Study On Microstructure And Properties Of SUS444 Stainless Steel Brazing Joint Based On Nickel Base And Iron Base Filler Metals

With the increasing strictness of Exhaust Gas emission regulations of engines at home and abroad,EGR(Exhaust Gas Recirculation)technology has been widely applied.EGR cooler is an important component of EGR system.At present,Ni-Cr-Si-P alloy is the main brazing filler metal,but the brazing of ferritic stainless steel based on this filler metal has not been studied yet.At the same time,due to the fluctuation of nickel price,the production cost of EGR cooler increases.Iron-based filler metal has become a trend to replace the traditional nickel-based filler metal,but there are still many problems in its wettability,brazing process and performance of joints.In this paper,the self-made 1#(31.5Fe29Cr27Ni6Si6P0.5Nb)and 2#(41.5Fe29Cr17Ni6Si6P0.5Nb)iron-based filler metal and BNi-16 nickel-based filler metal are used for vacuum brazing SUS444 and SUS304 dissimilar stainless steel,to verify and compare the wetability of the filler metals.The joint mechanism of element diffusion and solidification in the process of brazing was explored,and the influence of brazing temperature,holding time and lap gap on the microstructure and mechanical properties of the joint was analyzed.Finally,the corrosion behavior of the joint in acidic solution and the mechanical properties at high temperature were studied.The three kinds of brazing filler metal have good wettability at brazing temperature and can wet the joint well.After high temperature heating,the grain growth and mechanical properties decreased within the allowable range of the base metal.The diffusion bonding mechanism of the three typical brazing joints is different,that is,different microstructure is formed in different solidification stages.During the isothermal solidification stage,the nickel-based solid solution and iron-based solid solution are formed in BNi-16 and 1# filler metal,respectively,while the small size solid solution and eutectic microstructure are formed in 2# filler metal.In the cooling and solidification stage,BNi-16 and 1# filler metal all form massive solid solution,intermetallic phosphide and eutectic structure in the center of brazing joint,while 2# filler metal only forms eutectic structure in the center of brazing joint.According to the application requirements of the filler metal,the three kinds of filler metal have good bonding strength,among which the shear strength of BNi-16 reaches 209.3MPa,the shear strength of 1# and 2# brazing alloy is 164.7MPa and 160.0MPa,respectively,which is more than 75% of BNi-16.In shear failure of joints,a large amount of intermetallic phosphide is the main cause of joint fracture.Increasing brazing temperature can promote the formation of fusion,liquid phase diffusion and isothermal solidification microstructure of base metal at brazing joint interface,and the solid diffusion of elements Cr,Ni,Si and P towards SUS444 side also increases,which affects the distribution of eutectic microstructure in the center of brazing joint.The shear strength of the joint increases first and then decreases with increasing brazing temperature.The main reason for the decrease of the joint strength is the increase of phosphide accumulation between metals in the brazing center of BNi-16 and 2# filler metal.The strength of 1# filler metal decreases obviously due to the formation of large virtual holes in the brazing joints.The extension of holding time promotes the homogenization of chemical composition and microstructure in brazing joints,and increases the diffusion amount and diffusion distance of brazing elements to the base metal of SUS444 side.The proportion of solid solution in brazing joints of BNi-16 and 1# brazing joints is higher,and the microstructure of 2# brazing joints tends to form similar to that of SUS444 base metal.The hard brittle phase in the brazing joint tends to disappear under the longer holding time,which makes the shear strength of the joint increase,even the strength of 1# and 2# brazing filler metal will exceed that of BNi-16.Although increasing the gap can promote the growth of isothermal solidification microstructure into the brazing joint to a certain extent,the proportion of cooling solidification microstructure in the brazing joint will still increase.The shear strength of the joint decreases with the increase of the gap value.When the gap is 200μm or above,the isothermal solidification structure of the staggered convex makes the strength basically stable.The brazing corner of T-joints is mainly composed of the mass solid solution and eutectic structure.The solid solution in eutectic structure is easy to corrode as the anode in the micro-galvanic corrosion,while the intermetallic phosphide as the cathode does not corrode.With the increase of corrosion duration,the contact area between the corrosion solution and the microstructure in the brazing corner will increase due to the increase of corrosion current,resulting in faster corrosion rate.Due to the chemical composition difference between the interface region and the base material diffusion region,micro-galvanic corrosion may also occur at the interface between the two regions or on the base material.Taking the maximum corrosion depth as the evaluation of corrosion resistance,the corrosion resistance of BNi-16 is relatively good,and that of 1# filler metal is slightly worse than that of BNi-16,2# filler metal is relatively poor.With the increase of test temperature,1# and 2# filler metal has the same high temperature shear strength as BNi-16,and can meet the requirement of shear strength ≥50MPa at 700℃.