Study On Brazing Process And Mechanism Of High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steel?HNS?uses nitrogen element instead of nickel element to obtain stable austenite structure,which improves strength and corrosion resistance without sacrificing plasticity and toughness.Due to its excellent properties,high nitrogen steel has broad application prospects in the fields of machinery,chemical industry,energy,life science and so on,and has great pot ential to replace the traditional austenitic stainless steel?such as 304,316,etc.?.Of course,the industrial application of high nitrogen steel is inseparable from the development of connection technology.However,the traditional fusion welding method has the problems of nitrogen loss,nitrogen hole and so on.It is not suitable for the connection of complex shape parts.Therefore,in this paper,the brazing method with lower connection temperature is proposed to connect high nitrogen steel.The formation mechanism of microstructure of high nitrogen steel joint was studied.The internal relationship between solder composition,brazing process parameters,microstructure and mechanical properties of joint was revealed.The results are great significance to the development of high nitrogen steel and brazing technology.The Ni-Cr-B-Si amorphous filler metal was used for vacuum brazing of high nitrogen steel.The microstructure analysis of the joint shows that Cr₅B₃ compound is formed in the center of the joint,its content decreases with the increase of brazing temperature.When the brazing temperature is 1100?,Cr₅B₃ compound disappears completely.BN compound is formed at the interface of base metal and solder,its content increased with the increase of brazing temperature.The test results of the mechanical properties of the joint show that the BN compound at the joint interface plays a decisive role in the strength of the joint.When the brazing temperature is1020?,the best joint shear strength?176.7MPa?is obtained.The high nitrogen steel was vacuum brazed with Ni-Cr-P solder without B element.The results show that?Ni,Cr?₃P compounds are continuously distributed in the center of the solder joint.Cr₂N compounds are formed at the interface between the base metal and the solder.The content of both compounds decreases with the increase of brazing temperature.When the brazing temperature increased to 1050?,Cr₂N compound disappeared because of dissolution,and?Ni,Cr?₃P compound also decreased significantly,so the best joint shear strength?163.0MPa?was obtained.High nitrogen steel and 316 L stainless steel were vacuum brazed with Ag-Cu eutectic filler metal,the latter was used for comparative experiment.Compared with Ag-Cu/316L,Ag-Cu/HNS system shows lower equilibrium wetting angle and shorter wetting time.The reason is that Mn element in high nitrogen steel can dissolve into Ag-Cu solder.The microstructure analysis shows that 316 L/316 L joint is composed of?-Cu and fine Ag-Cu eutectic structure,while HNS/HNS joint is composed of?-Ag and thick Ag-Cu eutectic structure.At the same time,there is a diffusion layer composed of Ag,FeCr and?-Fe in the base metal near the interface.The optimum shear strengt h of HNS/HNS joint is 290.0MPa,much higher than that of 316L/316L joint?175.0MPa?.