Study On Microstructure And Properties Of Flash Welded Joint Of Rail

With the rapid development of seamless rail,the demand for Flash Welded Joints of U71 MnG hot-rolled rail and U75 V heat-treated rail is increasing.Various quality problems often occur in the production process of these two rail joints in rail base.In order to ensure the quality of rail joints produced by rail base,it is of great significance to study the welded and normalized joints of these two rails.In this paper,the microstructures and properties of Flash Welded Joints and normalized joints of U71 MnG hot rolled rail and U75 V heat treated rail were studied.At the same time,the fracture failure analysis of U75 V joints which exceeded the standard by ultrasonic inspection and on-line service damaged U75 V joints was carried out.The microstructures of Flash Welded Joints of U71 MnG hot-rolled rail and U75 V heattreated rail are "reticular ferrite+ pearlite" and that of heat-affected zone are pearlite.The grains of weld and heat-affected zone are relatively large,the impact absorbing power and fracture elongation of joints are low,and the plastic toughness is poor.After heat treatment,U71 MnG hot-rolled rail and U75 V heat-treated rail joints have uniform distribution of "ferrite + pearlite" in the weld structure,and pearlite in the heat-affected zone.The grain size in the weld and heat-affected zone is obviously refined.After normalizing,the strength of the joint is increased,and the properties of the plastic toughness are enhanced,which meet the requirements of the standard TB/T1632.2-2014.The micro-shear strength of U71 MnG hot-rolled rail and U75 V heat-treated rail can directly reflect the mechanical inhomogeneity of the longitudinal section.The micromorphology of the shear fracture tear zone shows parabolic dimple pattern which is elongated along the shear direction.By analyzing the fracture surface of the joints that exceed the standard by ultrasonic testing,it is found that brittle fracture is the main form of drop hammer fracture of the "failure" joints.The main reason for the excess of ultrasonic testing echo equivalent is the presence of silicate inclusions and silicon manganese oxides in the joints.Fatigue initiation is the initial form of crack initiation for on-line service "failure" joints,and the energy spectrum at the initiation source shows that there are silicon manganese oxides.It is analyzed that the silicon manganese oxide formed during flash welding of rail is not extruded.