Research On Vacuum Diffusion Welding Technology Of Low Activation Martensitic Steel

Because of their better void swelling resistance,thermo-physical and thermo-mechanical properties,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.The Test Blanket Module(TBM)has a complex structure and a relatively poor service environment,different components need to be solid connected by welding or other solutions.For the past decades,welding properties of RAFMs has been studied in many institutes and universities,including hot isostatic pressing joining(HIP),plasma arc welding(PAW),electron beam welding(EBW)and laser beam welding(LBW).Because of the relatively high contents of alloy elements,the low activation martensitic steel needs more stringent requirements of welding technical.Liquid-solid thermal cycling and non-equilibrium solidification of weld zone exist in traditional fusion welding,which would cause the degradation of microstructure and properties,thus affect the safety and reliable operation of the fusion.And solid-phase joining technique is expected to replace the traditional welding process and application in fusion experimental reactor blanket module on account of the advantages,such as the lower welding temperature and high precision assembly.In this paper,the phase composition and microstructure of the weld zone,the microstructure evolution during heating and heat preservation are analyzed,through the observation of optical microscopy(OM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and X ray diffraction(XRD),the diffusion bonding specimens of low activation martensitic steel;The effects of different process parameters and grain structure on the mechanical properties were investigated,through the tensile and impact test of the weldments and the comparison of mechanical properties of weldments after heat treatment.Results show that,the microstructure of the original parent material is lath martensite.The low activation martensitic steel happens Recrystallization and austenite in the process of vacuum diffusion welding.When the welding temperature is high or the holding time is longer,it will promote the growth of austenite,and the coarse austenite grains can damage the mechanical properties of the diffusion welded joints.At the end of the heat preservation time,the microstructure of the weld zone is martensite,residual austenite and a large number of precipitated carbides.However,the retained austenite structure disappeared after the heat treatment.When the welding temperature at the range of 950~1100?,with the increase of the welding temperature,the tensile strength increases,and the maximum value is reached at 973 MPa when the welding temperature is 1050?,when the welding temperature continues to rise to 1100?,the tensile strength decreases.When the holding time is from 90 min to 180 min,the tensile strength increases with the increase of holding time,however,when the holding time is more than 150 min,the tensile strength of the joint is not enhanced,and it tends to be gentle.Under the pressure of 10~20MPa,the tensile strength of the welded joint has been improved steadily with the increase of welding pressure.To improve the welding temperature,the diffusion effect can be enhanced by increasing the number of atoms,and the tensile strength can be improved to some extent,however,the higher welding temperature will also promote the merger and growth of austenite grains,and the coarse grain structure of the coarse austenite grains will reduce the tensile strength.Therefore,when the welding temperature is reached,the tensile strength will decrease with the increase of welding temperature.Similarly,the longer holding time for good diffusion provides the necessary conditions,but when the holding time is too long,although active atom diffusion more fully,but because of coarse austenite,the tensile strength has no obvious improvement.When the welding pressure is greater,the greater the degree of plastic deformation of the weld surface,the greater the actual contact area between the welding surface,thus,the active atoms in the vicinity of the weld surface can obtain enough diffusion channels to carry out the solid state self diffusion through the irregular migration and across the interface.The impact toughness of welding mainly depends on the grain size of the weld zone of diffusion welding at the end,when the austenite grain increased with prolonging holding time and welding temperature is more and more big,the impact toughness of weld metal is getting worse.