Microstructure And Properties Of Vacuum Brazed Joints For 42CrMo Steel Piston

With the continuous increase of diesel engine heat load,the load borne by the piston,the key component in the cylinder,is greatly increased,which leads to frequent high-temperature failure of the traditional aluminum alloy piston.Therefore,a new type of steel structure piston is obtained by vacuum brazing method,which is made of high-strength steel material,due to its characteristics including high hardness,low thermal conductivity and thermal expansion coefficient.In this paper,a new type of steel piston is developed by vacuum brazing method,and vacuum brazing connection of 42CrMo steel piston is realized by nickel base filler metal.The microstructure and fracture morphology of 42CrMo steel brazed joints were characterized by means of optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectroscopy(EDS).Tensile strength and microhardness were evaluated by universal tensile testing machine and microhardness tester.It is hoped that the results of the present work would provide a theoretical foundation and experimental basis for the brazing of 42CrMo steel,as well as broaden the engineering application scope of steel piston.The results indicated that Ni-based filler metal(BNi-2-1 and BNi-2-2)had a good wetting with 42CrMo steel,and there were good bonding interface without crack or micro-hole observed in the brazed joint.According to the solidification characterization and microstructure,42CrMo steel piston brazed joint was divided into three zones:isothermal solidification zone(ISZ),athermal solidification zone(ASZ)and diffusion affected zone(DAZ).When 42CrMo steel piston was brazed with Ni-Cr-Si-B filler metal,the diffusion of boron from brazing filler into base metal is the main controlling factor pertaining to the microstructural evolution of the joint interface.When the brazing temperature was 1060?,the brazed seam was mainly made up of y-Ni solid solution,Ni3B,CrB,Ni3Si.The petal Ni3Si particles dispersed in solid solution,and some boride was precipitated in the DAZ.When the brazing temperature increased,the more diffusion of element B,the less eutectic structure,the wider DAZ and more precipitated phase.But lower the temperature,there will be residual part of molten solder.Accordingly,temperature increase lead to loss of liquid solder,even defects like micro-hole.The maximum microhardness appeared in the center of brazed joint,and the microhardness of connection interface(362HV0.5)was higher than base metal(200HV0.5).Eutectic structure formed in brazed joint was of high hardness,and with the diffusion of B,boride formed with Fe and Cr in DAZ played a role in precipitation strengthening.Increasing temperature promoted the elements diffusion,resulting in the decrease of eutectic and hardness in the center of brazed joint,and the growing of precipitated phase and hardness at the joint interface.42CrMo steel piston brazing temperature was 1060?,the tensile strength was 406 MPa and tensile fracture showed obvious brittle fracture.The fracture located in the brazing seam and originated from the interface near the base metal.The microfracture was mainly characterized by brittle transgranular fracture of cleavage steps and quasi-cleavage river pattern,and a little plastic dimples fracture occurred locally,which was a mixed fracture combining brittle and ductile fracture.