Study On Microstructure Formation Rules Of Solid-state Welding Joint Of 9Cr Ferritic/Martensitic Steel

9Cr ferritic/martensitic steels are considered as the most promising structural material in the nuclear reactors for their outstanding thermal physical properties.Due to geometrical complex structure in test blanket module,the welding requirements for9Cr ferritic/martensitic steels are high in nuclear power components.Generally the composition and microstructure of the joints are required to be uniform and good to resist radiation brittleness,meanwhile,the residual stress is as small as possible.Therefore,the maturity of welding technology and process is crucial for the practical application of 9Cr ferritic/martensitic steels in the nuclear reactor.This paper explores new welding technology of 9Cr ferritic/martensitic steels to provide the experimental foundation and new idea for the welding method of 9Cr ferritic/martensitic steels.Diffusion bonding with an electro-deposited Ni interlayer and linear friction welding are employed to join 9Cr ferritic/martensitic steels.The weldability of 9Cr ferritic/martensitic steel using diffusion bonding and linear friction welding as well as the microstructure evolution and mechanical properties of bonded joints are systematically characterized by optical microscope?OM?,scanning electron microscope?SEM?and transmission electron microscope?TEM?,respectively.The main conclusions can be summarized as follows:The diffusion bonding of 9Cr ferritic/martensitic steels with an electro-deposited Ni interlayer was developed.For the steel/Ni interface,the inter-diffusion of elements formed Ni-rich solid solution at the transition region that would transform into retained austenite at room temperature.The retained austenite along the joint interface may release the local stress concentration and reduce the microcrack formation opportunities in the steel during deformation due to its greater plasticity.In addition,the effect of tempering treatment after diffusion bonding on the microstructure evolution and mechanical properties of the joint is also studied.After tempering treatment,the joint interface in steel still has high density of dislocations,due to the pinning effect of solute atoms.The tempering treatment can improve interface bonding at the joints according to the tensile tests.Since the joints prior to tempering treatment fractured at the interlayer,while the joints subjected to tempering treatment fractured in the base steel.No intermetallic compounds were observed at the bonding interface.9Cr ferritic/martensitic steels can be effectively joined by linear friction welding,the joints that are free of cracks and pores can be obtained under appropriate welding parameters.The high-temperature plastic deformation and the dynamic recrystallization are caused by high temperature and high stress during welding,resulting in the smaller grains near the weld zone.Due to the strengthening of fine grains,the strength of the joints has been improved and the weld zone exhibits the highest microhardness.The heat input gradually decreases with the increase of the distance from the weld zone,partial dynamic recrystallization occurs in the thermo-mechanically affected zone and massive M₂₃C₆ carbide particles precipitated along the prior austenite and martensite lath boundaries in the heat affected zone.The tensile strength of joints is about 478 MPa and the fracture of the joints takes place at the base metal.Tensile strength tests at room temperature verify the reliability of the joint attained by linear friction welding.