Analysis of carbon migration phenomenon in dissimilar steel joints of 22MnB5 and QStE550TM
Carbon migration has always been a key issue in the welding of dissimilar steels,and the migration of car-bon elements will lead to uneven mechanical properties of welded joints.The carbon migration phenomenon of 22MnB5 and QStE550TM low alloy high strength steels after welding is investigated,which provides theoretical sup-port for the application of dissimilar steel welding of 22MnB5 and QStE550TM in actual automobile manufacturing.H08Mn2Si welding wire was used to weld QStE550TM steel and 22MnB5 steel by argon tungsten arc welding(TIG).Optical microscope(OM),scanning electron microscope(SEM),electron backscattering diffraction(EBSD)and electron probe analysis(EPMA)were used to study the carbon migration degree and microstructure of dissimilar steel joints,and universal testing machine was used to analyze whether the carbon migration phenomenon had obvi-ous influence on the overall mechanical properties of the dissimilar steel joints.The results show that the weld area and the heat affected zone on both sides of the welded joint are mainly composed of bainite,and the weld area is domi-nated by large angle grain boundaries,and the grain size is relatively uniform.Ferrite is the main component phase in the weld zone and heat affected zone,and the second phase(Fe,Ni)is also precipitated.C element segregation exists at the fusion interface of 22MnB5,and the thickness of carburization layer is about 70 μm.Besides lath bainite,feathery bainite and acicular ferrite are also observed in the carburization layer.The tensile and bending properties of the joints are good.The fracture mode of the tensile specimens is ductile fracture at QStE550TM side.The bending specimens are still intact and no cracks appear after face bending and back bending tests.Based on the above analysis results,it can be seen that the carbon migration phenomenon of this dissimilar steel joint does not obviously affect the overall mechanical properties.Combined with the fact that this joint does not serve in extreme environments such as high temperature and high pressure,the dissimilar steel joint can be obtained by using H08Mn2Si welding wire,which is in line with practical production and application.
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