Study On The Electroslag Welding Process And Joint Performance Of U71Mn Rail

With the development of rail transit towards high speed and heavy load,seamless rail has become an important guarantee for the safe and high speed operation of trains.At present,the welding methods of rail mainly include flash welding,aluminothermic welding and pneumatic welding.Flash welding and pneumatic welding equipment is huge,expensive,not suitable for on-site welding;Thermite welding is easy to operate,but slag is easy to exist in weld metal and its performance is poor.Electroslag welding is a new and important method to build seamless railway track.This paper mainly studies the welding process of rail steel three wire electroslag welding and the structure and performance of welded joints.Analyzes the defects under different welding process parameters,measures the static bending strength of rail welded joints under different process parameters.Analyzes the microstructure of welded joints,and optimizes the appropriate welding parameters.The microstructure and properties of welded joints under different heat treatment temperatures were studied.The welding parameters of U71 Mn rail steel three wire electroslag welding are optimized.When the welding current is less than 375 A and the voltage is less than 33 V,the heat input is small,and the rail bottom is prone to fusion defects.When the welding current of rail head is lower than 285 A and the voltage is lower than 27 V,the rail head has the defect of incomplete fusion.When the welding current is greater than 425 A and the voltage is greater than 35 V,the heat input is relatively large,and the rail bottom is prone to over-melting,with serious undercut.When the welding current of rail head is higher than335 A and the voltage is higher than 29 V,the rail head has undercut phenomenon.When the welding current is 375~425A and the voltage is 33~35V,the rail bottom and the rail waist are well formed without any welding defects.When the welding current of rail head is 285~330A and the voltage is 27~29V,the rail head is well formed and no welding defects appear.The microstructure and properties of rail welded joints with three wire electroslag welding were studied systematically.With five kinds of heat treatment process design:respectively welded joint heated to 500?,600?,700?,800?,900?,and then keep warm 1h,then air cooling.The microstructure under the mode of welding is a small amount of ferrite+pearlite,heat affected zone mostly lamellar pearlite.At 500?heat treatment,weld microstructure is acicular ferrite+ pearlite,heat affected zone for lamellar pearlite;At 600? heat treatment,weld microstructure is acicular ferrite+ pearlite,heat affected zone for lamellar pearlite + spheroidal pearlite.At 700? heat treatment,weld microstructure is ferrite+pearlite,heat affected zone group is a large amount of spheroidal pearlite;At 800?heat treatment,weld microstructure is reticular ferrite + pearlite,heat affected zone for lamellar pearlite.At 900? heat treatment,weld microstructure is block ferrite + pearlite,heat affected zone for lamellar pearlite.In the as-welded state,the tensile strength of the weld metal reached 729 MPa,the elongation after fracture is 6.8%,the impact toughness is 16.4J,the tensile strength in the heat-affected zone is 1021 MPa,the elongation after fracture is 6.3%,the impact toughness is 10.8J,the width of the softening zone of the welded joint was 20 mm,and the static bending strength is 1210 KN.Mechanical properties at 900? heat heat affected zone,best tensile strength reached 1114 MPa,impact toughness for 13 J,respectively by 8.9% and20.4% higher than that of welding condition,softening zone width is reduced to 4mm,heat treatment after the static bending strength of 1299 KN,the welding condition of welded joint by 7.4%.