| High-speed and over-loaned are the main targets of the modern railway. To achieve these two targets, building super long no-seam rail line is necessary. In this paper, the mechanical properties and microstructure of flash butt welding joint of the austentic manganese steel frog and the track steel had been researched by the metallographical examination, TEM, SEM and tensile, impact and hardness test. Its result has the very important practice meaning.The results showed that the microstructure of the HAZ was consisted of granular austenite in the austentic manganese steel side, but no carbide precipitates. The microstructure of the coarse grained zone is consisted of coarse pearlite and little ferrite in the track steel side; the microstructure of the granular grained zone is consisted of granular pearlite and little granular sorbite.Hardness test showed that the hardness of HAZ of the track steel was higher than the hardness of HAZ of austentic manganese steel, but it was not uniform on whole HAZ, the hardness of the granular grained zone was higher than the hardness of coarse grained zone and the inadequacy re-crystal zone. But the hardness is uniform on the HAZ of austentic manganese steel side. The hardness of buildup welding layer on the rail bottom is higher than the hardness of austentic manganese steel, but it is lower than the hardness of track steel.The tensile strength of flash butt welding joint is about 60% of the tensile strength of the track steel, and the impact toughness of the flash butt welding joint is above 60% of the impact toughness of the track steel. The properties of the flash butt welding joint achieved the level of the foreign experiment results. It can meet the requests of the correlation technology condition. The scanning electron microscope test showed that the fracture of the flash butt welding joint was cleavage fracture and quasi-cleavage fracture. The flash butt welding joint has good plastic and impact toughness.
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