Effect Of Solution Treatment On High Temperature Creep Properties Of Inconel625/BNi-2 Brazed Joint

As the world’s energy and environmental problems intensify,high-temperature-resistant compact and high-efficiency heat exchange equipment is an important carrier for energy conversion and heat transfer,recycling high-temperature waste heat and improving energy efficiency,which has broad application development prospects.Under severe service environments such as high temperature and high pressure,brazed joints,as the weakest part of high-efficiency compact heat exchangers,are the key to high-reliability and long-term safe service of the equipment,and creep rupture is one of its main failure modes.Therefore,exploring the mechanism of strengthening and toughening of brazed joints and their influence on creep life and fracture mechanism,and realizing the essential improvement of the overall creep performance of joints,is of great significance to the development of precise life-based design and manufacturing.In this paper,Inconel625/BNi-2 brazed joint in the aero-engine regenerator is taken as the research object,the microstructure and creep properties of the brazed joints before and after the solution treatment are explored,the weak area and fracture behavior of brazed joints under the action of creep are proved.The main research contents and conclusions of this article are as follows:(1)The brazed joints were treated with high-temperature solid solution,and the microstructure of Inconel625/BNi-2 brazed joints was characterized by metallography,scanning electron microscopy,electron backscatter diffraction technology and other test methods.After solution treatment,the distribution density of a large number of dispersed rice grain and needle-like borides in the joint diffusion affected zone decreases,the overall corrosion resistance is improved,and the isothermal solidification zone and the diffusion zone expand.The alloying elements continue to diffuse with the solid solution treatment,and the element composition of each micro area of the joint tends to be uniform.The content of Nb,Cr and other elements in the isothermal solidification zone is significantly increased,and the content of B element in the diffusion zone is significantly reduced.As the solution treatment time increases,the grain size of each micro-area increases.(2)The uniaxial creep curve of brazed joints at 650℃ has an initial stage and a steadystate stage.The creep fractures under different solution treatment schemes are relatively smooth,and there is basically no necking and extension.The creep life of brazed joints after solution treatment for 3 h is the most prolonged,and the creep life under 140 MPa stress reaches 3501 h,which is more than twice as long as that of untreated brazed joints.The creep resistance of brazed joints before and after solution treatment is between base material Inconel625 alloy and solder BNi-2.The value of each creep performance parameter of the joint after treatment for 3h is closer to the base material parameter,so the creep resistance performance is better.The various creep parameters obtained in the test have been well verified in the simulation.(3)The variable fracture is divided into three regions according to the fracture mode: the initiation zone,the extension zone and the instantaneous fault zone.Creep fractures of untreated brazed joints all occur at the boundary between the isothermal solidification zone and the diffusion zone.The initial fracture is intercrystalline brittle fracture,and the latter is transcrystalline brittle fracture.The fracture position of the joint after solution treatment for 3h is the same as that of the untreated joint.The fracture is mostly intergranular ductile fracture at the initial stage and transgranular brittle fracture at the later stage.After solution treatment for 6 h,the borides in the diffusion zone have been basically eliminated in the joints,and the fracture position becomes the center of the braze joint,and ductility and brittle fracture interact.(4)For untreated brazed joints,a large amount of brittle borides reduces the bonding force of the grain boundaries in the diffusion zone,resulting in a decrease in the creep resistance and strength of the diffusion zone,and becomes the weakest creep position of the brazed joint.And there is a continuous and straight grain boundary at the junction of the brazing seam and the diffusion zone,which is easy to crack continuously.However,the presence of a small amount of borides between the crystals can act as a pinning point in the creep dislocation movement,resulting in a pinning effect,enhancing the durability of the creep process,and thereby increasing the creep life of the joint.Therefore,the improvement effect of solution treatment for 3 h of joint creep performance is better than solution treatment for 6 h.