Reaearch On Microstructures And Properties Of Electron Beam Welding Joints And Laser Welding Joints Of CLF-1 Steel

Deuterium and tritium,which exist ubiquitous in seawater,are regarded as the main raw materials for preparing nuclear fusion fuel.Nuclear fusion can release huge energy instantly,and it is clean,safe and reliable during the release process.It is considered to be one of the effective ways to solve the problem of energy shortage and improve environmental pollution.Reduced activation ferritic/martensitic（RAFM）steel is considered to be the preferred structural material for ITER（International Thermonuclear Experimental Reactor）TBM（Test Blanket Moduledue）to the excellent irradiation resistance,low thermal expansion coefficient and outstanding mechanical properties.CLF-1 steel,independently developed by China,has become the main candidate material for TBM structure in China.Due to the poor weldability of CLF-1 steel,the welding method with higher heat input can not meet the performance requirements.Therefore the high-energy beam electron beam and laser welding method with lower heat input become the optimal choice.To overcome CLF-1 steel welding core jam technology is of great scientific significance and practical value to promote the progress of China’s nuclear energy industry.The CLF-1 steels with 32 mm plate thickness were welded by vacuum electron beam.Under the optimum technological parameters,the welded joints were well formed and no metallurgical defects（such as cracks）occurred.After welding,the specimens were subjected to high temperature tempering（710℃/2 h/AC（air cooling）,abbreviation HTT）and incomplete annealing（840℃/2 h/AC,abbreviation IA）heat treatment.This work compared the microstructure and mechanical properties of the specimens under as-welded,HTT and IA states.The results showed that weld metal（WM）was composed of thick lath martensite,in addition,and the heat affected zone（HAZ）was composed of a large amount of massive martensite and dispersed fine MX and M₂₃C₆.Compared with the as-welded WM,the nanoscale M₂₃C₆and MX carbides precipitated in HTT WM had the effect of dispersion strengthening.Compared with the HTT state,the martensite structure of the WM and HAZ in the IA state became coarser,and both M₂₃C₆and MX tend to grow together.The welded joints under HTT state heat treatment reveal the best comprehensive mechanical properties,moreover,the hardness range,the tensile strength,elongation and area shrinkage are approximately 190～250 Hv,705.89 MPa,19.65%,and 241 J at room temperature（23℃）,respectively.The CLF-1 steels with 6 mm plate thickness were welded by general laser welding and Ar environment laser welding.The welded joints were well formed under the optimum technological parameters and no metallurgical defects（such as cracks）occurred.After welding,welded samples of Ar environment were treated with high temperature tempering710℃/2 h/AC（air cooling）and normalizing 980℃/1 h/AC+tempering 740℃/2 h/AC.The results showed that the microstructure of the welded joint general laser welding was basically the same as welded joints of Ar environment laser welding.WM was composed of thick lath martensite,however,the width of martensite in WM of Ar environment laser welding was narrower,and the oxygen content of WM of Ar environment laser welding decreased by 16 times,resulting in the impact performance improved significantly.Compared with Ar environment welding sample,the microstructure of lath martensite in WM was finer after heat treatment at 710℃/2 h/AC,and a large amount of nanometer M₂₃C₆and MX carbides were precipitated,resulting in a significant increase in ductility.At the same time,WM structure is tempered martensite after heat treatment at 980℃/1 h/AC+710℃/2 h/AC.However,precipitated phase M₂₃C₆obviously accumulated and grew up,which weakened the effect of dispersion strengthening and the degree of toughness improvement.The grain orientation of laser welded joints was random under the four processing conditions,and heat treatment had no obvious effect on crystal orientation.In general,after laser welding in Ar environment,and the welded joints with 710℃/2 h/AC heat treatment has the best mechanical properties.The hardness range is about 240～280 Hv.The tensile strength,the elongation,the area shrinkage and the toughness are approximately 666.6 MPa,15.33%,73.64%and 239 J at room temperature（23℃）,respectively.The tensile strength,the elongation,the elongation and the toughness of the weld are about 348.66 MPa,19.2%,73.64%and 239 J at 550℃,respectively,and the mechanical performance meets the requirements of TBM.