Research On Effect Of Plastic Deformation Of Low Activation Martensitic Steel By Vacumm Diffusion Welding

Reduced Activation Ferritic/Martensitic?RAFM?steels have been considered as the primary candidate structural materials for the DEMO fusion reactor and the first fusion power plant because of its good void swelling resistance,thermo-physical and thermo-mechanical properties.The plastic deformation and welding processes are involved in the manufacturing and assembly processes of various parts of fusion reactors.Solid-state connection technology,represented by diffusion connections,plays an important role in the field of manufacturing of fusion reactor cladding modules.In this paper,vacuum diffusion welding test of low activation martensitic steels is carried out under different welding conditions and different pre-weld deformation conditions.By means of optical microscopy,scanning electron microscopy,energy spectrum testing,and X-ray diffraction observation of the diffusion welding sample,the microstructure,interface phase composition,and element distribution of the weldment base metal and weld zone were analyzed.Through the microhardness,room temperature tensile and impact test of the weldment joint,the difference in the mechanical properties of the weldment before and after the heat treatment was compared,and the influence of the pre-weld plastic deformation and the influence of the welding process parameters on the microstructure and mechanical properties was investigated.The results show that the original microstructure of the low activation martensitic steel is mainly lath martensite.During the vacuum diffusion welding heating and heat preservation process,the low activation martensitic steel will undergo recrystallization and austenite phenomenon.When the welding temperature is high or the holding time is longer,roughened austenite grains will be produced.The microstructure of the weld zone is tempered martensite and retained austenite after post weld heat treatment,.With the decrease of deformation temperature and the increase of deformation before welding,the martensite laths are gradually oriented,grains are gradually refined,and the dislocation density is significantly increased.The lattice distortions and defects introduced by the pre-weld plastic deformation can accelerate the process of diffusion bonding as a channel for atomic diffusion and facilitate the good bonding of the welding interface.There was no significant diffusion reaction at the interface of the weldment weld and certain precipitation of carbides of the M₂₃C₆ and MX phases at the weld joint and in the base material.Through the microhardness,room temperature tensile and impact test of the weldment joint,the difference in the mechanical properties of the weldment before and after the heat treatment was compared,and the influence of the pre-weld plastic deformation and the influence of the welding process parameters on the microstructure and mechanical properties was investigated.The tensile strength of the weldment under all welding process parameters and pre-weld deformation is higher than that of the base material.The tensile strength increases first and then decreases with the increase of the welding temperature.With the increase of the holding time and welding pressure,the tensile strength increases,and degree of promotion gradually became flat.The average micro-hardness and tensile strength of welded joints are higher than that of the material,and the fracture of all tensile samples occurs at the position of the base material.With the increase of the amount of deformation before welding,the dimples in the tensile fracture gradually become deeper and increase in size.The post-weld microstructure inherits the high dislocation martensite which has been deformed before welding.The weld with larger deformation before temperature deformation shows better tensile properties,and the retained austenite after heat treatment improves the the fracture toughness of weldment.With the increase of the deformation temperature before welding,the impact energy of the weldment has increased slightly.The post weld heat treatment failed to effectively improve the impact toughness of the weldment,coarsened austenite grains and the formation of inclusions and oxides at the diffusion interface.This is the reason for the drop in impact toughness.