Thermal Ageing And Irradiation Damage Of Nuclear Grade 308L Stainless Steel Weld Metal

308L stainless steel weld metal has been widely used in inner overlay weld cladding and nozzle/safe-end weld joint of reactor pressure vessel in pressurized water reactors(PWRs),due to its good mechanical and corrosion properties.However,308L weld metal suffers from thermal ageing during the long-term service at 280?330?,leading to an increase in hardness but a decrease in toughness as well as a degradation of corrosion resistance,and thus affecting the structural integrity of the relevant weld joints.In addition,irradiation damage by neutron irradiation may occur in the weld overlay cladding and affect its corrosion and stress corrosion cracking(SCC)behaviors.Therefore,elucidating the mechanisms of thermal ageing and irradiation damage and their effects on microstructure and corrosion and SCC behaviors in high temperature high pressure water,along with the recovery mechanism of post-ageing/irradiation annealing,is essential to the service safety,life prediction and life extension of relevant weld joints.Firstly,thermal ageing of nuclear grade 308L stainless steel weld metal was studied.In this work,accelerated thermal ageing treatment at 440? was performed on 308L weld metal up to 7000 h,followed by microstructure characterization by using transmission electron microscope(TEM)and atom probe tomography(APT)to study the effect of thermal ageing on microstructures of 308L weld metal.After that,double loop electrochemical potentiokinetic reactivation(DLEPR)test was carried out to quantitatively evaluate the degree of thermal ageing.Then corrosion experiments were performed in simulated PWR water containing 2.3 ppm dissolved hydrogen(DH)and 200 ppb dissolved oxygen(DO),respectively,for 480 h,in order to investigate the effect of thermal ageing as well as dissolved gas on corrosion behavior of 308L weld metal.The results of microstructure characterization suggest that austenite is nearly unaffected by thermal ageing,while spinodal decomposition of ?-ferrite into Fe-rich ? phase and Cr-rich ?' phase as well as precipitation of Ni/Si/Mn/P/Cu-rich G-phase in ?-ferrite occur.In addition,thermal ageing results in segregation of Ni,Mn and C at austenite/?-ferrite phase boundary,but has little effect on the concentration of Cr,Si and P.The results of DLEPR test indicate that the activation current peak is formed mainly by the general dissolution of austenite,while the reactivation peak is induced by the selective dissolution of ?-ferrite.Before 5000 h,a linear relation fits well between the nanoindentation hardness of ?-ferrite and the reactivation ratio obtained from the DLEPR curves,but it deviates after 5000 h due to the saturation of hardness.Nevertheless,the transition point of growth rate of the reactivation ratio corresponds to the saturation point of hardness of ?-ferrite.The results of corrosion experiment show that under DO condition the oxide particles formed on austenite are less than that on ?-ferrite regardless of thermal ageing,while the inner oxide thickness of austenite is larger than that of ?-ferrite.Some dissolved regions are found at the interface between inner oxide layer and austenite or ?-ferrite matrix,and a Cr-rich layer is often observed on the surface of oxide particles.In addition,slight localized corrosion occurs at the austenite/?-ferrite phase boundary for both unaged and 7000-h aged specimens.Under DH condition,however,the oxide particle number and inner oxide thickness of austenite are larger than that of ?-ferrite regardless of thermal ageing.No localized corrosion along phase boundary is found either in unaged or 7000-h aged specimens.On the whole,thermal ageing has little effect on the corrosion behavior of 308L weld metal,since slight enhancement of ?-ferrite corrosion is found only under DH condition.Secondly,irradiation damage of nuclear grade 308L stainless steel weld metal was studied.In this work,proton irradiation was performed on 308L weld metal up to 3 dpa(displacement per atom)so as to emulate neutron irradiation in service.The irradiation effect on microstructure of 308L weld metal was studied by combining TEM with APT.After that,corrosion experiment and slow strain rate tensile test in simulated PWR primary water were carried out to study the irradiation effect on corrosion and SCC behaviors,respectively.The microstructure characterization indicate that numerous voids,Frank dislocation loops and Ni/Si-rich solute clusters are caused by irradiation in austenite but not in ?-ferrite.Nevertheless,the irradiation enhances the spinodal decomposition of ?-ferrite and induces the precipitation of Ni/Si/Mn/P-rich G-phase in?-ferrite.By increasing the irradiation dose,coarsening of G-phase occurs,followed by inverse coarsening due to the enhanced irradiation recoil effect.In addition,similar enrichment of Ni and Si but depletion of Cr is observed at austenitic grain boundary and three types of phase boundaries(austenite/?-ferrite,austenite/carbide and ?-ferrite/carbide phase boundaries)in the irradiated specimen.The corrosion experiments suggest that irradiation obviously enhances the corrosion of austenite but has little effect on the corrosion of ?-ferrite,indicating that ?-ferrite has a higher corrosion resistance than austenite.In addition,localized corrosion along the grain boundary and the phase boundary without carbide is caused by irradiation.For the phase boundary with carbide precipitation,however,severe localized corrosion occurs in the unirradiated specimen and remains nearly unchanged after irradiation.The SSRT results suggest that the SCC susceptibility of 308L weld metal is evidently increased by irradiation by causing slip steps on the specimen surface and various deformation structures in the specimen matrix including dislocation channels,deformation twins,stacking faults and atomic plane rotation.The propagation of intergranular SCC crack is closely related with the environment of crack tip.For the crack tip located in the specimen interior,the localized deformation is presumably the dominant factor affecting the crack propagation.For the crack tip on the specimen surface,however,the localized corrosion along grain boundary is most likely the key factor of crack growth.The SCC crack along phase boundary initiates either by the cracking of the phase boundary itself,or by the crack propagation from grain boundary to phase boundary.It is noted that all the phase boundaries where the SCC occurs contain a large number of carbides and are severely corroded,suggesting that the localized corrosion plays a key role in the SCC along phase boundary.Finally,the annealing recovery of thermal ageing and irradiation damage of nuclear grade 308L weld metal was studied.In this work,an annealing treatment at 550? for 1h was performed on 7000-h aged specimen and 3-dpa irradiated specimen,and the annealing effect on the microstructure and corrosion behavior of aged and irradiated specimens was investigated.The results of post-ageing annealing suggest that austenite presents a high thermal stability with no structural or chemical changes induced by annealing.On the other hand,the spinodal decomposition and G-phase in ?-ferrite and all the element segregation at austenite/?-ferrite phase boundary are eliminated by annealing.In addition,the hardening of ?-ferrite induced by thermal ageing is recovered by annealing,but the corrosion behavior of 308L weld metal remains unchanged.The results of post-irradiation annealing show that both the number density and size of voids and the size of Frank loops are increased by annealing,while the number densities of Frank loops and Ni/Si-rich clusters are largely decreased.The spinodal decomposition and G-phase in ?-ferrite as well as the segregation at grain boundary are also eliminated.The post-irradiation annealing enhances the corrosion of austenite but has on obvious effect on the corrosion of ?-ferrite.In addition,the localized corrosion along the austenitic grain boundary and the austenite/?-ferrite phase boundary without carbide is recovered by annealing,but it still exists along the phase boundary with carbide precipitation.According to the annealing work mentioned above,it can be drawn that the annealing treatment of 550?/1h presents a good recovery for the thermal ageing by restoring the microstructure of thermally aged 308L weld metal to the unaged state.However,the same annealing treatment only partially remove the irradiation structures,which requires more researches on post-irradiation annealing at elevated temperatures or for extended durations.