Microstructure Characteristics And Service Reliability Of Welded Joint Of 16MnDR Steel For Nuclear Lean Material Storage Tank

At present,nuclear power has become an important choice for many countries to promote clean and low-carbon transformation of power structure and reduce dependence on fossil fuels.Most nuclear power plants require 235 U of isotopically enriched uranium as fuel.However,the enrichment and conversion processes produce a large amount of depleted uranium waste,of which the lower level depleted UF6 is mostly stored in storage directly.The 16 MnDR steel storage tanks used to store UF6 require a lot of welding during the production process,and the welded joints are often weak spots for the mechanical and corrosion properties of the welded structures.In order to meet the requirements of hydrofluoric acid corrosion resistance and low temperature impact toughness of 16 MnDR steel welded joints under actual service conditions,the study on the structure characteristics of welded joints and the evaluation of service reliability provide theoretical guidance for the safe service of nuclear lean material storage tanks.As for the research on the corrosion resistance of 16 MnDR steel welded joints to hydrofluoric acid,this paper uses dual-frequency pulsed TIG priming,Submerged Arc Welding(SAW)filling and covering to weld 16 MnDR steel,and gets the welded joints with good weld shape.After SAW filling,the structure of the bottom layer changed from Acicular Ferrite + Sorbite to polygonal Ferrite + wormlike Troostite.The immersion corrosion test results show that the corrosion resistance of the bottom layer of SAW after filling is degraded seriously,and its service life is predicted to be only4.5 years,which cannot meet the requirement of 10 years of life of nuclear lean material storage tank.Through the analysis of the priming layer corrosion mechanism,the results show that after SAW filling,the formed bottom layer was tempered twice.During the recrystallization process of the bottom layer,the C element was diffused through a small distance to form Troostite with small lamellar spacing and lamellar thickness.The cementite in the dense distribution of Troostite act as a single cathode in the galvanic corrosion system.They form strong corrosive galvanic couples with the surrounding Ferrite,which accelerates the anodic dissolution of the Ferrite around Troostite.This eventually leads to the loosening and shedding of the Troostite,resulting in the formation of corrosion pits.For the study of low temperature impact toughness of 16 MnDR steel welded joint,this paper adopts the traditional TIG priming and Submerged Arc Welding filling different pass welding methods to weld 16 MnDR steel.When one pass is filled,the CCT chart shifts move to the right due to the addition of a large number of alloyelements in the weld center area of the welded joint,and its good pinning effect on grain boundaries makes the grain in this area fine,which shows good low temperature impact toughness.However,due to the heat cycle at higher temperature,the grain size of the heat affected zone is obviously grown,which mainly presents the microstructure of thin and long lath Bainite.Moreover,a large number of white massive Bainite elementary structure formed in the microstructure provides a good channel to prevent the propagation of cracks,so its low temperature impact toughness is poor.When two passes are filled,the pinning effect of alloying elements on the grain boundary in the center area of the welded joint is weakened,resulting in coarse grains,a large number of grain boundary ferrites and carbide films forming on the grain boundary embrittle the grain boundary,which accelerates the brittle fracture of the samples in the impact test.Due to the small hot cycle,the lath Bainite generated in the heat affected zone is short and thick,and the stripped Bainite elementary structure generated in the microstructure can prevent the crack propagation,so that the region shows good low temperature impact toughness.Through the mechanism study of the relationship between the structure formation and joint performance,the theoretical foundation is laid for the low cost,high performance and safe service of nuclear lean material storage tank.