Investigation On Thermal Aging And Ion-irradiation Behaviors Of T92 F/M Steel

T92 ferritic/martensitic(F/M)steel is one of the most promising materials for critical components of outer casing in sodium cold fast reactor(SFR)nuclear power plants,due to its excellent resistance to irradiation induced void swelling,outstanding thermal stability,good mechanical properties,and compatibility with sodium.Serving in SFRs,the material suffers from high level neutron irradiation,long-term thermal exposure and complex stresses,which will result in changes in the microstructure and properties of the outer casing material,directly affecting its service behavior and related to the safe operation of SFRs.Therefore,it is vital to understand the complicated effects of long-term thermal aging and irradiation-induced degradation on the microstructure and mechanical properties in T92 F/M steel to ensure safe and efficient operation of SFRs.The results will provide a basis for the design and development of nuclear grade 9Cr F/M steel,which is of great significance for the development of China's SFRs.It is complicate,expensive,time consuming and poorly controllable to conduct the neutron irradiation experiment.In addition,the post-irradiated samples with strong residual radioactivity are not conducive to subsequent microstructural observation and performance evaluation.Based on the similarity of irradiation-induced damage in the materials induced by ion and neutron irradiation,ion irradiation has been more widely used to simulate neutron damage due to its high damage rate,high experimental controllability,and ease of sample analysis and testing.In this study,the T92 steel was thermally aged at 650 ? for as long as 15,000 hours(?1.71 years).Through scanning electron microscopy,transmission electron microscopy and high-energy X-ray synchrotron radiation diffraction and other microscopic structure analysis techniques,thermal aging mechanism of T92 steel was analyzed systematically,and the effect of microstructure evolution during thermal aging on the mechanical properties was revealed.The original and thermal-aged T92 steels were both ion-irradiated at roo,m temperature with 250 keV protons and 3 MeV Fe11+respectively.The evolutions of irradiation-induced defects were studied by the transmission electron microscopy,slow positron annihilation,and nanoindentation.The relationship between microstructure and properties after irradiation was constructed,and the effect of long-term thermal aging on the radiation damage behavior of T92 steel was investigated.It is found that,the martensite matrix in the T92 steel recovers and the width of the lathes grows with increasing thermal aging time.With recombination or offset of dislocations during thermal aging,the density of dislocations decreases,which also leads to the reorganization and subcrystallization of martensite laths and the increase in the concentration of vacancy-type defects.The dominant dislocation lines in both the original and aged T92 steels are 1/2 a0<111>type.The shape and size of the MX carbonitride precipitates remain stable during the thermal aging process,while the short-bar shaped M23C6 carbide precipitates gradually coarsen accompanied the reduction of long and short axis ratio.The Laves precipitates rich with W and Mo preferentially nucleate on the M23C6 phase and coarsen rapidly,whose average size in the late stage of thermal aging increases to 400 nm.All of the above microstructure evolutions are only obvious in the range of 0?9000 hours of thermal aging,and become stable after 9000 hours.The hardness of T92 steel gradually decreases with increasing thermal aging time,and the relationship between Vickers hardness Hv and nano hardness H0 fitted by the Nix-Gao model is HV ? 0.78 H0.The aged T92 steel exhibits a more pronounced indentation size effect than the original one due to its lower statistically stored dislocation density.Although the strength and ductility of T92 steel decreases slightly after long-term thermal aging,it still meets the SA-335 standard.The results of in-situ synchrotron radiation X-ray diffraction tensile tests show that the lattice strain in the aged sample is smaller than that of the original one.The degradation of tensile property for T92 steel during thermal aging can be mainly attributed to the coarsening of Laves precipitates.Both the unaged and 15,000 hours aged T92 steels were irradiated with protons to 0.01,0.05 and 0.20 dpa and with Fe ions to 0.25 dpa,0.50,1.00 and 5.00 dpa at room temperature respectively.Both the dislocation-type and vacancy-type defects were induced by ion irradiation.The dominant a0<001>type dislocation loops induced by proton irradiation distribute uniformly in the matrix,while the 1/2 a0<111>type loops distribute alon g the dislocation lines.All dislocation loops in T92 after irradiation by Fe ions for 5.00 dpa are a0(001)type.The concentration of vacancy defects and the proportion of multi-vacancy defects grow with increasing irradiation dose.Compared to the unaged sample,more dislocation-type defects and fewer vacancy-type defects induced by ion irradiation were found in the aged samples under the same irradiation conditions,and slight element segregation is only observed in the aged one.The microscopic hardness increases exponentially with increasing irradiation dose.The hardness increment per dpa decreases with increasing dose,resulting in the hardening saturation.After long-term thermal aging,the aged T92 sample exhibits a more pronounced irradiation hardening effect than the original one,due to its higher density and larger size dislocation loops induced by ion irradiation.The values of hardness in the aged sample are always lower than those in the original ones under the same irradiation conditions.