Irradiation And Corrosion Resistance Of 316LN Stainless Steel With A Nanostructured Surface

With the rapid development of nuclear industry,the safety of nuclear materials in service has become a major concern.In nuclear reactors,316LN austenitic stainless steel(316LN SS)is widely used in the manufacture of structural parts due to its excellent comprehensive properties.However,in the harsh environment,it may cease to be effective,such as stress corrosion,irradiation swelling,irradiation assisted stress corrosion cracking et al.The corrosion failure and radiation damage of the 316LN SS relate to the passive film characteristics on its surface and the absorption 'trap' density of irradiation defects.In theory,the irradiation and corrosion resistance properties of the 316LN SS can be improved by optimizing the surface structure of this steel using a surface nanocrystallization technology.In this dissertation,rotating accelerated shot peening(RASP)method was used to produce a nanostructured layer on 316LN SS.The evolution of microstructure and mechanical property of this steel during shot peening were researched.The effect of nanostructured layer thickness,grain size,grain boundary characteristic and surface roughness on the irradiation and corrosion resistance were analyzed.The relations between the special structures such as nanocrystalline,twin boundary,et al and the irradiation defect and passive film is revealed.The main research contents and conclusions are as follows:(1)The influence of RASP treatment on microstructure of 316LN SS was studied.The RASP did not produce any obvious crack on the 316LN SS specimen.The Bragg-diffraction peaks were broadened obviously and the grain size were refined tremendously for the RASP-processed specimens.After 15 min of RASP treatment,the average grain size of the specimen was reduced to 30 nm.The hardness of this specimen decreased gradually from its surface to the matrix,and gradually stabilized at about 3.24 GPa from 5.06 GPa.The thickness of the deformation layer for the specimens increased continuously with the RASP-processed time extension.The plastic deformation and grain refinement process of the 316LN SS specimens mainly included the formation of deformation twins and dislocations in coarse grains,the interaction of twins and dislocations,the degradation of twins in ultrafine grains and the division of twins.(2)The effects of grain boundaries and twin boundaries on the irradiation defects were investigated.The relationship between irradiated helium bubbles and the mechanical properties of 316LN SS was established.RASP treatment produced high densities dislocations and twin boundaries on the surface of the 316LN SS specimen,which improved the surface hardness and uniformity of the steel,thus avoided the preferred orientation of grains that caused by irradiation.With irradiation dose increasing,the hardness of the specimens increased steadily and then decreased abruptly.The irradiation softening effect appeared on both as-received and RASP-processed specimens surprisingly.It was found that under the action of shear force,the high density and large size helium bubbles introduced by irradiation can aggregate,coarsen,and finally connect to form unstable shear bands,which caused the softening and failure of material.High densities of dislocations and twins produced by RASP inhibited the formation and growth of helium bubbles,sequentially restrained the formation of shear bands and the appearance of irradiation softening,thus improving the mechanical stability of the steel.(3)The corrosion resistance mechanism of the nanostructured 316LN SS specimen was clarified.On the premise of eliminating the influence of surface roughness,the RASP-processed 316LN SS specimens showed a better corrosion resistance by the more positive Ecorr,higher Ep,Rp and lower Icorr.Passive films grown on the surface of nanocrystalline specimens contained less carrier density ND and more corrosion-resistant oxides.The formation of such a protective passive film was mainly attributed to the nanocrystallines and twins produced by the RASP process.The high density of grain boundary can provide more activity sites to reduce the nucleation work and expansion work required for the formation of the passive film and improve the adhesion between passive film and the nanostructured substrate.(4)The best corrosion resistance area and its microstructure characteristics for the gradient-nanostructured(GS)316LN SS specimen were determined.Thickness of the GS layer produced by RASP and cold rolling was about 210 ?m.For this layer,the defects produced by shot peening basically never existed in the area of 110 ?m distance from surface,while the nanostructure was retained,thus corrosion resistance of this area was optimum.The passive film formed on this area was flat and its thickness reached 13.9 nm.The defects produced by shot peening can increase the strain energy of the passive film.During the stress relaxation process,the passive film will be stretched and thinned to 5.5 nm,thus became the weak spot during corrosion.(5)In the environment of simulating primary water systems of pressurized water reactor,the effects of RASP on the oxide film of 316LN SS was investigated.The results showed that the RASP treatment did not alter the composition of the oxide films formed on the 316LN SS specimens,but promoted the nucleation and growth of the oxide film.With the prolongation of RASP treatment time,the density and size of oxide particles in the outer layer and the Cr2O3 content of the inner layer of the oxide film both increased.The electric resistance of the oxide film formed on the specimen that 10 min RASP-treated was highest whereas its Ecorr and Iorr were lowest,indicating the corrosion resistance of this oxide film was best compared with others formed on the specimens RASP-treated at different times.