| As the core component of nuclear reactor,the primary main pipe is the channel to maintain and restricts the coolant circulation and flow,which is called the "aorta"of a nuclear reactor.To meet the use requirements,the primary main pipeline is made of duplex stainless steel containing 10%to 20%ferrite.However,due to the harsh service environment(280～320℃,13～15MPa),duplex stainless steel will undergo thermal aging embrittlement after long-term operation,which is specifically shown by the reduction of critical crack size,the rise of ductile-brittle transition temperature,and the increase of brittle fracture probability.With the extension of service time,the performance of duplex stainless steel cannot meet the safety threshold required by nuclear power plant,and the reactor must be shut down for maintenance,even permanent shutdown.Therefore,the thermal aging embrittlement is the littleneck for the safe operation and service life of nuclear power plant.Through in-depth understanding of the thermal aging damage mechanism of duplex stainless steel,it is proposed that using effective treatment methods to regulate the microstructure of damaged materials and thus achieve the performance repair of service damaged components is fundamental to solve the thermal aging damage of main piping and extend the service life.After thermal aging treatment of duplex stainless steel at 400℃ and 475℃ for different times,the micro-hardness of ferrite gradually increases with aging time prolonging,and the fracture type changes from ductile to brittle fracture.However,there are some differences in the microstructure of ferrite phase caused by aging temperature,that is,when the aging temperature was at 400℃,the ferrite phase decomposes to spinodal structure and the spherical G-phase;When the steel was aged at 475℃,ferrite decomposes to form spinodal structure and the cubic G-phase.Further,the aged duplex stainless steels were annealed at 550℃ for 1h to eliminate the effect of spinodal decomposition on aging hardening,and thus investigating the effect of G-phase on thermal aging embrittlement.When the duplex stainless steel was aged at 400℃,550℃ annealing treatment could restore the degradation performance by more than 77%,and it is no direct correlation between the degree of repair and thermal aging time.For the samples aged at 475℃,annealing treatment could only partially repair the degraded properties of the aged samples,and with the increase of aging time,the repair degree of annealing treatment decreases from 42.2%to 18.7%.By comparing the mechanical properties and microstructure differences of duplex stainless steels aged at 400℃ and 475℃,it can be concluded that spinodal decomposition is the dominant mechanism for the embrittlement of duplex stainless steels aged at 400℃;while G phase is the main cause for aging embrittlement of duplex stainless steel aged at 475℃.Although 550℃ annealing treatment can eliminate the spinodal decomposition to achieve partial recovery of damage properties,due to the higher thermal stability of the G-phase,annealing treatment cannot dissolve it.If the annealing temperature is increased forcibly,it will induce the formation of σ-phase,χ-phases and carbides,which further deteriorates the properties of duplex stainless steels.In this study,the seriously aged duplex stainless steel was obtained through accelerated aging,and the pulsed electric current was used to explore methods to synchronously eliminate spinodal decomposition and G phase to achieve complete repair of performance.Using multiscale microstructure characterization,mechanical property tracking testing,and numerical calculations,the mechanism of microstructure regulation and the correlation between structure evolution and deteriorated performance repair under pulsed current were investigated.Duplex stainless steel suffered serious thermal aging embrittlement after aging at 400℃ for 20000 h,while pulsed electric current treatment could completely eliminate the aging embrittlement.The effects of pulsed electric current treatment on the mechanical properties and microstructure of aged duplex stainless steel were analyzed by nanoindentation,transmission electron microscopy,and atom probe tomography.Pulsed electric current treatment at 420℃ for 1h could completely eliminate the spinodal decomposition and G-phase,and thus fully recovering the degraded properties of duplex stainless steel.Due to the difference in electrical conductivity between the spinodal structure and matrix,the uneven distribution of current increases the system free energy,leading to the spinodal domain lines change,which promotes the complete dissolution of the decomposed structure.Based on the established service life evaluation model for duplex stainless steel,it is calculated that after pulsed electric current treatment,the thermal aging state of duplex stainless steel significantly decreases from 20000h to 12h,which implies that the thermal aging behavior of duplex stainless steel is almost completely removed,and the service life of duplex stainless steel is extended by nearly one time.When the duplex phase stainless steel was isothermally aged at 400℃ and 475℃,the morphology of the G-phase is spherical and cubic,respectively.Based on the relationship between the morphology changes of precipitate and interface energy,the transformation of the morphology of the precipitate is driven by the minimization of interfacial chemical energy and strain energy.To eliminate or reduce the hardening effects of different morphologies of G on the properties of duplex stainless steel,the aged samples were subjected to pulsed electric current and annealing treatment,annealing treatment at 550℃ for 1h can only promote further coarsening of the G-phase,resulting in an increase in the hazard of G-phase.Pulsed electric current can completely dissolve the fine spherical G-phase and remove the contribution of spherical G to hardening;Pulsed electric current can spheroidize the cubic G phase to reduce size,thereby decreasing the contribution of the cubic G-phase to hardening caused by more than 20%.When pulsed electric current was applied directly to the thermal aging process at 475℃,the ferrite did not suffer age-hardening behavior,and spinodal decomposition could not occur spontaneously.The thermodynamic analysis indicates that the introduction of pulsed electric current changes the system free energy,causing the contraction of the spinodal region and avoiding the occurrence of spinodal decomposition.Further,the life assessment model presents that the duplex stainless steel aged at 475℃ for 100h has worked for 18.2 years at the service temperature of 297℃,while the duplex stainless steel aged by a pulsed electric current is equivalent to only using for 1.2 years under the same conditions,which means that the pulsed electric current can reduce the thermal aging embrittlement rate～93.4%.In summary,using pulsed electric current to repair the deteriorated property of thermally aged duplex stainless steel in this study is an inspiration for the life extension management of the primary circuit main pipes of nuclear reactors. |
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