Study On Submerged Arc Welding Process And Welded Joint Properties Of N610CF Steel In Hydropower Station

By virtue of good comprehensive performance,N610CF steel is widely used in pressure steel pipe and other metal structures of hydropower stations.The welding quality of N610CF steel is directly related to the safety of hydropower stations.At present,single wire submerged arc welding is mostly used for the welding of hydropower pressure steel pipe.Double wire submerged arc welding can greatly improve the welding efficiency of of hydropower station pressure steel pipe,but there is no application report in domestic hydropower system.In order to ensure the quality of welded joints of pressure steel pipe and improve the welding efficiency,it is of great significance and value to study the welding process and joint performance of double wire submerged arc welding.In this paper,the single wire submerged arc welded joints of N610CF steel for hydropower and the double wire submerged arc welded joints under different welding speeds(V=60cm/min,V=70cm/min,V=80cm/min)were studied.The distribution of residual stress under different welding processes was analyzed by welding thermal cycle test and residual stress test.The microstructure and basic mechanical properties of welded joints under different welding processes were studied by metallographic test,hardness test,tensile test,bending test and low temperature impact test.The micro shear mechanical properties of joints under different welding processes were studied by micro shear test.The fracture toughness of weld zone and heat affected zone of joints under different welding processes was carried out by three-point bending loading method.The fracture toughness was analyzed by SEM and microstructure observation.And the welding cracks that appeared during the bottom welding were analyzed.The main conclusions are as follows:The weld bead of the joint sunder different welding process conditions are well formed,and the conventional mechanical properties of the joint all meet the standard requirements.With the increase of welding speed,the cooling time t_8/5 and t_8/3 of double wire submerged arc welding are gradually reduced,and the maximum residual stress gradually increased.Double wire submerged arc welding can improve welding efficiency and ensure welding quality.Each micro-zone of the joint under different processes has high shear strength and indentation rate,indicating that its strength and plasticity are good.The change trend of shear strength of double wire submerged arc welding joints under different welding speeds is the same.From the weld center to the base metal,the shear strength decreased first and then increased.The average shear strength of the heat affected zone is the lowest.Moreover,the shear strength of coarse grained zone and partial phase transition zone in the heat affected zone are lower.Both the lowest shear strength position and the tensile fracture position are in the heat-affected zone,indicating that the heat-affected zone is the weak area of the joint.The CTOD values of weld zone and heat affected zone of single wire submerged arc welding joint and double wire submerged arc welding joint at different welding speed are relatively high.The existence of a large number of dimples in the fracture indicates the fracture toughness of the joint is good.When the welding speed of double-wire submerged arc welding is 70cm/min,the CTOD value of the heat-affected zone reaches the maximum,with an average value of 0.775mm.As the welding speed of double-wire submerged arc welding increases,the fracture toughness value of the weld zone shows a decreasing trend,but the CTOD value changes very little.The CTOD value reaches the maximum when the welding speed is60cm/min,and the average value is 0.680 mm.The cracks appeared during the bottom welding are crystalline cracks.The main cause of crystal cracks is that the welding current is too large and the voltage is too small during the bottom welding,resulting in a smaller weld shape factor,and columnar crystals of opposite growth are formed in the weld.During the process of weld solidification and shrinkage,the"liquid film"at the center of the weld cracked due to the increased tensile stress.