions.LSP with gradient pulse energy can effectively reduce the crack growth rate and improve the corrosion fatigue life.The improvement of corrosion fatigue performance is mainly due to the synergistic effect of compressive residual stress and microstructural evolution.The crack propagation of the as-received specimen is mainly along the martensitic lath boundary,while the crack propagation of the LSPed specimen is mainly along the ultrafine grain boundary. |
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