Research On Low-Temperature Diffusion Bonding Of Zirconium Alloy And Corrosion Resistance Of Joints

Zr-4 alloy is widely used in water-cooled nuclear reactors due to its low thermal neutron absorption cross section,good mechanical properties and processing properties,and excellent corrosion resistance in high temperature and high pressure water and steam.At present,the commonly used connection method usually realizes the welding of Zr-4 alloy through a large heat input(such as TIG welding,etc.),which often causes the base material grain to grow and the performance decreases or the deformation is difficult to meet the accuracy requirements,so the realization of Zr-4 The low-temperature connection of the alloy is particularly critical,and the low-temperature diffusion bonding can obtain a joint with relatively ideal strength at a lower connection temperature.In this subject,the surface hydrogenation layer and the surface nanocrystalline layer were prepared by electrolytic hydrogenation and surface mechanical polishing.Then,the joints with the weld strength equivalent to the base metal were obtained at a temperature lower than the conventional connection temperature,and finally the surface treated Zr The corrosion resistance of-4 alloy was studied.First,the effect of surface treatment process parameters on the surface hydrogenation layer and nanocrystalline layer was studied,and the effect of surface hydrogenation and nanocrystallization on the performance of the Zr-4 zirconium alloy surface layer was explored,and the hydrogenation and mechanical polishing(SMAT)were determined.Process parameter range.The results show that the diffraction peak of the Zr-4 alloy after hydrogenation treatment changes significantly in the phase of 30°?40°.The hydrogen atom content exceeds its solid solubility in?and precipitates into?(Zr H₂)phase.The?phase is observed in the electron diffraction pattern.The surface roughness value of the sample after hydrogenation treatment increased slightly but did not change much,the change range was 0.1?0.2?m,and the microhardness of the surface layer hardly changed.After SMAT treatment,the diffraction peaks of Zr-4 alloy are broadened.When the treatment frequency is 50 Hz,and the treatment time is 4 hours,the surface layer grain size calculated by the Scherrer formula is below 30 nm.The surface roughness value of the sample after SMAT treatment increased first and then decreased with the extension of SMAT time,and the variation range was 0.1?0.2?m,and the microhardness of the surface layer increased.A low-temperature diffusion bonding experiment was carried out on the Zr-4alloy after surface treatment,and the influence of surface hydrogenation and nanometerization parameters on the microstructure and properties of the joint was explored,and the influence of the diffusion bonding process parameters on the microstructure and properties of the base metal surface treatment joint was investigated.The best low-temperature diffusion bonding process parameters for the surface treatment samples are presented.The results show that Zr-4 alloy has improved the interface diffusion efficiency after surface nano-treatment,and the joint welding rate has increased significantly.The sample after hydrogenation on the surface also greatly improved the interface diffusion due to the defects of hydrogen atom vacancies during diffusion bonding.effectiveness.In order to combine the advantages of the two,a surface composite treatment is used,that is,nano-treatment first and then hydrogen treatment.Under the conditions of connection temperature of 770?,holding time of 1h,and connection pressure of 5MPa,a joint with a shear strength of 470MPa is obtained.,Approximately equal to the strength of the Zr-4alloy base material.The oxidation kinetics behavior of Zr-4 alloys with different surface states at400?water vapor corrosion was fitted.The early oxidation behavior of Zr-4 alloys can be divided into two stages:"fast oxidation"and"slow oxidation".It is found that hydrogenation can slightly decrease the corrosion resistance of Zr-4 alloy,while nanometerization improves the corrosion resistance of Zr-4 alloy.The corrosion resistance of the welded joint of the diffusion joint after nanometerization treatment is similar to that of the base material.almost the same.